モードI/変動モードII混合荷重下での遅れ破壊き裂進展開始挙動に及ぼす応力波形の影響

概要

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The crack growth initiation behavior in delayed failure under superposed cyclic mode II stress of various stress wave forms was investigated on the precracked specimens of Ni-Cr-Mo steel quenched and tempered at 673K. When the range of mode II cyclic stress intensity factor ΔK_<II> was superposed on the static stress intensity factor K_I, the crack growth initiation time t_g decreased with an increase in ΔK_<II>, the degree of which increased with an increasing period at a constant frequency ; i. e. t_g decreased most under cyclic liad with positive saw tooth wave, followed by those with negative saw tooth wave and trapezoid. the intergranular fracture ratio ψ of crack growth initiation surface was affected little by stress wave forms of ΔK_<II>. The reason why the crack growth initiation time t_g decreased when the stress wave form of mode II had a long stress increasing period seems to be that the corrosive reaction is activated during the stress increasing period, since the protective film in the vicinity of crack tip is destroyed mechanically by cyclic stress.
1992-01-15