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A study of residual stress formation in three-dimensionally constrained epoxy resins
A study of the evolution of residual stresses in three-dimensionally constrained epoxy resins is presented. Visual observation of crack formation in an epoxy resin cured isothermally in a thick-walled glass tube suggested the possibility of large residual stress formation in resins cured under three-dimensional constraint. A novel strain-gaged tube technique was developed to measure the internal stresses evolved during the cure of three-dimensionally constrained epoxy resins. The stresses evolved during the cure of a typical epoxy/amine resin system developed as a result of restrained polymerization shrinkage and restrained thermal expansion. Hydrostatic tensile stresses as large as 12 MPa developed in the epoxy resin during isothermal cures due to post-gelation chemical shrinkage. Upon cooling from the reaction temperature, the thermal stress developed per degree of temperature change was large and remained nearly constant as the resin was cooled through its glass transition temperature. It was discovered that the residual stresses could be reduced by gelling the resin at a low temperature and then completing the cure by elevating the temperature using a slow temperature ramp. In addition to internal stress measurements, the bulk modulus and Poisson's ratio of the epoxy resin were calculated as a function of cure time. The evolution of residual stress was also measured in rubber-toughened, reactive thermoplastic oligomer-modified, and particulate-filled epoxy resins subjected to three-dimensional constraints. The addition of reactive rubber and thermoplastic oligomers had a negligible effect on the development of stress in the epoxy resin. In the particulate-filled systems, the average stress in the composite was reduced by the addition of fillers, increasing the compressibility of the composite. The principles related to the study of residual stress formation in three-dimensionally constrained epoxy resins were applied to an adhesion problem. The breaking strength of a highly constrained tubular joint bonded with epoxy resin was measured. The bond strength was improved by both the addition of a compressible filler to the epoxy adhesive and the modification of the cure schedule.
Plepys, Anthony Raymond, "A study of residual stress formation in three-dimensionally constrained epoxy resins" (1992). Doctoral Dissertations Available from Proquest. AAI9233133.