Novel Indicators & Nonlinear Method for Onset Damage Detection in Very High Cycle Fatigue Tests (VHCF)

Modern industry and demanding markets require engineering products characterizing their performance in Very High Cycle Fatigue (VHCF). An international testing standard must not be limited to the number of endured load cycles, but at a minimum, the probe’s damage onset and characterization should be detected. The author poses three criteria based on a proposed test set-up that adds another synchronous channel to the current axial (Ch1), an orthogonal bending monitoring at the probe’s tip (Ch2). The first proposed criterion uses axial response data, and its results perfectly coincide with the existing Nonlinear Acoustic Emission NAE criterion. The second relies on the bending BFSI indexes obtained from the author’s proposed added monitored direction, where the damage indexes can be used to estimate the crack-like gravity. The third criterion relies on trends and application of the NAE criterion to the bending amplitude, which is more sensible than the current monitored axial vibration. Criteria 2 and 3 are impossible to achieve should the current experimental set-up not add the (Ch2) bending response. To validate, these criteria are applied to a real test using PIMM Lab sophisticated VHCF equipment yielding PSB onset detections. Further validation, XCT tomography detects intergranular microcracks extending in a zigzag pattern; this damage pattern suggests a significant advance in the material degradation due to fatigue, as PSB, microcracks near to coalesce. Previous validation results are coherent with the proposed three initial damage criteria and this work’s analyses.