Residual stress effects on fatigue crack growth in a Ti‐6Al‐4V friction stir weld

Recent studies have illustrated a predominant role of the residual stress on the fatigue crack growth in friction stir welded joints. In this study, the role of the residual stress on the propagation of fatigue cracks orthogonal to the weld direction in a friction stir welded Ti‐6Al‐4V joint was investigated. A numerical prediction of the fatigue crack growth rate in the presence of the residual stresses was carried out using AFGROW software; reasonable correspondence between the predictions and the experimental results were observed when the effects of residual stress were included in the simulation.     

Comparative study of fatigue and fracture in friction stir and electron beam welds of 24 mm thick titanium alloy Ti‐6Al‐4 V

In this study, friction stir welding of Ti‐6Al‐4 V was demonstrated in 24 mm thickness material. The microstructure and mechanical properties, fatigue, fracture toughness and crack growth of these thick section friction stir welds were evaluated and compared with electron beam welds produced in the same thickness material. It was found that the friction stir welds possessed a relatively coarse lamellar alpha transformed beta microstructure because of slow cooling from above the transus temperature of the material. The electron beam welds had a fine acicular alpha structure as a result of rapid solidification. The friction stir welds possessed better ductility, fatigue life, fracture toughness and crack growth resistance than the base meal or electron beam welds. Thus, even though friction stir welding is a relatively new process, the performance benefits it offers for the fabrication of heavy gage primary structure make it a more attractive option than the more well‐established electron beam welding method.     

Fracture toughness and fatigue crack growth rate properties in wire + arc additive manufactured Ti‐6Al‐4V

This paper presents an experimental investigation of the fracture and fatigue crack growth properties of Ti‐6Al‐4V produced by the Wire + Arc Additive Manufacture (WAAM®) process. First, fracture toughness was measured for two different orientations with respect to the build direction; the effect of wire oxygen content and build strategy were also evaluated in the light of microstructure examination. Second, fatigue crack growth rates were measured for fully additive manufactured samples, as well as for samples containing an interface between WAAM® and wrought materials. The latter category covers five different scenarios of crack location and orientation with respect to the interface. Fatigue crack growth rates are compared with that of the wrought or WAAM® alone conditions. Crack growth trajectory of these tests is discussed in relation to the microstructure characteristics.     

Fracture and fatigue crack growth behaviour of PMMC friction stir welded butt joints

The paper is focused on the evaluation of the fracture and Fatigue Crack Growth (FCG) properties of butt joints of particulate metal-matrix composite (PMMC) obtained by friction stir welding (FSW). The materials considered are two aluminum alloy matrix/alumina particle PMMCs (AA6061/Al₂O₃/20p and AA7005/Al₂O₃/10p). Tests were conducted on unwelded and welded PMMCs using CT and Extended CT (ECT) specimen geometries, respectively. The crack growth rate was monitored by means of compliance with a strain gage attached on the back of the specimen. FCG experiments were carried out both at the centre and in the Thermo-Mechanically Altered Zone (TMAZ) at the side of the weld. The comparison between unwelded and welded PMMCs showed that FSW influences fracture toughness and FCG rate in a different fashion depending on the material. In particular, the FSW AA6061/Al₂O₃/20p butt joint exhibited comparable fracture toughness and higher FCG threshold with respect to the unwelded material, while in the case of AA7005/Al₂O₃/10p the behaviour is the opposite. The interpretation of this trend has been carried out by optical analysis of the crack path roughness and its correlation with the FCG rate. The dynamic recrystallization of the aluminum matrix and particle shaping operated by the FSW tool are at the ground of the explanation.     

