Single Overload Cycle Research Articles

Characterization of discontinuous crack closure behavior after the application of a single overload cycle

This paper aims to analyze the discontinuous crack closure behavior observed after a single tensile overload (OL) under otherwise constant ΔK loading. The investigation was performed on a disk-shaped compact tension DC(T) specimen made of Al6351-T6 with thickness of 2 mm, in order to simulate the case of a crack propagating under plane stress conditions. The Digital Image Correlation (DIC) technique was used in situ to obtain the full-field displacement and strain solution around the crack surfaces during the fatigue crack propagation. The experimental data obtained from DIC analysis revels that the residual hump of stretched material left in the wake of the propagating crack by the overload acts like a compliant spring, leading to a discontinuous crack closure mechanism. Moreover, this residual hump has a significant effect on the post-overload transient crack growth, even at distances much larger than the plastically deformed area created by the overload in front of the crack tip.

Investigation of effective stress intensity factors during overload fatigue cycles using photoelastic and DIC techniques

This work uses DIC and photoelastic techniques to investigate the effect of single overload cycles applied during constant amplitude fatigue. Effective values of the range of stress intensity factor were calculated using the CJP model of crack tip stress and displacement fields, as this model considers both wake contact and compatibility-induced influences on the applied elastic field arising from the plastic enclave generated around a fatigue crack. Values of the effective stress intensity factor are related to the observed crack growth acceleration and retardation. In addition, the paper compares the CJP results with those obtained using a compliance-based method. The present work demonstrates the utility of the CJP model in characterising fatigue crack growth rates during variable amplitude loading. It is also possible with the CJP model, through changes in the coefficient values and hence, for the first time, to shed explicit light on the contributions made by different mechanisms to the shielding effects of an overload.

Effect of Tensile or Compressive Overload on the Fatigue Crack Growth of Friction Stir Welded 19501 Aluminum Alloy

Abstract In this paper, crack growth tests on 19501 aluminum alloy friction stir welded butt joints after a single overload cycle in tension or in compression on compact tension (CT) specimens, along with constant amplitude tests, are investigated. The effect of the tensile overload or compressive overload applied just after 20,000 load cycles on the crack growth rate, crack growth path, and growth profile are investigated. The overload cycle exhibits a significant influence on fatigue life endurance. Increases of 39.14 % and 131.58 % in fatigue life with delayed cycles of 13,874 and 46,646 are seen because of the application of 1.78 times tension or compression overload. The crack propagation after overload cycles is investigated by inspection of the fatigue crack propagation rate. Results are discussed with reference to the retardation characteristic parameters, such as retardation ratio, plastic zone size, crack deviation, etc. Compressive overload is found to be more effective in retarding the growth rates compared to tensile overload of the same magnitude in friction stir welded joints. Secondary crack formation is seen after the tensile overload, whereas no such secondary crack formation is observed with compressive overload. The possible reasons for the crack growth behavior due to tensile or compressive overload are further analyzed in terms of scanning electron micrographs and plastic zone size.

A study of overload effect on fatigue crack propagation using EBSD, FIB–DIC and FEM methods

Abrupt increase in the maximum load during fatigue cycling modifies the deformation conditions at the crack tip, causing plastic flow that may lead to crack closure, introducing residual stress and hardening. The net consequence of these effects is notable crack growth retardation. Despite decades of research in the field, controversy persists regarding the role of each specific mechanism and their interaction. Resolving these issues with the help of experimental observation is related to the difficulty of obtaining local residual stress information at appropriate resolution. The present study examines the effect of overload on fatigue crack grown in a Compact Tension (CT) specimen of aluminium alloy AA6082 (BS HE30). Fatigue crack was grown in the sample under cyclic tension (R=0.1). After the application of a single overload cycle, fatigue loading was recommenced under previous cycling conditions. The crack morphology was investigated using Scanning Electron Microscopy (SEM). Electron Backscattered Diffraction (EBSD) was used to map grain orientation and crystal lattice distortion (pattern quality) in the vicinity of the crack. EBSD analysis of intra-granular misorientation allowed the qualitative analysis of the region around the crack tip location at the time of the overload application. Observations are discussed with a view to identify the roles of crack closure and residual stress effects. Residual stress was evaluated at salient locations around the crack retardation site using the FIB–DIC method which combines the use of Focused Ion Beam (FIB) and Digital Image Correlation (DIC) for residual stress measurement at the (sub)micron-scale. The residual stress field due to overload occurrence was also simulated using Finite Element Method (FEM), and the results compared with experimental observations.

