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A method for measuring chemical shrinkage and coefficient of thermal expansion during cure and postgelation is presented based on digital image correlation to record the in situ stress-free strain field in a thermosetting polymer. An independent determination of the resin cure kinetics was required to relate the chemical shrinkage strain and coefficient of thermal expansion to the degree of cure. The changes in the coefficient of thermal expansion were observed from the initial heating to final cooling of the sample. The results obtained are shown to compare favorably with results previously found by other techniques. The proposed method provides a simple and reliable procedure to measure the evolution of thermochemical shrinkage properties of the resin during the cure cycle.

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The compressive behavior of poly(HIPE) foams was studied using the developed micromechanics based computational model. The model allowed identifying the morphological parameters governing the foam compressive behavior. These parameters comprise: (i) foam density, (ii) Sauter mean diameter of voids calculated from the morphological analysis of the polydispersed microstructure of poly(HIPE), and (iii) polymer/strut characteristic size identified as the height of the curvilinear triangular cross-section. The model prediction compared closely with the experiments and considered both the linear and plateau regions of the compressive poly(HIPE) behavior. The computational model allows the prediction of structure-property relationships for poly(HIPE) foams with various relative densities and open cell microstructure using the input parameters obtained from the morphology characterization of the poly(HIPE). The simulations provide a pathway for understanding how tuning the manufacturing process can enable the optimal foam morphology for targeted mechanical properties.

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Abstract:Curing rates of an epoxy amine system were varied via different curing cycles, and glass-fiber epoxy composites were prepared using the same protocol, with the aim of investigating the correlation between microstructure and composite properties. It was found that the fast curing cycle resulted in a non-homogenous network, with a larger percentage of a softer phase. Homogenized composite properties, namely storage modulus and quasi-static intra-laminar shear strength, remained unaffected by the change in resin microstructure. However, fatigue tests revealed a significant reduction in fatigue life for composites cured at fast curing rates, while composites with curing cycles that allowed a pre-cure until the critical gel point, were unaffected by the rate of reaction. This result was explained by the increased role of epoxy microstructure on damage initiation and propagation in the matrix during fatigue life. Therefore, local non-homogeneities in the epoxy matrix, corresponding to regions with variable crosslink density, can play a significant role in limiting the fatigue life of composites and must be considered in the manufacturing of large scale components, where temperature gradients and significant exotherms are expected.