Size-Effects on the VHCF Response of Flat Metallic Specimens for Automotive Applications: Analysis of Fatigue Data with a Method Based on the Stress Gradient

The design against fatigue failures at very high number of cycles (VHCF) is fundamental to guarantee the integrity of components used in structural applications (aerospace, energy production, automotive). Experimental tests to assess the VHCF response of materials are generally carried out on small specimens with sizes that are significantly different from those of the components to be designed. Size-effect, which is widely known to affect the VHCF response, must therefore be properly modeled and accounted when components are designed to ensure their structural integrity. Size-effects in VHCF have been generally investigated by testing specimens with circular cross-sections and modeled by considering the probabilistic increment of the defect size with the loaded volume. In the present paper, ultrasonic fatigue tests have been carried out on hourglass flat specimens and larger dog-bone flat specimens to investigate size-effects. One aluminum alloy and four steels used for automotive applications have been tested. The experimental results have been analyzed with an innovative statistical model based on the weakest-link principle and on the stress gradient within the specimens, which does not require the size of the defect at the origin of the fatigue failure, generally not available if the specimen fails from the surface. Size-effects were found to significantly influence the VHCF response of the investigated materials, further confirming that it is strongly material dependent, even for flat specimens. A general rule for size-effect in VHCF of flat specimens was not found, proving that it must be properly experimentally verified and safely accounted when designing large components against VHCF failure.