Characterization of three-dimensional microstructure of composite materials by X-ray tomography

Abstract

Analysis methods for X-ray microtomographic images of short fibre composite materials were developed. The methods enable estimation of microstructural properties of the material, e.g., aspect ratio and orientation of fibres. Being based on X-ray microtomography and image analysis, the methods are nondestructive and do not require user intervention. In particular, a method for determination of the aspect ratio of fibres was first developed. The method contains an assumption about similarity of the shape of the fibres. The assumption was relaxed in an improved method that can estimate cross-sectional properties of fibres, too, e.g., cross-sectional area. Additionally, the effect of finite image volume on the results of the measurements was discussed. It was concluded that fibre length is the quantity that is most biased by it. A method for correcting the bias was proposed. The developed algorithms were tested and applied in estimation of parameters for a micromechanical model and in quantification of morphological degradation of wood fibres in injection moulding process. It was demonstrated that the methods can be used to measure the parameters of a specific micromechanical model for Young’s modulus of flax fibre composites. The modelling results were compared to those calculated with parameters determined manually, and to results of tensile tests. Morphological degradation of wood fibres in injection moulding process was studied. It was observed that both the length and the aspect ratio of the fibres decrease considerably during processing. Finally, a special sample holder was fabricated for studying the hygroexpansion of fibres in a wood fibre composite material. Tomographic images acquired with the sample holder were used to estimate expansion parameters of the fibres. The parameters were applied in validating a finite element model. In all cases discussed above, results obtained using the developed methods are in agreement with those from independent reference measurements.