The Influence of Strain Conditions in Steel Samples on the Fatigue Crack Growth and Delay after Overload

The work presents the results of the analysis of fatigue crack growth in flat steel samples for different strain conditions on crack tip in the sample. Growth of fatigue crack in the samples was studied in conditions of a plane strain state, plane stress state and in the transitory zone between those states. Test results showed a clear influence of these conditions on the fatigue crack growth rate. The paper also includes the test results of effectiveness of retardation of fatigue crack growth after single overload cycles used in different strain conditions.

Characterisation of crack tip fields under non-uniform fatigue loading

The paper analyses previously reported work, which uses digital image correlation to measure fatigue crack closure. As well as determining crack opening loads, the information on crack shape may be used to estimate the stress intensity factor, as well as other parameters in more complex models of crack tip fields. A number of specimens were subjected to single overload cycles, which produced a significant retardation in crack growth rate. The method previously applied to the analysis of constant amplitude loading is here used to analyse the single overload case. The stress intensity factor history is found to be very different in the two cases and the consequences of this observation for analysis of fatigue crack propagation are discussed.

Overload effect on the fatigue crack propagation in large‐scale tubular joints

ABSTRACTThis paper examines the overloading effect on the fatigue crack propagations monitored in a large‐scale tubular X‐joint specimen under two separate cyclic tests. The first cyclic test applies a constant‐amplitude brace in‐plane bending to the joint, with a single cycle of 150% overload before the crack depth reaches the mid‐thickness of the chord. The second fatigue test applies two batches of cyclic loads, with the amplitude of the second batch at 66% of the former. The X‐joint specimen experiences a 150% overload cycle during the first batch of loading, followed by the second batch after it has recovered from the overload effect. The experimental results reveal that deep surface cracks experience more significant overload retardation than does a shallow fatigue crack. The Paris law estimation indicates that the single overload cycle applied in the first specimen leads to a 7% increase in the fatigue life of the X‐joint.

Fatigue crack growth “overload effect”: mechanistic insights from in-situ synchrotron measurements

Synchrotron-based, high-energy X-ray diffraction measurements are used to study the local strain fields underlying the transient fatigue crack growth rate retardation produced by a single overload cycle known as the overload effect. Specifically, 4140 steel compact tension specimens fatigued for varying levels of crack growth after an overload cycle have been studied with in-situ diffraction under varying external loads. The load responses of the strain at the overload-position, versus at the crack tip, are focused upon in detail. The large compressive residual strain at the overload-point is observed to remain essentially unchanged even after the overload-point is left in the wake of the propagating crack tip. The differential strain-load response at the crack-tip/overload position before and immediately after the overload is seen to be unchanged. Once the overload point is behind the crack tip, a highly nonlinear behavior is observed in which the load response of the strain field transfers from the overload -point to the crack tip when the load exceeds a critical value. The results are discussed in terms of plasticity-induced crack face contact at the overload point as an important local mechanism contributing to the “overload effect” in this specific system.

Role of Internal Stresses on the Incubation Times during Stress Corrosion Cracking

We have examined the incubation times in two alloys, 7075-T651 aluminum alloy and 4140 steel, as a function of applied K, using the published data in aqueous environment. The role of overloads was compared with the results from those without overloads, for a given environment. Effect of environment (NaCl vs deionized water) was also examined. The results show that in a constant K test, the incubation time increases with decreasing K. When a single overload cycle was applied, the time increased with percent overload for a constant background K, indicating that overload cycle affected the crack tip driving forces. These effects varied with the environment. The changes in the incubation times are analyzed considering one-to-one correspondence between the crack tip driving force and the times. Overloads contributed to compressive residual or internal stresses, thereby affecting the crack tip driving force. The stresses are related to changes in the plastic zone (PZ) sizes formed before and after the overloads. The effective stress intensity due to internal stress, Kint, is defined and is shown to be a function of PZ size. Similarly, condition for crack initiation is expressed as Ktotal = Kapp ± Kint ≥ KIscc. A detailed methodology for the determination of Kint is outlined.

