The measurement of cohesive and interfacial toughness for bonded metal joints with epoxy adhesives

The types of crack growth in adhesive joints are reviewed and three are identified, namely central cohesive, asymmetric cohesive and interfacial. Test methods for measuring fracture toughness associated with these cracks are then outlined and include a Tapered Double Cantilever Beam (TDCB) test for a central cohesive crack and peel tests on flexible laminates for the other types of crack. In particular, fixed arm and mandrel peel tests are used. Two aerospace adhesives are used to prepare test specimens in order to conduct these tests. For one of these adhesives, all three types of crack growth were recorded and this provided an opportunity to make detailed comparisons of the three associated fracture toughness values. Of particular interest was the use of the mandrel peel method because it enabled a fracture transition (asymmetric cohesive to interfacial fracture) to be observed during the test. The fracture toughness value associated with a central cohesive crack was similar in magnitude to that for an asymmetric cohesive crack. However, the fracture toughness for interfacial fracture was much lower, but similar in magnitude to the expected value of half the fracture toughness from a TDCB test.     

Investigations of epoxy-based adhesives with PLEPS

Contamination-tolerant adhesives are of ever increasing importance in industrial applications. The possible failure mechanisms of adhesive bonds in these adhesives are however still poorly understood. Results of a series of investigations with our pulsed low energy positron beam system (PLEPS) in an epoxy-based contamination-tolerant adhesive are presented.     

Effects of roughness on interfacial performances of silica glass and non-polar polyarylacetylene resin composites

The influence of roughness on interfacial performances of silica glass/polyarylacetylene resin composites was investigated. In order to obtain different roughness, silica glass surface was abraded by different grits of abrasives and its topography was observed by scanning electron microscopy and atomic force microscopy. At the same time, the failure mechanisms of composites were analyzed by fracture morphologies and the interfacial adhesion was evaluated by shear strength test. The results indicated that shear strength of silica glass/polyarylacetylene resin composites firstly increased and then decreased with the surface roughness of silica glass increased. The best surface roughness range of silica glass was 40-60 nm. The main mechanism for the improvement of the interfacial adhesion was physical interlocking at the interface.     

Detachment of stretched viscoelastic fibrils

New experimental results are presented about the final stage of failure of soft viscoelastic adhesives. A microscopic view of the detachment of the adhesive shows that after cavity growth and expansion, well adhered soft adhesives form a network of fibrils connected to expanded contacting feet which fail via a sliding mechanism, sensitive to interfacial shear stresses rather than by a fracture mechanism as sometimes suggested in earlier work. A mechanical model of this stretching and sliding failure phenomenon is presented which treats the fibril as a nonlinear elastic or viscoelastic rod and the foot as an elastic layer subject to a friction force proportional to the local displacement rate. The force on the stretched rod drives the sliding of the foot against the substrate. The main experimental parameter controlling the failure strain and stress during the sliding process is identified by the model as the normalized probe pull speed, which also depends on the magnitude of the friction and PSA modulus. In addition, the material properties, viscoelasticity and finite extensibility of the polymer chains, are shown to have an important effect on both the details of the sliding process and the ultimate failure strain and stress. Electronic supplementary material Appendix B is only available in electronic form at and are accessible for authorised users.     

Improving Epoxy Adhesives with Zirconia Nanoparticles

Zirconia nanoparticles were synthesized by a sol–gel route and dispersed into an epoxy base for structural adhesives. Nanoparticles were used as-synthesized or after calcination. Moreover, the effect of silane functionalization was also investigated. According to preliminary tensile mechanical tests on bulk nanocomposite samples, calcined and untreated zirconia nanoparticles were selected for the preparation of adhesives with various filler contents. The glass transition temperature increased up to a filler content of 1 vol% and then decreased, probably due to the concurrent and contrasting effects of chain blocking and reduction of the crosslinking degree. Also tensile modulus, stress at break and fracture toughness of bulk adhesives samples were positively affected by the presence of an optimal amount of zirconia nanoparticles. Mechanical tests on single lap aluminium bonded joints indicated that zirconia nanoparticles led to relevant enhancements of the shear strength of the joints. In particular, the shear strength increased by about 60% for an optimal filler content of 1 vol%, and an adhesive failure mechanism was evidenced for all the tested specimens. Concurrently, a significant decrease of the equilibrium contact angle with water was observed for adhesives containing zirconia nanoparticles. It can therefore be concluded that the addition of zirconia nanoparticles can effectively improve epoxy adhesives, both by increasing their mechanical properties and by enhancing the interfacial wettability with an aluminium substrate.     

