Adhesive joint fracture behavior during setting and aging

This study focuses on the changes in the fracture toughness of a room-temperature-set urea-formaldehyde resin adhesive during setting and aging. Toughness was measured in cleavage using a contoured double-cantilever beam specimen formed of a wood laminate and contoured aluminum beams. After initial bonding at room temperature, the fracture toughness of the adhesive was measured periodically during extended setting at room temperature and also during wet and dry accelerated aging. The fracture toughness of the adhesive reached a peak after about 2 weeks. The crack-growth rate reached its most unstable level at the same time. These points were related to the development and relief of internal stresses caused by cross-linking and shrinkage of the resin. In specimens set at room temperature, changes in fracture toughness and crack-growth behavior continued for possibly 1 year. During aging, the rate of fracture toughness loss was almost 20 times more rapid in water than in dry air, despite a lower aging temperature. These patterns of changing fracture toughness and crack-growth behavior during setting and aging provide insight into the chemical processes and physical changes underlying setting and aging.

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