Relationship between closure stress of small fatigue crack and ultrasonic response

This paper describes a new methodology for analyzing the closure stress of small fatigue cracks using ultrasonics. Surface connected back-wall cracks in steel specimens are considered. The crack-corner echo of ultrasonic shear wave beam of 50degrees incidence in the material is used. In order to account for the exact effect of closure stress on ultrasonic response, the echo amplitude is analyzed based on artificially applying additional closure stress. The initial closure stress at no load condition is determined by the standard compliance technique. A relationship between the closure stress and the crack-echo response is established which provides the facility of getting a true model of crack closure stress. The relationship is found to be independent of crack depth. The present approach can be used to overcome the difficulties encountered by crack closure in recent nondestructive ultrasonic techniques and may lead the way in quantitative evaluation of crack depth as well as exact closure stress.