FRACTURE OF METAL/POLYMER/METAL ASSEMBLIES: VISCOELASTIC EFFECTS

The purpose of this work is to link the polymer viscoelastic properties (especially its relaxation time) and the adhesive behaviour of steel/polymer/steel assemblies. A wedge test device developed in the laboratory allows one to introduce the wedge into the assembly at a controlled speed and to follow the crack propagation with a camera-equipped microscope. The adherence energy (calculated from the equilibrium crack length) and the crack propagation rate are measured for different wedge introduction rates. Polymer equivalent relaxation time is determined for each introduction rate according to the time-temperature superposition principle. Relations between adherence energy, crack propagation rate, and calculated equivalent relaxation time values are proposed. These quantitative relations confirm the major influence of polymer viscoelastic properties on the rate sensitivity of adhesive behaviour.     

Adhesion in Polymer/Steel Sandwiches

Polymer/steel sandwiches are able to reduce the nuisance due to vibrations and noise in automotive applications, for example. Thin layers of polymer are inserted between two metal sheets. The deformation of the polymer is responsible for the damping properties of the sandwiches and, therefore, the viscoelastic behavior of the polymer is of major importance. However, adhesion between the two materials is also required. The polymer studied in the present work is a copolymer of ethylene and vinyl acetate (EVA) containing 28 wt% of vinyl acetate grafted with maleic anhydride (1 wt%). A wedge test is used to measure the interfacial strength and the durability of the adhesive bond. The influence of the surface treatment of the steel substrate on the adhesive behavior and the effect of water has been studied. FTIR surface analysis after cleavage helped us to identify the nature of the interfacial bonds.     

Adhesion in Polymer/Steel Sandwiches

Polymer/steel sandwiches are able to reduce the nuisance due to vibrations and noise in automotive applications, for example. Thin layers of polymer are inserted between two metal sheets. The deformation of the polymer is responsible for the damping properties of the sandwiches and, therefore, the viscoelastic behavior of the polymer is of major importance. However, adhesion between the two materials is also required. The polymer studied in the present work is a copolymer of ethylene and vinyl acetate (EVA) containing 28 wt% of vinyl acetate grafted with maleic anhydride (1 wt%). A wedge test is used to measure the interfacial strength and the durability of the adhesive bond. The influence of the surface treatment of the steel substrate on the adhesive behavior and the effect of water has been studied. FTIR surface analysis after cleavage helped us to identify the nature of the interfacial bonds.     

ADHESIVE CONTACT OF VISCOELASTIC SPHERES: A HAND-WAVING INTRODUCTION

We give an overview of the general features of the linear viscoelastic adhesive contact model. The two main features are (1) a delay between the contraction of the contact radius and the onset of the indenter retraction, and (2) the enhancement of the adherence force. We emphasize the role played by stress relaxation within the contact zone in these phenomena and give simple forms of the viscoelastic adhesive contact equations to account for it. Two characteristic timescales are identified, respectively associated with the crack tip and the contact zone. Their asymmetric roles in the growing and receding contact phases is evidenced. Energy release rates for both phases are calculated together with their irreversible components.     

ADHESIVE CONTACT OF VISCOELASTIC SPHERES: A HAND-WAVING INTRODUCTION

We give an overview of the general features of the linear viscoelastic adhesive contact model. The two main features are (1) a delay between the contraction of the contact radius and the onset of the indenter retraction, and (2) the enhancement of the adherence force. We emphasize the role played by stress relaxation within the contact zone in these phenomena and give simple forms of the viscoelastic adhesive contact equations to account for it. Two characteristic timescales are identified, respectively associated with the crack tip and the contact zone. Their asymmetric roles in the growing and receding contact phases is evidenced. Energy release rates for both phases are calculated together with their irreversible components.     

