The Effect of PEEK Fibres and Powder on Joints Made with a High Temperature Adhesive

High temperature adhesives typically exhibit low levels of peel strength since they tend to be more brittle than typical toughened adhesives used for lower temperature applications. It was found that incorporating thermoplastic fibres or powder into the bondline of a joint made with a high temperature epoxy-based adhesive resulted in significant improvements in peel strength. Poly(ether ether ketone) (PEEK) fibres and powder were incorporated into the adhesive resin and used in aluminium joints. These were tested in peel and single lap shear using a range of fibre lengths, orientations and volume fractions. It was seen that large increases in peel strength could be achieved but that lap shear strength was degraded with most types of modification. However, some modifications resulted in significant increases in peel strength with limited decrease in lap shear strength. These improved properties have been achieved using physical modifications rather than chemical alteration of the resin.     

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.     

Hygrothermal Properties of Adhesively Bonded Joints and Their Correlation with Bulk Adhesive Properties

The combined effects of heat (50[ddot]C) and humidity (95% R.H.) on the lap shear and T-peel strengths of 120[ddot]C, 150[ddot]C and 215[ddot]C service epoxy film adhesives have been characterized. Experimental results have indicated that effects of hygrothermal conditioning on lap shear and peel properties vary with exposure time and final testing temperatures and type of adhesive tested. In the cases where cohesive failure was observed in the shear and peel specimens, a correlation could be established between the bulk properties of the adhesives (tensile strength and elongation) and their adhesively bonded joint properties (shear and peel). When testing was carried out at room temperature, a general correlation between the tensile elongation and T-peel or shear could be obtained. At below freezing temperatures, lap shear strength seemed to be correlated with bulk tensile strength while peel correlated with bulk tensile elongation. At elevated temperatures, the relative contributions of bulk strength and elongation were the decisive factors as far as shear and peel strengths are concerned.     

New Developments in Structural Adhesives for the Automotive Industry

Recently developed epoxy paste adhesives, reactive hot melts, adhesive film tape and polyurethane adhesives are presented for structural bonding in the automotive industry. Paste adhesives usually require a precure stage to obtain handling strength of the joints and to guarantee wash-out resistance of the adhesive in the paint baths. This step can be omitted with reactive hot melts and adhesive film tape, which are solid before and after their application. In addition they allow an improved working hygiene. Some mechanical properties of the adhesives are shown such as lap shear strength and peel strength as well as lap shear strength as a function of the bondline thickness. Results of the excellent durability of epoxy one-component pastes, reactive hot melts and adhesive film tape are given from cyclic environmental and salt spray tests.     

The application of the iosipescu shear test to structural adhesives

The Iosipescu shear test method, adapted for adhesives, was used to evaluate shear stress–strain properties of five epoxy-based film adhesives. The shear strength of some adhesives, determined by this technique, was found to differ greatly from that determined by either lap shear and/or thick adherend tests. Accurate measurements of the adhesive bond-line deformation enabled evaluation of the adhesive elastic and plastic properties as well as the calculation of shear modulus. A high-speed video camera was employed to augment observation on the joint deformation.     

Optimization of adhesively-bonded single lap joints by adherend notching

The effect of adherend notching on the strength and deformation behavior of single lap joints was investigated. First, a parametric study was conducted using finite element analysis (FEA). This initial part of the research into the effect of notches on joint behavior involved determination of the optimum notch location and notch dimensions. This was done by using FEA in a series of models with different notch positions and geometries. The results of this parametric study were used to select the most promising lap geometries for further study. Next, more detailed FEA were conducted on the selected lap geometries. These data were compared with the experimental single-lap shear test results to assess the applicability of different failure criteria. Three different model adhesives were used: a rubber toughened film epoxy with nylon carrier, a styrene-butadiene-styrene block copolymer based deformable 'gel' adhesive, and a two-part, metal filled brittle epoxy adhesive. The FEA for single lap joints containing 'top notches' on the unbonded, top side of the adherends, at locations corresponding to the overlap ends, and bonded with the two-part metal filled epoxy provided the best agreement with the experimental results. The experimental results showed a 29% increase in joint strength with the introduction of the notches, which matched very well with the 27% decrease in the peak peel stress observed by the FEA results. For this brittle adhesive, the peel stress is almost certainly the governing failure stress. This was confirmed by matching of the FEA peak peel stress ratios with the experimental load ratios, for both the notched and unnotched specimens.     

