A Novel Procedure for Identification of 3D Moisture Diffusion Parameters on Thick Composites: Theory, Validation and Experimental Results

Abstract

This paper presents a novel method for the identification of the parameters governing Fickian moisture diffusion in composite materials. In the first part of the manuscript, the theoretical background is given and closed-form solutions are produced for 3D Fickian diffusion. Then, the different existing identification alternatives are reviewed and a novel procedure is proposed that takes advantage of all the gravimetric curve data points instead of just initial slope and saturation. The method relies on the solution of an optimization problem. In the second part of the paper, validation of this identification technique is performed from simulated gravimetric curves. The main advantage of the method when compared to previously published materials is that diffusion coefficients and saturation level can be identified from gravimetric curves obtained from unsaturated coupons. This is particularly useful when considering thick coupons that will take a long time to saturate or when non-Fickian diffusion mechanisms occur after some time. Finally, the method is applied to experimental results obtained from glass/epoxy composites. It is shown that although the diffusion of moisture in the composite is basically non-Fickian, it can be considered Fickian up to about 10 days and that these data are sufficient to retrieve the 3D diffusion coefficients as well as the Fickian saturation level.

Characterisation of the hygro-thermo-mechanical behaviour of organic matrix composites instrumented with optical fibres: A study of interfacial bonding

“…This numerical method was intended for identifying both the macroscopic transverse moisture diffusion coefficient (D 2 ) and the maximum moisture absorption capacity (M s ) of the samples. The numerical method was based on the comparison between the 3D Fick's solution and the experimental measurement of the weight gain occurring during the diffusion process [20]. The method consists of finding out the unknown values of the problem by minimizing the standard deviation (Equation 1) using a GaussNewton algorithm:…”

Characterization of principal diffusivities of composites by suspension method

“…An important factor in the theory is that the material diffusivities that dominate the diffusion rate are required in predicting the hygric strain resulting from moisture absorption or desorption. The governing equation of diffusion in an orthotropic plate described by Fick's diffusion law [2,11] is…”

“…A usual approach of evaluating the principal diffusivities is based on fitting the theoretical solution for the moisture concentration of a square laminate immersed in water to the data of its weight-gain ratio of moisture absorption verse immersion duration [2]. The specimen for immersion test is prepared in the way that its width, length, and thickness are in the three material principal directions, fiber's longitudinal direction, in-plane fiber's transverse direction, and out-of-plane fiber's transverse direction (Fig.…”

“…1), respectively. The theoretical analysis based on Fick's diffusion law indicates that the moisture concentration in the immersed specimen due to moisture absorption is a function of its geometry, principal diffusivities, and immersion duration [1][2][3][4][5]. The two principal diffusivities in the two fiber's transverse directions are the same, because unidirectional composite laminate is normally assumed to be transversely isotropic [1].…”

“…When a wet specimen is retrieved from water, surface water must be wiped off, and the specimen must be laid in ambient air and weighted only after the remaining surface moisture has evaporated [2,6]. When the surface of the wet specimen becomes dry, it starts to dewater, and its inner moisture distribution begins to change.…”