2013 年 99 巻 4 号 p. 312-321
A study to evaluate the effect of pre-strain on crack growth is carried out based on fracture mechanical method using 3 point bend test. Two kinds of hot rolled high tensile strength steel sheets were used as samples. One has relatively uniform microstructure strengthened by precipitates, and the other has relatively non-uniform microstructure strengthened by hard secondary phase particles. The effect of pre-strain on fracture mechanical resistance to crack growth was measured, and the mechanism involved in the result was discussed.
And the following conclusions were obtained.
1) The steel sheet with uniform microstructure has a higher resistance to initiation of crack growth (Jq), and tearing modulus (Tmat) than the other. Pre-strain reduces the resistance to initiation of crack growth of the both steel sheets. However, the decrease in Jq due to pre-strain is more significant in steel sheets with non-uniform microstructure.
2) The main factors determining crack growth in steel sheet with uniform microstructure are large precipitates and inclusions, and that for the other steel sheet is the secondary hard phase.
3) The reason for decline of Jq due to pre-strain is attributed to the increase of density of voids in the vicinity of crack front due to pre-straining. The difference in the influence of pre-strain on Jq may be understood as the difference in the density of void nucleation sites. Namely, larger density of void nucleation sites in the steel with non-uniform microstructure leads to lower Jq.
