Overload effects on a ferritic-baintic steel and a cast aluminium alloy: Two very different behaviours

Load controlled fatigue tests were performed up to 107 cycles on flat notched specimens (Kt=2.5) under constant amplitude and variable amplitude loadings with and without periodical overloads. Two materials are studied: a ferritic-bainitic steel and a cast aluminium alloy. These materials have a very different cyclic behaviour: the steel exhibits cyclic strain softening whereas the Al alloy shows cyclic strain hardening. The fatigue tests show that, for the steel, periodical overload applications reduce significantly the fatigue life for fully reversed load ratio (R\'03=-1), while they have no influence under pulsating loading (R\'03=0). For the Al alloy overloads have an effect (fatigue life decreasing) only for variable amplitude loadings. The detrimental effect of overloads on the steel is due to ratcheting at the notch root which evolution is overload's dependant.