Influence of process parameters on mechanical performance and bonding area of AA2024/carbon-fiber-reinforced poly(phenylene sulfide) friction spot single lap joints

The effects of friction spot joining process parameters on the bonding area and mechanical performance of single lap joints were investigated using full-factorial design of experiments and analysis of variance. On one hand, the main process parameters with significant influence on the bonding area were joining pressure, tool rotational speed and joining time. On the other hand, tool rotational speed and joining pressure displayed the highest influence on the lap shear strength of the joints followed by tool plunge depth, whereas the joining time was not statistically significant. The interaction between the rotational speed and joining time was the only interaction with a significant effect on the mechanical performance. Joints with ultimate lap shear forces varying between 1698 ± 92 N and 2310 ± 155 N were obtained. It was observed that generally a larger bonding area as a result of higher heat input leads to an increased mechanical performance of the joints. The generated regression model by the analysis of variance was used to identify an optimized set of parameters for increasing the lap shear strength of the joints to 2280 ± 88 N. Furthermore, the process temperature was monitored, which varied in the range of 370–474 °C.     

Prediction of fatigue life in aircraft double lap bolted joints using several multiaxial fatigue criteria

In this research, the effects of torque tightening on the fatigue strength of 2024-T3 aluminium alloy double lap bolted joints have been studied via experimental and multiaxial fatigue analysis. To do so, three sets of the specimens were prepared and each subjected to different levels of torque i.e. 1, 2.5 and 5 N m and then fatigue tests were carried out at various cyclic longitudinal load levels. A non-linear finite element ANSYS code was used to obtain stress and strain distribution in the joint plates due to torque tightening of bolt and longitudinal applied loads. Fatigue lives of the specimens were estimated with six different multiaxial fatigue criteria by means of local stress and strain distribution obtained from finite element analysis. Multiaxial fatigue analysis and experimental results revealed that the fatigue life of double lap bolted joints were improved by increasing the clamping force due to compressive stresses which appeared around the hole.     

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 different carbon nano-fillers on rheological properties and lap shear strength of epoxy adhesive joints

In this work, the rheological properties, thermal stability and the lap shear strength of epoxy adhesive joints reinforced with different carbon nano-fillers such as multi-walled carbon nanotubes (CNT), graphene nanoplatelets (GNP) and single-walled carbon nanohorns (CNH) have been studied. The nano-fillers were dispersed homogeneously using Brabender® Plasti-Corder®. The epoxy pre-polymer with and without the nano-fillers exhibited shear thinning behavior. The nano-filler epoxy mixtures exhibited a viscoplastic behavior which was analyzed using Casson’s model. Thermo-gravimetric analysis indicated an increase in the thermal stability of the epoxy with the addition of carbon nano-fillers. Carbon nano-fillers resulted in increased lap shear strength having high Weibull modulus. The joint strength increased by 53%, 49% and 46% with the addition of 1 wt.% CNT, 0.5 wt.% GNP and 0.5 wt.% CNH, respectively. The strength of the joints having high filler content (>1 wt.%) was limited by mixed mode type of failure.     

Experimental and numerical investigations about the combined effect of interference fit and bolt clamping on the fatigue behavior of Al 2024-T3 double shear lap joints

In this paper the fatigue behavior of double shear lap joints treated by different combinations of interference fit and bolt clamping have been investigated both experimentally and numerically. To do so, specimens made from aerospace structural material of aluminum alloy 2024-T3 plates were interference fitted at the sizes of 1.5% and 4.7% and torque tightened with 2 and 4 N m to be prepared for fatigue tests. Consequently, the joints were subjected to cyclic load at different levels to obtain fatigue life. Finite element (FE) analysis was also performed to find the stress and strain distributions and the results were used to help explain the trends observed in the experimentally obtained S–N data. The experimental tests showed that during the interference fit process a protruded region is created at around the hole in the exit plane due to directional material plastic flow as a consequence of the oversized bolt force fitting. This protruded region has a bigger height for the bigger interference fit size. The finite element results showed that the protruded region generally localizes the compressive effect of bolt clamping and reduces its capability in fatigue life enhancement, by relaxing the clamping force. The fatigue test results showed that a better fatigue life improvement was achieved by employing the combination of a smaller interference fit size and bigger clamping force.     

