Glass/epoxy composites and aluminium alloy joint using Kevlar® intermediate layer and nanoclay-reinforced epoxy adhesive

Adhesive lap joint between glass fibre/epoxy composites and aluminium alloy (2014 T4) was prepared by an in situ moulding process using a matched die mould. The surface of aluminium alloy was treated with chromic acid before adhesive bonding. Lap shear strength and fatigue life were evaluated in tensile mode and tension–compression mode (at 40% of lap shear load of adhesive joint), respectively. Knurling on the surface of aluminium alloy improved the lap shear strength of the adhesive joint but did not influence the fatigue life of the same. Lap shear strength and fatigue life of adhesive joint made with neat epoxy adhesive and reinforcement of an intermediate layer of Kevlar^® between glass/epoxy composite and aluminium alloy were observed to be 0.44?kg/mm² and 3.6?×?10⁵ cycles, respectively. In another case, lap shear strength and fatigue life of similar type of adhesive joint made from nanoclay (Cloisite 30B)-reinforced epoxy adhesive and without reinforcement of an intermediate layer of Kevlar^® were observed to be 0.38?kg/mm² and 2.3?×?10⁵ cycles, respectively. Whereas, lap shear strength and fatigue life of adhesive joint made from nanoclay-reinforced epoxy adhesive along with the reinforcement of an intermediate layer of Kevlar^® were 0.48?kg/mm² and 3.9?×?10⁵ cycles, respectively. Therefore, adhesive joint made from nanoclay-reinforced epoxy adhesive along with the reinforcement of an intermediate layer of Kevlar^® was the best.     

Benchmark tests on adhesive strengths in butt, single and double lap joints and double-cantilever beams

The RC99 committee of the Japan Society for Mechanical Engineers conducted the benchmark tests on strengths of adhesive joints using different testing methods. The effects of joint configuration, loading mode, adherend yield strength and so on, on the strength and data scatter were investigated using two typical epoxy adhesives. The strengths obtained by various tests were compared with each other. The relationships among strengths of butt, single lap and double lap joints and fracture toughness were given. Thirteen member institutes of the committee participated in this project. The benchmark results allow us to recognize that the joint strengths are strongly affected by the curing process. The key to obtaining the appropriate joint strength, is precise temperature control inside the adhesive layer for curing. Toughened adhesives do not always give higher joint strengths than untoughened adhesives. The yield strength of adherends much affects the observed lap joint strength of adhesives.     

Photoelastic Determination of Flaw Size and Distribution Effects on Adhesively Bonded Lap Joints

It is well known that the load carrying capacity of adhesively bonded lap joints can be influenced by the presence of flaw-like defects which are often created during its bonding process. To design an effective adhesive joint containing possible bonding defects, adequate knowledge and understanding of the shear stress distribution along the entire lap joint are necessary.

This paper describes an investigation into the effects of internal adhesive flaw size and distribution on the fracture behaviour of adhesively bonded lap joints. Photoelasticity is used to gain a quantitative understanding of the localized shear stress concentrations due to the presence of the internal flaws along the bonding layer. It is observed that a 20% increase in the maximum shear stress may be induced when an isolated central flaw of S. O mm was extended to 37.5 mm representing a flaw size of 75% of the lap length. For the presence of multiple flaws along the bonding line, there is no significant effect of the flaw separation distance on the maximum shear stresses. There is, however, a marked increase in the maximum shear stress up to about 45% when a flaw size is increased from 2.5 mm to 7.5 mm.     

Mechanical behaviour of a combined joint composed of mechanical fastening and adhesive bonding

A study has been made of ‘combined’ joints consisting of mechanical fastening and adhesive bonding between rigid adherends. The mechanical characteristics of such ‘hybrid’ butt and lap shear joints have been derived by theoretical analysis. Results from experimental tests in combined joints giver reasonable agreement with the theoretical equations relating joint load to deformation.     

Experimental study of adhesively bonded CFRP joints subjected to tensile loads

The tensile performance of adhesively bonded CFRP joints has been investigated experimentally. In this study, overlap length, adherend thickness, adherend width and scarf angle were chosen as design parameters. All load–displacement curves are linear, except that the thicker single-lap joints behave slightly nonlinearity due to the bending effect caused by eccentric loading. The lap shear strength is not directly proportional to overlap length, adherend thickness, adherend width and scarf angle for the brittle adhesive studied in the paper. The major failure mode includes adhesive shear failure and adherend delamination failure, sometimes accompanying with some fiber pull-out. Finally, the lap shear strength of three different lap types with similar bonding area (W=25 mm, L=10 mm, θ=5.71°) and adherend thickness (0.96 mm) was analyzed. It is found that the double-lap joint has the highest ultimate failure load. However, when considering the lap region weight, the scarf-lap joint is the most efficient.     

