The effect of thermal contact resistance (TCR) correlated to the degree of intimate contact (DIC) between the incoming tape and the substrate on the temperature history during laser-assisted fiber placement (LAFP) was investigated. A novel experimental methodology was designed to understand the effect with a non-contact method which did not influence the local consolidation quality. To assess the influence of TCR numerically, a three-dimensional optical-thermal model was developed. Experimental results indicated that, for the same tape temperature near the nip point, an increase in the compaction force resulted in a decrease in the temperature at the roller exit and the following cooling phase, in correlation with an increase in the final DIC. Also, the effect of the laser power on the final DIC was less pronounced than the compaction force. In the thermal model, when TCR at the tape-substrate interface was not considered, the temperature predictions underestimated the experimental measurements.

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The temperature history during the laser assisted fiber placement (LAFP) process is very important since it significantly influences the final quality of the structure. Air pockets between subsequent plies, caused by the lack of intimate contact, act as insulators and reduce the through-thickness heat transfer. This phenomenon is commonly referred to as the inter-ply thermal contact resistance (TCR). So far, the link between the degree of intimate contact, the corresponding TCR and its influence on the thermal history has not been clearly demonstrated specifically for LAFP. The results indicate that TCR influences the cooling period of the process.@en

It is a common practice in fusion bonding of thermoplastic composites to add a matrix layer between the two substrates to be joined. The aim is to ensure proper wetting of the two parts. The effect of this additional matrix layer on the mechanical performance was studied by mode I fracture toughness measurements. The additional matrix was inserted at the interface in the form of films of various thicknesses. Three different manufacturing techniques, namely autoclave consolidation, press consolidation and stamp forming, were used to prepare different sets of specimens with varying resin-rich bond line thickness. The occurrence of fibre migration towards the matrix rich interface was induced by the manufacturing techniques used due to their different processing times. The interlaminar fracture toughness was observed to increase with increasing amount of extra-matrix at the interface, while no effects of the fibre migration on the fracture toughness were observed.

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The present work focuses on the applicability of the mandrel peel test to quantify the interlaminar fracture toughness of 5 harness satin woven fabric Carbon/PEEK composites. For this purpose, the Mandrel Peel (MP) test was compared to the Double Cantilever Beam (DCB) and End-Loaded Split (ELS) test in terms of experimental procedure and results obtained. The interlaminar toughness of the 5 harness Carbon/PEEK was measured both parallel and perpendicular to the predominant fibre direction at the interface. While stable crack propagation was observed in the ELS test, unstable crack propagation (stick-slip) was observed during both the DCB and the mandrel peel tests. In the case of the mandrel peel test, however, the unstable propagation was immediately arrested by the mandrel, limiting the instability and providing numerous crack re-initiation values per unit of crack length. This effect is expected to increase the statistical relevance of a single test and thereby to increase the reliability of the measured values as compared to DCB tests. A fractographic analysis was performed to study the nature of the crack propagation for the different testing techniques. The mandrel peel test was found to be a potentially plausible alternative to the DCB test for woven fabric reinforced composites.

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The effect of cooling rate on the interlaminar fracture toughness of Carbon reinforced PPS laminates was investigated experimentally. A typical stamp forming process was utilised in a novel manner to achieve high average cooling rates, of up to 3500 °C/min, while ensuring a good consolidation quality. Differential scanning calorimetry measurements were used to characterise the degree of crystallinity of the samples, while the interlaminar fracture toughness of the laminates was characterised under mode I using the Double Cantilever Beam test. Finally, micrographic analysis of the fracture surfaces was carried out to correlate the degree of crystallinity to the failure modes. A strong correlation between fracture toughness and degree of crystallinity was found. The samples with a low degree of crystallinity showed a high interlaminar fracture toughness and large plastic deformation of the matrix during fracture.

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Carbon fibre reinforced thermoplastic composite laminates have been press-consolidated using various release media. The potential contamination of the laminate's surface by the release media and its effect on the performance of the joint after fusion bonding was studied. Before bonding, the physical and chemical state of the bonding surfaces were analysed by measurement the surface energy and roughness. The laminate surfaces chemical composition was investigated by X-ray photoelectron spectroscopy (XPS). Subsequently, the laminates were fusion bonded through an autoclave cycle. The mechanical performance of the joints was characterized by mode-I fracture toughness and short beam strength tests. The surface of some of the composite laminates were found to contain high concentration of the release medium residues after consolidation. This contamination showed a significant effect on the mechanical performance of fusion bonded joints. It was demonstrated that the performance of the joint can be regained by employing a simple cleaning method.@en