VHCF of spray formed hypereutectic aluminium silicon alloy

Abstract

Materials and methodUltrasonic fatigue experiments in the very high cycle fatigue (VHCF) regime are performed with spray-formed hypereutectic aluminium silicon alloy DISPAL® S232-T6x. DISPAL exhibits excellent mechanical properties with low weight and can achieve higher stiffness, greater high temperature strength, excellent wear resistance and a reduced coefficient of thermal expansion compared to cast or wrought aluminium alloys. T6x indicates peak ageing with improved ductility.In the present investigation specimens are cycled in resonance in constant (CA) and variable amplitude (VA) tests at ultrasonic frequency in the high cycle fatigue (HCF) and VHCF regime up to 2 × 10 10 cycles [1]. The cycling frequency of approximately 20 kHz allows accumulating fatigue data within reasonable testing times. Means of non-linear acoustics are employed for detecting early stages of fatigue damage and for monitoring vibration properties during fatigue life. Miner calculation and lifetime prediction is performed to describe mean VA lifetimes. Early failures caused by large materials defects are considered by using an adapted crack growth model. Results and discussionFigure 1 (left panel) shows the fatigue properties in CA experiments for stress amplitudes /2 between 296 MPa and 139 MPa, yielding lifetimes between 3.2 × 10 5 and 7.6 × 10 9 cycles. Lifetimes in VA tests for maximum stress amplitudes max /2 between 296 MPa and 185 MPa were found between 4.3 × 10 7 and 1.7 × 10 10 . Additionally, means of non-linear acoustics are employed in computer control and data acquisition for monitoring and analysis of the specimens' vibration properties in CA experiments [2]. The method allows deriving the resonance frequency as well as the non-linearity parameter rel , which may serve for early detection of fatigue damage [3]. Changes in resonance frequency and non-linearity parameter rel are indicators for microstructural changes and are used to monitor progress of fatigue damage in the VHCF regime. Figure 1 (right panel) shows an exemplary course of resonance frequency and rel versus number of cycles of a specimen that failed after 1.3 × 10 9 cycles at /2 = 162 MPa. From the very beginning of the test, the resonance frequency slowly decreases, while rel continuously increases. The appearance of increasing numbers and lengths of fatigue cracks affects resonance frequency as a consequence of increased specimen compliance. This indicates that fatigue cracks are a Corresponding