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Using polymethyl methacrylate (PMMA) as the test material, three-point bending tests were carried out on single hole and double hole specimens with prefabricated cracks at the bottom. The fracture process of the specimen was observed by using a new digital laser dynamic caustics system (DLDC). The ABAQUS was used to simulate the double hole fracture experiment. The experimental results and simulation result showed that for single-hole specimens, smaller S and larger r had a more obvious attraction to cracks. For the double-hole specimen, the variation of S/r and r parameters had different effects on the crack. [ABSTRACT FROM AUTHOR]

Holes have an important influence on dynamic fracture process of brittle materials. In this paper, the influence of the hole offset distance (HOD) on the movement process of the explosive crack is studied. Polymethyl methacrylate (PMMA) is used as the experimental material, and different HOD holes are drilled on the PMMA plate. A new digital laser dynamic caustics system is employed to observe the crack tip dynamic stress intensity factor and crack growth velocity of PMMA plates with different HODs under explosive loading. The results indicate that holes reduce the peak velocity of explosive crack propagation. Moreover, the interaction between the stress wave reflected by the hole and the explosion stress wave is the main cause of crack deflection. Lastly, as the HOD increases, the crack motion trajectory is weakened by the interference of the holes, which gradually decreases corresponding stress intensity factors. With the increase of HOD/d, the crack arrest effect of empty hole decreases gradually. [ABSTRACT FROM AUTHOR]

An underground roadway usually contains defects of various types, and when the roadway is subjected to external loading, the locations of those defects influence the roadway by differing degrees. In this study, to study how the locations of defects affect crack propagation in a roadway, specimens with tunnel-type voids were made using polymethyl methacrylate, and the stress wave produced by a bullet impacting an incident rod was used as the impact load. Meanwhile, the variations in crack speed, displacement, and dynamic stress intensity factor during crack propagation were obtained using an experimental system of digital laser dynamic caustics, and the commercial software ABAQUS was used for numerical simulations. From the experiments and numerical simulations, the crack propagation path was verified and the impact fracture behavior of a semicircular-arch roadway with different defect positions was presented. The results show that when the pre-fabricated crack is on the central axis of the sample, the crack propagation is purely mode I; when the pre-fabricated crack is 5 mm from the central axis, the crack propagation alternates between mode I and a mixture of modes I and II; when the pre-fabricated crack is at the edge of the semicircular-arch roadway, the crack propagation follows the I–II mixed mode. [ABSTRACT FROM AUTHOR]

Crack defects make it difficult to predict the dynamic fracture of tunnel specimens under an impact load. To study the impact of the velocity and crack location on a roadway under dynamic load, specimens with tunnel-type voids were made using polymethyl methacrylate. The split-Hopkinson bar was used as the loading method, and a digital laser dynamic caustics system was used to observe the fracture process of the specimens. The dynamic fracture process was evaluated by the crack propagation velocity, displacement, and dynamic stress intensity factor. To predict and verify the test results, ABAQUS was used to simulate the test process. It was found that the results of the simulated combinations of the crack propagation path and initial fracture toughness change law are consistent with the test results. The initial fracture toughness and the peak value of the crack propagation velocity increased with the increase of the impact velocity. The crack propagation law and trajectory were affected by the location of the prefabricated cracks. [ABSTRACT FROM AUTHOR]

• The explosive loading system and caustics system are employed together. • The empty hole can absorb the energy of stress wave and diverge again. • The distance between two holes have an important effect on crack propagation. • The numerical simulation is similar to the image of caustics. In this paper, the crack propagation behavior of a polymethyl methacrylate plate with double holes under explosive load is investigated. The propagation process of the explosion crack is recorded using a new digital laser dynamic caustics system, in which a high-speed camera and the explosion loading are employed. Firstly, the interaction process between the explosion stress wave and the empty hole reflected stress wave is analyzed using caustics. Moreover, the deflection law of the explosion crack is discussed. Secondly, the relationship between the propagation distance, velocity, stress intensity factor of the explosion crack tip, and the distance between the two holes is analyzed. Finally, the explosion process is analyzed using numerical simulations. It is concluded that the evolution law of the explosion stress wave in during the simulation process is similar to the caustics diagram. Lastly, the time-varying law of the stress between the double holes is obtained. [ABSTRACT FROM AUTHOR]