Quantitative nondestructive testing of small, tight cracks using ultrasonic angle beam technique

This paper describes a new ultrasonic angle beam technique and a method for quantitative nondestructive testing of small, tightly closed cracks that are situated on the surface of a material. The angle beam technique uses an oblique longitudinal wave that has the capability of dealing with very small, tightly closed cracks sensitively, which is beyond the ability of the conventional methods of normal incidence and large angle beam techniques. Using the present testing method, both the crack size and the closure stress can be determined simultaneously from the same measurement by analyzing the inverse problem. The method is fully automated and computer controlled and can be employed to practical applications. The sensitivity limit of the testing technique, especially for the closed cracks, is discussed in relation to specimen thickness. The accuracy and reliability of this new NDT method are checked by comparing the evaluated results of rightly closed smaller fatigue cracks with the actual crack sizes observed on the fractured surfaces. The ultrasonic method developed here proves to be a very powerful tool for quantitatively evaluating very small tight cracks.