Experimental and numerical analyses of mean stress relaxation in cold expanded plate of Al‐alloy 2024‐T3 in double shear lap joints

In this paper, the mean stress relaxation behavior of simple Al‐alloy 2024‐T3 specimens and also the mean stress relaxation around the hole of cold expanded specimen are studied. The analyses are performed through the combination of the nonlinear isotropic hardening and Chaboche nonlinear kinematic hardening model accompanied by the results of experimental tests. The strain‐controlled axial tests are performed at two different strain amplitudes, while the stress‐controlled tests of cold expanded specimens are performed for three different imposed load amplitudes. The constitutive equations of the hardening model are coded as a UMAT subroutine in FORTRAN programming language and implemented in the commercial finite element code of ABAQUS. The accuracy of the hardening model has been proved in two steps: first by simulations of mean stress relaxation during the uniaxial strain‐controlled cyclic tests and second by simulation of strain ratcheting during the stress‐controlled cyclic loading. The stress and strain distributions after cold expansion process are examined as well as the mean stress relaxation due to cyclic loading. The results show the influences of imposed stress amplitude on increasing mean stress relaxation and also the effect of cold expansion level on reducing the mean stress relaxation.     

Investigation of cold expansion of short edge margin holes with pre‐existing cracks in 2024‐T351 aluminium alloy

The United States Air Force has requirements to inspect and cold expand potentially thousands of fastener holes for an aircraft fleet, and the presence of existing cracks at those fastener holes is expected. Fatigue experiments were performed to investigate the resulting fatigue crack growth life of a fastener hole that contained a representative ‘unknown’ crack at the time of inspection (approximately 0.050 in. in length) at a short edge margin hole that was then cold expanded and compare that to a non‐cold expanded hole and a cold expanded hole with no pre‐existing cracks. The United States Air Force analytical approach used to account for the benefit due to cold expansion was compared to the experimental data and does not consistently provide conservative predictions.     

Strain ratcheting and stress relaxation around interference‐fitted single‐holed plates under cyclic loading: experimental and numerical investigations

Longitudinal strain ratcheting and stress relaxation in interference‐fitted single‐holed plates were investigated both experimentally and numerically. In the experimental part single‐holed plates made from Al‐alloy 7075‐T6 were force‐fitted with oversized pins to create 1% and 2% nominal interference fit sizes. Then these plates (specimens) were instrumented with dynamic strain gauges in longitudinal direction around the hole to measure the strain during interference fit and strain ratcheting during subsequent cyclic loading. In the numerical part, 2D finite element code has been written to simulate the interference fit process and subsequent cyclic loading to obtain strains and stresses around the force fitted hole. To predict the strain ratcheting, Ohno–Wang kinematic hardening model was applied for simulation of stress/strain path. The strain ratcheting predicted from the finite element code and experimental test results were compared. The results showed that there is a good agreement between the measured and numerically evaluated strains, and the strain ratcheting is bigger for higher cyclic load level, but it is smaller for larger interference size.     

Effect of stress amplitude on uniaxial ratcheting of aluminum alloy 2124‐T851

This paper presents the latest research results of ratcheting behavior of aluminum alloy 2124‐T851 under uniaxial loading. All fatigue tests were carried out at the Technical University of Ostrava mainly under block loading in order to determine the effect of stress amplitude and mean stress on the strain response of the material. Experimental results show that the critical value of the axial deformation is close to the ductility of the material investigated. The aim of this paper also is to draw important conclusions in terms of ratcheting modelling for the material investigated.     

Effect of cold expansion on the fatigue life of Al 2024-T3 in double shear lap joints: Experimental and numerical investigations

In this paper the effect of cold expansion on fatigue life improvement of aluminum alloy 2024-T3 plates used in double shear lap joints is investigated experimentally by conducting fatigue tests and numerically by implementing finite element simulations. In the experimental part, fatigue tests were carried out on the plates with cold expansion levels of 0%, 1.5% and 4.7% which were used in double shear lap joints. In the numerical study, three-dimensional finite element models were employed to predict stress distributions in the cold expanded plates used in the double shear lap joint. The results obtained from finite element simulation, have been employed to explain the trends which were observed in the experimentally attained S–N data and the fatigue crack initiation location. The experimental and numerical results showed that cold expansion improves fatigue life at low load levels and the life enhancement is more for the bigger cold expansion size. However, the fatigue life improvement is smaller in double shear lap joints compared to a single cold expanded plate.     