Effect of build direction on the fracture toughness and fatigue crack growth in selective laser melted Ti‐6Al‐4 V

Experimental investigation was conducted to evaluate the fracture toughness and fatigue crack growth characteristics in selective laser‐melted titanium 6Al‐4 V materials as a follow‐on to a previous study on high cycle fatigue. For both the fracture toughness and crack growth evaluation, the compact tension specimen geometry was used. It was found that the fracture toughness was lower than what would be expected from wrought or cast product forms in the same alloy. This was attributed to the rapidly cooled, martensitic microstructure, developed in the parts. At low stress ratios, the crack growth rates were faster than in wrought titanium but became comparable at higher ratios. The fracture toughness appears to be higher when the crack is oriented perpendicular to the build layers. The difference in the average threshold and critical stress intensity values for the crack growth results for the three orientations was within the scatter of the data, so there was essentially no difference. The same was true for the empirically derived Paris Law constants. Residual stresses were likely to have overshadowed any variation in crack growth because of microstructural directionalities associated with build orientation.     

Prediction of short fatigue crack growth of Ti‐6Al‐4V

It is observed that the short fatigue cracks grow faster than long fatigue cracks at the same nominal driving force and even grow at stress intensity factor range below the threshold value for long cracks in titanium alloy materials. The anomalous behaviours of short cracks have a great influence on the accurate fatigue life prediction of submersible pressure hulls. Based on the unified fatigue life prediction method developed in the authors' group, a modified model for short crack propagation is proposed in this paper. The elastic–plastic behaviour of short cracks in the vicinity of crack tips is considered in the modified model. The model shows that the rate of crack propagation for very short cracks is determined by the range of cyclic stress rather than the range of the stress intensity factor controlling the long crack propagation and the threshold stress intensity factor range of short fatigue cracks is a function of crack length. The proposed model is used to calculate short crack propagation rate of different titanium alloys. The short crack propagation rates of Ti‐6Al‐4V and its corresponding fatigue lives are predicted under different stress ratios and different stress levels. The model is validated by comparing model prediction results with the experimental data.     

Local strain accumulation in fatigue crack propagation process of Ti‐6Al‐4V alloy

Fatigue crack growth behaviours of the titanium alloy Ti‐6Al‐4V, with two different microstructures, at different maximum stresses were identified by digital image correlation technique. Full‐field strains were monitored around fatigue cracks after consecutive cycles in fatigue crack growth experiments. Results indicated that the Ti‐6Al‐4V alloy with a bi‐modal microstructure had a better fatigue resistance than that with a primary‐α microstructure. Typical behaviours of small cracks and the evolution of multi‐scale fatigue cracks were clarified. The strain accumulations around the micro‐notch and fatigue crack increased with increasing number of load cycles. On the basis of von Mises strain mapping, it was found that crack growth rate could be characterized by crack‐tip plastic zone size.     

Probabilistic modeling of fatigue crack growth in Ti–6Al–4V

This paper proposes an approximate approach to efficient estimation of some variabilities caused by the material microstructural inhomogeneities. The approach is based on the results of a combined experimental and analytical study of the probabilistic nature of fatigue crack growth in Ti–6Al–4V. A simplified experimental fracture mechanics framework is presented for the determination of statistical fatigue crack growth parameters from two fatigue tests. The experimental studies suggest that the variabilities in long fatigue crack growth rate data and the Paris coefficient are well described by the log-normal distributions. The variabilities in the Paris exponent are also shown to be well characterized by a normal distribution. The measured statistical distributions are incorporated into a probabilistic fracture mechanics framework for the estimation of material reliability. The implications of the results are discussed for the probabilistic analysis of fatigue crack growth.     

Macro and microscopic observations of fatigue crack growth in friction stir welded aluminum joints

This paper presents the macro and microscopic fractography performed on fractures from fatigue cracks through friction stir welded joints. The welds were placed under different angles in the various specimens to study the influence of the yield strength and residual stress on fatigue crack growth. As a result, different behavior was observed at the macro level, depending on the type of alloy and orientation of the weld. The variations in rotation of the crack plane raised a number of questions regarding the mode of loading, i.e. mode I or mode II. The purpose of this study was to investigate the fracture surfaces at microscopic level to find explanations for the local macro behavior. Special focus was placed on the fracture surfaces on which features were observed indicating mode II fatigue crack growth.     

