陽電子消滅法によるステンレス鋼SUS316 の微小疲労き裂発生挙動評価

疲労き裂は金属構造の突発的な破壊を引き起こすが，一般的に降状応力よりも低い応力の繰返しで発生する.疲労の初期段階では，格子欠陥（転位の）移動と増殖，すなわち微視的なすべりが発生し，それに続いて疲労き裂が発生する.陽電子消滅法は，様々な材料における空孔欠陥の検出に広く用いられている.本研究では，ＳＵＳ316ステンレス鋼の回転曲げ疲労試験を実施し，疲労損傷の蓄積を陽電子消滅法で定義されるＳパラメータを用いて検討した.応力繰返し数の増加とともに，疲労損傷の蓄積，すなわち転位密度の上昇によってＳパラメータも増加した.疲労き裂が発生すると，き裂部位で測定されるＳパラメータの値は，疲労き裂が無い部分よりも大きくなった.疲労き裂先端に存在する塑性域は非常に高い転位密度を有している.き裂部位における大きなＳパラメータ値は，そのような疲労き裂先端における塑性域の存在に起因している.

Fatigue cracks, leading to catastrophic fracture of metal structure, generally initiate by the cyclic stress even under yield stress. At the first stage of fatigue process, movement and increase of lattice defects (dislocations), namely microscopic sliding takes place, and crack initiation follows thereafter. The positron annihilation spectroscopy has been widely used for probing open volume lattice defects in various materials. In this study, rotating bending fatigue test of Type 316 stainless steel was conducted, and fatigue damage accumulation has been investigated using S parameters defined in positron annihilation spectroscopy. S-parameter increased with increasing number of loading cycles due to the fatigue damage accumulation, namely increase of dislocation density. When fatigue crack initiated, S-parameter measured at the crack initiation site became higher than that at the location without fatigue crack. The plastic deformation zones at the fatigue crack tips had extremely higher dislocation densities. Higher S parameter at the crack initiation site was attributed to those plastic zones at the fatigue crack tips.

Positron annihilation spectroscopy, Fatigue, Fatigue damage evaluation, Crack initiation,Non-destructive testing