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.     

Mixed-mode fatigue crack growth behaviour in aluminium alloy

Fatigue crack propagation tests in compact mixed-mode specimens were carried out for several stress intensity ratios of mode I and mode II, K_I/K_II, in AlMgSi1-T6 aluminium alloy with 3 mm thickness. The tests were performed in a standard servo-hydraulic machine. A linkage system was developed in order to permit the variation of the K_I/K_II ratio by changing the loading angle. Crack closure loads were obtained through the compliance technique. A finite element analysis was also done in order to obtain the K_I and K_II values for the different loading angles. Crack closure increases under mixed-mode loading conditions in comparison to mode-I loading due the friction between the crack tip surfaces. Moreover, the crack closure level increases with the K_I/K_II ratio decrease. Correlations of the equivalent values of the effective stress intensity factor with the crack growth rates are also performed. Finally, an elastic–plastic finite element analysis was performed to obtain the plastic zones sizes and shapes and model the effect of mixed-mode loading on crack closure.     

Effects of particle size on fatigue crack initiation and small crack growth in SiC particulate-reinforced aluminium alloy composites

Fatigue crack initiation and small crack growth were studied under axial loading using powder metallurgy 2024 aluminum-matrix composites reinforced with SiC particles of three different sizes of 5, 20 and 60 μm. The 5 and 20 μm SiC_p/Al composites exhibited nearly the same fatigue strength as the unreinforced alloy, while the 60 μm SiC_p/Al composite showed a significantly lower fatigue strength due to its inferior crack initiation resistance that could be attributed to interface debonding between particles and the matrix. Small crack growth behaviour was different depending on stress level. At a low applied stress, the addition of SiC particles enhanced the growth resistance, particularly in the composites reinforced with coarser particles, while at a high applied stress, the 60 μm SiC_p/Al composite showed a considerably low growth resistance, which could be attributed to interaction and coalescence of multiple cracks. In the 5 μm SiC_p/Al composite, small cracks grew avoiding particles and thus few particles appearing on the fracture surfaces were seen, particularly in small crack size region. In the 20 and 60 μm SiC_p/Al composites, they grew along interfaces between particles and the matrix and the number of particles appearing on the fracture surfaces increased with increasing crack size or maximum stress intensity factor.     

Fatigue crack growth in residual stress fields

A fatigue crack growth (FCG) model for specimens with well-characterized residual stress fields has been studied using experimental analysis and finite element (FE) modeling. The residual stress field was obtained using four point bending tests performed on 7050-T7451 aluminum alloy rectangular specimens and consecutively modeled using the FE method. The experimentally obtained residual stress fields were characterized using a digital image correlation technique and a slitting method, and a good agreement between the experimental residual stress fields and the stress field in the FE model was obtained. The FE FCG models were developed using a linear elastic model, a linear elastic model with crack closure and an elastic–plastic model with crack closure. The crack growth in the FE FCG model was predicted using Paris–Erdogan data obtained from the residual stress free samples, using the Harter T-method for interpolating between different baseline crack growth curves, and using the effective stress intensity factor range and stress ratio. The elastic–plastic model with crack closure effects provides results close to the experimental data for the FCG with positive applied stress ratios reproducing the FCG deceleration in the compressive zone of the residual stress field. However, in the case of a negative stress ratio all models with crack closure effects strongly underestimate the FCG rates, in which case a linear elastic model provides the best fit with the experimental data. The results demonstrate that the negative part of the stress cycle with a fully closed crack contributes to the driving force for the FCG and thus should be accounted for in the fatigue life estimates.     