Effect of post-weld heat treatment on fracture toughness and fatigue crack growth behaviour of electron beam welds of a titanium (α+β) alloy

The effects of a post-weld heat treatment on the fracture toughness and fatigue crack growth behaviour of electron beam welds of an α + β titanium alloy, Ti–6.5Al–1.9Zr–0.25Si have been studied. Welds in the stress-relieved condition exhibited poor fracture toughness due to poor energy absorbing capacity of the thin α and α' phases. Post-weld heat treatment which resulted in the decomposition of α' to α + β and the coarsening of intragranular and intergranular α resulted in improved toughness. This improvement in the toughness is related to improved ductility leading to crack blunting, crack path deviation at the thick intragranular and intergranular α phase. Fatigue crack growth resistance of welds was superior to the base metal in the α + β heat-treated condition. The superior crack growth resistance of the welds is due to the acicular α microstructure which results in a tortuous crack path and possible crack closure arising from crack path tortuosity.     

搅拌摩擦与氩弧焊铝合金接头疲劳性能的比较

建立了铝合金焊接接头的S-N曲线,对比分析了搅拌摩擦和氩弧焊两种工艺对其焊接接头疲劳性能的影响,结果表明:在载荷相同的条件下,铝合金搅拌磨擦焊接接头的疲劳性能优于氩弧焊接头,搅拌摩擦焊接头的疲劳寿命N=106次的疲劳强度为59-65 MPa,搅拌摩擦焊接接头具有比氩弧焊接头更为细小的晶粒和狭窄的焊接热影响区,阻碍了滑移带的形成和裂纹的扩展,从而提高了接头的疲劳性能,铝合金焊接接头的缺陷是主要的疲劳裂纹源.     

Fatigue behaviour of dissimilar friction stir spot welds between A6061‐T6 and low carbon steel sheets welded by a scroll grooved tool without probe

A6061 and low carbon steel sheets, whose thicknesses were 2 mm, were welded by a friction stir spot welding (FSSW) technique using a scroll grooved tool without probe (scroll tool). Tensile‐shear fatigue tests were performed using lap‐shear specimens at a stress ratio R = 0.1, and the fatigue behaviour of dissimilar welds was discussed. Tensile‐shear force of the dissimilar welds was higher than that of the A6061 similar ones. Furthermore, the dissimilar welds exhibited nearly the same fatigue strengths as the A6061 similar ones, indicating FSSW by a scroll tool was effective technique for joining aluminium to steel sheet. Fatigue fracture modes of the dissimilar welds were dependent on load levels, where shear fracture through the interface between A6061 and steel occurred at high load levels, while crack grew through A6061 sheet at low load level.     

Strength and fatigue strength of a similar Ti‐6Al‐2Sn‐4Zr‐2Mo‐0.1Si linear friction welded joint

Strengths for monotonic and cyclic loadings of similar overmatching Ti‐6Al‐2Sn‐4Zr‐2Mo‐0.1Si (Ti6242) linear friction welds (LFW) were studied and compared with the parent material (PM) behaviour. Non‐destructive synchrotron observations revealed the presence of pores in the weld interface. The weld centre zone (WCZ) showed a higher strength leading to lower macroscopic ductility of the cross‐weld samples. Local strain and normalized strain rate have been assessed by stereo digital image correlation (DIC) and revealed an early plastic activity at yielding in the vicinity of the WCZ attributed to residual stresses. For the target life, the fatigue strength was slightly reduced but compromised by a strong scatter. Indeed, an internal fish‐eye fatigue crack initiation was found on an unexpected dendritic defect that was very different from the PM microstructure and the known martensitic α′ in the WCZ. The dendritic defect was linked to surface contamination prior to welding and led to melting.     

