Void content measurements in thermoplastic composite materials through image analysis from optical micrographs

The principal interest of measuring the level of internal porosity in composite materials lies in the possibility of relating void content with macro and micro-structural properties and hence with composite manufacturing quality. In the particular case of commingled E-glass/polypropylene composites, processing parameters, such as moulding pressure and material temperature, have been demonstrated to influence void content, which is in turn affecting mechanical and impact properties of the material. It is of paramount importance therefore to dispose of a simple and reliable method for void content determination on these composites, which are increasingly used e.g., in automotive industry, for their energy absorbing properties and their low cost. In this paper, void measurements in E-glass/polypropylene commingled laminates are performed by analysing images from optical micrographs. Voids were classified, due to their size and orientation, as microvoids, intratow voidage, coplanar voids and extensive voids. In practice, optical microscopy of specimens was able to detect void contents of less than 1% using a statistical point-counting technique. Computerised analysis of the micrographs was also possible, based on the grey scale difference existing between voids and the rest of the composite image (matrix, reinforcement). Possibilities and limitations of this approach are highlighted by discussing the relation between the sample of the material considered and the obtained accuracy, which depends also on image filtering and appropriate 1image thresholding. Due to the complex structure of Twintex laminates and to consequent large deviation in void content values, not less than 80% of any cross-section (20 mm. long) removed from the laminate should be examined to obtain a measurement of the void content percent with an error of 0.15. With void contents ranging from 0.6 to 15%, this corresponds to an accuracy of void content measurement variable from ±0.1 to ±2%. In practice, this accuracy can only be obtained, if a significant number of cross-sections are removed from different regions of the laminate.