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

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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
DSTO-TR-1477Examples of the Type IV process and the formula generated are shown in Figure 24 and Figure 25for one crack from each peak stress level. These values along with the curve fits for the Phase IIIparts of the curves will be presented and discussed in the following section. Although this methodwhen applied to the peened specimens would probably over estimate the effectiveness of a flawconsidering the variable residual stress (particularly the small region near the surface where theresidual stresses are less intense, see Figure 23), it does give a consistent picture that may betransferable to other spectra for this material.The crack growth curves with corrections for the measured flaw depth (Type I EPS) and theestimated EPS Type II are shown in Figure 26 to Figure 29 A & B. Table 6 gives the Type II EPSand flaw depths along with the means of these values. The Type IV EPS calculations will bepresented in the following section.0 10000 20000 30000 4000010KSIF 149 lowKSIF 149 highSFHy = 0.11611 * e^(0.011033x) R= 0.97153SFH0 5000 10000 15000 20000 2500010KSIF 140 lowKSIF 140 highy = 0.03347 * e^(0.041935x) R= 0.99540.1y = 1.1892e-09 * e^(0.17481x) R= 0.927170.11y = 8.5323e-06 * e^(0.18776x) R= 0.9974110.010.10.010.0010.10 20 40 60 80 100 120 140IARPO3a + 5ML Spectra0.010 10 20 30 40 50 60 70 80IARPO3a + 5ML SpectraFigure 24 Examples of the generation of Phase I and Phase III growth slopes and the Type IV EPS valuesfor the 360 and 290MPa peak stress levels.SFH0 2000 4000 6000 8000 1000010KSIF 191 lowKSIF 191 highSFH0 1000 2000 3000 4000 5000 6000 7000 800010KSIF 146 lowKSIF 146 high1y = 0.043288 * e^(0.089068x) R= 0.98739y = 7.2653e-05 * e^(0.33012x) R= 0.99410.11y = 0.049993 * e^(0.1354x) R= 0.98559y = 0.00028305 * e^(0.4503x) R= 0.944750.10.010.010.10.10.0010.0010.010 5 10 15 20 25 30 35IARPO3a + 5ML Spectra0.010 5 10 15 20 25IARPO3a + 5ML SpectraFigure 25 Examples of the generation of Phase I and Phase III growth slopes and the Type IV EPS valuesfor the 420 and 450MPa peak stress levels.23
Page 3: Fatigue Crack Growth in 7050T7451 A
Page 8 and 9: DSTO-TR-1477To build the required l
Page 10 and 11: DSTO-TR-1477than internal flaws. An
Page 12 and 13: DSTO-TR-1477127mm Radius (5’)230m
Page 14: DSTO-TR-1477At the surface large gr
Page 18 and 19: DSTO-TR-1477Figure 11 The fracture
Page 20 and 21: DSTO-TR-1477A (KSIF152)B (KSIF107)F
Page 22 and 23: DSTO-TR-1477life axes. The data poi
Page 24 and 25: DSTO-TR-14774. Discussion4.1 Introd
Page 26 and 27: DSTO-TR-14774.2.2 Shape of the crac
Page 30 and 31: DSTO-TR-1477Simulated flight hours0
Page 33 and 34: DSTO-TR-1477in the appropriate regi
Page 35 and 36: DSTO-TR-1477390MPa(higherslope)420M
Page 37 and 38: DSTO-TR-1477Table 9Specimen No.A co
Page 39 and 40: DSTO-TR-1477considerable concern to
Page 41 and 42: DSTO-TR-1477Barter, S. A., Fatigue
Page 43 and 44: DSTO-TR-1477126.5 41102 0.1638 0.25
Page 45 and 46: DSTO-TR-1477248.5 80743 0.1195 0.12
Page 47 and 48: DSTO-TR-147730.5 9910.1 0.0713 0.10
Page 49 and 50: DSTO-TR-1477104.5 33954 0.0733 0.08
Page 51 and 52: DSTO-TR-147743.5 14134 0.042 0.0468
Page 53 and 54: DSTO-TR-147716.5 5523.6 0.0406 0.05
Page 55 and 56: DSTO-TR-147718.5 6011 0.0264 0.0452
Page 57 and 58: DSTO-TR-147723.5 7635.6 0.0481 0.08
Page 59 and 60: DSTO-TR-1477Table A-13. Results of
Page 61 and 62: DSTO-TR-1477Table A-16. Results of
Page 63 and 64: DISTRIBUTION LISTFatigue Crack Grow
Page 65: Page classification: UNCLASSIFIEDDE

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