Adaptive Wing Design For A Morphing Mav Biology Essay

Adaptive Wing Design For A Morphing Mav Biology Essay The wing design of a Micro Air Vehicle is to be designed to aid in the assessment of earthquake damage. The MAV is required only to be powered by an electrical motor, limiting flight time and endurance/range, and must be able to fly to the scene quickly and efficiently. Therefore, the analysis of the wing structure is two-fold: 1) having an efficient wing design for cruise (faster) flight and 2) having an effective loiter (slower flight) wing design. The main methodology in ensuring the optimal wing structure would be to design two aerofoils with an actuator which will morph the wing dependant on the different requirements. The software package DESFOIL, on the University of Sheffield intranet, is considered a friendlier user interface for the original XFOIL package created by MIT Professor Mark Drela to aid in the understanding of low speed aerofoil flow solution. However, since XFOIL contains a less than friendly interface, DESFOIL, a MATLAB based software, creates a friendlier user interface, enabling aerofoil analysis to be more easily understood. NACA Aerofoils Since NACA aerofoils will be the predominant choice in attaining the aerofoils for our MAV, this section will concentrate on providing a breakdown on the key aspects of such aerofoils. The 4 digits associated with the NACA aerofoil provide information as to the physical structure of the aerofoil. The four digits are broken up as such The first number offers information about the maximum camber as a percentage of the chord length. The second digit provides information as to the position of this maximum camber as a function of the overall chord length (in tens of units) The final two digits provide information on the maximum thickness of the aerofoil as a percentage of the overall chord length, as a percentage. Thus, it can easily be seen that the chord length is pivotal in determining the correct NACA aerofoils. Centre of Pressure and Aerodynamic Centre Other important aerofoil characteristics are the centre of pressure and aerodynamic centre. If we consider them individually, we can attain a clearer picture of their importance. Starting with the centre of pressure, it is known that the positioning of this point changes with lift, namely it moves towards the leading edge as lift increases. However, using this same theory, it can be understood that this centre of pressure can move outside of the aerofoil geometry. More specifically, at low levels of lift generation, the pressure centre can be behind the aerofoil. Such a parameter is important to understand since it allows for trimming of the potential aircraft on which the aerofoil will be mounted. The centre of pressure can be calculated using calculus provided a pressure distribution is understood. Since we are dealing with a distribution, the length of the chord becomes increasingly important and it is normal to talk about the pressure as a function of chord length, generally starting with the leading edge. The aerodynamic centre contains a more direct link with the stability of the aircraft/aerofoil. The aerodynamic centre is the point on the aerofoil where the pitching moment of the aerofoil is independant of the angle of incidence. Understanding of this point is crucial due to its large contribution to the balancing and therefore stability of the aerofoil/ aircraft. Since the majority of aerofoils which will be considered within this journal are cambered, it is interesting to note that the aerodynamic centre is approximately situated at a point  ¼ of the chord length. Drag When considering drag, the main thought is of a retarding force to motion in a certain direction. The relationship is simple; the greater the drag (retarding) force, the greater impedance on motion. From an aerofoil design point of view, we wish for such force to be as minimal as possible since a lower retarding force allows faster speeds (longer endurance and/ or range) and more efficient flight. To understand more about drag, we need to understand more about the different components of the force. If we can understand the forces main constituent parts, this may help us lower the drag our aerofoil experiences. The drag force can be broken up into several different components. Some of these are Parasitic drag Lift induced drag As we will later calculate, the lift our aerofoil produces will vary depending upon which flight attitude regime we are in i.e. loiter or cruise, and thus the drag each regime experiences will vary. Since this is important in understanding how the aerofoil will react to regime changes, the lift induced drag will be looked at more closely within this analysis. The concept of parasitic drag is broken into many different parts. Such constituent parts include skin friction and pressure drag. The concept of skin friction comes about due to the interaction of fluid molecules contacting the surface of the aerofoil, bringing local wall shear stresses into consideration. It can thus be seen that the faster the motion of molecules past the aerofoil, the larger wall shear stress. The skin friction coefficient shares an inverse parabolic relationship with the speed of the aircraft The contribution from pressure drag will be considered in terms of flow separation points further into this report. Such drag will take precedence in the analysis of drag within this report since it provides a more rigorous depiction of the drag at different angles of attack and different DESFOIL accuracy parameters (panel number). Since DESFOIL offers only this type of drag, it will be assumed that this pressure drag is the overall coefficient of drag, when discussing analysis of graphical data. This is a reasonable assumption since the drag values and pressure distributions compliment each other. Furthermore, since we will later consider the 3D effects of the aerofoil, it is important to note that there will be different drag factors which will increase the amount of drag experienced by the aerofoil. A major form of drag which the aerofoil will experience while in flight is the vortex drag, more specifically the drag due to the mismatch of pressure along the upper and lower surfaces of the aerofoil. More specifically, this drag arises due to an overspill of high pressure on the lower surface of the aerofoil to the upper surface, which is abundant in low pressure areas. Thus as the aerofoil moves through the fluid, in our case air, this overspill will manifest itself into tip vortex, increasing the drag experience by the aerofoil. Thus, although only drag will be termed in this journal, there may be separate underlying factors involved. 3D and 2D Calculations Although DESFOIL is only applicable to 2D aerofoils, adjustments can be made such that the results from DESFOIL can be used within 3D situations. Since we are designing an actual aerofoil, such considerations need to be taken into account, and are during the later parts of this journal. The importance of using such a program lays in its simulation of the aerodynamics the aerofoil experiences. Therefore, using such a program allows the possibility to determine what coefficient of lift (or, 2D and 3D analysis respectively) and coefficients of drag, subscriptd, or D accordingly, are needed for optimal flight. As we will determine in this report, optimal coefficients will be calculated and a wing structure designed accordingly. Design Brief The following reading is an analysis of the software package DESFOIL on the suitability of difference NACA 4 digit aerofoils on an MAV of certain design specifications. These include Cruise Speed, = 15 Loiter Speed, = 8 Wing Area, S = 0.13 We will assume a rectangular planform for our aerofoil. Furthermore, we will assume the aerofoil as the main form of lift, i.e. neglecting fuselage, tail plane or rudder lift generation Wing Characteristic/ Structure Lift is defined as the aerodynamic force that a surface produces in the presence of a perpendicular velocity vector. Since lift is defined as a force, , we can assume that lift is some function of the density of the medium it is produced within, , the size of the object producing such a force, , and the before mentioned velocity, Therefore, (1) Where x, y and z are unknown parameters defining the relationship outlined in the equation. Through dimensional analysis we can deduce the values of such unknowns. (2) (3) In terms of lift forces, the constant of proportionality is termed the coefficient of lift, deriving the lift equation (4) It is also possible to consider a more rigorous analysis of the coefficient of lift taking into account symmetrical and cambered aerofoils, which yield and respectively. However, such equations only apply to thin aerofoils and since the thicknesses of the aerofoils are unknown in this assignment, the generic formulae will be used. Similarly, derivation of the drag forces can yield an equivalent drag version of equation (4). (5) To deduce our optimal lift coefficient, we will assume the lift generated will equal the weight of the aircraft, a reasonable assumption when considering straight and level (cruise) flight and the loiter regime. Therefore, the lift coefficients can be calculated for the respective flight conditions (6) Equation (6) yields a cruise coefficient of lift of 0.285, while similar analysis for loiter conditions yields a lift coefficient, of 1. Since we are initially more concerned with the wing aerodynamics with respect to wing structure (aspect ratio), we will consider the induced drag, , whereby, (7) Where e is the Oswald efficiency of the aerofoil, a correctional factor added since the wing shape differs from the elliptical wing used for the derivation, and A is the aspect ratio, calculated by the length to width ratio. To select the best aspect ratio for our aerofoil, the induced drag variation with aspect ratio changes is shown in Figure 1. It can easily been seen from Figure 1 that an aspect ratio of 5 would be acceptable since there is negligible variation in terms of the two dimensionless concepts. However, if we consider this in terms of the actual MAV, an aspect ratio of 5 would yield a span of 0.8m and a chord length of 0.16m. Evidently, while this is the longest and thinnest allowed in this particular investigation, possible structural problems may occur. However, if we consider the capabilities of the aircraft, there are advantages too. As Figure 1 has shown the induced drag in flight would be decreased, enabling better endurance and longer range. The structural instability could be overcome by careful selection of materials and designing of the structure. Thus, although problems may arise from such an aspect ratio, these problems can be overcome and do have their own advantages. Such data allows calculation of cruise and loiter Reynolds number and Mach number to be calculated. Figure 1: Induced Drag and Aspect Ratio relationship (8) (9) Similarly, and. Panel Number Since DESFOIL is the primary tool in determining which aerofoil will be used and its aerodynamic characteristics known, it would be wise to research the capabilities of the software and which system (panel number) to use to ensure the results obtained are of relevant accuracy. Another important aspect of using DESFOIL is the time taken for results to be determined. This will be analysed next. If we consider the effects of panel number on the lift, drag and pressure distribution respectively, we can clearly see a relationship shown in Figures 2, 3 and 4. Considering an angle of incidence of 10 degrees, it is evident to see that the most accurate