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.     

Single and dual adhesive bond strength analysis of single lap joint between dissimilar adherends

The emerging trends for joining of aircraft structural parts made up of different materials are essential for structural optimization. Adhesively bonded joints are widely used in the aircraft structural constructions for joining of the similar and dissimilar materials. The bond strength mainly depends on the type of adhesive and its properties. Dual adhesive bonded single lap joint concept is preferred where there is large difference in properties of the two dissimilar adherends and demanding environmental conditions. In this work, Araldite-2015 ductile and AV138 brittle adhesives have been used separately between the dissimilar adherends such as, CFRP and aluminium adherends. In the dual adhesive case, the ductile adhesive Araldite-2015 has been used at the ends of the overlap because of high shear and peel strength, whereas in the middle of the bonded region the brittle adhesive AV138 has been used at different dimensions. The bond strength and corresponding failure patterns have been evaluated. The Digital Image Correlation (DIC) method has been used to monitor the relative displacements between the dissimilar adherends. Finite element analysis (FEA) has been carried-out using ABAQUS software. The variation of peel and shear stresses along the single and dual adhesive bond length have been captured. Comparison of experimental and numerical studies have been carried-out and the results of numerical values are closely matching with the experimental values. From the studies it is found that, the use of dual adhesive helps in increasing the bond strength.     

Studies on the adhesion behavior of water-based adhesives blended with asan gum

The influence of asan gum, a locally available waste material obtained from the Terminalia alata tree, in blends with waterborne natural rubber adhesive and poly(vinyl acetate), on the lap shear strength and the peel strength has been investigated. Both the strength values increase, even with a small quantity of the gum. At a higher gum content, both these parameters, however, decrease. Fourier Transform Infrared studies reveal that there is no covalent bonding between the gum and the adhesives, although some hydrogen bonding exists in the poly(vinyl acetate) blend. Morphological studies reveal mechanical interlocking of the adhesive in the substrates. The pseudoplastic nature of the gum-modified waterborne adhesives has been confirmed from rheological studies using a Brookfield viscometer. The higher lap shear and peel strength values of the gum-modified adhesives compared to the control adhesives are attributed to the higher shear modulus of the former. The 100% modulus and tensile strength of the adhesives blended with the gum are also higher, compared to their controls.     

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.     

A study on the lap shear strength of a co-cured single lap joint

In this paper, the lap shear strength of a co-cured single lap joint subjected to a tensile load was investigated by experimental analysis. Co-cured joint specimens with several different bonding parameters such as bond length, surface roughness, and stacking sequence of the composite laminate were fabricated and tested. The dependence of the lap shear strength of the co-cured joint on the bonding parameters was investigated from the experimental results. The failure mechanism of the co-cured single lap joint was partially cohesive failure. The lap shear strength of the co-cured single lap joint was significantly affected by the bond length and the stacking sequence of the composite laminate. However, the effect of surface roughness on the lap shear strength of the co-cured single lap joint was not so significant.     

