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Experiments have been recently completed which characterize the thick adherend bonded joint behavior of two structural adhesives, FM73M and FM300M, subjected to temperature and moisture environments. All the testing was done from room temperature up to the glass transition temperature, and after exposure to three different moisture levels. It was found that the stiffness behavior followed the time-temperature-moisture relationship typical of glassy polymers. The strength of the joint was modeled using fracture mechanics even though there was no initial flaw in the bondline. It was assumed that the crack initiated at one end of the joint and it propagated along the midplane of the bondline. The joint was modeled with finite elements and the critical strain-energy-release rate was calculated using the crack closure technique. For each adhesive, the critical strain-energy-release rate was essentially constant up to the glass transition temperature, and became heavily dependent on the drop in stiffness and reduced failure loads at temperatures beyond the glass transition.
adhesive joints, environment effects, strain-energy-release rate, crack closure technique
Member of Technical Staff, Alcoa Defense Systems, Inc., San Diego, CA