Thermoplastic composites (TPCs) evolved as an alternative to structural thermoset polymer composites. These materials offer their inherent toughness and do not have the drawbacks of chemistry during part production and poor recyclability because they use a thermoplastic matrix, and thus can be processed by repetitive heating, forming and cooling. Therefore, TPCs use in aerospace industry has grown credit to their excellent specific stiffness and strength, as well as their potential for cost-effective and automated manufacturing. However, there are still some challenges in TPCs manufacturing including the occurrence of defects such as fiber waviness during part production. Ideally, fiber waviness in thermoplastic composite parts can originate from the manufacturing stage, even when the part has been made following the design and manufacturing specifications. Components with such nonconformities need expensive additional non-destructive inspection. These parts are generally scrapped, as the influence of fiber waviness on the mechanical performance is poorly understood. It is highly desirable that the scrap rate be reduced by minimizing or even preventing fiber waviness; a process which demands thorough understanding of the underlying governing mechanisms.
Previous studies have already shown that flat laminate consolidated using a heated press or an autoclave may already suffer from fiber waviness, even in the absence of compressive fiber loads due to forming of the laminate. Generally, such waviness is considered unacceptable due to the large negative influence on part performance and uncertainties in accurately determining the knockdown. Unfortunately, the amount of literature on the formation of waviness is less abundant; particularly one focusing on TPCs. Nonetheless, full control of the process is needed to reduce the occurrence of waviness in TPC parts. On this account, Dutch researchers from the University of Twente: Dr. E. Krämer, Dr. B. Grouve, Dr. L. Warnet and led by Professor Remko Akkerman, in collaboration with Dr. S. Koussios at the ThermoPlastic composites Research Center looked carefully the formation of in-plane waviness during consolidation of a flat laminate. Their focus was describe accurately the governing mechanisms. Their work is currently published in the research journal, Composites Part A.
In previous studies, the proposed waviness formation mechanisms mentioned can only be deducted posteriori since observation of an actual part during processing cannot been performed. Bearing this in mind, the research team designed an experimental setup to visually observe the formation of waviness during consolidation of flat TPC laminates. The authors hoped that the physical mechanisms governing the formation of waviness could possibly be deduced from the visual observation.
For C/PEEK unidirectional composites, the authors observed that waviness formed during cooling between the maximum process temperature and the crystallization temperature. Further, the researcher noted that a higher processing temperature lead to waviness with a larger misalignment angle. Nonetheless, no such trends were observed for the wavelength. In fact, they noted that waviness did not form or increase in severity after crystallization.
In summary, the study presented the detailed experimental setup for real-time observation of waviness formation during the production of thermoplastic composite laminates. The presented approach was successfully used to produce laminates with patches of wavy fibers, to observe the formation of waviness and to determine the temperature interval in which waviness formation took place in C/PEEK prepregs. In a statement to Advances in Engineering, Professor Remko Akkerman, the corresponding author said: with this experiment, we were for the first time able to observe how these defects are actually formed and to discover the driving mechanism: something that puzzled us for years. This knowledge enables us (and our industrial partners) to take the necessary countermeasures preventing these defects arising during manufacturing.
E.T.M. Krämer, W.J.B. Grouve, S. Koussios, L.L. Warnet, R. Akkerman. Real-time observation of waviness formation during C/PEEK consolidation. Composites Part A, volume 133 (2020) 105872.