Abstract
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.
Highlights
Fatigue crack closure has been recognized as one of the most influential operating mechanisms during fatigue for a long time
The sensitivity of modulation intensity (MI) to crack closure confirmed the feasibility of estimating crack opening stress using vibro-acoustic modulation (VAM) testing
The crack opening stress was estimated according to the procedure mentioned in subsection ‘‘Procedure of approximating opening stress considering plasticity-induced crack closure (PICC).’’ Figure 9 shows the normalized MI versus applied tensile load when the half-crack length varies from 2.55 to 4.33 mm
Summary
Fatigue crack closure has been recognized as one of the most influential operating mechanisms during fatigue for a long time. In the early 1970s, Elber[1] discovered that when a closed crack is loading, the crack surfaces will remain partly closed until all the compressive stresses were overcome under the corresponding crack opening stress sop. The crack will be fully open until the applied load decreases to the crack closure stress scl, which is usually assumed to be approximately equal to sop. It has been testified that the effective stress range Dseff = smax À sop is the actual driving force of fatigue crack growth rather than the applied (external) stress range Dseff = smax À smin. The crack closure parameters (e.g. sop and scl) are the key factors of crack propagation, and accurately estimating that the crack closure behavior is significant in fatigue life evaluation.
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