The role of alumina nanoparticles in epoxy adhesives

Both untreated and calcined fumed alumina nanoparticles were dispersed into an epoxy-based adhesive at various percentages. The glass transition temperature of the nanofilled adhesives increased up to an optimal filler loading and then decreased, probably due to concurrent and contrasting effects of chain blocking and reduction of the crosslinking degree. Tensile modulus, stress at break, and fracture toughness of bulk adhesive were positively affected by the presence of untreated alumina nanoparticles at an optimal filler content. Mechanical tests on single-lap aluminum bonded joints indicated that untreated alumina nanoparticles markedly improved both the shear strength and fatigue life of the bonded joints. In particular, the shear strength increased by about 60% for an optimal filler content of 1 vol.%, and an adhesive failure mechanism was evidenced for all the tested specimens. Concurrently, a relevant decrease of the equilibrium contact angle with water was observed for nanofilled bulk adhesives. In summary, alumina nanoparticles can effectively improve the mechanical performances of epoxy structural adhesives, both by increasing their mechanical properties and by enhancing the interfacial wettability with an aluminum substrate.     

Surface characterisation of carbon fibre recycled using fluidised bed

X-ray photoelectron spectroscopy (XPS) was used to investigate the surface of carbon fibres recycled using a high-temperature fluidised bed. The interfacial shear strength of the recycled carbon fibres with epoxy resin was examined using a micro-droplet test. The corresponding as received carbon fibres were used as control samples. It was shown that the recycling process converted some of the surface hydroxyl groups into carbonyl and carboxylic groups due to the effect of heat in atmosphere of air. The overall O/C ratio was not changed significantly. The interfacial shear strength with epoxy resin was not affected by the change of surface oxygen composition. It was also shown that surface texture may play a dominant role in interfacial bonding performance.     

Measuring the critical energy release rate in mode II of tough,structural adhesive joints using the tapered end-notched flexure (TENF) test

This paper shows and discusses results of the Tapered End-Notched Flexure (TENF) test, investigating the fracture behaviour of high-strength structural adhesive joints under shear loading. The TENF test has been previously applied to brittle joints by different authors and has been re-designed to be applicable to ductile adhesives in the presented work. Furthermore, the tests are performed at two velocities, a quasi-static and a dynamic one, to investigate rate effects on the fracture behaviour of the joint. All experimental work has been performed using the structural adhesive DOW Betamate 1496V.     

Thermosets as compatibilizers at the isotactic polypropylene film and thermomechanical pulp fiber interphase

The objective of this study was to improve interfacial adhesion properties at the interface of thermomechanical pulp (TMP) fiber and isotactic polypropylene (iPP) using thermoset adhesives such as phenol formaldehyde (PF) and urea formaldehyde (UF). This study also attempted to achieve fiber-to-fiber adhesion using thermoset adhesives before the molten iPP would flow into the fiber web. The fracture surfaces with thermoset adhesive showed identical differences in terms of fracture modes at the interface. An increased TMP fiber failure was observed with increased thermoset quantity at the interface. Using one percent resin content of weight fraction of TMP fiber handsheet, the tensile strength properties increased almost two fold higher than the strength of control samples. Additional adhesive contents of three and five percent showed gradual strength enhancement. However, the enhanced strength was statistically insignificant. UF resin showed slightly better strength performance over PF resin. This result may be caused by solid contents and additional pigments of resins.     

Modeling on debonding dynamics of pressure-sensitive adhesives

We propose a simple mechanical model describing viscoelasticity and cavitation during the debonding process in pressure-sensitive adhesives (PSA). Our calculation qualitatively reproduces typical stress-strain curves in the probe-tack test, such as the steep stress maxima and the following plateau region. It is shown that in the thin-film geometry the stress-strain curve is essentially determined by the cavities created by the large negative pressure. Effects of pre-existent air bubbles due to surface roughness are also discussed.     

