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Laboratory tests were performed on 5/8-inch- (16mm) diameter scale models of cylindrical structures fielded in Mighty Epic. The model structures were tested in 4-inch- (0.1-m) diameter specimens of SRI RMG 2C2, a tuff simulant. Results from these tests show that (1) structures are damaged less during dynamic loading than during static loading, (2) if yielding occurs in the free field, tunnel closure is not greatly influenced by structure strength, and (3) structures can effectively resist deformation under repeated loading. Theoretical analyses were performed for axisymmetric loading of a deep-based structure in SRI RMG 2C2. Results show that (1) the analyses can predict tunnel closures measured in the isotropic loading laboratory tests, (2) theoretical tunnel closures under plane strain loading (radical pressure at infinity and axial strains identically zero) and isotropic loading (hydrostatic pressure at infinity) differ only slightly, but (3) tunnel closure and rock stress field under end-on loading differ substantially from closure and stress under isotropic loading; thus substantial theoretical extrapolation of rock response is needed to predict tunnel closures in the field during end-on loading using isotropic loading laboratory data, even though both loading types produce symmetric tunnel response., See also Volume 1, ADA063487.

An analysis based on a one-dimensional beam-mass model was developed to predict the early-time response of penetrator structures in angle-of-attack impacts. The model was verified by comparison with the strain response measured in a reverse ballistics test and with the strain response measured in idealized scale model penetrators. Loads were simulated by a device that produces the resultant force-time history near the front end of the penetrator. The tests indicated that the loader could be built in a larger size to test full-scale penetrators. The analysis was then used to investigate penetrator response. It was found that the peak compressive strain, which determines whether or not the penetrator casing fails, depends on the magnitude of the lateral load produced by impacts at an angle of attack, the load rise time (which is inversely proportional to impact velocity), and the relative mass of the nose and aft sections. Finally, a procedure was devised to characterize the strength of penetrator structures in terms of impact velocity and angle of attack. The resulting critical impact curves can be used to make tradeoffs among structural dimensions (e.g., length and wall thickness), to select the best structure for a particular applications, and to provide a framework for planning and interpreting experiments and more detailed calculations.

Laboratory studies were performed in support of DIABLO HAWK structures and cable-hardening experiments to investigate: (1) the influence of loading rate on tunnel closure in both water-saturated and dry specimens of SRI RMG 2C2, a tuff simulant; (2) borehole collapse mechanisms and borehole/cable interaction; and (3) the influence of joints and joint orientation on the closure of circular tunnels in specimens of jointed 16A rock simulant. Results for SRI RMG 2C2 show that greater pressure must be applied in dynamic tests than in static tests to achieve a given tunnel closure. The percentage increase in pressure is twice as large for saturated specimens than for dry specimens. This larger loading rate effect in saturated specimens is due, in part, to no porewater migration and drainage in the dynamic tests. Static and dynamic borehole collapse and borehole/cable interaction test results show that springline collapse is the dominant borehole closure mechanism and that the most severe loading on the cable is from the inward motion of springline rubble. Observed critical loads for cable damage suggest that the TRW fielding ranges in DIABLO HAWK should provide the desired range of damage from none to severe.

Theoretical and laboratory studies were performed to investigate: (1) effects of lateral confinement and rock specimen-to-tunnel diameter ratio in laboratory testing of reinforced tunnels in rock, (2) tunnel response in jointed rock and (3) response to repeat loading of various reinforced tunnels in intact and jointed rock. Results of the laboratory study of effects of lateral confinement and rock specimen-to-tunnel diameter ratio show that: (a) Small deviations from the uniaxial strain lateral confining pressure (10% to 20% over- or underconfinement) cause correspondingly small deviations in the loading needed to produce a critical design crown-invert tunnel closure, (b) The specimen-to-tunnel diameter ratio has a small but measurable effect (less than 20%) on critical loads for tunnel closure in the range used in laboratory testing, and (c) The presence of the tunnel does not cause the laboratory specimen to bulge. Specimen lateral boundaries remain straight to within the accuracy required for uniaxial strain tunnel response (to within 200 microstrain for tunnel closures of interest.

