Fibre-orientation measurements in short-glass-fibre composites—II: a quantitative error estimate of the 2D image analysis technique

Fibre-orientation measurement by two-dimensional (2D) image analysis of polished cross-sections is a rapid and highly efficient method for determining the fibre orientation distribution over large sample areas. In a recent paper, a new technique was presented for measuring fibre orientation to a high level of accuracy by the use of a confocal microscope. In this paper, the confocal technique is used to evaluate independently the performance of the 2D image analysis technique and the errors are presented in full. The results reveal a significant systematic error in orientation measurements and the effect of image resolution is considered. A method is proposed for correcting this systematic error and its validity is verified experimentally. A technique for determining the sampling error of a fibre-orientation measurement is presented, enabling the calculation of confidence limits about the derived orientation tensor components. It is shown how confidence limits aid the comparison of two fibre-orientation measurements of similar samples. Furthermore, the sampling error should enable a more meaningful comparison of numerical simulation of injection moulded composites and their experimentally manufactured counterparts.