EFFECT OF NONPROPORTIONAL OVERLOADING ON FATIGUE LIFE

The effect of non-proportional overloading on both low cycle and high cycle fatigue life has been studied. Low cycle multiaxial fatigue test s were performed on EN 15R (a low alloy steel) using sequential loadin g blocks which comprised uniaxial ''ordinary'' cycles and torsion ''ov erload'' cycles, and vice versa. In high cycle fatigue, the behaviour of mode I crack growth in a medium carbon steel subjected to mixed (I and II) mode overloading was examined. Under tension-torsion sequentia l overloading, crack growth behaviour shows an earlier transition from Stage I to Stage II with a pronounced reduction in accumulated fatigu e life. Tensile overloading on torsion cycles was found to be more dam aging compared to torsion overloading on repeated tensile cycles. The crack-load interaction in sequential overloading and its influence on crack growth and fatigue life is discussed. In low strain fatigue, Sta ge II crack growth retardation closely relates to the overload plastic zone size, crack tip blunting and crack surface shielding. Mixed mode overloading is shown to have a significant effect only if the mode I component of overloading is large enough to keep the crack open. Under both low cycle and high cycle fatigue conditions non-proportional ove rloading is shown to be more damaging than proportional overloading.