results come about with the higher panel numbers. Since the maximum panel number within DESFOIL is 280, it would seem this would be the optimal choice. However, upon closer analysis, it is the time taken for such accurate results to come back from the software, which is of greater importance. For example, a panel number of 280 will provide the most accurate answer, but also take the longest to deduce. Therefore, if we consider the (negligible) variation of values, we can deduce that a panel number of 180 is significantly lower, thus, allowing quicker results, but still retains a high level of accuracy. For example, for the lift coefficient, 180 yields 1.0012, while 280 yields 1.0028. Thus the accuracy difference is negligible. Figure 2: Variation of Lift with different panel number on NACA0012 aerofoil Figure 3: Variation of Drag with panel number on NACA0012 aerofoil When analysing the pressure distribution, fewer panel numbers were considered, since the graphical representation would have become severely hard to differentiate between the different graphs. On the other hand, the before mentioned negligible differences is perhaps clearer in Figure 4. With the panel number at 280, the pressure distribution is most smooth, allowing finer details to be seen, which would otherwise be lost in lower panel numbers. Thus, a panel number of 180, the lowest without losing significant accuracy, is optimal. Figure 4: Variation of Pressure Distribution with panel number on NACA0012 aerofoil Reynolds/ Mach number So far, we have considered only the cruise aspect of the MAV. Since the aircraft will experience loitering stages also, analysis must be considered into different Reynolds and Mach numbers. Both of these are necessary in understanding the aerodynamics of the aerofoil since they both alter the way in which the aerofoil will react to airflow. For example, consideration of transition points, the onset of turbulent flow, boundary layer thickness and laminar flow needs to be understood to optimise the aerofoil design. Therefore, changes in the behaviour of the aerofoil/ airflow must be modelled and simulated within DESFOIL. For further understanding of such phenomenon, XFOIL will be used to pictorially show the effects of Reynolds number and Mach number on boundary layer, amongst other sets of information. More specifically, larger Reynolds and Mach numbers will be taken into consideration to visualise compressibility effects. To observe such results, i.e. how changes in density with regards to the pressure distribution, comparisons will be made to show how the compressibility effects (large Reynolds/ Mach number values) alter the characteristics/ performance of the aerofoil. An angle of attack of 10 degrees was considered when undertaking the computations in all examples. Incompressibility/ Compressibility Effects Figures 5 and 6 visually show the variation of the boundary layer with a high Reynolds and Mach number. If we consider Figure 5, we can see the specific values of coefficients of lift, drag and pitching moment at the angle of attack mentioned before. Another helpful mode shown within Figure 5 is the description of the change in boundary layer over the length of the chord of the aerofoil. This pictoral view shows the general formation of turbulent flow from laminar flow. As will be seen later in the report, there is a relationship between the boundary layer thickness and the Reynolds number. This relationship is important to note since a thinner laminar boundary layer ensure lower drag. Again, this concept will be further investigated later. Figure 5: XFOIL graph showing pressure distribution along aerofoil Figure 6: XFOIL graph showing variation of other aerofoil characteristics Reynolds Number Mach Number Cl Cd lift/drag 1000000 0.4 1.083 0.01965 55.13 169412 0.04 1.0266 0.03469 29.59 90353 0.024 0.9415 0.05289 17.8 Figure 7: How lift and drag vary with different Reynolds and Mach numbers From Figure 7, we can see the direct impact the differing Reynolds numbers and Mach numbers have on the generation of lift and drag. Quite clearly, as the Reynolds/ Mach number decreases, so does the coefficient of lift, and thus lift generated. Also of significant importance is the increase in drag with decreasing Reynolds/Mach number. Due to these variations, the lift to drag ratio also decreases. However, it is important to note that the results are non-linear. This non-linearity can be explained from the transition from incompressible flow to flow whose density changes with respect to the pressure distribution. Thus, such characteristics cannot be extrapolated or calculated; they must be experimentally defined, or computationally simulated, since consideration of compressibility effects adds complexity to calculations. Boundary Layer Analysis Although there is little difference between the values of lift coefficients (in the first two examples), there seems to be a drastic difference between the lift: drag ratios. Since the coefficients of lift are similar, varying by less than a magnitude of value, the only possible change must come from the drag experienced on the aerofoil. Experimental data, treating the aerofoil as a flat plate, shows that as the Reynolds number increases, the boundary layer thickness decreases, shown in Equation (10). (10) Thus, a decrease in the Reynolds number causes a larger boundary layer around the aerofoil, which in turn causes a greater disturbance to the free stream air. Since the boundary layer cannot handle a large adverse pressure gradient without separation, the higher values of Reynolds number cause separation earlier, even though they have thinner, boundary layers. This is due to greater adverse pressure gradients which are responsible for the larger values of lift coefficients attained. The separated flow causes larger amounts of drag, which is obviously undesirable, since the flow is no longer uniform along the chord. Once the pressure gradient exceeds a critical point, the boundary layer will separate from the aerofoil, therefore reducing the magnitude of the pressure gradient, reducing lift generation. Therefore, the lift: drag ratio decreases as drag will increase upon separation. The drag experienced at higher Reynolds numbers is still considerably smaller than the drag experienced at lower Reynolds numbers due to the thickness of the boundary layer. Although separation of the flow is a factor with regards to drag, the boundary layer thickness, as seen in Figure (7) using Equation (10), is a larger factor. Since this separation point (transition from laminar to turbulent flow) is an area of interest with regards to the amount of drag experienced by the aerofoil, Figure 8 shows the movement of such a point with regards to the Reynolds number. The black lines only show the separation points on the upper surface of the aerofoil since this is the surface of most interest. Figure 8: Transition point. 1) Re=1000000, M=0.4 2) Re=169412, M=0.04 3) Re=90353, M=0.024 At this point it is important to note that the DESFOIL parameters were changed to ensure a completely accurate result from the simulation. To ensure the accuracy was maximised, the transition detection was 100% the length of the chord, and not simply the default 20%. This allowed DESFOIL to look throughout the whole length of the chord for the transition/ separation point as opposed to the default 20%. As we can see, for the same angle of attack, the higher Reynolds/ Mach numbers cause the separation point to be significantly closer to the leading edge. Similar XFOIL graphs were constructed as that in Figure 6 for the other Reynolds/ Mach numbers. From Figure 6, we can see that at an angle of attack of 0 degrees, there is a separation point at 0.637, i.e. 63.7% away from the leading edge as a function of the chord length. When the Reynolds number is 169412, this separation point is 91% as a function of the chord length, while the loiter Reynolds number remains laminar at 0 degrees angle of attack. Geometries If we consider other three dimensional geometries with respect to the drag each produces, we can understand why an aerofoil is an optimal shape in terms of reducing drag. Figure 9: Drag values for various 3D geometries If we consider streamlining any given shape, we can possibly reduce the amount of drag experienced, as shown in Figure 9, by an order of magnitude. For example, if we consider the sphere, hemisphere and teardrop shapes, although all have the same frontal geometry, it is the streamlining of the teardrop which contributes most to a significant reduce in drag, due to the prolonged attachment of the airflow. Since the airflow after the sphere/hemisphere is suddenly separated (due to the non gradual geometry behind the shape), there is a significant amount of drag experienced. This is why Figure 5 depicts such a prolonged attachment of the airflow, only becoming separated towards the trailing edge of aerofoil. To further reinforce the advantageous effects of streamlining, Figure 10 shows the geometrical differences which can be obtained with intelligent streamlining. Figure 10: Two different geometries with the same aerodynamic drag force Lower Aerofoil Surface Another important feature found from graphs similar to Figure 5 highlights the relationship between the angle of attack, Reynolds/ Mach number and flow over the lower wing section. It was found that at lower Reynolds numbers, the flow is relatively laminar across the length of the chord length. This makes sense at high angles of attack since the underside of the aerofoil has a larger wetted area. Possible further investigation and research may lie in determining the flow over the lower surface of the aerofoil in negative angles of attack. Perhaps such an investigation will help understand the landing/ descending section of a flight path. It may be interesting to learn whether separation points play such a major role on the lower surface as they do on the upper surface, in terms of lift and drag. Such understanding can provide insight into painting a complete picture of the airflow surrounding a wing. Furthermore, since a NACA0012 aerofoil was considered giving all the results mentioned previously, changes in airflow with varying NACA aerofoils could help determine a more complex relationship. For example, as thickness, camber and camber position change, how does the transition point vary on the underside of the aerofoil? Such variations are made within the next section with regards to the overall lift and drag. However pressure variations could be conducted in a similar fashion. Designing using DESFOIL Since DESFOIL allows the user to design, test and evaluate their own chosen design (one of the many reasons it was chosen for undertaking of this particular investigation), it is important to understand how the different parameters affect the aerofoil characteristics. From this, we can deduce what the optimal aerofoil for our application could be. Furthermore, it allows for reinforcement of aerodynamic theory into the reaction of airflow over changing geometries of aerofoils. This could be seen as a measurement of DESFOILs accuracy in its simulations. If its simulation results were to vary from known aerodynamics, then the softwares validity would be questionable. Throughout the analysis, therefore, the aerodynamic theory will be called upon to explain the results given from DESFOIL. Since the software allows for three different design features, it was deemed necessary, to gain a full understanding, to adjust and examine one parameter at a time and comment on the results obtained. Since different values of lift were