Microstructural evolution of aluminum/magnesium lap joints welded using MIG process with zinc foil as an interlayer

Dissimilar material welding between 1 mm-thick magnesium and aluminum alloy plates in lap form was performed using the MIG process with zinc foil as the interlayer material. The zinc foil acted as a barrier layer that restrained reactions between the aluminum and magnesium atoms, and a crack-free lap joint of dissimilar materials was obtained. The interfacial layer between the fusion zone and the unmelted magnesium substrate was mainly composed of Mg–Zn binary intermetallic compounds. The tensile strength of the lap joint was 64 MPa, and a fracture occurred at the interface between the fusion zone and the unmelted magnesium alloy.     

Evaluation of microstructures and mechanical properties of friction stir welded lap joints of Inconel 600/SS 400

The microstructures and mechanical properties of friction stir welded Inconel 600 and SS 400 lap joints were evaluated in this study. Friction stir welding was carried out at a tool rotation speed of 200 rpm and a welding speed of 100 mm/min. Application of friction stir welding was notably effective in reducing the grain size of the stir zone, as a result, the average grain size of Inconel 600 was reduced from 20 μm in the base material to 8.5 μm in the stir zone. The joint interface between Inconel 600 and SS 400 was soundly welded without voids and cracks, and MC carbides with a size of 50 nm were partially formed in the region of the lap joint interface in Inconel 600. In addition, a hook from SS 400 was formed on the advancing side of the Inconel 600 alloy, which directly affected an increase in the peel strength of the weld. In this study, we systematically discussed the effect of friction stir welding on the evolution of the microstructures and mechanical properties of friction stir lap jointed Inconel 600 and SS 400.     

Environmental effects on fatigue behavior of adhesively-bonded pultruded structural joints

The fatigue response of adhesively-bonded pultruded GFRP double-lap joints has been investigated under different environmental conditions. Tests were performed at ?35 °C, 23 °C and 40 °C. A fourth set of fatigue data was collected from tests on preconditioned specimens in warm (40 °C) water. The tests were performed at 40 °C and at 90% relative humidity. Specimens were instrumented with strain and crack gages to record fatigue data. In addition to the S–N curves, stiffness fluctuations and crack initiation and propagation during fatigue were monitored. The dominant failure mode was a fiber-tear failure that occurred in the mat layers of the GFRP laminates. In the presence of high humidity, the failure shifted to the adhesive/composite interface. Although the testing temperature was lower than the glass transition temperature of the adhesive, its influence on the fatigue life and fracture behavior of the examined joints was apparent and was aggravated by the presence of humidity.     

Investigation the effect of tightening torque on the fatigue strength of double lap simple bolted and hybrid (bolted–bonded) joints using volumetric method

In this research, the effect of the tightening torque on the fatigue strength of 2024-T3 double lap simple bolted and hybrid (bolted–bonded) joints have been investigated experimentally by conducting fatigue tests and numerically by implementing finite element analysis. To do so, three sets of specimens were prepared and each of them subjected to tightening torque of 1, 2.5 and 5 Nm and then fatigue tests were carried out under different cyclic longitudinal load levels. In the numerical method, the effect of the tightening torque on the fatigue strength of the considered joints has been studied by means of volumetric method. To obtain stress distribution around the notch (bolt hole) which is required for the volumetric method, nonlinear finite element simulations were carried out. In order to estimate the fatigue life, the available smooth S–N curve of Al2024-T3 and the fatigue notch factors obtained from the volumetric method were used. The estimated fatigue life was compared with the available experimental test results. The investigation shows that there is a good agreement between the life predicted by the volumetric method and the experimental results for different specimens with a various amount of tightening torques. The results obtained from the experimental analysis showed that the hybrid joints have a better fatigue strength compared to the simple bolted joints. In addition, the volumetric method and experimental results revealed that the fatigue life of both kinds of the joints were improved by increasing the clamping force resulting from the torque tightening due to compressive stresses which appeared around the bolt hole.     

