Abstract
While it is well known that fatigue crack growth in metals that display confined slip, such as high strength aluminium alloys, develop crack paths that are responsive to the loading direction and the local microstructural orientation, it is less well known that such paths are also responsive to the loading history. In these materials, certain loading sequences can produce highly directional slip bands ahead of the crack tip and by adjusting the sequence of loads, distinct fracture surface features or progression marks, even at very small crack depths can result. Investigating the path a crack selects in fatigue testing when particular combinations of constant and variable amplitude load sequences are applied is providing insight into crack growth. Further, it is possible to design load sequences that allow very small amounts of crack growth to be measured, at very small crack sizes, well below the conventional crack growth threshold in the aluminium alloy discussed here. This paper reports on observations of the crack path phenomenon and a novel test loading method for measuring crack growth rates for very small crack depths in aluminium alloy 7050-T7451 (an important aircraft primary structural material). The aim of this work was to firstly generate short- crack constant amplitude growth data and secondly, through the careful manipulation of the applied loading, to achieve a greater understanding of the mechanisms of fatigue crack growth in the material being investigated. A particular focus of this work is the identification of the possible sources of crack growth retardation and closure in these small cracks. Interpreting these results suggests a possible mechanism for why small fatigue crack growth through this material under variable amplitude loading is faster than predicted from models based on constant amplitude data alone.
Highlights
Fatigue crack growth in metals that display localisation of slip, develop fracture surface morphologies which reveal that crack path changes are dependent on the loading direction and the local microstructural orientation of the grains, and on the loading history In these materials, certain simple variable amplitude (VA) loading sequences can produce localised plasticity in the form of strong confined slip bands ahead of the crack tip as a result of both the loading and unloading of each load cycle
B y adjusting the pattern of loads involving sequences of high stress ratio Constant amplitude (CA) cycles followed by; underloads, bands of different stress ratio cycles or bands of VA loading, distinct fracture surface features or progression marks can be produced at very low stress intensity factor range levels
Examination of the crack paths on the fracture surfaces produced by small cracks in AA7050-T7451 found that path changes were sensitive to the crystal orientation of the grain through which the crack was growing, and by using simple loading sequences the path could be changed
Summary
Fatigue crack growth in metals that display localisation of slip, (or confined slip) develop fracture surface morphologies which reveal that crack path changes are dependent on the loading direction and the local microstructural orientation of the grains, and on the loading history In these materials, certain simple variable amplitude (VA) loading sequences can produce localised plasticity in the form of strong confined slip bands ahead of the crack tip as a result of both the loading and unloading of each load cycle. These marks, for small cracks, are usually associated with either a local change in the crack path that leads to a visible feature on the fracture surface or a change in the fracture surface texture Such a local crack path change may take the form of a relatively prominent ridge, and a corresponding depression on the matching face, where a large change in load amplitude or peak load is/are followed by cycles that are significantly different in range or peak, i.e. with low or negative R [15, 16]. Such features, often have considerable variety, form the markings that are visible on the fracture surface of cracks grown with complex VA loading spectra. This suggests an average crack growth rate of approximately 10-9 m/cycle if all the loads cause growth, which for the case of the example shown has been found to be likely due to the growth
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