ENGINEERING CORRELATIONS FOR THE INFLUENCE OF AGEING ON THE FATIGUE AND FRACTURE BEHAVIOUR OF AN ALUMINIUM CAST ALLOY

摘要

Over the last years, lightweight design has become highly important in automotive engineering, which has led to a continuously increasing use of aluminium alloys. In conventional lifetime estimation methods, S/N curves are widely used. These are obtained from standardized smooth, defect-free specimens. In lifetime estimation concepts based on fracture mechanics, a crack is assumed in the material, whose size is estimated from the detection limit in nondestructive testing. To enable S/N curves to account for cracks or flaws in the component, a combination of fracture mechanics and stress-based approaches is needed. For internal combustion engines, where elevated temperatures of locally up to 250 °C can occur, also the accelerated ageing behaviour of precipitation hardened aluminium alloys has to be considered. The influence of temperature and time on the static stress-strain behaviour of the material can be described by different ageing models, e.g. the one proposed by Sher-cliff & Ashby. However, these models are unable to quantify or predict how the fatigue andfracture behaviour is affected by ageing. To combine damage mechanics and fracture mechanics, a correlation between the fatigue limit and the fatigue crack growth threshold stress intensity factor has to be found that is able to account for different ageing conditions. Based on the El Haddad equation, an engineering estimate is proposed which allows to calculate the fatigue limit of a die cast aluminium alloy depending on flaw size and temperature and duration of ageing.With few additional data from tensile and crack growth tests it is possible to define the behaviour of the S/N curve for different influences such as flaw size, temperature and duration of ageing, or stress ratio.