CRACK GROWTH IN ADHESIVELY BONDED JOINTS SUBJECTED TO VARIABLE FREQUENCY FATIGUE LOADING

The main aim of this article is to investigate the effect of frequency on fatigue crack propagation in adhesively bonded joints. Adhesively bonded double-cantilever beam (DCB) samples were tested in fatigue at various frequencies between 0.1 and 10 Hz. The adhesive used was a toughened epoxy, and the substrates used were a carbon fibre-reinforced polymer (CFRP) and mild steel. Results showed that the crack growth per cycle increases and the fatigue threshold decreases as the test frequency decreases. The locus of failure with the CFRP adherends was predominantly in the adhesive layer, whereas the locus of failure with the steel adherends was in the interfacial region between the steel and the adhesive. The crack growth was faster, for a given strain energy release rate, and the fatigue thresholds lower for the samples with steel adherends. Tests with variable frequency loading were also carried out, and a generalised method of predicting crack growth in samples subjected to a variable frequency loading was introduced. The predicted crack growth using this method agreed well with experimental results.     

Adhesive and Rheological Properties of Lightly Crosslinked Model Acrylic Networks

The viscoelastic and adhesive properties of a series of model, lightly crosslinked acrylic polymer networks have been investigated. The model networks were statistical copolymers of 2-ethyl-hexyl acrylate and acrylic acid or terpolymers of 2-ethyl-hexyl acrylate, acrylic acid, and stearyl acrylate synthesized in solution. All were lightly crosslinked after the polymerization was completed to obtain typical properties of pressure-sensitive adhesives. The bulk rheological properties of the networks were characterized by dynamical mechanical spectroscopy and in uniaxial extension. Their adhesive properties were tested with an instrumented probe tester fitted with a cylindrical steel probe. The presence of acrylic acid in the copolymer caused an increase in both elastic modulus and resistance to interfacial crack propagation characterized by the critical energy-release rate G_c and the incorporation of stearyl acrylate caused a decrease in both modulus and G_c. In both cases, however, the modification of G_c controlled the overall behavior. The analysis of the nonlinear elastic properties of the adhesives with the Mooney-Rivlin model provided new insights on the role played by the ratio between entanglements and crosslink points in controlling the formation and extension of the bridging fibrils observed upon debonding.     

Durability of titanium adhesive bonds with surface pretreatments based on alkaline anodisation

The most recent works suggest that the alkaline anodizing process (NaTESi) based in a bath of sodium hydroxide may be an attractive alternative to chromic acid anodizing (CAA) for surface pretreatment of titanium alloys for preparing hybrid adhesive bonds Ti6Al4V/Carbon Fiber Reinforced Composite (CFRC). This work compares several anodizing processes used for surface preparation, such as CAA, NaTESi and two modified NaTESi processes. The surface morphology, roughness, surface free energy and, especially, the initial strength adherence and durability under the wedge crack tests have been characterized. Wedge crack tests were performed in three different ageing media that may be representative of the environment that adhesive joints based upon Ti6Al4V/CFRC have to withstand during aircraft service life environments: hot/wet conditions; CTB3+TS test, that combines wet-dry cycles with exposure to a corrosive environment (CTB3) and thermal shocking (TS); and immersion tests in a Lap Joint Simulant Solution (LJSS). The results indicate that despite the morphological differences of the oxide grown by CAA and NaTESi, the initial adhesive strength with an epoxy adhesive and the durability of the bond are similar for both anodizing processes. Conversely, higher initial adhesive forces are exhibited for both modified NaTESi anodizing processes.     

Crack growth in adhesively bonded joints subjected to variable frequency fatigue loading

The main aim of this article is to investigate the effect of frequency on fatigue crack propagation in adhesively bonded joints. Adhesively bonded double-cantilever beam (DCB) samples were tested in fatigue at various frequencies between 0.1 and 10 Hz. The adhesive used was a toughened epoxy, and the substrates used were a carbon fibre-reinforced polymer (CFRP) and mild steel. Results showed that the crack growth per cycle increases and the fatigue threshold decreases as the test frequency decreases. The locus of failure with the CFRP adherends was predominantly in the adhesive layer, whereas the locus of failure with the steel adherends was in the interfacial region between the steel and the adhesive. The crack growth was faster, for a given strain energy release rate, and the fatigue thresholds lower for the samples with steel adherends. Tests with variable frequency loading were also carried out, and a generalised method of predicting crack growth in samples subjected to a variable frequency loading was introduced. The predicted crack growth using this method agreed well with experimental results.     

