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UBC Theses and Dissertations

Effect of environment on the fracture of brittle solids MacMillan, Robin Frederick George


The effect of specific active environment on the fracture strength of glass and polymethyl methacrylate was investigated using an indirect tensile testing technique. The strength of glass was not affected by exposure to dry gaseous N₂ and CO₂. At low water vapour coverages, (<1/3 monolayer), the tensile strength of glass was reduced by approximately 50%. Further increase in water vapour pressure did not weaken the solid to a much greater extent. The existence of surface microcracks governs the absolute tensile fracture strength, and any process which varies the flaw geometry acts to vary the tensile fracture strength. Soaking in the liquid has the same effect as adsorption from the vapour phase near saturation. All vapour adsorbates caused a weakening, the magnitude of the decrease increasing with increasing ability of the adsorbate to screen the surface Si++++ cores. Moisture was the most active environment encountered. Polymethyl methacrylate did not weaken in the vapour phase despite multilayer adsorption, but stressing in wetting liquids did cause drastic failure, with a 57% decrease in tensile strength. Non-wetting liquids do not affect the strength of the acrylic plastics. Fracture experiments on a quartzitic rock in aqueous solutions of surfactant, (quartenary ammonium salts), show that the weakening due to surfactant adsorption is negligible, since water itself causes the maximum strength reduction. The adsorption of surfactant is only a secondary effect. A mechanism has been proposed for the stress-environmental failure of brittle solids. This mechanism recognizes the existence of micro-cracks, regards the stable crack propagation stage of the fracture process to be environment sensitive, and involves the strain-activated adsorption resulting in a decrease in cohesion at the flaw apex. The magnitude of the weakening is critically dependent on the nature of the bonding in the solid surface. A literature review of stress-sorption cracking, with an emphasis on non-metallic systems, is included.

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