Peel Stresses Research Articles

Numerical modelling and experimental validation of step bonded joints

For the joint strength prediction of bonded joints, Fracture Mechanics-based techniques are often used. In this context, the tensile and shear toughness of the adhesives are two of the most important parameters to predict the joint behavior. The Finite Element Method has been used for strength prediction in the last decades. Cohesive zone modelling coupled to a FEM analysis is generally accepted as an accurate method. More recently, the Extended Finite Element Method (XFEM) has emerged. This work aims to validate the XFEM to predict the behavior of stepped-lap joints bonded with the adhesive Araldite® 2015. Peel and shear stresses in the bondline were evaluated, which allows an analysis of the behavior of the adhesive under different geometrical conditions. For the XFEM strength prediction, different damage initiation criteria were used based on either stresses or strains. The damage law shape was also evaluated, namely the linear and exponential damage propagation laws. The XFEM was found to be adequate to predict the joint strength using the Quadratic Stress and Maximum Stress damage initiation criteria.

Shear Strength of an Aluminum Alloy Bonded with a DP-460 Adhesive: Single Lap-shear Joints

Single lap-shear joints (SLJ) specimens with and without partial round fillets were fabricated to measure the average shear strength of adhesives. The effects of the length of the adherend on the SLJ specimens were also investigated. An epoxy adhesive was used to bond aluminum alloy. Tensile tests were performed on the adhesive bulk specimens to measure the mechanical properties. The finite element analysis (FEA) method was used to measure the adhesive stress distributions, i.e., the peel and shear stresses, on the bonded part. The experimental results revealed that the specimen consisting short length of adherend and without the partial round fillets exhibited the smallest average shear strength of adhesive among the investigated specimens. FEA revealed that the low average shear strength for the specimen with a short adherend length was caused by high stress concentrations on the adhesive at the edge of the bonded part.

Thermoelastic effect on inter-laminar embedded delamination characteristics in Spar Wingskin Joints made with laminated FRP composites

This paper presents two sets of full three-dimensional thermoelastic finite element analyses of superimposed thermo-mechanically loaded Spar Wingskin Joints made with laminated Graphite Fiber Reinforced Plastic composites. The study emphasizes the influence of residual thermal stresses and material anisotropy on the inter-laminar delamination behavior of the joint structure. The delamination has been pre-embedded at the most likely location, i.e., in resin layer between the top and next ply of the fiber reinforced plastic laminated wingskin and near the spar overlap end. Multi-Point Constraint finite elements have been made use of at the vicinity of the delamination fronts. This helps in simulating the growth of the embedded delamination at both ends. The inter-laminar thermoelastic peel and shear stresses responsible for causing delamination damage due to a combined thermal and a static loading have been evaluated. Strain energy release rate components corresponding to the Mode I (opening), Mode II (sliding) and Mode III (tearing) of delamination are determined using the principle of Virtual Crack Closure Technique. These are seen to be different and non-self-similar at the two fronts of the embedded delamination. Residual stresses developed due to the thermoelastic anisotropy of the laminae are found to strongly influence the delamination onset and propagation characteristics, which have been reflected by the asymmetries in the nature of energy release rate plots and their significant variation along the delamination front.

Numerical studies on weak bond effects in single and dual adhesive bonded single lap joint between CFRP and aluminium

In this paper, Araldite-2015 and AV138 adhesives are used individually, Araldite-2015 ductile adhesive has been used in single adhesive bonded joint between Carbon Fiber Reinforced Plastics (CFRP) and aluminium. Then Araldite-2015 ductile and AV138 brittle adhesives are used together as separate portions in the bonded region in dual adhesive bonded joints. Different weak bonds are introduced in single and dual adhesive bonded joints. The tensile loads have been applied at the free end of the adherends to find the bond strength of the adhesives. Variations of peel and shear stresses have been captured along the single and dual adhesive bonded joints with varied areas of the weak bond. This numerical study is useful for preliminary research and also reduces time and cost involved to conduct the experimental work. From the studies, it is found that the bond strength decreases with increase in the weak bond area. The weak bond effects in dual adhesive bonded joints are not significant when compared to single adhesive bonded joints. © 2019 Elsevier Ltd. All rights reserved.

