The effect of compressive peak stress on fatigue behaviour

The effect of compressive peak stress on the maximum stress at the endurance limit, crack propagation rate, threshold stress intensity and crack closure was studied in a laboratory environment using two steels (SAE1045 and SAE1010) and two aluminium alloys (2024-T351 and 7075-T651).As the compressive peak stress, Scp, was increased in magnitude, the maximum stress at the endurance limit, Sfa, decreased linearly. Compression-compression cycling did not initiate any cracks in a centre-notched SAE1010 steel specimen but initiated cracks, which gradually became non-propagating, in the notched 2024-T351 aluminium alloy specimens. In compression-tension tests, the crack propagation rate increased, and the threshold and the crack opening stress intensities decreased linearly with increasing compressive peak stress. During compression-compression cycling the load/displacement curves were not linear, indicating that the crack was not fully closed.     

A MODEL FOR THE FATIGUE LIMIT AND SHORT CRACK BEHAVIOUR RELATED TO SURFACE STRAIN REDISTRIBUTION

A model based on surface strain redistribution and the reduced closure stress of short cracks is shown to successfully predict the fatigue limit and short crack growth behaviour for aluminium alloy 2024-T351. Using this approach, the length of non-propagating cracks can be anticipated. The local stress intensity range may be resolved into two components (first the linear elastic fracture mechanics component and the second is due to surface strain concentration). Consequently, the local stress intensity range of aluminium alloy 2024-T351 is a maximum at a depth of approximately half a grain diameter and a minimum at a depth slightly in excess of three grain diameters. The reduced closure stress for short cracks coupled with the increased applied stress intensity caused by surface strain redistribution accounts for the variation of the effective stress intensity parameter as a function of crack depth. This parameter is a maximum for the smallest possible crack (3 μm) and decreases as crack length increases.     

IN SITU SEM MEASUREMENTS OF CRACK CLOSURE FOR SMALL FATIGUE CRACKS IN ALUMINIUM 2024-T351

Abstract Crack closure has been measured for a range of small, self-initiated fatigue cracks using in situ SEM loading. Cracks were grown at positive R ratios in the aluminium alloy 2024-T351 and at nominal ΔK levels that extend substantially below the corresponding long crack threshold. The crack closure stress of the small cracks decreased and the K_cl level increased with increasing crack size until the long crack value near threshold was reached. For cracks of depth larger than about one grain size, a good correlation was obtained between small and long crack growth rate data in terms of ΔK_eff     

THRESHOLD STRESS INTENSITY BEHAVIOUR OF CRACKED STEEL STRUCTURAL COMPONENTS

Abstract— The effects of stress variables on the fatigue design of steel structural components (SAE 1010 steel) in the threshold region are investigated. The threshold and the closure threshold stress intensity ranges both decreased linearly as the stress ratio is increased. The threshold and opening threshold stress intensities also decreased linearly as the magnitude of compressive peak stress is increased. Crack opening stress measurements using a mechanical extensometer showed that the crack is not fully closed throughout the stress cycle at the threshold level. The crack opening stress is found to be independent of the crack length up to a certain crack length depending on the loading conditions. It is also found that the threshold stress intensity consists of two components: opening or closure stress intensity required to overcome crack closure, and intrinsic stress intensity range required to grow the crack. Linear relationships are obtained for the intrinsic stress intensity range as a function of stress ratio or compressive peak stress.     

Weld residual stress effects on fatigue crack growth behaviour of aluminium alloy 2024-T351

The interaction between residual stress and fatigue crack growth rate has been investigated in middle tension and compact tension specimens machined from a variable polarity plasma arc welded aluminium alloy 2024-T351 plate. The specimens were tested at three levels of applied constant stress intensity factor range. Crack closure was continuously monitored using an eddy current transducer and the residual stresses were measured with neutron diffraction. The effect of the residual stresses on the fatigue crack behaviour was modelled for both specimen geometries using two approaches: a crack closure approach where the effective stress intensity factor was computed; and a residual stress approach where the effect of the residual stresses on the stress ratio was considered. Good correlation between the experimental results and the predictions were found for the effective stress intensity factor approach at a high stress intensity factor range whereas the residual stress approach yielded good predictions at low and moderate stress intensity factor ranges. In particular, the residual stresses accelerated the fatigue crack growth rate in the middle tension specimen whereas they decelerated the growth rate in the compact tension sample, demonstrating the importance of accurately evaluating the residual stresses in welded specimens which will be used to produce damage tolerance design data.     