Debonding dynamics of pressure-sensitive adhesives: 3D block model

We develop a 3-dimensional mechanical model which describes cavity expansions in a viscoelastic solid medium during the debonding phase of the probe-tack test. The stress-strain curves are in good agreement with experiments for the typical pressure-sensitive adhesives. We also show that the separation speed dependence can be explained by viscous dissipations due to large strain rates around the cavities.     

Cavity growth in soft adhesives

The growth process of cavities nucleated at the interface between a rigid surface and a soft adhesive layer has been investigated with a probe method. A tensile stress was applied to the highly confined layer resulting in a negative hydrostatic pressure in the layer. The statistics of appearance and rate of growth of cavities as a function of applied negative stress were monitored with a CCD camera. If large germs of cavities were initially present, most of the cavities became optically visible above a critical level of stress independent of layer thickness. Cavities grew simultaneously and at the same expansion rate as a function of applied stress. In the absence of large germs, cavities became optically visible one after another, reaching a limiting size controlled by the thickness of the layer independently and very rapidly. Although, for each sample, we observed a statistical distribution of critical stress levels where a cavity expanded, the mean cavitation stress depended both on surface topography and more surprisingly on layer thickness. We believe that this new and somewhat surprising result can be interpreted with a model for the growth of small germs in finite size layers (J. Dollhofer, A. Chiche, V. Muralidharan et al., Int. J. Solids Struct. 41, 6111 (2004)). This model is mainly based on the dual notion of an energy activated transition from an unexpanded metastable state to an expanded stable state and to the proportionality of the activation energy with the elastic energy stored in the adhesive layer.     

Adhesive fracture of heterogeneous interfaces

We have studied the effect of interface heterogeneity on fracture, at both local and global scales. The single cantilever beam adhesion test was used to investigate interfacial fracture between polycarbonate plates and an elastic/fragile epoxy adhesive. Two surface treatments were applied to a (given) polycarbonate plate giving zones of strong and weak adhesion parallel to the crack direction. Calculated fracture energies differed from those expected from a simple rule-of-mixtures. A perturbation method, proposed by Rice, was used and results compared with crack fronts observed in situ. The technique was applied successfully but the difference in values of stress intensity factor between the zones was found substantially different from the experimental value. In an attempt to explain discrepancies, specimens with discontinuous crack fronts (adhesive and/or plates severed along the strong/weak adhesion frontier) were tested. Good agreement was found with the rule-of-mixtures predictions raising questions about the role of crack front continuity in load transfer.     

Stress analysis at the interface between Ni-P coating and SiCp/Al substrate of space mirror

This paper investigates the mechanical properties at the interface of the coating-substrate system, which comprises the electroless nickel-phosphorus (Ni-P) coating and the aluminum matrix composite substrate reinforced by the silicon carbide particles (SiC_p/Al), and is used for the space mirror. To estimate the adhesion of Ni-P coating on SiC_p/Al substrate, the scratch adhesion testing has been performed by drawing a spherically tipped diamond indenter with a radius of 200 μm over the coated surface. The influence of the coating thickness on the interfacial stress induced by the inertial accelerations, temperature gradients and thermal soaks has been evaluated by simulation analysis based on the finite element method. The results of the scratch testing indicate that the adhesion strength of Ni-P coating to SiC_p/Al composite is more than 3.0 GPa. Compared the maximum value of the interfacial stress obtained by simulation analysis with results of the scratch testing, it is can be seen that the mirror has enough safety margin. Furthermore, the most significant conclusion that can be drawn from this work is that the coating thickness should not exceed 45 μm in order to ensure the performance and reliability of Ni-P coating and SiC_p/Al substrate system for space applications.     

Interface Separation in Residually-Stressed Thin-Film Structures

Plasticity is a significant contributor to the interfacial fracture resistance of multilayer thin-film structures containing ductile layers. Salient parameters affecting plasticity contributions to interfacial fracture energy including the ductile layer thickness, yield strength, and the maximum cohesive stress governing interface separation, have been reported. However, the effects of residual stresses in the thin-film layers on such plasticity contributions have not been considered. We explore the effect of residual stresses on debonding with particular attention to the relationship between the stress state in both ductile and elastic layers and the resulting macroscopic debond energy. Using multiscale simulations it is shown that residual thin-film stresses can alter plasticity in the ductile layer and significantly influence the macroscopic fracture energy. A simple yield-based model to account for this behavior is proposed.     

