چكيده به لاتين
Due to mismatches of thermal and mechanical properties, polymeric composites are notoriously susceptible to initiation of the matrix cracking. Matrix cracking can even appear during the manufacturing process and before starting the service life. For this reason, polymeric composites with the capability of mitigating damages before propagation of them have gained attention in the recent decade. In this thesis, the effect of the cross-links conversion degree on the healing efficiency of a thermally remendable polymer based on the Diels-Alder (DA) reaction consists of reversible and irreversible bonds was studied. Using the differential scanning calorimetry (DSC) results along with an autocatalytic model for the reaction, the degree of conversion was predicted as a function of time and temperature. To enhance the mechanical properties of the polymer, monomers with the capability of reversible bond forming were blended with a hot-cured epoxy resin with a specific molar ratio. To investigate the healing efficiency at different conversion degrees, three-point bending specimens were fabricated under certain curing condition, which guarantees the formation of both reversible and irreversible bonds. The specimens failed and healed up to the certain conversion degrees many times considering the conversion degree, but it would not have a considerable effect on the flexural modulus. Moreover, it was shown that the synthesized polymer has the capability to be healed for multiple times.
After characterizing the synthesized polymer, fiber-reinforced composites was fabricated using this self-healing polymer. To find the healing efficiency of composites, three-point bending specimens with the stacking sequence of [903/0/903] were prepared. The specimens were initially subjected to tensile loading until reaching the characteristic damage state. Then the flexural modulus and strength after the healing up to different conversion degrees were obtained and compared with the virgin specimens. The results revealed over 95% recovery of the flexural strength and modulus after the complete healing. The effect of multiple healing on healing efficiency was also investigated. It was observed that in the absence of the fiber breakage, composites can be completely healed. To obtain the residual flexural modulus of the damaged specimens as a function of the crack density, a model was developed utilizing the shear-lag analysis accompanied by the classical lamination theory. Moreover, the proposed model was extended to predict the recovered flexural stiffness after thermal treatment for a given time at the desired temperature. By conducting a series of tests, it was proved that the present model is able to predict the residual and recovered flexural moduli with an error of less than 10%.