NEAR-THRESHOLD FATIGUE CRACK GROWTH IN A12O3 PARTICLE REINFORCED 6061 ALUMINIUM ALLOY

Abstract— The influence of Al₂O₃ particle reinforcement on the fatigue crack growth properties of 6061-T6 aluminium alloy in the near threshold regime has been investigated at a load ratio of R =– 1 using an alloy with 15 vol.% fine particles (6061/Al₂O₃/15p) and one with 21 vol.% coarser particles (6061/Al₂O₃/21p). The Al₂O₃ particles act as obstacles for fatigue crack growth and are especially effective at very low cyclic loads. For the reinforced alloy with fine particles the threshold of the stress intensity amplitude is higher than that for the alloy containing coarse particles, and the lowest threshold value of K _max was obtained for pure 6061-T6. Fracture of ceramic particles and interfaces between matrix and Al₂O₃ particles, both more frequent for coarser particles, may serve as an explanation for the more effective improvement of fatigue crack growth properties by fine particles. At maximum stress intensity factors above 6.5 MPa√m, fatigue crack growth in the particle reinforced alloys is faster than in the unreinforced alloy 6061-T6, which is attributed to more frequent particle and interface fracturing.     

The effects of environment and load frequency on the crack propagation law for macro fatigue crack growth in aluminium alloys

Data are presented for fatigue crack propagation of two aluminium alloys (2024-T3 Alclad-7075-T6 clad) in different environments and for several loading frequencies. Correlation of the data was attempted by the stress-intensity factor by application of the equation for crack growth proposed by Forman. The result was that the influence of the stress ratio on the crack growth was predicted by the equation in agreement with the test results but that the effects of the environment and the load frequency could not be coped with by changes in the constants of the equation. There may be a threshold level of the stress-intensity factor, strongly dependent on the environment and the alloy, in order to make the crack grow.     

Fatigue crack growth and crack closure in an AlMgSi alloy

This study reports an experimental investigation of fatigue crack propagation in AlMgSi1-T6 aluminium alloy using both constant and variable load amplitudes. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. For the constant amplitude tests four different stress ratios were analysed. The crack closure parameter U was calculated and related with Δ K and the stress ratio, R . The threshold of the stress intensity factor range, Δ K _th , was also obtained. Fatigue crack propagation tests with single tensile peak overloads have been performed at constant load amplitude conditions. The observed transient post overload behaviour is discussed in terms of the overload ratio, Δ K baseline level and R . The crack closure parameter U trends are compared with the crack growth transients. Experimental support is given for the hypothesis that crack closure is the main factor determining the transient crack growth behaviour following overloads on AlMgSi1-T6 alloy for plane stress conditions.     

Crack closure in fibre metal laminates

GLARE is a fibre metal laminate (FML) built up of alternating layers of S2-glass/FM94 prepreg and aluminium 2024-T3. The excellent fatigue behaviour of GLARE can be described with a recently published analytical prediction model. This model is based on linear elastic fracture mechanics and the assumption that a similar stress state in the aluminium layers of GLARE and monolithic aluminium result in the same crack growth behaviour. It therefore describes the crack growth with an effective stress intensity factor (SIF) range at the crack tip in the aluminium layers, including the effect of internal residual stress as result of curing and the stiffness differences between the individual layers. In that model, an empirical relation is used to calculate the effective SIF range, which had been determined without sufficiently investigating the effect of crack closure. This paper presents the research performed on crack closure in GLARE. It is assumed that crack closure in FMLs is determined by the actual stress cycles in the metal layers and that it can be described with the available relations for monolithic aluminium published in the literature. Fatigue crack growth experiments have been performed on GLARE specimens in which crack growth rates and crack opening stresses have been recorded. The prediction model incorporating the crack closure relation for aluminium 2024-T3 obtained from the literature has been validated with the test results. It is concluded that crack growth in GLARE can be correlated with the effective SIF range at the crack tip in the aluminium layers, if it is determined with the crack closure relation for aluminium 2024-T3 based on actual stresses in the aluminium layers.     

A model for fatigue crack growth with a variable stress intensity factor

A model for fatigue crack growth, similar to that of Majumdar and Morrow, is proposed where the crack growth rate is determined from the low cycle fatigue and cyclic stress-strain response of the material. The model is for a constant stress range at infinity, but does allow for a variable stress intensity factor due to the changing crack length. The study also includes an analysis of the strain range in the neighborhood of the crack tip. Further it is shown that the model predicts the critical stress intensity factor. A prediction of the crack growth rate is made for 2024-T351 aluminium, copper and CU-6.3 AL alloy and is compared to the experimental observations.     

