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Abstract

Exploiting very high speed digital videography, an in situ examination of the hole exit delamination at the bottom-most ply during drilling holes in the selected woven CFRP epoxy laminates is presented. At the beginning, a rotating elastic bulge of the carbon fibers at the bottom-ply, which is just the impression of the protruding drill chisel edge, was always observed. Following the elastic bulging, a few, initial cracks along the weak fiber/matrix interfaces appeared. Thereafter, tensile failures in the carbon fibers were seen. The exact location of the initiation of these fiber failures specifically depends on the actual drill-hole position with respect to the woven configuration of the bottom-ply. A visual model for the weak interstitial or undulated regions at the bottom-ply is also proposed in this paper, showing the undulating fibers, which are susceptible to mostly tensile failures under the drilling loads. During a sub-completion drilling-phase at the bottom-ply, various cracks were seen to be propagating mostly via linear paths. Also, the exit delamination at the bottom-ply during a sub-completion drilling-phase was always observed as to be divided into various small, independent localized contours—each of which propagated almost independently through within several individual warps/wefts during drill-feed. Also, the shape of each such tiny contour within a single warp/weft was identified as elliptic, which is observed around an entire drill-hole in unidirectional (UD) composites as reported in literature. It was also observed that the overhanging cantilever-like fibers at the bottom-ply are really difficult to cut, once their base-location or their exit delamination contour reaches outside the hole nominal diameter. Moreover, by referring to some very basic cutting angle configurations for the orthogonal trimming of UD-composites as found rarely in archival literature, an illustrative model diagram is also proposed for the drilling of the selected laminate material. This idea is also approximately validated via a few visual observations. The proposed visual model is generally an attempt in correlating the observed peripheral hole quality (delamination) with various instantaneous tool/fiber engagement configurations occurring across the entire drill-hole periphery at the bottom-ply, during the very last drilling-phase.