Fatigue assessment of refill friction stir spot weld in AA 2024‐T3 similar joints

Refill friction stir spot welding is a solid‐state process technology that is suitable for welding lightweight materials in similar or dissimilar overlapped configuration. In this study, the fatigue behaviour of single overlapped spot joints of AA2024‐T3 was studied. To statistically analyse the fatigue data, a 2‐parameter Weibull distribution was deployed, considering several reliabilities (R_e = 0.99, R_e = 0.90, R_e = 0.5, R_e = 0.10). To obtain an optimized weld parameter according to the fatigue behaviour, 2 different weld conditions were studied, taking into account the effect of the hook formation. The microstructure analyses and microhardness profiles showed great similarity in both weld conditions. However, these conditions presented distinct interfacial hook profiles, in which the interfacial hook downward represented better fatigue life and infinite fatigue life at 15% of the maximum strength load. The fracture surfaces obtained from 3 different fracture modes were investigated by using scanning electron microscopy; the crack was tracked and described according to its fracture mechanisms from its initiation until the final failures. It was observed that the crack is initiated at hook profile.     

Experimental characterisation of a CuAg alloy for thermo‐mechanical applications. Part 1: Identifying parameters of non‐linear plasticity models

Despite the wide use of copper alloys in thermo‐mechanical applications, there is little data on their cyclic plasticity behaviour, particularly for CuAg alloys. This prevents the behaviour of the materials from being correctly described in numerical simulations for design purposes. In this work CuAg0.1 alloy used for thermo‐mechanical applications was tested by strain‐controlled cyclic loading at 3 different temperatures (room temperature, 250°C, 300°C). In each test, stress‐strain cycles were recorded until the alloy had completely stabilised. These cycles were then used to identify material parameters of non‐linear kinematic and isotropic models. The focus was on plasticity models (Armstrong‐Frederick, Chaboche, Voce) that are usually implemented in commercial finite element codes. Simulated cyclic responses with the identified material models were compared with experiments and showed a good agreement. The identified material parameters for the CuAg alloy under investigation can be used directly in finite element models for cyclic plasticity simulations, thus enabling a durability analysis of components under thermo‐mechanical loads to be performed, particularly in the field of steel‐making plants.     

An experimental investigation of the bolt clamping force and friction effect on the fatigue behavior of aluminum alloy 2024-T3 double shear lap joint

In this article, the effect of bolt clamping force on the fatigue life of bolted double shear lap joints was investigated. To do so, fatigue tests were carried out on the bolt clamped double shear lap joint specimens made of aluminum alloy 2024-T3. These fatigue tests were conducted with applied torques of 0.25, 2 and 4 N m at different cyclic longitudinal load levels in un-lubricated and lubricated states. From these tests the stress–life (S–N) data for different clamping forces for un-lubricated and lubricated states were obtained. The results show that clamping force increases fatigue life compared to clearance fit specimens. In general, at higher tightening torque higher fatigue lives were achieved, however, below a certain load level the life improvement was discontinued because of fretting phenomenon. Also lubricating the parts of the specimens reduces the advantage of clamping force or torque tightening.     

Effect of hole lubrication on the fretting fatigue life of double shear lap joints: An experimental and numerical study

In this research the affect that lubrication at a hole and pin connection has on the fatigue life of a double shear lap joint is studied both experimentally and numerically. The study focuses on the joint middle plate item, which is connected via a central hole to the outer plates by means of a clearance fitting pin, thereby placing the hole in double shear. In the experimental work three identical batches of fatigue specimens, which are made from aluminum alloy 2024-T3, were fatigue tested. In the first batch the surface of the fastener hole was not lubricated whilst the hole in the other two batches was lubricated – each batch using a different lubricant. The three batches of double shear lap joint specimens were fatigue tested and their S–N curves established. The results show that the specimens in which the holes were lubricated have better fatigue lives than the non-lubricated hole specimens. In the numerical study, FE simulations were performed to include hole lubrication effect on the stress distribution by using different friction coefficient at the interface of the hole and its fastener (pin). The FE results have helped to gain an understanding of the reasons for fatigue life improvement and also have helped to quantify the level of improvement.     

