The influence of yield strength and negative stress ratio on fatigue crack growth delay in 4140 steel

Abstract

Single tensile overloads were applied to AISI 4140 compact steel specimens with three yield strengths of 110, 165 and 205 ksi and three stress ratios R of 0, −1/2, and −1. Fatigue crack growth delay was significantly longer for the low yield strength steel with R=0. However, at R=−1 only slight differences in delay occurred for all three yield strengths. Thus delay from single tensile overloads with R≥0 can be quite misleading when compared with negative stress ratio delay results. This implies that small compressive cyclic stresses, which often have only a small influence on constant amplitude crack growth behavior, may have significant influence in spectrum loading due to reduction of delay from high tensile overloads. This reduction in delay may be attributed to cyclic relaxation of both crack region residual stresses and crack closure. Under constant amplitude testing, fatigue crack growth rates were more dependent upon yield strength than upon R ratio.