Papers by Author: Masahiro Endo

Abstract: Effect of occasional mode II loading on subsequent mode I fatigue crack growth behavior was investigated by using a thin-walled tube made of 7075-T6511 aluminum alloy. Careful observation of crack growth behavior revealed that the occasional mode II loading has two contradictory effects for crack growth behavior. The first is a retardation effect that is associated with the plastic deformation near crack tip. However, this effect is negligibly small for the crack growth life as a whole. The second is an acceleration effect caused by mode II fatigue crack growth itself. It was found that under relatively high ΔK level, the mode II crack growth was about an order magnitude faster than mode I crack growth. Therefore, to properly evaluate the effect of occasional shear loading in the 7075 alloy, the mode II crack growth should be taken into account.

Abstract: Effect of hydrogen-charging was investigated with respect to the tensile properties of three types of cast irons: JIS FCD400, FCD450 and FCD700. In this study, hydrogen charging led to a marked ductility loss in all the cast irons. The thermal desorption spectroscopy and the hydrogen microprint technique revealed that, in the hydrogen-charged specimens, most of solute hydrogen was diffusive and mainly segregated at graphite, graphite/matrix interface zone and pearlite. In the fracture process of non-charged specimen, neighboring graphites were interconnected with each other mainly by ductile dimple fracture. On the other hand, in the fracture process of hydrogen-charged specimen, the graphites were interconnected by cracks. The difference in the fracture morphology between the non-charged and the hydrogen-charged specimens is attributed to the presence of diffusive hydrogen in graphite and graphite/matrix interface. During early stage of fracture process in hydrogen-charged specimen, the interspace between graphite and matrix is filled with hydrogen gas, which leads to the ductility loss of matrix in the vicinity of graphite. Even after the initiation of crack from graphite, hydrogen is continuously outgassed from graphite and supplied to the crack tip. Therefore, concerning the hydrogen effect on the strength of cast irons, a role of subsurface graphite as a “local hydrogen supplier” should be taken into consideration.

Abstract: Flaking and spalling caused by rolling contact fatigue associate with a small crack, and a special testing method and machine are required to study the small fatigue crack behavior under shear mode loading. It was found by authors that the behaviors of small shear-mode fatigue cracks from the inclusions and the artificial defects could be successfully observed by applying the fully-reversed torsion coupled with static axial compressive stress. However, the servo-hydraulic fatigue testing machine is quite expensive for purchase and maintenance, and large installation space is necessary for the hydraulic and cooling systems. Moreover, the presence of axial compression significantly lowers the frequency of torsional loading, which consequently results in low testing speed. In this study, a cost-effective, space-saving and high-speed fatigue testing method was newly proposed, and the shear-mode fatigue crack growth tests were carried out by using the developed machine. Based on the obtained experimental data, the potential of the new testing machine is discussed.

Abstract: In this study, a series of experimental studies was conducted to investigate the fatigue behavior of Ti-6Al-4V alloy at room temperature. Specifically, by inspecting the cylindrical specimens with a circumferential notch of different depths (20-200µm) and notch root radii (20-100µm), the notch effect was systematically investigated with tension-compression fatigue tests (R = –1). To quantify the effects of small notch, the -parameter model was adopted and its applicability for Ti-6Al-4V alloy was examined. Finally, the fatigue characteristics are discussed in conjunction with the behavior of small fatigue cracks at notches.

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: Friction stir welding (FSW) is a new solid-state welding process that can produce low-cost and high-quality joints of especially aluminum and mgnesium alloys. The welding zone consists of different regions with characteristic microstructuralal details such as a weld nugget, a thermo-mechanically-affected zone (TMAZ) and a heat-affected zone (HAZ). Tension-compression fatigue tests were performed using FSW aluminum alloy AA5454 sheet specimens at a stress ratio of –1. To investigate the propagation behavior of small fatigue cracks in those regions, an artificial defect was introduced into different defined locations in the FSW specimens as well as into the parent material specimens. The crack propagation rates depended on the defined locations and were a function of the hardness; that is, the lower the hardness was, the higher the propagation rate was. The crack paths were mostly perpendicular to the applied stress axis, but some crack paths exhibited deviations by the influence of the local anisotropy of the microstructure.

Abstract: The plastic zone size (PZS) at the tip of a crack emanating from stress concentrators subjected to uniform tension was calculated based upon the Dugdale model. A method was proposed for the prediction of fatigue notch effects in terms of the McEvily method that has widely been used for the analysis of small fatigue crack growth. In modifying this method, the elastic-plastic effects due to stress concentration were taken into account using the values of PZS calculated in this study. The results predicted by this method accounted successfully for the behavior of small fatigue cracks near the threshold levels observed in the fatigue tests conducted using notched steel specimens.

Abstract: Titanium has widely been used as a biomaterial because of its excellent corrosion resistance and biocompatibility. However, problems with respect to biological reaction and fitness of elastic modulus for human bone or tooth have yet to be solved. Porous titanium is expected to be a promising material to solve these problems. The aim of this study is to clarify the effect of the porous structure of this material on the biomechanical compatibility. The spherical pure titanium powder, with an average particle size of 100 µm, was sintered by spark plasma sintering. The sintered porous titanium compacts had a porosity of 33 %. The specimens were machined from the sintered compacts for the evaluation of the mechanical properties. The elastic modulus indicated a value close to human bone, while the tensile and compressive strengths showed lower values than those of human bone.

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.