Abstract
High performance composite materials are highly orthotropic and can show complex failure behaviours. Matrix dominated failure has been extensively investigated, whereas fibre dominated tensile failure has mainly been investigated under simple stress states. However, composite materials are increasingly loaded under complex stress states, which result in the need to also understand fibre dominated tensile failure under three-dimensional loading.This PhD project investigates the influence and interaction of combinations of through-thickness compression and shear stresses on the in-plane fibre tensile strength of fibre reinforced polymer composites. Such critical stress states can occur in different industrial applications, in particular in component regions where high loads are transferred into a structure via friction or locking, which are the principals of, for example, clamped or dovetail joints, respectively.
A bespoke test method has been developed with the help of a finite element analysis model and applied on a biaxial test machine. High speed camera imaging was used to determine a range of suitable loading conditions which result in fibre-dominated tensile failure. Then, a digital twin was generated within a finite element analysis model for each different loading condition and experiment. For this, an automated programme was developed to translate three-dimensional volumetric and deformation data, which was obtained with the help of Digital Image Correlation, onto the finite element mesh and displacement-based boundary conditions representing the digital twin. By solving these finite element analysis models the failure stress state for each loading condition could be determined, and the general stress interaction at failure derived.
Stress interactions, particularly combinations of compression and shear stresses, can lead to complex non-linear stress strain behaviour. Capturing this complex non-linear constitutive material behaviour is important to predict the correct stress states based on the material deformations. For this, a recently developed non-linear material model has been further improved in collaboration with the University of Oxford [1] and implemented to accurately predict the three-dimensional stress states when fibre tensile dominated failure occurs. The improvement is based on through-thickness compression tests of unidirectional, cross-ply and inclined cross-ply carbon and glass fibre reinforced specimens (IM7/8552 and S2/8552). These tests gave the opportunity to capture the influence and interaction of different compressive and shear stress states, stacking sequences and materials on the stress-strain behaviour.
The results show that through-thickness compression and shear can reduce the strength in longitudinal fibre tensile direction. These novel results will help to understand the stress-strain and failure behaviour of composites under complex three-dimensional loading better, which can improve the design and reliability of composite structures.
| Date of Award | 25 Jan 2022 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Stephen R Hallett (Supervisor) & Michael R Wisnom (Supervisor) |