The Interfacial Bond Strength in Glass Fibre-Polyester Resin Composite Systems

The interfacial bond strength in glass fibre-polyester resin composites has been investigated using various experimental techniques. These included blocks of resin containing fibre (in which, depending on the geometry of the specimen, failure occurs in either a shear or tensile mode) the pullout of a fibre from a disc of resin and a short beam shear test for interlaminar shear strength determination.

Low power optical microscopy and optical retardation measurements of stress induced birefringence were used to detect the difference between intact and debonded fibre resin interfaces. The shear modulus and shear strength of the resin were obtained from torsion tests on cylindrical rods of the resin.

The single fibre shear debonding specimen and the short beam shear test are shown to be the most viable test methods but interpretation of the results is complicated by the various modes of failure possible and by the different stress states which exist in the area of the specimen where debonding starts. Stress concentration factors obtained by finite element analysis and photoelastic analysis have been applied to the results from these tests and the corrected interfacial bond strengths are in close agreement.

The real interfacial bond strengths of well bonded glass-fibre polyester resin systems is shown to be of the order of 70 MN m^?2.     

The Interfacial Bond Strength in Glass Fibre-Polyester Resin Composite Systems Part 2. The Effect of Surface Treatment

The effect of surface treatments on the bond strength in glass fibre-polyester resin composites has been investigated using single fibre interfacial shear strength specimens and the short beam shear test for interlaminar shear strength.

A range of bond strengths was obtained by using, either alone or in combination, the various components of the size formulation which is normally applied to the fibres, so that the interaction between the glass surface and the polyester ranged from Van de Waal forces through hydrogen bonding to covalent bonding, the bond strength increasing in that order.

The relative contribution to bond strength of mechanical bonding due to thermo-mechanical mismatch between the two components and of chemical bonding or physical interaction between the three phases, glass-surface treatment-resin, has been evaluated and found to be one third and two thirds respectively.     

The Effect of Sodium Ions on the Stability of the Interphase Region of Glass Fibre Reinforced Composites

The single embedded filament fragmentation and the short beam shear strength tests together with angle-resolved X-ray Photoelectron Spectroscopy (XPS) have been used to investigate the interfacial region of vinyl ester composites reinforced with sized AR-glass fibres, with and without amino and vinyl functional adhesion promoters.

The 7-aminopropyltriethoxysilane (APS) deposit on AR-glass is susceptible to a thermal degradation during post-cure, which has been attributed to a base catalysed equilibration of the siloxane bonds. The functional groups of APS required for resin compatibility were buried beneath the surface layers, contributing to a low bond strength, furthermore, mobile sodium ions existed within the interfacial region. Aqueous extraction prior to fabrication enhanced the composite bond strength by removing the soluble silane oligomers, the sodium ions, and exposing the organo-functional groups for co-reaction with the matrix.

The silane deposit on AR-glass is made hygroscopic by the presence of sodium ions. This increased the equilibrium moisture content of AR-glass composites, and diminished their retained short beam shear strength in contrast to the E-glass control which retained its properties after redrying.     

A failure criterion for debonding between encapsulants and leadframes in plastic IC packages

A failure criterion for debonding initiation between molding compounds and copper leadframes in plastic encapsulated integrated circuit (IC) packages is proposed. The leadframe pull-out test is used to evaluate the bond strengths between molding compounds and leadframes in plastic encapsulated IC packages. The normal and shear stress fields along the interface are analyzed using the finite element method. An average stress approach is employed for the interface failure criterion and the tensile and shear interface bond strengths are obtained from the experimental failure loads. In a parametric study, the effects of specimen loading geometry, moisture, and surface contamination of copper leadframes on the interfacial bond strengths are specifically analyzed. The results show that the interface bond strengths determined for the two specimen geometries are consistent.     

Regeneration of Adhesive Bonding in Fiber/Resin Systems

The bond between a fiber and a surrounding polymer matrix can be weakened or completely broken by mechanical shearing. In some cases bond strength can be reduced by exposure to active environments (for example, hot water or steam). Obviously, it would be of considerable practical value if weakended or lost bonding could be regenerated. This paper presents the first results of an ongoing study of the possibility of bond repair in fiber/resin systems.

