Chapter 22 Materials Selection and Design Considerations
Chapter 22 Materials Selection and Design Considerations
Chapter 22 Materials Selection and Design Considerations
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W100 • <strong>Chapter</strong> <strong>22</strong> / <strong>Materials</strong> <strong>Selection</strong> <strong>and</strong> <strong>Design</strong> <strong>Considerations</strong><br />
Stress (10 3 psi)<br />
100<br />
80<br />
60<br />
40<br />
20<br />
0<br />
� max = 92,200 psi<br />
� aa = 25,600 psi<br />
� min = 41,000 psi<br />
Time<br />
� m = 66,600 psi<br />
Figure <strong>22</strong>.10 Shear stress<br />
versus time for an automobile<br />
valve spring.<br />
Now let us determine the actual stress amplitude for the valve spring using<br />
Equation 8.16 modified to the shear stress condition:<br />
taa �<br />
92,200 psi � 41,000 psi<br />
� � 25,600 psi 1177 MPa2<br />
2<br />
tmax � tmin 2<br />
(<strong>22</strong>.23)<br />
Thus, the actual stress amplitude is slightly greater than the fatigue limit, which<br />
means that this spring design is marginal.<br />
The fatigue limit of this alloy may be increased to greater than 25,300 psi<br />
(175 MPa) by shot peening, a procedure described in Section 8.10. Shot peening<br />
involves the introduction of residual compressive surface stresses by plastically<br />
deforming outer surface regions; small <strong>and</strong> very hard particles are projected onto<br />
the surface at high velocities. This is an automated procedure commonly used<br />
to improve the fatigue resistance of valve springs; in fact, the spring shown in<br />
Figure <strong>22</strong>.7 has been shot peened, which accounts for its rough surface texture.<br />
Shot peening has been observed to increase the fatigue limit of steel alloys in<br />
excess of 50% <strong>and</strong>, in addition, to reduce significantly the degree of scatter of<br />
fatigue data.<br />
This spring design, including shot peening, may be satisfactory; however, its adequacy<br />
should be verified by experimental testing.The testing procedure is relatively<br />
complicated <strong>and</strong>, consequently, will not be discussed in detail. In essence, it involves<br />
performing a relatively large number of fatigue tests (on the order of 1000) on this<br />
shot-peened ASTM 232 steel, in shear, using a mean stress of 66,600 psi (460 MPa)<br />
<strong>and</strong> a stress amplitude of 25,600 psi (177 MPa), <strong>and</strong> for 10 cycles. On the basis of<br />
6<br />
the number of failures, an estimate of the survival probability can be made. For the<br />
sake of argument, let us assume that this probability turns out to be 0.99999; this<br />
means that one spring in 100,000 produced will fail.<br />
Suppose that you are employed by one of the large automobile companies that<br />
manufactures on the order of 1 million cars per year, <strong>and</strong> that the engine powering<br />
each automobile is a six-cylinder one. Since for each cylinder there are two valves,<br />
<strong>and</strong> thus two valve springs, a total of 12 million springs would be produced every<br />
year. For the above survival probability rate, the total number of spring failures<br />
would be approximately 120, which also corresponds to 120 engine failures. As a<br />
t aa