Finite element investigations of bolt clamping force and friction coefficient effect on the fatigue behavior of aluminum alloy 2024-T3 in double shear lap joint

In this paper, the effect of bolt clamping force on the fatigue life of bolted double shear lap joints was investigated numerically. To do so, finite element simulation results were used to illustrate the trends occurred in experimental fatigue tests showing the effect of bolt clamping on improving the fatigue life of double shear lap joints. The results show that clamping force decreases the resultant longitudinal stress at the hole edge thus the fatigue life increases compared to clearance fit specimens. In general, at higher tightening torque longer fatigue lives were achieved, however, below a certain load level the life improvement was discontinued because of fretting occurrence. Also lubricating the specimens reduces the advantages of the clamping force.     

Impact of slide diamond burnishing additional parameters on fatigue behaviour of 2024‐T3 Al alloy

One of the methods for increasing fatigue life of symmetric rotary metal components is slide diamond burnishing (SDB). This method is implemented on conventional and computer numerical control machine tools by means of simple equipment, which is its main advantage. The SDB basic parameters are diamond insert radius, burnishing force, feed rate, and burnishing velocity. The additional ones are number of passes, working scheme, and lubrication conditions. The effect of SDB additional parameters on the fatigue behaviour of 2024‐T3 Al alloy was experimentally studied. Groups of smooth and notched hourglass‐shaped specimens were slide burnished using different combinations of additional SDB parameters and then were subjected to bending fatigue tests. The residual stresses, introduced by SDB, were measured by X‐ray diffraction technique. The near‐surface microstructure of the slide‐burnished specimens was investigated. Based on the results obtained, it was established that SDB produces two main effects, which depend on SDB additional parameters. The essence of the macroeffect is creation of residual compressive stresses in the superficial and subsurface layers. This stresses retard the formation and growth of fatigue macrocracks and thus increase the lifetime of slide‐burnished components. The microeffect is expressed in modifying the microstructure of the surface and subsurface layers, correspondingly, refining the grain and homogenizing and reducing the pores in the material. Such microstructure is characterized by increased plasticity and fatigue crack resistance. The fatigue life depends on the combination of these two effects. Thus, the desired fatigue behaviour of the slide‐burnished component can be ensured through an appropriate selection of the governing additional SDB parameters.     

Local deformation at micro‐notches and crack initiation in an intermetallic γ‐TiAl‐alloy

The relation is studied between crack initiation from micro‐notches in a fully lamellar intermetallic γ‐TiAl alloy and the local strain field. These micro‐notches were introduced using femtosecond‐laser ablation and had dimensions below the average colony size. The specimen under investigation was then subjected to fatigue loading. Continuous monitoring using a travelling optical microscope allowed detecting microcracks at an early stage. Prior to fatigue loading, a sustained load was applied and the local strain field was determined using digital image correlation. This was supplemented by a Finite Element analysis of the notches and their neighbourhood. It was found that a crack was initiated from a notch causing high normal strains in lamella direction, whereas no crack was initiated from notches with high shear strains.     

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.     

Numerical and experimental investigation of the cold expansion process with split sleeve in titanium alloy TC4

A series of uniaxial fatigue tests were carried out using specimens containing non-cold expanded and cold expanded holes to assess the effect of split sleeve cold expansion on fatigue behavior of titanium alloy TC4. The fracture surfaces of specimens were observed by scanning electron microscope (SEM). 3D finite element models were also used to analyze the residual stress fields around cold expanded holes. Based on the qualitative finite element analysis (FEA), the multi-axial fatigue lives of the non-cold and cold expanded holes have been predicted by Smith–Watson–Topper (SWT) method and Wang–Brown (WB) method respectively. The effects of the friction between the split sleeve and the hole’s surface were also considered. The results reveal that crack of cold expanded specimen always initiates near entrance face and the crack propagation speed along transverse direction is faster than along axial direction. The lowest compressive stress occurs at the entrance face where crack is preferentially initiated. The mandrel entrance face is the most sensitive region to friction between the split sleeve outer surface and the hole. After cold expansion, fatigue life of TC4 open hole was increased to 1.7–2.2 times. Compared with the result of SWT theory, the result of WB theory is more conservative and reliable.     

