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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1. Introduction Various concepts developed in the field of fracture mechanics play important roles also in understanding the source mechanisms of earthquakes. Seismic shaking of ordinary earthquakes lasts a few tens of seconds, and in seismology, as an earthquake source model, one single, relatively large fracture region is usually assumed in a brittle solid to expand quasi-statically or dynamically and radiate seismic waves. Such a single large-scale fracture model causes most likely one sole, short seismic event (Uenishi and Nagasawa, 2022). However, in reality, although not often, we have very long seismic shaking, typically lasting about two minutes, which is related to an interlocking earthquake with intermittent crustal fractures. For example, the off the Coast of Fukushima Prefecture earthquake at 23:36 JST (Japan Standard Time) on 16 March 2022 itself (Japan Meteorological Agency (JMA) magnitude 7.4, epicentral latitude and longitude of 37.697  N and 141.622  E with a focal depth of 57 km) has records of long duration of shaking of about two minutes, with a foreshock (JMA magnitude 6.1, epicentral latitude and longitude of 37.680  N and 141.605  E with a focal depth of 57 km) just two minutes before, at 23:34 JST, causing the derailment of the Shinkansen high-speed train. In this case, the total duration of shaking for this earthquake might be considered to be as long as four minutes (Uenishi, 2024). For such earthquakes with multiple seismic events one after another, including a cluster of earthquakes and earthquake swarms, a single large-scale fracture model may not be sufficient, and therefore, for a deeper understanding of the generation mechanisms of this and other seemingly complex fracture processes, we have been performing global observations of large-scale material behavior and more local, smaller scale tracing of development of fractures and waves in linear elastic brittle solid specimens in the two-dimensional context. The solid materials have sets of digitally fabricated multiple small-scale parallel cracks modeling large-scale geological fault planes that are under external (quasi-)static tensile loading and dynamic impact. So far, we have illustrated that the fracture behavior significantly depends not only on the loading conditions but also on the initial inclination angle and distribution pattern of the sets of parallel cracks, and the behavior can be very complex especially in and around the sets of cracks (Uenishi et al., 2020; Uenishi and Nagasawa, 2022, 2023). In this contribution, we show three cases of our latest series of experiments observing the dissimilar (un)fracture behavior associated with preset multiple small-scale parallel cracks in brittle birefringent solid specimens that are subjected to In Fig. 1(a), one of the transparent photoelastic polycarbonate specimens, prepared using a digitally controlled laser cutter for the new series of laboratory fracture experiments, is depicted. Here, the specimen has small-scale parallel cracks with an inclination (dip) angle of 45 degrees and it is under external static tensile or compressive load that is exerted by a testing machine and acting parallel to the free surfaces. Then, in addition to the static load, dynamic impact is given to one of the free surfaces of the specimen by a projectile (sphere of diameter 6 mm, mass 0.2 grams) that is launched with an airsoft gun. The initiation and evolution of fractures and possibly the fracture induced waves are observed using a high-speed video camera (Photron FASTCAM SA-Z) at a frame rate of 100,000 frames per second. In addition to the cases with an inclination angle of 45 degrees, a variety of different distribution patterns of preexisting cracks with different inclination angles are scrutinized, but in this contribution, as mentioned above, characteristic three cases with the configuration of the specimen shown in Fig. 1(a) are introduced. In the first case (Fig. 1(b)-(c)), the specimen is subjected to uniaxial static tensile strain, 0.027 (tension positive), that is slightly smaller than the static limit strain, 0.028, for the specimen. When a strain above this limit is exerted by the testing machine, the specimen is fractured in tension. Dynamic impact is additionally given by the projectile with the impact velocity of some 66 m/s. Although in some photographs the opening displacement of the newly fractured section is small and fracture development is not clearly identified, the photographs taken by the high-speed camera, shown in Fig. 1(b), indicate that the dynamic fracture develops in an intuitive prograde fashion, i.e. upwards from the bottom near the impact point, but it does not always follow the perforation lines composed of small-scale cracks with an inclination angle of 45 degrees. Instead of “unzipping”, at some sections, the fracture propagates vertically or perpendicularly to the direction of the static tension and causes the total split of the specimen into two. The fracture path is indicated in red in Fig. 1(c). external static loading in tension or in compression and dynamic impact. 2. Dynamic impact imparted under static tension or compression