Suppression of fatigue crack growth in aluminium alloy sheets

The effect of crack bridging reinforcing members on the growth of a transverse fatigue crack in an aluminium alloy sheet has been examined. The specimens have been subjected to zero tension cyclic loading applied in the reinforcement direction. In the case of some specimens initial tensile stresses were developed in the substrate aluminium sheet by the reinforcing members. In other cases initial compressive stresses were developed. Fatigue crack growth rates in the unreinforced aluminium alloy sheet were also determined. In the case of both types of reinforced structures the reinforcing elements suppressed the growth of fatigue cracks compared with the unreinforced material. This effect was particularly marked in the case where the substrate aluminium sheet was prestressed in compression-the crack growth rates approaching zero as the peak applied tensile strain approached the value of the initial compressive pre strain.The mechanisms whereby fatigue crack growth in the substrate aluminium sheet is suppressed are discussed and their effectiveness related to the behaviour of the unreinforced material by comparing crack growth rates in terms of effective stresses intensity factors.The compressive stresses were developed in the substrate sheet using two part core/tube reinforcing members. The cores were held in residual tension with the rest of the structure in corresponding compression. The prestress condition appears to be achievable with negligible weight penalty in structures of appreciable size where the engineering requirement is to limit the deformation of a structure under load. Where increased elastic deflections and correspondingly higher stresses are acceptable, reductions in structure weight together with an improved resistance to fatigue crack growth compared with the unreinforced material appear possible.The cores are designed not to be fracturable as a consequence of tensile deformations of any magnitude applied to the composite structure. Hence, they provide a residual load bearing capability and also inhibit crack extension in the fracturable part of the structure under all conditions of tensile loading.

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Résumé On a examiné l'effet de pontage d'une fissure occasionné par des éléments de renforcement, dans le cas de la croissance de fissure de fatigue transversale dans une tôle d'alliage d'aluminium. Les éprouvettes étaient soumises à une sollicitation de traction cyclique pulsée appliquée dans la direction du renforcement. Dans le cas de certaines éprouvettes, des contraintes initiales de traction étaient développées dans la tôle d'aluminium servant de substrat directement par l'élément de renforcement. Dans d'autres cas des contraintes initiales de compression étaient développées. La vitesse de croissance de la fissure de fatigue de la tôle d'alliage d'aluminium non renforcée a également été déterminée. Dans les deux types de structure renforcée qui ont été examinés les éléments de renforcement suppriment la croissance de fissure de fatigue, quand on se compare au matériau non renforcé. Cet effet a été particulièrement marqué dans le cas d'une tôle d'aluminium pré-comprimée: dans ce cas, les vitesses de croissance de fissuration approchaient de 0 lorsque la déformation appliquée en traction approchait la valeur de la pré-déformation de compression initiale.On discute les mécanismes par lesquels la croissance d'une fissure de fatigue dans un tôle d'aluminium peut être supprimée ainsi que l'efficacité de leur comportement dans le matériau non renforcé en comparant les vitesses de croissance de fissure en termes de facteur d'intensité de contrainte.Les contraintes de compression étaient introduites dans la tôle de base en utilisant des éléments de renforcement constitués d'un tube et d'un noyau. Le noyau était maintenu en tension résiduelle tandis que le reste de la structure était amené en compression. Les conditions de précontrainte apparaissent réalisables avec une faible pénalisation en poids dans des structures d'une dimension déjà appréciable lorsque les exigences de la conception limitent la déformation de la structure sous contrainte. Lorsqu'on admet des déflexions élastiques plus importantes et des contraintes correspondantes plus élevées, il apparait possible d'obtenir des réductions dans le poids de la structure ainsi qu'une résistance accrue à la croissance d'une fissure de fatigue, par rapport à une structure non renforcée.Les noyaux sont conçus de manière à résister aux déformations de traction appliquées à la structure composite. Dès lors, ils apportent un complément de résistance aux charges appliquées et empêchent l'extension d'une fissure dans la partie de la structure susceptible de se rompre, sous toutes les conditions de mise en charge.

     

Influence of interfacial stress transfer on fatigue crack growth in SiC-fibre reinforced titanium alloys

An important damage mechanism during fatigue of unidirectional SiC-fibre reinforced titanium alloys is the formation of matrix cracks transverse to the fibre direction. Due to the relatively low fibre/matrix bond strength these matrix cracks initially do not break the fibres, so that matrix cracks bridged by fibres develop. It is shown experimentally, that the strong drop in fatigue strength is caused by the formation of a bridged crack of a critical size and the crack propagation rate (da/dN) for a single load level has been determined. A prediction of da/dN on the basis of finite element calculation of the stress intensity factor range of the bridged matrix crack ΔK_m and the ΔK_m–da/dN relationship of the used titanium alloy (Timetal 834) has been performed. Calculation of ΔK_m assuming a negligible fibre/matrix bond strength and considering shear load transfer at the fibre/matrix interface due to Coulomb friction (coefficient of friction μ=0.5 and μ=0.9) led to a large discrepancy between the measured and predicted crack growth rate. It can be concluded, that the assumed conditions of stress transfer at the fibre/matrix interface neglecting bonding is the reason for this discrepancy.     

