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

Fatigue crack growth tests have been carried out on a medium carbon structural steel over a wide range of stress ratios, i.e. from 0 to 0.7. All tests were conducted under constant amplitude loading conditions corresponding to growth rates in the Paris regime. Crack closure behaviour was observed experimentally by a surface strain gauge technique, and numerically by a finite element analysis under plane stress condition. While the crack closure could not be detected by experimental measurements at stress ratios equal to or greater than 0.5, the numerical results showed that closure occurred even at high stress ratios up to 0.7. The differences between experimentally and numerically determined crack opening levels were found for each stress ratio. A cause for these differences is discussed. In addition, two new types of crack tip parameters which have been proposed recently are evaluated by finite element analysis and their relevance to fatigue crack growth are discussed. It is concluded that fatigue crack growth rates are substantially determined by the effective stress intensity factor range which is based on the crack closure concept. NOMENCLATURE a = crack length d, = length of closed crack during loading da/dN = crack growth rate F = geometry factor AK = stress intensity factor range AK,= effective stress intensity factor range AKRP = stress intensity factor range which corresponds to the applied stress range (0--uRpG) AKtt = stress intensity factor range which corresponds to the applied stress range (urn -utt) L, = crack length that is close at umin P-, Pmax =minimum and maximum applied load R = stress ratio (umin/umu) ~y = strain in y-direction u,,, = crack opening stress uII =applied stress at which stress at crack tip becomes tensile oy =stress in y-direction ays = yield stress u-, urn= =minimum and maximum applied stress uRpG = applied stress just when plastic deformation occurs in the vicinity of crack tip

Solid state bonding between superplastic duplex stainless steel with carbon steel under low pressures is studied for the objectives to understand bonding behaviors and to determine mechanisms controlling bonding through the activation energy analysis. Experiments were carried out by combining several bonding parameters such as bonding temperatures, bonding pressures and surface roughness. Bond quality was evaluated by its tensile strength. Bonding strength increased with temperature, time and pressure but decreased with surface roughness. Sound bonds comparable to that of the parent metal were obtained at considerably short bonding time and low and 220 s bonding time, producing the parent metal strength with around 1% of deformation ratio. Changes in microstructures and hardness were observed across the bonding interface, due to diffusion of atoms, mainly C, from the carbon steel side to the duplex stainless steel side. A hard carburized layer formed at the DSS side and a soft decarburized layer at the carbon steel side. From the value of activation energy and experimental data, bonding was controlled mainly by two mechanisms, which were the superplastic deformation at the early stage and the diffusion of carbon at the second stage.

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