Active microwave-excited thermography with coupled finite element calculations to determine the spatially resolved moisture distributions in polyamide 6 and their effects on the moisture-dependent deformation and failure behavior

• Applicants: Professor Dr.-Ing. Hans-Georg Herrmann; Professor Dr.-Ing. Markus Stommel
• Subject Area: Plastics Engineering
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 555260573

The material behaviour of polyamide 6 (PA 6) is strongly dependent on the present moisture content. The hygroscopic PA 6 absorbs the polar water molecules from its environment. The water molecules diffuse along the concentration gradient in the amorphous region of the semi-crystalline thermoplastic in order to bind to the equally polar carbonamide groups of the molecular chains until equilibrium with the environmental humidity is reached through saturation of the amide groups. In real, constantly changing environmental conditions, this results in transient moisture distributions in PA 6 components. Water absorption increases the intermolecular chain spacing and the cooperative chain segment mobility increases. As a result, the glass transition temperature Tg is reduced, so that the thermomechanical properties are also strongly changed. Simultaneously, the deposition of water molecules results in a material swelling and causes residual stresses locally in the transient state due to the space requirement of these molecules. In order to evaluate the influence of water on the deformation or failure behaviour, it is thus not only the percentage of absorbed water that is relevant, but the transient distribution within a component in particular. The project proposal aims to develop a fast, non-destructive characterisation method using active microwave excited thermography (AMET) to detect inhomogeneous water concentration distributions in PA 6. The method is based on the different heating of PA 6 by microwave radiation depending on the local water content. The numerical computation of concentration distributions using the finite element method (FEM) has already been established and experimentally validated for a few constant environmental conditions. FE calculation of the moisture distribution in known, validated conditions provides a possibility to apply the AMET also for quantitative analyses of the locally existing distribution, once the specific correlation between water distribution and temperature increase due to microwave excitation is known. Furthermore, approaches from literature for solving inverse problems are used to generate depth information. With precise knowledge of the concentration distribution from the AMET, a basis is created for validating the FE calculation of the water distribution in the material for different times, even for varying relative humidities. Concentration-dependent characterisation of the thermomechanical material behaviour with homogeneous water distribution is performed to determine parameters that are coupled to the water concentration and, via this, to the moisture-dependent Tg in the FE calculation. In addition, the material behaviour under external mechanical load is computed numerically, considering the existing inhomogeneous water distribution, and simultaneously validated in experiments.

• DFG Programme: Research Grants