Composites Part A: Applied Science and Manufacturing
A method for the direct measurement of the fibre bed compaction curve of composite prepregs
Introduction
Flow and compaction behaviour during autoclave processing of thermoset matrix composites has a significant effect on the final thickness profile, residual stress distribution and distortion of composite structures. The prediction and control of both the mean value and the variability of these effects is a major concern to industry. To address this problem, science based process models have been developed [1], [2], [3], [4], [5], [6]. Typically, these models treat the composite as a deformable fibre bed saturated with a curing resin. The resin flow relative to the fibre bed is governed by Darcy's law and is coupled with the compaction behaviour of the fibre bed. At any point in the composite, the total through thickness stress σ is shared by the fibre bed and the resin (Fig. 1). The simple viscoelastic system presented in Fig. 1 implies that during compaction the resin flow and fibre bed compaction are the viscous and the elastic component, respectively. Theoretically, in a very low fibre volume fraction prepreg, or perhaps in a system with ideal straight fibres, the fibre bed carries no through the thickness stress. In practice, the relatively high fibre volume fraction, and the wavy geometry of real fibre beds mean, significant stress can be borne by the fibre bed, especially if there is any loss of resin. This leads to the following equilibrium relation:where is the effective stress in the fibre bed and P the resin pressure. The relationship between the effective stress and the fibre bed deformation is given by the fibre bed compaction curve. This parameter is a critical material property input for any flow-compaction model. Compaction curves have been measured by several investigators for carbon fibre beds [7], [8], [9], [10], [11], [12] and glass fibre fabrics [13], [14]. Kim et al. [15] did numerous experiments on dry and lubricated glass and carbon fibre beds. Gutowski and Dillon [16] propose a “universal” model that fits all the fibre bed compaction curves found in the literature. They emphasize that this model is valid only if the fibre bed configuration is not modified by the condition of the experiment.
The principal difficulty in determining the fibre bed compaction curve is how to measure the response of the fibre bed alone. The resin plays a significant role, and it is difficult to determine the resin pressure P. Therefore typically, the fibre bed is tested in a dry form or is impregnated with silicone oil after dissolving out the polymer matrix. However the act of dissolving the resin out of the prepreg can change the fibre arrangement and could affect the fibre bed compaction behaviour. It is also a time-consuming and tricky operation. The use of a very low viscosity fluid (e.g. μ<0.1 Pa s) or no fluid, and the loading of the specimen at a very low rate (e.g. <0.1 mm/min) are steps normally taken to minimize the viscous response effects caused by the fluid flow out of the sample. When a wetting fluid is used, the fluid pressure is measured and subtracted from the total applied stress to obtain the fibre bed effective stress.
In the present work, an experimental procedure is presented which allows the measurement of the compaction behaviour directly from the actual prepreg. This technique allows the measurement of the material as is, without any steps that could alter the fibre arrangement in the matrix. In addition, the technique is very simple, and requires little sophisticated instrumentation or equipment. Results obtained for a commonly used composite system are presented, followed by a series of verification simulations.
Section snippets
Experimental procedure
The testing apparatus developed to obtain the compaction curve (Fig. 2) is similar to that used in Ref. [7]. With this apparatus, a unidirectional composite specimen is loaded in the vertical direction (z), which is the main deformation mode of the fibre bed. To obtain a uniaxial testing condition, the deformation in the transverse (y) direction is blocked by the mould walls and the fibres are oriented in the longitudinal (x) direction. Since, the fibres are very stiff in the longitudinal
Results and discussion
The specimen final strains calculated using Eq. (2) are presented in Table 1. In all cases, εx and εy were negligible as expected by the physical constraints of the mould and by the fact that the fibres prevented any motion in the longitudinal direction. Therefore, the measured vertical deformation of the specimen (εz) were caused by the volumetric strains which are a consequence of the bleeding of excess resin. A typical variation of the displacement uz and the applied load Fz with time for
Compaction curve validation
Independent validation tests were conducted to verify the validity of the obtained compaction curves. The tests consisted of using the compaction fixture (Fig. 2) to load a specimen at different temperatures. The specimens were loaded in load control mode to simulate an autoclave condition where a prescribed pressure (rather than a prescribed deformation) is applied to the laminate. The first tests named V100 and V140, were conducted for the following temperatures: 100 and 140°C with an 8 ply
Compaction test finite element model
For each specimen tested, a finite element mesh was constructed. An example of the mesh used is shown in Fig. 9. One half of the specimen was modelled due to the symmetry of the problem. The mesh was divided in two regions: the piston and the specimen. The piston was modelled to reproduce the uniform compaction of the specimen during a compression test. The stiffness and impermeability of the steel piston was achieved by setting the element properties accordingly. The load measured during the
Discussion
Table 2 compares the volumetric strains predicted by the flow-compaction model with the experimental data. In Fig. 10, Fig. 11, the compaction predictions are compared with the experimental results at 100 and 140°C. In both cases, the agreement between the numerical and the experimental data is very good. The load predicted by the model for the test at 100°C (Fig. 10) is slightly higher in the early stages of compaction (uz<0.2 mm). The actual fibre bed compaction curve was added to the graph
Conclusions
The method presented here is simpler than other methods and has the added advantage that it directly provides the actual compaction curve of the unmodified prepreg. The method does not require any complex and delicate sample preparation that could alter the fibre bed structure and therefore the measured compaction curve. This method can be applied to other types of composite systems including woven or non-woven fabrics. The compaction curve obtained can be used in process models to predict the
Acknowledgements
This work was supported by funding from the Natural Sciences and Engineering Research Council of Canada. We would also like to gratefully acknowledge the significant interaction and support from our colleagues at The University of British Columbia, The Boeing Company and Integrated Technologies Inc. We would like to acknowledge the contribution of Mrs. Laura Petrescue for her work on the measurement of the compaction curves.
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