The use of the TRI microbond shear strength measurement technique makes it possible to study bond regeneration with individual fiber/resin specimens. Since the microdrop is displaced only a very short distance along the fiber during the shear strength measurement, it is a simple matter to treat a sheared drop without removing it from the fiber and then perform a second shear strength evaluation. Several systems have been studied in this manner, involving both thermosetting and thermoplastic resins. Examples of significant regeneration of both mechanically-sheared and hydrolytically-weakened bonds are given, and possible mechanisms for the bond strength regeneration are discussed.     

Peel Strength of Aluminium-Aluminium Joints Bonded by Adhesive Based on Carboxylated Nitrile Rubber-Chlorobutyl Rubber Blend

The peel strength of aluminium-aluminium joints bonded by an adhesive based on carboxylated nitrile rubber and chlorobutyl rubber was found to depend on surface topography and use of a silane primer. Anodization causes a marginal increase in bond strength while the silane primer improves the adhesive joint strength remarkably.

The peel strength was also found to be dependent on test conditions (test rate and temperature). The threshold peel strength value obtained by measurements at low peel rate and high test temperature was found to depend on the type of failure during peeling (cohesive or interfacial) which, in turn, is controlled by the presence of silica filler in the adhesive. Two different threshold values of peel strength were obtained: 60 N/m for interfacial failure (in silica-filled adhesive), 140 N/m for cohesive failure (in unfilled adhesive).     

Stress Transfer Function for Interface Assessment in Composites with Plasma Copolymer Functionalized Carbon Fibres

Radio-frequency-induced plasma copolymerization of acrylic acid/1,7-octadiene was used to produce a range of functionalized plasma copolymer coatings with controlled degree of adhesion. The single-fibre fragmentation test was used to characterize the adhesion of plasma copolymer coated fibres to epoxy resin. The cumulative stress transfer function (CSTF) and Kelly-Tyson approaches were used to evaluate the degree of adhesion. By continuous monitoring of the fragmentation process, it was found that the mechanical performance of a composite material could be evaluated using the CSTF methodology at strain well below saturation. The degree of debonding was a good measure of relative interface/interphase adhesive strength. The trend in the CSTF is consistent with the propagation of interfacial debonds during the test. For a completely debonded fibre a normalized CSTF value, referred as stress transfer efficiency (STE), was found to provide a more consistent analysis that was able to differentiate between fibres with similar degrees of debonding. The calculated values of interfacial shear strength (IFSS) were only valid for a fully debonded fibre (1,7-octadiene plasma homopolymer coating) where the assumption of a constant shear stress, as in the Kelly-Tyson model, applied. However, IFSS did not provide the same ranking. Where debonding does not occur, the stress transfer efficiency also provides a sensitive measure of the interface/interphase performance. Improved adhesion over the untreated-unsized carbon fibre was observed for both of the plasma copolymer-coated and commercially treated carbon fibres. Since there is a concentration dependence of carboxyl groups on adhesion, the mechanism appears to relate to covalent bond formation with the epoxy group. Plasma copolymer coatings on carbon fibres also causes an increased tensile strength and Weibull modulus.     

Stress Transfer Function for Interface Assessment in Composites with Plasma Copolymer Functionalized Carbon Fibres

