Experimental Study on Tearing Circular Steel Pipes by Internal Explosion

Literature Overview

The paper by Wang Debao and colleagues from the University of Science and Technology of China, published in Blasting in 2013, presents an experimental investigation into the structural response of circular steel pipes subjected to internal explosion loading. The study was supported by the National Natural Science Foundation of China Key Project (51134012) and General Project (51174183). The research objective was to understand how internal explosions can selectively tear circular steel pipes without completely destroying them, which has practical applications in controlled demolition and structural disassembly. The experimental setup involved using a custom-made inner explosive shell with grooves on its outer surface, filled with emulsion explosive, placed inside a larger circular test steel pipe.

Experimental Setup and Methodology

The experimental configuration is carefully designed to achieve the target of tearing rather than complete destruction of the test pipe. The inner explosive device, or shaped charge, is made from 45# steel tubing with dimensions of 37 mm outer diameter, 3.8 mm wall thickness, and 20 to 30 cm in length. The outer surface of this inner shell is machined with grooves that serve as pre-designed weak zones to control the direction and pattern of tearing. The test pipe is made from 45# steel with dimensions of 180 mm outer diameter, 9.6 mm wall thickness, and 1 meter in length. The test pipe is filled with either water or air as the pressure transmission medium, and detonation is initiated by a detonating cord detonator.

The use of grooves on the outer surface of the inner shell is an innovative approach to controlling the explosion-induced failure pattern. These grooves act as stress concentrators that guide the tearing process, ensuring that the pipe splits along predetermined lines rather than failing in an uncontrolled manner. This is analogous to the concept of pre-notching in fracture mechanics, where controlled crack initiation is achieved by introducing geometric discontinuities.

Key Findings and Defect Analysis

The experimental results reveal several important findings. First, the primary force responsible for tearing the steel pipe is the explosion shock wave rather than the fragments from the inner shell. This is a significant finding because it indicates that the pressure medium (water or air) plays a crucial role in transmitting the detonation energy to the pipe wall. Second, water as a pressure medium produces significantly better tearing results than air, which is consistent with the higher bulk modulus and incompressibility of water compared to air. The shock wave propagation characteristics differ substantially between the two media, affecting the peak pressure and impulse delivered to the pipe wall.

The grooves on the inner shell surface serve as effective constraint weak zones that enable directional tearing of the pipe. The number of grooves, their depth, and their spacing influence the tearing pattern and the number of segments produced. Different explosive charges were tested to determine the optimal amount for achieving clean tearing without excessive fragmentation. The length of the inner shell relative to the test pipe also affects the tearing pattern, with longer shells producing more uniform tearing along the pipe length.

From a welding and materials perspective, the tearing behavior of the steel pipe is governed by the material's fracture toughness, yield strength, and strain rate sensitivity. The high strain rate loading from the explosion shock wave can significantly alter the material's mechanical response compared to quasi-static loading. The dynamic fracture behavior of 45# steel under such extreme loading conditions is relevant to understanding the limits of material performance under blast loading.

Engineering Practice and Safety Considerations

The practical application of internal explosion tearing is primarily in controlled demolition of steel pipe structures, such as large-diameter pipelines, storage tanks, or structural columns. The ability to achieve predictable tearing patterns is essential for safe and efficient demolition operations. Engineers must carefully design the explosive configuration, including the charge amount, placement, and initiation sequence, to ensure that the tearing follows the intended pattern and does not produce uncontrolled fragmentation that could endanger personnel or surrounding structures.

Safety protocols must be strictly followed during such operations, including adequate standoff distances, blast shielding, and personnel exclusion zones. The selection of water as the preferred pressure medium, while producing better results, also introduces additional safety considerations related to the handling and containment of large volumes of water within the pipe. The experimental findings provide a valuable foundation for developing practical guidelines for explosion-induced demolition of steel pipe structures, which can reduce the need for cutting equipment, lower demolition costs, and minimize environmental impact.

Study Insights and Reflections

This research bridges the fields of explosives engineering, structural dynamics, and materials science, and its findings have practical relevance for steel pipe engineers involved in demolition and structural modification projects. The emphasis on the shock wave as the primary tearing mechanism, rather than fragment impact, is a counterintuitive finding that challenges conventional understanding of explosion-induced damage. The use of pre-designed weak zones on the inner shell demonstrates a systematic approach to controlling failure patterns, which is analogous to controlled fracture concepts in materials engineering. For engineers planning controlled demolition of steel pipe structures, the experimental parameters and results from this study provide a valuable starting point for designing safe and effective explosion configurations. The work underscores the importance of understanding dynamic material behavior and shock wave physics in practical engineering applications involving controlled structural failure.