Low-speed bending impact behavior of adhesively bonded single-lap joints

This study addresses the low-speed impact behavior of adhesively bonded single-lap joints. An explicit dynamic finite element analysis was conducted in order to determine the damage initiation and propagation in the adhesive layers of adhesive single-lap joints under a bending impact load. A cohesive zone model was implemented to predict probable failure initiation and propagation along adhesive–adherend interfaces whereas an elasto-plastic material model was used for the adhesive zone between upper and lower adhesive interfaces as well as the adherends. The effect of the plastic deformation ability of adherend material on the damage mechanism of the adhesive layer was also studied for two aluminum materials Al 2024-T3 and Al 5754-0 having different strength and plastic deformation ability. The effects of impact energy (3 and 11 J) and the overlap length (25 and 40 mm) were also investigated. The predicted contact force-time, contact force-central displacement variations, the damage initiation and propagation mechanism were verified with experimental ones. The SEM and macroscope photographs of the adhesive fracture surfaces were similar to those of the explicit dynamic finite element analysis.     

Stress analysis at the interface of metal-to-metal adhesively bonded joints subjected to 4-point bending: Finite element method

This paper presents a study of stress states in two-dimensional models of metal-to-metal adhesively bonded joints subjected to 4-point flexural loading using the finite element (FE) method. The FE simulations were carried out on adhesive bonded joints of high support span to specimen thickness ratio undergoing extensive plastic deformations. Two different adhesive types with eight different adhesive layer thicknesses each varying between 50 μm and μm were considered. The lower interfaces in the brittle adhesive were observed to be under a lower stress state because of the constraint exerted by a relatively stiff lower adherend. The ductile adhesive layers were under a lower state of stress as a result of the lower elastic modulus. It is concluded that the degree of plastic deformation in the adhesive is dictated by the adherend stiffness and the load transfer along the interface. The effect of load and support pins is noticeable at all adhesive thicknesses. High stress localisation exists in the vicinity of the load pins. The constraint exerted by the adherends dictates the deformation gradient through thickness of the adhesive layer. Adhesive joint behaviour as determined by the adhesive properties is investigated and also experimentally validated. Conclusions were drawn by correlating the adhesive and adherend stress states.     

Modeling of cohesive failure processes in structural adhesive bonded joints

This paper presents an approach to predicting the strength of joints bonded by structural adhesives using a finite element method. The material properties of a commercial structural adhesive and the strength of single-lap joints and scarf joints of aluminum bonded by this adhesive were experimentally measured to provide input for and comparison with the finite element model. Criteria based on maximum strain and stress were used to characterize the cohesive failure within the adhesive and adherend failure observed in this study. In addition to its simplicity, the approach described in this paper is capable of analyzing the entire deformation and failure process of adhesive joints in which different fracture modes may dominate and both adhesive and adherends may undergo inelastic deformation. It was shown that the finite element predictions of the joint strength generally agreed well with the experimental measurements.     

Metallic multi-material adhesive joint testing and modeling for vehicle lightweighting

While adhesive bonding has been shown to be a beneficial technique to join multi-material automotive bodies-in-white, quantitatively assessing the effect of adherend response on the ultimate strength of adhesively bonded joints is necessary for accurate joint design.In the current study, thin adherend single lap shear testing was carried out using three sheet metals used to replace mild steel when lightweighting automotive structures: hot stamped Usibor® 1500 AS ultra-high strength steel (UHSS), aluminum (AA5182), and magnesium (ZEK 100). Six combinations of single and multi-material samples were bonded with a one-part toughed structural epoxy adhesive and experimentally tested to measure the force, displacement across the bond line, and joint rotation during loading. Finite element models of each test were analyzed using LS-DYNA to quantitatively assess the effects of the mode mixity on ultimate joint failure. The adherends were modeled with shell elements and a cohesive zone model was implemented using bulk material properties for the adhesive to allow full three-dimensional analysis of the test, while still being computationally efficient.The UHSS-UHSS joint strength (27.2 MPa; SD 0.6 MPa) was significantly higher than all other material combinations, with joint strengths between 17.9 MPa (SD 0.9 MPa) and 23.9 MPa (SD 1.4 MPa). The models predicted the test response (average R2 of 0.86) including the bending deformation of the adherends, which led to mixed mode loading of the adhesive. The critical cohesive element in the UHSS-UHSS simulation predicted 85% Mode II loading at failure while the other material combinations predicted between 41% and 53% Mode II loading at failure, explaining the higher failure strength in the UHSS-UHSS joint.This study presents a computational method to predict adhesive joint response and failure in multi-material structures, and highlights the importance of the adherend bending stiffness and on joint rotation and ultimate joint strength.     

