Fracture Behavior Of Joints Research Articles

Prediction of Mechanical Properties and Fracture Behavior of TC17 Linear Friction Welded Joint Based on Finite Element Simulation.

TC17 titanium alloy is widely used in the aviation industry for dual-performance blades, and linear friction welding (LFW) is a key technology for its manufacturing and repair. However, accurate evaluation of the mechanical properties of TC17-LFW joints and research on their joint fracture behavior are still not clear. Therefore, this paper used the finite element numerical simulation method (FEM) to investigate the mechanical behavior of the TC17-LFW joint with a complex micro-structure during the tensile processing, and predicted its mechanical properties and fracture behavior. The results indicate that the simulated elastic modulus of the joint is 108.5 GPa, the yield strength is 1023.2 MPa, the tensile strength is 1067.5 MPa, and the elongation is 1.98%. The deviations from measured results between simulated results are less than 2%. The stress and strain field studies during the processing show that the material located at the upper and lower edges of the joint in the WZ experiences stress and strain concentration, followed by the extending of the stress and strain concentration zone toward the center of the WZ. And finally, the strain concentration zone covered the entire WZ. The fracture behavior studies show that the material necking occurs in the TMAZ of TC17(α + β) and WZ, while cracks first appear in the WZ. Subsequently, joint cracks propagate along the TC17(α + β) side of the WZ until fracture occurs. There are obvious tearing edges formed by the partial tearing of the WZ structure in the simulated fracture surface, and there are fracture surfaces with different height differences at the center of the joint crack, indicating that the joint has mixed fracture characteristics.

Deformation behavior study of SAC305 solder joints under shear and tensile loading by crystal plasticity finite element method

Micro solder joints experience various loads during operation, and the failure of a single micro solder joint can impact the overall reliability of the entire microsystem. This study utilized the Crystal Plasticity Finite Element Method (CPFEM) to create a polycrystalline model that accurately represents the shape of solder joints. By calibrating the crystal plasticity characteristics of SAC305 material solder joints using macroscopic stress-strain curves, the model successfully simulates the deformation mechanisms of micro-solder joints under both tensile and shear loads. This paper highlights that equivalent plastic strain serves as a key indicator of solder joint fracture behavior, revealing that strain components have varying effects on cracking: shear strain was found to cause crack initiation, while normal strain was found to promote crack extension. Moreover, Grain boundary features and orientation lead to uneven plastic strain by affecting the slip system and the Schmidt factor, especially small Schmidt factor grains at triple grain boundary intersections are more susceptible to crack initiation. Additionally, the study found that solder joints exhibit more pronounced mechanical responses under tensile loading compared to shear loading. These insights offer valuable guidance for the design and manufacturing of micro-solder joints.

Study on the microstructure and properties of spray formed 7055-T76 aluminum alloy joints by rotating magnetic field assisted friction stir welding

This study explores an electrically-magnetically-assisted friction stir welding (EMAFSW) method, conducting butt welding experiments on T76 tempered spray-formed 7055 aluminum alloy. Compared to traditional friction stir welding (FSW) processes, the EMAFSW method maintained basically consistent weld hardness, while tensile strength and elongation increased by 13.1% and 72.3%, respectively. The rotating magnetic field enhanced the mechanical stirring action of the welding tool, reduced the size of precipitates, promoted dislocation pinning, and increased the mobility of dislocations. The stirred zone (SZ) and thermo-mechanically affected zone (TMAZ) of EMAFSW weld joint showed significant grain refinement and increased recrystallization proportion, enhancing the weld joint toughness. The joint's fracture behavior shifted from brittle to ductile fracture.

