Mean Stress Effects in Stress-Life and Strain-Life Fatigue 2004-01-2227
Various approaches to estimating mean stress effects on stress-life and strain-life behavior are compared with test data for engineering metals. The modified Goodman equation with the ultimate tensile strength is found to be highly inaccurate, and the similar expression of Morrow using the true fracture strength is a considerable improvement. However, the Morrow expression employing the fatigue strength coefficient σ′ƒ may be grossly non-conservative for metals other than steels. The Smith, Watson, and Topper (SWT) method is a reasonable choice that avoids the above difficulties. Another option is the Walker approach, with an adjustable exponent γ that may be fitted to test data, allowing superior accuracy. Handling mean stress effects for strain-life curves is also discussed, including the issue of mathematical consistency with mean stress equations expressed in terms of stress. A new and mathematically consistent method for incorporating the Walker approach into strain-life curves is developed. With γ = 0.5, this result gives a new strain-based interpretation of the SWT method.