Co-authors​ :

 Anirudh KOHLI (IRELAND), Partha P PAUL (FRANCE), Yunhui YUNHUI CHEN , Katherine KATHERINE NELMS (UNITED KINGDOM), Philip WITHERS (UNITED KINGDOM), Jose JOSÉ HUMBERTO S. ALMEIDA JR  

Compression failure is a critical aspect in composite materials, and understanding its importance is crucial for designing and engineering reliable structures. The main reasons why the study of compression failure in composites is relevant include i) material characterization, ii) design considerations, and iii) failure analysis. Concerning the latter, failure of a composite system is highly dependent on several factors, such as the constituents of the composite material, fibre orientation, fibre type, matrix system, and manufacturing process. Understanding their roles on the compressive failure process of a composite is crucial for further design considerations. The main reason for poor compressive strength of composites is due to premature failure by fibre kinking. Kink bands forms suddenly and catastrophically, and it is known that fibre misalignment is vital to its early onset. Catastrophic kink-band initiation is known to form in less than 0.2 ms, requiring extremely fast in situ X-ray imaging, to resolve the event with sufficient high spatial resolution. In this work, we aim to understand the failure process of two different composite materials made by 3D printing. The utilised 3D printed follows the fused filament fabrication (FFF) technique. The composites in study are: 1) glass fibre reinforced nylon composites with unidirectional continuous fibres, and 2) carbon fibre reinforced nylon composites using chopped carbon fibres. The samples made of glass/nylon material is in cylindrical shape and v-notched at the middle of the sample, while the carbon/glass composite is in rectangular shape and double v-notched at the centre of the specimen. The notched are aimed to induce failure to occur within the field of view (FoV) of the synchrotron detector. Both samples have a gauge length of 10 m. They are embedded into steel end caps to provide stability to the sample during the test and to transfer stiffness from the sample ends to the gauge area to avoid edge effects. The in-situ synchrotron X-ray experiments was performed at the European Synchrotron Radiation Facility (ESRF). The samples are placed into a testing rig developed at INSA-Lyon for in-situ tension or compression tests. The samples are compressed at a speed rate of 1 mm/s while radiographs are collected at 20,000 fps using a Photron FASTCAM SAZ. A pink x-ray beam with beam energy of 18 keV has been used. The projections are recorded continuously. The time-resolved images were able to capture the main failure mechanisms for both samples, in which the glass/fibre samples with long fibres have their failure dominated by a large kinking band, whilst the composites with short fibres failed by fibre micro-buckling and micro-cracking, matrix plastic deformation, and fibre/matrix interfacial failure.