Photoelastic Stress Analysis of Bonded Lap Shear Joints Having Thermoplastic Adherends

The effects of substrate stiffness and modulus on joint strength and stress distribution were investigated for a series of nylon substrates bonded with an epoxy adhesive. Substrate stiffness and modulus were controlled by the level of glass filler in the resin. Single lap shear samples having both identical (“self-bonded”) and dissimilar (“cross-bonded”) substrates were investigated. For the self-bonded samples, lap shear strength was found to increase with increasing substrate modulus and stiffness. The strengths of the cross-bonded samples were intermediate to the strengths of the corresponding self-bonded samples. Photoelastic techniques were used to observe stress patterns in the lap joints during testing. One type of stress pattern was observed for all self-bonded samples regardless of substrate stiffness. Two patterns, one for the stiff substrate and one for the more flexible substrate, were observed for cross-bonded samples. The photoelastic analysis agreed qualitatively with predictions of stress distributions based on linear elastic and linear elastic/perfectly plastic theoretical models.     

Adhesive Bonding to Galvanized Steel: I. Lap Shear Strengths and Environmental Durability

Initial (i.e., unaged) adhesion, as well as adhesion after seven day, 60°C water immersion and six week scab corrosion accelerated environmental exposures, has been assessed for five different one and two-part epoxy adhesives, bonded to three different types of galvanized steel substrates. We have shown that adhesion, as measured by lap shear strength, is specific to the galvanized substrate type. In general, for a given adhesive, adhesion to “hot-dipped” galvanized substrates is harder to achieve and maintain under accelerated environmental exposure than is adhesion to “electroplated” galvanized. Also, for a given type of galvanized steel, the one-part epoxies evaluated generally showed higher initial strengths, as well as better strength retention under environmental exposure than did the two-part epoxies.     

Direct Measurement of Longitudinal Strains and Stresses within Single Lap Shear Adhesive Joints Using Neutron Diffraction

The neutron diffraction technique has been used to investigate the longitudinal stresses in the adherend produced as a result of cure and due to the application of a tensile load in a single lap shear joint. A comparison has also been made between the stress distributions in loaded “aged” and “unaged” joints. The neutron diffraction technique is the only viable method of investigating these stresses within metal adherends and enables comparisons between predicted and measured stresses to be made. The results of these experiments cast doubt on some of the predictions from finite element modelling of adherend stress levels.     

The Adhesive Bonding of Steel with Polysulfone

The bonding of cold rolled steel with polysulfone, a thermoplastic adhesive has been studied. The single lap shear strength of bonded samples where the substrate was gritblasted was 40% lower than for the case where the substrate was hydrothermally oxidized. Both surface preparations resulted in diminished lap shear strength on exposure to high humidity conditions. However, the hydrothermally oxidized substrates resulted in more durable bonds.     

Adhesive Bonding to Galvanized Steel: II. Substrate Chemistry, Morphology and Bond Failure Analysis

Galvanized substrate morphology, oxide layer chemistry, bond failure modes, failure loci, and bondline corrosion have been investigated for adhesive bonds to galvanized steel. Significant differences in surface morphology were observed between the relatively smooth surfaces of “hot-dipped” substrates and the considerably rougher texture of “electroplated” substrates. The hot-dipped substrates were also chemically heterogeneous, with significant amounts of Al, Mg, Ca, and Pb, in addition to Zn, constituting the surface layer. For electroplated substrates, on the other hand, Zn was the major constituent. It was concluded that, for a given adhesive, low strengths and poor bond durability generally correlated with the minimum surface roughness and maximum chemical heterogeneity of the hot-dipped substrates. Higher strengths, and better durability, on the other hand, were observed for electroplated substrates, which showed the greater roughness, as well as chemically the more uniform surface.

Significantly, ESCA spectroscopy of fracture surfaces of unaged samples established that failure loci for both one and two-part epoxy adhesives included the oxide layer of the substrate. This was true for both hot-dipped, as well as electroplated substrates. For aged samples, scanning electron microscopy and X-ray diffraction analysis of failure surface identified zinc-based corrosion products present in the original bond area.     

Effect of adherend's rigidity on the shear strength of single lap adhesive joints

The present paper compares the tensile shear strength of single-lap joints with different adherends. Three materials were combined in the single lap joints: a carbon/epoxy laminated composite, a high elastic limit steel and the 6082-T6 aluminium alloy. The shear strength of joints was influenced by the adherend stiffness and the highest shear strengths were obtained using high stiffness adherend materials. The overlap length influenced the shear strength in different ways depending on the adherend materials. Numerical analysis concluded that the increase in the rigidity of the adherends decreases the rotation of the specimen and promotes a more uniform distribution of stresses in the glue. In joints with distinct materials, the less stiff material was found to determine the strength of the appropriate joint.     

