Bilinear S-N Curves and Equivalent Stress Ranges for Fatigue Life Estimation

With the magnitude of some stress cycles in a live load stress range spectrum being higher than the constant amplitude fatigue limit (CAFL) of a structural detail, the development of a fatigue crack could be expected. The current procedure in the AASHTO specifications for fatigue life estimation utilizes an equivalent constant amplitude stress range with the direct extension of the S-N lines of slope −3 to below the CAFL. This approach is conservative; however, it often results in overprediction of fatigue damage and underestimation of the safe useful life of structural details. A set of bilinear S-N curves with the knee at the CAFL for the AASHTO fatigue strength categories and a slope of −4 below have been produced analytically for estimating fatigue life of existing structures subjected to variable live load stresses. This paper provides information regarding the development of the new equivalent constant amplitude stress ranges for the bilinear S-N curves. The results from a few examples are presented. Adoption of the procedure presented herein for fatigue life estimation is recommended.