1988 年 37 巻 417 号 p. 606-611
To investigate the influence of mean stress on the tensile and compressive fatigue strength of the heat-treated high-speed steel, fatigue tests were conducted on the test pieces of 9mm and 6mm in diameter which had been heat-treated at two levels of temperature. The fatigue strength at a finite life was evaluated at three different levels of mean stress by using the stair case method for small sample size (JSME Standard S02-1981 14S-N testing method). Furthermore, to clarify the fracture mechanism, fracture toughness tests were conducted on the test pieces of 1/2 compact tension specimens. The following results were obtained.
(1) It was found that the fatigue life for 50% survival was strongly affected by mean stress. The fatigue life obtained was much shorter than the estimated value by the modified Goodman law but close to that by the Soderberg diagram. The effect of mean stress was stronger in the harder material.
(2) In the tensile and compressive fatigue tests, “Fish eye” was found on the fracture surface. By assuming that “Fish eye” was an initial crack, the value of fracture toughness KIC was estimated from the radius of Fish eye, which showed 15-25 MPa√m for the specimen 1 (HRC60) and 22-30MPa√m for the specimen 2 (HRC56).
(3) From the fracture toughness tests on the test pieces of 1/2 compact tension specimens, it was found that the values of KIC for the specimens 1 and 2 were 20.9-21.2MPa√m and 23.2-26.9MPa√m, respectively. These values were in good agreement with the mean value of fracture toughness KIC estimated by the tensile and compressive fatigue tests.