Abstract
The fatigue or durability life of a few critical structural metallic components often sets the safe and/or economic useful life of a military airframe. In the case of aluminium airframe components, growth rates, at or soon after fatigue crack nucleation are being driven by near threshold local cyclic stress intensities and thus are very low. Standard crack growth rate data is usually generated from large cracks, and therefore do not represent the growth of small cracks (typically <1mm). Discussed here is an innovative test and analysis technique to measure the growth rates of small cracks growing as the result of stress intensities just above the cyclic growth threshold. Using post-test quantitative fractographic examination of fatigue crack surfaces from a series of 7XXX test coupons, crack growth rates and observations of related growth phenomenon in the threshold region have been made. To better predict small crack growth rates under a range of aircraft loading spectra a method by which standard material data models could be adapted is proposed. Early results suggest that for small cracks this method could be useful in informing engineers on the relative severity of various spectra and leading to more accurate predictions of small crack growth rates which can dominate the fatigue life of airframe components.
Highlights
The life of type (LOT) of a military airframe in service maybe based on the fatigue or durability life of a few critical structural metallic components, due to safety and/or maintainability concerns [1]
The modification resulted in the intertwined spectrum being predicted to be 19% more severe than the blocked spectrum. This compares to the un-modified DST16 model that only predicted a difference of 3% between the spectra for small crack growth rates. (Only the un-modified DST16 model for the intertwined crack growth is shown in Fig.8 since the blocked result is almost identical and hard to differentiate at this scale)
Shown in the results presented here are the significant influences that load sequence has on crack growth rates of small cracks in AA7050-T7451
Summary
The life of type (LOT) of a military airframe in service maybe based on the fatigue or durability life of a few critical structural metallic components, due to safety and/or maintainability concerns [1]. For many airframe aluminium alloys, microstructurally small fatigue cracks initially have very low growth rates; when loaded with representative load spectra These low growth rates are often being driven by near threshold local cyclic stress intensities (∆K), and are not well represented by standard fatigue lifing methodologies, further such methods often under estimate growth rates in this regime [6,7,8], resulting in un-conservative fatigue life predictions. A validation of the crack growth data model is presented to predict a fatigue crack growth in similar coupons that were tested with a variable amplitude (VA) spectrum
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