Development of Ultrasonic Fatigue Test Method on Biaxial Bending
Thin-walled metal components frequently undergo multiaxial bending fatigue during opera-tion,necessitating an experimental method to replicate loading conditions for investigating material proper-ties.In this study,a novel biaxial bending test method using ultrasonic fatigue technology was proposed.The design involved a cruciform TC4 titanium alloy specimen tuned to a natural frequency of 20 kHz based on the principle of harmonic vibration,featuring vertically superimposed fourth-order and third-order ben-ding modes to ensure the maximum stress region remained in the test section.Arc transitions were utilized in other regions to mitigate stress concentration.Finite element simulations and strain gauge tests were conducted to calibrate stress amplitudes in the specimens.Analysis of S-N curves,crack propagation paths,and fracture morphologies revealed the failure mechanisms of biaxial bending fatigue in the very high cycle regime under varied loading conditions.It was found that TC4 titanium alloy exhibited no fa-tigue limit in the very high cycle regime,showing a continuous downward trend.Owing to gradient stress distribution,biaxial bending fatigue demonstrated significantly longer fatigue life compared to uniaxial ul-trasonic fatigue.Meanwhile,crack propagation behavior resembled conventional biaxial fatigue,producing H-shaped or Y-shaped cracks.Fracture surfaces exhibited a brittle characteristic in a large area of the crack initiation zone,with morphology primarily characterized by facets and tearing ridges formed through facet coalescence.In contrast to uniaxial bending fatigue,the facets in biaxial bending showed a batten pattern akin to multiaxial fatigue failure.
very high cycle fatiguebiaxial fatiguebending fatiguecrack initiationcrack path
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