Experimental Analysis of the Bondline Stress Concentrations to Characterize the Influence of Adhesive Ductility on the Composite Single Lap Joint Strength

Adhesively bonded composite single lap joints were experimentally investigated to analyze the bondline stress concentrations and characterize the influence of adhesive ductility on the joint strength. Two epoxy paste adhesives—one with high tensile strength and low ductility, and the other with relatively low tensile strength and high ductility—were used to manufacture composite single lap joints. Quasi-static tensile tests were conducted on the single lap joints to failure at room temperature. High magnification two-dimensional digital image correlation was used to analyze strain distributions near the adhesive fillet regions. The failure mechanisms were examined using scanning electron microscopy to understand the effect of adhesive ductility on the joint strength. For a given surface treatment and laminate type, the results show that adhesive ductility significantly increases the joint strength by positively influencing stress distribution and failure mechanism near the overlap edges. Moreover, it is shown that high magnification two-dimensional digital image correlation can successfully be used to study the damage initiation phase in composite bonded joints.     

Strength prediction of bonded single lap joints by non-linear finite element methods

A non-linear finite element technique has been used to predict the mode of failure and failure load of single lap joints made from three aluminium alloys and four epoxy adhesives, and the results compared with those obtained from experiment and closed-form analyses. The finite element program used was able to account for the large displacement rotations that occur in a single lap joint under load, and allowed the effects of elasto-plasticity in both the adhesive and adherends to be modelled. A failure criterion based on the uniaxial tensile properties of the adhesive was used: for two untoughened adhesives a maximum stress criterion was found to be appropriate while for two toughened adhesives a maximum strain criterion was employed.     

The determination of dynamic strength of single lap joints using the split Hopkinson pressure bar

The current investigation focuses on the determination of the strength of adhesive-bonded single lap joints under impact with the use of a split Hopkinson pressure bar (Kolsky bar). For this, experiments were conducted at different loading rates, for identical metallic adherends bonded by a two-part epoxy adhesive. Four different types of specimens were adopted, all with a given adhesive thickness. The length of overlap and the width of the adherends were varied resulting in four different areas of overlap. It was found that the average strength, as calculated from the readings obtained from a Kolsky bar, increases with decrease of overlap area. An elastodynamic model for the shear strain of the adhesive-bonded single lap joint was developed to investigate this drastic effect of overlap area on the average strength of the joint. The mathematical model was found to be dependent on both the material properties of the adherend and adhesive, as well as the structural properties of the joint, viz. the width and the thickness of the adhesive layer. A combined experimental-numerical technique was used to predict the strain distribution over the length of the bond in the adhesive. It was found that the edges of the adhesive were subjected to maximum strain, while a large part of the adhesive was found to exhibit zero shear strain. The effect of the lap length and the width was studied individually. The cumulative effect of averaging the strain over the entire overlap area, was decreased shear strain for an increased overlap area. The Kolsky bar was identified to give conservative values of the shear strength of an adhesive bonded lap joint under high rates of loading.     

Effect of Temperature on the Joints Strength of an Automotive Polyurethane Adhesive

In this paper, the performance of an automotive polyurethane adhesive was studied through adhesive joints tests. Butt joints and single lap joints were fabricated and tested at seven temperature measuring points (TMPs). It is shown that both the tensile strength and lap shear strength decrease with the increasing of temperature. Quadratic polynomial expression obtained by the least square method can represent the tensile and lap shear strength as a function of temperature very well. ?40°C, 0°C, and 90°C were selected as the most ideal TMPs for this adhesive through the comparison of the residual sums of squares of 35 fitting curves with different combination of TMPs. Scarf joints with adhesive angles of 60° and 30° were fabricated and tested at ?40°C, 0°C, and 90°C. It also showed a decrease in joint strength with the increasing temperature. Joint strength as a function of adhesive angle is presented. It was found to closely follow a linear behaviour. A three-dimensional surface, consisting of temperature, adhesive angle, and joint strength, is presented finally to facilitate the design of automotive bonding structures.     