optimal for the different stages within our flight path, both the cruise and loiter conditions were looked at. From the template aerofoil NACA0012, the thickness was the first parameter to be changed. Figures 11 and 12 below graphically shows the variation in lift and drag over the four different aerofoil thicknesss chosen. Cruise Conditions Figure 11: Lift variation with different NACA aerofoils thicknesses Figure 12: Drag variation with varying thicknesses Firstly, the cruise conditions will be investigated. As we can see from the figures above, the thickness of the aerofoil plays an important role in determining such characteristics as stall angle and maximum coefficient of lift. If we consider both graphs simultaneously, we can deduce the thicker the aerofoil, the greater the values of lift can be obtained. This is shown with the increase in coefficient of lift values from 12% thickness to 15-21% thickness. This is down to the curvature of the aerofoil being the main form of lift generation, i.e. the more curved (thicker in this instance since camber position is constant) the aerofoil, the larger amounts of lift generated, within limits. Also, nose shape effects help the generation of high lift coefficients. Furthermore, it is important to note that the thinner aerofoil has also stalled significantly harder than the thicker aerofoils. Since stalling is undesirable, perhaps thicker aerofoils would be best for use in the chosen aerofoil. Concentrating on the graphs from a drag point of few, we can again see that thinner aerofoils are undesirable due to the drag they produce/ experience. The sharp rise in drag experienced by the thinner NACA0012 aerofoil is complimentary of the stall it experienced at an angle of attack of 13. Furthermore, it is important to note that there are slight variations in the small angle of attack region with respect to lift and negligible difference in the corresponding drag section. Since a definitive relationship was deduced from the thickness investigation, it was reasonable to continue the designing experiments. Next, the camber thickness was investigated. Figure 13: Lift variation with angle of attack with different camber thicknesses Figure 14: Drag variation with angle of attack with different camber thicknesses From the above figures, certain relationships can be deduced between the camber thickness and the effect such parameters have on the lift and drag experienced on the aerofoil. Firstly, lets consider the adverse effects on the lift and drag, shown here by the NACA-2012, whereby the -2 denotes a negative camber. From Figure 13, we can see a significantly lower lift attained flight with an earlier stall, which compliments Figure 14, whereby the drag significantly increases due to the separated flow resulting from the stall. For the other three aerofoils shown, the aerodynamic drag force experienced by each has negligible difference, since all follow the same shape. The differences can more obviously be seen through analysis of Figure 13. Here, we can see the larger the camber, the greater values of lift can be obtained. However, it is important to note that only the NACA4012 aerofoil does not experience a stall. On the other hand, the other two positive aerofoils, while although experiencing a stall, do not stall extremely harshly, and so a stall of this kind, while although not optimal, can be considered negligible in terms of lift generated. The camber position was investigated next Figure 15: Lift variation with angle of attack under different camber positions Figure 16: Drag variation with angle of attack under different camber positions As we can see from the above two figures, the effect of camber position is not as drastic as the other previous analysed parameters. From Figure 15, we can see the highest lift is attained by the NACA4212 aerofoil, although all the aerofoils have the same similar low angle of attack lift generation. It is only towards angles of attack greater than 7 where there is greatest deviation. On the other hand, it can also be seen that the NACA4212 aerofoil, while giving the highest lift value, also stalls. As mentioned before, this is undesirable. From a drag perspective, the NACA4212 aerofoil performs best towards larger angles of attack however performs worst at low angles of attack. Depending on where the greatest emphasis needs to be placed upon the cruise aerofoil conditions, this may be an important factor. Chosen Cruise Aerofoil Since we have analysed the effects of the three different parameters within DESFOIL, we can now evaluate what lift and drag characteristics we want from our chosen aerofoil. Since the actual aerofoil will be 3D, we need to take into consideration 3D effects. For this instance, we are going to assume the 3D coefficient of lift is 90% of the 2D coefficient of lift, namely, (11) One reason there is a decrease in the change from two dimensional to three dimensional bodies is the appearance of an extra plane, i.e. the z plane. Thus, the lift generation needs to distribute the lift over three planes instead of two. Thus resulting in less lift overall. Therefore, we can calculate a coefficient of lift of 0.3167 to be found using DESFOIL. Taking what was found from the above investigation, various NACA aerofoils were tested. The final aerofoil chosen was the NACA2615 aerofoil for reasons clearly shown using Figure 17. Figure 17: NACA2615 aerofoil characteristics From this figure, we can see the optimal design characteristics we want from our cruise aerofoil. These characteristics include a significantly low drag, as compared to the lift generated, which can be seen as a direct result of no stall being present. Furthermore, if we consider the lift we wanted to generate, namely 0.3167, we can see this aerofoil manages to attain such lift at a low angle of attack, something we want from our aerofoil since the quicker the optimal lift can be generated, the quicker the aerofoil will start behaving to optimise its performance. Since this optimal lift is generated at an angle between 1 and 2, the lift: drag ratio was calculated for these two angles. They are 17 and 39 respectively. These high values show the positive performance of our aerofoil in the cruise condition. Loiter Condition As we can see from the previous section, a detailed investigation and analysis was undertaken to establish the best NACA aerofoil for our cruise purposes. If we now go on to consider the loiter condition, there are certain parameters which need to be considered, namely the 2D lift we wish to aim for, at the lowest angle of attack, to find the optimal aerofoil. As shown in the previous section, the thickness, camber and camber position were all varied individually and the resulting effect on the lift and drag analysed. Using Equation 11, we can calculate the required lift (needed to be found in DESFOIL) as 1.11. Firstly, the thickness was adjusted. It was found, just like the cruise condition investigation that an increase in thickness resulted in higher levels of lift being attained. However it was also found that the aerofoils under loiter conditions tend to stall, regardless of thickness. However, the greater the thickness the higher angle of stall. Since all aerofoils tested stalled, the drag associated with each was indicative of this phenomenon. It is also important to note that the aerofoils tested were the NACA0010, NACA0012, NACA0018 and NACA0021. Considering the position of maximum camber next, it was found that the NACA4212 aerofoil stalled significantly earlier than the NACA4012, NA

Breastfeeding: Advantages and Disadvantages

Breastfeeding: Advantages and Disadvantages This essay is potentially vast in its scope as the advantages and disadvantages of breastfeeding can vary enormously depending on which authority one chooses to consult. (1) We shall therefore take an overview and present the generally accepted arguments in this area. There is a substantial evidence base for the benefits of breastfeeding for not only the infant, but also the mother, the families and society as a whole. (2). The benefits are not only nutritional, but cover other areas such as development, immunity, psychological well-being, overall health risks, social and environmental areas (3) Human milk is widely recommended as the food of choice for the otherwise healthy full-term newborn. Apart from providing the optimal nutrition it contains a number of cellular and humoral components such as phagocytes, immunocompetant cells, immunoglobulins and hormones and also plays a (as yet, not fully defined) role in promoting intestinal mucosal maturation. (4) Because of these (and other) considerations, the World Health Organisation recommends exclusive breastfeeding for at least the first six months of life. (5) There is widespread evidence that the trend towards universal breastfeeding is increasing (viz. 6) in most of the developed countries, Fewtrell gives a global figure of 2% per year. but studies show that this trend is not equal across all socio-economic and demographic groups. If we specifically consider the UK situation then we can show that the commencement of breastfeeding is equal in the working and non-working mothers groups, although if one considers the situation of how many mothers are still breastfeeding at six months post partum, the incidence in the working mothers group is less than half that in the non-working group. (7) What then is the evidence base for the benefits of breastfeeding? In addressing this question we shall confine our comments to those relevant to the developed world. If we consider the Kramer study, which compared the benefits of continued breastfeeding from 3 months to 6 months, the authors were able to demonstrate that the additional 3 months produced benefits in terms of greater weight gain, greater growth (length) together with a reduced incidence of gastrointestinal infection. (8) In absolute terms however, the benefits of breastfeeding over formula milks (or cows milk) is much greater. A huge number of potential childhood infections have been demonstrated to have a lower incidence in the breastfed child including bacterial meningitis, diarrhoea, respiratory tract infections, otitis media, urinary tract infections (9) as well as less common entities such as necrotising enetrocolitis (10). It is also significant that the all-cause mortality rates are 21% less in breast fed infants. We should note that this figure, although accurate, is misleading, as many high risk babies are not breast fed because of their intercurrent problems. (11) Apart from infections, many other health benefits can be demonstrated. There is a reduction in the incidence in sudden infant death syndrome under the age of 1 yr. (12). Diabetes (Type I and Type II) is less common amongst breastfed babies (in later life) as are the incidences of conditions such as lymphoma, leukaemia and Hodgkin’s disease. (13). Breastfeeding tends to be associated with a lower incidence of obesity, hypercholesterolaemia and asthma. (14) There is a slightly weaker evidence base to support the benefit of breastfeeding in terms of neurological development. Some authorities suggest that it can improve cognitive development. (15) and it may also reduce the perception of some painful stimuli. (16) In addition to benefits for the child, there are also a number of demonstrable benefits for the mother who breastfeeds. If suckling occurs at the time of birth, the resultant release of oxytocin reduces the incidence of post partum haemorrhage and increases the speed of uterine involution. (17). In the period after the birth, breastfeeding reduces both