Compressive failure of unidirectional NCF composites
Doctoral thesis, 2019
When longitudinally arranged fibres in a composite are wavy, local misalignments are generated with respect to the load axis. These induce shear stresses, critical to compressive failure. The sensitivity to fibre misalignments is generally well known. Yet, no systematic measurements have previously been conducted of its spatial distribution. Existing models for strength prediction
consider fibre misalignment representations as either, a scalar value, periodic or random. Our approach is instead based on measurements of fibre misalignment with high accuracy and high spatial resolution in a large number of samples. Misalignment data has been used for statistically based
direct assessments of compressive strength. The misalignment data has also been used to calibrate models for strength prediction and for numerical studies to increase understanding. The diversity in studied fibre misalignments are not generated by artificial means, but instead reflect upon the
relevant material architecture and processing principles.
Many studies on compressive failure seek to model or understand details on kink-band formation. We have instead maintained a clear focus on failure initiation, relevant to aero-engine components. We have addressed the extreme sensitivity of kink-band initiation to fibre misalignment angle that subsequently lead to compressive failure within a ply. We conclude that kink-band initiation in practical fibre composites is a coordinated kinematic event. It requires studies of regions with real (measured) spatial distributions of fibre misalignment angles. These studies are preferably conducted on 2D micrographs parallel to the kink-plane.
Chalmers, Industrial and Materials Science, Material and Computational Mechanics
A high resolution method for characterisation of fibre misalignment angles in composites
Composites Science and Technology,; Vol. 165(2018)p. 214-221
An experimental study of fibre waviness and its effects on compressive properties of unidirectional NCF composites
Composites Part A: Applied Science and Manufacturing,; Vol. 107(2018)p. 665-674
Fibre waviness induced bending in compression tests of unidirectional NCF composites
Proceedings of the 21st International Conference on Composite Materials, ICCM-21, Xi’an, China, 2017,; (2017)
Paper in proceedings
Influence of in-plane shear on kink-plane orientation in a unidirectional
Composites Part A: Applied Science and Manufacturing,; Vol. 119(2019)p. 283-290
Wilhelmsson D, Talreja R, Gutkin R, Asp L. E. "Compressive strength assessment of fibre composites based on a defect severity model"
is to reduce the energy in flight by lighter composite materials such as carbon fibre reinforced polymers (CFRP). Compared to metals, CFRP has a superior strength-to-weight ratio due to the very strong carbon fibres and low density.
The most common type of CFRP in aircraft is very expensive and the current trend is to replace these with textile based composites to reduce costs. The textile composites consist of carbon fibres, equally strong to the ones in the expensive material. However, the manufacturing principles are different, which cause the fibres to be wavy. Waviness of the fibres have a negative effect on material properties such as strength. The reduction in strength when the material is loaded in compression is the most critical case. In this project we address this problem such that reliable dimensioning methods for this type of materials can be developed for use the aerospace industry.
The reason that compressive strength is particularly affected by the fibre waviness is related to the interaction between the matrix material (the epoxy polymer) and the carbon fibre. The carbon fibres provide the strength of the composite material and it is the role of the matrix material to distribute the load between fibres. In compression, the matrix material must support the fibres or else they will collapse. This collapse is very sensitive to the orientation of the fibres with respect to the direction of the compressive load.
Existing models for strength prediction consider the fibre waviness in a way which is too simple. We have developed a method to measure the fibre waviness in a very accurate and detailed manner. Based on this information, we have gained fundamental knowledge on compressive failure. We have
further developed new models that are able to consider the small variations in fibre orientations and the large impact they have on compressive strength.
Compressive failure of complex NCF composite structures
GKN Aerospace Sweden, 2013-12-02 -- 2019-02-15.
VINNOVA, 2013-12-02 -- 2019-02-15.
Areas of Advance
Composite Science and Engineering
C3SE (Chalmers Centre for Computational Science and Engineering)
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4546
Chalmers University of Technology
VDL, Hörsalsvägen 7, Göteborg
Opponent: Prof. Michael R. Wisnom, Department of Aerospace Engineering, University of Bristol, England