PSI - Issue 82

Koji Uenishi et al. / Procedia Structural Integrity 82 (2026) 72–78 Uenishi et al. / Structural Integrity Procedia 00 (2026) 000–000

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racture- waves

1.760 ms

11.385 ms

11.390 ms

11.395 ms

11.400 ms

11.435 ms

12.045 ms

Fig. 3. (continued).

and 0.830 ms, respectively. However, the fracture-induced waves are relatively weak and they do not induce further waved-induced fracture. The fracture repeats this arrest-extension process, and further fracture extension is found e.g. at 0.845, 3.085, 3.745, 3.780, 5.150, 5.205, 6.860, 6.995, 9.770 and 10.970 ms. The fracture reaches the last small scale crack or the notch (length 1 mm) at 12.165 ms, and dynamic stresses (and waves) quickly disappear (see the photograph at 13.920 ms). Thus, at least in the polycarbonate specimen, fracture extension is due to stress concentration by the advancing “axe”, not by impact-/fracture-induced waves, but the “axe” does not split the specimen all at once along the single perforation line. Rather, fracture repeats extending and arresting unidirectionally downwards until the specimen is totally split along the perforation line. Note that the isochromatic fringe patterns are generally symmetric with respect to the splitting vertical perforation line. In Fig. 3, isochromatic fringe patterns are shown for the case where two parallel perforation lines exist. In this case, the specimen is dynamically split along (one of) the perforation line(s) more quickly. As indicated at time 0.510 ms, the tip of the extending fracture is identifiable again by the distinctive isochromatic fringe pattern or stress concentration, but now due to the existence of the second (left) perforation line, the stress concentration seems larger. That is, dynamic stresses seem to be confined in and blocked at the region between the perforation lines and now the isochromatic fringe patterns are asymmetric with respect to the perforation lines. However, repetition of the arrest extension fracture process can be found also in this case. For instance, the fracture is arrested between 0.570 and 1.185 ms and then propagates straightforward downwards to a position very close to the bottom that is well ahead of the “axe”. At 1.745 and 1.760 ms, a fracture-induced surface-type wave moving upwards along the left perforation line can be identified. Finally around time 11.385 ms, the “axe” approaches the unbroken section of the right perforation line and the specimen is totally split. Concentric but energetically asymmetric fracture-induced body waves can be observed at time 11.395 and 11.400 ms. Both in the single (Fig. 2) and double (Fig. 3) perforation line(s) cases, the last bottom part of the line remains unbroken until the “axe” comes very close to that part. 3. Conclusions The experimentally taken isochromatic fringe patterns suggest that the mechanical process of wood chopping may be controlled by the advancement of the “axe”, not by fracture-induced waves (and spallation), as simply imagined. It is said that tougher firewood can be easily split by inserting the axe at the bottom and applying repeated loads to the axe upwards in the vertical direction. We try to observe this fracture behavior and elucidate the possible mechanism of collapse of wooden structures caused by vertical shock or impact. Acknowledgements The research has been financially supported by the Japan Society for the Promotion of Science (JSPS) through the “KAKENHI: Grant-in-Aid for Scientific Research (C)” Program under grant number 23K04021.