Mapping and load response of overload strain fields: Synchrotron X-ray measurements

High energy synchrotron X-ray diffraction measurements have been performed to provide quantitative microscopic guidance for modeling of fatigue crack growth. Specifically we report local strain mapping, along with in situ loading strain response, results on 4140 steel fatigue specimens exhibiting the crack growth retardation “overload effect”. Detailed, 2D, ε yy -strain field mapping shows that a single overload (OL) cycle creates a compressive strain field extending millimeters above and below the crack plane. The OL strain field structures are shown to persist after the crack tip has grown well beyond the OL position. The specimen exhibiting the maximal crack growth rate retardation following overload exhibits a tensile residual strain region at the crack tip. Strain field results, on in situ tensile loaded specimens, show a striking critical threshold load, F c, phenomenon in their strain response. At loads below F c the strain response is dominated by a rapid suppression of the compressive OL feature with modest response at the crack tip. At loads above F c the strain response at the OL position terminates and the response at the crack tip becomes large. This threshold load response behavior is shown to exhibit lower F c values, and dramatically enhanced rates of strain change with load as the crack tip propagates farther beyond the OL position. The OL strain feature behind the crack tip also is shown to be suppressed by removing the opposing crack faces via an electron discharge cut passing through the crack tip. Finally unique 2D strain field mapping (imaging) results, through the depth of the specimen, of the fatigue crack front and the OL feature in the wake are also presented.

Prediction of fatigue crack growth and residual life using an exponential model: Part II (mode-I overload induced retardation)

Almost all load bearing components usually experience variable amplitude loading (VAL) rather than constant amplitude loading (CAL) during their service lives. A single overload cycle introduced in a constant amplitude fatigue loading retards fatigue crack growth and increases residual fatigue life. Although many models have been proposed on this subject, but life prediction under these complex situations is still under constant improvement. The present study aims at evaluating retardation in fatigue life due to application of a single tensile overload spike by adopting an exponential model. The proposed model calculates not only parameters related with overload induced retardation in fatigue crack growth, but also residual life in case of 7020-T7 and 2024-T3 aluminum alloys with reasonable accuracy without integration of rate equation. The model also covers stage-II and stage-III of post-overload period.

Fatigue history and in-situ loading studies of the overload effect using high resolution X-ray strain profiling

High-energy synchrotron X-ray diffraction experiments are used to perform local crack plane strain profiling of 4140 steel compact tension specimens fatigued at constant amplitude, subjected to a single overload cycle, then fatigued some more at constant amplitude. X-ray strain profiling results on a series of samples employing in-situ load cycling are correlated with the crack growth rate (d a/d N) providing insight into the d a/d N retardation known as the “overload effect”. Immediately after the overload, the strain under maximum load is greatly reduced but the range of strain, between zero and maximum load, remains unchanged compared to the pre-overload values. At the point of maximum retardation, it is the strain range that is greatly reduced while the maximum-load strain has begun to recover to the pre-overload value. For a sample that has recovered to approximately half of the original d a/d N value following the overload, the strain at maximum load is fully recovered while the strain range, though partially recovered, is still substantially reduced. The dominance of the strain range in the overload effect is clearly indicated. Subject to some assumptions, strong quantitative support for a crack growth rate driving force of the suggested form [( K max) 1− p (Δ K) p ] γ is found. A dramatic nonlinear load dependence in the spatial distribution of the strain at maximum retardation is also demonstrated: at low load the response is dominantly at the overload position; whereas at high loads it is dominantly at the crack tip position. This transfer of load response away from the crack tip to the overload position appears fundamental to the overload effect for high R-ratio fatigue as studied here.

Measurement of crack closure after the application of an overload cycle, using moiré interferometry

The crack closure behaviour on the application of a single overload cycle was studied in a Ti-6Al-4V specimen. Moiré interferometry with photoresist gratings was used to measure crack displacements. During the overload cycle a large crack opening displacement was observed at the maximum load. This was similar to predictions from a Dugdale-type crack closure model. When the load was taken back to zero, the crack was open at the crack tip due to the high levels of plastic deformation during the overload cycle. As the crack was grown there was some evidence of the deformed material on the crack faces. Moiré interferometry provided displacement data close to the crack faces, even when the crack had grown to over two-and-a-half times the overload crack length. When the overload was applied the crack bifurcated, and the Dugdale-type model under-predicted the crack opening.

Strain profiling of fatigue crack overload effects using energy dispersive X-ray diffraction

Synchrotron based energy dispersive X-ray diffraction has been used to profile the strains around fatigue cracks in 4140 steel test specimens. In particular strain field comparisons were made on specimens prepared: with initial constant stress intensity fatigue; with this initial fatigue followed by a single overload cycle; and with this fatigue-overload sequence followed by an additional constant stress intensity fatigue. The strain profiles behind, at and in-front-of the crack tip are discussed in detail. Selected strain profiles measurements under in situ applied tensile stress are also presented. The technique of optical surface height profiling reveals surface depression effects which can be correlated with the interior strain profiles.