Investigation of the Thermal Loading and Random Vibration Influences on Fatigue Life of the Solder Joints for a Metal-Oxide-Semiconductor-Field-Effect Transistor in a DC-DC Power Boost Converter

This study presents the effects of the vibration and thermal cycling on the fatigue life of a power Metal Oxide Semiconductor Field Effect Transistor (MOSFET) in a power converter circuit. The fatigue mechanism in per loading mode was investigated separately and based on the overlap approach, the synchronous effects were analyzed. The solder creeps’ attitudes are depended on the fatigue life for the thermal loops. The success of the deposited strain per thermal loop is in direct relation with the fatigue lifetime. The main source of the stress in the packaging process is the differences between the components’ thermal coefficients. To evaluate the effects of the vibration on the fatigue life for the solder layers, the RMS value of the peeling stress was considered. According to the results, the maximum stress and main affected points realized at the corners of the layers. It has been identified that the assembling of the thermal effects and mechanical loads are quickened the failure rate at the solder joints for this device. The Finite Element Method (FEM) is used for the simulation and the results confirm the estimated crack formation places in the layers.

A nonlinear semi-analytical model for predicting debonding of FRP laminates from RC beams subjected to uniform or concentrated load

A semi-analytical model is developed to determine in FRP retrofitted reinforced concrete (RC) beams the interfacial shear and peeling stresses, the FRP laminate and the RC section strain and stresses at all loading stages up to failure. The FRP is assumed to be externally bonded to the beam but can undergo slip and relative vertical displacement at its interface with the concrete. The model is developed by satisfying the requirements of equilibrium and strain compatibility while concurrently allowing for interfacial deformations. FRP is treated as a linear elastic, steel as elasto-plastic strain hardening and concrete as fully nonlinear material in compression and tension, including tension stiffening. The governing equations are formulated as two second order differential equations with their dependent variables being the strain in the FRP and the relative normal displacement of the interface. The equations are solved for discrete states (uncracked, cracked, yielded) experienced by the RC section and their associated level of interfacial slip. The model results are compared with available experimental results for several beams retrofitted with carbon FRP or steel reinforced polymer (SRP) laminates subjected to either four point bending or simulated uniform load, with satisfactory agreement between them.

Mode I cohesive zone model parameters identification and comparison of measurement techniques based on uncertainty estimation

Adhesive bondline mechanical behaviour is frequently described with cohesive zone models (CZM). For mode I loading condition these phenomenological laws simply represent the evolution of the peel stress as a function of the two adherends relative displacement normal to the joint. Generally, these laws are identified rather than really measured using experimental data obtained from crack initiation and propagation experiments such as the Double Cantilever Beam Test (DCB). The uncertainty on parameter estimation are generally not indicated, as for a DCB test it is only the critical energy release rate that has the most influence on the results. However, the uncertainties on the other parameters prevent the use of the identified TSL for other mechanical tests where mode I solicitations are predominant. In this article, the purpose is to evaluate the methodologies reliability for the assessment of mode I CZM. To do so, several methods used to evaluate CZM parameters are compared in terms parameter estimation reliability. Synthetic noisy data are considered for a χ² function minimisation. Then, sensitivity calculations are performed to determine the estimated parameters standard deviation. By applying this procedure on different type of synthetic measurements (respectively P(Δ), J(δ,θ), backface strain and DIC) the ability of these different techniques to capture the best parameters for a chosen CZM shape can be rigorously evaluated.

Adhesively- and hybrid- bonded joining of basalt and carbon fibre reinforced polybenzoxazine-based composites

Polybenzoxazines-based composites are an excellent alternative in lightweight design due to their outstanding properties in terms of chemical stability, inherent flame resistance, as well as mechanical performance. The present work provides an investigation of different joining techniques for polybenzoxazine-based composites, namely adhesively - and hybrid bonded joining. The effects of fibre reinforcement (carbon or basalt), and substrate thickness (2 or 3 mm) on the lap-shear strength of joints are taken into consideration. Here, basalt (BFRP) and carbon fibre reinforced plastics (CFRP) were manufactured by vacuum infusion technique followed by a convection oven curing. Mechanical characterisation of substrates included tensile and interlaminar shear strength testing. Hybrid joints were fabricated by rivet insertion in cured joints bonded with a structural adhesive. Adhesive bonding showed to be a proper joining technique presenting a good adhesion with substrates, and adequate cohesive strength. Hybrid bonded joints had a fail-safe fracture and higher energy absorption prior to failure than their adhesively bonded counterparts. No effect of substrate thickness on lap-shear strength was observed. The dominating failure mechanism for the joints was the delamination of the topmost-layer due to peel stresses. CFRP joints exhibited higher lap-shear strength than BFRP joints, which was consistent with interlaminar shear strength results.

