Transmission and dissipation of stress wave energy at a percussive drill rod joint

Abstract

Transmission and dissipation of stress wave energy at a percussive drill rod joint is determined for three joint types, three joint preload levels, three hammer lengths (∼ incident wave lengths) and a range of impact velocities (∼ incident wave amplitudes). The energies of the incident, reflected and transmitted waves are evaluated from measured strains, and then the dissipated energy is determined as the deficit of energy in the two latter waves compared with the first. The accuracy needed is obtained by introducing a compensation factor and determining its value from the requirement of momentum conservation. The experimental results for threaded standard joints are compared with experimental results for threadless dummy joints, made in one piece, and also with theoretical results. The latter are based on one model which represents the joint as a well on a one- dimensional elastic rod (characteristics impedance or CI model) and on another model which represents the joint as a rigid mass between one-dimensional elastic rods (rigid mass or RM model). For the joints 70–100% of the incident wave energy is transmitted and 0–20% is dissipated. The corresponding figures for the dummies are 90–100% and (±)1%, repectively. The latter figure indicates the inaccuracy in the measurement and evaluation procedures. The two models agree fairly well with the dummies. Under certain conditions they also give reasonably accurate predictions for the energy transmitted through a joint. As they sometimes give quite inaccurate results for energy transmission and furthermore fail to predict energy dissipation they need be refined.