Residual static strength and the fracture initiation path in adhesively bonded joints weakened with interfacial edge pre-crack

This paper deals with the application of fracture mechanics approaches for predicting the residual static strength and the crack kinking angle of adhesively bonded joints containing interfacial edge pre-cracks. The interfacial cracks are created due to different factors such as inappropriate surface preparation which cause a significant reduction of the joint strength. To investigate the residual strength of interfacial cracked adhesive joints and predict the crack kinking angle, three different approaches including the maximum tangential stress (MTS), the minimum strain energy density (SED) and the maximum tangential strain energy density (MTSED) were assessed. To this end, single lap joints (SLJs) containing a brittle adhesive material and with different pre-crack sizes and various substrate thicknesses were manufactured and tested. The results were also verified by applying fracture mechanics approaches on previously published experimental data. According to the results, it was concluded that in mode II dominant cases, the predictions of kinking angle using the MTS method was in good agreement with the experimental observations, while in mode I dominant cases the mentioned approach provided poor predictions. It was also found that the SED criterion could be a precise model for predicting the crack extension angle in mode I dominant conditions. The results also showed that the MTS criterion predicts the residual static strength of interfacial cracked adhesive joints very well.     

Influence of multi-walled carbon nanotubes on creep behavior of adhesively bonded joints subjected to elevated temperatures

Polymeric materials are prone to creep loading. This paper is aimed to study the effect of multi-walled carbon nanotubes (MWCNTs) on creep behavior of adhesively bonded joints. Neat and MWCNTs-reinforced adhesively bonded joints were manufactured and tested under creep loading at elevated temperatures. Two MWCNT weight percentages of 0.1 and 0.3 were used for reinforcing the single lap joints (SLJs) and the joints were tested at different temperature and load levels. The results showed that 0.1 wt% of MWCNTs resulted maximum improvements in creep behavior of adhesive joints. Adding 0.1 wt% of MWCNTs into the adhesive layer caused maximum reductions of 57%, 60% and 47% in the steady-state creep rates of the joints tested at 30, 40 and 50°C, respectively. Furthermore, 0.1 wt% of MWCNTs resulted maximum reductions of 29%, 33% and 37% in the creep strains corresponding to a specific creep loading time and maximum reductions of 23%, 45% and 49% in the elastic strains corresponding to the time at which creep loading started.     

Taguchi analysis of bonded composite single-lap joints using a combined interface–adhesive damage model

The mechanical behaviour of bonded composite joints depends on several factors, such as the strength of the composite–adhesive interface, the strength of the adhesive and the strength of the composite itself. In this regard, a finite element model was developed using a combined interface–adhesive damage approach. A cohesive zone model is used to represent the composite–adhesive interface and a continuum damage model for the adhesive bondline. The influence of the composite–adhesive interfacial adhesion and the strength of the adhesive on the performance of a bonded composite single-lap joint was investigated numerically. A Taguchi analysis was conducted to rank the influence of material parameters on the static behaviour of the joint. It was found that the composite–adhesive interfacial fracture energy and the mechanical properties of the adhesive predominantly govern the static performance of the joints. A parametric study was performed by varying the most important material parameters, and a response surface equation is proposed to predict the joint strength. It is shown that the influence of experimental parameter variations, e.g. variation in adhesive curing and surface preparation conditions, can be numerically accommodated to investigate the static behaviour of bonded composite joints by combining finite element and statistical techniques. The methods presented could be used by practicing engineers to describe the failure envelope of adhesively bonded composite joints.     

Analysis and design of adhesively bonded joints for fatigue and fracture loading: a fracture-mechanics approach

An experimental–computational fracture-mechanics approach for the analysis and design of structural adhesive joints under static loading is demonstrated by predicting the ultimate fracture load of cracked lap shear and single lap shear aluminum and steel joints bonded using a highly toughened epoxy adhesive. The predictions are then compared with measured values. The effects of spew fillet, adhesive thickness, and surface roughness on the quasi-static strength of the joints are also discussed. This fracture-mechanics approach is extended to characterize the fatigue threshold and crack growth behavior of a toughened epoxy adhesive system for design purposes. The effects of the mode ratio of loading, adhesive thickness, substrate modulus, spew fillet, and surface roughness on the fatigue threshold and crack growth rates are considered. A finite element model is developed to both explain the experimental results and to predict how a change in an adhesive system affects the fatigue performance of the bonded joint.     

