A numerical model to prevent the thermal degradation of CFRPs at extreme heating rates – The laser processing of CF/PEEK

Data availability: Data will be made available on request.The work was enabled through, and undertaken at, the National Structural Integrity Research Centre (NSIRC), a postgraduate engineering facility for industry-led research into structural integrity established and managed by TWI through a network of both national and international Universities.This study proposes a coupled thermal-chemical numerical model for preventing the thermal degradation of carbon fibre (CF) reinforced polymers at extreme heating rates. Its applicability is demonstrated in a laser-heating case study of CF-reinforced poly-ether-ether-ketone (CF/PEEK). The kinetic parameters of the PEEK matrix, derived from thermogravimetric analysis at conventional heating rates, are introduced in the model and an extrapolation approach is applied to investigate the laser heating of CF/PEEK. The results show that the model captures the heating rate effect on the decomposition of the material, and is used to identify the processing conditions that can reach high temperatures without triggering the thermal degradation mechanisms of the PEEK matrix. Then, a multi-technique experimental investigation takes place to identify the processing conditions that first trigger the thermal degradation mechanisms of CF/PEEK in the examined laser-heating case study. Interestingly, a good agreement is found between the experimental and numerical investigations which validates the model and the applied extrapolation approach.TW

Influence of rapid high-temperature processing on the interface of CF/PEEK, a quick and effective method for enhancing the IFSS

Data availability: Data will be made available on request.The work was enabled through, and undertaken at, the National Structural Integrity Research Centre (NSIRC), a postgraduate engineering facility for industry-led research into structural integrity established and managed by TWI through a network of both national and international Universities.This study explores the effect of rapid high-temperature processing on the interface of carbon fibre (CF) reinforced poly-ether-ether-ketone (PEEK). Specimens that have been thermally treated at slower and faster heating rates and specimens that have not been post-manufacturing treated (virgin) are examined with single fibre pull-out tests. A comparison between their interfacial shear strength (IFSS) and their failure modes takes place. Scanning electron microscopy is used to assess the surface morphology of the thermally treated specimens, and partly cross-polarised microscopy is employed to investigate the development of transcrystallinity. Furthermore, to identify the extent of thermal degradation the specimens are examined with attenuated total reflection Fourier transform infrared spectroscopy. At faster heating rates, an improved interfacial adhesion up to 25% is found at temperatures where a low-level thermal damage is induced. At higher temperatures and despite the increased thermal damage, an IFSS increase of up to 10% is still identified. This is due to the beneficial formation of thermal residual stresses upon high-temperature processing, and overall, especially rapid high-temperature processing could effectively serve for enhancing the interface properties of CF/PEEK.TW

Parametric analysis of cohesive zone model method for CFRP stiffened panel CAI behaviour

This paper examined the effect of the stiffness of the cohesive elements on the accuracy and the computational efficiency of Carbon Fibre Reinforced Polymer (CFRP) stiffened panels under ,Compression After Impact (CAI). Abaqus® software was used and the Cohesive Zone Model (CZM) method was applied to capture the damage initiation and propagation of the panels. Various case studies were examined and the effect of the stiffness parameters of the cohesive elements wascritically assessed. Moreover, the required number of cohesive zones to fully capture the damage mechanisms of the impacted and pristine panels under compressive loading was examined. The results showed that a wrong set of parameters can even lead to neglecting the induced damage and can cause severe convergence problems in the numerical mode

A multi-technique and multi-scale analysis of the thermal degradation of PEEK in laser heating

Data availability: Data will be made available on request.Copyright © 2023 The Author(s). The present work studies the thermal degradation of laser-heated poly-ether-ether-ketone (PEEK) as the heating duration increases. Its damage morphology, chemical composition, crystallinity content, and mechanical properties are examined with optical microscopy, attenuated total reflection-Fourier transform infrared spectroscopy, differential scanning calorimetry, Raman spectroscopy, and continuous stiffness measurement nanoindentation. The applicability of those methods in detecting the thermal degradation of laser-heated PEEK and assessing the induced thermal damage is highlighted. Results show that short-time laser heating acts as an annealing process that improves the crystallinity and hardness on the affected surface of PEEK by up to 5.1% and 10.8% respectively. With a further increase in the heating duration, surface carbonisation occurs and a char layer is formed. Surface carbonisation is associated with the thermal limits of PEEK in laser heating decreasing by up to 50% its hardness and by 45% its indentation modulus. Finally, the char layer is found to act as a shielding mechanism that protects the bulk PEEK from the applied thermal load, resulting in mostly superficial thermally induced damage.This publication was made possible by the sponsorship and support of TWI. The work was enabled through, and undertaken at, the National Structural Integrity Research Centre (NSIRC), a postgraduate engineering facility for industry-led research into structural integrity established and managed by TWI through a network of both national and international Universities. Dimitrios Gaitanelis and Dr Angeliki Chanteli would like to thank Young European Research Universities Network (YERUN) for being awarded the YERUN Research Mobility Award 2021 to proceed to this collaboration. Dr Angeliki Chanteli and Professor Paul M. Weaver would like to thank Science Foundation Ireland (SFI) for funding Spatially and Temporally VARIable COMPosite Structures (VARICOMP) Grant No. (15/RP/2773) under its Research Professor programme

Detecting, characterising and assessing PEEK’s and CF-PEEK’s thermal degradation in rapid high-temperature processing

The work was enabled through, and undertaken at, the National Structural Integrity Research Centre (NSIRC), a postgraduate engineering facility for industry-led research into structural integrity established and managed by TWI through a network of both national and international Universities.Copyright © 2022 The Author(s). In this study, a methodology is proposed that can identify and characterise the extent of thermal degradation that takes place in rapid high-temperature processing of PEEK and CF-PEEK. Initially, their decomposition mechanisms are examined in air with thermogravimetric analysis (TGA). Then, PEEK and CF-PEEK samples are heated in static air conditions up to temperatures that are close to and above the onset of thermal degradation with heating rates up to 100 °C/min. The samples are then examined with attenuated total reflection - Fourier transform infrared (ATR-FTIR) spectroscopy and a new fluorenone peak is detected at 1711 cm-1 that is directly linked with the progress of degradation. A correlation between its intensity and the resulting thermal degradation takes place and it is shown that the 1711 cm-1 peak can be safely used as a tool for characterising the extent of thermal degradation at the examined heating conditions. Finally, an investigation with differential scanning calorimetry (DSC) is conducted to identify the extent of thermal degradation that would not induce a severe thermal damage on the two materials. In both PEEK and CF-PEEK, faster heating rates are found to have a reduced effect on their crystallinity content and a degradation around 1% of their PEEK matrix content is found tolerable.TW