Short fatigue crack growth in aluminium alloy 6082-T6

Fatigue crack growth tests on 6082-T6 aluminium specimens have been carried out. The single edge notched tension specimens had a spark eroded 0.2 mm deep starter notch. In order to measure the crack growth, a special direct current potential drop method was used. The experiments were carried out at four different stress ratios. The crack growth data validate the long crack growth data given in Eurocode 9 [CEN. ENV 1999-2: Eurocode 9: Design of aluminium structures. Part 2: Structures susceptible to fatigue. European Committee for Standardisation, 1998.] and some indication of accelerated crack growth for short cracks was found. An effective stress intensity factor, which is a generalisation of an approach proposed by El Haddad et al. [El Haddad MH, Topper TH, Smith KN. Prediction of nonpropagating cracks. Eng Fract Mech 1979; 11:573–84], has been applied to correct for short crack growth behaviour. It has been shown that the potential drop technique can be successfully used to measure crack growth in aluminium specimens. The mean stress dependence was found to be less pronounced than for the 7075-T6 and 2024-T3 aluminium alloys.     

Laser shock peening on fatigue crack growth behaviour of aluminium alloy

The effect of laser shock peening (LPS) in the fatigue crack growth behaviour of a 2024‐T3 aluminium alloy with various notch geometries was investigated. LPS was performed under a ‘confined ablation mode’ using an Nd: glass laser at a laser power density of 5 GW cm^?2. A black paint coating layer and water layer was used as a sacrificial and plasma confinement layer, respectively. The shock wave propagates into the material, causing the surface layer to deform plastically, and thereby, develop a residual compressive stress at the surface. The residual compressive stress as a function of depth was measured by X‐ray diffraction technique. The fatigue crack initiation life and fatigue crack growth rates of an Al alloy with different preexisting notch configurations were characterized and compared with those of the unpeened material. The results clearly show that LSP is an effective surface treatment technique for suppressing the fatigue crack growth of Al alloys with various preexisting notch configurations.     

Particle size dependence of fatigue crack propagation in SiC particulate-reinforced aluminium alloy composites

Fatigue crack propagation (FCP) and fracture mechanisms have been studied for two orientations in powder metallurgy 2024 aluminium alloy matrix composites reinforced with three different sizes of silicon carbide particles. Particular attention has been paid to make a better understanding for the mechanistic role of particle size. The FCP rates of the composites decreased with increasing particle size regardless of orientation and were slightly faster in the FCP direction parallel to the extrusion direction. After allowing for crack closure, the differences in FCP rate among the composites and between two orientations were significantly diminished, but the composites showed lower FCP rates than the corresponding unreinforced alloy. Fracture surface roughness was found to be more remarkable with increasing particle size and in the FCP direction perpendicular to the extrusion direction. Taking into account the difference in the modulus of elasticity in addition to crack closure, the differences in FCP rate between the unreinforced alloy and the composites were almost eliminated.     

Effect of notch geometry on short fatigue crack growth in 8090 Al–Li alloy

The short-crack propagation behaviour of 8090 Al–Li alloy under different ageing conditions has been investigated. The effect of notch geometry on short fatigue crack growth was also studied. The results show that the geometrical configuration of the notch significantly affects the growth behaviour of the short crack, the growth rates of notched short cracks being much higher than those of long cracks at the same stress intensity factor range ΔK level. The orientations of the specimens had a stronger effect on the growth rate of long cracks than on that of short cracks. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effect of stress ratio on fatigue behaviour in SiC particulate-reinforced aluminium alloy composite