胶粘剂特性和厚度对劈裂载荷作用下胶接接头应力分布的影响

运用三维弹塑性有限元法对劈裂栽荷作用下的胶接接头(即劈裂接头)承载后的应力分布特征进行了分析，重点研究了胶粘剂层厚度对劈裂接头应力分布的影响。结果表明，胶粘剂的性能对应力分布有较大影响，提高胶粘剂强度和减小胶层厚度，均导致胶层应力集中加剧，各向正应力峰值呈上升趋势，各向剪切应力则正好相反；并且劈裂接头中应力分布以三向主应力为主，剪切应力的存在亦不可忽略。故在不引起过大应力集中和较大胶层缺陷条件下采用高强度的胶粘荆和较厚胶层对提高劈裂接头强度有利，实验结果与有限元分析相吻合。     

Improvement of the Durability of Zinc-Coated Steel/Epoxy Bonded Joints

The durability properties of bonded lap shear joints made from an epoxy/dicyandiamide adhesive and hot-dipped galvanized (G2F) or electroplated-phosphated (EZ2) steel have been investigated. The degradation mechanisms have been studied after three accelerated ageing tests: the “cataplasme humide” (“C.H.T.”), immersion (“I.T.”), and salt spray (“S.S.T.”) tests. X-ray photoelectron spectroscopy (XPS) analysis of fracture surfaces after ageing have shown that anodic dissolution of the zinc-coating is responsible for debonding in all cases and that intergranular corrosion phenomena account for poorer performances of the hot-dipped galvanized substrate during “C.H.T.” and “I.T.” Silane coupling agents were successfully used as primers on both substrates to increase the hydrolytic stability of the metal/adhesive interface. XPS results indicate that both the interfacial dissolution of the phosphate coating of EZ2 and intergranular corrosion of G2F are delayed for silane-primed specimens. The observed improvements do not appear to depend on the nature of the silane coupling agents. Alkylsilanes have been found to perform as well as silanes having a group capable of reacting with the epoxy/dicyandiamide system.

Additional tests were carried out in view of the possible application of organosilane reagents as additives in corrosion-protective oils. Good durability properties have been obtained by priming the metal coupons with a standard oil/silane mixture prior to bonding.

When corrosion was the controlling degradation mechanism as is the case during the salt spray test, silane treated specimens did not generally perform better than control specimens.     

Graphite Fibre-Epoxy Matrix Interface Interactions

Studies of fibre-epoxy resin matrix model composites show that the “tensile debond” test is not applicable to carbon or graphite fibres. Fibre fracture occurs under the compression strains involved relieving interface stresses and precluding subsequent debond. Calculated minimum bond strengths for pitch-based graphite fibres are similar to results for boron and glass fibres. Interfacial failure is obtained with the “shear debond” test for low and intermediate modulus graphite fibres, but compression fracture also occurs first with high modulus fibres. Pitch-based graphite fibres show a decreasing adhesive interaction with epoxy resin the more oriented the fibre, but results compare favorably with those of other fibres. Surface characterisation shows that all pitch-based graphite fibres exhibit a surface-oriented skin, although surface roughness increases with fibre modulus. The fibres all exhibit similar apparent surface energy characteristics which suggests that wettability does not play a significant role in determining interfacial bond strengths.     

Measurement of adhesive shear properties by short beam shear test based on higher order beam theory

This paper refers to the measurement of the shear properties of adhesive bonding by a new beam theory using the short beam shear (SBS) test configuration. A novel higher-order sandwich beam theory has been developed to analyze the adhesive bonded beam that consists of two adhered laminates and a single layer of adhesive in between. The closed form analytical solution for the SBS test model of the adhesively bonded beam is obtained in terms of deflection and stress distribution. The present theory has been used for calculating the adhesive shear modulus from the structural compliance. The initiation of stiffness degradation for the short beam shear test model was used as the critical load value for deriving the adhesive shear strength. A finite element model is built for validating the present model, and to evaluate its suitability for measuring adhesive shear properties. The present theory shows better accuracy for measuring the shear modulus than existing theories for both thin and thick adhesive layers. The measured strength values are more accurate than those obtained from the single lap joint shear test model. This theory can be used for adhesive materials with linear elastic deformation behavior.     

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     

Finite element analysis of adhesively bonded composite joints subjected to impact loadings

The main concern of this paper is to explore the geometrical and material effects on composite double lap joints (DLJ) subjected to dynamic in-plane loadings. Thus, three-dimensional finite element analyses were carried out at quasi-static and impact velocities. The DLJ alone was used for quasi-static case while an output bar was added for impact case. Elastic behavior was assumed for both adhesive and adherends. Average shear stress and stress homogeneity were extracted and compared. It was observed that the adhesive shear stiffness increases the average shear stress. Moreover, it makes the stress heterogeneity more important. On the other hand, higher values of the substrates longitudinal stiffness make the average shear stress higher; whereas, the stress homogeneity in the joint is better achieved for lower substrates’ shear stiffness.     

