Hygrothermal Ageing and Damage Characterization in Epoxies and in Epoxy-Glass Interfaces: a Micromechanical Approach Using Embedded Optical Sensors

Nowadays, composite materials are increasingly used in high technology fields such as aerospace and automotive because of the combination of excellent mechanical performance and lightweight. For these characteristics, classical metal alloys are progressively replaced in primary structural components by reinforced polymer composites. The service life of such structural components is more and more increased thus raising questions on the composites' durability since it is well known that they are susceptible to long term aggressive environments. In fact, UV radiations, high temperature, solvents, fuel, humidity and water synergistically act with mechanical loads to reduce the resistance of composite materials. To address this problem two main approaches are used. On one hand, the macromechanical approach is used for the long term reliability of composite materials that experimentally is characterized by means of accelerated ageing tests that reproduce the service environments. On the other hand, the micromechanical approach is aimed at understanding the intimate degradation mechanisms triggered by the environmental conditions. This objective is partially accomplished by reducing the geometrical complexity of the material microstructure subdividing it into elementary unit cells. In this work, the combination of moderate high temperature and moisture is chosen as the ageing environment. The effects of the hygrothermal ageing on the properties of an epoxy resin are studied using a single fiber composite (SFC) unit cell. The reinforcement of the SFC is an optical glass fiber that presents, along a portion of its core, a fiber Bragg grating (FBG) sensor capable of gathering information about the deformation field inside the unit cell. This particular configuration allows to study evolution of the internal stress state during cure, at high temperatures and during the ageing process. The residual deformation field in the SFC is investigated by using a mixed experimental-numerical technique. A series of radial cuts are introduced into the SFC geometry while the induced strain perturbation is recorded by the embedded FBG sensor. This knowledge is used in an identification scheme in order to determine a shrinkage function capable of reproducing the initial deformation state in the axysimmetric configuration. The calculated deformation field is in good agreement with similar studies in litterature. The study of the thermomechanical response of the SFC allows to determine the resin's coefficient of thermal expansion. The procedure is validated in the case of an epoxy resin reinforced with different weight percent of SiO2 nanoparticles and rubber microparticles. It is shown that the method provides more stable results if compared to the classical TMA analysis. The resin response to the wet environment at 50°C is characterized firstly by means of a gravimetric analysis that led to the determination of the water diffusion kinetic parameters. Secondly, the mechanical property evolution as function of humidity is determined by means of tensile, multiple relaxation tests and microindentations. Lastly, the resin coefficient of moisture expansion is determined and displays a non linear trend. A residual-thermo-hygro mechanical model is built on the basis of the determined parameters showing excellent agreement with the recorded experimental data. Moreover, the hygrothermally induced fiber fracture is analyzed by means of the linear elastic fracture mechanics and the shear lag theory. Finally, the fiber-matrix debonding, triggered by the reinforcement failure, is characterized by the use of cohesive elements whose mechanical properties are varied as function of the concentration at the interface. The calculated redistribution of stresses after the fiber failure and the debonding kinetic reproduces accurately the recorded experimental behaviour.

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