Mahoor Mehdikhani (KU Leuven)

Latest publications by this author

Longitudinal debonding in unidirectional fibre-reinforced composites: Numerical analysis of the effect of interfacial properties

Sina AhmadvashAghbash, Christian Breite, Mahoor Mehdikhani, and Yentl Swolfs

Longitudinal fibre-matrix debonding is governed by interfacial strength, fracture toughness, thermal residual stresses, friction, and matrix plasticity. The proposed finite element model for fibre-matrix longitudinal debonding associated with fibre breakage accounts for these features, retrieving more realistic results for the stress redistribution around a fibre break. In contrast with the majority of the available finite element models, the current model does not impose the debond length and enables debond propagation based on the assigned interfacial properties. Several parametric studies have been performed to assess the effect of input parameters in two configurations: single- and multi-fibre packings. Higher values for interfacial friction coefficient, thermal residual stress and interfacial fracture toughness restrain the debond propagation and consequently accelerate the stress recovery. Conversely, including matrix plasticity facilitates the debond propagation. A prescribed matrix crack, concentric with the broken fibre and as large as thrice the fibre radius, has no significant effect on the extent of the debond but increases the stress concentration on the nearest intact fibres in the multi-fibre model. The results of the proposed finite element model match the reported laser Raman spectroscopy literature data. The current study improves the prediction capability of models for the longitudinal tensile failure of unidirectional composites.

Influence of Test Specimen Geometry on Probability of Failure of Composites Based on Weibull Weakest Link Theory

Rajnish Kumar, Bo Madsen, Hans Lilholt and Lars P Mikkelsen

This paper presents an analytical model that quantifies the stress ratio between two test specimens for the same probability of failure based on the Weibull weakest link theory. The model takes into account the test specimen geometry, i.e., its shape and volume, and the related non-constant stress state along the specimen. The proposed model is a valuable tool for quantifying the effect of a change of specimen geometry on the probability of failure. This is essential to distinguish size scaling from the actual improvement in measured strength when specimen geometry is optimized, aiming for failure in the gauge section. For unidirectional carbon fibre composites with Weibull modulus m in the range 10–40, it can be calculated by the model that strength measured with a straight-sided specimen will be 1–2% lower than the strength measured with a specific waisted butterfly-shaped specimen solely due to the difference in test specimen shape and volume.

Towards separator-free structural composite supercapacitors

Olivier Hubert, Nikola Todorovic, Alexander Bismarck

Structural supercapacitors can both carry load and store electrical energy. An approach to build such devices is to modify carbon fibre surfaces to increase their specific surface area and to embed them into a structural electrolyte. We present a way to coat carbon fibres with graphene nanoplatelets by electrophoretic deposition in water. The effect of time and voltage on the mechanical properties of the carbon fibres, the structure of the coating and the specific surface area of the coated carbon fibres are discussed. A specific capacity of 1.44 F/g was reached, which is 130% higher than state-of-the-art structural electrodes. We demonstrate the scalability of the deposition process to continuous production of coated carbon fibres. These carbon fibre electrodes were used to realise large (21 cm long) structural supercapacitor demonstrators without the need for a separator, having a specific capacity of 623 mF/g.