The effect of abrading treatment method on the bonding surface characteristics and fracture behavior of glass fiber-epoxy composite adhesive joints

The effect of the mechanical abrading method on the bonding surface characteristics and fracture response of glass fiber-reinforced polymer (GFRP) composite adhesively bonded joints was investigated. Double cantilever beam specimens were manufactured by bonding GFRP substrates with an epoxy adhesive to evaluate the mode-I adhesive joint fracture response. The bonding surfaces were treated with degreasing and abrading methods utilizing sandpapers with various grit sizes ranging from P60 to P800. The results showed that the sandpaper grit size can considerably change the bonding surface properties comprising the surface energy, roughness, morphology, and water drop contact angle and consequently, it can affect the joint fracture behavior. Reducing the sandpaper grit size continuously increased the surface roughness but there were optimum values for the surface energy and water drop contact angle. The fracture resistance of adhesive joints followed the trend of bonding surface energy as the higher the bonding surface energy, the higher the joint fracture resistance. The fracture energy and load-bearing capacity of adhesive joints were improved by 62 and 34%, respectively, when the joint bonding surfaces were abraded with P240. However, further reducing the sandpaper grit size to P60 caused a decrease in the fracture energy and load-bearing capacity of adhesive joints by 18 and 44%, respectively, compared to the joint with degreased and non-abraded bonding surfaces. Moreover, the abrading method considerably changed the microscopic and macroscopic failure modes of adhesive joints.

Fracture prediction of solder joints using two-parameter fracture mechanics

Two-parameter fracture mechanics theory is a novel approach to investigate the fracture behavior of solder joints. The present study showed that the single-parameter fracture mechanics was not valid for evaluating the fracture behavior of these joints. Therefore, the ability of the two-parameter (J-Q) fracture mechanics method was assessed. J (critical strain energy release rate) and Q (The constraint parameter) parameters were calculated for the double cantilever beam (DCB) specimens. The two-parameter fracture mechanics theory was observed to be valid for the DCB specimens examined in this research. The J-Q loci extracted from two distinct experimental data sets (adherend thickness and loading arm variation) were coincident. The overlap of two J-Q locus curves indicates the uniqueness of the J-Q locus and capability to predict fracture load.

Application of data mining techniques for assessment of fracture load and energy in double cantilever beam solder joints

Predicting the exact values of fracture load and energy has a significant effect in preventing solder joint failure. In this research, various double cantilever beam (DCB) solder joints with different geometric constraints, including adherend thickness, adherend width, loading arm length, and solder thickness were tested under mode I crack propagation and at a strain rate of 0.03 . According to ANOVA test results, all the mentioned geometric factors influence the fracture load, but adherend width and solder thickness don't change the failure energy remarkably, considering the type I error is equal to 0.05. The failure load and energy forecasting using the random forest algorithm showed that the prediction accuracy is 92% and 81% respectively. Linear regression and lasso regression were also utilized to identify the solder joint's fracture behavior. The coefficient of determination in both methods is acceptable for solder joint fracture force prediction, but for fracture energy, it has decreased to about 70%. Finally, multi-objective optimization was done with the help of the NSGA-II method, and the Pareto front diagram drawn according to the problem's constraints to find the optimal values for fracture force and energy.

Influence of laser welding power on steel/CFRP lap joint fracture behaviors

Dual-phase steel DP590 and carbon fibre reinforced polymer (CFRP) were successfully joined by laser welding. Experiments and numerical simulations were performed to clarify the influence of laser power on the joint characteristic, bonding mechanism and fracture behavior. The results indicate that the melting width and depth of CFRP increased with the enhancement of laser power. With relatively low laser power, un-bonded steel/CFRP interface was formed due to the insufficient melting of resin matrix and interfacial residual stress resulted from thermal expansion mismatch between steel and CFRP. Compact-bonding was produced with the laser power range of 400–700 W. Decomposition of CFRP at the interface occurred with the further increase of laser power to 800 W due to excessively high peak temperature. The highest tensile-shear peak load of 3855 N was produced at 700 W laser power. Three different fracture modes were found in the joints with different laser powers, i.e., interfacial fracture (IF) at 300 W and 400 W, cohesion fracture (CF) in CFRP at 500 W and 600 W, and interface fracture at inner region together with cohesion fracture at outer region of the bonding interface (IF + CF) at 700 W and 800 W. X-ray photoelectron spectroscopy results showed that C-M and O-M chemical bonds were formed at the interface due to the reaction between resin matrix and DP590. Stronger O-M and C-M chemical bonds produced at higher laser power were beneficial for the improvement of fracture load.

