Very High Cycle Fatigue and Ultrasonic Testing in Multiaxial Loading

Fatigue damage has special relevance on the lifespan of mechanical components and struc-tures, as it takes responsibility for the majority of registered structural failures. Most fatigue testing today still uses uniaxial loads, nonetheless it is generally recognized that multiaxial stresses occur in many full-scale structures. Nowadays, the growing need for greater lifespans forced the study of material behavior under what is nowadays very high cycle fatigue (VHCF) regime, fatigue research beyond 10E07 up to 10E10 cycles. However, most classic fatigue ma-chines are often very costly, both in terms of time and energy, for such an elevated number of cycles. The high costs of equipment and time to conduct such experiments have seen a massive improvement with the introduction of high frequency ultrasonic machines. Presently, it is al-ready possible to characterize materials VHCF response under axial and multiaxial loading con-ditions in a fraction of the time. Ultrasonic testing machines were first introduced for simpler uniaxial loading and more recently adapted for multiaxial loading conditions. The present work reviews the current background of ultrasonic fatigue testing machines working at 20-kHz fre-quency, with emphasis on multiaxial fatigue and VHCF. A review of a series of fatigue tests are presented, respectively, axial, shear, axial/shear, and in-plane bi-axial fatigue tests with differ-ent combining dimension specimens allowing different shear/axial stress ratios. Special atten-tion will be put into the performance of multiaxial classical cylindrical specimens under tension/ torsion and flat cruciform specimens under in-plane bi-axial testing using low-cost piezoelec-tric transducers. By comparing all so far developed and successfully reached multiaxial spec-imens and respective methodologies, two base approaches were possible to be established. When designing new specimens or ultrasonic machines, or both, one of the two methods must be followed. To have a well-comprehended methodology and successful design approach will undoubtedly guide future multiaxial ultrasonic method ideas.