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Tensile tests are a basic characterization method for composite materials, but the specimens often do not fail properly. Obtaining reliable results in tensile testing requires avoiding stress concentrations near the grips. The material near the tabbed section is under longitudinal, transverse and shear stress concentrations, which leads to underestimated results and conservative design. This study therefore examines different geometries to find the best testing method that yields the maximum failure strain. The experimental results show that the novel arrow shape end tabs and continuous tab specimens allow reaching the highest failure strain value.

The complexity of the microstructure in composites has led to major generalizations when predicting the strength of composites. In recent years composite strength predictions have progressed by improving the fidelity of randomly packed unidirectional models. Taking it one step, further we created digital twin models of real microstructures for discontinuous aligned composites. In these models we evaluated the impact of the mean fiber direction on the local stress concentration factors. We found that the local stress concentration factors can vary 71% within a 20 ° range of rotation. Nonetheless, this variation is load dependent and will be reduced with load progression.

The present work investigates the tensile properties of a novel type of hybrid composites that contain stiff but brittle webs of discontinuous carbon fibres, and tough but compliant plies of self-reinforced polypropylene (SRPP). The goal is to achieve a better balance between stiffness, strength and toughness. In the scope of this study, the effect of fibre volume fraction on the ductility of the hybrid was evaluated, and failure mechanisms that govern the transition between brittle and ductile behaviour were assessed. Hybrids with volume fraction of less than 7 % exhibited quasi-ductile response, while reaching the modulus of 6.1 GPa and the failure strain of 10 %. Through the modelling work it was shown that the initial elastic behaviour of the hybrid can be estimated using the incremental rule-of-mixtures. It was found that catastrophic interply delamination is not a dominant failure mode, and consequentially, the strength of the hybrid can be predicted from the failure strengths of the web and the SRPP. Overall, this family of hybrids has attractive properties that can be tuned to specific applications by controlling the fibre volume fraction. ispartof: pages:1-7 ispartof: Proceedings ECCM-17, 17th European Conference on Composite Materials pages:1-7 ispartof: European Conference on Composite Materials (ECCM) location:Munich, Germany date:26 Jun - 30 Jun 2016 status: published

Combining a reasonable stiffness and a high ultimate failure strain in a single material is challenging task. Intralayer hybridisation of self-reinforced polypropylene (SRPP) with carbon fibre is proposed as a new strategy to increase stiffness while maintaining a high strain to failure. The bonding between SRPP and the carbon fibre prepregs was found to be a crucial parameter in these hybrids. It can be altered either directly by replacing the homopolymer polypropylene (PP) matrix in the prepregs by a maleic anhydride PP, or indirectly by changing the carbon fibre volume fraction. Weak bonding was key to preserving a high ultimate failure strain and impact resistance. Strong bonding reduced the ultimate failure strain and impact resistance, but improved flexural properties. These results reveal a delicate balance in optimising the bonding in hybrid composites to achieve optimum performance. ispartof: pages:1-10 ispartof: Proceedings of the 20th International Conference on Composite Materials vol:2015-July pages:1-10 ispartof: 20th International Conference on Composite Materials location:Copenhagen, Denmark date:19 Jul - 24 Jul 2015 status: published

A non-deterministic multiscale methodology is proposed to predict variability in longitudinal tensile stiffness and strength of unidirectional composite plies. Local composite strength and stiffness are predicted at the microscale and assigned to finite elements models, which represent the material at the scale of a ply. In doing so, fibre volume fraction variations and fibre misalignment are accounted for. The local strength is computed using a fibre break model able to reproduce the onset and accumulation of the fibre breaks at the microscale. A large number of simulations were performed at both scales to study the convergence of the mean values of strength and the variability. It is shown that the local volume fraction variation has a major role in the material failure prediction, while very small misalignment can be considered negligible. ispartof: Proceedings ECCM 18, 18th European Conference on Composite Materials ispartof: ECCM 18, 18th European Conference on Composite Materials location:Athens, Greece date:25 Jun - 28 Jun 2018 status: published

© 2017 International Committee on Composite Materials. All rights reserved. Both discontinuous fiber-reinforced composites and hybrid composites show the potential to overcome the stiffness-toughness dilemma that conventional fiber reinforced polymer composites suffer from. To fully utilize the advantages of discontinuities and hybridization, careful design of the configuration of discontinuities is required. In this paper, high-precision fiber cuts with a staggered pattern were introduced in the unidirectional carbon fiber layer of hybrid carbon fiber/self-reinforced polypropylene composites. This aimed to control the failure mechanisms, hence targeting a gradual failure in tension. The effect of the geometrical parameters of the patterns, including cut length, cut fraction and step length, were investigated. The tensile behavior of the hybrids was influenced by a combined effect of these three parameters. The hybrids exhibited three types of the failure behavior: 1) single fracture of the carbon fiber layer followed by single delamination growth, 2) carbon fiber layer fragmentation accompanied by multiple delamination growth and 3) multiple delamination growth without carbon fiber layer fracture. In the regime of carbon fiber layer fragmentation accompanied by multiple delamination growth, a gradual failure with relatively high damage resistance was achieved. ispartof: Proceedings ICCM21 vol:2017-August ispartof: International Conference on Composite Materials (ICCM) location:Xi'an, China date:20 Aug - 25 Aug 2017 status: published

Current applications of carbon fibre reinforced plastics (CFRP) can be found mostly in sectors where their use is not principally cost-driven and which have limited production volumes, such as aerospace and sports cars. In order to achieve a step-change in the application of high-performance composites in larger-volume applications, new materials systems are needed that combine very short production cycle times with performance that meets automotive requirements. The EU-FP 7-project HIVOCOMP is developing two material systems that show unique promise for cost effective, higher-volume production of high performance carbon fibre reinforced parts. These materials systems are: Advanced polyurethane (PU) thermoset matrix materials offering a combination of improved mechanical performance and reduced cycle times in comparison with conventional matrix systems, Thermoplastic PP- and PA6-based self-reinforced polymer composites incorporating continuous carbon fibre reinforcements offering increased toughness and reduced cycle times in comparison to current thermoplastic and thermoset solutions. In this introduction, the global concept of the HIVOCOMP-project is presented, and an overview is given of the achievements during the first three project years. Emphasis is then put on the demonstrators (see figures) and how they show the advantages of both innovative material concepts, both in processing (reduced cycle times) and in properties (increased toughness). Finally, the environmental impact of these new material concepts is evaluated by reporting on the results of the LCA (life cycle assessment) and LCC (life cycle costing) studies.