Investigation of the Credibility of In-Situ Measurements of Radial and Tangential Stress in a Salt Test Bed.

Abstract : Eighteen ytterbium piezoresistant sensors in nine Teflon-steel flatpack stress gage-salt core assemblies were fielded in a spherical HE test in a salt medium. The objective was to examine the credibility of in-situ stress measurements by comparing measured stress with stress inferred from independent particle velocity measurements and with stress from computational simulations of the experiment. Criteria for gage emplacement were examined by finite element computational simulation of the emplacement configuration. This simulation indicated that two critical parameters that influence the relationship between free-field stress and the stress in the core at the plane of the stress gage are the bonding of the cores to the native salt and the size of the gage relative to core diameter. Static loading calibration of the stress gages yielded a response essentially the same as static calibrations of ytterbium in uniaxial strain loading. Unloading calibration of the gages showed no hysteresis, unlike the uniaxial strain response of ytterbium foil. A gage sensitivity of .000005 Omega/Omega MPa was found to represent the loading and unloading data and was used to convert the experiment waveforms to stress histories. The experiment produced radial stress histories that are characterized by a slowly rising compression, often containing a precursor of approximately 10 bars (1 MPa), a larger main wave peak of 90 to 350 bars, and a release wave to an apparent tensile stress of 30 to 40 bars.