Fatigue Crack Closure and Crack Growth Following an Overload

Abstract The present investigation studies the effect of a single tensile overload cycle on fatigue crack closure and its mechanism. Attempts have also been made to study the crack growth kinetics following the overload application. It is concluded that Stage I-type of crack propagation occurs in the early stage of crack growth subsequent to overloading and the same is dominated by asperity-induced closure. A material - and loading-independent relationship has also been attempted between an overload-induced closure parameter and the overloading ratio.

Fatigue crack growth in a structural steel under single and multiple periodic overload cycles

Preliminary results of a research program on fatigue crack growth in a low‐carbon steel under a variable amplitude loading are presented. First, test results are reported on crack growth under simple loading sequences containing single and multiple tensile overloads applied periodically between smaller, constant amplitude cycles. Next, the observed crack growth behaviour is compared to predictions from a theoretical model developed by the authors.

EFFECT OF SPECIMEN THICKNESS ON FATIGUE CRACK GROWTH BEHAVIOUR UNDER CONSTANT AND VARIABLE AMPLITUDE LOADING

Mode I fatigue crack growth behaviour was experimentally investigated under constant and following the application of a single spike overload. Different specimen thicknesses were investigated at different overload ratios. A previously developed crack tip deformation model to correlate fatigue crack growth behaviour following k a single overload cycle was used to correlate that behaviour for different specimen thicknesses. It was found that the effect of specimen thickness on the fatigue crack growth behaviour under constant amplitude of loading is marginal especially at growth rates greater than le mm/cycle. The retardation due to the application of a tensile single overload decreases with the increase in specimen thickness. Good agreement between relevant experimental results and those predicted using three dimensional crack tip deformation parameter was found.

Deformation behaviour at the tip of a physically short fatigue crack due to a single overload

ABSTRACTA two‐dimensional elastic–plastic finite element analysis was utilized to investigate the transition behaviour of a physically short fatigue crack following the application of a single overload cycle. The deformation accommodated at the tip of a crack artificially advancing with a fully reversed load was considered. The development of the cyclic crack tip opening displacement was computed and then modelled to include the effects of the stress level of the base cycles, overload pattern and crack length at which the transient cycle was applied.The cyclic crack tip opening displacement was initially of a relatively high value. It decreased and then increased to match the behaviour under the base load cycles. The extent and location of both the minimum and matching points were dependent on the overload crack length and the stress compared with the material’s yield stress. In the case of the yield stress being exceeded by the overload, the minimum and the‐return‐to‐normality points are identical.A previously developed crack tip deformation parameter was invoked to predict relevant experimental fatigue growth rates of short cracks reported in the literature.

Effect of rest time after application of single overload cycle on fatigue life : Kumar, Rl, Kumar, A. and Singh, K. Eng. Fract. Mech. (1996) 54 (1), 147–153

Research on the basic fatigue formula, the criterion for omitting small loads, the rule for cumulative fatigue damage, and models for predicting the fatigue crack initiation (FCI) life and probability distribution under variable-amplitude loading are comprehensively studied and summarized in the present paper. First, the formula for FCI life is given as a function of the equivalent stress amplitude, and the procedures for investigating overload effects are introduced. Using the above formula to analyze test results of the overload effect on FCI life yields the formula for FCI life containing the overload effect factor, z, which can be adopted to characterize the load interaction effect when Miner's rule is used to compute the cumulative fatigue damage. Fatigue test results and analysis of the underload effect show that the FCI threshold in the FCI life formula can be taken as the criterion for omitting small loads in life prediction and in compilation of the load spectrum. Then, the fatigue damage function is defined, the cumulative fatigue damage can be computed and the FCI life of structure members under variable-amplitude loading can be obtained by using Miner's rule. Procedures for predicting the probability distribution of FCI life or fatigue life are described and examples are introduced. Furthermore, factors affecting the value of z and the applicability of Miner's rule are briefly discussed. Finally, further areas for fatigue research are tentatively proposed. Most interest should be focused on research into generally applicable life prediction models and the approach for fatigue reliability assessment of structure details made of the metals with continuous strain-hardening characteristics, which have been widely adopted in aeronautical engineering, under service load and environments.

Effect of rest time after application of single overload cycle on fatigue life

Effect of rest time after application of single overload cycle on fatigue life