JOUI•NAL OF GEOPHYSICAL I•ESEARCI• VOL. 71, NO. 18 S•P•'•aR15, 1966 Stress Relaxation behindElastic ShockWaves in Rocks TI-IOMASJ. AI-IRElVS PoulterLaboratories, Stanford Research Institute Menlo Park, California G•o•, E. DUVALI, Department ofPhysics, Washington State University, Pullman Theamplitude of elastic shock waves in Arkansas novaculite is observed to decrease at a rate of ,•3.3kb/mm for shock propag•.tion path lengths of 6 to 12mm. Theamplitude of the final shock state in the experiments is held near the 155-kb pressure level. A totalvaria- tionof elastic shock waveamplitude (ttugoniot elastic limit) of ,•40 kb (from 110to 70 kb) is observed in •1 cm of shock travel. The intrinsic attenuation term in a constitutive equation for a stress-relaxing elastoplastic materiM is found to account for ,•90% of the ob- served peak pressure attenuation of the elastic shock, as compared with the •10% which is predicted from the instantaneous elastic shock profile. The ttugoniot elastic limits of Sioux and Eureka quartzites, which were notas intensively studied as the Arkansas novaculite, are also found to decrease with shock propagation pathlength. INTRODUCTION When a large pressure, p.. is applied suddenly io the plane bo•dary of a rock or other type ofsolid having a normal pressure-volume curve, theshock wave disturbance that propagates in thex direction forms, in certain stress ranges, two shock wave fronts. The first shock wave front, which has the greater propagation ve- locity U•, is called the elastic shock wave or elastic precursor. This shock is assumed to en- compass material whichis initially at rest (par- ticle velocity u --- 0) and at zero pressure (p, = p, - 0); the shock fransforms this material into the elastic shock sta!e. (The y direction is per- pendicular to the shock propagation direction.) The elastic shock state has a particlevelocity u -- • and longitudinal compresslye stress p, -- p•. The second, or deformational, shock front, propagating at a slowervelocity U• with re- spect to laboratory coordinates, brings lhe rock from the elastic shock state to the deforma- riohal or final shock state. The secondshock front imparis a particle velocity u -- u• and longitudinal stress p, ----p.. to the rock.The amplitudeof the first shock front is often called thet!ugoniot elastic limit and is believed to represent the maximum stress that a rock or other solid material can withstand under conditions of rapid one-dimensional compres- sion without internal rearrangement taking place in the material at the shock front. As the recording techniques usedin deter- mining profiles of shockwaves in solidshave beenrefined it has become possible to record a decrease of elastic shock waveamplitude in rocks with increasing propagation path length when the final shock state is at some nearly con- stant value. The decreasein elastic shock wave amplitude with increasing propagation path length is of special significance for a plane wave system because, in general,when the elastic shock is drivenby a second deformafional shock in two or three dimensions, the attenuation is dueto geometrical spreading. In one dimension, however, the geometrically produced attenua- tion is absent and the decreasein elastic shock amplitude must reflecta property of the rock rather than be a result of a limitation in the energy available from the shock source.This intrinsic amplitude decrease (or attenuation) of the elastic shock wave has been named elastic wave siress relaxation. In some cases a pronounced increase in elastic shock wave amp- litude with increasing final shock state has been observed when the shock propagation path length was fixed. Both an increase in elastic shock wave amplitude with increasing final shock state and a decrease in elastie shock wave 4349
Stress Relaxation behind Elastic Shock Waves in Rocks
Jun 24, 2023
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Related Documents
