Characterisation and Modeling of the Moisture Dependent Thermomechanical Behaviour of Poylamide

• Applicants: Professor Dr.-Ing. Stefan Diebels; Professor Dr.-Ing. Hans-Georg Herrmann; Professor Dr.-Ing. Markus Stommel
• Subject Area: Plastics Engineering
• Term: from 2016 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 321154911

The aim of this project is to provide a broad basic knowledge on the coupled thermo-mechanical behaviour of polyamide-66 (PA). The influence of moisture and temperature on the mechanical behaviour of PA has been reported by many workgroups so far. Focus of more recently published papers lies on processes of moisture transport, which are influenced by the free volume inside the bulk material. Besides, there is a strong interrelation of mechanical behaviour and free volume. Therefore, the interrelation of moisture transport mechanisms and applied loadings is to be elaborated by appropriate test methodologies with the application of non-destructive testing and included in a subsequent material modelling.The LKT will provide the specimens and develop test methodologies and procedures to determine the influence of moisture content on the mechanical behaviour and vice versa. In a first step, experiments on conditioned specimens will be conducted to gain general information on the moisture dependent macroscopic material behaviour, followed by the adaption of in-situ moisture measurement methods on the test device for considering microscopic effects such as moisture transport. Furthermore, the influence of dimensions as well as superposed loading states on this behaviour will be determined by developing appropriate specimens and test methods.The LLB will develop a measurement and reconstruction method based on terahertz waves in order to retrieve the parameters of the distribution and dynamics of the moisture in PA (local and global moisture content, depth gradient). The THz development is comprised of several work packages, starting with integral moisture determination, then its lateral distribution and finally determination of the depth gradient. A sub-contractor will perform Positron Annihilation Lifetime Spectroscopy (PALS) measurements in order to obtain the individual amount of free volume as a reference. The LLB results will complement the stress-strain-data from LKT and will serve as input data for the continuum mechanical material model from LTM, thus helping to improve and verify it.Aim of work of the LTM is the continuum mechanical modelling of moisture transport and the moisture dependent thermo-mechanical properties of PA. Therefore, a diffusion model will be fully coupled to a viscoelastic material model for the PA. On the one hand, the diffusion model is coupled to the free volume, one the other hand, the stiffness and relaxation times of the viscoelastic model will depend on the moisture concentration. In this context strong coupling between moisture transport and mechanical properties as well as dependency on the load has to be considered. Data obtained from a multitude of experimental tests, made by LLB and LKT, enables the identification of material parameters by using inverse methods by means of a robust numerical implementation of the multifield problem.

• DFG Programme: Research Grants