University of Twente Student Theses


Enhancing the Ti-PEKK hybrid joint by changing the PEKK crystal morphology

Visser, F.F. (2022) Enhancing the Ti-PEKK hybrid joint by changing the PEKK crystal morphology.

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Abstract:The weight of an aircraft determines its fuel consumption and emissions. Therefore, the aviation industry is constantly looking for lightweight and durable material alternatives. Composite structures in general and thermoplastic composites, in particular, are finding increased use in the next generation of aircraft. However, the successful adaptation of thermoplastic composites into the superstructure depends on the development of reliable joining methods. Co-consolidation is developed as an attractive method where a metal and composite part are joined during the consolidation step. In the current hybrid thermoplastic composite joint between CF/PEKK and Titanium, a thin interlayer of PEKK material is used to allow full wetting of the interfaces. This interlayer was shown to be a limiting factor in increasing the overall joint strength. To produce a tougher joint it is necessary to increase the toughness of the PEKK interlayer. Research has shown that changing PEKK crystal structure by decreasing the spherulite size can be beneficial for the toughness. Herein, GNP is used as a nucleation agent for the PEKK polymer, decreasing the spherulite size by creating a dense nucleation field. Through the use of only 0.01 wt% GNP in the polymer the typical diameter for nucleation is shrunk from 46 µm to 9 µm. However, the nucleation density of the material was not homogeneous. To accurately characterise the inhomogeneities in the nucleation field a wider view of crystal distributions is combined with image recognition to create a numerical map of the nucleation field. Each map contains more than one million nuclei for the nucleated samples. Areas of low and high nucleation density were present, creating an inhomogenity in the material. The cause was determined as the polymer particle size which is a limiting factor in the mixing procedure of PEKK and GNP. To decrease the inhomogeneities of the the as received polymer was seived to a small aggregate size, below < 140 µm. Using this sieved grade of material created an on average denser nucleation field due to the disappearance of the low-density areas. These results were confirmed by the thermal characterisation of the nucleation process in DSC testing. An alternative mixing method was also utilised, powders were pressed out to a film, such that the polymer and GNP can mix under melt conditions. This method is found to work and created even better results than through the use of the sieved polymer and is recommended for further research. Second, the polymer materials with the altered crystalline structure are mechanically tested to evaluate the mechanical performance of the PEKK material with a small crystal size. In tensile testing, the performance of the PEKK/GNP polymers with the inhomogeneous crystal structure showed very poor mechanical performance. Most of these samples showed brittle failures. The samples produced from the sieved polymer with GNP showed much better performance but did not outperform the pure PEKK samples.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:52 mechanical engineering
Programme:Mechanical Engineering MSc (60439)
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