Papers by Author: Hisao Matsunaga

Abstract: Geometrical discontinuities in the engineering components, such as holes, fillets, grooves, and keyways, are unavoidable in design. In essence, they act as a stress-raiser that causes the fatigue cracks. Accordingly, the geometrical discontinuities trigger a significant amount of reduction for the fatigue strength. It is well known that the fatigue limit of the notched components is governed by either the initiation or propagation of a small crack at the root of a notch. Since the elastoplastic behaviors and the crack closure effect should be properly taken into consideration, the behavior of such a small crack cannot be characterized solely by linear elastic fracture mechanics. To overcome the difficulty mentioned above, in this study, a novel method is proposed to investigate the notch effect by making use of the McEvily method, which has been widely used for the analysis of small fatigue crack growth. Further, to modify the McEvily method, the plastic zone size of a crack is calculated based on the Dugdale model to incorporate the effect of the plastic yielding near the crack tip. Finally, the predictive capability of the proposed method is demonstrated by comparing our theoretical predictions with the available experimental data.

Abstract: The near-threshold fatigue behavior of small, semi-elliptical surface cracks in a bearing steel was investigated under cyclic shear-mode loading in ambient air. Fully-reversed cyclic torsion was combined with a static axial compressive stress to obtain a stable shear-mode crack growth in the longitudinal direction of cylindrical specimens. Shear stress amplitude was gradually decreased with an increase in crack length and the crack finally became non-propagating. Abrasive wear on the crack faces was inferred by debris and also by changes in microstructure in the wake of crack tip. Further, it was found that these effects resulted in a significant decrease in the crack growth rate. In this study, we shed light on the important role of the crack size and crack face interference on the crack growth behavior.

Abstract: Semi-elliptical shear-mode fatigue cracks were promoted in the axial direction of round specimens of SAE52100 bearing steel by fully-reversed cyclic torsion tests under a static axial compressive stress. Non-propagating cracks smaller than 1 mm were obtained in two ways; (i) stress amplitude decreasing tests of notched specimens, or (ii) constant stress amplitude tests of smooth specimens. The threshold stress intensity factor ranges, (KIIth and (KIIIth, showed a crack size dependency.

Abstract: A study has been made of the striations and fissures developed in the aluminum alloy 2024-T3 during fatigue crack growth. Fissures were found to form on inclined facets. They were uniformly spaced as the result of a shielding process. Striation spacings were in accord with da/dN values at the higher levels of
K investigated, but at low
K levels striation spacings were larger than the corresponding da/dN values. The percentage of the fracture surface containing striations varied with the
K level, ranging from less than 1 % at low
K levels to 80 % at higher
K levels. The reason for the discrepancy between the spacing of striations and the corresponding da/dN values is discussed.

Abstract: Shear-type fatigue crack behavior in a bearing steel, SAE52100, was investigated in a biaxial fatigue testing machine using cylindrical specimens. Either of the following two types of artificial defect with the total length of 400 ~ 440 %m and the depth of 200 ~ 300 %m was introduced into the specimen surfaces: (a) a semi-elliptical pre-crack emanating from 2 adjacent holes by a tension-compression loading, (b) 3 adjacent holes oriented in the axial direction which had slits made by the focused ion beam technique at the both ends. Fully reversed torsion with a shear stress amplitude at specimen surface, τa = ~ 600 MPa, was applied to the specimens under the static axial compressive stress σm = −1000 ~ −1200 MPa. In case of the defect (a), a shear-type crack propagated from the pre-crack in direction perpendicular to the specimen axis. At the specimen surface, the shear-type crack periodically branched from the crack tip and propagated 5~10 %m in Mode I directions. The shear-type crack growth was decelerated with an increase in the crack length and finally stopped at N ≅ 7.5×106 cycles and 2a ≅ 600 %m. On the other hand, in case of the defect (b), the shear-type crack propagated in axial direction, and the crack growth was accelerated with an increase in the crack length. In addition, the threshold stress in case without the compressive stress on crack-face was determined by a τa-decreasing test. The results revealed that the shear-type crack growth was strongly influenced by the crack-face friction and the existence of the Mode I blanched cracks.

Abstract: In order to investigate the influence of hydrogen on the fatigue strength of Type 304 meta-stable austenitic stainless steel, specimens were cathodically charged with hydrogen. Hydrogen-charging led to a marked decrease in fatigue crack growth life. Crack growth paths and slip bands morphology were changed by the hydrogen-charging. To elucidate the mechanism of the degradation by hydrogen, the surfaces of both the uncharged and charged specimens were examined by the hydrogen microprint technique (HMT). In the uncharged specimen, no hydrogen emission from specimen surface was observed. On the other hand, in the hydrogen-charged specimen, a hydrogen emission was observed, especially in the vicinity of fatigue cracks. Hydrogen was mainly emitted from slip bands. These results suggest that the degradation of fatigue crack growth resistance in hydrogen-charged specimens was caused by the diffusion of hydrogen to slip bands, which accelerated the dislocation mobility and thereby facilitated the fatigue crack growth.