Defect formation, microstructure evolution, and mechanical properties of bobbin tool friction–stir welded 2219-T8 alloy

Bobbin tool friction stir welding (BT–FSW) is a novel and effective welding method for aluminum alloys, but microvoid defects are usually formed in the nugget zones (NZ). Therefore, it is important to understand the formation mechanism of microvoid defects and their effect on the fracture behavior of joints. In the present study, 6 mm thick 2219–T8 aluminum alloy plates were subjected to BT–FSW under a constant rotation rate of 300 rpm and welding speeds of 150–500 mm min–1 with the objective of analyzing the evolution of microvoid defects and their effect. The results showed that, lower welding speeds resulted in microvoid defects in the band pattern structure on the retreating side (RS) of the NZ, and the size of the defects decreased with increasing the welding speed, while sound joints were obtained under higher welding speeds. In contrast to conventional FSW precipitation–hardened aluminum alloys with the lowest hardness zone (LHZ) at the heat affected zone (HAZ), the LHZs of the BT–FSW 2219–T8 joints were located at the thermal–mechanically affected zone (TMAZ). Digital image correlation (DIC) of large tensile samples showed that the microvoid defects exhibited an obvious effect on the tensile deformation and fracture behavior of the joints due to the decrease in the maximum strain and lack of necking. Mini-samples in different layers further confirmed that cracks initiated from the microvoid defects in the NZ. However, the sound BT–FSW 2219–T8 joints fractured along the LHZs on the RS, with a maximum joint strength coefficient of 78.1% being achieved.

Fracture behavior of thermally aged Ag–Cu composite sinter joint through microscale tensile test coupled with nano X-ray computed tomography

Herein, we investigated the fracture behavior of thermally aged Ag–Cu composite sinter joints through nano-X-ray computed tomography (nano-CT) combined with microscale tensile testing. Nano-CT results showed that the joint comprised Ag/oxidized Cu flake sintered layer and Cu oxide, which was formed on the Cu substrate; further, pores were present at the interface between the substrate and Cu oxide layer. The tensile test showed that the stress increased with plastic deformation up to a maximum of 308 MPa, before a subsequent steep decrease due to crack initiation inside the sintered layer. The nano-CT and in situ observation results suggest that the microscale joint fractured through the interface between the oxidized Cu flake and Ag. Results obtained herein suggest that microscale tensile testing coupled with nano-CT is an effective approach to describe the intrinsic fracture behavior of joints.

A DIC method to determine the Mode I energy release rate G, the J-integral and the traction-separation law simultaneously for adhesive joints

The quasi-static Mode I fracture behaviour of joints bonded with either a brittle or toughened epoxy adhesive or a ductile polyurethane adhesive has been investigated by means of digital image correlation (DIC). A novel method to measure the crack length using DIC analysis is proposed. By measuring the crack tip separation, beam rotation and crack length, the energy release rate G and the J-integral are obtained and are compared to analyse the validity of Linear Elastic Fracture Mechanics (LEFM) methods. Simultaneously the traction-separation laws (TSLs) for the adhesive joints were measured. The TSLs were then used as input data for FE modelling to evaluate their accuracy by comparing with experimental results. It is shown that LEFM is valid for the joints bonded with either the brittle or toughened epoxy adhesives but is invalid for joints bonded with the polyurethane adhesive. The procedure proposed here to measure the crack length via DIC shows great promise and can be automated readily in practice.