Effect of periodic surface cracks on the interfacial fracture of thermal barrier coating system

Periodic surface cracks and interfacial debonding in thermal barrier coating (TBC) system may be induced during cooling process. The objective of this work is to investigate the effect of periodic surface cracks on the interfacial fracture of TBC system. The finite element method (FEM) incorporating cohesive zone model is used in analysis. It is found that surface crack spacing has significant effect on the initiation and propagation of short interface crack. Three different regions are identified for describing the effect of surface crack spacing. In Region I the interface crack driving force is dramatically reduced due to high surface crack density. In this case, the initiation of interfacial delamination can be delayed. Region II applies as the surface crack spacing is moderate. Analysis of this transition zone brings to the definition of normalized critical surface crack spacing. Region III arises for sufficient large surface crack spacing. In this case, the interface crack driving force reaches a steady state, where the effects of adjacent surface cracks are relatively insignificant and can be ignored. It can be concluded that an appropriately high surface crack density can enhance the durability of TBC system.     

Electro-mechanical response of functionally graded beams with imperfectly integrated surface piezoelectric layers

The time-dependent behavior of a simply-supported functionally graded beam bonded with piezoelectric sensors and actuators is studied using the state-space method. The creep behavior of bonding adhesives between piezoelectric layers and beam is characterized by a Kelvin-Voigt viscoelastic model, which is practical in a high temperature circumstance. Both the host elastic functionally graded beam and the piezoelectric layers are orthotropic and in a state of plane stress, with the former being inhomogeneous along the thickness direction. A laminate model is employed to approximate the host beam. Moreover, the coupling effect between the elastic deformation and electric field in piezoelectric layers is considered. Results indicate that the viscoelastic property of interfacial adhesives has a significant effect on the function of bonded actuators and sensors with time elapsing.     

Kinetics of adhesive contact formation between thermoplastic melts and solid surfaces: Effect on bond strength

Formation of an adhesive contact between a polymer melt (or solution) and reinforcing fibers is considered from the viewpoint of kinetics. A two-stage model of this process has been proposed, and an expression for the interfacial bond strength as a function of time and temperature is derived. Experimental data on bond strength in adhesive joints between thermoplastic polymers and reinforcing fibers formed under various conditions were obtained, and the concept of activation energy was used to analyze them. Since the process is controlled by the stage having the larger activation energy, the adhesive contact formation between fibers and polymer solutions is governed by the rate of adhesive bonding, whereas that between fibers and polymer melts is governed by the rate of the melt spreading.     

Z箍缩动态黑腔负载装配用胶粘剂

由于Z箍缩动态黑腔负载中的低密度聚合物泡沫柱具有多孔结构和极低的力学强度,在装配过程中对胶粘剂有一定的特殊要求。实验以聚氨酯丙烯酸酯为主体树脂,丙烯酸异冰片酯为活性单体,配以光引发剂和偶联剂制得了适用于低密度泡沫粘接用的紫外光固化胶粘剂。测试表明,所研制的胶粘剂的体积收缩率仅为2.25%,且具有较高的固化速率和适当的粘接强度,能够满足稳固、快速装配的要求。通过对粘接界面的观测发现,胶粘剂在诊断孔内没有明显的扩散,且在低密度致密结构泡沫中的扩散厚度较小,均匀性好。     

Z箍缩动态黑腔负载装配用胶粘剂

由于Z箍缩动态黑腔负载中的低密度聚合物泡沫柱具有多孔结构和极低的力学强度,在装配过程中对胶粘剂有一定的特殊要求。实验以聚氨酯丙烯酸酯为主体树脂,丙烯酸异冰片酯为活性单体,配以光引发剂和偶联剂制得了适用于低密度泡沫粘接用的紫外光固化胶粘剂。测试表明,所研制的胶粘剂的体积收缩率仅为2.25%,且具有较高的固化速率和适当的粘接强度,能够满足稳固、快速装配的要求。通过对粘接界面的观测发现,胶粘剂在诊断孔内没有明显的扩散,且在低密度致密结构泡沫中的扩散厚度较小,均匀性好。