Fatigue behavior and modeling of short fiber reinforced polymer composites including anisotropy and temperature effects

Effects of anisotropy and temperature on cyclic deformation and fatigue behavior of two short glass fiber reinforced polymer composites were investigated. Fatigue tests were conducted under fully-reversed (R = −1) and positive stress ratios (R = 0.1 and 0.3) with specimens of different thicknesses, different fiber orientations, and at temperatures of −40 °C, 23 °C, and 125 °C. In samples with 90° fiber orientation angle, considerable effect of thickness on fatigue strength was observed. Effect of mold flow direction was significant at all temperatures and stress ratios and the Tsai–Hill criterion was used to predict off-axis fatigue strengths. Temperature also greatly influenced fatigue strength and a shift factor of Arrhenius type was developed to correlate fatigue data at various temperatures, independent of the mold flow direction and stress ratio. Micromechanisms of fatigue failure at different temperatures were also investigated. Good correlations between fatigue strength and tensile strength were obtained and a method for obtaining strain–life curves from load-controlled fatigue test data is presented. A fatigue life estimation model is also presented which correlates data for different temperatures, fiber orientations, and stress ratios.     

Achieving superior mechanical properties in friction lap joints of copper to carbon-fiber-reinforced plastic by tool offsetting

It is a challenge to achieve a sound welded metal/carbon-fiber-reinforced thermoplastic (CFRTP) joint with high strength and few bubbles. In this study, sound lap joints of Cu and CFRTP were obtained by friction lap joining (FLJ) directly at rotation rates of 600–2000 rpm, with the welding tool at the joint center and offsetting the tool 7 mm away from the center toward the retreating side, respectively. Tool offsetting reduced the non-uniform temperature distribution in the lap joints resulting from the high conductivity of Cu, which not only enhanced the tensile shear force from 0.89–2.25 kN to 1.71–3.54 kN, with the maximum increasing rate of 135%, but also reduced the bubble area to only 19% of the original level of 2000 rpm. It is the first time to report a high-quality Cu/CFRTP joint with a high strength and few bubbles. The large increase of the strength after tool offsetting was attributed to the increase of the joining area, the decrease of bubbles and the decrease of the CFRTP degradation. The details on the generation, quantitative distribution and expulsion of the bubbles in the FLJ joints were discussed.     

Friction Spot Joining of aluminum AA2024/carbon-fiber reinforced poly(phenylene sulfide) composite single lap joints: Microstructure and mechanical performance

Friction Spot Joining is a promising alternative joining technology for polymer–metal hybrid structures. In this work, the feasibility of Friction Spot Joining of aluminum AA2024-T3 (bare and alclad)/carbon-fiber reinforced poly(phenylene sulfide) is reported. The process temperature and the microstructure of the joints were investigated. Lap shear tensile strength as high as 27 MPa was achieved by using aluminum bare specimens. Sand blasting was also performed as an effective mechanical surface pre-treatment on aluminum surfaces, which resulted in higher surface roughness and accordingly improved mechanical performance for the selected conditions. In addition, the alclad specimens exhibited promising mechanical performance (lap shear strength of up to 43 MPa) that justifies further investigations. Finally, the bonding and failure mechanisms of the joints are briefly discussed.     

Parametric study of the creep failure of double lap adhesively bonded joints

In this paper the influence of adhesive thickness and adhesive fillet on the creep deformation and creep life time of the adhesively bonded double lap joint have been studied experimentally. Also finite element modeling was used to simulate creep behavior of bonded joints and the results are compared with those obtained from experimental tests. The adhesive used in this research was Araldite 2015 which is an epoxy based adhesive. Research procedure is carried out in two major stages. Firstly, uniaxial creep tests were conducted in 63 °C to obtain the creep characteristics and constitutive equation parameters of the adhesive at 63 °C. An empirical based rheological model based on Maxwell and Zener’s model is proposed to simulate the creep behavior of the adhesive and it is used to predict the creep behavior of the bonded joint using finite element method. Numerical results show good agreement with experimental data. It was observed that applying fillet increases creep life and decreases joint creep deformation, however increasing adhesive thickness has slight effect on the creep life time of the joint.     