Development of a Failure Model for the Adhesively Bonded Tubular Single Lap Joint

The accurate calculation of the stresses and torque capacities of adhesively bonded joints is not possible without understanding the failure phenomena of the adhesive joints and the nonlinear behavior of the adhesive.

In this paper, an adhesive failure model of the adhesively bonded tubular single lap joint with steel-steel adherends was proposed to predict the torque capacity accurately.

The model incorporated the nonlinear behavior of the adhesive and the different failure modes in which the adhesive failure mode changed from bulk shear failure, via transient failure, to interfacial failure between the adhesive and the adherend, according to the magnitudes of the residual thermally-induced stresses from fabrication.     

Finite element analysis of adhesive joints in four-point bending load

Nonlinear finite element analysis (FEA) was applied to the adhesively bonded Single Lap Joint (SLJ) in bending load. Two adhesives, one stiff and one flexible, with very different mechanical behaviors, and hard steel as adherend with four different thicknesses, were analyzed for the joint configuration. For comparison, experimental work was also undertaken. It was shown that adherend thickness played an important part in the joint performance; while the stiff adhesive gave stronger joint strength when using thick adherends, the opposite was the case for the flexible adhesive when using thin adherends. These results were related to the mechanical behaviors of the adhesives used. It was shown that the results from the FEA and the experimental works were in a good agreement.     

Stress analysis and strength evaluation of single-lap band adhesive joints of dissimilar adherends subjected to external bending moments

Single-lap band adhesive joints of dissimilar adherends subjected to external bending moments are analyzed as a four-body contact problem using a two-dimensional theory of elasticity (plane strain state). In the analysis, the upper and lower adherends and the adhesive which are bonded in two regions are replaced by finite strips. In the numerical calculations, the effects of the ratio of Young's moduli of the adherends, the ratio of the adherend thicknesses, and the ratio of the band length to the half lap length on the stress distributions at the interfaces are examined. A method for estimating the joint strength is proposed using the interface stress and strain obtained by the analysis. An elasto-plastic finite element analysis (EP-FEA) was conducted for predicting the joint strength more exactly. Experiments to measure strains and the joint strength were also carried out. The results show that the strength of a single-lap band adhesive joint is almost the same as that of a single-lap adhesive joint in which the two adherends are completely bonded at the interfaces. Thus, the single-lap band adhesive joints are useful in the design of single-lap joints.     

FINITE ELEMENT ANALYSIS OF ADHESIVE JOINTS IN FOUR-POINT BENDING LOAD

Nonlinear finite element analysis (FEA) was applied to the adhesively bonded Single Lap Joint (SLJ) in bending load. Two adhesives, one stiff and one flexible, with very different mechanical behaviors, and hard steel as adherend with four different thicknesses, were analyzed for the joint configuration. For comparison, experimental work was also undertaken. It was shown that adherend thickness played an important part in the joint performance; while the stiff adhesive gave stronger joint strength when using thick adherends, the opposite was the case for the flexible adhesive when using thin adherends. These results were related to the mechanical behaviors of the adhesives used. It was shown that the results from the FEA and the experimental works were in a good agreement.     