Fatigue behaviour of friction stir welded AA2024‐T3 alloy: longitudinal and transverse crack growth

The fatigue crack growth properties of friction stir welded joints of 2024‐T3 aluminium alloy have been studied under constant load amplitude (increasing‐ΔK), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka's method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold ΔK values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K_C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non‐conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non‐conservative crack growth rate predictions next to K_C instability. At threshold ΔK values non‐conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered.     

Initiation and growth behaviour of small internal fatigue cracks in Ti‐6Al‐4V via synchrotron radiation microcomputed tomography

Small internal fatigue cracks initiated in Ti‐6Al‐4V in the very high cycle regime were detected by synchrotron radiation microcomputed tomography (SR‐μCT) at SPring‐8 in Japan. The initiation and growth behaviours of the cracks were nondestructively observed, and the da/dN‐ΔK relationship was measured and compared with that obtained in a high vacuum environment. SR‐μCT revealed that more than 20 cracks were initiated in one specimen. The crack initiation life varied widely from 20% to 70% of the average fatigue life and had little influence on the growth behaviour that followed. The initiation site size of each internal crack detected in one specimen was comparable with the size of the fracture origins obtained in ordinary fatigue tests. These results suggest that the surrounding microstructures around the initiation site are likely a dominant factor on the internal fracture rather than the potential initiation site itself. The internal crack growth rates were lower than 10^?10 m/cycle, and extremely slow rates ranging from 10^?13 to 10^?11 m/cycle were measured in a lower ΔK regime below 5 MPa√m. The internal crack growth rate closely matched that of surface cracks in a high vacuum, and the reason for the very long life of internal fatigue fractures was believed to result from the vacuum‐like environment inside the internal cracks.     

Fracture analysis of dissimilar Al‐Al friction stir welded joints under tensile/shear loading

In this research, fracture of dissimilar friction stir welded (FSWed) joint made of Al 7075‐T6 and Al 6061‐T6 aluminum alloys is investigated in the cracked semi‐circular bend (CSCB) specimen under mixed mode I/II loading. Due to the elastic‐plastic behavior of the welded material and the existence of significant plastic deformations around the crack tip at the propagation instance, fracture prediction of the FSWed specimens needs some failure criteria in the context of the elastic‐plastic fracture mechanics which are very complicated and time‐consuming. For this purpose, the Equivalent Material Concept (EMC) is used herein by which the tensile behavior of the welded material is equated with that of a virtual brittle material. By combining EMC with the 2 brittle fracture criteria, namely the maximum tangential stress (MTS) and mean stress (MS) criteria, the load‐carrying capacity (LCC) of the FSWed CSCB specimens is predicted. Comparison of the experimental results and theoretical predictions from the 2 criteria showed that both criteria could accurately predict the LCC of the cracked specimens. Moreover, as the contribution of mode II loading increases, the size of the plastic region around the crack tip at failure increases, leading to increasing the LCC.     

Fretting fatigue crack analysis in Ti–6Al–4V

A study was conducted to verify the efficacy of a fracture mechanics methodology to model the crack growth behavior of fretting fatigue-nucleated cracks obtained under test conditions similar to those found in turbine engine blade attachments. Experiments were performed to produce cracked samples, and fretting fatigue crack propagation lives were calculated for each sample. Cracks were generated at 10⁶ cycles (10%-of-life) under applied stress conditions previously identified as the fretting fatigue limit conditions for a 10⁷ cycle fatigue life. Resulting cracks, ranging in size from 30 to 1200 μm, were identified and measured using scanning electron microscopy. Uniaxial fatigue limit stresses were determined experimentally for the fretting fatigue-cracked samples, using a step loading technique, for R=0.5 at 300 Hz. Fracture surfaces were inspected to characterize the fretting fatigue crack front indicated by heat tinting. The shape and size of the crack front were then used in calculating ΔKth values for each crack. The resulting uniaxial fatigue limit and ΔKth values compared favorably with the baseline fatigue strength (660 MPa) for this material and the ΔKth value (2.9 MPa√m) for naturally initiated cracks tested at R=0.5 on a Kitagawa diagram.Crack propagation lives were calculated using stress results of FEM analysis of the contact conditions and a weight function method for determination of ΔK. Resulting lives were compared with the nine million-cycle propagation life that would have been expected in the experiments, if the contact conditions had not been removed. Scatter in the experimental results for fatigue limit stresses and fatigue lives had to be considered as part of an explanation why the fatigue life calculations were unable to match the experiments that were modeled. Analytical life prediction results for the case where propagation life is observed to be very short experimentally were most accurate when using a coefficient of friction, μ=1.0, rather than for the calculations using μ=0.3     

