Effect of small defects on the fatigue strength of martensitic stainless steels

The defect tolerance of three different martensitic stainless steels (17-4PH, X20Cr13 and AISI403) under cyclic loading was investigated. Fatigue tests were performed with specimens containing diverse artificial defects (corrosion pits, single and multiple drilled holes, sharp circumferential notches and pre-cracked holes). The measured cyclic strengths are compared to predictions based on the root area parameter model proposed by Murakami and Endo. This model is well applicable to small sharp defects if the threshold condition for crack propagation determines the fatigue limit. The transition size from small to large defects, root area(trans), - above which the threshold stress intensity factor range, Delta K-th, becomes a constant value - is between 80 and 166 mu m for the investigated steels. For larger defects, the threshold for long cracks, Delta K-th,K-lc serves well to predict the fatigue strength under fully reversed loading, i.e. at a load ratio of R = -1. Comprehensive tests were performed to study the influence of mean stress, and threshold conditions were determined that enable to predict the fatigue limit at different load ratios. A simple equation is introduced to estimate the critical defect size, root area(critical), which is the size above which defects become detrimental under cyclic loading. It has been found that root area(critical) increases with increasing load ratio while root area(trans) is independent of R. Furthermore, higher values of the threshold stress intensity factor range were determined for large defects (with sizes larger than root area(trans)) compared to long cracks if the load ratio is higher than R = -1. This could be explained by the plane stress condition at the material surface which is prevalent for surface defects, while the condition for long cracks can be assumed as plane strain. It is further demonstrated that the effect of small defects on the fatigue strength of stainless steels is highly sensitive to the notch root radius, rho. Small holes with diameters of 100 mu m (p = 50 mu m) or larger are less detrimental for the fatigue strength than defects with high stress concentrations such as corrosion pits.