Radio-frequency-induced plasma copolymerization of acrylic acid/1,7-octadiene was used to produce a range of functionalized plasma copolymer coatings with controlled degree of adhesion. The single-fibre fragmentation test was used to characterize the adhesion of plasma copolymer coated fibres to epoxy resin. The cumulative stress transfer function (CSTF) and Kelly-Tyson approaches were used to evaluate the degree of adhesion. By continuous monitoring of the fragmentation process, it was found that the mechanical performance of a composite material could be evaluated using the CSTF methodology at strain well below saturation. The degree of debonding was a good measure of relative interface/interphase adhesive strength. The trend in the CSTF is consistent with the propagation of interfacial debonds during the test. For a completely debonded fibre a normalized CSTF value, referred as stress transfer efficiency (STE), was found to provide a more consistent analysis that was able to differentiate between fibres with similar degrees of debonding. The calculated values of interfacial shear strength (IFSS) were only valid for a fully debonded fibre (1,7-octadiene plasma homopolymer coating) where the assumption of a constant shear stress, as in the Kelly-Tyson model, applied. However, IFSS did not provide the same ranking. Where debonding does not occur, the stress transfer efficiency also provides a sensitive measure of the interface/interphase performance. Improved adhesion over the untreated-unsized carbon fibre was observed for both of the plasma copolymer-coated and commercially treated carbon fibres. Since there is a concentration dependence of carboxyl groups on adhesion, the mechanism appears to relate to covalent bond formation with the epoxy group. Plasma copolymer coatings on carbon fibres also causes an increased tensile strength and Weibull modulus.     

Flexibilized Copolyimide Adhesives

Two copolyimides, LARC-STPI and STPI-LARC-2, with flexible backbones were prepared and characterized as adhesives. The processability and adhesive properties were compared to those of a commercially available form of LARC-TPI.

Lap shear specimens were fabricated using adhesive tape prepared from each of the three polymers. Lap shear tests were performed at room temperature, 177°C, and 204°C before and after exposure to water-boil and to thermal aging at 204°C for up to 1000 hours.

The three adhesive systems possess exceptional lap shear strengths at room temperature and elevated temperatures both before and after thermal exposure. LARC-STPI, because of its high glass transition temperature provided high lap shear strengths up to 260°C. After water-boil, LARC-TPI exhibited the highest lap shear strengths at room temperature and 177°C, whereas the LARC-STPI retained a higher percentage of its original strength when tested at 204°C [68% versus 50% (STPI-LARC-2) and 40% (LARC-TPI)].

These flexible thermoplastic copolyimides show considerable potential as adhesives based on this study and because of the ease of preparation with low cost, commercially available materials.     

Non-destructive Inspection of Adhesively-bonded Joints

The objective of any system of non-destructively examining an adhesive joint must be to obtain a direct correlation between the strength of the joint and some mechanical, physical or chemical parameter which can readily be measured without causing damage. Faults or defects are defined as anything which adversely affect the short or long term strength of a joint. There are two basic areas for examination, the cohesive strength of the polymeric adhesive, and the adhesive strength of the bond between polymer and substrate.

Adhesive strength is very difficult to measure since it is an interfacial phenomenon involving a very thin layer of material, thin even in comparison with bond-line dimensions. Effectively, it would be necessary to assess intermolecular forces and this is not readily possible with existing techniques. This aspect of quality control is usually reduced to assessing the nature of the adherend surfaces prior to bonding.

The cohesive strength of the adhesive is really the only parameter which can be estimated with any degree of confidence, and it is this which features most on destructive tests of bonded joints.

In this paper, defects including porosity, surface un-bonds, zero-volume unbonds, poor cure and so on are discussed, together with the various methods currently used (and some new methods) for physical non-destructive testing.     

Effects of Extraction on Wettability and Gluability of Apitong (Dipterocarpus Grandiflorus Blanco)

An investigation was made of the effects of extraction and various chemicals applied on veneer surface on the wettability and gluing properties of apitong, Dipterocarpus grandiflorus Blanco, using urea formaldehyde resin. Wettability was determined by measuring contact angles with distilled water.

It was found that extraction with methanol-benzene greatly improved the wettability and gluability of apitong veneer. Likewise, surface treatment with methanol-benzene significantly increased the wettability of the veneer as well as the dry and wet shear strengths of the resulting bond. Treatment with sodium hydroxide increased both wettability and dry shear, but decreased the wet shear strength of the bond. Acetone did not have a significant effect on both wettability and dry shear, but decreased wet shear strength. On the other hand, ether had adverse effects on the wettability and gluability of apitong veneer.

