SONIC VELOCITY AND PENETRABILITY OF SIMULATED LUNAR ROCK DUST

Abstract

Theoretical considerations are given for the design of an ultrasonic pulsing system for noncohesive dusts. In addition, measurements of less than one percent error of sonic velocity were made in volcanic rocks of different textures. Pumice containing elongate vesicles exhibits marked sonic anisotropy. In contrast, basaltic scoria with approximately equidimensional vesicles exhibits sonic isotropy. These anisotropy contrasts may be significant in interpreting Surveyor return data and in evaluating lunar base requirements. Sonic velocity with a simultaneous recording of porosity in rock dusts under ambient conditions ranges from 150 m/sec (500 ft/sec) to 700 m/sec (2,100 ft/sec). Absolute measurements of shear strength, using the vane‐type shear tester, were made at critical density obtained by vibrating the simulated lunar dust samples for specified time periods. Relative shear strengths were also compared at maximum density obtained by long‐period vibration of dust samples. Comparson of stress curves of dusts of different grain shapes indicates that relatively little strain occurs before the plastic zone of the stress curve is reached for volcanic dust consisting of angular glass shards. For purpose of shear strength analysis, grain shapes may be grouped as spherical, prismatic, and vesiculate. Shearing resistance curves are analyzed for each of these groups, and the vesiculate shapes have the highest shear strengths at critical density. Moreover, the sliding habit during shear appears to reflect the shape of the grains. Grain shape is apparently a first‐order parameter affecting shear strength of dust particles >0.1 mm in diameter. Adhesion effects due to hard vacuum ([Formula: see text] to [Formula: see text] torr) are small but measurable for the grain sizes considered. At equivalent porosities, vacuum at [Formula: see text] torr apparently does not markedly influence the peak shear strength of dust composed of vesiculate particles greater than 0.1 mm in diameter. Once sonic velocity and shear strength data are obtained by a lunar probe or astronaut, practical studies can be made on bearing capacity, trafficability, and lunar base construction.