Surface Silanization of Polyethylene for Enhanced Adhesion

Low density polyethylene has been treated using a novel surface treatment process “SICOR” (“SIIane-on-CORona” treated polymer) in order to enhance adhesion with a range of adhesives including polyure-thane, methacrylate and cyanoacrylate. The process comprises two steps, i.e corona discharge followed by application of an organo-functional silane. The incorporation of surface hydroxyl groups onto the polymer surface enables organo-silane to create the hydrogen or covalent bonds with the oxidized polymer surface. The possibility of the creation of these bonds has been investigated using FTIR, XPS and wettability studies. The adhesion enhancement due to the new process is significant. Frequently, the strength increase exceeds 200% compared with the corona discharge treatment and more than 300% compared with LDPE priming using the “Loctite 770” polyolefin primer. The process is shown to be as good as, or better than, plasma treatment in terms of the strength increase following substrate treatment prior to adhesive bonding.     

Durability of Adhesive Bonds to Zinc-Coated Steels: Effects of Corrosive Environments on Lap Shear Strength

The effects of corrosive environments on adhesive bonds to electro-galvanized, zinc/aluminum alloy coated, coated electro-galvanized, and cold-rolled steels have been investigated. Bonds prepared using a rubber-modified dicyandiamide-cured epoxy adhesive, an epoxy-modified poly(vinyl chloride)-based adhesive, an acrylic-modified poly(vinyl chloride)-based adhesive a one-part urethane adhesive, and a two-component epoxy-modified acrylic adhesive were exposed under no-load conditions to constant high humidity or cyclic corrosion exposure for 50 days or 50 cycles (10 weeks) respectively.

Over the course of this study, exposure to constant high humidity had little effect on lap shear strength for any of the systems studied. Bond failures were initially cohesive, and with few exceptions remained so.

Bond strength retention under the cyclic corrosion exposure conditions employed was strongly dependent on adhesive composition and on substrate type. On galvanized substrates, lap shear strengths for the poly(vinyl chloride)-based adhesives were reduced by 90-100% during the course of the corrosion exposure, and a change in the mode of bond failure (from cohesive to interfacial) was observed. On the coated electro-galvanized steel substrate, the poly(vinyl chloride)-based adhesives showed about 50% retention in lap shear strength and a cohesive failure throughout most of the corrosion test. The dicyandiamide-cured epoxy adhesive used in this study generally showed the best lap shear strength retention to zinc-coated substrates; bonds to cold-rolled steel were severely degraded by corrosion exposure. The performance of the acrylic and urethane adhesives were intermediate to the dicyandiamide-cured epoxy and poly(vinyl chloride)-based adhesives in strength retention.     

Durability of Adhesive Bonds to Zinc-Coated Steels: Effects of Corrosive Environments on Lap Shear Strength

The effects of corrosive environments on adhesive bonds to electro-galvanized, zinc/aluminum alloy coated, coated electro-galvanized, and cold-rolled steels have been investigated. Bonds prepared using a rubber-modified dicyandiamide-cured epoxy adhesive, an epoxy-modified poly(vinyl chloride)-based adhesive, an acrylic-modified poly(vinyl chloride)-based adhesive a one-part urethane adhesive, and a two-component epoxy-modified acrylic adhesive were exposed under no-load conditions to constant high humidity or cyclic corrosion exposure for 50 days or 50 cycles (10 weeks) respectively.

Over the course of this study, exposure to constant high humidity had little effect on lap shear strength for any of the systems studied. Bond failures were initially cohesive, and with few exceptions remained so.

Bond strength retention under the cyclic corrosion exposure conditions employed was strongly dependent on adhesive composition and on substrate type. On galvanized substrates, lap shear strengths for the poly(vinyl chloride)-based adhesives were reduced by 90–100% during the course of the corrosion exposure, and a change in the mode of bond failure (from cohesive to interfacial) was observed. On the coated electro-galvanized steel substrate, the poly(vinyl chloride)-based adhesives showed about 50% retention in lap shear strength and a cohesive failure throughout most of the corrosion test. The dicyandiamide-cured epoxy adhesive used in this study generally showed the best lap shear strength retention to zinc-coated substrates; bonds to cold-rolled steel were severely degraded by corrosion exposure. The performance of the acrylic and urethane adhesives were intermediate to the dicyandiamide-cured epoxy and poly(vinyl chloride)-based adhesives in strength retention.     

The effect of flexible substrates on pressure-sensitive adhesive performance

The adhesive fracture energy (fracture toughness) of tapes during globally elastic unpeeling is often calculated from the relation “G=P/b(1−cos θ)”. We show that while this expression is correct for elastic peeling from rigid substrates, it gives misleading results when peeling from reversible flexible substrates. A two-dimensional analysis is presented for peeling from non-linear elastic substrates that give consistent fracture energies from experimental data.     

