Bi-material Joints Research Articles

Effect of Pressure on the Linear Friction Welding of a Tool Steel and a Low‐Alloy Carbon Steel

This study investigates the effect of pressure (burn‐off and forging) on the mechanical properties of the joint between a wear‐resistant tool steel and a low‐alloy steel using linear friction welding. The authors have previously demonstrated the feasibility of joining these dissimilar materials, but the impact of pressure on the mechanical properties of the bimaterial joint remains unclear. To address this, weld samples are prepared using different pressures and are characterized through microstructural analysis, microhardness, tensile testing, and fractography. The results show that the strength of the joint between the wear‐resistant tool steel and the low‐alloy carbon steel increases as the pressure increases up to a certain point, after which a decrease is observed. The highest joint strength is achieved at a pressure of 360 MPa. The microhardness profile measurement reveals a distinct transition zone at the interface between the two materials, with varying hardness values. The hardness of the low‐alloy carbon steel increases near the interface, while that of the wear‐resistant tool steel decreases. This transition zone is found to be narrower at higher pressures. Microstructural characterization shows that the grain structure near the interface differs from that of the starting base materials.

The morphology and evolution in Al-Cu and Al-Fe magnetic pulse weld interfaces characterized through phase-contrast micro-tomography

Magnetic Pulse Welding (MPW) facilitates the permanent joining of dissimilar metallic materials through the sudden impact generated by a magnetic pulsed field. The process can introduce distinct morphological features at the interface of bi-material joints, which subsequently affect the joint’s quality and durability. This article delves into the investigation and quantification of various interfacial morphologies in Aluminum/Copper and Aluminum/Steel joints, using high-energy phase-contrast synchrotron micro-tomography. Surface topography is extracted from 3D tomographic datasets between dissimilar materials, enabling a comprehensive comparison between different material pairings and various locations within the weld. The study analyses and compares the roughness parameters of these surfaces. Moreover, it describes the interface’s waves and vortexes through diverse morphological metrics, encompassing their shape and size. The results provide evidences that vortexes evolve in three dimensions, with lateral growth and collapse. The waves and vortexes shapes promote material interlocking, increasing the contact area between the dissimilar materials by up to 20%. The interface morphology of Al/Cu joints exhibits higher roughness and a greater number of vortexes compared to Al/Fe joints. Lastly, the findings reveal the presence of interface damage in the form of pre-existing discontinuities.

Mode decoupling in interlaminar fracture toughness tests on bimaterial specimens

The present work has a two-fold objective: (i) to critically review the methods for fracture mode decoupling in unconventional laboratory specimens, such as the asymmetric double cantilever beam (ADCB) specimen; and (ii) to propose mode decoupling conditions and associated specimen design formulae to obtain pure fracture modes when bimaterial specimens are tested in ADCB and asymmetric end-notched flexure (AENF) configurations. In the first part of the paper, the literature on fracture mode decoupling is reviewed to shed light on some controversial points. We start with discussing various pure-mode conditions suggested by different authors and continue with the simplest case of the bimaterial joint. Our review also considers complex cases, such as the presence of bending–extension coupling or residual (hygrothermal) stresses. In the second part of the paper, bimaterial specimens loaded in ADCB and AENF test configurations are investigated. Employing energetically orthogonal mode decomposition, Engesser–Castigliano’s theorem, and the laminated beam theory, we illustrate specimen design criteria enabling to obtain pure fracture modes. The obtained specimen design formulae are validated through finite element analyses.

INTERFACIAL FRACTURE TOUGHNESS OF UNCONVENTIONAL SPECIMENS: SOME KEY ISSUES

Laboratory specimens used to assess the interfacial fracture toughness of layered materials can be classified as either conventional or unconventional. We call conventional a specimen cut from a unidirectional composite laminate or an adhesive joint between two identical adherents. Assessing fracture toughness using conventional specimens is a common practice guided by international test standards. In contrast, we term unconventional a specimen resulting from, for instance, bimaterial joints, fiber metal laminates, or laminates with an elastically coupled behavior or residual stresses. This paper deals with unconventional specimens and highlights the key issues in determining their interfacial fracture toughness(es) based on fracture tests. Firstly, the mode decoupling and mode partitioning approaches are briefly discussed as tools to extract the pure-mode fracture toughnesses of an unconventional specimen that experiences mixed-mode fracture during testing. Next, we elaborate on the effects of bending-extension coupling and residual thermal stresses often appearing in unconventional specimens by reviewing major mechanical models that consider those effects. Lastly, the paper reviews two of our previous analytical models that surpass the state-of-the-art in that they consider the effects of bending-extension coupling and residual thermal stresses while they also offer mode partitioning.

