Engineering Student

MAE 3344-001 HW-01 Nandi Huang I. Case study 1.2 – U.S. Pennies According to the book, millions of pennies are produced in order to fulfill the needs of the nation and the material used to produce the pennies has changed from time to time. 100% copper were used at the beginning but now 97.5% zinc plated with copper are used to produce pennies. According to U.S. Department of Treasury, pennies are made with three manufacturing process. Firstly, materials are washed by a chemical solution for protection. Next, planchets are put through edge-rolling operation. Lastly, they are pressed into the same dimensions and into same inscriptions. II. Answer the following questions 1. A unit cell is the smallest repeating structure of a solid. And single crystal might contain many unit cells. 2. Strain hardening is when a higher shear stress is required, there is an increase in the overall strength and hardness of the metal. Greater the deformation, the greater the number of entanglements which means higher increase in the metal’s strength. 3. If there is more entanglements of dislocations occur with grain boundaries, the strength of the metal would increase. So polycrystalline metal with large grains will not likely to have many entanglements which means less strength. 4. For cold working in this problem, the melting temperature of the metal should be at least three times the boiling point of water which is 300°C. There are many metals that have higher melting point thus it is possible to cold work a metal at 100°C. 5. The number of grains for grain size number 12 is twice the grains number for grain size 11 which is 32800 grains/mm^3 (16400*2). The volume of the sphere is V = (4/3)*pi*(r^3) = (4/3)*pi*(0.5mm^3) = 0.523 mm^3. Total number of grains = 32800 grains/mm^3 * 0.523 mm^3 = 17174. The number of grain is 17174. 6. The engineering stress is the ratio of the applied load P to the original cross-sectional area A0. The true stress is the ratio of the load P to the actual cross-sectional area A. As the stress increases, the difference between engineering stress and true stress increases. 7. Ductile fracture occurs with plastic deformation. Ductile materials tens to neck down to a point before failing in a tension test and the ductile fractures take place along planes on which the shear stress is a maximum. However, brittle fracture occurs with little or no plastic deformation. Also, brittle fracture takes place along the crystallographic plane on which the normal tensile stress is maximum. 8. The highest temperature would occur at the neck region because at that point strain is the highest which means the energy dissipated is highest which means the temperature is highest. 9. There are two basic behaviors of smart materials. 1. When subjected to an electric current, the material undergo a reversible change in shape, by as much as 4%. 2. When deformed by an external force, the materials emit a small electric current.