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Stress and deformation build-up in bonded composite patch repair Curiel, Tomer Maurice

Abstract

Bonded composite patch repairs have many advantages over traditional riveted doubler repairs and have been used successfully in a number of repair programs. A mismatch in the coefficients of thermal expansions of the patch and substrate causes thermally induced residual stresses, which are detrimental to the long-term service life of the repair. A dynamic mechanical thermal analyzer bimaterial beam technique is developed that can be used in a variety of different configurations and loading conditions for innovative and versatile characterization of time, temperature, and cure dependant material properties. The technique is first used to determine the stress relaxation modulus of a viscoelastic material bonded to an elastic substrate. The relaxation modulus of Lexan specimens are characterized, first as monolithic beams and then bonded to an elastic substrate. Results show that the relaxation modulus of Lexan can be determined from bimaterial beam relaxation tests. The temperature and cure dependant modulus of FM300 adhesive is then characterized by subjecting a bimaterial beam to a dynamic displacement while curing isothermally at a variety of temperatures. The results are fit to a model that defines the instantaneous modulus as a function of two variables - the instantaneous temperature and the instantaneous glass transition temperature. The technique is then extended to quantify the development of process induced residual stresses in beam specimens designed to simulate a bonded composite patch repair. In these beam specimens, residual stresses correspond to an out of plane deflection that can be monitored in-situ throughout a complete cure cycle. Specimens, consisting of a steel shim, an FM300 adhesive layer, and an AS4/3501-6 [0°]₂ composite patch, are subjected to a variety of cure cycles to determine the effects of cure time and temperature on the out of plane deflection in single- and multi-hold cycles. The experimental results are then compared to those obtained from a cure hardening instantaneously linear elastic (CHILE) model modified to include thermal softening. Results show that a reduction in thermally induced residual stresses is possible by modifying the cure cycle. Model sensitivities, cycle times, real versus idealized cycles, and the effects of thermal softening are also investigated. The DMA beam technique is shown to be an effective means of material characterization, as well as monitoring the out of plane deflection of bonded composite patch repair specimens throughout a cure cycle. Insight gained from these measurements can be used to optimize cure cycles so as to reduce the thermally induced residual stresses in real applications of bonded composite patch repairs.

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