抄録
A number of techniques have been used in recent years for the measurement of crack length in notched laboratory specimens during fatigue or fracture at room temperature. However, at elevated temperatures, useful and simple means of monitoring crack growth are limited. The optical measurement is the most widely used technique, and it has been used by the authors for studying creep and monitoring high-temperature fatigue cracks in ductile materials and is still being used together with a TV monitoring system. However, this method is not always advantageous. In this study, the applicability of the d. c. electrical potential method was examined to the crack initiation and propagation tests at elevated temperatures. The kinds of tests and the results of the measurement are as follows.
(1) Creep crack propagation tests were conducted using CT specimens of the brittle aluminum alloy (2014-T6) at 150°C. Although the detection of crack tip was difficult by the optical method due to very small COD, the d. c. potential drop method was successfully used to monitor not only the crack length but also the crack initiation time.
(2) In ductile steels, the effect of creep deformation on the change in potential drop was quite large. The increase in d. c. potential with strain was found in the creep tests of the smooth specimens of 304 stainless steel at 650°C.
(3) For the same 304 stainless steel, crack propagation tests of low cycle fatigue were carried out at 650°C using CCP specimens with large amplitudes of push-pull load conditions. It was found that the contribution of cyclic deformation to the change in potential in each cycle was significant, and that the calibration curve must in general be determined for each test condition. On the other hand, the crack initiation time could be determined reliably by positioning the potential leads close to the notch root.
