R. Ohtani

Analysis on the applicability of direct current electrical potential method to the detection of damage by multiple small internal cracks

The direct current electric potential around a number of penny-shaped cracks was analyzed by Crack-Flow Modification Method (CFMM) in order to investigate the applicability of the Direct Current Electrical Potential Method (DC-EPM) to the detection of damage due to multiple small internal cracks. The potential difference was calculated for the cracking process in a smooth round bar specimen of a Type 304 stainless steel under a creep-fatigue condition. It was clarified by the analysis that, in the case of the present creep-fatigue test, damage could be detected at about a half of the lifetime assuming that a change of 1 percent in the potential difference was measurable by a voltmeter.

Fatigue crack propagation in a cast TiAl intermetallic compound at high temperature

High-temperature fatigue crack propagation tests were conducted on Ti-34 wt% Al intermetallic compound with lamellar structure, in order to elucidate the effects of lamellar boundaries and creep on the resistance to translamellar cracking. The results obtained are summarized as follows. (1) Slow crack growth during fatigue is found to traverse the lamellae, and its rate is independent of whether the cracking direction is perpendicular to the lamellae or along them. (2) The fatigue crack propagation rate, d l /d N , at 1073 K is correlated well with the effective stress intensity factor range, Δ K eff . (3) The fatigue crack propagation rate at room temperature is little slower than that at 1073 K. (4) Under creep-fatigue conditions, the crack propagation exhibits a time-dependence and its rate per cycle, d l /d N , is governed by the creep J -integral range, Δ J c . (5) In transverse crack propagation, the d l /d N -Δ J c relationship is independent of the crack direction to the lamellar surface. (6) Compared with some heat-resisting steels and alloys, the resistance of the present TiAl to fatigue crack propagation is remarkably low at high levels of stress amplitude while the resistance to creep- fatigue crack propagation is almost the same.

Characterization of Creep Fatigue Cracking in Type 304 Stainless Steel

The behaviour of initiation and growth of small cracks in high-temperature push-pull low-cycle fatigue (LCF) was investigated on a Type 304 stainless steel. The characterization of cracking was done both on the basis of the precise observation on the surface of as well as inside the specimens which were subjected to a wide range of isothermal loading conditions, and on the basis of the numerical simulation for initiation and growth of small cracks. Similar fundamental properties must be found in thermal fatigue and in other kinds of polycrystalline steels and alloys.

Analysis of direct current potential field around multiple random cracks

A method for evaluating the distribution of electrical potential around multiple through cracks is proposed. The method gives accurate potential values for cracks with different lengths at random positions by combining theoretical potential functions for a single crack in an infinite plate. The validity of the method is numerically confirmed with respect to problems of a single and three cracks existing in a square plate subjected to a current flow, and the method is applied to a problem of random cracks in an infinite plate.