Deformation, yield and fracture of unidirectional composites in transverse
loading
1. Influence of fibre volume fraction and test-temperature
J.M.M. de Kok
*
, H.E.H. Meijer
Centre for Polymers and Composites, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands
Received 9 April 1994; accepted 30 July 1998
Abstract
The influence of the fibre volume fraction and test-temperature on the transverse tensile properties of glass fibre reinforced epoxy is studied
using experimental and numerical techniques. The numerical analyses are based on micromechanical models with square and hexagonal fibre
packings. Special attention has been directed towards the identification of the necessary failure criteria. Using a von Mises failure criterion,
an increase in transverse tensile strength is predicted at higher fibre volume fractions with both models. This is in good quantitative
agreement with experimentally determined transverse flexural strengths. With decreasing test-temperatures, higher transverse strengths
are obtained. This is primarily caused by the temperature dependence of the yield stress of the matrix. The counteracting influence of the
residual thermal stresses and the temperature dependent matrix ductility consequently proved to be less significant for the transverse strength.
᭧1999 Published by Elsevier Science Ltd. All rights reserved.
Keywords: C. Micromechanics; B. Strength; A. Glass fibres; Epoxy
1. Introduction
High performance composites are generally composed of
unidirectional fibre reinforced laminates. Such laminates
possess a pronounced anisotropy given the large differences
in fibre and matrix properties, and especially the perfor-
mance in transverse direction are poor. Therefore in struc-
tural applications usually stacked plies with different fibre
orientations are used, allowing for a considerable stiffness
and strength in more than one direction. However, even in
these laminates low off-axis strains can lead to premature
failure in the individual layers. Therefore, the low transverse
failure strain of unidirectional composites can be regarded
as one of the major limitations in the application of compo-
site materials.
To improve the transverse failure strain, an intensive
experimental study would be required in order to determine
the influence of parameters such as the fibre volume frac-
tion, fibre–matrix bonding, fibre coating properties and
matrix ductility. Considering the large number of para-
meters involved, it is useful to combine experiments on
well defined (model) composite systems with micromechan-
ical analyses. Since in such analyses a parameter variation is
easily accomplished, the amount of experimental work can
significantly be reduced.
The transverse deformation and fracture of metal–matrix
[1–5] and polymer–matrix [6–16] composites have been
studied using finite difference or finite element methods
(FEM) on micromechanical models. These numerical
analyses offer the opportunity to reveal the deformation
(and fracture) on a microscale and may lead to a consider-
able improvement in the fundamental insight of the influ-
ence of the distinct parameters. However, in most studies
hypothetical strength criteria are used, such as a maximum
principle stress criterion for polymer matrices [8–13], and
many papers do not contain sufficient experimental data to
verify the numerical results [12–16]. As a result of the
complex three-dimensional stress situations in composites
the choice of a failure criterion like the maximum principal
stress might lead to the wrong conclusions. To avoid this,
attention must be focused on the determination of the
materials failure criteria [17,18].
This investigation is part of a detailed study that focuses
on the transverse tensile properties of unidirectional compo-
sites and that combines experiments and finite element
analyses on various composite systems. In this first paper
a system is studied based on glass fibres in a relatively brittle
epoxy matrix. The objective is to develop an appropriate
Composites: Part A 30 (1999) 905–916
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* Corresponding author.