From thin to extra-thick adhesive layer thicknesses: Fracture of bonded joints under mode I loading conditions

The fracture behaviour of joints bonded with a structural epoxy adhesive and bond line thicknesses of 0.1–4.5 mm has been studied. However, limited research is found on similar joints with thicker bond lines, which are relevant for maritime applications. Therefore, the effect of the adhesive bond line thickness, varying from 0.4 to 10.1 mm, on the mode I fracture behaviour of steel to steel joints bonded with a structural epoxy adhesive was investigated in this study. An experimental test campaign of double-cantilever beam (DCB) specimens was carried out in laboratory conditions. Five bond line thicknesses were studied: 0.4, 1.1, 2.6, 4.1 and 10.1 mm. Analytical predictions of the experimental load-displacement curves were performed based on the Simple Beam Theory (SBT), the Compliance Calibration Method (CCM) and the Penado-Kanninen (P-K) model. The P-K model was used to determine the mode I strain energy release rate (SERR). The average mode I SERR, GIav., presented similar values for the specimens with adhesive bond line thicknesses of 0.4, 1.1 and 2.6 mm (GI av.=0.71, 0.61, 0.63 N/mm, respectively). However, it increased by approximately 63% for 4.1 mm (GI av.=1.16 N/mm) and decreased by about 10% (in comparison with 4.1 mm) for the 10.1 mm (GI av.=1.04 N/mm). The trend of the GIav. in relation to the bond line thickness is explained by the combination of three factors: the crack path location, the failure surfaces features and the stress field ahead of the crack tip.

Parametric study of size, curvature and free edge effects on the predicted strength of bonded composite joints

This paper presents the effects of size, curvature and free edges of laboratory lap joints on the debond fracture behaviour of joints that more realistically represent fuselage skin structures than conventional flat, narrow specimens. Finite Element Analysis is used in conjunction with Cohesive Zone Modelling (CZM) to predict the strength of selected joint features. The modelling approach was verified by simple single lap joint geometry. Four realistic joint features were then modelled by this validated modelling approach. The results show that moderate curvature has negligible effect on the peak load. There is a significant difference in the load vs displacement response of flat lab coupon joints with free edges and realistic curved joints with constrained edges. Further detail design features were investigated in this study, including (i) the joint runout and (ii) the presence of initial damage (thumbnail delamination). The modelling results show that the joggle configuration has an effect on the distribution of interlaminar stresses that affect the damage initiation and propagation. Fracture behaviour from different initial crack geometries associated with wider specimens has been simulated. From a design standpoint, an expansion of modelling capability is suggested to reduce the number of component tests in the traditional test pyramid.

Shear strength and fracture behavior of reflowed Sn3.0Ag0.5Cu/Cu solder joints under various strain rates

The effects of strain rate on the shear fracture behavior of Sn3.0Ag0.5Cu/Cu solder joints with thin and thick interfacial intermetallic compound layers were investigated. A single lap-shear sample was designed to conduct the study. The Cu/Sn3.0Ag0.5Cu/Cu joints were reflowed at 250 °C and 280 °C for 10 min, respectively, resulting in formations of a thin (6.08 μm) and thick (13.25 μm) intermetallic compound layer between the solder and Cu substrate. The single lap-shear tests were performed at the shear strain rates of 5 × 10−4 s−1, 5 × 10−3 s−1, 2 × 10−2 s−1, 1 × 10−1 s−1 and 2 × 10−1 s−1 to observe the shear fracture behavior. Experimental results revealed that both the thickness of the interfacial intermetallic compound layer and strain rate had influence on the shear strength and failure mode of the solder joint. The shear strength of the solder joints increased with increasing strain rate, but decreased with thickness of the interfacial intermetallic compound layer. It was observed that the solder joints broke in a ductile manner at low strain rates (5 × 10−4 s−1, 5 × 10−3 s−1), although they exhibited a ductile-brittle mixed manner at intermediate (2 × 10−2 s−1) and a brittle manner at high strain rates (1 × 10−1 s−1, 2 × 10−1 s−1). In the case of low strain rates, the ductile fracture behavior of joints with thin and thick intermetallic compound layers were similar. Nevertheless, more cleavage and broken Cu6Sn5 grains were detected on the fracture surface of the solder joint with a thick intermetallic compound layer as the strain rate increased to 2 × 10−2 s−1, 1 × 10−1 s−1 and 2 × 10−1 s−1. Furthermore, when the strain rate of 2 × 10−1 s−1 was applied, the solder joint with a thin intermetallic compound layer was completely broken inside the Cu6Sn5 layer, but partly broken at the Cu6Sn5/Cu3Sn interface for the joints with a thick intermetallic compound layer. Four modes were proposed to explain the detailed ductile-to-brittle transition in failure behavior. In addition, the strain rate sensitivities of the solder joints with thin and thick interfacial intermetallic compound layers were also investigated. It was found that the strain rate of a solder joint with a thick interfacial intermetallic compound layer was lower than that of one with a thin interfacial intermetallic compound layer.

