Adhesive Fracture Energy Research Articles

The Fatigue and Durability Behaviour of Automotive Adhesives. Part I: Fracture Mechanics Tests

A fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively-bonded joints, which consisted of aluminium-alloy or electro-galvanised (EG) steel substrates bonded using toughened-epoxy structural paste-adhesives. The adhesive systems are typical of those being considered for use, or in use, for bonding load-bearing components in the automobile industry. The results were plotted in the form of the rate of crack growth per cycle, da/dN, versus the maximum strain-energy release rate, Gmax , applied in the fatigue cycle, using logarithmic axes. Of particular interest was the presence of a threshold value of the strain-energy release rate, Gth , applied in the fatigue cycle, below which fatigue crack growth was not observed to occur. The cyclic fatigue tests conducted in a relatively dry environment of 23°C, and 55%; RH were shown to cause crack propagation at far lower values of Gmax compared with the value of the adhesive fracture energies, Gc , which were determined from monotonically-loaded fracture tests. Cyclic fatigue tests were also conducted in a “wet” environment, namely immersion in distilled water at 28 C. The “wet” fatigue tests clearly revealed the further significant effect an aggressive, hostile environment may have upon the mechanical performance of adhesive joints, and highlighted the important influence that the surface pretreatment, used for the substrates prior to bonding, has upon joint durability. The development and standardisation of “wet” fatigue tests may provide the basis for a very effective accelerated-ageing test.

The effect of bond formation on the tack of polymers

The tack of polymers to be used as adhesives is measured by a two-stage process of bond formation and bond separation. Bond formation is governed by the contact time, the contact force, the roughness of the surfaces, surface and interfacial tensions, and the mechanical or viscoelastic properties of the adhesive and substrate. This paper presents experimental studies of the contact formation of various model polymers on steel surfaces with well-defined and different degrees of roughness. The tack was measured with an instrument of the probe tack type, which determines the adhesive (interfacial) fracture energy per unit of interface as a measure of the tack and by means of which the most important parameters during bond formation and separation, such as the contact time, contact force, rate of separation, and temperature, can be adjusted and measured over sufficiently wide ranges. In the typical time interval for the contact time, the polymers are found in the plateau range of their viscoelastic spectrum. This means that entanglements strongly affect their bonding behaviour. Good agreement was found between the experimental results presented in this study and a model of contact formation on rough surfaces, published recently by Creton and Leibler [1], especially concerning the dependence of the adhesive fracture energy on the contact force and the contact time for smooth and rough substrate surfaces. The influence of the surface roughness becomes significant at low contact forces, where full contact is not yet developed on a rough substrate surface, and for polymers with comparatively high moduli. The fracture energy increases with the contact time and shows the same time dependence as the reciprocal modulus.

Fracture mechanics of single-fibre pull-out test

Pull-out of an elastic fibre from an elastic matrix was investigated. A simple pull-out mechanics has been developed, based on the fracture energy criterion, to describe the debonding process, including friction. Experiments were carried out using polytetrafluoroethylene (PTFE) fibres embedded in a polypropylene (PP) matrix. It was found that growth of an interfacial crack was stable after the initiation of a debond at the loaded fibre end. At first, the debonding force increased linearly with the crack length due to friction in the debonded region. However, the crack accelerated after reaching a critical length, dependent on the embedded fibre length. At this point, the force required to propagate the debond levelled off. Thus, further increase in the debonding force was not necessary to further complete the debonding process. The debonding force was found to be in good agreement with that predicted by the present theory. Techniques for determining the interfacial properties, including adhesive fracture energy, compressive residual stress and coefficient of friction, were considered. In addition, a simple criterion has been derived to predict which fibre end, either embedded end or loaded end, will debond first when the specimen is subjected to an axial load.