Axial fatigue tests have been performed at three different stress ratios, R, of ?1, 0 and 0.4 using smooth specimens of an aluminium alloy composite reinforced with SiC particulates of 20 μm particle size. The effect of stress ratio on fatigue strength was studied on the basis of crack initiation, small crack growth and fracture surface analysis. The stress ratio dependence of fatigue strength that has been commonly observed in other materials was obtained, in which fatigue strength decreased with increasing stress ratio when characterized in terms of stress amplitude. At R=?1, the fatigue strength of the SiC_p/Al composite was the same as that of the unreinforced alloy, but at R= 0 and 0.4 decreased significantly, indicating a detrimental effect of tensile mean stress in the SiC_p/Al composite. The modified Goodman relation gave a fairly good estimation of the fatigue strength at 10⁷ cycles in the unreinforced alloy, but significantly unconservative estimation in the SiC_p/Al composite. At R= 0 and 0.4, cracks initiated at the interfaces between SiC particles and the matrix or due to particle cracking and then grew predominantly along the interfaces, because debonding between SiC particles and the matrix occurred easily under tensile mean stress. Such behaviour was different from that at R=?1. Therefore, it was concluded that the decrease in fatigue strength at high stress ratios and the observed stress ratio dependence in the SiC_p/Al composite were attributed to the different fracture mechanisms operated at high stress ratios.     

Limited weld residual stress measurements in fatigue crack propagation: Part II. FEM-based fatigue crack propagation with complete residual stress fields

Using a limited set of residual stress measurements acquired by neutron diffraction and an equilibrium‐based, weighted least square algorithm to reconstruct the complete residual stress tensor field from the measured residual stress data, the effect of weld residual stress on fatigue crack propagation is investigated for 2024‐T351 aluminium alloy plate joined by friction stir welding. Through incorporation of the least squares, complete equilibrated residual stress field into a finite element model of the Friction Stir Weld (FSW) region, progressive crack growth along a direction perpendicular to the welding line is simulated as part of the analysis. Both the residual stress redistribution and the stress intensity factor due to the residual stress field, K_res, are calculated during the crack extension process. Results show that (a) incorporation of the complete, self‐equilibrated residual stress field into a finite element (FE) model of the specimen provides a robust, hybrid approach for assessing the importance of residual stress on fatigue crack propagation, (b) the calculated stress‐intensity factor due to the residual stress field, K_res, has the same trend as measured experimentally by the ‘cut‐compliance method’ and (c) the da/dN results are readily explained with reference to the effect of the residual stress field on the applied stress intensity factor.     

Near-threshold fatigue crack growth at room temperature and an elevated temperature in Al---Li alloy 8090

The effects of slip distribution and crack tip shielding mechanisms on the near-threshold fatigue crack growth of the Al---Li alloy 8090 have been studied at both room temperature and an elevated temperature. The slip distribution has been varied by changing the distribution of the S phase, through prior stretching or by means of a duplex heat treatment. Fatigue crack growth (FCG) tests were conducted at a high stress ratio to reduce possible effects due to crack closure.

At room temperature the changes in FCG rates are interpreted as arising from the changes in the degree of planarity of slip in the materials.

At 150°C, the microstructural changes due to the long exposure to elevated temperature appear to dominate the effects observed. At lower ΔK, where the time at temperature is greatest, lower ΔK thresholds than those found at room temperature are obtained. These have been attributed to increased slip homogenization due to the increased precipitation and coarsening of the incoherent S phase together with loss of toughness due to the growth of coarse grain boundary phases and the formation of the associated δ′ precipitate free zone.

At higher ΔK, where the time at temperature is low and microstructural changes are minimal, slower FCG rates than those found at room temperature are obtaine. These are explained in terms of increased crack tip shielding which arises because of the increase in tortuosity of the crack path, the increased slip homogenization and the climb and cross-slip within the crack tip plastic zone.     

