Fatigue of Silane Bonded Epoxy/Glass Interfaces
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291
Mat. Res. Soc. Symp. Proc. Vol. 563 © 1999 Materials Research Society
a 0.1 M aqueous solution of the silane coupling agent 3-aminopropyltriethoxysilane (3-APES). The beams were bonded together with an epoxy adhesive (2-Ton, Devcon Corp.) by pressing the bonded beam between stops that controlled the adhesive thickness to be 40 gim (± 10 pim). After curing the epoxy for 24 hours in ambient air, the edges of the specimens were polished to eliminate any excessive adhesive that had squeezed out between the glass beams. A diamond core drill (radius 0.79 mm) was used to drill a hole through the center of each specimen. All specimens were preconditioned in a high humidity environment (> 95% RH) for 1 hour prior to testing. With the DCDC test, compressive loading causes tensile stresses to develop at the poles of the drilled hole. Cracks then nucleate, extend from the poles, and propagate axially along the interface in the sandwich specimens. Finite element analysis [5] shows that the energy release rate G for a monolithic glass specimen is given by -(0.235 G~E
--
R
R
0.259)-
R(1
(-
where cr is the compressive stress, R is the hole radius, E is the elastic modulus, a is the crack length, and 2w is the specimen width. The phase angle y for the monolithic glass specimen is 00 [5], which corresponds to pure normal, i.e. mode I, loading at the crack tip. Since the adhesive layer is thin, G for the epoxy/glass sandwich is approximately equal to that of the monolithic glass specimen [6] and the phase angle for the epoxy/glass interface, given by the asymptotic solution of Suo and Hutchinson [6], is -11'. The compressive load was applied in a servo-hydraulic Instron testing machine (Model 1321) that applied a sinusoidal load with a frequency of 3 Hz. Graphite foil was placed on the ends of the specimens to compensate for any surface roughness. The minimum load for all tests was set at 130 N, corresponding to a G of about 0.008 J/m2. All tests were carried out at high humidity (>95% RH) by enclosing the test fixture with a plastic envelope and then piping in moist air. After establishing a precrack to a length of about 3 mm, the load was cycled and crack growth was measured as a function of time using a Questar telescopic microscope, coupled with a television monitor. An analysis of this data using the finite difference between data points then gave the crack growth rate as a function of the applied G. RESULTS AND DISCUSSION Environmentally assisted crack growth under static loading, known as stress corrosion, is relatively well understood in silicate glasses [7] where crack growth results from a chemical reaction between the environmental species, typically water molecules, and the stressed Si-O-Si bonds near the crack tip. The crack growth rate, Vs, under static loading, is generally characterized by a simple power law function of the applied energy release rate, G: V.s = A G"
(2)
where A and n are constants dependent on the glass and environmental conditions. Because of the lack of plasticity in silicate glasses,
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