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

DSTO-TR-1477poorly defined areas on the fracture surface. The interactive examination of the crack surface is astrong advantage of this method, while the dependence of the result on the skill and consistencyof the fractographer may be a disadvantage as far as reproducibility is concerned.After the data have been collected and the flaw depth and initial best-fit EPS have beenanticipated other methods were investigated to estimate EPS. These are listed below along withthe two prior types mentioned as measures of the flaw effectiveness. Each method has beendesignated as an EPS Type:Type I.‘Measured flaw depth’. This is usually the depth of the flaw at the point from whichcrack measurement was started, and has a level of uncertainty associated with theflaw shape and where along the edge of the flaw the measurements were startedfrom, as well as the position of the true surface; either the peaks between the peeningdents or the base of the peening dents or somewhere between. (It should be notedthat this is not necessarily the deepest point of the flaw). In some historicalpresentations of this approach, particularly Barter et al, 1991 and Athiniotis et al,1991 the measured flaw size was included in the data and a exponential curve fit wasapplied (equation 1) to the data to determine the a o (zero life intersect). Thisapproach, although not used here is designated as a Type IA EPS.Type II. The measure that was used during the collection of the QF data, which was the‘anticipated EPS’. This measure is subjective and depends on the skill of thefractographer.Type III. Fitting the data to the exponential model (equation 1) using a curve fitting program,and optimising the initial crack depth added to the raw crack size (offset value),checking the ‘goodness of fit’ (R 2 value), and refining the depth added until themaximum R 2 is reached. This method could be used to produce the curve that bestfitted the total data, and gave a zero life intercept value, which was thought to be thebest measure of the two possible EPSs (offset (Type IIIA) or intercept (Type III)). Thismethod would produce an ‘optimised EPS’ for all the data which clearly was notappropriate here due to the non-single exponential growth rate for the cracksgrowing through the peening.Type IV. The data for each curve were divided into two at the middle of the transition Phase(II) between the slower accelerating early Phase I of cracking and the faster latterPhase III. The ‘anticipated EPS’ was included in the early Phase I data in order togive an approximately correct starting point and an exponential curve was fitted toPhase I data ignoring the effect of the small amount of transition data included. Thezero life intersect of the curve a 0 was then used as the ‘fitted EPS’. This process wasdifferent to the process followed in Barter, 2003 (Type III EPS).While the Type III EPS is probably a more refined method for growth curves that areclose to an exponential over their entire life, the amount of data to use in the case ofthe peened slopes needed to be considered carefully since the optimisation routineused in the Type III EPS calculation, when applied to the early Phase I data,occasionally resulted in a runaway of the of the amount added since the curve simplybecomes a more perfect exponential fit with greater additions when the data is notvery exponential in nature. This was particularly the case when the early part of thecurve is not entirely straight (log depth/linear life) such as some of the curves for the360MPa peak stress.22