AC Potential Method Research Articles

Ｍｏｄｅ ＩＩ疲労き裂進展下限界値ΔＫＩＩｔｈの測定

In order to measure the Mode II fatigue crack growth resistance of various materials, a particular chevron notched double-cantilever type specimen with side grooves was designed. Mode II fatigue crack growth tests by using the newly designed specimens were conducted by a conventional closed-loop type tension-compression fatigue testing machine under a constant load amplitude which corresponds to the ΔKII-decreasing test with increase in crack length. The crack length was measured by the AC potential method. The particular morphology of Mode II crack fracture surface was investigated. The distribution of shear stress in crack tip and Mode II stress intensity factor were analyzed using a FEM software, ABAQUS. The relationships between Mode II crack growth rate (da/dN) and ΔKII for various materials were obtained, and the values of ΔKIIth were determined. It was found that ΔKIIth is larger than ΔKIth for all materials tested in this study.

A NEW METHOD FOR THE MEASUREMENT OF MODE II FATIGUE THRESHOLD STRESS INTENSITY FACTOR RANGE ΔKτth

Abstract— In order to evaluate the threshold value ΔKτth for mode II fatigue crack growth, a new measurement method of mode II fatigue crack growth has been developed. This method uses a conventional closed‐loop tension—compression fatigue testing machine without additional loading attachments. Mode II fatigue tests for structural steel and rail steel have been carried out. This method has proved successful and has reproduced mode II fatigue fracture surfaces similar to those found in the spalling of industrial steel‐making rolls. The crack length during testing was measured by an AC potential method. The relationships between da/dN and ΔKτ and AKτth for several materials have been obtained.

金属間化合物ＴｉＡｌにおける切欠き底微小疲労き裂の伝ぱ挙動

Single-edge-notched specimens of an intermetallic compound TiAl were fatigued under cyclic axial tension compression. The effect of microstructure on the propagation behavior of short fatigue cracks formed at the notch root was investigated. The crack length was measured with the AC potential method and the development of crack closure with crack extension was measured through the compliance method. The crack path is tortuous in lamellagrains, while straight in gamma-grains. When compared at the same non-propagating crack length, the crack opening stress intensity factor at the threshold increases with increasing lamella volume fraction. The relation between the crack propagation rate and the effective stress intensity range for short cracks was identical to that for long cracks. The resistance-curve method was used to predict the fatigue strength of notched specimens. The predicted values of the fatigue limits for crack initiation and for fracture, and the non-propagating crack length agreed well with the experimental results.

破壊力学 せん断（ＭｏｄｅＩＩ）疲労き裂伝ぱ抵抗値ΔＫτｔｈの測定方法とその応用

In order to measure the Mode II fatigue crack growth resistance of SS41 steel and a rail steel, a particular double-cantilever type specimen with side grooves was designed. Mode II fatigue crack growth tests by using the newly designed specimens were conducted by a conventional closed-loop type tension-compression fatigue testing machine under a constant load amplitude which corresponds to ΔKII-decreasing test with increase in crack length. The crack length was measured by the AC potential method. Mode II crack growth rate gradually decreased as the crack grew. The typical fracture surface of Mode II fatigue crack growth which was quite different from Mode I fatigue fracture surface was observed. But the trace of local branching of crack growth mode from Mode II to I was observed on the overall fracture surface of Mode II. The relationship between Mode II crack growth rate (da/dN) and ΔKII was obtained.

Measurement of short surface crack lengths by an AC potential method.

An alternating current (ac) potential technique was developed for the measurement of short surface cracks in smooth specimens of a ferromagnetic material. For a constant crack length, the potential difference (E0) between two probes located across the crack was found to be directly proportional to the operating ac frequency. The relation between E0 and crack length (α) changed specimen by specimen, because the value of E0 was very sensitive to placement of lead wires for potential measurement. The relation of ΔE0(=E0-E0I) vs. a, however, was uniquely determined, independent of specimen, where E0i Was the potential difference for a reference crack length, e. g., initial crack length. Therefore, surface crack lengths were able to be determined through the equation for ΔE0 vs. a. Best accuracy was obtained for the operating ac frequency of 2 kHz in this study. In fatigue crack growth tests, the potential difference (E) was found to be a function of loading conditions. In this case, the crack length was calculated with good accuracy through the equation for ΔE0 vs. a by substituting ΔE (=E-Ei) for ΔE0, where E was the potential difference under loading, and Ei was the value at reference crack length.