Fig. 3: Pin insertion test.

a Sketch of the pin insertion test in a tow of width \(2b\). Insertion of a pin of diameter \(d\) results in a crack of length \(2a\) forming in the tow. The inset shows a sketch of a magnified view of the crack along with the shear deformation mode of the crack. b Optical image of the tow A with an inserted pin of diameter \(d=1.4\,{{{\rm{mm}}}}\). c XCT images of a region within the crack with negligible bridging and d near the crack tip region to show the bridging zone. In (d), we include images for the bridging zone created by two different pin diameters, \(d\). In both cases, \(\lambda \sim 2.7\,{{{\rm{mm}}}}\). d The distribution of \(|\varphi |\) is overlaid on the XCT reconstruction for the region ~2 mm ahead of the crack tip. All results here are for tow A.