fertility and menstrual loss, it facilitates a return to pre-pregnancy weight, it reduces the risk of both ovarian and breast cancer (18) and may well reduce the incidence of osteoporotic fractured hips (19) In the opening segment of this essay we alluded to the benefits to the community as a whole. These can be defined in terms or reduced health costs to the community by virtue of the protective effects of breastfeeding. There are also less definable benefits in terms of reduced employee absenteeism. Some authorities have gone as far as to point to the environmental benefits of reduction in energy expenditure on production, distribution and disposal of formula feeds and their packaging. (20) Thus far we have considered the positive benefits of breastfeeding but in order to provide a balanced argument, we should also consider the disadvantages. The pre-term infant or severely underweight or ill baby has special needs and there are a number of reasons why they should not be breastfed. The pre-term infant has immature physiological systems and the kidneys may not be able to handle the osmotic gradients that are required to excrete the amount of fluid necessary to remove the amount of nutritional load required for adequate growth. This may result in respiratory problems and exacerbation of any pre-existing cardiac conditions (viz. patent ductus arteriosus). For this reason, most pre-term infant are electively parentrally fed and then weaned onto enteral feeding when their gastrointestinal tract and other physiological systems are mature enough to handle the fluid load. (21) If the mother is ill or has a potentially communicable illness such as HIV/AIDS or TB, then breastfeeding is contraindicated, as it is in conditions when certain drugs (both medicinal and recreational) are taken by the mother. (22). We should also record that the evidence for HIV/AIDS spread is not secure, as some studies have suggested that breastfeeding actually confers a degree of protection against HIV/AIDS for the child. This is still an area of considerable debate. (23) It is also clear from an examination of the literature on the subject, that there is a great deal of misinformation on the subject of breastfeeding in the popular press (and to a lesser extent in the medical press). Breastfeeding is not contraindicated in conditions such as Hepatitis B or C +ve. (24). Most febrile conditions are not a contraindication to breastfeeding as the maternal immune response will be passively given to the child in any event. Some authorities suggest that tobacco smoking is a contraindication to breastfeeding. If we remove considerations of general health from the consideration, there is no reason why tobacco smoke should be considered a bar to breastfeeding as such. (it clearly may be ill-advised however) (25). Some authorities suggest on theoretical grounds that breastfeeding should be suspended during the period of physiological jaundice of the newborn. A number of recent studies have shown that this is not necessary and may cause insurmountable difficulties in rehabilitation-establishing breastfeeding after the event. (26) In essence, within the scope of the exclusions referred to above, healthcare professionals should actively encourage and support mothers in their ability to breastfeed their offspring. In doing so, one should always consider the autonomy of the mother (27) and attempt to provide empowerment and education in order to facilitate the best result rather than compulsion or emotional blackmail. (28). This should help to ensure the maximum possible take up of breastfeeding from mothers who have been able to make a fully informed decision. If direct breastfeeding is not possible, then, generally speaking, expressed breast milk is the preferred substitute. In this essay we do not presume to have covered anything like an exhaustive presentation of the arguments, but there is no doubt, from an overview of the evidence base on the subject, that in the vast majority of cases, both mother and child will derive substantial benefits from being able to breast feed for at least the first six months of life. References (1) Duerbeck N B (1998) Breast-feeding: what you should know so you can talk to your patients. Comp Ther 1998 ; 24 : 310 318 (2) Kramer M S, Chalmers B, Hodnett E D, et al. (2001) Promotion of Breastfeeding Intervention Trial (PROBIT): a randomized trial in the Republic of Belarus. JAMA. 2001 ; 285 : 413 – 420 (3) Schanler R J. (2001) The use of human milk for premature infants. Pediatr Clin North Am. 2001 ; 48 : 207 – 219 (4) Margolis L H and J. B. Schwartz (2000) The Relationship Between the Timing of Maternal Postpartum Hospital Discharge and Breastfeeding. J Hum Lact, May 1, 2000 ; 16 (2) : 121 128. (5) Fewtrell M S , J. B Morgan, C. Duggan, G. Gunnlaugsson, P. L Hibberd, A. Lucas, and R. E Klein man (2007) Optimal duration of exclusive breastfeeding: what is the evidence to support current recommendations? Am. J. Clinical Nutrition, February 1, 2007 ; 85 (2) : 635S 638S. (6) Bonuck K A, K. Freeman, and M. Trombley (2006) Randomized controlled trial of a prenatal and postnatal lactation consultant intervention on infant health care use. Arch Pediatr Adolesc Med, September 1, 2006 ; 160 (9) : 953 960. (7) Kramer M S, Kakuma R. (2001) The Optimal Duration of Exclusive Breastfeeding. Geneva, Switzerland: World Health Organization ; 2001 (8) Kramer M S , Guo T, Platt R W et al. (2003) Infant growth and health outcomes associated with 3 compared with 6 mo of exclusive breastfeeding. American Journal of Clinical Nutrition, Vol. 78, No. 2, 291 295, August 2003 (9) Heinig M J. (2001) Host defense benefits of breastfeeding for the infant. Effect of breastfeeding duration and exclusivity. Pediatr Clin North Am. 2001 ; 48 : 105 –123 (10) Dewey K G, Heinig M J, Nommsen-Rivers L A. (1995) Differences in morbidity between breast-fed and formula-fed infants. J Pediatr. 1995 ;126 : 696 – 702 (11) Chen A, Rogan W J. (2004) Breastfeeding and the risk of postneonatal death in the United States. Pediatrics. 2004 ;113 (5) (12) Horne R S, Parslow P M, Ferens D, Watts A M, Adamson T M. (2004) Comparison of evoked arousability in breast and formula fed infants. Arch Dis Child. 2004 ; 89 (1) : 22 –25 (13) Davis M K. (1998) Review of the evidence for an association between infant feeding and childhood cancer. Int J Cancer Suppl. 1998 ; 11 : 29 – 33 (14) Toschke A M, Vignerova J, Lhotska L, Osancova K, Koletzko B, von Kries R. (2002) Overweight and obesity in 6- to 14-year old Czech children in 1991: protective effect of breast-feeding. J Pediatr. 2002 ; 141 : 764 – 769 (15) Horwood L J, Darlow B A, Mogridge N. (2001) Breast milk feeding and cognitive ability at 7–8 years. Arch Dis Child Fetal Neonatal Ed. 2001 ; 84 : F23 – F27 (16) Carbajal R, Veerapen S, Couderc S, Jugie M, Ville Y. (2003) Analgesic effect of breast feeding in term neonates: randomized controlled trial. BMJ. 2003 ; 326 : 13 (17) Labbok M H. (2001) Effects of breastfeeding on the mother. Pediatr Clin North Am. 2001 ; 48 : 143 – 158 (18) Rosenblatt K A, Thomas D B. (1993) Lactation and the risk of epithelial ovarian cancer. WHO Collaborative Study of Neoplasia and Steroid contraceptives. Int J Epidemiol. 1993 ; 22 : 192 – 197 (19) Paton L M, Alexander J L, Nowson C A, et al. (2003) Pregnancy and lactation have no long-term deleterious effect on measures of bone mineral in healthy women: a twin study. Am J Clin Nutr. 2003 ; 77 : 707 – 714 (20) Jarosz L A. (1993) Breast-feeding versus formula: cost comparison. Hawaii Med J. 1993 ; 52 : 14 – 18 (21) Denne, S. C., Karn, C. A., Ahlrichs, J. A., Dorotheo, A. R., Wang, J. Liechty, E. A. (1996) Proteolysis and phenylalanine hydroxylation in response to parenteral nutrition in extremely premature and normal newborns. J. Clin. Invest 97 : 746 -754 (22) Read J S; (2003) American Academy of Pediatrics, Committee on Pediatric AIDS. Human milk, breastfeeding, and transmission of human immunodeficiency virus type 1 in the United States. Pediatrics. 2003 ; 112 : 1196 – 1205 (23) Coutsoudis A, Rollins N. (2003) Breast-feeding and HIV transmission: the jury is still out. J Pediatr Gastroenterol Nutr. 2003 ; 36 : 434 – 442 (24) Pickering : (2003) American Academy of Pediatrics. Transmission of infectious agents via human milk. In: Pickering LK, ed. Red Book: 2003 Report of the Committee on Infectious Diseases. 26th ed. Elk Grove Village, IL: American Academy of Pediatrics; 2003 : 118 – 121 (25) American Academy of Pediatrics, (2001) Committee on Drugs. Transfer of drugs and other chemicals into human milk. Pediatrics. 2001 ; 108 : 776 – 789 (26) American Academy of Pediatrics, (2004) Subcommittee on Hyperbilirubinemia. Management of hyperbilirubinemia in the newborn infant 35 or more weeks of gestation. Pediatrics. 2004 ; 114 : 297 – 316 (27) Sines D (1995) Impaired autonomy: the challenge of caring. Journal of Clinical Nursing 4 (2) 109 -115. (28) Marinker M.(1997) From compliance to concordance: achieving shared goals in medicine taking. BMJ 1997 ; 314 : 747 – 8. ############################################################### 12.05.07 PDG Word count 2,183 Polycrystalline Ceramics: Sub Grain Structure Polycrystalline Ceramics: Sub Grain Structure Pure un-doped zirconia is a polymorph which has three allotropes namely: Monoclinic, Tetragonal and finally Cubic. These phases tend to transform into each other when exposed to certain temperature ranges and such transformation is important for the processing and mechanical properties of zirconia. The monoclinic phase of pure un-doped zirconia is stable at room temperature and remains so up to about 11700C, where it then transforms into tetragonal phase. It becomes stable tetragonal at this temperature and remains so up to 23700C, where it turns to cubic. The cubic phase occurs up to the melting temperature of 26800C. Monoclinic phase The monoclinic form also referred to as baddeleyite, is a thermodynamically stable phase at a temperature range between room temperature and approximately 9500C. It contains four ZrO2 molecules per unit cell and has a space group of P21/c. Figure 2.1 shows the lattice parameter of monoclinic form. Its structure is described as a distorted fluorite (CaF2 structure). It is difficult to define the crystal structure of monoclinic zirconia because of its complexity as well as the problem of making a monoclinic single crystal with the satisfactory qualities due to: micro-cracking, low purity, twinning and disproportionate solid solution formation. Tetragonal phase This is a high temperature phase (t) firstly discovered by a group of scientist during its transformation from the lower temperature monoclinic phase over a temperature of about 11500C. Figure 2.1 shows the lattice parameter of tetragonal form. The structure is similar to that of monoclinic polymorph in the sense that it is also distorted CaF2 structure. Hence, tetragonal zirconia (t-ZrO2) can be described using the face centred tetragonal Bravais lattice as oppose to the body centred tetragonal lattice, that contains a unit cell with volume twice the size of the primitive cell. (3) Figure 2.2b shows a simple schematic of a tetragonal unit cell. Its structure comprises of eight oxygen ions surrounding a zirconium ion, with half at a distance of 0.2455nm forming an elongated tetrahedron and the remaining four are at a distance of 0.2065 forming a flattened tetrahedron (the elongated and flattened tetrahedron are rotated 900 to each other). The transformation from tetragonal to monocli nic can start (Ts) and finish (Ts) over a range of temperatures. This reaction can be measured using the following experimental techniques: DTA, XRD and dilatometry. (3) Cubic phase Unlike the other structures, the cubic polymorph is quite easy to explain as it has a fluorite structure (CaF2). Figure 2.2c shows a simple schematic of a tetragonal unit cell. It has a lattice parameter of the order 0.508nm (this however depends on the temperature purity of zirconia that is partially stabilised zirconia at room temperature or pure zirconia at elevated temperature) and a crystal symmetry of Fm3m.   