Strengthening of the adhesive bond using a mixture of adhesives between dissimilar adherends in a single lap joint

Abstract

Adhesive bonding is the best alternative to riveting in aircraft structures but the strength of the adhesive bonded joint is low and is limited by strength of adhesive. Strengthening of adhesive bonding is an important requirement. In this work, an attempt has been made to strengthen the adhesive bonding by mixing different quantities of brittle adhesive in the ductile adhesive and vice-versa. Two different adhesives, one brittle (AV138) and another ductile (Araldite-2015) adhesive have been considered. Initially single lap joint has been constructed between the CFRP and aluminium with individual adhesives, then the mixture of adhesives have been used in the bonded region in varied proportions. The X-ray radiography and ultrasonic testing have been performed to check the quality of bonding. Uniaxial tensile tests have been conducted on the lap joints along with Digital Image Correlations (DIC) to obtain the individual and mixed adhesive bond strength. The failure patterns have been identified using optical and scanning electron microscope. These studies indicate that strengthening of the adhesive bonding achieved by mixing of two adhesives and highest bond strength obtained when the mixture of AV138 and Araldite-2015 adhesives are used in equal proportions.     

Experimental and numerical study on the composite adhesive joint reinforcement using wavy edge

The joints are usually the weakest part of the engineering structures. In this study, the employment of wavy edges for increasing the adhesive joint load-bearing capacity is considered. The effects of geometric parameters of the wavy edges on the strength of the adhesive joints were investigated, experimentally. Two different adhesives, Araldite 2015 and Epoxy RL440/HY441 as ductile and brittle adhesives were used, respectively. The finite element model was also developed for more investigation. The joint stress distributions were used successfully to explain the experimental observations. For the appropriate wavy joint configuration, compressive peel stress on the both ends of the adhesive led to a considerable delay in damage initiation and consequently increased the joint strength. The effects of geometrical parameters of the wavy edge on the joint strength were also examined. For the optimum configuration, the joint with wavy edge offered 32% more strength than the flat single lap joint.     

The Effect of Lateral Constraint on the Strength of a Single Lap Joint

The mechanical performance of a single lap joint (SLJ) is mainly affected by the lateral normal tensile stresses acting at the edges of its interlaminar adhesive layer (IAL). Owing to these stresses, the delamination failure which initiates at the IAL edges and propagates inward, is predominantly of the peel type. The subject of this study is the effect of constraint of the lateral deflection of adhering edges applied by tightly binding them together.

Experimental results showed that the effect of this type of constraint is a reduction in the extent of peel and an overall increase in the joint tensile strength. This effect is more pronounced in the case of brittle than in the case of ductile adhesives.     

P2352W-19预浸料与可剥布匹配性研究

为了进一步了解P2352W-19预浸料匹配可剥布对胶接质量的影响,使用干态和湿态聚酯可剥布处理待胶接表面,并通过Ⅰ型双悬臂梁断裂韧性试验和双搭接剪切试验来考察可剥布处理后试验件的胶接表面力学性能表现,将试验结果结合试验件破坏模式进行分析。研究了层板材料匹配两种高温胶膜的共胶接及二次胶接质量,确定了P2352W-19预浸料最佳匹配可剥布。     

Preparation and characterization of novel film adhesives based on cyanate ester resin for bonding advanced radome

In this paper, the novel film adhesives based on phenolphthalein poly(ether sulfone) (PES-C) and epoxy (EP) modified cyanate ester resin (CE) were prepared for bonding an advanced radome. The film adhesives are convenient for applying to manufacture, possessing good adhesion strength, thermal durability and excellent dielectric property. The curing behaviors were confirmed by differential scanning calorimetry (DSC), showing that the main reaction pathways are not varied with adding PES-C but the reaction rates are evidently accelerated, and the film adhesives can be well cured at lower temperature of 177 °C. The adhesion strength was evaluated in lap shear strength and peel strength, indicating that the better adhesion strength is obtained with increasing in PES-C. The maximum value of lap shear strength is 33 MPa at room temperature. The thermal durability was determined by thermal aging tests of lap shear specimens, showing that the decrease in strength gets faster with adding PES-C, and the usability of film adhesives over 2000 h at 200 °C. The dielectric properties were measured by dielectric resonator methods, finding that the introduction of PES-C brings a positive effect on dielectric properties. The lowest value of determined dielectric loss is 0.0075 at 10 GHz.     

