Performance Enhancement of Abrasive Waterjet Rock Cutting Considering Properties of Abrasives and Targets

Abstract

Population densities have led to an increase in and aging of structures, which requires urban construction and its maintenance. Noise and vibration caused by construction and maintenance work in urban areas results in social problems. Abrasive waterjet is a low-noise, vibration-free, and high-efficiency method, suitable for urban construction. Abrasive waterjets are conventionally applied to precision machining of hard materials or cleaning of soft materials

therefore, if they are used for geotechnical purposes, large-scale material removal is required for hard materials such as rock. However, abrasives are expensive, and removal performance varies with abrasive and rock properties

thus, it is very important to understand the characteristics of abrasive and rocks, select the proper abrasive type, and determine the input abrasive rate. Therefore, this study conducted literature research on the principles of material removal, and aimed to improve the efficiency by evaluating the erosion characteristics according to the properties of abrasion and rock in the overall process employing the abrasive waterjet.Kinetic energy of the accelerated abrasive is the main erosion energy in the abrasive waterjet

therefore, calculating the exit velocity of the abrasive is critical. The abrasive properties determine the acceleration characteristics and the exit velocity. Thus, based on the acceleration and the drag force applied to particles in a fluid, a time-dependent velocity model of the abrasive was developed. The proposed model considers the abrasive properties such as the diameter, density, and drag coefficient and calculates the velocity and traveled distance of abrasives so that the exit velocity is along the focus length. According to this model, the velocity of the abrasive is greatly influenced by the early acceleration time, depending on the abrasive properties, and the abrasive leave the focus before reaching the terminal velocity. The proposed model can also be used to select the abrasive type and the economical abrasive flow rate.The abrasive flow rate and the geometry of the focus (or the focusing tube), which is the space where the abrasive is mixed and accelerated by a high-velocity fluid stream, affect the mixing efficiency. For efficient rock cutting, the effect of the abrasive flow rate and the focus geometry were evaluated experimentally. As a result, for efficient mixing, a focus of sufficient inner diameter is required, whereas a short length of 76.2 mm or less is suitable to prevent efficiency reduction owing to excessive mixing. In this study, it was found that the optimum abrasive flow rate was dependent on the impact frequency from the number of abrasive particles. In addition, a model was proposed to determine the momentum transfer parameter according to the abrasive flow rate, from the relationship between the maximum kinetic energy and the cutting rate, thereby enabling the estimation of the momentum transfer parameter, which was obtained only through complex experiments.Hardness acts as a resistance force for material erosion by abrasive kinetic energy. Therefore, the effective erosion energy depends on the hardness ratio between the abrasive and the object. Based on this concept, a rock-like brittle material erosion model was proposed that can consider the abrasive properties such as diameter, density, and hardness so that erosion depth can be estimated under various conditions. The proposed model was verified via comparison with the representative brittle erosion model, i.e., the elasto-plastic erosion model, and the numerical results. In addition, the parameter study evaluated the erosion characteristics of a wide range of materials such as abrasives and rocks. The results of this study can be used to select proper abrasives based on hardness and to predict cutting performance.Since waterjet cutting performance is determined by the properties of the rock, it is very important to understand the effect of the rock properties. In this study, uniaxial compressive strength, tensile strength, and hardness, which are the representative physical properties of rocks, were selected as the major physical properties. The effects of the rock properties were numerically evaluated by modeling rocks of various strengths and hardnesses and abrasives of different densities. The numerical results were verified via comparison with the published experimental results of other researchers. As a result, a parabolic regression model expressed the erosion characteristics depending on the main physical properties. This model can be used to analyze the rock cutting efficiency of excavation and tunneling.In this study, the influence of properties of abrasives and targets on erosion characteristics was investigated throughout the acceleration and erosion processes. The results of this study are expected to be used for the selection of the proper abrasive type, the estimation of momentum transfer parameter and effective erosion energy, the determination of the optimum abrasive flow rate, the design of waterjet system configuration, and the prediction of erosion performance. This dissertation contributes to the development of geotechnical applications of abrasive waterjets.