In situ observation of defect formation during composite manufacturing


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.

About the author

Erik Krämer is a PhD candidate at the ThermoPlastic composites Research Center (TPRC) and in the Production Technology (PT) group at the University of Twente, The Netherlands. Between 2009 and 2016 he studied at the University of Twente where he obtained his BSc and cum laude MSc degree in mechanical engineering. Three awards were issued for ‘best master thesis’ by SAMPE Benelux, SAMPE Europe, and KIVI Engineering Society, for his work on shape distortions in the thermoplastic composite stamp forming processing. His research interests now focus on material characterization and process modeling of thermoplastic composite part manufacturing in general. His current research efforts focus on the formation of defects during the production of thermoplastic composite laminates, with a particular focus on fiber buckling. The goal is to understand the underlying mechanisms of fiber buckling, to mathematically describe these mechanisms, and to establish processing guidelines for the production of waviness free laminates.

About the author

Dr W.J.B Grouve

Wouter Grouve received his PhD degree in 2012 on the topic of advanced fiber placement of thermoplastic composites. He was then employed by the ThermoPlastic composites Research center, before returning to the University of Twente in 2018 to take up a role as an assistant professor in the field of thermoplastic composites. His research efforts revolve mostly around the processing of thermoplastic composites, with the aim to develop predictive simulation tools that allow first-time-right manufacturing.

Wouter has received the ESAFORM PhD Prize for Industrial Research and the IOM Composite award in 2014, and is a member of the program committee of the ITHEC conference on thermoplastic composites.

About the author

Dr S. Koussios

With a BSc. In Mechanical Engineering, a BSc., MSc. and PhD. in Aerospace Engineering, Sotiris Koussios’ research and teaching is focused on the structural analysis of composite materials, fracture of composites, filament winding and robotic applications in composites manufacturing. After 20 years of teaching, researching and project leading at the Faculty of Aerospace Engineering, Delft University of Technology, he joined the research group of TPAC (Thermoplastic Composites Application Centre) at the Faculty of Mechanical Engineering, Saxion University of Applies Sciences. He is the (co-)author of over 150 publications. His current research focusses on 3D printing and winding with continuous fiber-reinforced thermoplastics, repair technologies and progressive failure of composites.

About the author

Dr L.L. Warnet

Laurent Warnet received his MSc degree in Mechanical Engineering in 1992 at the UTC in France and his PhD in Mechanical Engineering in 2000 from the University of Twente in the Netherlands. He is since affiliated to the University of Twente as an Assistant Professor in the chair of Production Technology led by Prof. Remko Akkerman.

His main research activities are in the field of fibre reinforced polymeric materials, focusing on the relation between processing and performance. He is secretary of the ESIS Technical Committee 4 on fracture mechanics of polymer, adhesive and composites.

About the author

Dr R. Akkerman

Remko Akkerman is full professor in Production Technology at the University of Twente, Faculty of Engineering Technology, in the Netherlands. He has spent his entire academic career at this university, where he obtained his MSc degree in Mechanical Engineering in 1988, as well as his PhD on viscoelastic flow simulations in 1993. Afterwards he started working as an assistant professor in the chair of Engineering Design in Plastics and in 2003 he was appointed as full professor in Production Technology.

His research focuses on lightweight materials for structural applications, with a clear emphasis on processing and performance of composite materials. Since June 2009, Akkerman also serves as the scientific director of the ThermoPlastic composite Research Center (TPRC), founded by Ten Cate, Fokker, Boeing and the University of Twente. In 10 years’ time, the center has grown to more than 20 members, covering the whole thermoplastic composites value chain, and to a globally respected institute on TPCs.

He is engaged in various international scientific committees and boards, such as Esaform, FPCM and Texcomp. He is a member of the KMHW and of the management team of 4tu.htm, the research centre for high-tech materials of the Dutch 4tu Federation. He further participates in the international advisory boards of large research programmes in the UK (Cimcomp) and Japan (SM4I).


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.

Go To Composites Part A

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