INFLUENCE OF MODE-I, MODE-II AND MODE-III OVERLOADING ON THE FATIGUE THRESHOLD OF MODE-I

The fatigue thresholds of specimens were measured under overloading in Modes I, II and III. The results were that: (1) overloading increased the threshold values under Modes II and III as well as under Mode I; (2) the effect of overloading under Modes II and III was greater than that under Mode I; (3) the effect of overloading increased with the ov erload ratio. The results are explained as follows. Overloading genera tes a greater number of cell dislocations and microvoids in the overlo aded zone than under normal loading. The effect of the microvoids is d ominant; it decreases the stress at the crack tip, which results in an increase in the effective threshold value, DELTAK(th.eff.ov). Overloa ding also generates residual plastic deformation, which produces resid ual compression stress surrounding the crack tip. The residual stress in front of the crack tip increases DELTAK(th.eff.ov). The residual st ress in the wake of the crack increases crack closure. Mode I overload ing does not produce plastic deformation in the crack wake, but it doe s produce opening of the crack tip, which decreases the closure. This is why the effect of overloading is less under Mode I than under Modes II and III.