Experimental and Numerical Study of Adhesively Bonded CFRP Scarf-Lap Joints Subjected to Tensile Loads

The tensile performance of adhesively bonded CFRP scarf-lap joints was investigated experimentally and numerically. In this study, scarf angle and adherend thickness were chosen as design parameters. The lap shear strength is not directly proportional to scarf angle and adherend thickness for the brittle adhesive studied in the paper. The major failure mode includes cohesive shear failure and adherend delamination failure. The results present a stepped failure morphology along the bondline in the adhesive layer. A finite element model based on cohesive zone model was established to further investigate the stress distribution of scarf-lap joints with different lap parameters. The numerical results were compared with the experiment results, showing a good agreement, thus verifying the validity of the established numerical model.     

Effect of material on the mechanical behaviour of adhesive joints for the automotive industry

One parameter that influences adhesively bonded joints performance is the adherend material and its effect should be taken into consideration in the design of adhesive joints. In this work, the effect of material on the mechanical behaviour of adhesive joints was investigated experimentally and numerically by single lap joints (SLJs) with different adherend materials (high strength steel, low strength steel and composite). The adhesives selected were two new modern tough structural adhesives used in the automotive industry. It was found that, for relatively short overlaps in SLJs bonded with structural modern tough adhesives, failure is dominated by adhesive global yielding and the influence of material on joint strength is not significant. For larger overlaps, the failure is not anymore due to global yielding and the effect of material becomes more important. Moreover, it was possible to evaluate which adhesive is more suited for each material.     

On the non-linear elastic stresses in an adhesively bonded T-joint with double support

Structures consisting of single or more materials, such as adhesive joints, may undergo large displacements and rotations under reasonably high loads, although all materials are still elastic. The linear elasticity theory cannot predict correctly the deformation and stress states of these structures, since it ignores the squares and products of partial derivatives of the displacement components with respect to the material coordinates. When these derivatives are not small, these terms result in a non-linear effect called geometrical non-linearity. In this study, the geometrically non-linear stress analysis of an adhesively bonded T-joint with double support was carried out using the incremental finite element method. Different T-joint configurations bonded to a rigid base and to a flexible base were considered. For each configuration, linear and geometrically non-linear stress analyses of the T-joint were carried out and their results were compared for different horizontal and vertical plate end conditions. The geometrically non-linear analysis showed that the large displacements had a considerable effect on the deformation and stress states of both adherends and the adhesive layer. High stress concentrations were observed around the adhesive free ends and the peak adhesive stresses occurred inside the adhesive fillets. The adherend regions corresponding to the free ends of the adhesive–plate interfaces also experienced stress concentrations. In addition, the effects of the support length on the peak adhesive and adherend stresses were investigated; increasing the support length had a considerable effect in reducing the peak adhesive and adherend stresses.     

Effect of overlap length on durability of joints bonded with a pressure-sensitive adhesive

In this study, the effect of overlap length on durability of a film type adhesive, Structural Bonding Tape (SBT) 9244, which possesses pressure-sensitive and visco-elastic properties, was investigated. Single-lap joints with 1.62 and 3.2 mm adherend thicknesses and at 12.5, 25 and 50 mm overlap lengths consisting of AA2024-T3 alloy as the adherend were exposed to two environmental conditions for exposure times of up to 90 days. The exposure environments were 100% relative humidity (RH) and 3.5% NaCl solution. At the end of exposure times, the failure surfaces were examined by Scanning Electron Microscopy (SEM) after the strength of joints was determined with the lap shear test. It was observed that with increasing overlap length, not only the failure load increased, but also the degradation rate decreased. In addition, as the metal adherends do not absorb any water and moisture from the environments, the metal adherend thickness had no effect on durability of the adhesively bonded joints.     

