Suitability of Split Box Direct Tension Testing for Measuring Tensile Strength of Regolith Simulants

Abstract

Under low effective stress conditions, the often-neglected soil tensile strength may significantly influence the behavior of lunar and Martian regoliths. Therefore, regoliths’ tensile strength in extraterrestrial environments may pose many challenges for future space operations and exploration such as in situ resource utilization (ISRU). To explore regolith tensile strength for physical modeling on Earth, this study assessed the efficacy of the split box direct tension test method in measuring small amounts of tensile strength of F-75 Ottawa sand with added crystalline silica powder (CSP) at 30% and 50% by weight, and more commonly used lunar simulants JSC-1A and GRC-3. Findings indicate an increase in tensile strength with increasing regolith density. However, JSC-1A and GRC-3 forced open the split box prior to applied loading at low relative densities. This highlights the importance of considering system friction and lateral forces in the specimen during analysis. Although split box testing provided reasonable estimates of tensile strength, the testing method is more appropriate for soils with significantly greater tensile strengths than those tested here. NASA’s in situ resource utilization (ISRU) efforts deal with the challenges from the lack of immediate access to supplies during space exploration and habitation. To use lunar regolith as an in situ resource, a more robust understanding of the strength properties is required. Due to the low gravitation forces acting on the lunar surface (roughly 1/6 the gravity of Earth), it is hypothesized that soil tensile strength may play a larger role in lunar regolith strength properties. Therefore, a simple method for testing the tensile strengths of lunar regolith is needed. To accomplish this goal, this research modified the simple split box test method to reduce the system friction by more than 10-fold for measuring the tensile strength of extraterrestrial regolith simulants. Due to the reduction in the system friction, low tensile strength soils that naturally fall into a conical shape forced the split box to open prior to loading. Therefore, this research indicates that prior split box testing may have reported higher than actual tensile strength values, and that future split box testing should consider the action of lateral earth pressures along the failure plane, especially in soils predicted to have low tensile strength.