# Effect of Holes on Dynamic Crack Propagation under Impact Loading
## 1 Introduction  
In civil engineering, geotechnical engineering, and mining engineering, the structure strength and the fracture properties are dominated by materials with several macro and micro defects, such as concrete, rock mass, and other construction building materials. Much attention is paid to investigating the defects' effect on crack propagation behaviors, with numerous experimental and numerical approaches conducted [1; 2; 3; 4; 5; 6; 7; 8]. Among those defects, holes are universal defects in materials which are shaped naturally or designed for a specific function, such as blast holes, observation holes, pilot holes, etc. Under dynamic loading, the induced crack might propagate and encounter holes, and the crack propagation behaviors could be influenced by the interaction between the crack and holes. It is significant to investigate the crack-hole interaction and predict the crucial behaviors in the crack propagation process for engineering security and hazard mitigation. In 1981, the propagation of a transverse crack with an opposite hole was investigated by [PERSON] [9]. Benefitting from a caustics system collaborated with a high-speed camera, the results were uncovered that the crack momentarily arrests when arriving at the discontinuity, then reinitiates later with a different velocity. In 1988, [PERSON] [10] studied crack propagation and crack-hole interactionunder different dynamic loads with the caustics method, and the crack-hole interaction characterized by the process of attraction revealed that the repulsion of the crack toward the hole was interrupted by a momentary crack arrest at the hole boundary. In 2008, [PERSON] [11] proposed a technique to arrest crack initiation by drilling ancillary holes near the stop-drilled holes. The calculations demonstrated that the holes' effects could be applied in the crack arrest technical method. In 2015, [PERSON] [12], conducted an experiment with an empty hole on a compact tension specimen to test crack initiation and growth under the hole's effect, pointing out that the stress concentration at the stop-hole edge was influenced by the loading condition and the diameter of the hole. In 2016, [PERSON] [13] proposed that the hole's influence on cracks should be noted and showed a method for determining the appropriate size of a stop hole for cracked marine and offshore structures. In 2016, [PERSON] [14] focused on investigating the crack-defect interaction mechanism of an empty hole before the pre-crack with a digital-laser dynamic caustics system, revealing the relationship between hole size and the dynamic stress intensity factor and the crack propagation behavior when the crack encounters a single hole. In 2017, with [PERSON]'s work [15], the effect of the original prefabricated hole on dynamic crack was investigated using an impact drop hammer combined with a dynamic caustics system. Polymethyl methacrylate (PMMA) specimens were employed in the experiment with the crack stress intensity factor and extending speed at the crack tip obtained for analysis. However, the mechanism of holes on running cracks under dynamic loading is not very clear, and there still remain some unanswered or partially unanswered aspects. For example, how do the cracks and holes interact as they approach each other? What is the effect of the holes on the running cracks? Under what conditions would a running crack completely stop? Under what conditions would a running crack bifurcate to sub-cracks connecting with the holes? In this paper, a single-cleavage semi-circle specimen with a pre-crack and two empty holes is proposed to investigate the effect of the holes on crack propagation. Notably, the holes' locations with respect to the crack were well designed with different distance parameters to realized quantitative analysis. The Split-Hopkinson pressure bar (SHPB), along with crack propagation gauges (CPGs), was employed in the impact experiment, and the crack fracture phenomena and propagation velocity data were collected.