損傷挙動観察に基づく延性き裂発生・進展特性を支配する鋼材機械的特性の解明: 延性き裂進展シミュレーション手法の構築

摘要

The effort of this study is to reveal the material properties controlling the resistance to ductile crack initiation and growth (CTOD-R curve) on the basis of the microscopic observation of crack growth, so that the CTOD-R curve can be numerically predicted only from those properties. The crack growth resistance tests using 3-point bend specimens with fatigue pre-crack for two steels provide different ductile crack growth resistance, CTOD-R curve, whereas both steels have the same "mechanical properties" in terms of strength and work hardening. The crack-tip constraint effect, in this case the pre-crack length effect, on the CTOD-R curve is also different between both steels. The observation of crack growth behaviors reveals that micro-mechanisms of ductile crack initiation from fatigue pre-crack and subsequent extension can be different. It is shown that two "ductile properties" of steel associated with ductile damage can control the CTOD-R curve. The one is "critical local strain" evaluated with the notch-tip strain for ductile cracking obtained with bending test of notched specimen, which controls ductile crack initiation of pre-cracked specimen. The other one is "stress triaxiality dependent ductility" obtained with circumferentially notched round-bar specimens subjected to tension, which controls the ductile crack extension for pre-cracked specimen.%近年の高靭性材料の開発により，延性き裂の進展を伴うrn場合の構造性能評価の重要性が益々増してきている。そのrnため，延性き裂の進展抵抗を定量的に予測する技術の確立rnが期待される。欧州の構造健全性評価手法であるFIT-rnNETで採用されているFracture assessment diagram（FAD）rn法や，Crack driving force（CDF）法では，延性き裂進展量をrnJ積分やCTODなどの破壊力学パラメータの関係で表したrn延性き裂発生・進展抵抗曲線（J-R curve，CTOD-R curve）をrn材料抵抗値として用い，構造部材が不安定延性破壊を開始rnする限界負荷レベルや限界き裂長さが評価されている。