1978 年 64 巻 8 号 p. 1209-1218
In relation to clarifying the cause of the scattering of energy absorbed in Charpy transition temperature range, a study was made of finding the cleavage crack initiation site. Main results were as follows.
1) The cleavage crack always initiated at the pearlitic second phase independently of the energy level absorbed in Charpy test. Heterogeneity such as inclusions and so on did not contribute to the cleavage crack initiation.
2) The cleavage microcracks initiating at the effective grain larger than mean size could grow to final fracture, while the microcracks initiating at the smaller effective grains could not propagate.
3) Frequency of cleavage microcrack initiation was very small. It was one or two cracks per area of 0.4mm2 of ductile fracture surface. On the basis of experimental results described above, statistical analysis was made. It may be concluded that Gaussian distribution of effective grain size was the direct cause of the scattering cf energy absorbed in Charpy test.