PSI - Issue 42

3

Koji Uenishi et al. / Procedia Structural Integrity 42 (2022) 755–761 Uenishi et al./ Structural Integrity Procedia 00 (2022) 000–000

757

a

Reflected tension (inbound)

Tension (outbound)

b

Blast hole

500 mm

4.0  10 -5

Tension

Cartridge

Cartridge

 4.0  10 -5

c

d

100 mm

Inbound crack

Outbound crack

Fig. 1. (a) Top and side views of the cylindrical concrete specimen used for the basic fracture experiment using electric discharge impulse (EDI) [unit: mm]. (b) Numerically generated contours of dynamic volumetric strain (tension positive) in the specimen approximately at time 80 (left) and 100 (right)  s after the initiation of pressurization due to the action of EDI. Contrary to the impression of the snapshot taken by a normal video camera (c), the photograph (top view) by a high-speed camera at a frame rate of 50,000 frames/s (d) clearly indicates the outbound crack propagating from the blast hole (center) as well as the inbound one from the edges. The inbound crack is guided by the reflected EDI-induced waves that are numerically predicted in (b) (modified after Uenishi et al. (2014)). empty dummy holes (diameter 18 mm) that are set at the corners of a square of side length 200 mm (top view of Fig. 2(a); every blast hole is located at the center of the square). The important point here is that dummy holes are not placed on the expected main crack path but they are rather set to assist the main crack in connecting the three blast holes. This specimen is geometrically symmetric with respect to the virtual central horizontal plane (at a height of 150 mm) as well as to the virtual central vertical plane containing the blast holes except for the stemming sections and screw holes drilled for the carriage of the specimen. The influence of dummy holes and free surfaces on wave motion and possible crack extension can be foreseen again numerically by the finite difference code with the same calculation settings as above. The contours of volumetric strain taken at 100  s (Fig. 2(b)) after the start of action of EDI indicate that tensile regions in the vicinity of the free surfaces and dummy holes as well as those close to the middle blast hole appear almost simultaneously, possibly because of the existence of waves reflected from the nearby holes and approaching to the middle blast hole. Thus, a rather flat tensile fracture plane connecting the three blast holes may be generated as desired. In reality, the top view of the final fracture pattern taken after the dynamic experiment (Fig. 2(c)) shows that the main crack path is well controlled and rather smooth as expected, especially in the central part. Near the edges, however, due to the effect of the free surfaces and screw holes on the sides, unnecessary cracks that are expected also from the calculations are generated (Uenishi et al., 2018).