Fracture toughness of ISO 3183 X80M (API 5L X80) steel friction stir welds

Friction stir welding (FSW) is a solid-state joining process with numerous advantages such as good dimensional stability and repeatability, which is widely used Al alloys and with a great potential for critical joining applications involving high melting temperature alloys. Twelve millimeter thick plates of ISO 3183 X80M (API 5L X80) steel was friction stir welded using two passes on both sides of the plate using ceramic tools. Different heat inputs were obtained using a fix travel (welding) speed in combination with several spindle speeds. The fracture toughness of the two-pass joints was evaluated at 25 °C using the critical crack tip opening displacement (CTOD_m), revealing that joints produced with lower spindle speeds presented higher toughness at the heat-affected zone (HAZ) and stir zone (SZ), which are comparable with the base metal (BM) toughness. On the other hand, joints produced using higher spindle speeds presented low fracture toughness at the SZ and elevated CTOD_m toughness at the HAZ. The joints produced with low spindle speeds showed CTOD_m-values above the offshore standard (DNV-OS-F101) requirements.     

Fatigue performance of hybrid overlap friction stir welding and adhesive bonding of an Al‐Mg‐Cu alloy

The need for weight reduction and leaner manufacturing and assembly processes in aircraft construction has led to the pursuit of welding technologies. One such technology that has been considered for this application is friction stir welding (FSW). Since it is a solid‐state joining method, it creates high performing joints in a wide range of materials while avoiding overlap lengths and added weight from fasteners, crack stoppers, doublers, etc. However, the adoption of this technology to the assembly of large fuselage shell components is challenging, due to geometric tolerance management requirements. In this paper, a hybrid joining method is proposed for such application, involving FSW and adhesive bonding. Fatigue performance of single lap joints of AA2024‐T3 Al‐Mg‐Cu alloy was assessed and benchmarked against FSW overlap and adhesive bonded joints. Significant strength and ductility increase was achieved through the hybridization of the overlap FSW joints. Fatigue strength of the hybrid joints was also higher than FSW overlap joints, although not as high as adhesive bonded joints.     

Effects of stress ratio and temperature on fatigue crack growth in a Ti–6Al–4V alloy

Fatigue thresholds and fatigue crack growth (FCG) rates in corner notched specimens of a forged Ti–6Al–4V aero-engine disk material were investigated at room temperature and 350 °C. The threshold stress intensity range, ΔK_th, was determined by a method involving a step change in stress ratio (the ‘jump in’ method). It was found that for three high stress ratios (R=0.7–0.9), where crack closure effects are widely accepted to be negligible, there were similar ΔK_th values at room temperature and 350 °C under the same R. For a given temperature, ΔK_th was observed to decrease from 3.1 to 2.1 MPam with R increasing from 0.7 to 0.9. The fatigue crack growth rate was influenced by increasing temperature. For high stress ratios, FCG rate at 350 °C was higher than that at room temperature under the same ΔK. For a low stress ratio (R=0.01), higher temperature led to higher FCG rates in the near-threshold regime, but showed almost no effect at higher ΔK. The influence of stress ratio and temperature on threshold and FCG rates was analysed in terms of a K_max effect and the implication of this effect, or related mechanisms, are discussed. In light of this, an equation incorporating the effects of the K_max and fatigue threshold, is proposed to describe FCG rates in the near-threshold and Paris regimes for both temperatures. The predictions compare favourably with experimental data.