Structural integrity assessment of a full-scale adhesively-bonded bi-material joint for maritime applications

The present study proposes a comprehensive integrity assessment approach for a full-scale adhesively-bonded bi-material joint for maritime applications. The joint represents a cross-section of the bond-line connection of a ship with a steel hull and a sandwich composite superstructure. The full-scale joint consists of a sandwich composite core adhesively bonded to two U-shaped steel brackets. The joint was subjected to a quasi-static loading profile including 6 load cycles up to the final failure. Each load cycle was followed by a dwell segment holding the joint at the maximum displacement for 30 s and then unloading to 50% of the maximum displacement. Three Structural Health Monitoring (SHM) techniques including Acoustic Emission (AE), Fiber Optic Sensor (FOS), and Digital Image Correlation (DIC) were employed during the test to assess the damage state of the joint. Moreover, a Finite Element Model (FEM) was developed to simulate the evolution behavior of different damage mechanisms in the joint and the FE results were compared against the experimental findings. The obtained results showed that the integration of all the employed techniques could successfully detect the damage initiation, assess the severity of the damage, localize the critical regions of the joint, and distinguish the different damage mechanisms.

Influence of accelerated corrosion on bi-material steel-CFRP double-lap joints bonded with thick adhesive

Bi-material steel-composite joints attract interest for marine applications. The marine environment imposes corrosion which is a prolonged process. This work presents a two-electrode electrochemical setup for accelerating free corrosion of the steel surfaces of bi-material joints. It is used to study the impact of accelerated corrosion on the mechanical performance of bi-material double lap specimens subjected to quasi-static tensile testing. Several test series have been conducted to evaluate the influence of overlap length and bond line thickness on shear strength and failure mode. Sixty specimens with a thick layer of methyl methacrylate adhesive have been fabricated and cured at room temperature for at least 24 h. Subsequently, 30 specimens were aged by subjecting them to electrochemical corrosion for 24 h. All specimens were tested for failure in quasi-static tensile loading while monitoring the strain fields in the joint area using digital image correlation. The measurements reveal a homogeneous shear strain field at the onset of loading, with a rapidly increasing shear strain concentration near the edges of the bond line preceding final failure. Both a decrease in the adhesive thickness and an increase in the overlap length increase the shear strength. Higher shear strength was observed for the electrochemically aged specimens than that of non-aged specimens. This is attributed to the faster residual curing of the adhesive during ageing because an increasing percentage of copper ions (released from the anode) accelerates the curing of the MMA adhesive. The electrochemically aged specimens showed mixed failure modes, i.e. cohesive failure and adhesive failure at the interface between steel and adhesive, and skin failure within the composite laminates.

Mode I tensile fracture behavior of adhesively-bonded metal–metal, metal–CFRP, and CFRP–CFRP bi-material combinations analyzed by size effect method

Understanding the adhesive and interfacial fracture is important for developing better adhesive performance in bi-material joints, i.e. adhesive joint between dissimilar materials. However, to characterize the fracturing behavior of various adhesively-bonded materials, it was shown in this work that the Mode I fracture energies estimated from conventional methods (e.g., work-of-fracture, (modified) compliance calibration method, (modified) beam theory, etc.) can be strongly affected by adherend thickness, adhesive bond length, and adherend material type. Consequently, the proper understanding of fracturing in bi-material joints has been hindered since the estimated fracture energies can exhibit unreasonable difference among various material combinations depending on the method of fracture energy calculation. This has led to confusion in the literature due to the unfair comparison of these non-objective results estimated by leveraging conventional methods on the specimens with different geometries or dissimilar adherend materials.This work introduced the size effect method to adhesively-bonded joints and compared the results with conventional methods of calculating the Mode I fracture energies of metal–metal, metal–CFRP, and CFRP–CFRP material combinations via Double Cantilever Beam (DCB) tests. The results showed that the estimated fracture energies obtained using the size effect method were not dependent on the specimen geometries, whereas conventional methods were strongly dependent on specimen geometry for most material combinations. The size effect method allowed objective comparison particularly on the interfacial fracturing between metal/adhesive and CFRP/adhesive combinations. The difference was further explained by and correlated with the damage morphology on the material surface after failure, which was quantified by three-dimensional profilometer.