Parametric study of adhesive joints with composites

Adhesively-bonded joints are increasingly used in aeronautical industry. Adhesive joints permit to join complex shapes and reduce the weight of structures. The need to reduce the weight of airplanes is also increasing the use of composites. Composites are very anisotropic: in the fibre directions, unidirectional composites can be very strong and stiff, whereas the transverse and shear properties are much lower. Bonded joints experience peel loading, so the composite may fail in transverse tension before the adhesive fails. That is why it is important to study these joints and try to find reliable ways to predict the strength of joints with composite adherends. The main goal of this study was to understand the failure in adhesive joints with composites, bonded with adhesives with different characteristics, and find reliable ways to predict them. Experimental tests were carried with single lap joints with composite adherends and different adhesives, brittle and ductile, with several overlap lengths. A Cohesive Zone Model (CZM) was taken into consideration to predict the results observed during the experimental tests. The experimental results were also compared with simple analytical models and the suitability of each model was evaluated for each bonded system.     

Influence of chemical structure of hardener on mechanical and adhesive properties of epoxy polymers

The mechanical and adhesive properties of epoxy formulations based on diglycidyl ether of bisphenol A cured with various aliphatic amines were evaluated in the glass state. Impact and uniaxial compression tests were used to determine the impact energy, elastic modulus and yield stress, respectively. The adhesion tests were carried out in steel–steel joints using single‐lap shear, T‐peel, and impact adhesive joints geometry. The better mechanical and adhesive behavior of the networks is obtained when exists high flexibility of chain between crosslink and/or high elastic modulus. The 1‐(2‐aminoethyl)piperazine epoxy network presents the best adhesive properties, high flexibility, and the largest impact energy. However, it possesses low elastic modulus and yield stress. Also, exhibits increases in peel strength and impact energy while reductions in lap shear strength. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of Adhesive Type and Thickness on the Lap Shear Strength

The effect of the adhesive thickness on the bond strength of single-lap adhesive joints is still not perfectly understood. The classical elastic analyses predict that the strength increases with the adhesive thickness, whereas experimental results show the opposite. Various theories have been proposed to explain this discrepancy, but more experimental tests are necessary to understand all the variables.

The objective of the present study was to assess the effect of the adhesive thickness on the strength of single-lap joints for different kinds of adhesives. Three different adhesives were selected and tested in bulk. The strain to failure in tension ranged from 1.3% for the most brittle adhesive to 44% for the most ductile adhesive. The adherend selected was a high-strength steel to keep the adherends in the elastic range and simplify the analysis. Three thicknesses were studied for each adhesive: 0.2, 0.5, and 1 mm.

A statistical analysis of the experimental results shows that the lap shear strength increases as the bondline gets thinner and the adhesive gets tougher.     

The effect of curing temperature on thermal,physical and mechanical characteristics of two types of adhesives for aerospace structures

The effects of cure temperatures on the thermal, physical and mechanical characteristics of two types of thermosetting structural epoxy film adhesives were determined in detail. The aim of this paper is to assess the effect of cure temperatures (82–121 °C) on the degree of cure of the two adhesives and the relevant void formations that need to be addressed in bonded part production and repair. Two thermal parameters were used to characterize the advancement of the reaction, such as degree of cure and glass transition temperature. The joint properties with respect to the cure temperatures were characterized by void content and bond-line thickness measurements and lap shear strength tests. Experimental results presented that all lap shear strengths were well within minimum shear strength (29 MPa) required by the specification of the film-type adhesive. However, the lap shear strength testing after aging at 82 °C and 95%R.H for 1000 h showed that the improved durability when the adhesive is cured at 121 °C did not occur for the 82 °C cure. Low curing conversion (75–77% degree of cure) combined with high voids (over 2 areal%) has a catastrophic effect on the bonding qualities at the metal-adhesive interface and due to lack of cohesion in the adhesive. The changes in the interface caused by the low temperature curing may contribute to an increased susceptibility of the bonded joint to moisture and consequent bond-line degradation.