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Thin-walled metal components frequently undergo multiaxial bending fatigue during operation, necessitating an experimental method to replicate loading conditions for investigating material properties. 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 bending 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 fatigue 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 ultrasonic 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.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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