Transient Fatigue Crack Growth Behaviour Following Single Overloads at High Stress Ratios

Abstract

Fatigue crack growth tests with constant amplitude loading and single overload have been performed on a long mode I crack in 2017-T3 aluminium alloy at various stress ratios from 0 to 0.7. Two crack tip parameters of uop and u, were evaluated using a finite element analysis for a growing crack under these loading conditions. The former is the crack opening stress and the latter is the applied stress level at which the stress at the crack tip becomes tensile. It was found that transient crack growth behaviour following single overloads at high stress ratios was significantly different from that at a low stress ratio: at higher stress ratios, following the application of the overload, there was a rapid retardation which was followed by an acceleration in growth rate and then a faster return to the steady state level at baseline loading. The experimentally observed transient post-overload behaviour is discussed in terms of the two effective stress range ratios of Uop and U,, which are determined from uop and u,, respectively. For the stress ratios and overload ratios studied, the results indicate that the changes in U, with crack extension after the overload are reasonably consistent with the crack growth rate trends. The stress distribution at minimum applied stress would account for the transient changes in U,.Keywords-2017-T3 aluminium alloy; Overloads (single); Transient crack growth behaviour; Finite element analysis; High R ratios NOMENCLATURE a = crack length da/dN = crack growth rate aoL = crack length at which overload is applied KOL = stress intensity factor at peak load during the overload cycle AK = stress intensity factor range AK, = stress intensity factor range which corresponds to the applied stress range (u--utt) NoL = number of cycles at which overload is applied roL = size of maximum plane stress plastic zone associated with the overload N = number of cycles OLR = overload ratio defined as (uoL -U&)/(U,, -u-) R =stress ratio (u-/uwx) S = striation spacing U,, = effective stress range defined as (umx -u,,)/(u, -a*) U,, = effective stress range ratio defined as (a, -utt)/(u, -u-) uoL = peak stress during the overload cycle u,, = crack opening stress utt = applied stress at which stress at crack tip becomes tensile uy = stress in y-direction uys = yield stress u-, om,, = minimum and maximum applied stress

Evaluation of overload effects on fatigue crack growth and closure

“…Table 3, the described trends were observed for all the DK BL analysed in this work at both R ratios of 0.05 and 0.25. Therefore, the magnitude and extent of crack retardation increase with the level of the OLR, accordingly to many other studies [1,2,12,15,18].…”

“…Finite element analyses have shown that this phenomenon can occur depending on the loading variables [14,25]. Tsukuda et al [18] have also corroborated the existence of discontinuous closure at high stress ratios due to single tensile overloads by finite element analysis. This investigation indicates that the stress at the crack tip becomes tensile before the crack fully opens at the overload location.…”

“…Furthermore, the amount of available data related to the material studied in this work is reduced. The effects of single peak tensile overloads have been reported in many investigations [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] simply because this type of loading can lead to significant load interaction effects. Several mechanisms have been proposed to explain crack growth retardation, which include models based on residual stresses [19], crack closure [20], crack tip blunting [21], strain hardening [22], crack branching [23] and reversed yielding [24].…”

Effects of Shot-Peening and Stress Ratio on the Fatigue Crack Propagation of AL 7475-T7351 Specimens

“…Several mechanisms have been proposed to explain crack growth retardation; including models based on residual stresses, crack closure, crack tip blunting, strain hardening, crack branching and reversed yielding. The effect of residual plastic deformation leads to compressive stresses in the wake of the crack and raises the crack opening load on subsequent crack growth (crack closure), becoming the most important phenomena in explaining the variation of the characteristic features of postoverload transients [19][20][21][22][23]. Donald and Paris [23] observed for 6061-T6 and 2024-T3 aluminium alloys, that closure measurements produced a good data correlation between different stress ratio crack growths obtained with K-increasing conditions.…”

“…Overloads can lead to significant interaction effects on crack propagation, as have been reported in many [11][12][13][14][15][16][17][18][19][20][21][22]. Several mechanisms have been proposed to explain crack growth retardation; including models based on residual stresses, crack closure, crack tip blunting, strain hardening, crack branching and reversed yielding.…”