Original paper(Vol.47 No.8 pp.846)
Torsional fatigue strength of specimens containing an initial small crack introduced by tension compression fatigue
Murakami Yukitaka; Takahashi Koji
Abstract:Reversed torsional fatigue tests were carried out on the 0.47% steel specimens containing an initial small crack. The initial small semi elliptical cracks ranging from 200mm to 1000mm were introduced by the preliminary tension compression fatigue tests using specimens containing a hole of 40mm diameter. The crack growth behavior from a semi elliptical surface crack was determined. Further, the effects of artificial small defects on torsional fatigue strength were also determined in relation to small cracks. The conclusions are summarized as follows:
(1)In rotating-bending and tension-compression fatigue, the effects of artificial small defects and small cracks are identificial from the viewpoint of fatigue limit. However, in torsional fatigue these effects are not identical.
(2)Crack leading to fracture were Mode I branched cracks initiated at crack tips. Initiation of Mode II cracks at the pre-crack tip was also observed in most cases in addition to Mode I branched cracks. However, theses cracks stopped propagating as the Mode I branched cracks propagated.
(3)Torsional fatigue limit for a semi-elliptical small cracks is determined by the threshold condition for non-propagation of Mode I branched cracks.
(4)Torsional fatigue limit of specimens containing a small crack can be successfully predicted by the square root of area parameter model with a combination of sqmax criterion.
Key Words:fatigue, torsion, small cracks, small defects, crack propagation, threshold stress intensity factor range, square root of area parameter model