T. J. Lardner

Use of the microindentation technique for determining interfacial fracture energy

The microindentation technique was used to determine the interfacial fracture energy of epoxy coatings on soda‐lime glass substrates. An analytical model was developed for calculating fracture energy based on indenter load versus debond crack size measurements. Finite‐element analysis was used to determine the relative amounts of opening and shear loadings at the debond crack tip. The calculated fracture energies are compared to values determined by the double‐cantilever‐beam technique and the four‐point flexure‐beam technique.

Measurement of adhesion of thin polymer coatings by indentation

A microindentation technique was developed to measure the adhesive shear strength of thin polymer coatings on glass substrates. Indentation‐induced debonding of the coating was observed to occur under three different conditions: Type I was with the deformations underneath the indenter being essentially elastic; Type II was with the deformations underneath the indenter being plastic; and Type III was after the indenter penetrated the substrate. Stress analyses to calculate the interfacial shear stress for the indentation‐induced debonding of thin coatings are presented for the three types of debonding. All three stress analyses are based upon the linear, elastic analysis of the contact stresses arising from indentation of a soft coating on a rigid substrate. These analyses provide a basis for using controlled indentation debonding as a quantitative measure of the interfacial shear strength of thin coatings to rigid substrates.

Moisture-assisted crack growth at epoxy–glass interfaces

The double cleavage drilled compression (DCDC) test was used to measure the critical energy release rate, moisture-assisted crack growth, and fatigue threshold for epoxy–glass interfaces bonded with and without a silane coupling agent. The DCDC specimen consists of two glass beams (either soda-lime or fused silica) bonded together with an epoxy adhesive. A through-the-thickness hole is drilled in the centre of the specimen. In the DCDC test compressive loading causes tensile stresses to develop at the poles of the drilled hole. Cracks then nucleate in the epoxy–glass interface, extend from the poles, and propagate axially along the interface in primarily mode I loading. The resistance to moisture-assisted crack growth at untreated epoxy–glass interfaces is significantly less than that in monolithic glass specimens. However, the resistance to moisture-assisted crack growth at silane bonded epoxy–glass interfaces can be comparable with or greater than that in monolithic glass. Silane bonding of epoxy to glass is more effective with fused silica than soda-lime glass, with the fatigue limit of silane bonded epoxy–fused silica interfaces being about 2.5 times greater than that for silane bonded epoxy–soda-lime glass. These results are discussed in terms of possible interfacial crack growth mechanisms.

Fatigue Crack Propagation at Polymer Adhesive Interfaces

Abstract Fatigue (slow) crack growth in epoxy/glass, epoxy acrylate/glass and epoxy/PMMA interfaces was studied under constant and cyclic loading at both high and low humidities using the interfacial, four-point flexure test. Finite element analysis was used to determine the energy release rate and phase angle appropriate for the different crack geometries observed. The experimental results show that for the polymer/glass interfaces, the primary driving force for fatigue crack growth is the applied energy release rate at the crack tip and that increasing test humidity enhances crack growth under constant loading but has an insignificant effect under cyclic loading. At low humidity the crack growth rates under cyclic loading are significantly greater than under constant loading. For epoxy/PMMA interfaces the crack growth results were independent of the applied energy release rate, relative humidity, and cyclic vs. constant loading, within experimental scatter. In addition, for polymer/glass interfaces the ...

Subcritical crack growth along epoxy/glass interfaces

Subcritical crack growth behavior along polymer/glass interfaces was measured for various epoxy adhesives at different relative humidities. A four-point flexure apparatus coupled with an inverted microscope allowed for observation in situ of the subcritical crack growth at the polymer/glass interface. The specimens consisted of soda-lime glass plates bonded together with epoxy acrylate, epoxy (Devcon), or epoxy (Shell) adhesives. Above a threshold strain energy release rate, the subcritical crack velocity was dependent on the strain energy release rate via a power law relationship where the exponent was independent of the adhesive tested and the test humidity ( n = 3). However, the multiplicative constant A in the power law relation varied by over three orders of magnitude between the various adhesives with epoxy (Shell) having the smallest value and the epoxy (Devcon) the greatest value; in addition, A was very sensitive to humidity, decreasing by over two orders of magnitude from 80% to 15% relative humidity. At high strain energy release rates, the subcritical crack velocity reached a plateau at approximately 10 −6 m/s. The use of this subcritical crack velocity data in predicting thin film delamination is discussed.