The martensitic transformation For a martensitic transformation to occur, a change in shape is required which must also produce a plane that does not change during transformation. This is so that it is common to the phase produced as well as the parent phase. The phase transformation in zirconia involves a change in volume of between 4 to 5%. The matrix inhibits the transformed particle of zirconia causing a partial shape change. However, the transformation creates a strain which is held in the monoclinic and its surrounding grains. As a result of this, researchers have come up with the idea that transformation stresses are relieved by deformation twinning. When this happens, most of the lattice strain is then restricted to the monoclinic/matrix interface. Micro-cracks can be formed at this matrix/monolithic interface or in the monoclinic particle if this lattice strain increases. The twinning found in monoclinic is caused by deformation twinning, as the researchers have observed using TEM that a section of the st rain related with the transformation happens as a result of a mechanism known as slip. (3) The phase transformation particularly from tetragonal to monoclinic is of great importance, as it attributes the zirconias excellent properties. [from fulltext.pdf] It was firstly discovered by Garvie et al that the transformation of metastable tetragonal phase to monoclinic phase acts as a toughnening mechanism to crack propagation resistance in zirconia. The transformation is quick and results in a 4 to 5 percent increase in volume which leads to formation of micro-cracks and eventually macro-cracks in the material. This process induces compressive stresses and thus toughens the materials. Gupta et al backed this theory up. Studies showed that the transformation mechanism is highly dependent on grain size and by doping the ceramic material with stabilisers. Examples of stabilisers are yttria (Y2O3), magnesia (MgO), calcia (CaO), etc. Y-TZP ceramics is in the family of these toughened materials. Tetragonal zirconia doped with Yttria (Y-TZP) has great strength of over 1000MPa and tou ghness weighing between 6 and 10 MPa.m1/2. This makes it an ideal contender in medical applications, particularly in hip joints. ZrO2 Y2O3 The phase diagram shown in figure 3 was firstly discovered by Scott (1975), this study was agreed and used by many more researchers. The tetragonal phase field is the main aspect of figure 3. It shows that up to about 2.5mol% of Yttria can be produced in solid solution in addition with the low eutectoid temperature leading to the formation of a fully tetragonal ceramic, this will happen as long as the grain is of an appropriate size. The theory of transformation toughening produced some excitement in the materials industry however this excitement came to a halt when Kobayashi et al discovered a flaw in Y-TZP ceramics. Y-TZPs undergoes low temperature degradation during ageing at temperatures ranging from 100 to 4000C, this is particularly enhanced when it is exposed to water or is in humid environments. This degradation is due to the formation of flaws such as micro-cracks and macro-cracks (mentioned earlier) at the surface which gradually goes into the bulk of the material. These flaws are due to the spontaneous transformation from tetragonal phase to monoclinic phase. Material scientists have documented literature regarding the degradation however there have been contradictory views as to the mechanism of this phenomenon. Figure 4 is a graphshowing the low temperature degradation of different types of TZPs. Figure 5 shows ageing temperature against surface monoclinic levels. Some of these researchers focused on the interaction between water (or water vapour) and YTZP, whilst others focused on ways to prevent this from happened. Sato et al came up with a theory where the hydroxyl group from water (H2O) reacts with zirconia from the bonds between zirconia and oxygen (that is Zr-O-Zr bonds) forming Zr-OH bonds at crack tips. This accelerates the rate at which the metastable tetragonal phase transforms to monoclinic at low temperatures. They came up with the conclusion that there is a strain which stabilizes the tetragonal phase, however under certain circumstances it is released and with the combination other pre existing flaws accelerates the transformation. The theory put forward by Yoshimura et al is similar to that of Sato et al in the sense that the Zr-OH bonds are also formed. However, the reaction process which leads to the same outcome is what differentiates the two theories. Their research showed a comparison of the transformed monoclinic phase to the untransformed tetragonal ZrO2. Hydroxyl (OH-) was in the monoclinic ZrO2 whereas there was no trace in the latter. Due to their findings, they came up with the theory that the degradation process occurred in stages: upon exposure to water, Zr-OH bonds are formed as a result of H2O being adsorbed on the YTZP surface. This creates a stress site which builds up as the OH- ions diffuse through the surface and lattice causing the formation of nucleation sites for the phase transformation. This occurs until the stress reaches crack level causing the transformation to occur at the surface leading to the formation of micro and macro cracks all the way through to the bulk. Lange et al [7] witnessed ÃŽÂ ±-Y(OH)3 crystallites of about 20 50 nm in size forming and came up with the idea that the hydroxide formed creates a monoclinic nuclei by removing Yttria from the grains of the tetragonal phase on the surface. As Yttria is being withdrawn, growth of the nuclei continues until a critical size where it will grow spontaneuously, leading to the transformation of tetragonal grains to monoclinic. Micro cracks and macro-cracks begin to occur as the transformed grain gets large enough. This process happens over and over again as the micro and macro-cracks act as a site for water molecules to penetrate into to the grains. This process occurs only if the grains are larger than the critical size. However, if they are smaller, the transformation will be influenced by the diffusion of Yttria on the surface. Other researchers such as Winnubst and Burggraf support this theory, as they found traces of Yttria on surface layer of the YTZP specimen. Their specimen was exposed to temperature of 1770C in a nitrogen environment for over 5hrs and using an auger electron microscope, they found a yttrium rich surface layer. The listed theories were based on YTZPs mechanism during degradation. Whalen et al identified that the reason for this degradation is the spontaneous transformation from tetragonal phase to monoclinic phase at the surface which then eventually spreads to the bulk. They came up with the idea of stabilising the tetragonal phase. This could be done by either of the following two methods: the chemical factor which is increasing the stabiliser content on the surface or the microstructural solution which is reducing the grain size at the surface. The latter was decided upon and this was done by the process of post sintering grinding followed by annealing treatment. 2.45mol% Y2O3/ZrO2 was the material involved in the research. Samples of the material were made using isostatic pressing at pressure of 275MPa and then sintered at a temperature of 15000C for a time period of 2hrs. A 2mm disk was formed of which its two sides had different surfaces treatments, One side being polished and the other being surface grounded. The phase compositions at surfaces were then examined using XRD. The XRD result indicated there was a significant difference in the phase composition of both sides. The ground side showed little transformation change whereas there was 50% increase in monoclinic phase after annealing. This provided evidence that the ground and annealed surface hindered the process of phase transformation from tetragonal to monoclinic at the surface. As a result of this, there were no micro-cracks formed at the surface and hence the expected mechanical properties were achieved. Talk and compare it to mine later (TEM as oppose to XRD, advantages of process) The aim of this project is to provide evidence (if any) of the occurrence of refined grains (recrystallization) in Y-TZP structures as a result of deformation. The ideology used to explain the concept of recrystallization in metals can be used to explain its occurrence in ceramics as this is a new phenomena in the ceramic industry. Grain refinement requires certain conditions in its exposure in polycrystalline ceramics and they are: plastically deforming the material (as a result of applying a stress) and followed by heat treatment. Deformation is basically a change in body shape which occurs as a result of an applied force. Materials may experience either elastic which is impermanent deformation that upon the release of an applied stress is recovered or plastic deformation which is permanent deformation that is non recoverable when a stress is applied. YTZPs recrystallization behaviour can be explained by its ability to plastically deform. The stress and strain behaviour of a material is used to determine the start and the degree of plastic deformation. Figure 6 shows an example of a typical stress and strain curve. Yield tensile strength is the point at which elastic deformation ends and the material begins to plastically deformation. Most polymers and metals undergo elastic followed by plastic deformation but this is not the case for ceramics. They undergo elastic deformation followed by fracture with little or no plastic deformation. YTZP has superplasticity properties and this nature can be used to explain refinement in its microstructure. Plastic deformation is governed by the movement of large numbers of dislocations. Hindering dislocation motion will increase a materials strength. Ceramics are inorganic materials held together by both ionic and covalent bonds. The bonding combination results in hindering the motion of dislocations, hence their high strength but brittle behaviour. Dislocation is an important factor in understanding plastic deformation and so certain elements need to be examined in order to understand the concept. Most materials comprise of an arrangement of atoms referred to as a crystal structure (these can either be single or polycrystalline that is having multiple crystals as the name suggests). This project will focus on polycrystalline zirconia, however understanding single crystals help in explaining the behaviour of polycrystalline materials. All crystal structures have flaws that distort the regular arrangement of the atoms. These flaws can either be point defect (that is they may have vacancies or interstitials), surface, line (dislocations) and volume defects. The activities and effects of all these flaws are interconnected thus the importance in the need to understand them. As the dislocations move, they tend to interact with one another however this interaction is a complex as an amount of dislocations (rephrased from pdf). The collective motion of dislocations leads to gross plastic deformation. http://composite.about.com/library/PR/2001/blmpi1.htm Dislocations can