Fatigue crack path manipulation for crack growth rate measurement

It is accepted that fatigue crack paths in metals that display confined slip, such as high strength aluminium alloys, are responsive to loading direction and local microstructural orientation. It is less well recognised that crack paths in these alloys are also responsive to the loading history since certain loading sequences can produce highly directional slip bands ahead of the crack tip. By adjusting the sequence of loads, distinct fracture surface features or progression marks can result, even at very small crack depths. An investigation into the path a fatigue crack selects as it progresses through a material when it is cyclically loaded with particular combinations of constant and variable amplitude sequences has provided insight into the way these cracks grow. This makes it possible to design load sequences that allow very small advances of crack growth to be measured post-test by Quantitative Fractography, at growth rates well below the conventionally recognised threshold of the aluminium alloy examined here. This paper reports on observations of the dependence of the crack path on the loading history applied to aluminium alloy 7050-T7451 (an important aircraft primary structural material) coupons, which was made possible through the use of novel test loading sequences that produce distinct markings on the crack surface. These markings allow the measurement of crack growth rates at very small crack depths and very low rates of growth in this alloy. The aims of this work were firstly to generate small-crack constant amplitude growth rate data and secondly, to achieve a greater understanding of the mechanisms of fatigue crack growth in this material. A particular focus of this work was the identification of possible sources of crack growth retardation and acceleration in small cracks. The results suggest that for small cracks in this material, the rate of crack growth under variable amplitude loading will tend to be faster than predictions made based on the crack growth rate under constant amplitude loading, due to crack path considerations.