A positive linear correlation was found between wettability and gluability of apitong veneer.     

Microscale groove effect on shear strength of epoxy-bonded dissimilar metal plate

In this study, the shear strengths of Al 7075–HSS adhesive bonded, grooved and smooth plates were investigated. The proven toughness and durability of adhesives have drawn the attention of researchers who want to take advantage of the technology to benefit the development of ballistic resistance sandwich panels. However, the strength of the panel depends on the design of surface topography. Therefore, it is essential to understand the fracture upon loading parallel to the plane of the adhesive bonded metal plates. In this experiment, toughened epoxy was used to bond dissimilar metal plates at 1 mm thickness. The shear tests were performed with a universal-testing machine to identify the maximum fracture loads. The results showed that a shear lap joint specimen with a grooved surface yields a higher strength than a smooth specimen. From the fracture behaviour of all specimens, interfacial failure with some degree of cohesive failure was observed. This indicates that the strength of the adhesive-bonded metal plate driven by a mechanical interlocking effect and mode of failure for thick bondline was the result of interfacial strength rather than adhesive bulk strength. Shear value results and fractography for 1 mm bond thickness provide insights towards steel fibre application in epoxy.     

BOND STRENGTH MEASUREMENT BETWEEN GLASS FIBRES AND EPOXY RESIN AT ELEVATED TEMPERATURES USING THE PULL-OUT AND PUSH-OUT TECHNIQUES

Pull-out and push-out measurements were performed on glass fibres in an epoxy resin to determine the dependence of bond strength on test temperature and on fibre surface treatment. A comparative analysis of the two techniques was carried out to elucidate elementary processes of polymer-fibre debonding and to determine energy values for adhesional bonds. Differences in bond strength values for pull-out and push-out tests were attributed to failure mechanisms that were either interface-controlled or matrix-controlled. Evidence for the different failure mechanisms characteristic of the two test techniques was provided by an estimation of failure parameters, such as the activation energy for debonding. Failure mechanisms also were manifest in AFM images, showing differences in topography and roughness that depended on fibre surface treatment, test geometry, and test temperature.     

Bond strength measurement between glass fibres and epoxy resin at elevated temperatures using the pull-out and push-out techniques

Pull-out and push-out measurements were performed on glass fibres in an epoxy resin to determine the dependence of bond strength on test temperature and on fibre surface treatment. A comparative analysis of the two techniques was carried out to elucidate elementary processes of polymer-fibre debonding and to determine energy values for adhesional bonds. Differences in bond strength values for pull-out and push-out tests were attributed to failure mechanisms that were either interface-controlled or matrix-controlled. Evidence for the different failure mechanisms characteristic of the two test techniques was provided by an estimation of failure parameters, such as the activation energy for debonding. Failure mechanisms also were manifest in AFM images, showing differences in topography and roughness that depended on fibre surface treatment, test geometry, and test temperature.     

Development of a Failure Model for the Adhesively Bonded Tubular Single Lap Joint

The accurate calculation of the stresses and torque capacities of adhesively bonded joints is not possible without understanding the failure phenomena of the adhesive joints and the nonlinear behavior of the adhesive.

In this paper, an adhesive failure model of the adhesively bonded tubular single lap joint with steel-steel adherends was proposed to predict the torque capacity accurately.

The model incorporated the nonlinear behavior of the adhesive and the different failure modes in which the adhesive failure mode changed from bulk shear failure, via transient failure, to interfacial failure between the adhesive and the adherend, according to the magnitudes of the residual thermally-induced stresses from fabrication.     

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.     

A microdebonding test for in situ assessment of fibre/matrix bond strength in composite materials

A test technique which gives a quantitative measure of the in situ fibre/matrix bond strength in fibre-reinforced composite materials is described. The test involves the compressive loading of a fibre or region of fibres on a polished specimen surface to produce debonding. Results are presented for the debonding load for glass/epoxy, aramid/epoxy and graphite/epoxy composites. The change in debonding load is also followed as the interface degrades during moisture conditioning at different points through the thickness of the material. It is concluded that refinements to the technique are needed to simplify interpretation of the debonding force in terms of interface shear strength, and to make the test more reproducible.     

