Chapter 22 Materials Selection and Design Considerations

W100 • Chapter 22 / Materials Selection and Design Considerations
Stress (10 3 psi)
100
80
60
40
20
0
� max = 92,200 psi
� aa = 25,600 psi
� min = 41,000 psi
Time
� m = 66,600 psi
Figure 22.10 Shear stress
versus time for an automobile
valve spring.
Now let us determine the actual stress amplitude for the valve spring using
Equation 8.16 modified to the shear stress condition:
taa �
92,200 psi � 41,000 psi
� � 25,600 psi 1177 MPa2
2
tmax � tmin 2
(22.23)
Thus, the actual stress amplitude is slightly greater than the fatigue limit, which
means that this spring design is marginal.
The fatigue limit of this alloy may be increased to greater than 25,300 psi
(175 MPa) by shot peening, a procedure described in Section 8.10. Shot peening
involves the introduction of residual compressive surface stresses by plastically
deforming outer surface regions; small and very hard particles are projected onto
the surface at high velocities. This is an automated procedure commonly used
to improve the fatigue resistance of valve springs; in fact, the spring shown in
Figure 22.7 has been shot peened, which accounts for its rough surface texture.
Shot peening has been observed to increase the fatigue limit of steel alloys in
excess of 50% and, in addition, to reduce significantly the degree of scatter of
fatigue data.
This spring design, including shot peening, may be satisfactory; however, its adequacy
should be verified by experimental testing.The testing procedure is relatively
complicated and, consequently, will not be discussed in detail. In essence, it involves
performing a relatively large number of fatigue tests (on the order of 1000) on this
shot-peened ASTM 232 steel, in shear, using a mean stress of 66,600 psi (460 MPa)
and a stress amplitude of 25,600 psi (177 MPa), and for 10 cycles. On the basis of
6
the number of failures, an estimate of the survival probability can be made. For the
sake of argument, let us assume that this probability turns out to be 0.99999; this
means that one spring in 100,000 produced will fail.
Suppose that you are employed by one of the large automobile companies that
manufactures on the order of 1 million cars per year, and that the engine powering
each automobile is a six-cylinder one. Since for each cylinder there are two valves,
and thus two valve springs, a total of 12 million springs would be produced every
year. For the above survival probability rate, the total number of spring failures
would be approximately 120, which also corresponds to 120 engine failures. As a
t aa