Crack propagation in polymer adhesive/glass sandwich specimens

Abstract Moisture-assisted crack growth in polymer adhesive/glass interfaces was measured as a function of the applied energy release rate, G, using a four-point flexure test coupled with an inverted microscope. The specimens consisted of two glass plates bonded together with an epoxy or an epoxy-acrylate adhesive. It was found that cracks formed and grew on both interfaces if the glass surfaces were both smooth; however, roughening the surface of one of the glass plates increased the fracture resistance of the interface sufficiently so that crack growth occurred only on the remaining “smooth” interface (top or bottom). Finite element analysis was used to determine the G and ψ (phase angle) appropriate for the different crack geometries. It was found experimentally that crack growth rates for all crack geometries depended on the applied G via a power law relationship and that for a given applied G, crack growth rates were sensitive to the crack geometry. The results indicate that the primary driving force...

STRENGTH OF POLY(METHYL METHACRYLATE) WITH INDENTATION FLAWS

Controlled flaws were introduced into poly(methyl methacrylate) samples in the presence of liquid acetone using a Vickers indenter over a range of indentation loads from 100 to 1400 N. Due to the large plastic zone underneath the indenter, the radial crack formed by indentation consisted of two halves, known as Palmqvist cracks, instead of a single semicircular crack. The strengths of the samples were measured in air either immediately following indentation or after a stress-relief anneal. The strength of the as-indented samples was about 6% less than that of the annealed samples; however, the dependence of strength on indentation load was similar for both sets of samples. These results were interpreted in terms of an indentation fracture mechanics model. The analysis is consistent with poly(methyl methacrylate) having a rising fracture toughness with increasing crack size.

OBSERVATIONS ON FINGER-LIKE CRACK-GROWTH AT A URETHANE ACRYLATE GLASS INTERFACE

Abstract Interfacial crack growth behavior along a urethane acrylate/glass interface is characterized by the development of finger-like perturbations along the advancing crack front. The finger-like perturbations grow from a slightly irregular crack front until they reach a steady-state where the velocity of the finger tips equals the velocity of the finger valleys. Once the fingers reached steady-state, the crack velocity was dependent on the applied strain energy release rate via a power law relationship where the exponent was independent of test humidity; however, the multiplicative constant A decreased by an order of magnitude from 80 to 15% RH. The spacing of the fingers was found to be independent of the crack′s velocity and the relative humidity of the environment.

Fatigue resistance of silane-bonded epoxy/glass interfaces using neat and rubber-toughened epoxies

The influence that the energy absorbing ability of an epoxy has on the cyclic fatigue resistance of a silane-bonded epoxy/glass interface in moist air was studied using the double cleavage drilled compression (DCDC) test. The material properties of two epoxies with similar chemical structures were controlled through the manipulation of the molecular weight between cross-links, Mc, of the epoxy network. Two rubber-toughened epoxies with different nominal particle sizes (10–40 μm and 1–2 μm) were fabricated by adding a liquid butadiene/acrylonitrile copolymer to the epoxy resins. Mixtures of the silane-coupling agents 3-aminopropyltriethoxysilane (3-APES) and propyltriethoxysilane (PES) were used to treat the glass substrate in order to control the number of covalent bonds between epoxy and the glass. 3-APES has the ability to form primary bonds with both the epoxy and the glass, while PES forms primary bonds with only the glass. Experimental results showed that the manipulation of Mc had little effect on the cyclic fatigue resistance of the epoxy/glass interface. However, incorporation of rubber particles gave a significant improvement in the fatigue resistance. The rubber particles allowed the microscopic, non-linear deformation mechanisms of cavitation and shear yielding to dissipate energy during fatigue crack growth. Smaller particles gave the greatest improvement to fatigue resistance; about a 75% improvement compared to the neat (non-modified) epoxy. Adjustment of the number of silane bonds between the neat epoxies and the glass had little effect within experimental scatter on the fatigue resistance of the interfaces, suggesting that the energy dissipated through the breaking of bonds at the interface was insignificant compared to the energy dissipated through plastic and other inelastic deformation mechanisms in the epoxy.

Fatigue and Durability of Silane-Bonded Epoxy/Glass Interfaces

Abstract Fatigue (slow) crack growth in epoxy/glass interfaces bonded with the silane coupling agent 3-aminopropyltriethoxysilane was studied under static and cyclic loading at 23°C, 95% RH using the double cleavage drilled compression test. Crack growth rates under cyclic loading were significantly greater than under static loading, in contrast to crack growth rate results in monolithic glass. After aging up to 34 h at 94°C in distilled water, the silane-bonded epoxy/glass specimens exhibited somewhat greater resistance to fatigue crack growth than the unaged samples; however, after aging at 98°C in distilled water and at 70°C in an aqueous KOH solution at pH 10, crack growth became cohesive and exhibited fractal behavior. Mechanisms for fatigue crack growth at silane-bonded epoxy/glass interfaces are proposed.