Abstract: The primary objective of this study is to develop a quantitative model to predict the effects of
materials, environment and mechanics such as loading configuration on environmentally-assisted
cracking (EAC) of stainless steels in high-temperature water.
It has basically been accepted that crack propagation in oxygenated high temperature water is
controlled by a slip-dissolution and/or deformation-oxidation mechanism. According to this
mechanism, the crack-tip strain rate is an extremely important mechanical parameter for
determining the crack growth rates. Based upon a formulation obtained by combining Faraday’s
equation with an elastic plastic analysis of the strain singularity at a growing crack-tip in work
hardening materials, a theoretical formulation of crack-tip strain rate has been derived for plane
strain and plane stress conditions. The FEM analysis for 3D crack growth can be compared to the
theoretical 2D analysis.
In this paper, we first make a CCP (Center Crack Plane) model, and performed a
3-dimensional Finite Element Analysis (3D-FEA) to evaluate the crack-tip stain rate paying
attention to the element mesh size and to the loading history. After optimization these parameters,
the calculated crack-tip strain distribution, including its logarithmic singularity, was founded to
agree well with the theoretical distribution. The significance of the crack-tip strain rate upon the
crack-tip strain distribution and crack growth rate was demonstrated. The specimen size effects on
crack growth rates were discussed from this point of crack-tip strain distribution.
Finally, we focused on the importance of crack-tip strain rate as a unique mechanical
parameter that controls the crack growth rate.
Abstract: It was previously demonstrated by the authors that density of water and the density-related physical properties of water are ones of the major governing factors of corrosion of metals in supercritical aqueous solutions. The water density is expected to have significant effects also on stress corrosion cracking (SCC) of metals in supercritical water. In this study, we have looked into cracking behavior of sensitized and non-sensitized stainless steels in water under various pressures
and at fixed temperatures above and below the critical point by using SSRT technique, and discussed its correlation with dielectric constant of water. The experimental results have suggested two different cracking mechanisms of 316(L) stainless steels as follows;
For sensitized 316 SS - pure water system; (1) Effects of phase state of water and applied pressure, more essentially, physical property of water, were clearly observed. (2) SCC did not occur in the oxygenated 'gas-like', supercritical water at 400°C/25MPa. (3) Cracking occurred at 400°C/30MPa and the cracking severity was more pronounced as applied pressure was increased up to 60MPa at the same temperature. (4) This variation in cracking susceptibility being dependent on pressure was understood from dielectric constant of water. (5) The results give a strong evidence of the dissolution mechanism. For non-sensitized 316L SS - sulfuric acid water system; (1) Non-sensitized 316L SS severely cracked with IG even in a 'gas-like' supercritical water. (2) Dielectric constant did not affect cracking severity. (3) Cracking was more enhanced at higher temperature. (4)
The results suggested oxidation cracking.
Abstract: The fracture mechanics characteristics in the critical locations of the wheelset for high-speed train have not been studied enough yet despite of severe conditions due to increase in operating speeds. Moreover, the fracture mechanics characteristics with respect to the aging effects of wheelset materials have not been clearly studied.
In the present study, the following fracture mechanics characterization tests were carried out in accordance with various locations on the wheelset for high-speed train: fracture toughness depending on load rate, fatigue crack growth rate and fatigue thresholds. The results show that the fatigue crack growth rates in accordance to the locations on wheelset were not remarkably different, and the fatigue threshold in the region of the bolt-hole is lower than that in other regions. The
fracture toughness depending on load rate data shows that once the downward curve from quasi-static values was reached, subsequent values showed a slow increase with respect to the impact velocity. This means that dynamic fracture toughness should be considered in the design code of the wheelset material.
Abstract: The diverse techniques for condition monitoring of the test material have been applied in the material test loop (System Safety Benchmark Facility: SSBF) simulating the water condition that of the boiling water reactor. In this paper, two methods of condition monitoring are mainly described. One is a thinning process monitoring of material wall by using pulse echo methods with ultrasonic sensors. The other is early detection of a leakage by measuring and analyzing the SSBF parameter and the temperature and humidity in the protector. Although the experiment is still going on, the validity of the proposed methods of condition monitoring has been confirmed.
Abstract: In this paper, cracking behavior of distributed microcracks is discussed using a numerical simulation. The microcracks are initially distributed in a rectangle region. The directions and locations of the cracks are chosen at random. Three kinds of length distributions are used, such as a uniform length, a random length distribution and a fractal length distribution. The crack propagations from the initially distributed cracks are analyzed under a uniaxial tensile load using liner elastic fracture mechanics.
The global behaviors of various crack distribution are studied. Results obtained from the numerical calculations indicate that the effect of the crack length distribution is minor in term of the macroscopic behavior of the cracked body.
Abstract: It is well known, that thermally grown oxide (TGO) forms at the interface between the thermal barrier top coating (TBC) and the bond coating during service. In previous work, SEM images showed that the TGO layer contained at least two layers with different oxides. One layer was Al2O3, and the other was a mixed oxide of NiO, CoO, Cr2O3, and their spinels. It was supposed that a reason for macro-crack formation near an interface is due to a decrease in bond strength or to the formation of stress concentration sites caused by the formation of pores in the mixed oxide. In order to improve the bond strength, a modified bond coating material was developed, which is MCrAlY with Ce and Si added. Four- point bending tests were carried out to measure the bond strength of conventional TBC and of the modified TBC with MCrAlYCeSi bond coating. As a result, the TBC with modified bond coating had a bond strength superior to the conventional one. It is likely that the reason for the superior bond strength is due to a notable difference in oxidation behavior.
Abstract: Evaluation method of nano-scale internal cracks by ultrasonic atomic force microscopy (UAFM) is proposed based on two approaches. The first one is a linear vibration analysis of the contact stiffness calculated from a finite element method analysis of a model including a subsurface gap. The second one is a nonlinear vibration analysis of a stiffening or softening spring representing the opening-and-closing behavior of the gap. These methods were verified by obtaining the resonance frequency mapping, the load dependence of the resonance frequency and the resonance spectra in UAFM on a subsurface gap in highly oriented pyrolytic graphite. As a result, it was proved that the proposed method is useful for evaluating the opening-and-closing behavior of the gap. Although the present study is focused on a nano-scale gap, this method is applicable to larger scale cracks using a larger tip and more stiff support than those used in AFM.
Abstract: The rotating bending fatigue tests in high cycle region were carried out on alumina short fiber reinforced aluminum alloy composites (MMCs) at room and elevated temperatures of 200, 350, 400 and 450°C. The four kind of MMCs with 0%, 10%, 18% and 25% volume fraction were prepared in order to investigate the effects of alumina short fiber volume fraction on the fatigue property such as the fatigue strength, the crack initiation and propagation behaviors. As results, it was found that the fatigue strength at 107 cycles decreased with increase in the test temperature, but increased with an increase in alumina short fiber volume fraction at room and elevated temperatures. The crack initiation sites were large size alumina short fibers; some kind of cluster of short fibers and large size alumina particles (i.e. shots). And the crack growth paths were related to the distribution of the short fibers.