Development of Dynamic/Quasi-Static Fracture Toughness Testing Apparatus Using Miniaturized Specimens and Application to Evaluation of Local Toughness in Materials

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From the present research program the following conclusion can be drawn :1. We successfully developed the dynamic/quasi-static fracture toughness testing apparatus using miniaturized V-notch specimens where the fracture toughness can be evaluated under wide variety of deformation rate(4〜10^<-5>m/s).2. Small specimen impact tests using miniaturized specimens with 1.5, 1.0 and 0.7 mm square and 20 mm length were carried out to evaluate the toughness of a laser welded metal. The DBTTs and FATTs given by the miniaturized and standard specimens showed a simple linear correlation. The laser welded metal composed of an extremely fine acicular ferrite structure was estimated to have such an excellent toughness as 168K DBTT in conversion to the standard size specimen.3. Micro-structure, micro-hardness and micro-absorbed impact energy in the Japanese sword were investigated to clarify excellent mechanical properties of the Japanese sword. The HASAKI side had a lower value while the MUNE side showed a higher value in the micro-absorbed impact energies obtained with the 1.0 and 0.7 mm square miniaturized specimens. It was shown clearly that the TSUKURIKOMI and the TSUCHIOKI processes gave the excellent gradated balance of strength-toughness to the Japanese sword.4. In order to clarify influence of deformation rate on susceptibility to hydrogen assisted embrittlement(HAE)in low alloy steels, three points bend test using v-notched small specimen with a dimension of 1.5×1.5×20mm was applied high strength low alloy steels with cathodically hydrogen charged. The test was carried out at room temperature under a wide variety of deformation rates, i.e., from 2.0×10^0to 1.7×10^<-8>m・s^<-1>. The test results showed that there was a critical deformation rate for the HAE in the high stength low alloy steels, where the fracture energy suddenly drops off and the fracture mode also transits from ductile fracture to brittle one.