Single Lap Joints with Rounded Adherend Corners: Stress and Strain Analysis

One of the major difficulties in designing adhesive lap joints is the stress singularity present at the adherend corners at the ends of the overlap. One way to overcome this problem is to assume that the corners have a certain degree of rounding. The objective of the present study was to better understand the effect of the change in the geometry of the adherend corners on the stress distribution and, therefore, on the joint strength. Various degrees of rounding were studied and two different types of adhesives were used, one very brittle and another which could sustain a large plastic deformation. The study gives a detailed stress and strain distribution around the rounded adherends using the finite element method. The major finding is that the stresses or strains in the adhesive layer of a joint with rounded adherend corners are finite. In real joints, adherends generally have small rounded corners. Consequently, the model with small radius corners may be used to represent real adherends.     

Dynamic analysis of single-lap,adhesively bonded composite-titanium joints subjected to solid projectile impact

The transient stress in a single-lap, adhesively bonded composite-titanium joints subjected to solid projectile impact is analyzed using the three-dimensional finite element method. This method is constructed based on the progressive failure features of the composite adherend and the elastic-plastic property of the titanium adherend and adhesive. The effects of the thickness and overlap length of the adhesive layer, the solid projectile size and its velocity, and the strain-rate effect on the dynamic stress of the joints are examined. It is shown that the stress evolution with certain amplitude exists in the joint. During the impact process, compressive stress concentration is imparted at the point of the contact. Furthermore, experiments are carried out for measuring the strain responses of the adhesively bonded joints. A fairly good agreement is observed between the numerical and measured results.     

Single Lap Joints with Rounded Adherend Corners: Experimental Results and Strength Prediction

The objective of the present study was to better understand the effect of the change in the geometry of the adherend corners on the stress distribution in single lap joints and, therefore, on the joint strength. Various degrees of rounding were studied and two different types of adhesives were used: one very brittle and another which had a large plastic deformation. Experimental results on the strength of joints with different degrees of rounding are presented. For joints bonded with brittle adhesives, the effect of the rounded adherend corners is larger than that with ductile adhesives. The strength of joints with brittle adhesives with a large radius adherend corner increases by about 40% compared to that with a sharp adherend corner. It is shown that for joints bonded with brittle adhesives, crack propagation occurs for a short period before it grows into catastrophic failure. However, for ductile adhesives, there is large adhesive yielding and small crack propagation before final failure. Another important feature of joints bonded with ductile adhesives is that there may be more than one crack in the adhesive layer before failure. This makes strength predictions more difficult. The second part of the paper presents an approximate method for predicting the strength of joints bonded with brittle and ductile adhesives, with and without adherend corner rounding. The predictions, based on an average value around the singularity, compare well with the experimental results, especially for joints bonded with ductile adhesives.     

The effects of surface roughness and bond thickness on the fatigue life of adhesively bonded tubular single lap joints

Since the surface roughness of adherends greatly affects the strength of adhesively bonded joints, the effect of surface roughness on the fatigue life of adhesively bonded tubular single lap joints was investigated analytically and experimentally by a fatigue torsion test. The stiffness of the interfacial layer between the adherends and the adhesive was modelled as a normal statistical distribution function of the surface roughness of the adherends. From the investigation, it was found that the optimum surface roughness of the adherends for the fatigue strength of tubular single lap joints was dependent on the bond thickness and applied load.     

Bonding characteristics and corrosion resistance of silane–cerium-treated aluminum adherend

A silane–cerium treatment was applied on an aluminum adherend to simultaneously improve the bonding performance and corrosion resistance of the adhesively bonded aluminum joint in cryogenic applications, such as with liquefied natural gas containment tanks. The lap shear strengths and corrosion performances of the adhesively bonded joints composed of treated aluminum adherends were measured with respect to the silane–cerium treatment and the surface pretreatment on the aluminum adherend. The bonding characteristics of the aluminum adherend were investigated by measuring the water contact angle and conducting the potentiodynamic polarization test after the aluminum adherends with different surface treatments of silane–cerium were immersed in a 0.5?M NaCl solution. In addition, the surfaces were analyzed with scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy to characterize the chemical compositions of the silane–cerium-treated aluminum adherend. The experimental results show that an appropriate silane–cerium treatment on the aluminum adherend produces an effective corrosion-resistant layer and that it has a highly reliable bonding characteristic for the adhesive joint at a cryogenic temperature of ?150?°C.     