Effect of Surface Morphology on Crack Growth at a Sol-Gel Reinforced Epoxy/Aluminum Interface

The Boeing sol-gel conversion coating (Boegel-EPII), derived from an acid-catalyzed aqueous solution of organofunctional silane and zirconium alkoxide precursors, is being used as an adhesion promoter for adhesive bonding and painting applications in the aerospace industry. A unique advantage of the sol-gel process is that strong and durable bonds are produced without the hazardous chemical usage and rinse-water requirements of conventional anodizing or etching processes. In this study, a fracture mechanics method was used to investigate the adhesion properties of sol-gel-reinforced epoxy/aluminum joints. The Hugh Brown asymmetric double cantilever beam (ADCB) wedge test was employed, which allowed the measurements of the critical energy-release rate, subcritical crack-growth kinetics, and threshold energy-release rate on a single sample in a reasonably short period of time. These experiments were carried out with aluminum substrates on which the surface morphology was systematically varied by polishing, sanding, grit-blasting, and chemical etching. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to identify the locus of failure. The surface morphology of the substrates was characterized with SEM, optical profilometry, and spreading kinetics. The macrorough structures drive the crack to within a thin epoxy layer close to the polymer/metal interface, which enhances the initial strength of the sol-gel-reinforced interface. The microroughness of the substrate is, however, more effective than the macroroughness in enhancing the durability. Lastly, an attempt has been made to correlate the energy-release rate with the fractal dimension for sol-gel-reinforced joints with macrorough substrates.     

Effect of Surface Morphology on Crack Growth at a Sol-Gel Reinforced Epoxy/Aluminum Interface

The Boeing sol-gel conversion coating (Boegel-EPII), derived from an acid-catalyzed aqueous solution of organofunctional silane and zirconium alkoxide precursors, is being used as an adhesion promoter for adhesive bonding and painting applications in the aerospace industry. A unique advantage of the sol-gel process is that strong and durable bonds are produced without the hazardous chemical usage and rinse-water requirements of conventional anodizing or etching processes. In this study, a fracture mechanics method was used to investigate the adhesion properties of sol-gel-reinforced epoxy/aluminum joints. The Hugh Brown asymmetric double cantilever beam (ADCB) wedge test was employed, which allowed the measurements of the critical energy-release rate, subcritical crack-growth kinetics, and threshold energy-release rate on a single sample in a reasonably short period of time. These experiments were carried out with aluminum substrates on which the surface morphology was systematically varied by polishing, sanding, grit-blasting, and chemical etching. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to identify the locus of failure. The surface morphology of the substrates was characterized with SEM, optical profilometry, and spreading kinetics. The macrorough structures drive the crack to within a thin epoxy layer close to the polymer/metal interface, which enhances the initial strength of the sol-gel-reinforced interface. The microroughness of the substrate is, however, more effective than the macroroughness in enhancing the durability. Lastly, an attempt has been made to correlate the energy-release rate with the fractal dimension for sol-gel-reinforced joints with macrorough substrates.     

Experimental study of cohesive strain prior to crack initiation in constant force single cantilever beam test

Retarded deformation at the bond line edge of a thick, viscoelastic adhesive layer is observed using digital image correlation. Such information is required for proposing physical crack onset/propagation parameters which take into account viscoelastic and viscous mechanisms in the evaluation of dissipated energy. A complex stress/strain redistribution is observed in the early stage after specimen loading. The results illustrate that viscoelasticity dissipates a considerable amount of energy prior to crack nucleation and the lack of predictive models to account for that phenomenon.     

