Process-induced morphologies and fatigue properties of PEEK: 3D characterization and correlation

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Polyetheretherketone (PEEK) is a semi-crystalline thermoplastic whose importance as a structural and functional material in biomedical and lightweight applications is growing. The morphology in semi-crystalline thermoplastics has a significant influence on the fatigue behaviour. In the injection moulding process, the morphology can be influenced by the process parameters pressure and temperature. Shear-induced and thermally induced crystallisation take place competitively and form geometrically different structures. In PEEK, shear-induced uniaxial nematic structures (liquid crystals) are already formed in the melt. In contrast to thermally induced crystals, shear-induced crystals have strongly anisotropic properties. Due to the stiff behaviour in the chain direction in PEEK, this shear-induced anisotropy is probably pronounced. The current state of research indicates that under fatigue loading, the damage mechanism in other semi-crystalline polymers is based in the morphology. In addition, the ratio and the change in the ratio of elastic to dissipated energy determines the fatigue life. Therefore, the project aims to correlate the injection moulding-induced morphology with the fatigue behaviour of PEEK. The knowledge gained in this way contributes to the understanding of the morphology influence on the damage mechanisms and to the targeted influencing of the fatigue properties. Methods with destructive sample preparation are currently used to characterize crystal structures inside the sample. Due to the destruction of the sample, the damage development can only be examined to a limited extent. In contrast, high-frequency ultrasonic testing offers considerable added value and, based on promising preliminary investigations, is to be enabled for the three-dimensional characterisation of polymer morphology (flow lines and property gradients). During fatigue, the morphology is investigated by combining high-frequency ultrasonic testing with in situ lock-in thermography. The focus is on the correlation of the measurement signals with the polymer properties on the one hand and the correlation of the injection moulding-induced morphology with the fatigue behaviour on the other hand. The fusion of the expertise of the research partners enables the development of methods (linking measurement signals with process understanding) and the correlation of the process influence with the fatigue behaviour, through supporting non-destructive testing. Comprehensive fatigue tests are carried out in which the influence of the elastic and dissipative properties and the shear- or thermally induced morphology on the fatigue behaviour is investigated. These investigations enable the measurement methodology to characterise the local morphology and, at the same time, the non-destructive in-situ method adds value to fatigue testing. Finally, it will be investigated, if and how far the results are transferrable to Polypropylene.