either be screw, edge or a hybrid of both. Edge dislocation: in this dislocation, the line of defect is parallel to the shear stress. The dislocation movement is similar to that of a caterpillar in the sense that the motion is in small amounts at a time. Figure 7shows a typical schematic of the motion of dislocations. A is the extra half plane of atoms. As shear stress is applied, the bond between the upper and lower part of B is broken. The extra atom plane of atom A bonds with the lower part of B converting the lower part to an extra half plane. This motion causes the top half to move with respect to the bottom half. Screw dislocation: this is similar to that of edge in the sense that it also occurs with shear stress however, the defect line is perpendicular to the shear stress as oppose to being parallel. Just like the edge dislocation only a minute fraction of bonds are broken at a given time. Although the motions are different, the overall plastic deformation for both dislocations is the same. The primary mechanism that causes plastic deformation in crystals is called slip. As dislocations move across the crystals, they shear the crystals along their planes of motion. Slip System The degree of ease of motion of dislocations is different with in all crystallographic directions and crystallographic planes of atoms. Normally dislocation motion occurs in a preferred plane and within that plane there are specific directions at also which it occurs. The combination of the plane and direction is referred to as a slip system. The plane at which this motion occurs is referred to as slip plane, and the direction is referred to as slip direction. The slip system depends on the crystal structure of the material. Slip will only occur when the value of applied the shear stress exceeds a certain critical value. The mechanism at which slip occurs is different in single crystals that of polycrystalline materials. Schmid defined the critical shear stress in single crystals as shown in figure 9: Deformation is much more complicated in polycrystalline materials as the crystallography orientations of numerous grains have to be taken into account. This orientation is random and therefore causes the direction of slip to vary from one grain to another. Its complexity extends further more to the grain boundaries which acts as barriers to dislocation motion. Twinning is another mechanism at which plastic deformation can occur. The idea of twinning in plastic deformation is to allow further slip to occur by producing changes in plane orientations. It occurs when a fraction of the crystals adopts an orientation that is correlated to the orientation of the rest of the untwined lattice in an exact proportioned way. Figure 10 shows an illustration of an un-deformed crystal with one undergoing slip and twinning. There is a clear difference between slip and twinning. The crystal orientation in a slip is the same above and below the slip plane whereas in twinning differs across the twin plane. More differences is illustrated in figure 11 Slip Twinning Where it occurs Widely spread planes Every plane of region is involved Occurrence On many multiple slip systems simultaneously On a particular plane for each crystal Time required Milli seconds Micro seconds Size (in terms of inter atomic distance) Multiples Fractions. ANNEALING PROCESS LEADING TO RECOVERY, RECRYSTALLIZATION AND GRAIN GROWTH Annealing is a high temperature process that causes changes in a materials structure, leading to alterations in its properties. When a material is plastically deformed, majority of the energy is dissipated as heat, but a minute fraction is stored in the material as strain energy which is associated with a range of lattice imperfections established as a result of deformation. The deformation process as well as a number of various factors (such as temperature and rate of deformation) determines the amount of energy stored in the material. A reduction in deformation and an increase in intensity of deformation cause a vast increase in the amount of retained energy. The release of stored energy There are two main techniques of releasing the energy retained by a material due to plastic deformation and they are an-isothermal annealing and isothermal annealing. Anisothermal annealing occurs when the material is continuously heated from a lower temperature to that of a higher one (the energy discharged is determined as a function of temperature) whereas, Isothermal annealing occurs when the temperature is constant. The materials microstructure will undergo either or maybe all of these three restoration processes: recovery, recrystallization and grain growth. The extent of plastic deformation can sometimes determine the mechanisms of recovery and recrystallization. These processes require heat treatment to cause rearrangement of grain boundaries and dislocations. Recovery It is the initial stage of annealing that takes place at the low temperature stage of annealing. As a material is plastically deformed, a minute portion of mechanical energy is stored which exists in crystals as stacking faults, point defects (such defects are interstitials and vacancies) and dislocations. When a material is plastically deformed, it is at a thermodynamically unstable state of higher energy. This is converted to lower energy states by the application of annealing leading to a change in microstructure. There are two process involved in recovery: slip annihilating and polygonization. Slip annihilation occurs when dislocations of opposite signs (that is in the case of edge dislocations, the fusion of the positive and the negative edge dislocation or in the case of screw in which the right hand screw merges with the left hand screw) merge together thereby cancelling each other out. Polygonization is the rearrangement of dislocation after annihilation recovery to a lower energy configuration. During recovery, this strain energy built up is relieved to some extent by dislocation motion, due to enhanced atomic diffusion at high temperatures. Recovery leads to physical properties like thermal and electrical conductivities being recovered to their pre worked states. [ggbk] Recrystallization After recovery, grains are not entirely strain free. That is the energy state of the grains is relatively high. New sets of strain free grains having near equal dimensions in all directions with low dislocation densities are formed. This process is known as recrystallization. This mechanism of producing new equaxed grains is driven by the difference in internal energy between the unstrained and strained material. The process of recrystallization can occur after or during deformation. The manner at which recrystallization occurs is of two kinds which vary with materials. Firstly a continuous manner, at which the microstructure gradually evolves into a recrystallized one or a discontinuous manner at which distinct new grains nucleate and grow Recrystallization after deformation is referred to as static whereas the latter is known as dynamic. The extent at which recrystallization occurs is dependent on two factors namely: time and recrystallization temperature. The temperature at which recrystallization is completed in an hour is referred to as recrystallization temperature. It is usually a third to half the materials melting temperature. The rate at which recovery process occurs is inversely proportional to time (that is it reduces with increasing time). Recrystallization has an entirely different kinetic. During the isothermal annealing, recrystallization starts very slowly then builds up gradually up to a certain point where it slows down. This can be shown in figure 13 In some cases it can be as high 0.7th the melting temperature. An illustration of the relationship between recrystallization temperature and percentage cold work is shown in figure 14. It is understood that as the percentage cold work increases, the recrystallization temperature decreases. Other factors affect the rate and occurrence of recrystallization. The annealing temperature is one of a few factors that have an effect on recrystallization. A materials recrystallization temperature reduces annealing time. The stress applied is another factor both recrystallization and temperature, an increase in stress applied means a lower temperature is required to activate the process. Also, the deformation on the material must be enough to allow nucleation and growth. A process known as grain growth occurs in a polycrystalline material after recrystallization provided the annealing temperature is maintained. The restoration mechanism does not require prior deformation or recrystallization and therefore will occur during annealing in their absence in a polycrystalline material. Grain boundary is the driving force for recovery. Stored energy produced as a result of a material being plastically deformed is released during the process of annealing causing a change in microstructure. This energy released is as a result of various mechanisms due to crystal defect interactions: A decrease in crystal defects due to their reactions with each other. Dislocations with opposite signs interacting causing their annihilation and dislocation loop shrinkage. Relocation of dislocations causing the formation of lower energy configurations such as grain boundaries with low angles. The formation of grain boundaries with high angles. These reactions occur during the restoration process of recovery. After this process, the following can occur: Dislocations as well as point defects being absorbed as a result of the migration of high angle grain boundaries. A decrease in the overall grain boundary area. These micro-structural changes occur during the restoration process of recrystallization and recovery. As a result of these micro-structural modifications, an ideal definition of recrystallization is derived: Along with the micro-structural changes, the properties of the specimen also change correspondingly. Thus, deformation and annealing are important processing methods for producing desired properties of the material by controlling its microstructures. Recrystallization mechanism The start of recrystallization is referred to as nucleation and occurs when dislocations are rearranged so as to form low dislocation density sections that have a high angle grain boundary with great mobility and thus is capable of quick movement over the strained region or recovered matrix. Recrystallization has a low driving force and high grain boundary energies; as a result of these characteristics, thermal variations cannot explain regions surrounded by high angle grain boundaries that are free from defects upon annealing. Therefore, the formation of recrystallized grains does not occur during annealing but previously exists in the deformed state. Three methods can be used to describe nucleation and they are: Movement of high angle boundaries that already exist before annealing: this happens when pre existing grain boundaries move into grains that are highly strained as illustrated in figure 16 this process requires a favourable energy balance between an increase in the overall grain boundary surface and a reduction in stored energy as a result of the removal defects triggered by the migration of the boundary. Movement of sub boundaries (that is low angle boundaries): this model is based on the theory of polygonization where stored energy is reduced during annealing as a result of rearrangement and removal of defects. It occurs when sub grain boundaries besiege regions containing low dislocation densities. Upon formation of sub grains, with the help of sub