Static strength and fatigue life of optimized hybrid single lap aluminum–CFRP structural joints

Hybrid bolted/bonded joints are used to assemble structural components, commonly made by carbon fiber reinforced plastics (CFRP), with aluminum frames. Hence, they have become common solutions in a number of modern structural applications in the industrial fields, as well as civil constructions. Unfortunately, due to the lack of understanding of the relationships between the multiple parameters of influence that characterize their mechanical performance, only limited improvement have been achieved so far over classical bonding approaches, in terms of static and fatigue strength. As a result, further studies are needed in order to better exploit the potential of hybrid bolted/bonded joints and identify optimum joint configurations. This paper describes an optimization procedure of the joints, achieved through a systematic experimental analysis of hybrid single lap aluminum–CFRP structural joints. This, analyzing the effect of overlap length, stiffness imbalance, adhesive curing as well as of size, positioning and preload of the bolt, results in a significant rise of the strength, especially in presence of high cycles fatigue loading. Also, micrographic analysis and related numerical simulations have allowed to gain a better insight into the damage mechanisms occurring during the in-service tensile loading, corroborating the highest mechanical performance of the angle-ply lay-up proposed for the CFRP adherent.     

Failure of elastomeric sealants under tension and shear: Experiments and analysis

The mechanical behaviour of two elastomeric fuel tank sealants was investigated. Laboratory tests were conducted on two sealants, an epoxy cured polythioether and a manganese cured polysulfide. Tensile, planar tension and torsion tests were performed to characterize and understand the response of the sealants under quasi-static loading. Single lap joint tests in tension were also carried out using four different sealant layer thicknesses and two types of adherend (aluminium and steel). The adherends were either primed with an epoxy primer or otherwise conventionally treated (e.g. grit blasted). Experimental results showed a decrease in joint strength with increasing sealant layer thickness. Emphasis was given to the understanding of the failure and the prediction of the strength of the single lap joints. Predicted failure loads were derived by analytical calculations and finite element simulation of the joints.     

Effect of temperature on the shear strength of aluminium single lap bonded joints for high temperature applications

An experimental and numerical investigation into the shear strength behaviour of adhesive single lap joints (SLJs) was carried out in order to understand the effect of temperature on the joint strength. The adherend material used for the experimental tests was an aluminium alloy in the form of thin sheets, and the adhesive used was a high-strength high temperature epoxy. Tensile tests as a function of temperature were performed and numerical predictions based on the use of a bilinear cohesive damage model were obtained. It is shown that at temperatures below T_g, the lap shear strength of SLJs increased, while at temperatures above T_g, a drastic drop in the lap shear strength was observed. Comparison between the experimental and numerical maximum loads representing the strength of the joints shows a reasonably good agreement.     

Non-linear analysis and optimization of adhesively bonded single lap joints between fibre-reinforced plastics and metals

Non-linear finite element methods are applied in the analysis of single lap joints between fibre-reinforced plastics (FRP) and metals. The importance of allowing for both geometric and material non-linearities is shown. The optimization of single lap joints is done by modifying the geometry of the joint ends. Different shapes of adhesive fillet, reverse tapering of the adherend, rounded edges and denting are applied in order to increase the joint strength. The influence of the joint-end geometry is shown for different metal adherend/FRP adherend/adhesive combinations. The results of the numerical predictions suggest that with a careful joint-end design the strength of the joints can be increased by 90–150%.     

Experimental study of the influence of adhesive reinforcement in lap joints for composite structures subjected to mechanical loads

The paper deals with experimental investigations on reinforcing the adhesive in single lap joints subjected to mechanical loads such as tensile, bending, impact and fatigue. The adhesive used for bonding was an epoxy reinforced with unidirectional and chopped glass fibres as well as micro-glass powder. The adherends were glass reinforced composite laminates. The bonding surfaces were prepared before joining. In the case of unidirectional fibres in the adhesive region, the fibre orientations considered were 0°, 45° and 90°. The volume fraction of fibres in the adhesive layer in all the cases was 30%. The volume fractions of micro-glass powder were 20%, 30% and 40%. The tensile, bending, impact and fatigue tests on the prepared specimens were conducted according to ASTM standards. The results show that except the 90° unidirectional orientation, reinforcing the adhesive with glass fibres or powder increases the joint strength. The use of volume fraction of 30% of micro-glass powder gave the best performance in the above loading conditions. The fatigue life increased by 125%, the ultimate joint strength in tension increased by 72%, the bending ultimate joint strength increased by 112% and the impact joint strength increased by 63%. The microstructure of the debonded area was examined and three modes of failure could be observed namely cohesive failure, light fibre-tear failure and thin layer cohesive failure.     

