Microstructurally small fatigue crack growth rates in aluminium alloys for developing improved predictive models

Abstract

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..

An analysis of fatigue crack features at various crack lengths through mating fracture surface pairs

“…The calculation of these two values allowed crack growth rate curves to be produced as shown in Section 3. Similar methods have been used in other publications and shown to produce accurate results that allow for improved crack growth rate predictions at all crack lengths 3,26,39–41 …”

“…Similar methods have been used in other publications and shown to produce accurate results that allow for improved crack growth rate predictions at all crack lengths. 3,26,[39][40][41] A HIM 32 was the predominant imaging tool used in this study. It was utilized for both the quantitative fractography images, as well as general fractographic analysis.…”

A fractographic study of fatigue crack growth from a cold expanded fastener hole at an engineering “crack initiation” scale

“…30 The inputs and parameters used for these predictions are listed in Table 3 below and were chosen as the best match for the experimental conditions presented herein. A specific material data set for small and nearthreshold FCGRs 25,31 consistent with the findings of past complex fatigue lifing analysis 33 was chosen. To model the residual stress field arising from the cold expansion process, references were sought from the open literature for AA 7050 plate specimens of approximately similar material and dimensions having undergone FTI 4% split sleeve cold expansion and subsequent residual stress measurement.…”

Study on the short fatigue crack initiation and propagation behavior of 42CrMo

“…6 The differences of microstructure growth conditions at the crack tip are considered to be the root reason of the randomness of damage nucleation and short crack initiation. The stochastic analysis method for fatigue crack propagation mainly includes mathematical model [7][8][9] and physical simulation method. [10][11][12][13] At present, the micro-mechanism of fatigue damage has not been fully and systematically understood, and the rules of short fatigue cracks growing and population evolution need to be further clarified.…”

“…6 The differences of microstructure growth conditions at the crack tip are considered to be the root reason of the randomness of damage nucleation and short crack initiation. The stochastic analysis method for fatigue crack propagation mainly includes mathematical model 7–9 and physical simulation method. 10–13…”