Local Mean Fiber Orientation via Computer Assisted Tomography Analysis for Long Discontinuous Fiber Composites

Abstract

Platelet-based long discontinuous fiber (LDF) composites have a unique microstructure with fiber collimation within platelets, discontinuity at platelet boundaries, and a large degree of heterogeneity at the macro-scale. This extreme macro-heterogeneity requires microstructural characterization of all regions of a part, as compared to the assumption that a few sample regions sufficiently represent all present microstructures. In this work, nondestructive computed tomography (CT) scanning with a meso-scale, 53μm resolution (voxel edge length) from a North Star Imaging X-50 system is analyzed. This resolution will not allow for the distinct viewing of individual carbon fibers (5-7μm diameter) or even distinct platelets (~100μm thick), but is the highest resolution possible on the equipment used for the 65x65x65mm part studied. Two principles are used to extract micro-scale details from the meso-scale CT scan resolution: 1) There is fiber alignment locally within each platelet as evidenced in micrographs, and 2) The direction of least density change within a stack of aligned platelets is aligned with the fiber direction. Using Eigen analysis of the density gradients, the locally averaged fiber orientation at each voxel within the CT scan is determined. Optical microscopy of selected regions within the scanned part allow for comparison of the fiber orientation vector components in the global X, Y, and Z directions. The relatively low resolution CT scan does not provide exact characterization of distinct platelet boundaries, but still yields a mean local fiber orientation. This improved mapping of the entire part microstructure may be used to better understand the manufacturing process, compare with manufacturing simulations, or implement in a finite element model for simulations where discrete platelet boundaries are not required.