212 Budynas-Nisbett:Shigley's ll.Failure Prevention 5.Failures Resulting from T©The McGraw-Hil Mechanical Engineering Static Loading Companies,2008 Design,Eighth Edition 208 Mechanical Engineering Design Figure 5-5 Valve-spring failure caused by spring surge in an oversped engine.The fractures exhibit the classic 45 shear failure 5-1 Static Strength Ideally,in designing any machine element,the engineer should have available the results of a great many strength tests of the particular material chosen.These tests should be made on specimens having the same heat treatment,surface finish,and size as the ele- ment the engineer proposes to design;and the tests should be made under exactly the same loading conditions as the part will experience in service.This means that if the part is to experience a bending load,it should be tested with a bending load.If it is to be subjected to combined bending and torsion,it should be tested under combined bending and torsion.If it is made of heat-treated AISI 1040 steel drawn at 500C with a ground finish,the specimens tested should be of the same material prepared in the same manner. Such tests will provide very useful and precise information.Whenever such data are available for design purposes,the engineer can be assured of doing the best possible job of engineering. The cost of gathering such extensive data prior to design is justified if failure of the part may endanger human life or if the part is manufactured in sufficiently large quan- tities.Refrigerators and other appliances,for example,have very good reliabilities because the parts are made in such large quantities that they can be thoroughly tested in advance of manufacture.The cost of making these tests is very low when it is divid- ed by the total number of parts manufactured. You can now appreciate the following four design categories: 1 Failure of the part would endanger human life,or the part is made in extremely large quantities:consequently,an elaborate testing program is justified during design. The part is made in large enough quantities that a moderate series of tests is feasible. 3 The part is made in such small quantities that testing is not justified at all;or the design must be completed so rapidly that there is not enough time for testing.Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition II. Failure Prevention 5. Failures Resulting from Static Loading 212 © The McGraw−Hill Companies, 2008 208 Mechanical Engineering Design Figure 5–5 Valve-spring failure caused by spring surge in an oversped engine. The fractures exhibit the classic 45◦ shear failure. 5–1 Static Strength Ideally, in designing any machine element, the engineer should have available the results of a great many strength tests of the particular material chosen. These tests should be made on specimens having the same heat treatment, surface finish, and size as the ele￾ment the engineer proposes to design; and the tests should be made under exactly the same loading conditions as the part will experience in service. This means that if the part is to experience a bending load, it should be tested with a bending load. If it is to be subjected to combined bending and torsion, it should be tested under combined bending and torsion. If it is made of heat-treated AISI 1040 steel drawn at 500◦C with a ground finish, the specimens tested should be of the same material prepared in the same manner. Such tests will provide very useful and precise information. Whenever such data are available for design purposes, the engineer can be assured of doing the best possible job of engineering. The cost of gathering such extensive data prior to design is justified if failure of the part may endanger human life or if the part is manufactured in sufficiently large quan￾tities. Refrigerators and other appliances, for example, have very good reliabilities because the parts are made in such large quantities that they can be thoroughly tested in advance of manufacture. The cost of making these tests is very low when it is divid￾ed by the total number of parts manufactured. You can now appreciate the following four design categories: 1 Failure of the part would endanger human life, or the part is made in extremely large quantities; consequently, an elaborate testing program is justified during design. 2 The part is made in large enough quantities that a moderate series of tests is feasible. 3 The part is made in such small quantities that testing is not justified at all; or the design must be completed so rapidly that there is not enough time for testing