Research On Heat Curing In Molds And Demoulding Deformation Of Fiber/Resin Composites

With the development of nearly one century, fiber reinforced resin matrix composites have become one of the most important aviation structural materials owing to their excellent fatigue resistance, high specific strength and modulus, outstanding versatility and shock resistance, unique designability and so on.Due to the thermal effect of composites, chemical shrinkage of matrix and the interaction between tools and parts, structural parts may generate lots of complicated internal stress during cure, which will cause structural parts deformed when demoulding. The deformation has serious impact on the performance of parts, and then reduces the qualification rate of products. Therefore, it is of great scientific significance and project application value to establish the regulatory mechanism of the deformation after parts demoulding based on the analysis of the thermo-curing process in molds and the evolution of internal stress of fiber composite structural parts.In this study, the research object is a kind of epoxy/carbon fiber composites. The relation of structural parameters, performance parameters and curing process parameters during cure of the material system is established. And then, the evolution of process-induced stress in molds and the final deformation of composites are researched by the method of numerical simulation combined with experiments. Based on the above studies, the developments of internal stress and the deformations of materials in different restricted conditions are investigated.Based on the establishment of the relation of curing process, structural parameters and performance parameters during cure, the changes of these parameters during curing process including the effect of the curing temperature on them are investigated for pure resin in this project. In a word, the integrated research of curing process-cross-linked structure-mechanical properties has been achieved.By means of experimental data fitting and a secondary development of the finite element analysis software, a new viscoelastic constitutive model of the composite materials is founded. And then, a series of comparisons among the results calculated by the elastic models and viscoelastic models are made. It shows that the deformations calculated by viscoelastic models are always less than that calculated by the elastic models due to the viscoelasticity of the resin matrix, which will lead to stress relaxation of the composite during cure. The higher the volume fraction of the resin is, the more obvious the viscoelasticity of the composite will be.The mechanical properties of the resin and its composites always change much during cure. In this study, the evolutions of these mechanical properties and the stress and strain in different directions of the composite are analyzed. The deformations with different lay-up types and thickness of the laminates are discussed, which are calculated by the previously established viscoelastic model. The shapes of the deformed laminates are consistent with the experimental results. What’s more, the temperature and strain are monitored dynamically by the fiber Bragg grating sensor during cure.The different constraints, which are equivalent to different structures of molds, are applied to laminates. The result shows that the internal stress at the end of cure in molds is greater than that in the free condition. The laminates have significant deformations after demoulding and recover to their mechanical equilibrium. At this time, the residual stress in the laminates is smaller than that in the laminates in the free condition.