Comparison of solder joint fracture behavior in Arcan and DCB specimens

The mixed-mode fracture energies of copper–SAC305–copper solder joints were measured using both an Arcan-type specimen and a double-cantilever beam (DCB) specimen. The critical strain energy release rate, Jci, values obtained from Arcan-type specimens were in the range of 10–80J/m2, depending on the strain rate and phase angle of loading, while those obtained from DCB specimens were much higher, in the range of 380–2300J/m2. This large difference in the measured Jci values, which is consistent with the literature, was explained in terms of the stress distributions, constraint and plastic zone sizes in the solder layers in these two specimens.

Effects of Microstructure and Loading on Fracture of Sn-3.8Ag-0.7Cu Joints on Cu Substrates with ENIG Surface Finish

When dropped, electronic packages often undergo failure by propagation of an interfacial crack in solder joints under a combination of tensile and shear loading. Hence, it is crucial to understand and predict the fracture behavior of solder joints under mixed-mode high-rate loading conditions. In this work, the effects of the loading conditions (strain rate and loading angle) and microstructure interfacial intermetallic compound (IMC) morphology and solder yield strength] on the mixed-mode fracture toughness of Sn-3.8 wt.%Ag-0.7 wt.%Cu solder joints sandwiched between two Cu substrates with electroless nickel immersion gold (ENIG) metallization have been studied, and compared with the fracture behavior of joints attached to bare Cu. Irrespective of the surface finish, the fracture toughness of the solder joints decreased monotonically with strain rate and mode-mixity, both resulting in increased fracture proportion through the interfacial IMC layer. Furthermore, the proportion of crack propagation through the interfacial IMC layer increased with increase in the thickness and the roughness of the interfacial IMC layer and the yield strength of the solder, resulting in a decrease in the fracture toughness of the joint. However, under most conditions, solder joints with ENIG finish showed higher resistance to fracture than joints attached directly to Cu substrates without ENIG metallization. Based on the experimental observations, a fracture mechanism map is constructed correlating the yield strength of the solder, the morphology and thickness of the interfacial IMC, and the fracture mechanisms as well as the fracture toughness values for different solder joints under mode I loading.

A general methodology for calculating mixed mode stress intensity factors and fracture toughness of solder joints with interfacial cracks

Solder joints in electronic packages undergo thermo-mechanical cycling, resulting in nucleation of micro-cracks, especially at the solder/bond-pad interface, which may lead to fracture of the joints. The fracture toughness of a solder joint depends on material properties, process conditions and service history, as well as strain rate and mode-mixity. This paper reports on a methodology for determining the mixed-mode fracture toughness of solder joints with an interfacial starter-crack, using a modified compact mixed mode (CMM) specimen containing an adhesive joint. Expressions for stress intensity factor (K) and strain energy release rate (G) are developed, using a combination of experiments and finite element (FE) analysis. In this methodology, crack length dependent geometry factors to convert for the modified CMM sample are first obtained via the crack-tip opening displacement (CTOD)-based linear extrapolation method to calculate the under far-field mode I and II conditions (f1a and f2a), (ii) generation of a master-plot to determine ac, and (iii) computation of K and G to analyze the fracture behavior of joints. The developed methodology was verified using J-integral calculations, and was also used to calculate experimental fracture toughness values of a few lead-free solder-Cu joints.