The effect of the substrate material on the value of the adhesive fracture energy, Gc

The effect of the substrate material on the value of the adhesive fracture energy, Gc

Modelling the Fracture Behaviour of Adhesive Joints

Abstract The present work has defined an adhesive fracture energy, G a, for the peel testing of flexible laminates. The values of G a characterises the fracture of the laminate and is considered to be a “geometry-independent′ parameter which reflects (i) the energy to break the interfacial bonding forces and (ii) the energy dissipated locally ahead of the peel front in the plastic or viscoelastic zone. We have shown that in order to determine this true adhesive fracture energy, G a, that the following energy terms must be considered: (i) the stored strain-energy in the peeling arm, (ii) the energy dissipated during tensile deformation of the peeling arm, and (iii) the energy dissipated due to bending of the peeling arm. The analysis proposed yields quantitative expressions for these various energy dissipation terms and, in particular, considers the energy dissipated due to bending of the peeling arm. Another important feature of the analysis is the modelling of the region below the peel front as an elasti...

Crack growth and fracture behavior of fabric reinforced polymer composites

AbstractThe objective of this study is to understand and characterize crack deflection and sub‐crack growth in fabric laminate composites. The theory of crack digression based on the Cook‐Gordon mechanism of crack blunting and the criterion developed by Kendall were used to study the crack propagation phenomenon. A simple approach has been developed to evaluate the cohesive and adhesive fracture energies, which play a vital role in the study of strength and toughness of the fabric laminate composites. The effects of strain rate and quasi‐static crack velocity on these energy values were identified. This study explored the possibility of selftoughening in an otherwise brittle composite system. Two competing mechanisms have been identified that control crack propagation in fabric laminate composites.

Transcrystallization of PTFE fiber/PP composites. II. Effect of transcrystallinity on the interfacial strength

It has been found that transcrystallinity of polypropylene (PP) develops easily on the polytetrafluoroethylene (PTFE) fiber surface in spite of the low surface energy of the fiber. Effect of the transcrystallinity on the interfacial strength has been extensively investigated using a single-fiber pull-out test. By controlling the crystallization temperature, range 25-130°C, the thickness of the transcrystalline layer varied from 0 to 175 μm for thick specimens, ca. 1 mm thick. Measurements of the adhesive fracture energy, the interfacial shear strength and the frictional stress were carried out for specimens with different embedded fiber lengths. Results show that interfacial strength and fracture energy are independent of the transcrystalline thickness. The calculated value of interfacial shear strength is 3.6 MPa, and the fracture energy for debonding is 2.1 J/m 2 . The presence of transcrystallinity does not promote the level of adhesion in PTFE/PP composites. However, the frictional stresses at the debonded fiber/matrix interface increase with transcrystalline thickness. It is attributed to the residual stresses which arise from shrinkage when specimens are cooled from crystallization temperature to room temperature.

A fracture mechanics study of the influence of moisture on the fatigue behaviour of adhesively bonded aluminium-alloy joints

A fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively bonded joints, which consisted of aluminium-alloy substrates bonded using a toughened-epoxy structural adhesive. The results were plotted in the form of the rate of crack growth per cycle, da dN , versus the maximum strain-energy release rate, G max, applied in the fatigue cycle, using logarithmic axes. The cyclic fatigue tests conducted in a relatively dry environment of 23°C and 55% r.h. were shown to cause crack propagation at far lower rates of G max compared to the value of the adhesive fracture energy, G c, which was determined from monotonically loaded fracture tests. Cyclic fatigue tests were also conducted in a ‘wet’ environment, namely immersion in distilled water at 26°C. The ‘wet’ fatigue tests clearly revealed the further dramatic effect an aggressive, hostile environment may have upon the mechanical performance of an adhesive joint, and highlighted the important influence that the surface pretreatment, used for the substrates prior to bonding, has upon joint durability. The development and standardization of ‘wet’ fatigue tests may provide the basis for a very effective accelerated-ageing test.

