Fatigue Crack Growth in 7050T7451 Aluminium Alloy Thick Section ...

DSTO-TR-14774.3 Estimating the effect of the initiating flawsThe examination of the fatigue crack growth of the KS etched specimens revealed that a relativelysimple method could be used to estimate the crack like size of the flaws from which the cracksgrew. This is reported in Barter, 2003. In that case the flaws that started cracking were etch pitsassociated with grain boundaries and etched out inclusions. The method used to analyse theseverity of these flaws was to fit an exponential growth model of the form:a = a 0 e β (N) (1)Where ‘a’ is the crack depth, ‘a 0 ‘is the apparent crack size at the commencement of loading, ‘N’ isthe life and ‘β’ is the slope of the curve, or a measure of the rate of crack growth. This was handledin several ways; one of which was to fit the curve to all the data points and use the zero lifeintersect as the measure of the Estimated Pre-Crack Size (EPS). A more complete discussion of thisprocess as applied previously may be found in Barter, 2003.The flaws that initiated the cracking in the peened specimens varied, although the dominant flawtype was observed to be laps in the surface due to folding during peening (section 3.6.1).Although the lap depth or the depth of any of the other types of flaw found in these specimenswas usually fairly easy to measure, unlike the etch pit flaws, their length varied along with theirposition within the peening dents. This led to uncertainty about the effectiveness of these flaws ininitiating fatigue cracks, so again the use of the QF results to estimate the EPS seemed to bewarranted.The growth rates of the peened specimens were not consistently exponential over their entirecrack depths ruling out the use of all the data to estimate the EPS. Nevertheless, the growth didappear to be the result of two phases of exponential growth connected by a transition phase atabout the depth at which the retarding effect of the peening disappeared (as was predicted byprevious measurements of the typical residual stress such as reported in Wang, 2003).The EPS was estimated using the slope of the crack growth curves, which in essence assumed thatthe resulting EPS is a combination of the size and shape of the flaw, the flaw position and thenumber of initiations that grew together from the flaw, as well as any abnormality (from theexponential growth rate assumed) in the growth at the very start of the cracking. The confidencein the EPS size was highest for those cracks where the majority of the progressions close to theorigin/s were found.To achieve the most reasonable 4 EPS result for these crack growth curves, several processes havebeen investigated. The easiest approach for the fractographer who is interactively examining aparticular crack and the measurements taken from that crack, is to add the measured depth of theflaw to the raw data which has been measured from the flaw-to-fatigue surface interface.Alternatively, varying amounts of depth can be added until a reasonably straight line is achievedon a log depth versus linear life plot of the measurements for the early part of the crack growth.This ‘anticipation-by-eye’ approach allows a ‘first cut’ at the data, which is used to highlightobvious errors in the measurements so that re-assessment of the measurements can be carried outon the specimen in real-time. This helps prevent ‘uninformed’ data correction while re-measuring4 The most reasonable measure of the EPS is still to be fully defined, although it should have some of thefollowing attributes; should not be so subjective such that different workers would produce grosslydifferent measures of it from the same data, be independent of the spectrum or load level used todetermine it, be transferable to different items made of the same material, not be excessivelyconservative or unconservative, nor should it be grossly different to the real flaw size that started thecracking since this may lead to confusion in interpreting some problems.21