Deformation, yield and fracture of unidirectional composites in transverse loading: 1. Influence of fibre volume fraction and test-temperature
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 performance in transverse direction are poor. Therefore in structural 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 composite 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 fraction, fibre–matrix bonding, fibre coating properties and matrix ductility. Considering the large number of parameters involved, it is useful to combine experiments on well defined (model) composite systems with micromechanical 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], [2], [3], [4], [5] and polymer–matrix [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [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 considerable improvement in the fundamental insight of the influence of the distinct parameters. However, in most studies hypothetical strength criteria are used, such as a maximum principle stress criterion for polymer matrices [8], [9], [10], [11], [12], [13], and many papers do not contain sufficient experimental data to verify the numerical results [12], [13], [14], [15], [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 composites 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 micromechanical model that can be used for transverse strength predictions in subsequent studies on fibre reinforced epoxies. By using isotropic fibres like glass and an isotropic matrix material micromechanical modelling is easier but, more importantly, more reliable and comprehensible. Mechanical properties and failure criteria, necessary as input parameters for the micromechanical analyses, have been determined experimentally. Experimental results are compared with the results of numerical analyses that are based on square and hexagonal fibre packing arrays. The influence of parameters like fibre volume fraction and test-temperature will be presented to validate the modelling in wider applications. In three subsequent papers, we will deal with fibre–matrix adhesion, matrix ductility and interphase properties, respectively.
Section snippets
Neat materials
The composites studied consisted of E-glass fibres from PPG Industries Fibre Glass bv (084-M28) and an Araldite epoxy system from Ciba Geigy, based on diglycidyl ether of bisphenol-A (LY556) with tetra-hydro-methyl-phthalic-anhydride (HY917) as a curing agent and methyl-imidazole accelerator (DY070) in a weight ratio of 100:90:1. To obtain the necessary input parameters for the numerical analyses, the neat materials were characterized using tension tests at room temperature at a strain rate of
Modulus
All composites experimentally tested showed perfect linear elastic behaviour up to failure. The transverse modulus strongly increases with increasing fibre volume fraction, as could be expected; see Fig. 5. The modulus determined by three-point bending is systematically lower than the modulus determined by tensile testing, despite the large span-to-depth ratios used in the bending tests. Numerical predictions were made using both types of packing. Most experimental data are found between the
Conclusion
Three-point bending tests are essential to obtain good experimental data of the transverse stress and strain to failure of unidirectional composites. Transverse testing of glass fibre reinforced epoxy in uniaxial tension yields too low strength values as a result of premature failure caused by flaws and defects. With three-point bending tests, strengths are obtained, which are close to the matrix strength indicating that the “intrinsic” material performance is measured.
The transverse stress to
Acknowledgements
The authors gratefully acknowledge H.J. Schellens for the identification of the failure criterion of the epoxy matrix and A.E. de Jong for his large contribution in the both experimental and numerical work.
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