Dynamic compressive creep behaviour of structural silicone-to-aluminum and silicone-based sealant-to-aluminum joints

This paper outlines an experimental study on the static and dynamic compressive creep behaviour of structural silicone adhesively bonded joints. The silicone adhesives are subjected to dynamic compressive loading, which is a common case for structural façade and hybrid glazing system. Typical crack propagation of adhesives, relations between compressive load (stress) and displacement (creep strain) are examined experimentally. It is shown that the test specimen with adhesives featured by lower hardness and higher elongation at break exhibit notable crack distribution concentrated in the middle of the crack surface. The compressive behaviour consists of three regions as initial elastic, nonlinear transition and post linear, in which the latter has notable strength increase with the increase of compressive deformation. The secant compressive modulus are measured based on compressive stress and creep strain relations. It is demonstrated that the joint has higher secant compressive modulus due to less crack propagation. All test joints exhibit significant degradation of strength and energy absorption, which can be well fitted in similar exponential forms with normalized cycle numbers for test joints with different adhesives.     

Impact strength of joints bonded with high-strength pressure-sensitive adhesive

The impact strength of joints bonded with a double-coated high-strength pressure-sensitive adhesive (PSA) was experimentally investigated. PSA has recently been used to join parts of mobile devices such as smart-phones, which are often subjected to drop impacts. Consequently, the impact strength of PSA bonded joints has become important.Two types of specimens, butt joint specimens and double cantilever beam (DCB) specimens bonded with adhesives were utilized for the experiments. Quasi-static tests and impact tests of the specimens were carried out using a mechanical testing machine and an impact testing machine. The PSA layers in the specimens were observed using a high-speed digital camera. The deformation and strain distribution in the adherends of the DCB specimens were also measured using a novel high-speed digital camera with photoelastic imaging capability.Though the strength of the butt joints increased as the loading rate increased, the critical fracture energy of the DCB specimens decreased at high loading rates. This may be attributed to the transition to the brittle nature of the PSA in the loading range in which no cavitation occurred. To verify the critical fracture energy obtained with the DCB tests, finite element analyses (FEA) based on the cohesive zone model (CZM) were carried out, and the load–displacement curves of the DCB tests were simulated. The predicted results showed good agreements with the experimental results.     

Thermo-fracture analysis of composite-aluminum bonded joints at low temperatures: Experimental and numerical analyses

Adhesive joints have found extensive applications in aerospace structures because of important advantages such as uniform stress distribution, thermal, acoustic and electrical insulation as well as capability of joining dissimilar materials. These joints in aerospace structures frequently experience severe low temperatures. Lack of experimental data in this field motivated the study of the fracture of adhesive joint at low temperatures in this paper. Fracture parameters of carbon-fibre-reinforced polymer-based composites (CFRP) and aluminum bonded joints were investigated in a temperature range of −80 to +22 °C. In order to understand the mechanical behavior of different components of the bonded joint, firstly, the components (adhesive, composite, and aluminum) were characterized by conducting tensile tests. Subsequently, specimens of cracked bonded joint were tested at low temperatures in different loading modes (mode I, mode II, and mixed mode I/II). The finite element model of the bonded joint was developed in order to obtain the dimensionless functions of stress intensity factors at lower temperatures. The results showed that a reduction in temperature down to a particular value contributes to improved critical stress intensity factors, while any further reduction in the temperature tends to lower the critical stress intensity factors, eventually leading to decreased fracture energy absorption capacity of the structure. In the final section of this paper, a study on fracture surfaces and fracture mechanisms was performed via macroscopic and scanning electron microscopic (SEM) analyses.     

Experimental and numerical investigation of the static response of environmentally aged adhesively bonded joints

The aim of this research is to investigate the effect of moisture on the static response of adhesively bonded monolithic single lap joints and laminated doublers loaded in bending. All joints were made of aluminium alloy Al 2024-T3 bonded using epoxy film adhesive FM 73M OST. The joints were aged in deionised water at a temperature of 50 °C for up to 2 years exposure. The use of different widths of specimen (5 mm for monolithic single lap joints and 15 mm for laminated doublers) allowed both full and partial saturation of the adhesive layer. The bulk adhesive has been characterised to obtain the coefficient of moisture diffusion, the coefficient of thermal and moisture expansion and the moisture dependent mechanical properties. The testing results showed that the mechanical properties degraded in a linear way with the moisture content. The residual strength after exposure decreased with increasing moisture content (exposure time) and tended to level off towards saturation. The damage evolution and failure of the joint has been successfully monitored using the backface strain technique and in-situ video microscopy. Progressive damage finite element modelling using a moisture dependent, bilinear traction-separation law has been undertaken to predict the residual strength. Residual stresses due to thermal and swelling strains in the adhesive layer have been included; however their effect on the predicted static strength was not significant. Good agreement was found between the predicted residual strength and the experimental result.     

