7. Frequently the S-N curves for steel can be approximated by a straight line between N = 10 and N = 10° cycles when the data are plotted on a log-log scale, as shown in the figure for SAE 4140 steel....

Extracted text: 7. Frequently the S-N curves for steel can be approximated by a straight line between N = 10 and N = 10° cycles when the data are plotted on a log-log scale, as shown in the figure for SAE 4140 steel. This implies S= AN, where a and b are %3D %3D constants. A. Find b for the 4140 steel for a certain part made from 4140 steel (Fig- ure 17.33). B. Fatigue failures occur after 5 years. By what factor would the cyclic stress amplitude have to be reduced to increase the life to 10 years? Assume the number of cycles of importance is proportional to time of service. 700 600 Figure 17.33. S-N curve for a SAE 4140 steel. 500 4140 steel 400 300 102 105 103 104 106 107 N, cycles to failure Sress, MPa