Variation in Wave Propagation in Lunar Percussive Drilling Instrument Due to Presence of a Joint

This paper presents the results of analysis of percussive system designed for lunar exploration using finite element modeling. The system consists of a string of hollow circular tubes driven by a mechanism delivering high-frequency impact loading so as to drive the tube forward into the lunar regolith. The tubes are attached in an end-to-end fashion with some compressive preload applied at the joint between the strings. The primary focus of this paper is to determine effects of preload and presence of joints on stress and displacement wave propagations along the length of the tube. Two types of finite element models were developed — one, a single circular tube without any joint and the second, two tubes connected end to end. Static preloads (500 N and 1500 N) were applied at the joint for the second model. Separation of tubes is seen with preloading value of 500 N for the given dynamic loading. Sudden discontinuity of stress and displacement waves is also seen. Similar analysis with 1500 N preload shows no separation between tubes. This information has considerable significance in design of joints in drill strings.