Weak bond effects in adhesively bonded joints between the dissimilar adherends

ABSTRACT Adhesively bonded joints provide uniform stress distributions for the width in the bonded region, which is preferred for lightweight assemblages in aircraft structures, but the bonded region is hidden between adherends and suffers due to the existence of weak bonds. Adhesively bonded joints between dissimilar adherends are susceptible to weak bonds due to large differences in the properties and behaviour of adherends. Weak bonds present at the interfaces due to improper curing of adhesive, may go undetected and affect the bond strength. In this paper, an attempt has been made to characterize weak bond effects in adhesive bonded joints between dissimilar adherends. Adhesively bonded single lap joint specimens have been fabricated with CFRP and aluminium using Araldite-2015 adhesive with varied areas of weak bond. PTFE (polytetrafluoroethylene) films used to introduce weak bonds in the bonded region. X-ray radiography and ultrasonic tests have been performed on specimens to verify the presence of weak bond areas. Uniaxial tensile tests have been conducted on specimens to evaluate the mechanical response of adhesive joints in the presence of weak bonds and to obtain failure loads and corresponding failure patterns. Finite element analysis has been carried out to evaluate variations of peel and shear stresses along the bond length. From the studies, it is found that failure occurs at the interface where weak bonds are present irrespective of dimensions of weak bonds and the corresponding failure load decreases with increase in the weak bond area resulting in interface failure.

Viscoelastic analysis of stress distribution in balanced and unbalanced adhesively bonded single-lap joints with functionally graded adherends under the Reddy model

In this study, shear and peel stress distributions in the viscoelastic adhesive layer of a single-lap joint (SLJ) with functionally graded (FG) adherends are investigated. The study focuses on the effect of different adherend profiles and material composition on the time-dependent stress concentration/distribution in balanced and unbalanced SLJs. For this purpose, the Reddy model is applied to the FG adherends and a three-parameter solid viscoelastic model is used to simulate the adhesive layer behavior. Using the first-order shear deformation theory for the FG adherends, the governing differential equations are derived and then transformed into the Laplace domain. A finite element model of the joint was also developed to further backup the numerical solution. The numerical inverse Laplace transform method was used to extract the desired results that were then compared with those of finite element method (FEM) findings. Very good agreements were observed between the results of both methods. Results show that the geometric and mechanical properties of the FG adherends have an essential role in reducing the shear and peel stress concentrations as well as the uniformity of shear stress distribution in the overlap region. Results also show that either method (finite element or the proposed semi-analytical method) can be utilized with confidence for prediction of stress relaxation in the adhesively bonded SLJs with FG adherends.

The experimental and numerical analysis of the adhesively bonded three-step-lap joints with different step lengths

Adhesive bonding is one of the most used joining methods for bonding composite or different types of materials. The joint type commonly used in adhesive bonding is the single lap joint (SLJ) type. However, in adhesively bonded joins, peel stress occurs due to the eccentric loading, which causes damage to the ends of the overlap area. Different types of joints are used to reduce these peel stresses. One of these joint types is the step-lap joint type obtained by creating steps in the overlap area. In this study, mechanical properties of the single lap joint (SLJ), one-step lap joint (OSLJ) and three-step lap joint (TSLJ) with five different step lengths subjected to tensile loading were examined experimentally and numerically by keeping the bonding area same for all samples examined. In this experimental study, AA2024-T3 aluminum alloy was used as the adherend and DP460, a structural adhesive with two components, was used as the adhesive. In addition, the progress of the crack in the overlap area was observed with a high-speed camera. As a result, the TLSJ type was found to carry more load than other joint types. Also, the change in the step length in the three-step lap joint type had a significant effect on the failure load of the joint. When the failure loads obtained in the experiments and the numerical analysis were examined, it was concluded that their results were quite compatible with each other when the cohesive zone model was used in the numerical analysis. Another result obtained from the study is the optimum length of the first step created at the ends of the overlap area in the three-step lap joint.

Low-velocity impact response of friction riveted joints for aircraft application

In this paper, the sensitivity of carbon fiber reinforced polyether-ether-ketone (CF-PEEK) friction riveted joints to impact damage was assessed as well as the damage propagation during fatigue and quasi-static mechanical testing. The joints were impacted with energies between 5 J and 30 J at room temperature and the impact damage was evaluated through microscopy and ultrasonic C-scan. Two damage types were identified: barely-visible impact damage with mainly shear-driven damage in the first plies of the composite and visible impact damage with delamination and premature failure of the rivet-composite interface owing to peel stresses upon the impact event. The joint strength and fatigue life were not compromised by the barely-visible impact damage, while a 40% decrease of quasi-static strength and lower fatigue resistance were achieved for visible impact damage. Despite altered fatigue behavior of impacted joints, damage accumulated towards fatigue was not critical to the joint mechanical integrity, confirmed by the residual strength of up to 96% after 106 cycles for 20 J impacts.

