For decades, nuclear density gauges have been used to measure the density and moisture content of soil during compaction testing in road and embankment construction. Worldwide regulatory agencies consider nuclear density gauges as controlled devices because of their inclusion of radioisotope sources. Regulatory requirements, such as the need for licensing, special storage, special transportation procedures, gauge operator training, and personal dosimetry, have become burdens for gauge users. Recently, a new nuclear density gauge for measuring soil wet density was declared exempt from licensing and other nuclear regulatory requirements in the United States. The new gauge uses an extremely low-activity radioisotope source. It further incorporates a separate probe operating on electromagnetic principles for soil moisture measurement or any other moisture measurement method the user desires. This paper presents the design features and measurement properties of this low-activity nuclear density gauge and its associated moisture probe. The measurement properties were determined from laboratory and field studies that were conducted in North Carolina, United States. These studies showed that the sensitivity and precision of wet density measurements made by the low-activity nuclear density gauge were similar to those made by a conventional nuclear density gauge. The wet density measurements made by the low-activity gauge and conventional gauge showed a strong correlation and agreement. For the two field sites studied, a silty sand subbase and a cement-treated aggregate base course, the dry density agreement between the low-activity nuclear density gauge/moisture probe and the conventional nuclear density gauge was within 3 lb/ft3 (48 kg/m3).

SUMMARYSeveral Cb, Ta, Mo, and W base alloys have been creep tested with both constant and continuously increasing loads in a vacuum environment of < 1~1 0 -~ t o r r at temperatures and stresses chosen to provide between 1/2 and 1 percent creep i n times up to 20 000 hours. Isostatic design data a r e presented in the form of a.Larson-Miller plot for 1/2 o r 1 percent creep life for each of these refractory alloys. Elevated temperature tension test data from the tantalum base alloy T-111 showed a strain aging reaction in the 1400' to 2200' F (760' to 1204' C) range which produced an unusual transient creep behavior. Analysis of the isostatic steady state creep data for this material showed that the minimum creep rate can be described by the expression 5 of13. 17,-90 000/RT ?-in the temperature and stress ranges of 1600' to 2600' F (870' to 1427' C) and 500 to 45 000 psi ( 0 . 3 4~1 0~ to 31. OX107 N/m2). These values of activation energy and s t r e s s exponent suggest that the isostatic steady state creep of T-111 is governed by a diffusion controlled microcreep mechanism rather than by nonconservative dislocation motion. The creep tests which were run on T-111 alloy with continuously increasing loads provided continuously increasing creep rates which were comparable to the isostatic steady state creep rates at equivalent s t r e s s levels.Methods have been developed for prediction of the 1 percent creep life under varying s t r e s s conditions from isostatic creep test data, and comparison of these predictions with the experimental results shows the dependence of analytical techniques on the character of the isostatic creep curves.Based on work done under NASA contracts NAS 3-2545 and NAS 3-9439.

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High frequency fatigue tests were conducted in an ultrahigh vacuum environment on recrystallized mqlybdenum base alloy TZC and tantalum base alloy T-111 at temperatures between 1800' and 2200' F (982' and 1204' C). The results indicated that in this temperature range no well-defined endurance limit existed for either material. At A ratios of 0.45, fatigue strengths a t 2000' F (1093' C) and lo9 cycles were as low as 16 000 psi (1 1. 0~1 0~ N/m2) for the TZC and 34 000 psi (2 4. 8~1 0 N/m2) for the T-111. The application of a cyclic load to a staticallyloaded specimen produced a marked acceleration in the degree of specimen extension. In both the TZC and T-111 alloy the increase was characterized by a relatively rapid first stage creep extension (on the order of 1 to 10 percent) compared to the very small extensions (less than 0. 0 1 percent) observed at the same temperatures and peak stresses in the isostatic creep tests. In the TZC alloy the dynamic loading increased the steady state creep rate by approximately two orders of magnitude; however, this acceleration w a s not observed for the T-111 alloy. Possible reasons for this behavior a r e discussed in t e r m s of previous observations concerning the isostatic creep behavior of these materials.