Failure of Composite Materials by Kink Band Formation


Simon Peter Hald Skovsgård


Kink band formation, Localisation, Plasticity, Propagating instability, Instabilitiy, Fibre composites


Fibre-reinforced composites and layered materials have a heterogeneous composition of two or more constituents and have anisotropic material behaviour. This heterogeneous structure enables numerous failure mechanisms. One of the dominant compressive failure mechanisms of the latter composites is composed of so-called kink bands. These are bands of material where the fibres inside the band have rotated relative to the fibres outside.

This dissertation mainly concerns the examination of kink band formation in fibre composites and layered materials using several different methodologies. Five publications on the latter topic have arisen based on the work done in this project. The four introductory chapters in the dissertation are devoted to explanations and elaborations of the methods used and developed during the project. Several finite element models are developed specifically to examine either the peak strength of the composite or the post-buckling kink band behaviour. Two novel constitutive formulations are developed that can be used in a general framework to investigate the elastic-plastic behaviour of composites, including the study of kink bands.

The following results and conclusions are drawn from the project results. Compressive kinking strength is affected by:
• Fibre-misalignments
• Insufficient fibre-to-matrix bonding
• Matrix yield strength
• Multi-axial loading
The stress at steady-state kink band broadening can be seen as the lower bound of the compressive strength and is lowered by:
• Increasing band inclinations to a certain limit
• Decreasing matrix Poisson’s ratio
• Decreasing maximum tangent modulus of the matrix
As part of my visit to the Department of Engineering at the University of Cambridge, contributions are made to the field of joint mechanics, particular regarding composites made from ultra-high molecular weight polyethylene. The latter study is a secondary focus of this dissertation. The research questions, problems, methodologies, results and conclusions appear in an attached article.


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