Influence of corona treatment on adhesion and mechanical properties in metal/polymer/metal systems

The effect of corona treatment (CT) on the adhesion at the metal–polymer interface was studied. Metal/polymer/metal laminates were manufactured by the laboratory roll‐bonding process with preliminary corona surface treatment of the polymer core: a polyethylene and polypropylene sheet as well as steel sheet. It was treated with corona discharge to increase its surface energy and the adhesion to metal, an austenitic steel. The adhesion, which was measured by T‐peel and shear tests, was increased by 43% of crack peel and 22% of mean peel resistance respectively, after 120 s CT. On the basis of scanning electron spectroscopy observations, improvements in the adhesive properties were attributed to the change in the interfacial morphology. In mechanical tests, yield and tensile strengths were strongly influenced by CT, indicating that these laminates were sensitive to interfacial phenomena. However, elongation at rupture of the composites was found to be unchanged. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011.     

Influence of rheological properties of adhesive polymer on strain energy release rate of mode II and adhesive shear strength

It is well known that adhesive strength shows temperature and rate dependencies reflecting viscoelastic properties of an adhesive used. Specifically, a mechanical relaxation mechanism around the glass transition temperature (T_g) has a strong effect on the adhesive strength, which involves deformation of the adhesive layer. In addition, it is very interesting to know how viscoelastic properties of the adhesive affect the value of strain energy release rate since deformation and failure of the adhesive occur at the measurement of strain energy release rate for adhesive joints. In this study, adhesive tensile strength and strain energy release rate (G_IIC) in plain-shearing mode were measured under a constant experimental condition using adhesives consisting of two types of epoxy resins; the influence of viscoelastic properties on these two values was investigated, and we discuss the relationship between the adhesive shear strength and G_IIC. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 525–536, 1997     

Crack Path Selection in Adhesively-Bonded Joints: The Role of Material Properties

This paper investigates the role of material properties on crack path selection in adhesively bonded joints. First, a parametric study of directionally unstable crack propagation in adhesively-bonded double cantilever beam specimens (DCB) is presented. The results indicate that the characteristic length of directionally unstable cracks varies with the Dundurs' parameters characterizing the material mismatch. Second, the effect of interface properties on crack path selection is investigated. DCB specimens with substrates treated using various surface preparation methods are tested under mixed mode fracture loading to determine the effect of interface properties on the locus of failure. As indicated by the post-failure analyses, debonding tends to be more interfacial as the mode II fracture component in the loading increases. On the other hand, failures in specimens prepared with more advanced surface preparation techniques appear more cohesive for given loading conditions. Using a high-speed camera to monitor the fracture sequence, DCB specimens are tested quasi-statically and the XPS analyses conducted on the failure surfaces indicate that the effect of crack propagation rate on the locus of failure is less significant when more advanced surface preparation techniques are used. The effect of asymmetric interface property on the behavior of directionally unstable crack propagation in adhesive bonds is also investigated. Geometrically-symmetric DCB specimens with asymmetric surface pretreatments are prepared and tested under low-speed impact. As indicated by Auger depth profile results, the centerline of the crack trajectory shifts slightly toward the interface with poor adhesion due to the asymmetric interface properties. Third, through varying the rubber content in the adhesive, DCB specimens with various fracture toughnesses are prepared and tested. An examination of the failure surfaces reveals that directionally unstable crack propagation is more unlikely to occur as the toughness of the adhesive increases, which is consistent with the analytical predictions that were discussed using an energy balance model.     

Interfacial adhesion measurement of a ceramic coating on metal substrate

The interfacial adhesion measurement of a ceramic coating on a metal substrate is studied by three-point bending (3PB) technique. In the measurement, interfacial cracks are induced during the 3PB test, and the interfacial energy release rate is calculated from the released energy per unit crack surface area during crack extension under the fixed displacement conditions. A finite element analysis (FEA) model encompassing the plastic behavior of the metal substrate is developed to simulate the 3PB test and extract the energy data. The inputs to the FEA model include the crack length, the maximum and critical loads corresponding to crack initiation, and the mechanical properties of the coating and substrate. A MoB/CoCr ceramic coating/stainless steel substrate system is investigated by the technique for demonstrating the utility of the technique.     

