Direct observation of the fracture behavior of the polyether ketone ketone (PEKK) spherulites

This article reports the direct observation of the fracture of individual poly‐ether‐ketone‐ketone (PEKK) spherulites. A single layer of PEKK spherulites was obtained by bonding a PEKK film in‐between two sandblasted Ti alloy plates using an autoclave. The crack of an individual PEKK spherulite was achieved by opening the Ti/PEKK/Ti sandwich using a double cantilever beam test. The fracture morphology of the PEKK spherulite was characterized using scanning electron microscopy and atomic force microscopy. It was found that under tensile stress the crack of the individual spherulite propagates along the middle plane and crosses the nucleation core. This is due to the symmetric radial structure of the spherulite. Moreover, it was found that the fracture surface morphology at the core of the spherulite is strongly influenced by the local crystalline structure, which is anisotropic and determined by the initial nucleation growth direction. As a result, the area fraction experiencing plastic deformation during the fracture of PEKK spherulites at different orientations may vary by an order of 10. Our results show the important role of the initial nucleation growth direction on the mechanical properties of the polymer crystals and may provide a new approach to the design of high‐performance polymer materials with tailored crystalline structures

Effect of resin-rich bond line thickness and fibre migration on the toughness of unidirectional Carbon/PEEK joints

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.</p

Interlaminar fracture toughness of 5HS Carbon/PEEK laminates. A comparison between DCB, ELS and mandrel peel tests

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.Structural Integrity & Composite

The influence of thermal contact resistance on the thermal history in laser-assisted fiber placement

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

The influence of thermal contact resistance on the thermal history in laser-assisted fiber placement

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.Structural Integrity & CompositesAerospace Structures & Computational MechanicsAerospace Manufacturing Technologie