Size dependent creep deformation of elastically constrained compliant metallic joints

The present study proposes a novel method to predict the size dependent creep behavior of bi-material joints formed by constraining a creep compliant metal, such as Sn, having varied thickness ranging from 1.4 mm to 170 µm, between stiff elastic substrates, such as Cu. A dramatic reduction in the secondary or minimum creep rate was observed with decrease in joint thickness. Finite element (FE) analysis using continuum formulations attributed this strengthening to the geometric constraints imposed by Cu, which increases the triaxiality in the joints and hence reduces the effective stress. While FE results were in close agreement with experiment in thick joints, it significantly overpredicted the creep rate of miniature joints. Further, orientation imaging revealed that microstructure varied with length scale, from bulk Sn with multiple grains to miniature joints having a few grains. The additional strengthening was captured using dislocation-based crystal plasticity (CP) creep modeling of Sn-Cu joints by incorporating this microstructural length scale, in addition to the geometric constraints. CP simulations revealed that orientation anisotropy of Sn and the constraints imposed by substrates on dislocation motion lead to higher strength in the miniature joints. Using the insights from FE and CP modeling a unified length scale sensitive model, incorporating both geometric or continuum and microstructural factors, was developed that can accurately predict the creep response of the joints of macro and meso length scale.

Shear strength and failure modes of double-lap bi-material CFRP/Ni-Cr alloy joint under severe environmental conditions

This study was systematically conducted to investigate the dependence of mechanical behaviors of double-lap bi-material joints on various environmental conditions, including room temperature dry (RTD), cold temperature dry (CTD), elevated temperature dry (ETD), elevated temperature wet (ETW), salty spray (SS), and thermal shock cycles (TS) conditions. Compared to the RTD condition, the shear strength of the joint under the CTD and TS conditions shows increases of approximately 50.0% and 9.09%, respectively, but lower values are obtained under ETD (18.18%), ETW (72.73%), and SS (27.27%) conditions. Failure modes of lap-joint are analyzed on fractured surfaces and confirmed by finite element simulation in which the cohesive failure mode mainly occurs with the highest distributed stresses. However, an experimental adhesive failure mode partially appears under the ETD, ETW, and SS conditions. Furthermore, as the less prevalent conditions, the dependencies of the shear strength and failure mode on the exposure time under SS and TS conditions are more thoroughly evaluated based on the scanning electron microscope (SEM) and the energy-dispersive X-ray spectroscopy (EDS) to show microstructures and chemical compositions of the fracture surface, respectively. The effects of environmental conditions, particularly under SS and TS conditions, could provide new insight into designing specific bonded joints.

Predicting thermally induced edge-crack initiation using finite fracture mechanics

In temperature-loaded bi-material joints, highly localized stress concentrations occur at the bi-material junction at the free edge. This so-called free-edge effect may cause premature failure in form of an interface crack emanating from the free edge. In the present work, thermal crack initiation is investigated using a coupled stress and energy criterion within the framework of finite fracture mechanics by the example of a uniformly cooled epoxy–glass bi-material specimen. The mechanical analysis is based on the assumption of a plane-strain state, providing a simplified two-dimensional model. Interface stresses and the energy dissipation due to crack initiation are computed efficiently using the scaled boundary finite element method (SBFEM) and the effect of the layer thickness on crack initiation is studied. Dimensional analyses revealed that interface stresses and energy dissipation can be scaled to an arbitrary layer thickness such that only few model evaluations are required. For validation purposes, a finite element reference solution is provided and compared to the results obtained by the SBFEM approach. In addition, a cohesive zone model is applied to validate the predictions of the coupled stress and energy criterion.

Thermo-mechanical stress analysis of dissimilar material joints using FEM

Abstract: This article presents numerical investigation of isotropic dissimilar material joints. Dissimilar material joints are broadly used in in various structures, including offshore, nuclear, electronic packaging, IC chip and spacecraft various fields of science and technology. In bi-material joints two different material are bonded with common interface region. High stress concentration occur at the interface of the joint under thermo-mechanical loadings due to the difference in the elastic properties and the thermal expansion coefficients of dissimilar materials. The stresses acting along the interface of dissimilar material joints are very important to determine whether the structure is reliable or not for operation. The main purpose of this research is to provide finite element solutions to predict the stress distribution at the interface of the joint based on the theory of elasticity. Keywords: Numerical Investigation, Dissimilar material joints, Stress concentration, Stress distributions, Theory of elasticity.