Use of master curves based on time-temperature superposition to predict creep failure of aluminium-glass adhesive joints

Advancements in materials technology and the use of innovative designs have led to extensive application of adhesive bonding techniques in the electric appliance industry. While the static strength of such joints is sufficient for the intended applications, long term durability remains a major concern, mainly due to creep effects. Conventional creep testing can be performed at the service temperature but it is a long test that can take decades, although it can be accelerated using high temperatures. In this work, glass-aluminium joints were studied under static and creep loads. Glass-aluminium specimens were subjected to creep testing at various temperatures. Using the time temperature superposition principle, the results of these individual creep tests were combined in a master curve that approximates the creep behaviour of the adhesive joint in a long time period. These master curves were used to guarantee a minimum service life of the joint.     

Numerical and experimental analysis of bonded joints with combined loading

The metallic materials bonding using structural adhesives has become an increasingly used process, presenting advantages when compared to other fastening methods such as screws and rivets. The aim of this paper is the numerical evaluation of bonded joints with combined loading (traction and shear) using the finite element method, comparing the results obtained with the experiments performed at the same configurations. Considering adhesive joints with the same bonded area, but with different linear dimensions, the mechanical strength may be different, which characterizes the shape factor. In this way, the analyzes considered the bonded area shape factor in nine different configurations, being modified both the height and the width of the joint, considering two points of force application for each group. For the numerical simulation, the cohesive zone models (CZM) were used, which use the concepts of linear elastic fracture mechanics (LEFM). These models consider that one or multiple interfaces or regions of fracture may be artificially introduced into the structures, which is done through the separation-traction laws. For this purpose, DCB (double cantilever beam) and ENF (end notched flexure) tests were performed, measuring this way the essential cohesive properties to the numerical modeling, especially the critical energy release in I and II modes (normal and shear, respectively). The influence of some cohesive properties on the maximum load of the bonded joint was investigated. The good numerical and experimental concordance in different configurations studied confirms that the CZM provide consistent results with the bonded joint experiments for the presented conditions of adhesive thickness, surface treatment and load application point, not only in single lap joints, but also in combined loading joints, whose investigation was done in this work.     

Failure behavior of adhesively bonded joints with a rubber modified epoxy adhesive under tensile-shear combined cyclic loading

Rubber-modified epoxy adhesives are used widely as structural adhesive owing to their properties of high fracture toughness. In many cases, these adhesively bonded joints are exposed to cyclic loading. Generally, the rubber modification decreases the static and fatigue strength of bulk adhesive without flaw. Hence, it is necessary to investigate the effect of rubber-modification on the fatigue strength of adhesively bonded joints, where industrial adhesively bonded joints usually have combined stress condition of normal and shear stresses in the adhesive layer. Therefore, it is necessary to investigate the effect of rubber-modification on the fatigue strength under combined cyclic stress conditions. Adhesively bonded butt and scarf joints provide considerably uniform normal and shear stresses in the adhesive layer except in the vicinity of the free end, where normal to shear stress ratio of these joints can cover the stress combination ratio in the adhesive layers of most adhesively bonded joints in industrial applications.

In this study, to investigate the effect of rubber modification on fatigue strength with various combined stress conditions in the adhesive layers, fatigue tests were conducted for adhesively bonded butt and scarf joints bonded with rubber modified and unmodified epoxy adhesives, wherein damage evolution in the adhesive layer was evaluated by monitoring strain the adhesive layer and the stress triaxiality parameter was used for evaluating combined stress conditions in the adhesive layer. The main experimental results are as follows: S–N characteristics of these joints showed that the maximum principal stress at the endurance limit indicated nearly constant values independent of combined stress conditions, furthermore the maximum principal stress at the endurance limit for the unmodified adhesive were nearly equal to that for the rubber modified adhesive. From the damage evolution behavior, it was observed that the initiation of the damage evolution shifted to early stage of the fatigue life with decreasing stress triaxiality in the adhesive layer, and the rubber modification accelerated the damage evolution under low stress triaxiality conditions in the adhesive layer.     

