Inverse Analysis to Determine Hygrothermal Properties in Fiber Reinforced Composites

Fiber-reinforced composite laminates are often used in severe environments such as high temperature and humidity conditions. Therefore, it is of practical interest to establish a simple but robust method to determine material properties that define the hygrothermal behavior of composites under various conditions. In this article a novel inverse analysis approach is presented to identify the diffusivity, maximum moisture content, and coefficient of thermal expansion (CTE) and coefficient of moisture expansion (CME) of carbon fiber-reinforced epoxy matrix composites subjected to various conditions of environmental exposure. This procedure involves three distinct steps. First, the transient expansions and weight gains are experimentally measured under different temperatures and humidity conditions. Next, reference solutions are established from detailed computational models, which incorporate the heterogeneous nature of composite’s microstructure. Finally, the Kalman filter technique is utilized to extract best estimates of the material parameters of interest. The use of inverse analysis is necessitated by the fact that backward relations from the measured parameters to the unknown properties are not directly apparent. The results show diffusivity to be a strong function of temperature while the maximum moisture content to be a strong function of relative humidity. For the case of material expansion, moisture induced strains as well as thermal strains exhibit slight dependence on temperature. In addition to estimating the material parameters of interest, detailed studies are conducted to investigate the source of scatterings observed in the expansion measurements during heating and moisture absorption, and possible 3D effects in measurements.