A peridynamic modeling approach of solid state impact bonding and simulation of interface morphologies

Peridynamics was originally developed to model spontaneously emerged discontinuities such as crack propagation and progressive failure. In this paper, contrary to typical peridynamic applications in the realm of modeling of discontinuities, a modeling approach is proposed in the framework of non-ordinary state-based peridynamics to simulate impact-driven solid-state welding processes that can effectively join a wide variety of similar and dissimilar combinations of materials. A scheme to model formations of metallurgical bonds resulting from severe plastic deformation due to high-velocity impact is developed in virtue of peridynamic bonds. The proposed model is of combined Lagrangian-Eulerian nature, given that nonlocal interactions are established on both reference and deformed configurations. Impact bonding processes of single- and bi-material joints are successfully simulated and validated. Distinctive morphological features of bonding interface are reproduced. Effectiveness and robustness of the proposed modeling approach are demonstrated by a numerical study of influences of processes variables on the morphology and stability of interface features.

Role of adherend material on the fracture of bi-material composite bonded joints

The aim of this study is to investigate the effect of the adherend material on the mode I fracture behaviour of bi-material composite bonded joints. Both single-material (steel-steel and composite-composite) and bi-material (steel-composite) joints bonded with a structural epoxy adhesive are studied. Additionally, two adhesive bondline thicknesses are considered: 0.4 mm (thin bondline) and 10.1 mm (thick bondline). The Penado-Kanninen reduction scheme is applied to evaluate the mode I strain energy release rate. The results show that the mode I fracture energy, GIc, is independent of the adherend type and joint configuration (single or bi-material). GIc shows average values between 0.60 and 0.72 N/mm for thin bondlines and 0.90–1.10 N/mm for thick bondlines. For thin bondlines, the failure is cohesive and the similar degree of constraint that is imposed to the adhesive by the high-modulus (i.e., steel) and/or relatively thick (i.e., composite) adherends results in similar values of GIc for both single- and bi-material joint types. For thick bondlines, the crack grows closer to one of the adhesive-adherend interfaces, but still within the adhesive. The results show that the adhesive could deform similarly, although the crack has been constrained on one side by different types of adherends, either a steel or composite.

Stress analysis of double-lap bi-material joints bonded with thick adhesive

Mechanics of double-lap Steel-to-CFRP adhesively-bonded joints loaded in tension are investigated experimentally using Digital Image Correlation (DIC) and Acoustic Emission (AE), analytically using a one-dimensional closed-form solution and numerically with Finite Element analysis. The double-lap bi-material joints are fabricated of a steel core adhesively bonded to two CFRP skins with adhesive thickness of ~ 8 mm, using an Epoxy-based and MMA-based adhesives. In order to capture the in-plane deformation of the joint, full field strain/displacement maps are obtained using DIC. This data is used to validate the shear-lag model predictions of the adhesive shear stress/strain distribution as well as the linear-elastic Finite Element Model (FEM) results. In addition, they are used to capture the susceptible damage locations and their effect on the displacement contour maps, strain distribution and load transfer between the joint's different constituents. A correlation between the DIC displacement and the AE signals is obtained for damage detection in both joints. Moreover, a good agreement amongst the analytical, FE and DIC strain/stress distributions along the bond-line is observed. This study introduces the analytical shear-lag model as an alternative to predict the stress state in thick-adhesive double-lap joints, with an acceptable level of accuracy and robustness.

Damage characterization of adhesively-bonded Bi-material joints using acoustic emission

The aim of the present study is to characterize the damage in bi-material steel-to-composite double-lap adhesively-bonded joints using Acoustic Emission (AE). Two different structural adhesives, a ductile (Methacrylate-based) and brittle (Epoxy-based), were used to bond CFRP skins to a steel core. The fabricated joints were loaded in tension while damage evolution was monitored by AE. Due to the difference in the fracture nature of the adhesives “ductile vs. brittle”, different damage mechanisms were observed; including cohesive failure within the adhesive layer, steel deformation, failure at the adhesive/adherends interface (adhesive failure) and delamination in the CFRP skin. To classify these damages by AE, the AE features of each damage mechanism were first obtained by conducting standard tests on the individual constituents. Then, these AE reference patterns were used to train an ensemble decision tree classifier. The best parameters of the ensemble model were obtained by Bayesian optimization, and the confusion matrix showed that the model was sufficiently trained with the accuracy of 99.5% and 99.8% for Methacrylate-based and Epoxy-based specimens respectively. Afterwards, the trained model was used to classify the AE signals of the double-lap specimens. The AE demonstrated that the dominant damage mechanisms in the case of the Methacrylate-based were cohesive and adhesive failures while in the case of the Epoxy-based they were CFRP skin failure and adhesive failure. These results were consistent with the Digital Image Correlation, Fiber Optic Sensor and camera results. This study demonstrates the potential of AE technique for damage characterization of adhesively-bonded bi-material joints.

