Analysis and Interpretation of Aircraft Component Defects Using Quantitative Fractography

Abstract

Quantitative fractography techniques have been in use at the Aeronautical Research Laboratory (ARL), Melbourne, Australia, for more than 15 years, and they have been developed into a specialized facility for deriving crack growth histories from fracture surfaces. This capability has played a fundamental role in a number of pioneering life extension programs which have permitted the continued operation of several aircraft types in current service with the Royal Australian Air Force (RAAF). The increasing RAAF demand for advanced fractography has required the development over a number of years of computer-based semiautomated data acquisition and data processing facilities for fractographic analysis; these developments have permitted a significant improvement in the speed with which fracture surfaces may be analyzed and are now used routinely in ARL's quantitative fractography. This paper describes current ARL activities in the use and further development of quantitative fractographic techniques and the specialized facilities employed to derive and process data from fracture surfaces observed by both scanning electron microscopy and optical microscopy. The results able to be obtained are illustrated by reference to case histories of defects in components from service aircraft, from full-scale component tests, and from laboratory test samples. Procedures for minimizing or overcoming some of the difficulties associated with interpretation of fracture surface markings are discussed. The paper also describes the way in which fracture mechanics concepts need to be used in conjunction with fracture surface analysis to achieve a correct understanding of the crack growth history. In particular, some empirically based approaches developed for interpolation and extrapolation of incomplete crack growth histories may now be understood by reference to fracture mechanics concepts. Finally, current and future developments in fracture surface analysis are discussed, with particular reference to the benefits to be gained by the introduction of modern image processing and image enhancement techniques.

A method for evaluation of in-service fatigue cracks

“…The Australian Defence Science and Technology Organisation (DSTO) has evaluated a wealth of data (see for example [1], [2]) that confirms the methodology proposed by Frost-Dugdale [3,4] that as a first approximation a linear relationship exists between the log of the crack length or depth and the service history (number of cycles):…”

“…The above relationships have been shown to hold for a wide range of typical materials and geometries and for both constant amplitude and variable (i.e., in-service) loading [1]. This relationship in Eq.…”

“…While this relationship appears to be a reasonable approximation over most of the life of many typical cracks [1,2], growth rate acceleration towards the end of life can cause an upward trend in the crack growth acceleration rate. From fractographic observation of many cracks this appears to be caused, in part by the onset of static fracture [5] modes as well as ductile fatigue crack growth, such as inclusion fracture and ultimately tearing and changes in geometry and the coalescence of other cracks growing nearby to form a larger crack -i.e., a linear function of ffiffi ffi a p no longer applies.…”

“…In order to demonstrate the Frost-Dugdale relationship we can consider Fig.1 (from [6]) and Fig.2 (from [7]) as presented by Barter et al [1]. These data represent the quantitative fractography (QF) measurements of cracks and the resulting slopes of these measurements.…”

A multi-scale approach to crack growth

“…For physically short cracks, it was further shown in [14][15][16][17][18][19][20][21][22] that a near linear relationship between ln(a) and N can be used. In this context, researchers at DSTO have for many years observed, in a wealth of experimental data [23][24][25][26][27][28][29][30][31][32][33][34][35][36], that exponential growth rates are often found for most of the life of cracks grown in service or from full-scale fatigue tests, and coupon tests under service spectra, down to cracks in the order of a few microns in depth. Examples include the aluminium alloy 7050 crack growth data used in the F/A-18 aircraft (see for example the quantitative fractography (QF) results in With respect to MEMS technology it is now known that, although bulk silicon does not exhibit a significant susceptibility to cyclic fatigue, micron-scale structures made from silicon films are vulnerable to fatigue in ambient air environments.…”