Quantitative Evaluation of Closed Cracks on the Surface of Stainless Steel by Microwaves
Yang JU, Toru MIYADU, Yasuhiro UCHIMURA, Hitoshi SOYAMA and Masumi SAKA
Abstract:Crack is the most dangerous defect in machines and structures. Cracking often initiates from the surface, since the surface stress is often higher than the stress of inside when a metal is under load. Nondestructive evaluation of a small crack is always regarded as a difficult but an utmost important issue for the integrity assessment of structural components. In the present paper, crack closure stress was estimated based on the compliance technique, while crack depth was measured by means of the microwave dual frequency technique. The effect of crack closure stress on the detection and evaluation of 2-D closed cracks in stainless steel by using the microwave dual frequency technique was discussed. In addition, a method to evaluate the shape and size of small 3-D cracks by microwaves was demonstrated. By considering the interference phenomenon occurring in the case of 3-D cracks, a parameter reflecting microwave interference effect, which is a function of the position of the sensor on the crack length, was introduced into the usual dual frequency evaluation-equation. From the modified dual frequency equation, a new component named interference waveform, which is based on the amplitude of the reflection coefficient measured at two different frequencies was obtained. On the other hand, based on the interference model, a corresponding interference waveform, which can be calculated from the assumed shape and size of 3-D cracks, was also introduced. By comparing these two interference waveforms, the evaluation of the shape and size of 3-D cracks were carried out. Key Words:Crack, Closure stress, Nondestructive evaluation, Microwave, Dual frequency