Marker Loads for Quantitative Fractography of Fatigue Cracks in Aerospace Alloys

Abstract

Abstract:The selection of fracture surface marking methods based on exploiting or altering the required fatigue loads is of much interest for many fatigue test programmes. This is particularly true when crack growth measurements during testing are not possible or insufficiently accurate. In such cases, post-test Quantitative Fractography (QF) of the fatigue crack growth may then be needed, and this can be made possible and/or greatly facilitated by fracture surface markers.Here, several examples of fatigue loadings that create fracture surface markings both naturally, as sometimes happens, and intentionally are presented and discussed. While these examples are from fatigue life tests of aircraft alloy specimens and components, particularly high strength aluminium alloys, under normal environmental conditions (air at ambient temperatures), it is probable that some of the fatigue load histories may provide fracture surface markings for other materials and in other environments.The advantages and disadvantages of the various intentional marking methods are detailed with a view to obtaining guidelines and procedures for optimising quantitative fractography of fatigue crack growth. These guidelines are presented in this paper. Theory and Operational Practice, M. Bos (ed.), ICAF 2009, Bridging the Gap between

Investigations into the Fatigue Enhancement Provided by the Hole Cold Expansion Process Using Accurate 3D FEA Simulations and Fatigue Testing

“…After every 2000 fatigue cycles, a block of 1000 cycles at a stress ratio of 0.9 was applied to provide contrasting marker bands on the fracture surface. The applied marker load spectrum was based on the work of Barter and Wanhill [10].…”

Three-dimensional characterization of microstructurally small fatigue-crack evolution using quantitative fractography combined with post-mortem X-ray tomography and high-energy X-ray diffraction microscopy

“…The reader is referred to Ref. [22] for a review, examples, and guidelines of applying marker loads for quantitative fractography in various aerospace alloys (e.g. aluminum, nickel-based and titanium).…”

“…Compared to traditional projection-based marker-band analyses, which restrict the measurement of crack-propagation rates to a planar projection of the crack surface [21][22][23][24][25][26], the method described herein incorporates X-ray tomography measurements to account for the effect of 3-D crack-surface morphology on the rate of crack propagation. To quantify this effect, the two measurement techniques were compared using the Al-Mg-Si fatigue specimen.…”

“…Although marker banding has typically been used for self-similar, long-crack growth, recent studies [23][24][25][26] have used marker banding to characterize crack propagation near crack-initiation sites (within 20 lm) in an aluminum alloy. Traditional marker-band analyses [21][22][23][24][25][26] do not account for the true 3-D nature of crack propagation since 2-D projections of the marker bands are used to calculate crack-growth rates.…”

“…Nor does it provide information pertaining to the rate of MSFC propagation. Subsurface crack-growth rates can be measured in a retroactive manner using a technique known as marker banding [21][22][23][24][25][26], whereby a loading spectrum is designed to imprint unique patterns on the fatigue-crack surface that demarcate the crack-front shape at programmed cycle intervals. Following failure, crack-front profiles (marker bands) are identified using fractography.…”