The effect of microstructure and environment on fatigue crack growth in 7049 aluminium alloy at negative stress ratios

The influence of environment and microstructure on fatigue crack growth has been investigated on a high strength 7049 aluminium alloy. This aluminium alloy was artificially aged to underaged (UA) and overaged (OA) microstructures. The heat treatment procedure was performed in order to obtain an UA and OA microstructure having the same yield strength properties, but differing in the mode of slip deformation: the UA alloy deforms by planar slip and that of the OA alloy by wavy slip. The crack growth measurements were performed in MT specimens at constant load ratios for R=0, −1, −2, −3 near-threshold and Paris regime in ambient air and vacuum conditions. Crack closure loads were measured in order to determine the P_open for each R ratio. Micromechanisms of near-threshold crack growth are briefly discussed for several concurrent processes involving environmentally assisted cracking with intrinsic microstructural effects. The results showed that the presence of humid air leads to a larger reduction in ΔK_th for both the ageing conditions, but the UA specimens were superior probably because of crack branching. The role of environmental effect and microstructures near-threshold regime seems to be more significant than any mechanical contributions to the crack closure, such as plasticity, roughness, oxide, etc.     

Fracture toughness and fatigue crack propagation behavior of 8090 Al-Li alloy

We present results of fatigue tests of high-strength 8090 Al-Li alloy and data on its fatigue crack growth resistance. High strength combined with fairly high crack growth resistance and endurance limit results in much better service characteristics compared to other high-strength aluminum alloys. We discuss results of tensile and impact tests of Charpy specimens and the critical values of theJ-integral andK _1c for 10-mm-thick specimens in the T-L and L-T orientations subjected to complete and partial aging. The experimental results are compared with published data for 8090 and other high-strength aluminum alloys. We suggest a numerical method for the evaluation of fatigue strength according toda/dN-K diagrams.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 45–58, January – February, 1995.     

Evolution of residual stresses with fatigue loading and subsequent crack growth in a welded aluminium alloy middle tension specimen

Neutron diffraction has been used to measure the evolution of the residual stresses in a VPPA welded Al-2024 alloy middle tension (M(T)) specimen with fatigue loading and subsequent crack growth. The measurements were carried out on the diffractometer ENGIN-X, a time-of-flight instrument, at the ISIS Pulsed Neutron Source. Fatigue crack growth was performed in situ and strain measurements averaged through the thickness of the specimen were made along two orthogonal directions as the crack grew, allowing the stresses in the specimen to be calculated assuming plane stress. 2D finite element simulation of the evolution of the initial residual stress field with crack growth, using an elastic model produced predictions that were in reasonable agreement with the experimental results. The results further indicate that some re-distribution of the residual stress field occurred due to the crack tip plasticity associated with the fatigue loading.     

Production of SiC particulate reinforced aluminium composites by melt spinning

Melt spinning is successfully used for the preparation of a rapidly solidified SiC particle reinforced AlSi7Mg0.3 alloy. The composites are prepared by introducing SiC particles in a semi-solid matrix slurry (SiC volume fractions up to 0.15, particle size 10 or 20 m). Duralcan material (SiC volume fraction 0.20, particle size 12 m) was also used. After stirring in the semi-solid state the composites are heated above the liquidus temperature and subsequently melt-spun. Featureless, columnar and dendritic zones can be identified in the ribbons. A finer dendritic structure is found around the SiC particles. The SiC particles tend to segregate to the air side of the ribbons and the segregation effect is influenced by particle size and volume fraction. As interface velocities are higher than the critical velocities predicted by models on interface pushing, it is concluded that fluid flow in the melt puddle is responsible for the segregation effect.     

Influence of low magnitude plastic strains on residual stress in 7075 aluminium alloy

The effect of post quench delay on residual stress has been determined by subjecting four rectilinear blocks of quenched 7075, containing large residual stress magnitudes, to small (<1.0%) plastic strains applied by cold compression. One experiment kept the plastic strain the same in all samples (～0.12%) by cold compressing simultaneously in one pressing. The second pressed the blocks separately to the same strain (～0.45%). The post quench delays were <10, 60, 120 and 240 min. The residual stresses were characterised using neutron and X-ray diffraction. It was possible to detect the impact of the small plastic strains on all samples, but the diffraction measurement techniques could not conclusively isolate the influence of the post quench delay.