Influence of tool geometry and rotational speed on mechanical properties and defect formation in friction stir lap welded 5456 aluminum alloy sheets

Friction stir welding of AA5456 aluminum alloy in lap joint configuration is with two different tempers, T321 and O, and different thicknesses, 5 mm and 2.5 mm was investigated. The influences of tool geometry and various rotational speeds on macrostructure, microstructure and joint strength are presented. Specifically, four different tool pin profiles (a conical thread pin, a cylindrical–conical thread pin, a stepped conical thread pin and Flared Triflute pin tool) and two rotational speeds, 600 and 800 rpm, were used. The results indicated that, tool geometry influences significantly material flow in the nugget zone and accordingly control the weld mechanical properties. Of particular interest is the stepped conical threaded pin, which is introduced for the first time in the present investigation. Scanning electron microscopy investigation of the fracture location of samples was carried out and the findings correlated with tool geometry features and their influences on material flow and tension test results. The optimum microstructure and mechanical properties were obtained for the joints produced with the stepped conical thread pin profile and rotational speed of 600 rpm. The characteristics of the nugget zone microstructure, hooking height, and fracture location of the weld joints were used as criteria to quantify the influence of processing conditions on joint performance and integrity. The results are interpreted in the framework of physical metallurgy properties and compared with published literature.     

Mechanical,fatigue and microstructural properties of friction stir welded 5083-H111 and 6082-T651 aluminum alloys

In this study, 5083-H111 and 6082-T651 aluminum alloy plates in 6 mm thickness that are used particularly for shipbuilding industry were welded using Friction Stir Welding (FSW) method as similar and dissimilar joints with one side pass at PA position with the parameters of 1250 rpm tool rotation, 64 mm/min welding speed and 2° tool tilt angle. Tensile tests results showed sufficient joint efficiencies and surprisingly high yield stress values. Bending fatigue test results of all joint types showed fatigue strength close to each other. Fatigue strength order of the joints were respectively FSWed 5083-5083, and 6082-6082 similar joints and 5083-6082 dissimilar joint. Cross sections of the weld zones have been analyzed with light optical microscopy (LOM) and fracture surface of fatigue test specimens were examined by scanning electron microscopy (SEM). Although there were no voids in radiographic and microscopic analyzes, 5083-6082 joint showed rarely encountered voiding effect under fatigue load. Microhardness measurements revealed rare result for FSWed AW5083 and novel result for FSWed 6082 aluminum alloy.     

Fatigue assessment of root failures in HSLA steel welded joints: A comparison among local approaches

Fatigue tests were performed on welded joints made of high-strength, low-alloy steel (S690). Different welding processes were tested, resulting in welds with different defects essentially consisting in lack of penetration. Fatigue tests were run with both constant and variable amplitude loading. The experimental results were compared to predictions obtained by applying local approaches (local stress and local strain) and the concepts of fracture mechanics. The local stress approach allowed the fatigue strength of joints in constant amplitude loading (for fatigue above 2 × 10⁶) to be predicted, but the assumption of a constant value of the slope k = 3 for all S–N curves led to non-conservative predictions of shorter lives. The local strain approach allowed the fatigue strength of the joints under constant amplitude to be predicted. Although, these predictions matched the experimental data well for both small and large defects in the entire cycle number range, they failed to predict the behaviour of joints under variable amplitude loading. Conversely, the fracture mechanics approach proved to be more efficient in predicting the fatigue behaviour of welded joint under variable amplitude loading.     