Strain-based methodology for mixed-mode I+II fracture: A new partitioning method for bi-material adhesively bonded joints

ABSTRACTThe dissemination of composite materials introduces applications of hybrid structures with composite and metal parts. The development of reliable methodologies to evaluate the performance of these structures is required. In this work, the mixed-mode fracture behaviour of a bi-material adhesively bonded joint is investigated. A new strain-based criterion for the design of the mixed-mode bending (MMB) bi-material specimen is suggested. A new analytical partitioning method based on the ‘global method’ is proposed and tested on a composite-to-metal bonded joint and compared with a finite element model using the virtual crack closure technique (VCCT). The results show that the proposed strain-based design methodology can be successfully used in MMB test for bi-material joints. The fracture mode partitioning is accurately predicted by the analytical method. However, the absolute values of the strain energy release rate (SERR) predicted by the analytical method are only accurate if the shear deformation in the test is not significant.

Predicting crack patterns at bi-material junctions: A coupled stress and energy approach

Failure of bi-material joints is affected significantly by the elastic material properties of both adherends as well as by geometrical parameters such as material interface orientation. Addressing these effects, scarf joint specimens made of a polymer foam and either PMMA or aluminum are examined employing a four-point bending test setup with varying scarf angle. The joints’ effective strengths are predicted using a coupled stress and energy criterion within the concept of finite fracture mechanics defining failure as spontaneous formation of finite sized cracks. Since numerous competing crack configurations are admissible, efficient modelling is crucial. Crack initiation triggered by the singular stress field at the bi-material junction is modelled using a semi-analytical approach based on the method of matched asymptotic expansions. In order to accurately capture stresses in the vicinity of the notch and energy dissipation due to crack onset, higher order terms associated to complex deformation modes are considered in the asymptotic expansion. Results based on the asymptotic approach are compared to numerical reference data. Theoretical predictions of the effective joint strength as well as the orientation of the associated initiating cracks are validated against experimental findings from literature.

The Intensity of Stress Singularity in Plastic Region around A Singular Point in Bi-material Joints

Abstract The influence of hardening exponent on the intensity of stress singularity in a plastic region around a singular point in a sandwich structure was investigated using finite element method (FEM). A piece of copper is placed as an interlayer between ceramic layers and then exerted by a four-point bending load. The material properties in the plastic region of copper were varied by hardening exponents (n = 0 to 1). The result shows that the intensity of the stress singularity in the plastic region was high as the hardening exponent approached a perfect plastic.

How pure mode I can be obtained in bi-material bonded DCB joints: A longitudinal strain-based criterion

An essential question to predict the structural integrity of bi-material bonded joints is how to obtain their fracture properties under pure mode I. From open literature, it is found that the most commonly used design criterion to test mode I fracture is matching the flexural stiffnesses of the two adherents in a DCB coupon. However, the material asymmetry in such designed joints results in mode II fracture as well. In this paper, a new design criterion is proposed to obtain pure mode I fracture in adhesively bonded bi-material DCB joints by matching the longitudinal strain distributions of the two adherends at the bondline - longitudinal strain based criterion. A test program and Finite Element modelling have been carried out to verify the proposed design criterion using composite-metal bonded DCB joints. Both the experimental and numerical results show that pure mode I can be achieved in bi-material joints designed with the proposed criterion. GII/GI ratio is reduced by a factor of 5 when using the proposed longitudinal strain based criterion in comparison with the flexural stiffness based criterion.

Fracture initiation in bi-material joints subject to combined tension and shear loading

Linear elastic solution of the stress field near an interface corner of bi-material joints is of the form Hrλ−1, where r is the radial distance from the corner, H is the stress intensity factor and λ–1 is the order of the singularity. Finite element analysis is used to determine the magnitude of H for a butt joint subject to remote shear; the obtained solution complements existing solution for remote tension and uniform change in temperature. The theoretical solution of the singular shear stress is shown to be in good agreement with the corresponding finite element solution. The effect of combined remote tension, remote shear and uniform change in temperature on the failure loads and failure mechanisms is experimentally determined for brass/araldite/brass butt joint. It is shown that the failure envelope in tensile stress – shear stress space is elliptical and the failure loads decrease with increasing cure temperature due to thermal residual stress associated with the curing process. The application of the results to the assessment of onset of failure in composite patch repair is discussed.