In situ measurement of fatigue crack opening stresses by nonlinear vibro-acoustic modulation testing

The difficulty of accurately measuring crack closure regions has hindered measurements of opening or closure stresses, and ultrasonic methods have shown great potentialities in evaluating closure area. Many recent researches have confirmed the advantages of nonclassical nonlinear acoustic approaches in detecting short cracks and even micro- or meso-cracks. However, there are few reports on using nonclassical nonlinear acoustic approaches to measure crack closure. The aim of this article is to verify the applicability of vibro-acoustic modulation approach in estimating opening stresses of fatigue cracks. A procedure of approximating the opening stress by the applied tensile load of the inflection point of the modulation intensity curve is proposed. To study the performance of this measuring method experimentally, in situ vibro-acoustic modulation tests were carried out on a central cracked 5052-H32 aluminum alloy fatigue sample. The experimental estimations match well with other researches. Analysis of the experimental results suggests that the vibro-acoustic modulation technology is not only capable of measuring the fatigue cracks opening stresses, but also capable of analyzing the crack closure behavior, even for physically short cracks.