Effect of thin oblique moving plates on long rod projectiles: A reverse impact study

The geometry and motion of long rod projectiles after penetrating thin obliquely oriented and moving armour plates were studied. Plates moving in their normal directions towards as well as away from the projectile (scalar product of velocities negative and positive, respectively) were considered. The influences of plate velocity and obliquity (angle between the normal of the plate and the axis of the projectile) were investigated through small-scale reverse impact tests with tungsten projectiles of length 30 mm and diameter 2 mm, and with 2 mm-thick steel plates. The obliquity (30°, 60° and 70°) and the plate velocity (300 to −300 m/s) were varied systematically for a projectile velocity of 2000 m/s. The disturbing effect of the plate on the projectile was characterised in terms of changes in length, velocity, angular momentum, linear momentum and kinetic energy. Plates with obliquity 60–70° moving away from the projectiles with velocity 200–300 m/s were found to cause extensive fragmentation of the projectile and to have large disturbing effects in terms of all measures used.     

The preparation and mechanical properties of carbon–carbon/lithium–aluminum–silicate composite joints

Silica carbide modified carbon cloth laminated C–C composites have been successfully joined to lithium–aluminum–silicate (LAS) glass–ceramics using magnesium–aluminum–silicate (MAS) glass–ceramics as interlayer by vacuum hot-press technique. The microstructure, mechanical properties and fracture mechanism of C–C/LAS composite joints were investigated. SiC coating modified the wettability between C–C composites and LAS glass–ceramics. Three continuous and homogenous interfaces (i.e. C–C/SiC, SiC/MAS and MAS/LAS) were formed by element interdiffusions and chemical reactions, which lead to a smooth transition from C–C composites to LAS glass–ceramics. The C–C/LAS joints have superior flexural property with a quasi-ductile behavior. The average flexural strength of C–C/LAS joints can be up to 140.26 MPa and 160.02 MPa at 25 °C and 800 °C, respectively. The average shear strength of C–C/LAS joints achieves 21.01 MPa and the joints are apt to fracture along the SiC/MAS interface. The high retention of mechanical properties at 800 °C makes the joints to be potentially used in a broad temperature range as structural components.     

Effect of welding parameters on mechanical and microstructural properties of dissimilar AA6082–AA2024 joints produced by friction stir welding

The effect of processing parameters on the mechanical and microstructural properties of dissimilar AA6082–AA2024 joints produced by friction stir welding was analysed in this study. Different samples were produced by varying the advancing speeds of the tool as 80 and 115 mm/min and by varying the alloy positioned on the advancing side of the tool. In all the experiments the rotating speed is fixed at 1600 RPM. All the welds were produced perpendicularly to the rolling direction for both the alloys. Microhardness (HV) and tensile tests performed at room temperature were used to evaluate the mechanical properties of the joints. The mechanical tests were performed on the joints previously subjected to annealing at 250 °C for 1 h. For the fatigue tests, a resonant electromechanical testing machine was employed under constant loading control up to 250 Hz sine wave loading. The fatigue tests were conducted in the axial total stress–amplitude control mode, with R = σ_min/σ_max = 0.1. In order to analyse the microstructural evolution of the material, the welds’ cross-sections were observed optically and SEM observations were made of the fracture surfaces.     

Creep–fatigue damage for a structure with dissimilar metal welds of modified 9Cr–1Mo steel and 316L stainless steel

A creep–fatigue test with a structural specimen made of Mod. 9Cr–1Mo steel and 316L stainless steel has been carried out and the test results were compared with those of the evaluations by the high temperature design codes of ASME subsection NH and RCC-MR to quantify the conservatism. A specimen with a diameter of 500 mm, height of 440 mm and thickness of 6.3 mm was subjected to creep–fatigue loads with two hours of a hold time at 600 °C and a primary nominal stress of 30 MPa. The creep–fatigue behaviours of the dissimilar metal welds as well as the similar metal welds were investigated and the results of the test were compared with the evaluation results. Bimetallic (direct) transition metal joint and trimetallic transition metal joint for a dissimilar metal weld were employed for a specimen, and their behaviours under a creep–fatigue load were compared. The conservatism of the design codes on the creep–fatigue evaluation at the welded joints as well as at the base metal with an emphasis on Mod.9Cr–1Mo steel are highlighted through comparisons with the results from the observation and the evaluation.