Analysis of tubular adhesive joints with a functionally modulus graded bondline subjected to axial loads

The strength and lifetime of adhesively bonded joints can be significantly improved by reducing the stress concentration at the ends of overlap and distributing the stresses uniformly over the entire bondline. The ideal way of achieving this is by employing a modulus graded bondline adhesive. This study presents a theoretical framework for the stress analysis of adhesively bonded tubular lap joint based on a variational principle which minimizes the complementary energy of the bonded system. The joint consists of similar or dissimilar adherends and a functionally modulus graded bondline (FMGB) adhesive. The varying modulus of the adhesive along the bondlength is expressed by suitable functions which are smooth and continuous. The axisymmetric elastic analysis reveals that the peel and shear stress peaks in the FMGB are much smaller and the stress distribution is more uniform along its length than those of mono-modulus bondline (MMB) adhesive joints under the same axial tensile load. A parametric evaluation has been conducted by varying the material and geometric properties of the joint in order to study their effect on stress distribution in the bondline. Furthermore, the results suggest that the peel and shear strengths can be optimized by spatially controlling the modulus of the adhesive.     

Enhancement of Epoxy Resin/Copper Heterojunction by Introduction of Sulfur‐Containing Polymers

Summary: Epoxy resins are widely used in electronics and electric industries because of their superior properties. In recent years, excellent bonding properties for metals (e.g., copper, gold, aluminium, silver, etc.), to which it is hard to adhere, have become desirable for epoxy resins in the electric and electronics industrial fields. However, epoxy resins have only poor bonding strength to the metals. In order to increase the heterojunction strength between epoxy resins and copper, the introduction of sulfur, providing strong interactions with metals, was investigated. TDP polyester and MPS polythioesters containing sulfur moieties were employed as the sulfur‐containing modifiers for the epoxy resin. Epoxy resins containing a modifier (5–20 phr) and a curing agent were cured between copper plates at 120 °C for 2 h and at 170 °C for 2 h. The effect of the added modifiers was evaluated by the lap‐shear testing method. It could be demonstrated that MPS polythioesters have a beneficial effect on the enhancement of the epoxy resin/copper heterojunction. The most effective example was the addition of 20 phr of the MPS_7 polythioester, which increased the lap shear strength from 5.7 to 17 MPa.

Improvement of lap shear strengths of adhesive copper joints by sulfur‐containing modifiers.     

The Effect of Abrasion Surface Treatment on the Bonding Behavior of Various Carbon Fiber-Reinforced Composites

Abstract

In this paper we show that current abrasion surface preparation practices do not perform equally on all composite surfaces. The effect of abrasion on the adhesive bond strength of various carbon fiber (CF) composites was investigated. Cyanate ester composites were fabricated using a low, a high and an ultra high modulus carbon fiber (T300, M55J, K13C2U). XPS and contact angle measurements showed that the surface energy of all three composites increased due to the removal of contaminants as well as increased in surface roughness. However, the lap shear strength degraded sharply for a number of cases, irrespective of roughness, depending on the fiber used. Composites utilizing lower modulus carbon fibers increased in adhesive bond strength following abrasion in comparison to composites with higher modulus fibers. As the modulus of the fiber and the abrasive grit size increased, the degree of degradation caused by abrasion was shown to increase significantly. Scanning electron microscopy (SEM) and profilometry measurements showed the development of an abrasion-affected zone that was especially prevalent for higher stiffness composites. The failures for the higher modulus specimens were caused by subsurface damage located a few fiber diameters below the abraded surface. However, an alternate technique using atmospheric plasma surface treatment exhibited efficient removal of contaminants while showing no degradation of bond quality when treating these ultra high modulus composites.     

Effect of Flaws in the Adhering Interface on the Strength of Adhesive-bonded Butt Joints

This paper presents the strength of metal-to-metal bonded joints with a flaw in the interface between the adhesive layer and the adhering surface of adherend. The test specimens of butt joints are prepared by bonding two thin-wall metal tubes. The materials are carbon steel, aluminum alloy, brass and copper. The adhesive is epoxy resin. The tensile and shear strength of the joints are experimentally determined by subjecting the specimens to axial load and torsion for various flaw sizes and thickness of adhesive layers. Linear elastic fracture mechanics is applied to the experimental results. The stress intensity factors for a layered composite with a flaw in the interface are numerically calculated in terms of flaw size and loading by using Erdogan's formulas. The fracture stresses of joints with a flaw are predicted at the critical values of the stress intensity factors. The strength of joints without a flaw is also correlated with the stress intensity factors by use of a concept of “effective flaw size”.