Plane Strain Condition Based on the Cleavage Crack Initiation Sites and Ductile Crack Extension Shapes

摘要

In the past studies, the authors proposed that the criterion for obtaining the plane strain condition at the middle-section of the specimen was given by the following equation; B>=0.004(K_c(J)/#sigma#_(ys))~2+0.01. However, there are some obscure points, such as the applicable region of K_c(J) #sigma#)_(ys), in it. Therefore, fracture toughness tests were performed in the ductile-brittle transition temperature range and low-toughness temperature range for high strength steel (HT780).The main results obtained were as followed. (1) For HT780, the results of the cleavage crack initiation sites and the fibrous crack shapes also showed the similar tendency with the former one. (2) In the lower K_c(J)/ #sigma#_(ys) region, the above-mentioned equation was not fit to the experimental results. (3) It is inferred that the stress in the thickness direction (#sigma#_z)is distributed among the range which is decided by S_c and 1/2B_c. (4) Therefore, becoming the plane strain with increasing the thickness, there will exist a transition state which can be the plane strain and the no-plane strain.