Peel Adhesion of Solid Films-The Surface and Bulk Effects

The peel strength of rubber and paint films has been measured over a range of peeling velocities using a dead weight method. At low peel rates the peel force is fairly constant but rises rapidly at higher peeling speeds.

Experiments show that the peel strength is a function both of the energy of interfacial bonds which must be broken as peeling proceeds and of bulk energy losses in a viscoelastic peeling material.

The interfacial effect has two components: an equilibrium surface force which accounts for the peel strength at low velocities, and a viscous peeling force which depends on the peeling rate. This viscous interfacial force explains the increase in peel strength of purely elastic films at higher peeling velocities.

The energy loss in the bulk of the peeling film introduces two additional effects: a magnification of the peel strength in steady peeling over a certain velocity range, and a slowing down or stopping of peeling as transient relaxation occurs shortly after the application of the peel force.     

Some Effects of Heat Processing on Adhesively Bonded Automotive Steel

During a program to develop new structural adhesives that would meet the processing requirements of the current automotive “high heat” bake cycles, substantial differences in performance were noted between the “standard” cold rolled steel (CRS) of SAE1008 and certain Drawing Quality steels (DQSK).

In parallel tests, certain DQSK test specimens (bonded with 200°C heat cycle) failed consistently in the interfacial region, while the CRS samples failed center of bond.

The surface characteristics of the steels and failed adhesive specimens were examined with ESCA, AUGER, and ISS spectroscopic methods. The metal failure surfaces of the DQSK samples were shown to contain relatively high levels of silicon and oxygen, and smaller amounts of boron, with a lower concentration of iron, as compared to CRS which shows iron surface with minor contaminations.

In subsequent testing, with other samples of DQSK SAE1008, this effect was not observed. These samples did not exhibit the same levels of contamination. It is suggested that certain DQSK processes may involve processing steps that are detrimental to the surface properties of the rolled sheet stock.     

A Study of the Pull‐Out Performance of PPTA Fibres in Composites of Ethylene‐Type Ionomer Matrices/PPTA Fibres

The mean frictional shear stresses of six ionomer resins and sized Kevlar fibre were determined from fibre pull‐out tests. A study of the failure mechanisms occurring during pull‐out revealed that fibre delamination and fibre resin adhesion were factors which increased the measured frictional shear stresses and that there was a definite grouping of high and low frictional shear stress values. The low frictional shear stress values were used to calculate the mean frictional shear stress values, τ_B, because these were uncomplicated by fibre delamination and fibre resin adhesion, since these factors (delamination and adhesion) are certainly not unexpected in an ionomer/Kevlar composite. From these shear stress values, it was determined that critical fibre lengths should be between 35 and 72 mm for the high tensile strength Kevlar fibres within an ionomer matrix, for the composite to be used effectively. The ratio of the debonding force (F_B) to the frictional shear force (F_F), θ, did not vary significantly with the lengths of the embedded reinforcing fibres. Both debonding and frictional forces indicate increasing trends with the interfacial contact areas. The ratio of the interfacial bonding strength (τ_B) to the frictional shear stress (τ_F), ϕ, for the resin PEA‐6 compared to the surface modified poly(p‐phenylene terephthalamide) (PPTA) fibre ranged from 2 to 24. These ratios were grouped into two, viz: those where ϕ > 11 and those with ϕ < 7. Using only the τ_F where ϕ > 11 provided a mean frictional shear stress of 0.94 MPa and a standard deviation, s, of 0.23 MPa (the number of test samples, n, was 9). This value is little different from the frictional shear stresses measured for sized PPTA (0.84 MPa). The decrease in the values of ϕ is attributed to the decrease in τ_B, due to the surface modification reaction, without necessarily affecting the frictional shear stress, τ_F.