THE EFFECTS OF STRESS RATIO ON THE GROWTH BEHAVIOUR OF SMALL FATIGUE CRACKS IN AN ALUMINUM ALLOY 7075-T6 (WITH SPECIAL INTEREST IN STAGE I CRACK GROWTH)

Abstract— The growth behaviour of small fatigue cracks has been investigated on aluminum alloy 7075-T6 at stress ratios R of 0, −1 and −2. The effects of stress ratio are discussed with special interest in the stage I region of small crack growth. Cracks which initiated at R =−1 and −2, grew by a stage I mechanism up to a certain depth followed by stage II crack growth. The stage I to stage II transition occurred under a constant maximum stress intensity factor which was approximately consistent with the threshold effective stress intensity range, λ K _eff,th, for large cracks. At R = 0, on the other hand, stage I crack growth was not observed because of crack initiation at inclusions. Small cracks grew more rapidly than large cracks subjected to the same nominal stress intensity ranges at all the stress ratios, and they grew below the threshold stress intensity range, λ K _th, for large cracks. Stage I cracks, in particular, showed much higher growth rates than large cracks and grew even below λ K _eff,th. It is suggested that stage II crack growth rates should be characterized in terms of an effective stress intensity range, while a micromechanics approach will be necessary to evaluate stage I crack growth rates.     

The effect of overloads on threshold and crack closure

The effect of tensile and compressive overloads on the threshold stress intensity level and crack closure behaviour of one aluminium alloy and three steels has been investigated. A few tensile overloads significantly decreased the crack propagation rate and increased the threshold stress intensity. An initially decreased and then increased opening stress was mostly responsible for the delayed retardation and crack arrest. Intermittant compressive overloads significantly accelerated the crack propagation and decreased the threshold stress intensity which was a function of the frequency of overloading. The opening stress was decreased to below zero after a large compressive peak load, and it took >10⁵ cycles for the opening stress to return to its stable level. During this period an initially high crack propagation rate also gradually decreased to the stable value.     

STRESS STATE-RELATED FATIGUE CRACK GROWTH UNDER SPECTRUM LOADING

Abstract— The fatigue crack growth behaviour in aluminium alloy sheets of 2024-T3 and 7475-T761, subjected to standardized spectra (TWIST and FALSTAFF), was investigated using centre-cracked specimens. A strip crack closure model was used to interpret experimental data, and to make predictions for the crack growth.
The strip model is based on the Dugdale concept, but modified to keep plastically-stretched materials on the crack surface so that the crack opening load can be determined, and the fatigue crack growth can be analysed according to Elber's crack growth assumption. Differing from other models of the same kind, a variable constraint factor was introduced to account for the gradual transition of stress state at the crack tip resulting from the crack growth. It has been shown that the transition of stress state at the crack tip causes the unusual behaviour of the fatigue crack growth in sheets. Both experiments and predictions show that a crack may grow faster at a low load than at a higher one in a certain applied load range due to the crack tip stress state transition. The crack tip stress state also contributes to the thickness effect observed for the crack growth in sheets. In agreement with experimental results, it has been shown that a plane stress state will prevail at the crack tip in a thin sheet compared to that in a thick sheet. The plane stress state results in a higher crack opening level which leads to a longer fatigue life for thin sheets.     

COMBINED MODE FRACTURE OF A DUCTILE MATERIAL UNDER PLANE STRESS CONDITIONS

Abstract— On the basis of our experimental results on Ly12-CZ (similar to 2024-T3) aluminium alloy tensile sheet specimens and Tresca's theory, a fracture criterion for a combined mode crack in ductile materials under plane stress conditions is proposed.     

CRACK GROWTH AND CLOSURE BEHAVIOUR OF SURFACE CRACKS UNDER AXIAL LOADING

Abstract— Crack growth and closure behaviour of surface cracks in 7075-T6 aluminium alloy are investigated under axial loading, noting the difference in fatigue growth behaviour at the maximum crack depth point and at the surface intersection point and also with through-thickness crack growth behaviour. The plane strain closure response at the point of maximum depth of a surface crack is monitored using an extensometer spanning the surface crack at the midpoint of its length. The plane stress closure at the surface intersection point is observed by multiple strain gauges placed at appropriate intervals ahead of the crack tip and continuously monitored without interrupting the fatigue test. The crack opening ratio is found to be about 10% greater at the maximum depth point than at the surface intersection point. Under axial loading, the difference in plane strain crack closure behaviour between the surface crack and the through-thickness crack is relatively small. Growth rates of surface cracks can be well described by the effective stress intensity factor range based on the closure measurements made in this study. The growth rates in terms of the effective stress intensity factor range seem to be slightly slower in surface cracks than in through-thickness cracks.     

