Study On Ultrasonic Bending Fatigue Property Of Thin Sheet Metallic Materials

Researches show that in the10’cycles or ultra-high stage the material is still possible to damage of fatigue. In engineering application, the fatigue cycle of many dynamic mechanical components have researches as high as1010or even more. Therefore the study of the fatigue damage mechanism of engineering materials in more than107cycles does have a great significance in mechanical components.From the published literature, we can see that the fatigue research are mainly concentrate on the metal materials S-N curve characteristics of symmetric tension-compression loading, asymmetric symmetrical tension-compression loading, high cycle fatigue behavior, fatigue fracture morphology observation and fatigue fracture mechanism, but referring the metal sheet under the condition of symmetric bending loading, the related reports are rarely seen. Because the sample instability phenomena exists in the sheet metal under symmetric tension-compression ultrasound fatigue load, and three point bending test cant achieve symmetric loading, that is R≠-1. Therefore, the research developed a sheet metal bending fatigue test technology and it can provide some test basis for symmetric bend ultrasonic fatigue test. And fatigue Behaviors of50steel and LY12aluminum alloy sheet metal are researched by this test technology in ultra-high cycle, the influence of size effect to sheet metal fatigue performance is discussed. Follows are the research results:On the basis of three points bending, this paper design ultrasonic bending fatigue sample fitted the test resonance under pure bending load. The analytical method is used to calculate the resonant length of three point bending, the deficiency of the sample is discussed and the sample is improved. Combined with the stress distribution by the finite element analysis in the sheet samples, a variable cross-section of ultrasonic bending fatigue is designed. Finally, it makes further discussion about the influence of which provides reference for sample frequency.Fatigue properties of50steel and LY12aluminum alloy sheet under symmetric bending loading are tested, Compared the fatigue performance of materials in symmetrical bending and symmetrical tension-compression loading, influence of metal material fatigue performance under different loading condition is obtained.Form the analysis of fracture,I discovered that fatigue cracks initiate in surface and secondary surface of sample, it can be explained that the inclusion or other non-metallic within material fall off from matrix, hollow is formed in material interior, cavity at the formation of micro-cracks, which led to specimen fracture.By comparing the fatigue properties of two samples with different thickness, it show that the material has a certain size effect, sample size and loading method has significant effect to material fatigue performance. To LY12, the thinner, the thickness sample is, the stronger its fatigue performance is, and bending the fatigue property of loading is stronger than the tension-compression loading fatigue performance.