Effect of partial rolling on the microstructure, mechanical properties and fracture behavior of AZ31 Mg alloy joints

Abstract Partial rolling processes at different temperatures were conducted on the fusion zone (FZ) of double-side welded Mg–3Al–1Zn alloy joints with excess weld bead. The effect of rolling reduction and rolling temperature on the microstructure, mechanical properties and fracture behavior of the joints were investigated. Results show that both the strength and elongation of the joints increase significantly with rolling reduction owing to the microstructure evolution in the FZ. Once rolling reduction exceeds a critical value (CRR), tensile fracture location will be transferred from the FZ to the substrate and the joints can achieve tensile properties roughly the same as those of the initial substrate. Rolling at a lower temperature is preferred to obtaining a weak basal texture in the FZ, for the corresponding CRR is smaller.

Study of the Solid-State Joining Process for Endless Hot Rolling

In this study, the joint strength and microstructure of carbon steels welded by an super-deformation joining process in air were investigated. The two block of model steels are overlapped and heated. The steels are welded with 1000% deformation at approximately 1000°C.The joint strengths are evaluated by uniaxial tension testing. The joint microstructures and fractured surfaces are investigated with OM and SEM. The results revealed that the mechanical properties of joint are close to the matrix’s, the grains adjacent to the interface are merged into one grain, which indicates that both blocks are soundly joined,fracture surface was covered with dimple indicating ductile fracture occurred. Joint fracture behavior is initiated by the brittle fracture of scale inclusions and growth around the internal oxides. Surface scale is the most important factor of sound joining.

Influence of welding parameter on mechanical properties and fracture behavior of friction stir welded Al–Mg–Sc joints

8.1-mm thick Al–Mg–Sc plates were friction stir welded (FSW) at tool rotation rates of 400–800rpm and welding speeds of 100–400mm/min to investigate the effect of welding parameter on the microstructure, mechanical property and fracture behavior of joints. The lazy S and kissing bond, with quite different morphologies, were observed in the stir zone at 400 and 600rpm, attributing to insufficient material flow. However, neither of them were detected at 800rpm. Ultimate tensile strength of the joints was nearly equal to that of the base metal with the joint efficiency being 97–99%. Facture behavior of the joints changed with the welding parameter and it was strongly affected by the lazy S and kissing bond. For the joints at 400 and 600rpm, fracture occurred partially along the lazy S at 100mm/min or initiated at the kissing bond at 400mm/min. However, the joints at 800rpm exhibited a fracture independent of the lazy S or kissing bond. Furthermore, the present observations were compared with those made by other researchers and the reasons for insufficient material flow at lower rotation rates of 400 and 600rpm were discussed.

Effect of base steels on mechanical behavior of adhesive joints with dissimilar steel substrates

The hybrid steel materials (e.g., advanced high strength steels combined with mild steels) have been increasingly applied in vehicle components to achieve the compromise among mechanical performance, lightweight and material cost. A significant issue related to the bonding of dissimilar steel materials is that the differences in substrate physical properties often lead to bond separation at strength levels far less than the bond strength established by the adhesive manufacturer for the joint with similar substrates. This research studied several important factors influencing the strengths of adhesive-bonded lap shear (LS) and coach-peel (CP) joints. Four types of steel substrates were used to fabricate the unbalanced adhesive joints (dissimilar steels or same steels with different thicknesses). The quasi-static tensile tests of the unbalanced joints were performed. A finite-element model was proposed to characterize the fracture behavior of joints. The effects of substrate properties such as bending stiffness and load-carrying capability of substrates on the joint strength (peak force) and fracture modes were investigated. It was observed that the interfacial fracture is prone to occur on the substrate with the relatively weak yield strength. A cohesive fracture can be produced when the load-carrying capabilities of the two substrates are similar. In order to optimize the load-carrying capability of the unbalanced joint, a design guideline was put forward.