Comparison of fracture behavior between acrylic and epoxy adhesives

An experimental study was conducted on the strength of adhesively bonded steel joints, prepared epoxy and acrylic adhesives. At first, to obtain strength characteristics of these adhesives under uniform stress distributions in the adhesive layer, tensile tests for butt, scarf and torsional test for butt joints with thin-wall tube were conducted. Based on the above strength data, the fracture envelope in the normal stress-shear stress plane for the acrylic adhesive was compared with that for the epoxy adhesive. Furthermore, for the epoxy and acrylic adhesives, the effect of stress triaxiality parameter on the failure stress was also investigated. From those comparison, it was found that the effect of stress tri-axiality in the adhesive layer on the joint strength with the epoxy adhesive differed from that with the acrylic adhesive. Fracture toughness tests were then conducted under mode l loading using double cantilever beam (DCB) specimens with the epoxy and acrylic adhesives. The results of the fracture toughness tests revealed continuous crack propagation for the acrylic adhesive, whereas stick-slip type propagation for the epoxy one. Finally, lap shear tests were conducted using lap joints bonded by the epoxy and acrylic adhesives with several lap lengths. The results of the lap shear tests indicated that the shear strength with the epoxy adhesive rapidly decreases with increasing lap length, whereas the shear strength with the acrylic adhesive decreases gently with increasing the lap length.     

The effects of partial bonding in load carrying capacity of single lap joints

The effects of the presence and size of gaps in the band single lap joint geometry were studied. Two types of adhesives: a deformable, acrylic tape and epoxy putty were used as model adhesives. When using the epoxy putty, the substrate overlap end conditions were also varied by machining 10° end tapers in some joints. For both adhesive types, the introduction of the gap had a moderate negative effect on the load carrying characteristics of the joint, but joints utilizing the epoxy putty maintained joint strength as the gap size was increased to 9.53 mm (38% gap), while the highly deformable acrylic tape case displayed a constant decline and maintaining constant ultimate shear stress values. We suspect that this variation is due to a combination of the different failure modes of each adhesive and their differing moduli, as well as how these relate to the peeling stresses at the ends of the bond length. In the epoxy putty series, the samples with tapered substrates consistently carried higher loads than those with unmodified substrates. This improvement is a manifestation of the ability of the tapered joint geometry to reduce peeling stresses experienced within the adhesive layer.     

The behaviour of single lap joints under bending loading

Four-point bend tests were performed on single lap joints with hard steel adherends and a structural epoxy adhesive. The effect of the overlap, the adherend thickness and the adhesive thickness was studied. It was found that the length of the overlap has no significant effect on the strength of the joints. This is because the load transfer is occurring in a very localised area around the edges of the overlap, being the failure governed by peel mechanisms. The thickness of the adherends strongly affects the strength of the joints. The thicker the adherend, the stronger is the joint. The experimental results are compared with a finite element model and reinforce the fact that the failure takes place due to local strains at the ends of the overlap in tension. An analytical model is also given to predict in a simple but effective way the joint strength and its dependence on the adherend thickness.     

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.     

X-Ray Photoelectron Spectroscopy of the Fracture Surfaces of Iron/ Epoxy and Titanium/Epoxy Adhesive Joints

The failure path of iron/epoxy and titanium/epoxy lap joints prepared from adherends pretreated with dilute aqueous solutions of ω-aminopropyltriethoxysilane (ω-APS) and then exposed to water at 60°C for varying times was determined using X-ray photoelectron spectroscopy (XPS). Iron/epoxy lap joints prepared using T-APS primers applied at pH 8.0 and tested without exposure to water failed near the oxide/primer interface with only a small amount of adhesive left on the substrate failure surface. Iron/epoxy joints that were primed with T-APS at pH 10.4 and tested without immersion in water failed partly within the adhesive and partly near the interface. All of the iron/epoxy joints failed in the primer near the primer/oxide interface when tested after exposure to water for seven days but very little corrosion of the substrates was observed. When titanium/epoxy lap joints were tested without exposure to water, failure was partly within the adhesive and partly near the primer/oxide interface regardless of the pH at which the primer was applied. However, the failure path shifted to near the primer/oxide interface after the joints were immersed in water for seven days. It was concluded that failure of the joints after exposure to water was associated with hydrolysis within the primer rather than with extensive corrosion of the substrate.     

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.