Effect of MWCNT filled epoxy adhesives on the quality of adhesively bonded joints

Most adhesively bonded joints exhibit adhesive or cohesive failure, i.e. failure at the adhesive/adherend interface or within the adhesive, respectively. The main objective of this study is to investigate the effect of surface modification of the metal substrate accompanied by modification of the adhesive properties on the strength and failure mechanism of bonded joints. A 5061 aluminium alloy has been used as the metal substrate onto which two types of surface treatments were applied; chemical surface modification and gritblasting. A standard epoxy resin was used as the adhesive medium, in which multi-wall carbon nanotubes (MWCNTs) were dispersed, with a range of weight fraction content (from 0.03% to 0.5%). The resin was fully characterised by mechanical testing in order to determine the optimum weight fraction to enhance its properties. Aluminium to aluminium and glass fibre reinforced polymer (GFRP) composite to aluminium single lap joints bonded with either pure epoxy resin or MWCNT reinforced epoxy resin were subsequently manufactured and tested. The tests show a moderate increase of the joint strength when MWCNTs are added into the adhesive with the failure mechanism changing from cohesive to adhesive. In addition, the comparison between different surface preparation methods shows that gritblasting results in considerably improved adhesive strength over chemical treatment.     

Dynamic strength of single lap joints with similar and dissimilar adherends

The increased use of adhesives for joining structural parts demands a thorough understanding of their load carrying capacity. The strength of the adhesive joints depends on several factors such as the joint geometry, adhesive type, adherend properties and also on the loading conditions. Particularly polymer based adhesives exhibit sensitivity to loading rate and therefore it is important to understand their behavior under impact like situations. The effect of similar versus dissimilar adherends on the dynamic strength of adhesive lap joints is addressed in this study. The dynamic strength is evaluated using the split-cylinder lap joint geometry in a split Hopkinson pressure bar setup. The commercial adhesive Araldite 2014 is used for preparing the joints. The adherend materials considered included steel and aluminum. The results of the study indicated that the dynamic strength of the lap joint is influenced by the adherend material and also by the adherent combination. Even in the case of joints with similar adherends, the strength was affected by the adherend type. The strength of steel–steel joints was higher than that for aluminum–aluminum joints. In the case of dissimilar adherends, the strength was lower than that of the case of similar adherends. The results of this study indicate that the combination of adherend material should also be accounted for while designing lap joints.     

Mechanical properties of adhesively single lap-bonded joints reinforced with multi-‌‌‌walled carbon nanotubes and silica nanoparticles

This paper aims to investigate the effect of adding nanoparticles to the adhesive layer on the shear strength and elongation at failure of adhesively bonded single lap joints (SLJs). Two different toughening particles including the silica nanoparticles (SNPs) and the multi-walled carbon nanotubes (MWCNTs) were considered for reinforcing the adhesive joints. The experimental results showed that the highest improvements in the SLJ shear strength and elongation at failure were obtained for 0.2 and 0.8 wt% of MWCNTs and SNPs, respectively. The fractography results indicated that adding nanoparticles improved the failure mode from adhesive to dominant cohesive representing improved adhesion between the adhesive and adherends. Moreover, different damage mechanisms were observed for the adhesives reinforced with different toughening particles. Several mechanisms including crack growth deviation, shear yielding, plastic deformation, and pull out phenomena were observed from scanning electron microscope (SEM) fractography of the fracture surfaces of the joints reinforced by MWCNTs. While in the case of reinforcing by SNPs, the shear yielding, the particle debonding, and subsequent void growth were found as the effective energy absorbing mechanisms.