Internal stress effects on fatigue crack initiation at notches

In this paper the effects of the internal stresses on crack initiation at notches is investigated. Internal stresses are induced by applying two pre-loading cycles at a given load ratio, R = SMIN/SMAX, to the ‘virgin’ notched specimen. After pre-loading, blocks of 106 cycles with a constant nominal stress range, ΔS, slightly larger than the nominal threshold stress range, ΔSth, are applied with increasing minimum stress, SMIN, in each subsequent block until a crack initiates. The lowest SMIN above which a crack initiates is recorded as SIN. The process is repeated for a wide range of positive and negative load ratios in order to obtain SIN/SMAX vs. load ratio curve. This curve for positive load ratios surprisingly resembles the KPR/KMAX plot reported in the literature for a cracked specimen. The results are explained in terms of compressive internal stress and the associated local clamping action at the notch cyclic plastic zone. Both the analytical and finite element calculations show a fairly good agreement with the experimental results.