K-0526 鋳造アルミニウム合金AC4CHにおける疲労挙動の応力比依存性(S04-4 非鉄金属の組織と疲労特性)(S04 金属材料の組織と疲労強度信頼性)

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Axial fatigue tests have been conducted using smooth specimens of a cast aluminium alloy AC4CH at three different stress ratios, R, of -1, 0.05 and 0.4. Particular attention has been paid to crack initiation and small crack growth behaviour. The Goodman relationship gave fairly good or slightly conservative estimation of the fatigue strength at 10^7 cycles. Cracks initiated from a casting porosity or Sb-segregated defect and the latter defect acted more predominantly as crack initiation site. Regardless of stress ratio, small cracks initially grew faster than the corresponding large cracks characterized in terms of the effective stress intensity factor range, ΔK_<eff>, then gradually approached the da/dN-ΔK_<eff> relationship for large cracks and coincided with it at a ΔK value of 2〜3MPa√m. Based on detailed fractographic analyses, shear-type growth region following the crack initiation was clearly discerned, the size of which showed remarkable stress ratio dependence, with increasing crack size with decreasing stress ratio, particularly very large at R=-1. The shear-type crack growth was responsible for the acceleration of small cracks because of a different growth mechanism from large cracks.
2001-08-22