Nonlinear Elastic Analysis of the Adhesive Strength between Rubber-Like Materials and Rigid Surfaces

Abstract A nonlinear elastic finite element analysis of the adhesive strength between rubber-like materials and rigid surfaces has been conducted based on fracture mechanics principles. The finite element model is an elastomer half sphere adhering to a rigid flat surface. An analytical solution for the system was first developed by Johnson, Kendall and Roberts (JKR) in 1971, which agrees well with experimental data for small contact area. The objective of this paper is to elucidate the relationship between the adhesive fracture energy, tensile loads and debonding process of the system when the contact area is large. A general analysis procedure for nonconforming adhesion-contact problem has been established in this study, which allows us to determine the adhesive fracture energy from measurements of the final pull-off force Pc or the zero-load contact radius α0. Our results agree well with JKR's prediction when the zero-load contact radius is less than 25% of the radius of the sphere. However, as the contact radius increases, the debonding process deviates from JKR's prediction.

A new test method for determining the adhesive fracture energy when bonding thin or coated substrates

A new test method for determining the adhesive fracture energy when bonding thin or coated substrates

The plasma treatment of thermoplastic fibre composites for adhesive bonding

The adhesive bonding of three unidirectional carbon fibre composites — with thermoplastic polyetheretherketone, thermoplastic polyphenylene sulfide and, for comparative purposes, thermosetting epoxy matrices — has been examined. The adhesives were a room-temperature-curing epoxy paste and a hot-curing epoxy film; composite substrates were pretreated by light abrasion and solvent cleaning, followed by either an oxygen-plasma or corona treatment. It is shown that good adhesive bonding of the thermoplastic composites can be achieved if a plasma treatment is used prior to bonding. For the oxygen-plasma and corona treatments employed, the adhesive fracture energy, G c , for the thermoplastic composite joints increases steadily with the intensity of the treatment until a plateau value of G c is reached. On the other hand, the thermosetting-based composite needs only light abrasion and solvent cleaning to give high values of G c . Without the oxygen-plasma or corona treatment, the thermoplastic composite joints fail at the adhesive/composite interface at a very low applied load. The oxygen-plasma and corona treatments lead to an increase in the surface concentration of polar, oxygen-containing groups, resulting in both increased wetting by the epoxy adhesive on the substrate and greater intrinsic adhesion across the adhesive/composite interface. The enhancement in intrinsic adhesion is reflected in the locus of failure of the joints moving away from the adhesive/composite interface, and in far higher values of G c .

Adhesion of electrolessly deposited Ni(P) layers on alumina ceramic. I. Mechanical properties

The adhesion mechanism of electrolessly deposited Ni(P) on alumina ceramic substrates has been investigated. The adhesion was measured by direct pull-off tests and by 90° peel tests, which provided information on adhesion strength and fracture energy, respectively. An assessment is made of quantitative aspects of both adhesion measurement techniques. The observed mechanical behavior is rationalized using the Griffith–Irwin theory. Two types of alumina substrates with different roughnesses were used. Ni(P) was deposited from two types of electroless Ni(P) solutions, one with glycine and one with acetate as the complexing agent. The fracture surfaces were analysed with scanning electron microscopy, combined with energy dispersive analysis of x rays. The adhesion strength of the glycine-type Ni(P) was much higher and the fracture energy was lower than that of the acetate-type Ni(P), for both substrate types. This implies that the difference in adhesion strength is not caused by differences in interfacial chemical bonding, but rather by differences in flaw sizes. Since high adhesion strength was measured on smooth substrates, along with low peel strength, it is concluded that strong adhesion can be obtained without making use of mechanical interlocking. Further research should be aimed at controlling the interfacial flaw sizes.

