Analysis of geometric imperfections affecting the fibers in unidirectional composites

The compressive strength of unidirectional composites is partly limited by the presence of geometric imperfections, mainly including undulation and general mis-alignment of the fibers. By examining a number of regularly spaced cut sections under an optical microscope, the fiber paths within the material can be reconstructed using image processing techniques to determine the coordinates of the fiber centers in each cut. For each fiber, these coordinates are projected in main planes in inertia to determine the overall misalignment angle and smooth the undulation using a sinusoidal waveform. Four carbon fiber composites were examined in this way. The complete analysis of the parameters relative to misalignment and undulation shows the magnitude of the imperfections affecting each composite. Main component analysis is performed to identify the interdependence between several of these factors. In addition, correlating the positions of the different fibers show that they interact only at very short distances. These results call into question the approximation generally applied up to now, which was to consider unidirectional composites as a series of mutual parallel fibers undulating in phase. This method therefore opens the way to more realistic modeling of geometric imperfections as is required both for numerical and finite element analyses of the compression behavior of such materials.