Papers by Author: Masanobu Kubota

Abstract: The high-cycle fatigue properties of 0.35% carbon steel and work-hardened oxygen-free copper in 10MPa hydrogen were studied. The fatigue limit of the carbon steel in hydrogen was almost the same as that in air. The fatigue strength at 10⁷ cycles of the copper was higher in hydrogen than in air. The fatigue life of both materials is longer in hydrogen than in air. The reason was the delays in the crack initiation and the early propagation of the cracks in hydrogen. For both materials, the detrimental effect on the fatigue strength due to the hydrogen environment was small, however, it was determined that hydrogen participates in the slip deformation. The morphology of the slip bands was specific in hydrogen. In the copper, the slip bands, which are non-viable in air, developed in hydrogen.

Abstract: Fretting fatigue is one of the major factors in the design of hydrogen equipment. The effect of internal hydrogen on the fretting fatigue strength of austenitic stainless steels was studied. The internal hydrogen reduced the fretting fatigue strength. The reduction in the fretting fatigue strength became more significant with an increase in the hydrogen content. The reason for this reduction is that the internal hydrogen assisted the crack initiation. When the fretting fatigue test of the hydrogen-charged material was carried out in hydrogen gas, the fretting fatigue strength was the lowest. Internal hydrogen and gaseous hydrogen synergistically induced the reduction in the fretting fatigue strength of the austenitic stainless steels. In the gaseous hydrogen, fretting creates adhesion between contacting surfaces where severe plastic deformation occurs. The internal hydrogen is activated at the adhered part by the plastic deformation which results in further reduction of the crack initiation limit.

Abstract: Crack propagation of SCM440H low alloy steel under varying load is enhanced by absorbed hydrogen. Substantial acceleration of crack propagation rate up to 1000 times was observed compared with that of uncharged material. The role of factors affecting enhanced acceleration was investigated by changing hydrogen concentration absorbed in metal, specimen thickness and loading frequency. Results are as follows. (1) 0.2 mass ppm diffusible hydrogen in metal was enough to cause enhanced acceleration. The predominant fracture mode showing acceleration was quasi cleavage. (2) In the case of thin specimen thinner than 0.8mm, the tri-axiality of stress is weak, and the enhanced crack propagation did not appear. However, the introduction of side-groove to 0.8mm specimen in order to increase the tri-axiality resulted in enhanced acceleration. (3) Lower loading frequency resulted in higher crack propagation rate in cycle domain. The crack propagation rate in time domain was almost constant irrespective of loading frequency. Enough concentration of hydrogen, tri-axiality and low loading frequency resulted in enhanced acceleration of fatigue crack propagation.

Abstract: It has been recognized that the threshold stress intensity factor range Kth of a short crack is lower than that of a long crack. The short crack behavior in plain specimen has been studied by many researchers. However, the behavior of a short crack at the root of a long notch is not yet clear. The crack closure behavior is considered to be affected by the constraint at notch root and it is dependent on the length and the root radius of notch. In this study, fatigue tests on specimens with short pre-crack at long notch were done and the difference in crack closure behavior was studied. As a result, short crack effect appeared in any notch root radius. In a sharp notch, the crack opening point easily reached its stable condition after a small amount of crack extension. On the contrary in a dull notch, the opening point was lower than the stable condition and consequently short crack effect lasted in relatively wide range of crack extension. The small crack effect of notched specimen was discussed based on crack closure behavior.

Abstract: Hydrogen is considered to be a possible energy source in the coming future. However, it has been recognized that hydrogen has a detrimental effect on the fatigue strength of metal. The fatigue crack growth characteristic is an important property for the integrity assessment of hydrogen utilization machine. In this report, the effect of hydrogen on the fatigue crack propagation characteristic was studied using low alloy steel, carbon steels and A286 alloy. Especially in this study, very short pre-cracked specimen as small as 0.03 mm deep was used and the near threshold fatigue crack behavior was studied. As a result, materials whose Vickers hardness was higher than 300 were found to be susceptible to absorbed hydrogen.

Abstract: Although fatigue limit diagram is defined in principle for constant stress amplitude condition, it is often considered that fatigue failure would not occur even in varying loading if applied stresses were kept within the fatigue limit diagram. However, it was shown in the case of small-notched specimen and fretting fatigue that fatigue failure occurred in some special case of variable amplitude loading condition even when all stress amplitudes were kept within the fatigue limit diagram. The cause of this phenomenon was examined using two-step and repeated two-step stress patterns in which the first step stress was with zero mean stress and the second step stress had a high mean stress. A non-propagating crack was formed by the first step stress. This crack functioned as a pre-crack for the second step stress with high mean stress. Consequently, fatigue failure occurred even when all stress amplitudes were kept within the fatigue limit diagram. It was an unexpected fracture caused by the interference effect of non-propagating crack and mean stress change.

Abstract: The objective of this study is to evaluate the effect of stress relief groove on fretting fatigue strength. Fretting fatigue tests and finite element analyses were done. The shape of groove was controlled by groove radius R and tangential angle θ. The depth of groove was specified by R and θ. Fretting fatigue strength was increased with an increase of θ and then it turned into a decrease. The decrease was caused by the transition of failure mode from fretting fatigue at the contact part to plain fatigue at the groove root. The transition was caused by an increase of stress concentration at the groove root with an increase of the groove depth. Therefore, the maximum improvement of fatigue strength was achieved by the largest θ limited by fatigue strength of the groove root. In the analysis, the groove generates high compressive stress field at the contact edge, where small cracks never propagate. Therefore, assumptions to relieve the contact pressure concentration at the contact edge were taken into the analysis model. The values of stress intensity factor ranges for small cracks introduced near the contact edge were almost the same between grooved and non-groove specimens.