Stress-strain gage measurements on a Nevada Test Site (NTS) event using pairs of triple foil gages

摘要

A triple-material stress-strain gage containing foils of ytterbium, manganin, and constantan was tested at LLNL. This gage yields three independent piezoresistance measurements from which an independent set of principal strains and principal stresses can be inferred. We have analyzed the signals from a gage at a specific location buried at a distance from the center of energy of an event at NTS. We inverted the resistivity signals to calculate the stress and strain histories as sensed by the foils. Using the elastic material properties and the traction and displacement conditions at the various material interfaces from the foils to the host geological medium, we calculated the stress and strain histories in the host geological medium. However, because of uncertainties in the elastic properties of the layers of surrounding materials, we calculated significant variations in both the peaks and signatures of the stress and strain histories that could produce the foil stress and strain histories. We conclude that, because of the inclusion problem, accurate measurements of the stress-strain histories in the host geological medium can be adequately addressed only by minimizing, as much as possible, such uncertainties.