New interlaminar features and void distributions in advanced aerospace-grade composites revealed via automated algorithms using micro-computed tomography

Abstract X-ray micro-computed tomography (μCT) is used to quantify morphology in AS4/8552 (autoclave) and IM7/M56 (Out-of-Autoclave, OoA) aerospace-grade advanced unidirectional-ply carbon fiber prepreg composites, revealing several previously unreported features. The micron-scale (1 μm voxel size) three-dimensional datasets combined with automated, objective algorithms, revealed the following previously unreported features of AS4/8552 and IM7/M56 laminates, respectively: all ply interfaces analyzed have misplaced microfibers at densities of 1–2 per mm2 of interface area that can contribute to the mean thickness of the interlaminar regions of 8.6 μm and 14.4 μm; all ply interfaces have elongated (aspect ratio > 10 and presumed to extend indefinitely) periodic resin pockets along the microfiber direction of the plies bounding the interlaminar region that we term tow-aligned resin pockets (TARPs), with typical thicknesses that are 2–3X greater than the average interlaminar thickness; overall void fractions are low at ~0.002 vol% and ~0.001 vol%, comprised primarily of newly-quantified "sub-microvoids" with an average volume of 26–31 μm3 that are equally pervasive in both materials, numbering ~300 per mm3. The new interlaminar region and void tools were also utilized to analyze laminates with aligned carbon nanotubes (A-CNTs), termed "nanostitches", incorporated between plies to reinforce the interlaminar regions. The addition of A-CNTs increased the interlaminar thickness by 2.2 μm and 8.0 μm for the AS4/8552 and IM7/M56 systems, respectively, but did not affect the quantity or distribution of voids or TARPs. These newly-identified features are relevant to the mechanical performance of such composites, as they may have positive or negative effects on damage initiation and progression.