The peeling of flexible laminates

The present work has defined an adhesive fracture energy Ga for the peel testing of flexible laminates. The value of Ga characterises the fracture of the laminate and is considered to be a ‘geometry-independent’ parameter which reflects (i) the energy to break the interfacial bonding forces and (ii) the energy dissipated locally ahead of the peel front in the plastic or viscoelastic zone. We have shown that in order to determine this true adhesive fracture energy Ga that the following energy terms must be considered: (i) the stored strain-energy in the peeling arm, (ii) the energy dissipated during tensile deformation of the peeling arm, and (iii) the energy dissipated due to bending of the peeling arm. The analysis proposed yields quantitative expressions for these various energy dissipation terms and, in particular, considers the energy dissipated due to bending of the peeling arm. Another important feature of the analysis is the modelling of the region below the peel front as an elastic beam on an elastic foundation; such that the peeling arm does not act as a truly built-in beam and root rotation at the peel front is allowed. The analysis described in the present paper has been employed for four different laminates. The values of the local angle θ0 at the peel front from the theoretical calculations have been shown to be in excellent agreement with the experimentally measured values; a small-scale peel test rig having been built so that the peel test, as a function of applied peel angle θ, thickness h of peeling arm and rate of test, could be observed and photographed using a stereo-optical microscope. The value of the adhesive fracture energy Ga (i.e the ‘fully corrected’ value) for each laminate is indeed shown to be a ‘material parameter’.

Adhesion (fracture energy) of electropolymerized poly(n-octyl maleimide-co-styrene) coatings on copper substrates using a constrained blister test

The fracture energy required to separate electropolymerized n-octyl maleimide/styrene polymer film from various copper substrates was measured using a constrained blister test. A G value (strain energy release rate) of 75 J/m2 at 0.5 mm/min debonding rate was found for separation of the n-octyl maleimide/styrene polymer film from a smooth copper surface. This value is slightly higher than the G value (60 J/m2) of pressure-sensitive adhesive tape on copper, and much higher than the estimated 4.4 J/m2 of Kapton® polyimide on a similar copper surface. It was found that rough surfaces having a regular pattern substantially increased the adhesion strength of the polymer coatings. The polymer films adhered well to the rough surfaces; attempts to separate the polymer film from the copper surface resulted in cohesive failure.

A Technique for the Measurement of Adhesive Fracture Energy by the Blister Method

Abstract The adhesive fracture energy, G1c, of a model adhesive/adherend system, consisting of poly(methyl-methacrylate) plates bonded with a cyanoacrylate adhesive, has been evaluated using the Tapered Double Cantilever Beam and Blister test geometries. A refined Blister testing technique is described which, using relatively large diameter test plates [200mm], is capable of arresting the initial propagation of the-invariably less than naturally sharp-starter crack. This allows us to three subsequent Gc determinations for the same specimen from starter cracks of natural sharpness. Adhesive fracture energy values determined for the model system using TDCB test pieces, 0.110±0.017 kJm−2, were in good agreement with those obtained for Blister specimens in which arrested cracks had been repropagated, 0.119±0.013 kJm−2. As is generally observed, values calculated from the initial propagation of starter cracks were somewhat higher for the TDCB specimens, 0.140±0.045 kJm−2. Corresponding values for the Blister t...

Structural material requirements and related R and D for ITER plasma-facing components: Boutard, J.L. J. Nucl. Mater. Mar.–Apr. 1991 179–181, 1179–1184

This paper reports results from the mode II testing of adhesively-bonded carbon-fibre-reinforced composite substrates using the end-loaded split (ELS) method. Two toughened, structural epoxy adhesives were employed (a general purpose grade epoxy-paste adhesive, and an aerospace grade epoxy-film adhesive). Linear Elastic Fracture Mechanics was employed to determine values of the mode II adhesive fracture energy, GIIC for the joints via various forms of corrected beam theory. The concept of an effective crack length is invoked and this is then used to calculate values of GIIC. The corrected beam theory analyses worked consistently for the joints bonded with the epoxy-paste adhesive, but discrepancies were encountered when analysing the results of joints bonded with the epoxy-film adhesive. During these experiments, a microcracked region ahead of the main crack was observed, which led to difficulties in defining the true crack length. The effective crack length approach provides an insight into the likely errors encountered when attempting to measure mode II crack growth experimentally.

