PSI - Issue 61

Koji Uenishi et al. / Procedia Structural Integrity 61 (2024) 108–114 Uenishi et al. / Structural Integrity Procedia 00 (2024) 000 – 000

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the top free surface and moving along a perforation line dipping 45 degrees, illustrated in light blue. The secondary fracture merges into the primary one to cause the split of the entire specimen into two. Although in both cases 1 with the strain 0.027 and 2 with 0.028, the specimen is broken into two and without the assistance of a high-speed camera only prograde fracture propagation from bottom to top seems to have occurred, in reality, in the case 2, the fracture jumps to a remote place and propagates backwards in a retrograde manner to join the primary prograde fracture.

Compression

Projectile

0  s

20  s

40  s

60  s

80  s

100  s 10 mm

Fig. 3. The specimen (Fig. 1(a)) under uniaxial static compressive strain -0.028 and dynamic impact with the impact velocity of the projectile being some 58 m/s and the time of impact being 0  s. Due to the strong stabilizing effect of the static compression, the impact cannot induce dynamic fractures linking the small-scale parallel cracks. Thus, a slight change in the static loading condition can indeed have an influence on the local fracture behavior. A larger change in the static loading condition, of course, does have a great influence on the dynamic behavior of the specimen. In the case 3, shown in Fig. 3, compression, instead of tension, is applied uniaxially to the specimen. The static compressive strain is set at -0.028 and the impact velocity of the projectile is approximately 58 m/s. In this case, the dynamic impact is not sufficiently strong to overcome the stabilizing effect of the static compression on the mechanical behavior of multiple small-scale parallel cracks, and no dynamic fracture connecting the small-scale Three cases are shown for the dynamic fracture behavior in a brittle polycarbonate specimen containing preexisting small-scale parallel cracks that dip 45 degrees. Depending on the static and dynamic loading conditions, the specimen behaves mechanically totally differently. In the case 1 in static tension, dynamic fracture induced by an impinging projectile moves upwards from bottom to top. On the other hand, in the case 2 in slightly higher static tension, dynamic primary fracture starts traveling upwards similarly from bottom, but it is arrested in the middle of the specimen. Then, fracture jumps and propagates downwards from top along a perforation line consisting of small scale cracks and merges into the primary one. In both cases, at the final stage, the specimen is completely split into two. Note that jump, arrest and possible resumption of propagation of fractures or a cluster of fractures can be found often in polycarbonate specimens with small-scale cracks during careful local observations, but these rather abrupt local changes of fracture behavior cannot be recognized through global measurements such as drawing stress-strain curves using testing machines (Uenishi and Nagasawa, 2023). In the case 3, it is seen that static compression suppresses the effect of dynamic impact posed by the projectile. Waves are visible in some photographs in Figs. 1, 2 and 3, but their connection with the dynamic fracture development is still uncertain in the above three cases. Moreover, as pointed out by Uenishi and Nagasawa (2022), the relation between (quasi-)static and dynamic stability of solid materials (Gomez et al., 2020) should be more quantitatively addressed, and also, the fundamental difference cracks develops. 3. Conclusions