Plastic CTOD as fatigue crack growth characterising parameter in 2024‐T3 and 7050‐T6 aluminium alloys using DIC

The plastic range of crack tip opening displacement (CTOD) has been used for the experimental characterisation of fatigue crack growth for 2024‐T3 and 7050‐T6 aluminium alloys using digital image correlation (DIC). Analysis of a complete loading cycle allowed resolving the CTOD into elastic and plastic components. Fatigue tests were conducted on compact tension specimens with a thickness of 1 mm and a width of 20 mm at stress ratios of 0.1, 0.3 and 0.5. The range of plastic CTOD could be related linearly to da/dN independent of stress ratio for both alloys. To facilitate accurate measurements of CTOD, a method was developed for correctly locating the crack tip and a sensitivity analysis was performed to explore the effect of measurement position behind the crack tip on the CTOD. The plastic range of CTOD was demonstrated to be a suitable alternate parameter to the stress intensity factor range for characterising fatigue crack propagation. A particularly innovative aspect of the work is that the paper describes a DIC‐based technique that the authors believe gives a reliable way to determine the appropriate position to measure CTOD.     

Monotonic and low cycle fatigue behaviour of 2024‐T3 aluminium alloy between room temperature and 300 °C for designing VAWT components

In the present study, the results of the monotonic tension tests and low cycle fatigue tests performed on aluminium alloy EN AW‐2024‐T3 under various operating temperatures are presented in order to assess the fatigue behaviour of the aluminium alloy under evaluated temperatures. Monotonic tests were performed to determine the influence of temperature on mechanical properties of the material. The aim of cyclic tests was to acquire the parameters required for Manson–Coffin equation in order to plot strain–fatigue life curves. Moreover, stress–strain behaviour of the alloy and the cyclic hardening behaviour were evaluated using Ramberg–Osgood equation. Finally, P_SWT‐damage parameters for each temperature have been calculated for further investigation of the effects of the temperature on fatigue life using acquired data while taking the account of mean stress effect into calculations. Variations in the experimental data due to various test temperatures are presented for both monotonic and cyclic tests.     

Correlation between precipitate microstructure and mechanical properties in AA7075‐T6 aluminum alloy friction stir welded joints

Microstructural evolution and mechanical properties of friction stir welded AA7075‐T6 aluminum alloy were examined. Grain structure and precipitate evolution in the stir zone and heat‐affected zone were evaluated using optical microscope and differential scanning calorimetry. A significant grain refinement and dissolution of η′ precipitates in the stir zone were found, but chromium‐bearing dispersoids remained nearly unchanged. The main particles in the stir zone and heat‐affected zone were η precipitates as well as Guinier‐Preston zones formed during post‐weld natural aging. The small recrystallized grains were observed in the thermo‐mechanically affected zone next to the stir zone. A W‐shaped hardness distribution where soft region was produced in the heat‐affected zone at a short distance from the stir zone were obtained. Hardness profiles of the welds were explained by precipitate distributions. Friction stir welding resulted in the reversion and coarsening of η′ precipitates. The formation of Guinier‐Preston zones in the stir zone and some parts of the heat‐affected zone during post‐weld natural aging increased the hardness. In transverse tensile specimens, fracture occurred in a location with the minimum hardness at either advancing or retreating side randomly. Further, influences of welding parameters on mechanical properties were investigated.     

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.     

Fatigue performance and fatigue damage parameter estimation of spot welded joints of aluminium alloys 6111‐T4 and 5754

In the present study, fatigue behaviours of spot welded joints of aluminium alloys 6111‐T4 and 5754 have been experimentally investigated. Fatigue results indicate that fatigue strength of spot weld primarily depends on specimen loading type and gauge thickness. Effects of base material and load ratio on fatigue resistance of welded specimen are insignificant. An equivalent stress based fatigue damage parameter is derived to consolidate empirical data and develop predictive capabilities for automobile designers. The fatigue damage parameter defined in this study is proven effective in consolidating a large amount of fatigue data into a narrow band and is especially suitable for the comparative fatigue strength evaluation of components and specimens.     

A 3D full‐field study of cracks in a nuclear graphite under mode I and mode II cyclic dwell loading conditions

Three‐dimensional (3D) full‐field deformation around crack tips in a nuclear graphite has been studied under mode I and mode II cyclic dwell loading conditions using digital volume correlation (DVC) and integrated finite element (FE) analysis. A cracked Brazilian disk specimen of Gilsocarbon graphite was tested at selected loading angles to achieve mode I and mode II cyclic dwell loading conditions. Integrated FE analysis was carried out with the 3D displacement fields measured by DVC injected into the FE model, from which the crack driving force J‐integral was obtained using a damaged plasticity material model. The evolution of near‐tip strains and the J‐integral during the cyclic dwell loading was examined. Under cyclic dwell, residual strain accumulation was observed for the first time. The results shed some light on the effect of dwell time on the 3D crack deformation and crack driving force in Gilsocarbon under cyclic mode I and II loading conditions.