Page 114 - 3D Fibre Reinforced Polymer Composites

Micromechanics Models for Mechanical Properties 103
averaged to obtain an average compliance (or stiffness) for the micro-cell. The macro-
cell model is developed for the entire cross-section of the specimen. In this model,
variations in the braider yarn orientation along its length are taken into account by
introducing an average yam orientation angle. It was reported that good agreement
between the model predictions and test results were achieved for axial tensile modulus.
A lack of agreement between the predicted and measured shear moduli was reported.
The effects of the braid angle, yarn size and axial yam content on the mechanical
properties were investigated by Naik et a1 (1994) for 2D triaxially braided textile
composites. The numerical results showed that the mechanical properties are more
sensitive to variability in braid angle than to variations in axial yarn content, and are not
sensitive to change in yarn size. Increasing the braid angle decreases the longitudinal
modulus but increases the transverse modulus, and the in-plane shear modulus values
peak at the braid angle of +45O. An increase in the axial yarn content results in a higher
longitudinal modulus, but a lower in-plane shear modulus and Poisson's ratio. The out-
of-plane properties remain virtually unchanged with variations in the braid angle and the
axial yarn content.

4.5.2 Knitted Composites
As knitted fabrics are not often used in structural applications, and their geometric
architectures are more complex compared to woven and braided fabrics, only a limited
attention has been given to the modelling of the mechanical properties of knitted fabric
composites (Ruan and Chow, 1996).
A simple stiffness model was proposed by Rudd et al. (1990) for predicting the
mechanical properties of weft knit glass fibre/polyester laminates. This model was
developed based on the well-known rule of mixtures approach. The comparison
between the predicted and experimental results suggested that the model require
modification to take into account the fabric relaxation.
Ramakrishna and Hull (1994) created an analytic model for predicting the elastic
moduli and tensile strengths of knitted fabric laminates. In this model, the reinforcement
efficiencies of yarns are incorporated into the rule-of-mixture, and the effects of the out-
of-plane yarns are however neglected. The tensile strengths of composites were
estimated by the strength of straight resin-impregnated yarns. It was reported that the
predicted elastic moduli were in a reasonable agreement with experimental results while
a significant discrepancy existed between the experimental and predicted tensile
strengths.
Ruan and Chou (1996) developed geometric models for plain-stitch and rib-stitch
fabric composites. These models were developed using the yarn configuration and
microstructures of the preform observed using an optical microscope. In this analytical
model, it is assumed that an infinitesimal segment, which is formed by two parallel
planes perpendicular to the warp (loading) direction, is subject to a uniform strain.
Modelling of elastic behaviour was conducted using an averaging method. It was
reported that the tensile and shear moduli determined by the analytical model were
higher than the actual values.
KO et al. (1986) proposed a fabric geometry model for predicting the tensile
properties of the warp-knit fabric composites. This model was developed based on the
unit cell concept and laminate theory. It was reported that there was a good agreement
between the predicted and experimental test results.