球状黒鉛鋳鉄の疲労き裂進展特性に及ぼす試験片板厚の影響

Abstract

In order to clarify the influence of the thickness of specimen on the fatigue crack propagation properties in spherical graphite cast iron, fatigue crack propagation test was carried. Spherical graphite cast iron (FCD450) was used as specimen material. Therefore, this material was conducted two kind of heat treatment process. One is the austemper treatment (ADI) and another is quenching and tempering (QT). As-cast material is added to those, and three kinds of materials are used for the experiment. The fatigue crack propagation test is carried out on the stress ratio R at 0.05 in the room temperature. The specimen is used 1CT type fatigue specimen, which followed ASTM Standard. The thickness of specimens are 12.5mm, 8mm and 4mm·And, it is investigated the correlation between the fatigue crack propagation and measured crack closure of each specimen. The obtained results are as follows:(1) The influence of the thickness specimen is caused from threshold stress intensity factor range, ΔKth to II a stage. The crack propagation speed increase, as the thickness of specimen is less, and ΔKth is lowers.(2) The results of various thickness of specimen agree well in da/dN-ΔKeff curve. Therefore, the influence of the thickness of specimen is mainly depended on the crack closure. The difference of crack closure with thickness is not depended on graphite or fracture surface roughness. It is influenced by the presence of the oxide on fracture surface.(3) It will be cosidered that crack tip shape is influenced on the thickness of specimen.