Moisture Content Characterization in Composite Materials based on Transmission Measurements

Moisture absorption is a water diffusion pro- cess known to alter many of the mechanical properties of polymer-based composites. This study presents ultrasonic characterization of moisture content based on the trans- mission of acoustic waves through composite plates. The elastic constants of fourteen plates with different controlled water contents are obtained. First the redundancy of the results allows to validate the characterization process and the assumption made on the samples' symmetry. The sensitivity of the process to moisture content is then discussed. I. INTRODUCTION In industrial applications, materials can be subjected to various environmental conditions such as X or γ rays, temperature variations and light or mechanical loadings. Among those, moisture has a great influence on polymer-based composites. One of the major con- sequences of water absorption is a shift of the glass transition temperature (1). In studied composite samples, the moisture content can reach 11% of the dry mass inducing swelling and density variations (2), (3). Nuclear Magnetic Resonance spectroscopy (4) or weight mea- surements are the most commonly used characterization methods, but they are generally destructive. Because of the change of the material mechanical properties, moisture content can be estimated using acoustic waves. The method is non destructive and allows in situ mea- surements. In the present work, we report measurements based on the transmission of acoustic waves through plates. They allowed to identify the elastic constants of fourteen composite samples with controlled water contents, varying between 0% and 11% of the dry mass. The redundancy of the results is then used to validate the characterization process and the assumption made on the samples' symmetry, and the sensitivity of the char- acterization method to moisture content is discussed.