Constant and variable amplitude ultrasonic fatigue of 2024-T351 aluminium alloy at different load ratios

Ultrasonic fatigue testing equipment is presented that is capable of performing constant amplitude (CA) and variable amplitude (VA) experiments at different constant load ratios. This equipment is used to study cyclic properties of aluminium alloy 2024-T351 in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime at load ratios R=-1 and R=0.5. CA loading does not reveal a fatigue limit below 10¹⁰ cycles. Cracks leading to VHCF failure start at broken constituent particles. Specimens that survived more than 10¹⁰ cycles at R=-1 contain non-propagating cracks of lengths below grain size. Resonance frequency and nonlinearity parameter β(rel) show changes of vibration properties of specimens at low fractions of their VHCF lifetime. VA lifetimes are measured in the HCF and VHCF regime and compared with Miner calculations. Damage sums decrease with decreasing load (and increasing mean lifetimes) and are lower for R=0.5 than R=-1.

On the use of ultrasonic fatigue testing technique--variable amplitude loadings and crack growth monitoring

In the very high cycle fatigue regime high-frequency testing techniques are required. Using the ultrasonic fatigue technique, testing time could be reduced significantly in comparison to conventional servo-hydraulic machines. An ultrasonic fatigue testing system developed by the BOKU Vienna with load frequencies of about 20kHz is used for variable amplitude loading investigations in the VHCF regime. Therefore, the amplitude level during fatigue tests is controlled by a PC using an own developed software. Additionally, an in situ reconstruction of a damage equivalent load spectrum based on a load time history is introduced schematically. To optimize the experimental procedure a temperature-controlled pulse-pause adaption has been developed and implemented into the software. For quantifying the influence of variable amplitude loadings on the fatigue life, e.g. load interaction effects, the crack growth is measured by using the potential drop technique that is adapted to the ultrasonic fatigue testing system. Finally, the results of two-step-block loading tests are presented.