An Observation of Fish-eye Fracture Process in High Strength Steel SUJ2.

Abstract

Long-term fatigue tests have been conducted on a high strength steel, SUJ2, in order to clarify the fatigue behaviour in long life region, more than 107 cycles. The fatigue failure at the stress level under the ordinary fatigue limit occurred in long life region. Fracture mechanism in this region is fish-eye type, i. e. inner defect induced failure, while surface slip type fracture is found in short fatigue life region. At the center of fish-eye non-metallic inclusion is recognized, which is surrounded by an indefinite and grainy region. However, the fish-eye without grainy region was also observed exceptionally. Therefore, a fish-eye is divided usually into three parts, i. e. inclusion, grainy region and inner crack. Most of the inclusions generating the fish-eye were located within 0.1mm from the specimen surface. The sizes of inclusions calculated by the √(area) method are almost constant to the stress level and the number of cycles. The initial stress intensity factor range, ΔKini, at which crack was generated from the inclusion, is almost the same with increasing fatigue life, i, e. 2.3∼3.5 MPa √(m). On the other hand, the sizes of fish-eyes calculated by the √(area) method are scattered to the stress level and the number of cycles. Based on the results, a model of the fatigue failure at the stress level under the fatigue limit is proposed.