Crack Path Selection in Adhesively-Bonded Joints: The Role of Material Properties

This paper investigates the role of material properties on crack path selection in adhesively bonded joints. First, a parametric study of directionally unstable crack propagation in adhesively-bonded double cantilever beam specimens (DCB) is presented. The results indicate that the characteristic length of directionally unstable cracks varies with the Dundurs' parameters characterizing the material mismatch. Second, the effect of interface properties on crack path selection is investigated. DCB specimens with substrates treated using various surface preparation methods are tested under mixed mode fracture loading to determine the effect of interface properties on the locus of failure. As indicated by the post-failure analyses, debonding tends to be more interfacial as the mode II fracture component in the loading increases. On the other hand, failures in specimens prepared with more advanced surface preparation techniques appear more cohesive for given loading conditions. Using a high-speed camera to monitor the fracture sequence, DCB specimens are tested quasi-statically and the XPS analyses conducted on the failure surfaces indicate that the effect of crack propagation rate on the locus of failure is less significant when more advanced surface preparation techniques are used. The effect of asymmetric interface property on the behavior of directionally unstable crack propagation in adhesive bonds is also investigated. Geometrically-symmetric DCB specimens with asymmetric surface pretreatments are prepared and tested under low-speed impact. As indicated by Auger depth profile results, the centerline of the crack trajectory shifts slightly toward the interface with poor adhesion due to the asymmetric interface properties. Third, through varying the rubber content in the adhesive, DCB specimens with various fracture toughnesses are prepared and tested. An examination of the failure surfaces reveals that directionally unstable crack propagation is more unlikely to occur as the toughness of the adhesive increases, which is consistent with the analytical predictions that were discussed using an energy balance model.     

Relationship between Viscoelastic and Peeling Properties of Model Adhesives. Part 1. Cohesive Fracture

The viscoelastic and peeling properties of polybutadiene/tackifying resin compatible blends have been studied in detail. Viscoelastic properties have been described through the variations of the complex shear modulus, G*(ω), as a function of frequency, ω and peeling properties through the variations of peeling force (F) as a function of peeling rate (V).

After showing the objective character of the peeling curves obtained, the variations of the peeling force and peeling geometry have been studied as a function of volume fraction of the tackifying resin.

In this first paper, the analysis is focused on the first domain of the peeling curves, i.e. the cohesive fracture region. In this region, the peeling properties have been related to the viscoelastic properties in the terminal region of relaxation. It is shown that the longest relaxation time, τ_o, is a reducing parameter of the peeling curves, so a peeling master curve-which is independent of temperature, resin volume fraction and polymer molecular weight-may be defined. Furthermore, the variations of the test geometry as a function of peeling rate have been investigated: the variations of the radius of curvature of the aluminium foil have been analyzed with respect to the viscoelastic behavior of the adhesive, which in fact governs the test geometry.

A detailed analysis of all these features leads to a model which allows one to calculate the peeling curves in the cohesive domain from the adhesive formulation.     

Study of a 'T'-type peel test on a metal/polymer/metal sheet sandwich

This paper describes the specific 'T'-type peel mode in the case of a metal/polymer/metal sheet sandwich and gives experimental results on the influence of plastic deformation in the metallic substrates on the peel energy. We propose an experimental method of carefully determining the peel energy of a metal-polymer interface in a sandwich structure. Based on the mechanical properties of the stainless steel substrates and the maximum curvature of the metallic sheet measured experimentally during the peel test, several expressions for the clastoplastic deformation energy of the metal substrates are given. It is noteworthy that the curvature of the metal substrate layers depends not only on the mechanical properties of the material, but also on the work necessary to overcome the interfacial or cohesive forces. It is shown that even for thin metallic substrates (0.1 mm thick stainless steel), the work absorbed by the deformation represents roughly 50% of the total measured energy. During peeling the same specimen at different rates, the propagation peel force is higher or lower than the initiation force depending on the previous curvature of the metal sheets.