Chapter 22 Materials Selection and Design Considerations

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SUMMARY Summary • W129 Materials Selection for a Torsionally Stressed Cylindrical Shaft In this chapter, we have illustrated the protocol of materials selection using six diverse examples. For the first case, a torsionally stressed cylindrical shaft, an expression for strength performance index was derived; then, using the appropriate materials selection chart, a preliminary candidate search was conducted. From the results of this search, several candidate engineering materials were ranked on both strength-per-unit mass and cost bases. Other factors that are relevant to the decisionmaking process were also discussed. Automobile Valve Spring A stress analysis was next performed on a helical spring, which was then extended to an automobile valve spring. It was noted that the possibility of fatigue failure was crucial to the performance of this spring application. The shear stress amplitude was computed, the magnitude of which was almost identical to the calculated fatigue limit for a chrome–vanadium steel that is commonly used for valve springs. It was noted that the fatigue limit of valve springs is often enhanced by shot peening. Finally, a procedure was suggested for assessing the economic feasibility of this spring design incorporating the shot-peened chrome–vanadium steel. Failure of an Automobile Rear Axle The next case study was devoted to a failure analysis, which detailed an investigation conducted on a failed rear axle of a light pickup truck that had overturned; the problem was to determine whether the accident resulted from this failure, or vice versa. Impact and tensile specimens were fabricated from outer perimeter and interior regions of the axle, which were subsequently tested. On the basis of scanning electron and metallographic examinations of the actual failed axle surface, as well as the surfaces of these test specimens, it was concluded that the accident caused the axle failure. Artificial Total Hip Replacement For the fourth case study, the artificial total hip replacement was explored. The hip anatomy was first presented, which was followed by a discussion of the components and material requirements for the artificial replacement. Implant materials must be biocompatible with body tissues and fluids, must be corrosion resistant, and must also be mechanically compatible with interfacing replacement/body components. The femoral stem and ball are normally made of a cold-worked stainless steel, a cast Co–Cr–Mo alloy, or a hot-forged titanium alloy. Some recent designs call for a polycrystalline aluminum oxide or zirconium oxide ball. Ultrahigh molecular weight polyethylene is commonly used for the acetabular cup, whereas acrylic bone cement is normally the fixation agent for attachment of the femoral stem (to the femur) and acetabular cup (to the pelvis). Chemical Protective Clothing The fifth materials case study was concerned with materials to be used for chemical protective clothing—specifically, glove materials to protect against exposure to methylene chloride, a common ingredient in paint removers. Important parameters relative to the suitability of a chemical protective material are breakthrough time
W130 • Chapter 22 / Materials Selection and Design Considerations REFERENCES General and exposure rate. Equations were provided that allow computation of these parameters, and values were determined for seven common protective glove materials. Only two materials {multilayered [poly(vinyl alcohol)/polyethylene] and Viton rubber} were deemed satisfactory for this application. Materials for Integrated Circuit Packages Materials utilized for the integrated circuit package incorporating the leadframe design were the topics of the final case study. An IC chip is bonded to the leadframe plate using either a eutectic solder or an epoxy resin. The leadframe material must be both electrically and thermally conductive, and, ideally, have a coefficient of thermal expansion that matches the IC chip material (i.e., silicon or gallium arsenide); copper alloys are commonly used leadframe materials. Very thin wires (preferably of gold, but often of copper or aluminum) are used to make electrical connections from the microscopic IC chip contact pads to the leadframe. Ultrasonic microjoining welding/brazing techniques are used where each connection joint may be in the form of either a ball or wedge. The final step is package encapsulation, wherein this leadframe–wire–chip assembly is encased in a protective enclosure. Ceramic glasses and polymeric resins are the most common encapsulation materials. Resins are less expensive than glasses and require lower encapsulation temperatures; however, glasses normally offer a higher level of protection. Ashby, M. F., CES4 EduPack—Cambridge Engineering Selector, Granta Design Ltd.,Cambridge, UK, http://www.grantadesign.com. Ashby, M. F., Materials Selection in Mechanical Design, 2nd edition, Butterworth-Heinemann, Woburn, UK, 2002. Budinski, K. G. and M. K. Budinski, Engineering Materials: Properties and Selection, 8th edition, Pearson Prentice Hall, Upper Saddle River, NJ, 2005. Dieter, G. E., Engineering Design, A Materials and Processing Approach, 3rd edition, McGraw- Hill, New York, 1999. Mangonon, P. L., The Principles of Materials Selection for Engineering Design, Pearson Prentice Hall, Upper Saddle River, NJ, 1999. Optimization of Strength Ashby, M. F. and D. R. H. Jones, Engineering Materials 1, An Introduction to Their Properties and Applications, 3rd edition, Butterworth- Heinemann, Woburn, UK, 2005. Spring Design Juvinall, R. C. and K. M Marshek, Fundamentals of Machine Component Design, 4th edition, Chapter 12, John Wiley & Sons, Hoboken, NJ, 2005. Shigley, J., C. Mischke, and R. Budynas, Mechanical Engineering Design, 7th edition, Chapter 10, McGraw-Hill Companies, New York, 2004. Artificial Total Hip Replacement Black, J. and G. Hastings (Editors), Handbook of Biomaterial Properties, Chapman & Hall, London, 1998. Davis, J. R., Handbook of Materials for Medical Devices, ASM International, Materials Park, OH, 2003. Chemical Protective Clothing Forsberg, K. and S. Z. Mansdorf, Quick Selection Guide to Chemical Protective Clothing, 4th edition, John Wiley & Sons, Hoboken, NJ, 2003. Materials for Integrated Circuit Packages Electronic Materials Handbook, Vol. I, Packaging, ASM International, Materials Park, OH, 1989. Grovenor, C. R. M., Microelectronic Materials, Institute of Physics Publishing, Bristol, 1989. Reprinted by Adam Hilger, Ltd., Bristol, UK.
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Chapter 22 Materials Selection and Design Considerations

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