Ch02 - bud21932_ch02_027-066 07/28/2006 05:40 PM Page 27...

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Chapter Outline 2–1 Material Strength and Stiffness 28 2–2 The Statistical Significance of Material Properties 32 2–3 Strength and Cold Work 33 2–4 Hardness 36 2–5 Impact Properties 37 2–6 Temperature Effects 39 2–7 Numbering Systems 40 2–8 Sand Casting 41 2–9 Shell Molding 42 2–10 Investment Casting 42 2–11 Powder-Metallurgy Process 42 2–12 Hot-Working Processes 43 2–13 Cold-Working Processes 44 2–14 The Heat Treatment of Steel 44 2–15 Alloy Steels 47 2–16 Corrosion-Resistant Steels 48 2–17 Casting Materials 49 2–18 Nonferrous Metals 51 2–19 Plastics 54 2–20 Composite Materials 55 2–21 Materials Selection 56 27 2 Materials
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28 Mechanical Engineering Design 1 See ASTM standards E8 and E-8 m for standard dimensions. The selection of a material for a machine part or a structural member is one of the most important decisions the designer is called on to make. The decision is usually made before the dimensions of the part are established. After choosing the process of creat- ing the desired geometry and the material (the two cannot be divorced), the designer can proportion the member so that loss of function can be avoided or the chance of loss of function can be held to an acceptable risk. In Chaps. 3 and 4, methods for estimating stresses and deflections of machine members are presented. These estimates are based on the properties of the material from which the member will be made. For deflections and stability evaluations, for example, the elastic (stiffness) properties of the material are required, and evaluations of stress at a critical location in a machine member require a comparison with the strength of the material at that location in the geometry and condition of use. This strength is a material property found by testing and is adjusted to the geometry and con- dition of use as necessary. As important as stress and deflection are in the design of mechanical parts, the selection of a material is not always based on these factors. Many parts carry no loads on them whatever. Parts may be designed merely to fill up space or for aesthetic quali- ties. Members must frequently be designed to also resist corrosion. Sometimes temper- ature effects are more important in design than stress and strain. So many other factors besides stress and strain may govern the design of parts that the designer must have the versatility that comes only with a broad background in materials and processes. 2–1 Material Strength and Stiffness The standard tensile test is used to obtain a variety of material characteristics and strengths that are used in design. Figure 2–l illustrates a typical tension-test specimen and its characteristic dimensions. 1 The original diameter d 0 and the gauge length l 0 , used to measure the deflections, are recorded before the test is begun. The specimen is then mounted in the test machine and slowly loaded in tension while the load P and deflection are observed. The load is converted to stress by the calculation σ = P A 0 (2–1) where A 0 = 1 4 π d 2 0 is the original area of the specimen.
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This note was uploaded on 03/22/2010 for the course MEEN ISEN 302 taught by Professor Kim during the Spring '10 term at Texas A&M University–Commerce.

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Ch02 - bud21932_ch02_027-066 07/28/2006 05:40 PM Page 27...

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