Engineering Design and Void Characterisation of Thick Composites

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Abstract

Voids are defects that are difficult to completely eliminate during the manufacturing process of composite materials. With the increase of void content (i.e. porosity), the mechanical properties and service life of the composite materials will be significantly affected. It is thus of vital importance to improve the quality of composite materials and avoid the formation of voids. In this project, military-use Twaron®-epoxy thick composite panels have been manufactured and non-destructive testing (NDT) techniques have been used to assess the voids in the panels. Three modification suggestions to the vacuum assisted resin infusion (VARI) composite fabrication method have been proposed in order to improve the quality ofthe composite panels.X-ray computed tomography (CT) has been employed to assess the voids in composite panels, which enables the void content, the morphology and distribution of the voids to be examined in three dimensional views. The void content is found in a range from 0.39% to 1.75% in different composite panels. In composite panels with different thickness, the results show that the porosity increases with an increasing number of fabric layers. Moreover, the threading density and the linear density of the yarns used to weave the fabrics are shown to affect the impregnation process. The void content in the composite panels consisting of coarse fabrics is less than that in the panel made of fine fabrics. Ultrasonic C-scan is also used in this research to study the void distribution from a qualitative perspective, showing a good agreement with the X-ray CT results.The effect of three modifications to the VARI method has been studied. Firstly, cutting the resin inlet tube under the resin surface can effectively eliminate the air remaining in the tube and reduce the air drawn into the vacuum bagging system, and this will reduce the voidage from 1.62% to 0.91%. Moreover, double meshes have been used as the resin flow media in the infusion process, which can enhance the through-thickness resin flow. Compared with the panel made by the conventional VARI method, the voids are fewer and more localised in the panel made with double meshes. In addition, a second outlet has been used, which can alleviate the non-homogenous expulsion of voids and can avoid air being accumulated and trapped in the fabric. It has been observed that the quality in the central part of the panel has been improved using symmetrical outlets.In this research, the non-destructive characterisation capability of X-ray CT has been confirmed and the voids in the composite materials have been visualised in three dimensions. Based on the investigation into the distribution of voids in the composite panel and the formation mechanisms involved, the idea of modifying manufacturing techniques has been proposed to improve the quality of composite materials. The three-dimensional visualisation of the size, shape and distribution of voids can provide useful information for the modelling and numerical studies of void formation and highlights the effect of voids on mechanical properties.

Keyword(s)

Textile Composites; VARI; X-ray CT

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