Drop weight tensile impact testing of adhesively bonded carbon/epoxy laminate joints

Crashworthiness of composite structures is a key issue for the design of lightweight vehicles. In particular the joined parts of the structures must be able to absorb a high amount of energy in order to protect the passengers. In this paper the dynamic behavior of adhesively bonded carbon/epoxy laminate joints is investigated. The adherends are made of unidirectional plies, whose orientations are carefully chosen in order to assess the influence of the adherend mechanical properties on the joint behavior. A drop weight machine has been modified in order to impact specimens under tension. Single lap joints are tested under impact tension at velocities from 1 to 4 m/s. Results of the impact tests that are compared to reference quasi-static test results emphasize the rate-sensitivity of the joints. The stiffness, the failure load and the absorbed energy all increase with increasing loading rate. One major result is that the joint behavior is qualitatively the same under quasi-static and impact loading: the failure mode and the joint ranking (based on their strength) remain identical. Therefore the impact design of the adhesive joints could be based on a static design at moderate loading rates.     

A review of finite element analysis of adhesively bonded joints

The need to design lightweight structures and the increased use of lightweight materials in industrial fields, have led to wide use of adhesive bonding. Recent work relating to finite element analysis of adhesively bonded joints is reviewed in this paper, in terms of static loading analysis, environmental behaviors, fatigue loading analysis and dynamic characteristics of the adhesively bonded joints. It is concluded that the finite element analysis of adhesively bonded joints will help future applications of adhesive bonding by allowing system parameters to be selected to give as large a process window as possible for successful joint manufacture. This will allow many different designs to be simulated in order to perform a selection of different designs before testing, which would currently take too long to perform or be prohibitively expensive in practice.     

Investigation of the creep behavior of graphene oxide nanoplatelet-reinforced adhesively bonded joints

In this paper, the effect of adding graphene oxide nano-platelets (GONPs) into the adhesive layer was investigated on the creep behavior of adhesively bonded joints. The neat and GONP-reinforced adhesive joints were manufactured and tested under creep loading with different stress and temperature levels. 0.1?wt% GONPs revealed the highest improvement on the adhesive joint creep behavior amongst the studied weight percentages. Furthermore, the effect of GONPs on the creep behavior of adhesive joints was more significant at higher temperatures. It was found that adding 0.1?wt% of GONPs into the adhesive layer imposed reductions of 21%, 31% and 34% in the elastic shear strains and reductions of 24%, 31% and 37% in the creep shear strains of SLJs under testing temperatures of 30, 40 and 50?°C, respectively. The Burgers rheological model was employed for simulating the creep behavior of the neat and GONP-reinforced adhesive joints. The Burgers model parameters were obtained as functions of testing temperature, creep shear stress and GONP weight percentage using a response surface methodology. Reasonable agreement was obtained between the modeled and experimental creep behaviors of the adhesive joints.     

Influence of mechanical surface treatment on fatigue life of bonded joints

Adhesively bonded joints can support a longer fatigue life if compared to conventional joining techniques, provided that a set of requirements is fulfilled. One of the most important requirements is the mechanical preparation of the bonded joint surface, which improves the joint interface adhesion. The aim of this work is to investigate the influence of surface roughness of mild steel substrates on fatigue behavior in adhesive bonded plates. To accomplish this objective, three different surface treatments were used on A36 steel substrate specimens, namely sand blasting, grit blasting, and bristle blasting. Bonded plate specimens, using end-notched flexure format, with a thin adhesive epoxy layer were manufactured and tested, under mode II loading condition, in both static and dynamic tests. The results confirm the importance of surface treatment of the substrate on the fatigue life, confirming that adhesively bonded joints have significant performance differences when subjected to static and dynamic loadings.     

Characterization of fracture in adhesively bonded double-lap joints

A broad finite element study was carried out to understand the stress fields and stress intensity factors behavior of cracks in adhesively bonded double-lap joints, which are representative of loading in real aerospace structures. The interaction integral method and fundamental relationships in fracture mechanics were used to determine the mixed-mode stress intensity factors and associated strain energy release rates for various cases of interest. The numerical analyses of bonded joints were also studied for various kinds of adhesives and adherends materials, joint configurations, and thickness of adhesive and different crack lengths. The finite element results obtained show that the patch materials of low stiffness, low adhesive moduli and low tapering angles are desirable for a strong double-lap joint. In the double-lap joint, the shearing-mode stress intensity factor is always larger than that of the opening-mode and both shearing and opening mode stress intensity factors increase as the crack length increases, but their amplitudes are not sensitive to adhesive thickness. Results are discussed in terms of their relationship to adhesively bonded joints design and can be used in the development of approaches aimed at using adhesive bonding and extending the lives of adhesively bonded repairs for aerospace structures.     