grain boundary movement, they are able to grow at the expense their neighbouring grains. Dislocations are absorbed by migrating sub boundaries and because of this, their mobility, orientation differences and energies are increased until their transformation into high angle boundaries, thus illustrating nucleation. Sub grains coalescence: this occurs when two neighbouring subgrains merge leading to their crystal lattices coinciding. It is regarded as a slow process but when compared to migration of sub grains is favoured for annealing at low temperatures. it is illustrated in figure 17. In this method, stored energy is reduced leading sub boundaries disappearing, sub grains growing and increase in orientation differences between coalescence groups and their neighbouring sub grains. These lead to the formation of high angle boundaries which move at high speeds and cause the process of recrystallization nucleation. It is vital to identify the fact that the total energy balance that takes the disappearance of sub boundaries into account with the increase and orientation difference is favourable (that is it leads to a reduction in total free energy). This mechanism is illustrated in figure 18. The occurrence of these three models is relatively diverse and they will therefore occur under different conditions. The basic requirement for the occurrence of the movement of pre existing grain boundaries that is the existence of differences in large strain between neighbouring grains is well accepted by researchers. However, there is conflict as to when the mechanisms sub grain boundaries migration and the coalescence of sub grains occur. Researchers believed the coalescence of sub grain boundaries are linked with large dispersion of sub grain angles distribution, relatively moderate strain, and reasonably low annealing temperatures. Whereas the mechanism of sub grain migration is linked with high annealing temperatures, strains that are relatively high and large dispersion in the distribution in sub grain size. Growth of recrystallized regions The basic mechanism causing recrystallization and grain growth is the migration of grain boundaries with high angles. However their driving force is what differentiates them from each other. The energy of the high angled grain boundaries is the main driving force for grain growth whether it being abnormal or normal growth. Whereas that for recrystallization is the energy stored during straining appeared as crystalline defects. In defect free regions that are encircled by boundaries with high angle, recrystallization progresses by enlargement of this nucleus over the non recrystallized medium. Grain growth and recrystallizations migrating high angle boundary curvature signal is another important factor that differentiates the two. http://asmcommunity.asminternational.org/portal/site/www/AsmStore/ProductDetails/?vgnextoid=a75a7dcbe4e18110VgnVCM100000701e010aRCRD ASM Handbook Volume 14A, Metalworking: Bulk Forming (ASM International) http://www.accuratus.com/zirc.html http://www.totaljoints.info/ceramic_for_total_hips.htm#2 http://www.azom.com/details.asp?ArticleID=940 http://books.google.co.uk/books?id=eUZw4SgqaPYCpg=PA126lpg=PA126dq=phase+transformation+mechanism+leading+to+microcracks+zirconiasource=blots=fCFhf-satfsig=WNiOjbUtX06mA_d1NkXIEEHcOsshl=enei=r568S9zCKJHFsgaxkd3lCQsa=Xoi=book_resultct=resultresnum=4ved=0CBYQ6AEwAzgK#v=onepageqf=false EXPERIMENTAL PROCEDURE Equipment Three samples of YTZP, 0ne made from 3mol Y2O3/ZrO2 powder and the other two made from the same powder but by two other manufacturers. Focused Ion Beam (FIB): is a technique used by material scientists in the analysis of a materials microstructure. the samples obtained can either be analysed directly using the FIB or transferred and viewed under a TEM or SEM. This process is similar to that of an SEM however ions (particularly gallium ions) rather than electrons. Figures 19a and b show how the FIB operates. The surface of the sample being tested is subjected to primary gallium ion beams. This spits a small part of the material, leading to the formation of either secondary ions (either positive or negative) or neutral atoms on the surface. Secondary electrons (e) are also produced from the gallium beam. This is collected as well as the signal from the split ions to form an image which is either analyzed using the FIB itself, SEM or TEM. http://www.fibics.com/fib/tutorials/introduction-focused-ion-beam-systems/4/ images is from here. GNU image manipulation program (GIMP): is an image editing software used to measure grain sizes. Vickers indentation: is used to form indentations which are viewed and studied under an optical microscope. A square imprint is formed from the Vickers indenter, where the two diagonal lengths are measured. In this project, the Vickers indentation is used to form plastic deformation. The surface defects such as scratches and unevenness need to be controlled, hence the reason for polishing. High temperature furnace: used for sintering and annealing. TEM Cold isostatic pressing Sample preparation Three mol% Y2O3/ZrO2 solid solution powder was used in this experiment. The powder was pressed into a disc at 200MPa, then sintered at 14500C for 2 hours. Two discs were produced, each pressed with loads 7.5 tons and 5 tons respectively. The load was reduced to 5 inorder to reduce the chance of lamination, as it occurred with the 7.5. Measurements and dimensions Sample 1: broken Sample 2 Weight 25.7711g 19.6836g Diameter 39.23 39.11 Height (thickness) 7.1 6.51 Dimensions after sintering<

Wars throughout Time Essay -- essays research papers

Sometime during the course of World War II, the United States became the most powerful nation in the world. During the last two hundred or so years, the United States has fought ten major wars and innumerable smaller military actions. For the most part it has been successful, in some instances defeating some of the world’s most powerful countries. In other instances it has simply been lucky. A common threat throughout the nation’s existence has been the practice of indifference and neglect. Between wars the Army shrinks to a very small size. Funds and attention almost disappear. This policy of the US Army may well indeed danger the nation’s safety in the future Military strength includes not only the Army, but sea and air power as well which will not be considered here. The resources of a country such as population, size, wealth, and factories all contribute to a nation’s military might and to how fast it might be expanded. Military power for ground forces has traditionally been measured in terms like regiments, brigades, divisions, corps, and armies. In addition, an important distinction exists between the Regular Army (the permanent establishment officered with West Point graduates), the militia (very important to the nation’s military strength for the first hundred years, local part-time units), and the National Guard or Reserves (in modern times a strong back-up to the Regular Army). The combined sizes of these forces have fluctuated widely when the nation has been at a threat of war.   Ã‚  Ã‚  Ã‚  Ã‚  It was very important that they created an Army during the American Revolutionary War. The colonists had militias, which fought with the British against the French and Indians. But the revolt against England demanded much more. The Americans who wanted independence gave it a maximum effort. The population of the colonies barely numbered 3,000,000 in 1775. Of the available manpower, a very large proportion either fought with the British or stayed neutral. So for an estimated 184,000 men to have fought for the new nation before the war ended in 1783, the remainder had to contribute a lot. (Almanac 209)   Ã‚  Ã‚  Ã‚  Ã‚  The new government was disorganized and needed money. Nevertheless, it created the Continental Army. It was organized as infantry in many regiments of about 800 men each. At any one time there were only about 15-20,0... ...ar II was the turning point. The rest of the world started fighting two years before Pearl Harbor. This gave the U.S. the time to expand its Army. It just barely did so (remember the Draft Act passing by one vote). Good fortune and luck allowed the policy of tiny peacetime army to work one more time.   Ã‚  Ã‚  Ã‚  Ã‚  The Cold War changed this forever, reinforced by tough lesson in Korea. In the time since then, the state of the world and technology demands that Army not plan a grace period to expand. It has to plan to meet a potential crises with what it has ready. Technology has dropped the need for large numbers of divisions and fighting troops. But technology has conversely demanded that stockpiles of equipment and training be at high levels at all times. Future wars will be over long before new orders for weapons like cruise missiles can be placed.   Ã‚  Ã‚  Ã‚  Ã‚   The best way to prepare for the present is to learn and correct our mistakes from the past. The cost may be high, but the world remains a dangerous environment. Today the U.S. must have a ready-to-use Army of a size sufficient for any conflict ahead. This was not always true in the past.

Riot police clash with SLC crowd :: essays research papers

I’VE BEEN IN riots and this was not a riot,† Salt Lake City Police Chief Rick Dinse said at a news conference at midmorning Sunday. â€Å"†¦ I believe it was the right amount of force at the right time.† Dinse said windows in several buildings and at least one vehicle were damaged during the clash, which began when individuals in a boisterous crowd in the thousands became unruly after the Bud World beer garden attraction was closed after reaching capacity. Dinse estimated damage could reach â€Å"the low thousands,† but both he and Mayor Rocky Anderson objected to early reports that characterized the incident as a riot. 2002 Winter Games Full Salt Lake coverage †¢ Gold medals stripped from two skiers †¢ Police, crowd clash in SLC; arrests made †¢ Celizic: There should be gold medal for whining †¢ Sorry Ohno, but these are the No Games †¢ NBCOlympics.com: Torson Injury Library †¢ Ask the Olympic Expert †¢ NBCOlympics.com †¢ Official Salt Lake site †¢ Local Games coverage †¢ More on Olympics â€Å"We want to emphasize that this was a relatively minor disturbance,† Anderson said. â€Å"†¦ Frankly, I anticipated far worse than this and far earlier (in the Games).† Dinse said between 180 and 200 helmeted and riot-gear clad officers responded to the area near Main and 200 South Streets after receiving reports that the crowd was getting out of hand. The gathering was declared an â€Å"unlawful assembly† about midnight and officers began slowly pushing the crowd, many of whom were chanting â€Å"USA, USA!†, down Main Street to the south, Dinse said. Rob Garcia, 18, of Salt Lake City told the Associated Press the crowd was yelling at police, and officers forced people against cars. â€Å"The police were pushing people,† he said. â€Å"The cops just jumped across and set up a barricade.† Two revelers fight in the street during a riot. Several shop windows were broken, but no injuries were reported. While many in the crowd obeyed orders to disburse, some individuals began hurling glass beer bottles at the officers, who responded by firing about 40 rounds of â€Å"impact devices† at the lower extremities of members of the crowd, he said. The action quickly scattered the crowd. Eighteen adults and two juveniles were arrested during the clash, and most were charged with public intoxication and failure to disburse, Dinse said. Most of those arrested were locals, but at least one was a Canadian citizen, he said.