Predicting fatigue crack growth rate in a welded butt joint: The role of effective R ratio in accounting for residual stress effect

A simple and efficient method is presented in this paper for predicting fatigue crack growth rate in welded butt joints. Three well-known empirical crack growth laws are employed using the material constants that were obtained from the base material coupon tests. Based on the superposition rule of the linear elastic fracture mechanics, welding residual stress effect is accounted for by replacing the nominal stress ratio (R) in the empirical laws by the effective stress intensity factor ratio (R_eff). The key part of the analysis process is to calculate the stress intensity factor due to the initial residual stress field and also the stress relaxation and redistribution due to crack growth. The finite element method in conjunction with the modified virtual crack closure technique was used for this analysis. Fatigue crack growth rates were then calculated by the empirical laws and comparisons were made among these predictions as well as against published experimental tests, which were conducted under either constant amplitude load or constant stress intensity factor range. Test samples were M(T) geometry made of aluminium alloy 2024-T351 with a longitudinal weld by the variable polarity plasma arc welding process. Good agreement was achieved.     

THE DETERMINATION OF R-CURVES FOR SHORT, WIDE CENTRE CRACKED PANELS OF ALUMINIUM ALLOY SHEET

Abstract— The feasibility of conducting an R –curve test on centre crack test pieces with a length/width ratio of between 1.3 and 0.25 has been investigated. To allow for the undefined mixed mode loading present with short, wide test pieces, two experimental methods of determining the geometric correction for the stress intensity factor have been proposed and compared. Also an experimental determination of the compliance calibration used to calculate the effective crack length has been made for these geometries. These methods have been evaluated by conducting R -curve tests on clad 2024-T3 aluminium alloy centre crack sheet test pieces. 2 m and 760 mm wide panels with length/width ratios of > 1.3, 0.5 and 0.25 were tested. The derived R -curves agreed well for both widths and for length/width ratios as low as 0.5.     

Acoustic emissions of fatigue crack growth

Acoustic emissions of fatigue crack growth have been monitored and quantitatively correlated with growth rate and the applied range of stress intensity for high cycle fatigue of 2024-T851 aluminum alloy. The data suggest a more cogent relationship for acoustic emissions and the applied range of stress intensity rather than between acoustic emissions and the average crack growth rate. Since nearly all crack growth is expected during the maximum load portion of the fatigue cycles, only the emissions from the acoustic events in the vicinity of the peak load were incorporated in correlations with $\text{d}\text{a/}\text{d}\text{n}$ and $\text{ΔK}$. Large amplitude emissions in the proximity of the minimum cyclic load were also detected. Because of their characteristics, these emissions are attributed to crack surface interference and, consequently, were not included in the correlation analyses.     

FATIGUE CRACK GROWTH IN DUCTILE MATERIALS UNDER CYCLIC COMPRESSIVE LOADING

Abstract— Mode I fatigue crack growth has been studied in notched specimens of 7017-T651 aluminium alloy subjected to fully compressive cyclic loads. The specimens were first subjected to a deliberate compressive preload which causes plastic deformation at the notch tip. On unloading, this region developed a residual tensile stress field and on subsequent compressive cyclic loading in laboratory air, a fatigue crack was nucleated at the notch and grew at a diminishing rate until it stopped. The final crack length increased with an increase in the value of the initial compressive preload and with an increase in the negative value of the applied cyclic mean load. To gain a better understanding of crack growth in residual stress fields, the magnitude and extent of residual stress induced from compressive preloads have been analysed. This was achieved when extending the notch by cutting while recording the change in the back face strain. From residual strain models it was found that the fatigue crack growth was confined to a region of tensile cyclic stress within the residual stress field. The effective stress intensity range was investigated at selected mean loads and amplitudes, for correlating purposes, using both the compliance technique and by invoking the crack growth rate behaviour of the alloy. Finally, a brief discussion of the fracture morphology of cracks subjected to cyclic compression is presented.     

OVERLOAD RETARDATION IN A STRUCTURAL STEEL

The mechanisms causing crack growth retardation after an overload were examined for BS4360 50B steel. It was found that plasticity-induced crack closure is the main cause of retardation when the pre-overload growth rate is in the mid-regime of the growth rate versus stress intensity range plot. When the pre-overload growth rate is near threshold it is argued that retardation at the surface of the specimen is primarily due to strain hardening and to the build-up of a favourable residual stress distribution in the material ahead of the crack tip. Supporting evidence for this argument is provided by a preliminary test on 2014A-T4 aluminium alloy. Plasticity-induced crack closure may be a further cause of retardation in the bulk, plane strain regions of the specimens made from BS4360 50B steel and 2014A-T4 aluminium alloy, when the pre-overload growth rate is near threshold.