Energy transfer in percussive rock destruction—III: Stress wave transmission through joints

Abstract

Two different models representing a joint between drill rods are analyzed. The joint is first treated as an elastic swell and then as a rigid mass between two elastic rods. According to the rigid joint model, the joint is analogous to a simple low-pass filter. The efficiencies of stress wave energy transfer derived from the two models are shown to agree well in many situations of interest in hammer drilling, and since the rigid joint model leads to a simplified analysis, this model is used in solving two problems. First, for an incident stress wave of a given length, the pulse shape leading to maximum efficiency of energy transfer through a single joint is determined. This ideal pulse, which is represented by a symmetric portion of a half sine wave, is generally difficult to obtain in a practical situation. However, a rectangular pulse of the same length, which is easily generated by impact, leads to an efficiency which is never more than 0·04 lower than the efficiency for the ideal pulse. Secondly, stress wave energy transmission through an arbitrary number of joints is treated. The results show that when passing a joint, the stress wave is favourably modified with respect to further transmission. Measured transmitted stress waves agree well with theoretical results derived from each of the two models.