Numerical analysis and experimental tests for long-term strength of adhesive bonds

This paper presents selected numerical analysis results on static strength of adhesive layers which were subjected to long-term loads. The numerical calculations involved modelling the properties of the adhesive layer using the Burger's model. The coefficients of the Burger's model components were determined on the basis of the creep curves of the adhesive. Some results were verified experimentally. It was ascertained that it is possible to examine the problems of long-term strength of adhesive bonds using numerical analysis under certain limitations. Numerical tests should reduce the need for time-consuming experiments. The investigations pointed out adhesive joints long-term strength dependence on creep curves of adhesives. The long-term strength of adhesive joints increased by reinforcement of the adhesive layer with glass fabric.     

Numerical analysis of the strength and interfacial properties of adhesive joints with graded adherends

The strength and interfacial behavior of single lap joints with graded adherends subjected to uniaxial tensile loading are investigated in the present paper. A bilinear cohesive zone model coupled with the finite element method is adopted to describe the damage and failure process of the adhesive layer. The peak loading, the rotation angle between the overlap of the joint and the horizontal direction, as well as the failure energy are investigated comprehensively. It is interesting to find that adopting different variation law in the graded adherends may result in varying strength of adhesive joints. By means of choosing proper material and geometry parameters of adhesive joints, the peak loading, the rotation angle and the failure energy of joints can be greatly improved. What is more, the strength of the SLJ is found to depend much more on the property of the soft part near the adhesive layer. The results should be helpful to guide the design of novel structures of adhesive joints in present and potential applications.     

Strain energy release rate based damage analysis of functionally graded adhesively bonded tubular lap joint of laminated FRP composites

Strain energy release rate (SERR) based damage analyses of functionally graded adhesively bonded tubular lap joints of laminated fiber reinforced plastic (FRP) composites under varied loadings have been studied using three-dimensional geometrically non-linear finite element (FE) analyses. FE simulations have been carried out when a tubular joint is subjected to axial and pressure loadings. SERR is utilized as the characterizing and governing parameter for assessing damages emanating from the critical location. Individual and total SERR over the damage front have been computed using modified crack closure integral (MCCI) based on the concept of linear elastic fracture mechanics. Results reveal that damage initiation locations in tubular joints subjected to axial and pressure loadings are entirely different. Furthermore, modes responsible for propagation of such damages in tubular joints under axial and pressure loadings are also different. Based on the FE simulations, tubular joints under pressure loading are found to be more vulnerable for damage initiation and its propagation. Furthermore, the damage propagation behavior of tubular joints with pre-embedded damages at the critical location has been compared between conventional mono-modulus adhesives and functionally graded adhesives with appropriate material gradation profile. Results indicate that material gradient profile of the adhesive layer offers excellent reduction in SERR for shorter interfacial failure lengths in tubular joints under axial loading which is desired to delay the damage growth. Improved crack growth resistance in the joint enhances the structural integrity and service life of the tubular joint structure. However, considerable reduction in SERR has not been noticed in the said joint when subjected to pressure loading. Hence, the use of functionally graded adhesive along the bond layer is recommended for the designer/technologist while designing tubular joint under general loading condition.     

Effects of Different Curvature Patches on the Strength of Double-Strap Adhesive Joints

In general, the damage in adhesively bonded joints initiates from and propagates through the ends of the overlap area due to high stress concentration in that area. The reduction of these stress concentrations results in an increase in the strength of the joints. For this reason, the rounding of the overlap region before bonding and then applying compression during the bonding process will exert compressive residual stresses on the adhesive layer in the overlap end regions. It is known that the residual stresses formed in this process increase the failure strength of the joint and hence delay the initiation of the damage.

In this study, the effects of overlap length (L = 50,75, and 100 mm), patch thickness (h = 1.6, 3.2, and 5 mm) and patch materials (AA2024 aluminum alloy, AISI 304 steel, AISI 1040 steel) on bond strength were experimentally investigated for adhesively bonded double-strap joint (DSJ) and curvature double-strap joint (CDSJ) subjected totensile loading. The experimental study showed that the overlap length, patch thickness and patch materials have considerable influence on the failure strength and displacement capacity of the joints.