Effect of adherend deflection on lap-shear tensile strength of laser-treated adhesive-bonded joints

Laser surface treatment was used as a surface preparation method for adherend prior to adhesive bonding, which significantly improved the fracture mode from mixed to cohesive and consequently increased the lap-shear tensile strength by 27.5% and 28.2% for aluminum alloys AA7075 and AA6022 adhesive-bonded joints, respectively. Nevertheless, the laser-treated AA7075 joint exhibited a 13.9% higher strength compared to laser-treated AA6022 joint although both joints showed cohesive fracture modes, which was attributed to smaller adherend deflection of laser-treated AA7075 joint than that of laser-treated AA6022 joint. Digital image correlation technique was applied to monitoring the deflections of laser-treated AA7075 and AA6022 joints during lap-shear tensile testing, and the stress analysis of adhesive-bonded lap-shear joints was conducted by finite element simulation. It was found that the deflection angle of the laser-treated AA6022 joint reached to 6.1° at the peak tensile load and was ∼69% larger than that of laser-treated AA7075 joint (i.e. 3.6°), which resulted in a mixed stress state with higher peel stress (i.e. ∼8 MPa) on edges of adhesive bonding region and consequently led to a premature fracture due to the low peel resistance of adhesive-bonded joints. Furthermore, the effect of decreased adherend deflection on lap-shear tensile strength was investigated, and a linear increase of lap-shear tensile strength was found with the decrease of deflection angle for adhesive-bonded joints showing cohesive fracture modes.

Interfacial stress of carbon fiber patch bonded to transverse cracked steel plate reinforcement

A three-dimensional elastic-plastic finite element model was established in finite element software ANSYS to investigate the interfacial stress of carbon fiber patch bonded to transverse cracked steel plate reinforcement. The patch and adhesive were modeled as linear elastic. The steel plate was modeled as elastic-plastic. The load-deflection curve of the steel was obtained by bending test. The influence of crack depth, adhesive thickness, patch thickness and patch length on reinforcing efficiency was studied. The results show that the peel stress of the adhesive was distributed symmetrically along the center line, mainly concentrated on the crack center, near the crack and the patch ends. The peel stress at the crack center was always greater than the near crack. The shear stress is anti-symmetric along the center line, and the shear stress at the crack center is zero, mainly concentrated near the crack and plate end. The debonding may be initiated from the vicinity of the crack. Increasing the crack depth can significantly increase the peel stress and shear stress at the crack, but has little effect on the stresses at the end. Increasing the thickness of the adhesive can reduce the peel stress and shear stress at the crack center and near the crack, and the shear stress at the ends, but has little effect on the peel stress at the ends. Increasing the patch thickness can reduce the stress at the crack but increase the stress at the end. Increasing the patch length cannot reduce the stress at the crack, but significantly reduce the stress at the end.

Fracture behavior of TBCs with cooling hole structure under cyclic thermal loadings

The integrity of thermal barrier coatings (TBCs) plays an important role in the performance of turbine blades, nevertheless the effect of cooling hole on fracture of TBCs have been less studied. The present work aimed to study the stress evolution and fracture behavior of TBCs with cooling hole under cyclic thermal loadings. Cyclic thermal tests were conducted and the cracking behavior of TBCs around cooling hole was examined using scanning electron microscope (SEM). TBCs exhibited two types of cracks near the hole edge, i.e. surface crack in the top coat and interfacial crack, which could lead to local TBC spallation and shorten the TBC lifetime. To disclose the fracture mechanism, finite element (FE) analysis was also performed. The computed residual stress values were consistent with those measured by Raman spectroscopy tests. FE simulations indicated that the free-edge effect facilitated interfacial peeling and shear stresses near the cooling hole, and hence promoted the initiation of interfacial crack in TBCs. With the increase of thermal loading cycles, the interfacial crack propagated and then coalesced with the surface cracks in the top coat, leading to the final spallation of TBCs at the cooling hole.

