<p>Understanding the intrinsic microstructural mechanisms of fatigue damage in metallic components has a strong impetus in the research community. In addition to unravelling this internal mechanics, it is imperative to look at its macroscopic external effects that can be possibly used for online monitoring and early detection of fatigue damage. Therefore, along with the traditional stress- or strain-life approaches, the development and research towards metrics for online damage detection are crucial for a more reliable functioning. To that end, in the present study, the constant amplitude fatigue behavior of Al7075 specimens is studied using the force (F) and displacement (D) time histories until failure. The specimens used in the research have a one-side V-notch to induce a high stress concentration. The fatigue testing setup is equipped to provide the F and D signals through a load cell and a linear variable differential transformer (LVDT), respectively during the test. The setup is also equipped with a confocal microscope to visually capture the occurrence of a short crack opening (~10 mm) near the notch. A combined look at the F and D data presents a unique insight into the energy dissipation (or hysteresis) behavior of the components under constant loading. This hysteresis phenomenon inevitably has signatures of the material damage. To quantitatively extract the damage signatures from the F-D data, three metrics viz. (i) the cumulative energy dissipation (CED), (ii) the energy dissipation rate (EDR), and (iii) change in material stiffness, are evaluated. Using the images from the confocal microscope, these three metrics are calibrated to pinpoint the damage initiation. CED is observed to follow a monotonously (and almost linearly) increasing behavior indicating the buildup of fatigue damage. Near the point of crack initiation, CED grows with a higher rate. This drastic growth is clearly reflected in the EDR which is computed by taking the derivative of the CED. A steep growth in the EDR correlates with the occurrence of the short crack opening of ~10 mm. Similarly, the material stiffness is observed to degrade with the increase in damage, and an understandably catastrophic loss of stiffness is encountered with the emergence of the short crack. Using these observations, a data-driven online monitoring framework is proposed such that the emergence of the crack can be detected using any (or all) of the three metrics. This online detection is enabled by computing the optimal thresholds for all the metrics through multiple tests and defining the critical values that indicate the occurrence of a short crack. Although material hysteresis is conventionally studied with the stress and strain data, the present work shows the complementary metrics build from the raw force and displacement signals along with a novel correlation with the short crack. With preliminary data, the efficacy of such framework in the online detection of the short crack surpasses 95% accuracy.</p>

Energy Dissipation Metrics for Fatigue Damage Detection in the Short Crack Regime for Aluminum Alloys

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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