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

聚醚醚酮(PEEK)是一种半结晶热塑性材料，在生物医学和轻量化应用中作为一种结构和功能材料的重要性越来越大。半结晶热塑性塑料中的形态对疲劳行为有重要影响。在注塑成型过程中，工艺参数压力和温度都会对制品的形态产生影响。剪切诱导和热诱导结晶相互竞争地进行，形成不同的几何结构。在PEEK中，剪切诱导的单轴向列相结构(液晶)已经在熔体中形成。与热诱导晶体相比，剪切诱导晶体具有强烈的各向异性。由于PEEK中链方向的刚性行为，这种剪切诱导的各向异性可能是显著的。目前的研究状况表明，在疲劳载荷作用下，其他半结晶聚合物的损伤机制是基于形态的。此外，弹性能与耗散能之比及其变化也决定了疲劳寿命。因此，该项目旨在将注射成型诱导的形态与PEEK的疲劳行为相关联。通过这种方法获得的知识有助于理解形貌对损伤机制的影响以及对疲劳性能的有针对性的影响。破坏性样品制备方法目前用于表征样品内部的晶体结构。由于样品的破坏，只能在有限的程度上检查损伤的发展。相比之下，高频超声波检测提供了相当大的附加值，并且基于有希望的初步研究，将能够对聚合物形态(流动线和性能梯度)进行三维表征。在疲劳过程中，结合高频超声检测和原位锁定热像仪对其形貌进行了研究。重点是测量信号与聚合物性能的相关性，以及注射成型诱导的形态与疲劳行为的相关性。通过融合研究伙伴的专业知识，可以开发方法(将测量信号与过程理解联系起来)，并通过支持无损检测，将过程影响与疲劳行为相关联。进行了综合疲劳试验，研究了弹性和耗散性能以及剪切或热诱发形貌对疲劳行为的影响。这些研究使测量方法能够表征局部形貌，同时，非破坏性原位方法增加了疲劳试验的价值。最后，将调查结果是否以及在多大程度上可以转移到聚丙烯。

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.