Interface crack model using finite fracture mechanics applied to the double pull-push shear test

An analytical procedure predicting a debond (interface crack) onset and growth in an adhesive joint between two beams or plates is developed and applied to a specific configuration often used in reinforcement tests in civil engineering. The procedure is based on Timoshenko beam theory and Linear Elastic-(Perfectly) Brittle Interface Model (LEBIM) combined with the Coupled Criterion of the Finite Fracture Mechanics (CC-FFM) for mixed-mode fracture. First, a sixth order differential equation in the shear stresses along the adhesive layer is deduced and solved, leading to closed form expressions for both shear and normal stresses in the adhesive. Then, the critical value of the applied load necessary to produce debonding is predicted by coupling a stress and an energy condition based on: (i) the stress distribution produced in the interface before the debond onset and (ii) the energy released during the debonding process along the interface. Although the developed procedure can be applied to several types of joints with different geometries, materials and loads (e.g., double lap joint tests including adherents made of steel or composites), herein it is applied to the double pull-push shear test where the debond onset and growth between a Carbon Fibre Reinforced Polymer (CFRP) laminate and a concrete block occurs. For such a case, the debond is produced under predominant fracture mode II; nevertheless, it is shown that relevant normal (peeling) stresses associated to mode I may appear as well. A comparison of the present solution with a previous one by the shear-lag model is provided as well.

A novel approach for the interfacial stress analysis of composite adhesively bonded joints

This paper presents a novel approach for the interfacial stress analysis of composite single-lap joints (SLJ). The analytical modeling is based on the interface elasticity where both the shear and peel stresses are assumed to be variant across the adhesive layer thickness. The unknown interfacial parameters are determined by using an experiment-based inverse method. The finite element simulation is then performed to calculate the interfacial stress distributions and evaluate the effect of the structural parameters on the interfacial behavior of the composite bonded joints. It is shown that both the peel and shear stresses at the interface are influenced by the interface energy and geometry parameters of the composite bonded joint. Numerical results are presented both to demonstrate the advantages of the present model over existing ones and to illustrate the main characteristics of interfacial stress distributions.

Function reconsideration of indium bump in InSb IRFPAs

Indium bumps are generally accepted to possess functions of electrical connection, mechanical support and heat transfer in flip-chip devices. After comparing the distribution of stress components along different paths in InSb infrared focal plane arrays (IRFPAs), we ascertain that local enhancement effects of the stress components are remarkable in regions where the indium bumps are. More specifically, the local enhanced tensile stress in InSb chip connected with the indium bumps can lead to the local fracture of the InSb chip, and the locally enhanced shear and peeling stresses may give rise to the local interfacial delamination between the InSb chip and the indium bumps. These inferences are confirmed by the observed local failure characteristics, such as the distribution of the local delamination, the distribution of the crack widths, and the distribution of the cracks. In addition, the simulated Z-component strain distribution in InSb IRFPAs is also consistent with the backside surface profile of the InAs/GaSb IRFPAs fabricated in America with the identical structure, that is, the InSb chip glued with the indium bumps is concave downward, and the InSb chip glued with the underfill is convex upward. Judging from all these confirmed simulation results, we are confident that the indium bumps play a pivotal role in inducing the local failure of InSb IRFPAs. So the role of the indium bumps in causing the local failure of InSb IRFPAs needs to be supplemented and emphasized to comprehensively evaluate the structural reliability of InSb IRFPAs.

Stress analysis of the thermal barrier coating system near a cooling hole considering the free-edge effect

Due to the thermal mismatch between layers and the free-edge effect, interfacial peeling and shear stresses are generated locally around the edges of cooling holes in a thermal barrier coating (TBC)–film cooling system. These interfacial peeling and shear stresses may lead to modes I and II edge delamination, resulting in TBC spallation around the cooling hole. In this study, analytical and numerical models were built to study the stress and interfacial cracking behaviors of TBCs near the cooling hole. Analytical solutions for interfacial peeling moment and shear force at each layer were obtained to analyze the free-edge effect on the stress distributions in TBCs, and they were verified by the finite element calculations. The results showed that interfacial peeling moment and shear force were functions of the hole radius and thicknesses of top coat and oxide layer. The increase of interfacial peeling moment and shear force raised the likelihood of edge cracking around the hole. Derived by the local stresses, the interfacial cracks in TBCs initiated and propagated from the hole edge upon cooling.

Tunneling Crack Initiation in Trailing-Edge Bond Lines of Wind-Turbine Blades

Tunneling cracks are observed in the adhesive of the trailing-edge bond line of wind-turbine rotor blades subjected to high-cycle fatigue. To identify the root causes of these cracks, their initiation was calculated using a stress-based approach. The mechanical stresses caused by aerodynamic and gravity loads, which arise when operating in the field, were considered on the one hand, and the residual stresses caused by thermal material shrinkage during manufacture on the other. This work investigates the impact of each mechanical stress and thermal residual stress component on the fatigue stress exposure along the blade span. It was found that the thermal residual stresses make a significant contribution to the bond-line fatigue. Besides the dominating longitudinal stress, peel and shear stress also contribute to the fatigue.