A Technique for the Composite Materials Unsaturated Permeability Testing of Composite Preforms as a Function of the Filling Ratio

Technological Issues of the Flying Vehicles Composite Structures Manufacturing


Аuthors

Shabalin L. P.*, Puzyretskiy E. A.**, Khamidullin O. L.***, Salikhov N. F.****

Kazan National Research Technical University named after A.N. Tupolev, Kazan, Russia

*e-mail: lpshabalin@kai.ru
**e-mail: EAPuzyretskiy@kai.ru
***e-mail: olkhamidullin@kai.ru
****e-mail: NFSalikhov@kai.ru

Abstract

The article presents an original technique for determining the unsaturated permeability of composite preforms as a function of the filling ratio, which allows obtaining the entire functional dependence in a single experiment. The relevance of the development is stipulated by the need to predict the impregnation processes in manufacturing the products from polymer composite materials by the transfer molding (RTM). Exact knowledge of the permeability dependence on the filling ratio is critically important for computational optimization of the impregnation modes, preventing formation of the defects in the form of residual porosity and dry zones, which significantly reduce the products operational characteristics, especially in aviation technology. The proposed approach is particularly important when working with the 3D-woven preforms, which are being characterized by an additional structural parameter, namely the number of the weft threads, or “layering”. The change in layering affects both physical and mechanical characteristics of the material, including permeability, which requires testing for each layering zone of the product. With the traditional approach, at least three samples per zone are required to build a permeability power-law dependence on the filling ratio, which, in the presence, for example, of ten layering zones, leads to the need for the 30 samples manufacturing and testing, and this is an extremely time-consuming task.
The proposed technique is based on comparing the results of both unsaturated and saturated permeability tests of the same sample, followed by the  data recomputing using the proportionality coefficient. The experimental procedure includes six stages:
– the unsaturated permeability testing of a sample with the initial filling coefficient;
– saturated permeability testing the of the same sample with the same filling coefficient;
– conducting saturated permeability tests with several other filling coefficients (achieved by sample the thickness changing during one experiment);
– analyzing the results and determining permeability coefficients;
– determining interrelationship between the unsaturated and saturated permeability with the initial filling coefficient;
– recomputing the unsaturated permeability for the remaining filling coefficients through the found interrelation coefficient.
The key assumption of the technique consists in the assumption of the saturated and unsaturated permeability ratio constancy, independent on the preform filling ratio.
The technique was tested on the 3D-woven composite preforms with specialized equipment that allows simultaneous control of the impregnation process and change in sample thickness. The PMS-100 silicone oil was used as a model liquid. Position fixation of the flow front was being accomplished by the video recording through the transparent organic glass base with account for the optical distortion corrections. The experimental data processing was performed based on the Darcy's law for stationary and non-stationary filtration.
Comparison of the results obtained by the proposed technique with the data from the standard tests, in which a separate sample is being used for each filling ration, demonstrated satisfactory convergence: the discrepancy between the values of the unsaturated permeability does not exceed 11.7%. With a filling ratio of 51.1%, the error was 4.9%, at 55.7% – 8.8%, and at 60.2% – 11.7%. The obtained accuracy is considered acceptable for engineering calculations of the impregnation processes.
The developed technique ensures significant reduction in the testing complexity and the need for cost intensive materials, which is especially important when working with the 3D-woven preforms characterized by a variety of structural parameters. The proposed approach may be applied to optimize the computed impregnation models during transfer molding, providing a more effective quality prediction of the manufactured composite products. Further development of the technique involves its verification on a wider class of reinforcing fillers and binders, as well as studying the effect of the impregnation rate on the interrelation between the unsaturated and saturated permeability.

Keywords:

unsaturated permeability, saturated permeability, filling coefficient of composite material, permeability testing methods, transfer molding (RTM), 3D-woven composite preforms, Darcy's law, binder impregnation

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