High Cycle Fatigue Crack Paths in C35 Steel under Complex Loading

ABSTRACT. The aim of this work is to study damage mechanisms in high cycle fatigue (105 – 106 cycles) of C35 steel under complex loading (multiaxial non-proportional loading, blocks loading). Effects of out-of-phase loading on crack behaviour (initiation, propagation planes, bifurcation…) have been investigated. Results indicate that there are two principal damage modes; the diffuse damage mode is specific for the torsion loading whereas the localized damage is common for many loading cases. HCF strength is higher for out-of-phase than in-phase tension-torsion, in terms of applied stress amplitude. Stage 1 crack propagation occurs on the maximum shear stress plane in all loading cases (in-phase and out-of-phase). The role of maximum normal stress (stage 2) is less determined under out-of-phase loading. Loading sequence effect is not remarkable in C35 steel. For lifetime modelling, it should predict correctly the fatigue limit and model completely both stages of crack propagation in each loading case