The crack growth initiation behavior in delayed failure under superposed static or dynamic mode II stress on mode I static stress was investigated on the precracked specimens of Ni-Cr-Mo steel that was quenched and tempered at 673K. When the superposed mode II static stress intensity factor
KII or the range of mode II dynamic stress intensity factor Δ
KII on the static stress intensity factor
KI was relatively small, the crack initiation time
ti was longer than that when
KII or Δ
KII was zero. When
KII or Δ
KII was large, however,
ti decreased with an increase in
KII or Δ
KII. The crack propagation direction under superposed
KII or Δ
KII on
KI is dependent on fracture mechanisms. When hydrogen embrittlement governed the mechanism, the crack propagation angle θ
p once increased with an increase in Δ
KII /
KI and decreased with a further increase in Δ
KII/
KI. In the case of fatigue θ
p was small when Δ
KII/
KI was small and increased with an increase in Δ
KII/
KI. The crack propagation under superposed Δ
KII on
KI in water occurred by the hydrogen embrittlement type when Δ
KII/
KI was small, and changed to the fatigue type when Δ
KII/
KI became large.
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