The adhesion of thermoplastic fibre composites

Fibre composites based upon thermoplastic polymeric matrices containing continuous fibres of carbon or ‘ Kevlar' are being increasingly used in engineering structures. Engineering applications frequently require that they be joined to components fabricated from similar fibre composites, or to other types of materials, and the use of structural adhesives, typically based upon epoxy resins, offers many advantages compared with other methods of joining. The present paper describes in detail the mechanics and mechanisms of the adhesion of thermoplastic fibre composites. The surface topography and chemistry of the composites have been characterized using contact angle measurements and X-ray photoelectron spectroscopy, both before and after using various surface treatments. Joints have then been prepared using epoxy adhesives and the adhesive fracture energies, Gc, of the joints have been measured. Two major aspects of the observed results, with wide applicability to many adhesion problems, have been analysed in detail. First, the need to apply a critical intensity of surface treatment to the thermoplastic fibre composite to prevent interfacial failure, and hence give relatively high values of G has been interpreted by relating the chemical composition of the surface of the composite to the corresponding value of the polar force component of the surface free energy. It is thereby shown that the fundamental requirement is that a critical value of the surface polarity has to be attained. Secondly, after this critical value is reached, it is shown that the locus of joint failure may then be accurately predicted from a knowledge of the stress field in the joint and the experimentally measured interlaminar fracture stress of the fibre composite substrates.

Self-assembling monolayer silane films as adhesion promoters

The structure of adsorbed silane films formed by the adsorption of trichlorosilane molecules containing long alkyl chains from solution onto aluminium substrates has been investigated. The structure of the absorbed films is dependent on the alkyl chain length. When the chain length is equal to or greater than 18, the molecules form a film in which the hydrocarbon chains are densely packed together and orientated away from the substrate. As the chain length is reduced the films become progressively more disorientated. When a terminal vinylic group is present on the molecules, the films can be activated after adsorption to yield a hydroxyl group that is available for further reaction with a polyurethane resin. The ability of 10-undecenyltrichlorosilane and 18-nonadecenyltrichlorosilane to act as adhesion promoters has been investigated using the blister test. Both these silanes yielded adhesive fracture energies of 18 J m −2 under dry conditions. When water was present, only the joints pretreated with 18-nonadecenyltrichlorosilane exhibited a high resistance to moisture attack. In these joints the dense packing of the hydrocarbon chains impeded the diffusion of water to the interface. In the case of the adsorbed 10-undecenyltrichlorosilane films, the disorientated nature of these films allowed the water to reach the interface more easily, resulting in rapid joint failure.

Nonlinear elastic analysis of adhesive blister test

A nonlinear elastic analysis of a blister test is conducted for rubber-like material described by the Ogden-Tschoegl strain energy function. A finite element program is applied to the analysis which is formulated based on the total Lagrangian procedure and capable of treating deformation-dependent pressure loading problems. The dependence of specific adhesive fracture energy γa on critical pressure pcris analyzed for a blister test specimen with different geometry and different material constants. It is suggested that the unbonded radius a in the dimensionless representation p cr 2 a/Eγa should be replaced by an average value ā in the numerical analysis. The numerical analysis of the blister test is also discussed in general.

Effect of Cross-Linking on Tack and Peel Strength of Polymers

Abstract In this work the influence of cross-linking on the adhesive fracture energy and the peel strength is studied choosing polydimethylsiloxane (PDMS) as a model polymer. A series of samples was prepared by electron-beam irradiation which covers the whole range from a viscoelastic liquid to a cross-linked rubber. The mechanical behaviour of these PDMS samples was characterized by mechanical spectroscopy. Tack measurements with an instrument described elsewhere5 and peel measurements show that the adhesive fracture energy after short contact times as a measure of tack and the peel strength have a pronounced maximum in the range above the gel point, where the PDMS consists of a very loose and imperfect network and a high fraction of soluble polymer. In this range debonding is connected with the formation of fibrillar structures within the polymer.