Design and fabrication of large-scale testing machine for static and dynamic testing of rock cavity reinforcement is described. The machine is patterned after a smaller prototype developed previously for DNA. The larger-scale machine can be used to apply static and dynamic triaxial loads on specimens that are 12 inches (0.3 m) in diameter and 12 to 18 inches (0.3 to 0.45 m) high, with rock cavities 2 inches in diameter. A variety of triaxial loadings are possible which permit laboratory simulation of different loadings imposed on deep-based structures in the field. The machine can apply vertical pressures up to 2 kbar (0.2 GPa) statically and to 1 kbar (0.1 GPa) dynamically. The maximum lateral pressure is 1.5 kbar (0.15 GPa) statically or dynamically. The testing machiine can also be used to apply dynamic loading superimposed on a static preload. The static preload pressures can be as large as 0.2 kbar (0.02 GPa). The tunnel cavity is maintained at ambient pressure, with access at both ends for photography and electrical instrumentation. (Author)

An experimental apparatus was developed to simulate the impact loads on earth penetrating structures. The device has been built to test 1/4-scale model structures, but the design is suitable for fabrication in a larger size for testing full-scale structures. The device uses the controlled flow of high-pressure explosion product gases acting against a piston to apply either an axial load or combined and lateral loads to a model structure. Loading parameters such as rise time and peak load can be easily varied from test to test by simple adjustments of the device. The technique allows hardwired measurement of the applied load and the response of the structure. A series of initial calibration tests conducted without a model structure indicates that the device operates properly. (Author), See also report dated Nov 75, AD-B016 967L.

The response of monocoque direct-contact and backpacked liners as hardened buried structures was studied through experiments on scale models (5/8-inch diameter tunnel), guided by analyses of structure and rock response assuming a Mohr-Coulomb rock strength characterization. Two loading machines were modified to extend their capabilities and improve their performance. The modifications to the machines produced improvements in tunnel access, in sealing between vertical and lateral pressure chambers, in dynamic pressure pulses, and in general test procedures.

Two new analyses have been developed to predict the surface loads produced by a laser supported detonation (LSD). The first analysis is based on the self-similar theory of expanding blast waves and is very simple to use. It is recommended for systems studies of target damage. The second is a combination of the method of characteristics and self-similar blast wave analysis and provides a more accurate picture of the plasma expansion process. Both models predict pressure pulse and impulse intensity distributions on the target surface with more realistic shapes than those predicted by the model of Pirri. Measurements of surface pressure inside the laser spot show that the pressure histories are close to those predicted by the more complete theory, but with a more rapid decay. This difference between analyses and experiment is attributed to the complex beam profile and pulse shape in the experiments and the comparable uncertainty in measuring laser energy. Analysis of the LSD explosive simulation technique (ESD) showed that it is possible to model both near field and far field effects in the same experiment. However, if near field loads are simulated precisely, far field loads will be 25 percent lower than those of an LSD.

Techniques are described for simulating blast-type transient surface loads of nearly exponential pulse shape having characteristic times (impulse/ peak pressure) ranging from 10 to 1000 microsec (the quasi-impulsive range for cylindrical shells about 1 foot in diameter). Pressure-time histories are measured at various positions around and along cylindrical models 3.5, 6, and 12 inches in diameter. A basic set of loads is obtained consisting of two limiting pressure distributions, an asymmetric distribution typical of side exposure to a normally incident blast wave, and a symmetric distribution typical of nose-on exposure. All of the loads are obtained using sheet explosive charges of various forms, from flat to semicylindrical to completely cylindrical surrounding the model and flat charges suspended at various standoffs in a shock tube. In support of the experiments, the self-similar solutions for blast waves from intense explosions are used to calculate the range of sheet charges needed to produce loads of interest and to show that the corresponding spherical charges become much too small and much too large for practical application near the extremes in load duration. Approximate formulas are also derived (the Korobeinikov theory) for the variation of peak pressure with distance from plane, cylindrical, and spherical charges. The range of validity of the formulas extends from high pressures, where the self-similar solutions are valid, to acoustic shock pressures. Experimental measurements from the present program and from compiled blast data show excellent agreement with the theory over the entire range.

Theoretical and experimental structural response investigations of space-vehicle-type structures under suddenly external surface loads are described. The simulation of a simultaneous impulsive load by a traveling load such as produced by an explosive is analyzed for the string and membrane. Three dynamic buckling problems are investigated: (1) dynamic plastic-flow buckling of flat plates due to in-plane flow, (2) dynamic elastic buckling of a thin cylindrical shell under axial impact, and (3) dynamic buckling of cylindrical shells of a strain-rate sensitive material. A scheme for correlating the results of structural response investigations concerned with dynamic failure loads of structures is presented and a brief review of available results is given.