Management of Field Construction Projects Essay -- Construction Manage

The objective of this report is to present and discuss the management of field construction projects. These projects involve a great deal of time and expense, so close control and management is paramount if they are to be completed within the established time and cost limitations. The term construction management is applied to the provision of professional management services to the owner of a construction project with the objective of achieving high quality with low costs. A specialist project manager organises, schedules, and controls the field work and is responsible for getting the project completed within the time and cost limitations. He acts as the focal point for all facets of the project and brings together the efforts of all organisations having input into the construction process. According to the Journal Your Pathway to qualifying in Project Management, RICS, July 2006. ‘Project Managers occupy a central role in driving successful completion of projects. Project Management is a thriving professional discipline much in demand throughout the world. Consequently, these chartered professionals can chose from a variety of potential employers including clients and consultants from both the public and private sectors with either commercial or not for profit aims. Typically, project managers will be appointed at the beginning of a project and will assist the client in developing the project brief and then selecting, appointing and co-ordinating the project team. He or She will then usually represent the client throughout the full development process managing the inputs from the client, consultants, contractors and other stakeholders.’ A key skill that every project manager should possess is being able to keep a proj... ...anager comes in as an enforcer. Subsequently if the client is looking to have more of an input in the construction and allow it to develop or mature over the course of the construction, a member of the design team will be employed to make the design changes either where necessary or where the client has had a change of heart. The member of the design team will also have the ability to ensure the contractor is fully aware of where all the design risks lie over the course of the construction. Works Cited 1. Latham, Michael (1994), Constructing the team 2. Egan, John (1998), Rethinking Construction 3. S. Keoki Sears, Richard Hudson Clough, Glenn A. Sears (2008), Construction Project Management: A Practical Guide to Field Construction Management 4. Sidney M. Levy (2006), Project Management in Construction 5. Raymond Joyce (2001), CDM Regulations Explained

18th century English literature Essay

1. There was a sense of relief and escape, relief from the strain of living in a mysterious universe and escape from the ignorance and barbarism of the Gothic centuries –not referring only to Gothic literature. The dark period provokes that people want to change and improve their lifestyle when they entered the 18th century. There was a general desire to emancipate from the dark aspects of rural and dark living. 2. Sanity, culture, and civilization had revived. There was a general feeling of emancipation from historic specters, a sense of security from the upheavals of the Civil War period. 3. Dryden wrote in 1668 â€Å"We have been so long together bad Englishmen that we had not leisure to be good poets†. This quote exemplifies that 17th century men were occupied with complete other things than humanities. â€Å"Nature†Ã¢â‚¬â€œphilosophical concept/religious concept that rule the 18th century. Western thinking– has been a controlling idea in the Western thought ever since antiquity, but it has probably never been so universally active as it was from the Renaissance to the end of the 18th century. The laws of â€Å"Nature† are the laws of reason; they are always and everywhere, and the axioms of mathematics they have only to be presented in order to be acknowledged as just and right by all men. This was the Golden Age of natural theology and deistical freethinking: Spinoza, Boyle, Locke, etc. During the Christian centuries religion has rested upon revelation; now it rested largely upon â€Å"Nature† and even the Orthodox who retain the supernatural basis felt that faith must be grounded firmly upon â€Å"Nature† before one had recourse t super-Nature. The 18th century is the century of Reason. If we want to apply reason, it has to be stable. Everything ought to be structured in logic axioms. It is the Golden Age of liberal thinking, also in religion which one had the power and gave divine explanations but they will not provide the answers anymore, but science will do. The scientific movement of the 17th and 17th centuries: Copernicus, Kepler, Galileo, Bacon, Descartes, Newton produced a â€Å"climate of opinion† in which supernatural and occult explanations of natural phenomena ceased to satisfy. The Universe came to be regarded as the Great Machine, working by rigidly determined laws of material causation –laws of Physics; everything has a cause. The supernatural, in both its divine and its diabolical forms, was  banished from Nature. Another relevant issue: the state power passed from the king gradually to the Parliament and the Cabinet ministers. A huge expansion abroad of British colonies in Asia, Africa and North America caused the Industrial Revolutio n. The basis consists of democratic principles. ! London became more and more the center of the literary and intellectual life of the country and writers came to look upon â€Å"polite† London society as their chief, if not their sole, audience. The opposite of natural living, cultivated people lived in London. Aristocracy in the old sense has been transmuted into gentility and wealth becomes the main motivating power in society –aristocracy regarded as gentile; educated and cultured people. Wealth becomes the motor of society -> new social class that centers in commercialization. Economics and Ethics are finally separated. The new economists prove to their own satisfaction that the individual desire to make money can produce in the long run nothing but good, and poverty can only be the result of idleness. In London, the coffeehouse replaces the Court as the meeting place of the men of culture. The journalist makes his appearance, and poetry becomes social and familiar. There was a correlation, between social class and education and between elegance –which was related to education; e.g. people went to the theatre– and learning that has not always existed in subsequent periods –people wanted to be cultivated so they started reading. The English novel coins in the 18th century thanks to journalism. And if poets were to use references to the Latin and Greek classics as well as to the events in the contemporary world of learning, they had to consider themselves addressing a very limited audience. That is why classic and contemporary studies were mixed in order to have a broader audience. Merchants and tradesmen of the town came to play a very important part in the life of the country. But the middle class were not yet the landed aristocracy, the country gentlemen and big state owners though they ruled only with the permission of and in alliance with the commercial interests. London The education and the entertainment of the middle classes became a legitimate objective of literature. The period between 1660 and 1785 was a time of amazing expansion for England. The world seemed different in 1785. A sense of new, expanding possibilities transformed the daily life of the British  people. And offered them fresh ways of thinking about their relations to nature. The city of London became the center of business, pleasure and the emerging consumer society. Samuel Johnson said that â€Å"when a man is tired of London, he is tired of life fir there is in London all that life can afford†. With growing prosperity, London turned into a city where everything was for sale. Its elegant shops dazzled tourists, supplying not only heaps of goods but also a perpetual source of amusement. Varieties of spectacles and shows drew larger and larger crowds, and theatres expanded to meet the competition. At the London playhouses, the audience itself was often part of the entertai nment. The Royal Exchange, in the hart of the city (financial district) of London, was not only a hub for business and shopping but also a symbols for â€Å"globalization†. The increasing importance of international commerce to the British economy. Addinson’s idyllic picture of the Exchange, written in 1711, celebrates the way in which the whole world seems to revolve around the blessings of trade. But many English people also worried that foreign luxuries might sap the national spirit of independence and self-sufficiency (Practice 1). There was a shift in population from the country to the town, and it reveals how far the life of the city, where every daily newspaper brought news sources of interest had moved from traditional values (London life is reflected in the newspaper). Formerly, the tastes of the court had dominated the art (!): the monarch stands for the nation. But the 18th century witnessed a turn from palaces to pleasure gardens that were open to anyone with the p rice of admission. New standards of taste were set by what the people of London wanted, and art joined with commerce to satisfy those desires. Artis Willams Hogarth made his living, not as earlier painters had done through portraits of royal and noble patrons, but by selling his paints to a large and appreciative public. London itself –its beauty and horror, its ever changing moods –became a favorite subject of writers (!). The sense that everything was changing was also sparkled by a revolution in science. In earlier periods, the universe had often seemed a small place, less than 6000 years old, where a single sun moved about the earth, center of the cosmos. Now time and space exploded, the microscope and telescope opened new fields of vision and the â€Å"plurality of world†, became a doctrine endlessly repeated. The authority of Aristotle was broken, their systems could not explain what Galileo and Kepler saw in  the heavens. As discoveries multiplied, it became clear that the moderns knew things of which the ancients had been ignorant. This challenge to received opinion was thrilling as well as di sturbing. In Paradise Lost, Book 8, the angel Raphael warn Adam to think about what concerns him, not to dream about other worlds. Yet, despite the warning voiced by Milton through Raphael, many later writers found the new science inspiring. It gave them new images to conjure with and new possibilities of fact and fiction to explore. Explorers were travelling around the earth, where they discovered unknown countries and ways of life. These encounters with other peoples often proved vicious. The trade and conquests that made Europeans powers like Spain and Portugal immensely rich also brought the scourge of racism and colonial exploitation. In the 18th century, Britain’s expansion into an empire was fueled by slavery and the slave trade, a source of profit that belied the national self-image as a heaven of liberty, and turned British people against one another. Rising prosperity at home had been built on inhumanity across the seas. At the end of the 18th century as many writers joined the abolitionist campaign, a new humanitarian ideal was forged. The modern world invented by the 18th century brought suffe ring along with progress. We still live with its legacies today. England was a nation of shopkeepers. But the stylish and lavish shops that filled 18th century London were also a visible sign of growing national power. The cutting edge of a consumer revolution, they showed the public that the modern world was to be welcomed, not feared. There was something for everyone to desire and possess in this new world of fashion. During the successful run of The Tatler (1709-1711)Germen de la novella de ficcion, Steele’s and Addison’s predecessor to The Spectator, The Female Tatler was published 3 times a week attributed to an imaginary â€Å"Mrs. Crackenthrope, a Lady that knows everything†. Its authors, who probably included both women and men, aimed to amuse and instruct female readers, as shown in the following piece on shops from 1709. -> Joseph Addison, from The Spectator, No. 69. Questions: Consider the satisfaction which Addison takes in The Royal Exchange. Why does he love so much to visit it? Are you persuaded that his pleasure comes from being â€Å"a great Lover of Mankind,† or is wealth itself what stimulates him? Many historians describe what happened in 18th century England as The Birth of a Consumer Society. According to this analysis, the widespread pursuit of good and entertainment turned England into the first truly modern nation, in which commercialization drives art as well as the economy. How well does this premise account for what you see in this topic?

Paleolithic Societies

Although the San and the Chumash were both hunter-gatherers, their culture and lifestyle had significant differences. Many cultures around the world had their fair share of differences. If you look closer though, you can find certain patterns that occur in many of the first societies that emerged in the world. Even these days, you can find the same types of patterns in people. The San lived in Southern Africa. They are also known as Bushmen, Sho, Barwa, Kung, or Khwe.Their lifestyle consisted of much leisure time, and the children did nothing but play. Women were greatly respected and admired in the San culture. The women would take part in both gathering and hunting, but mainly gathering. They used everything very wisely, including parts of ostrich eggs to hold fluids. Overall, the San were your typical hunter-gatherer society of the Paleolithic era. The Chumash lived in Southern California. They were also a hunter-gatherer society. They also were very good fisherman.They painted on cave walls and some of the drawings are still there today. Some of the Chumash settlements are believed to be over 10,000 years old. They were visited by the Spanish, which brought many diseases to the Chumash. They were also believed to be visited by Polynesians. Comparing these two cultures, we can see some similarities and many differences. They were both hunter-gatherer people. As their main food sources migrated, they followed along with them. Some differences would be that they lived in completely different parts of the world.The San did not fish, yet the Chumash did. They both have many more similarities and differences. The cultures I have compared and contrasted are only two of the many Paleolithic societies out there. They all can be recognized by their hunting and gathering ways. They followed their food, and obviously made it work for themselves. I think it would be an interesting journey to go back in time and see how they lived and interacted with each other before th e emerging of some of the technology that is common to us today.