Assessment of stable tearing on fatigue fracture surfaces in aluminium alloy

Fractographic analysis of fatigue fracture surfaces is used extensively in aircraft accident investigation to correlate various progression markings, associated with the crack front position, with the load cycle history which was experienced by the failed component in service. Matching the results of this analysis to the predicted fatigue crack growth, however, is often complicated by stable tearing crack growth. Bands of stable tearing are often observed on fracture surfaces in a range of structural metals but their growth is not incorporated in fatigue predictive models. Therefore, the key challenge in fracture surface analysis is to relate the multiple tears, of different lengths, to the loads present in the load history. The main objective of this research is to develop improved analytical and prognostic models for predicting the stable tearing jump length [Delta]<i>a</i> in aluminium alloys. This research involved a series of tests which produced stable tearing in 7075 aircraft aluminium alloy under constant amplitude (CA) and variable amplitude (VA) loading.<br><br>Macroscopic and microscopic characteristics of CA and VA tearing were studied and the main conclusion relates to the notable differences between tearing under CA and VA loading. This study revealed that the stress intensity factor was one of the key controlling parameters in tearing onset and arrest. The loading conditions also have been observed to impose different effects on the size of tearing. This study suggests that for similar<i> K</i>, the CA tearing at initiation has smaller tearing crack jump length [Delta]<i>a</i>, than the VA tearing, but as the crack progresses, the size of [Delta]<i>a</i> under VA conditions is markedly larger than that sustainable under CA conditions. The CA condition seems to confer apparent resistance to tearing, which results in smaller tearing crack jumps, than in VA loading conditions. The static tearing curve is developed based on the standard <i>K</i>_R curve test method. This study shows that the<i> K</i>-value at which the static tearing commences is approximately equivalent to the first onset stress intensity factor for VA tearing and the <i>R</i>-curve method can be used to estimate the [Delta]<i>a</i> of both CA and VA tearing during fractographic analysis, but this technique requires the <i>R</i>-curve to be developed for particular configurations.<br><br>The complex crack front curvatures observed at tearing arrest distort simple estimates of stress intensity factor and hence a three-dimensional (3D) finite element (FE) analysis has been undertaken to estimate the through thickness stress intensity factor <i>K</i>_3D variation. Based on the parametric finite element analysis of the stress-intensity factor <i>K</i>_3D at the mid-thickness of three VA tearing, this study presents a new validated stable tearing model for predicting the crack jump length [Delta]<i>a</i> during stable tearing. The main features of this new model are that the tongue­ shaped region of stable tearing is idealised as a trapezoidal shape and the average of areal ratio of tearing is approximately constant. Comparisons between the model predictions and experimental results indicate that this new model produces satisfactory prediction of stable tearing crack jump length [Delta]<i>a</i> in aluminium alloys of different cross-sectional thickness.<br>