A New Design for Mitigating Interfering Modes in Cruciform Specimens to Enhance Ultrasonic Fatigue Testing

Cruciform specimens have been extensively used to simulate biaxial loading conditions in Ultrasonic Fatigue Testing (UFT), particularly within the Very High Cycle Fatigue (VHCF) regime. However, these specimens are often affected by interference from unintended flexural modes, such as the ‘flapping mode,’ which occur at frequencies near the desired axial mode, compromising the accuracy and reliability of the tests. To address this issue, a new specimen design has been developed to effectively separate the axial and flexural modes, thereby transforming the precision of fatigue testing. Finite Element Analysis (FEA) and experimental validation using Digital Image Correlation (DIC) were employed to optimise the geometry of the specimens, resulting in a substantial frequency separation between the interfering flexural modes and the axial mode. Through this redesign, mode coupling has been virtually eliminated, ensuring that the specimens deform as intended during testing. This breakthrough enables in-plane biaxial testing across the full range of biaxiality ratios, overcoming the previous challenges posed by mode coupling. Equibiaxial in-phase biaxial UFT with this type of cruciform specimen, previously considered largely theoretical in its practical application, has now been successfully realised through the innovations presented in this work.