Ramesh B. Malla

Subgrade resilient modulus prediction models for coarse and fine-grained soils based on long-term pavement performance data

Resilient modulus (M R) of subgrade soils is the elastic modulus based on the recoverable strain under repeated loads and depends on several factors such as soil properties, soil type and stress states. This paper presents the prediction equations to estimate M R from a set of soil physical properties for the unified soil classification system soil types namely coarse-grained and fine-grained subgrade soils. Data extracted from long-term pavement performance information management system database for 259 test specimens of reconstituted soil samples from 19 states in New England and nearby regions in the USA and two provinces in Canada were used in this study. Generalised constitutive model consisting bulk stress and octahedral shear stress was used to predict the M R of subgrade soils by developing equations for the regression coefficients (k-coefficients) in the constitutive model that relates them to various soil properties. Prediction models were developed by conducting multiple linear regression analysis using computer software SAS®. To verify the prediction models, a set of fresh laboratory M R tests were conducted on representative New England subgrade soils using AASHTO standards. The laboratory test results show that the developed models predict M R values fairly well for the soils with their properties values within the range used in developing the prediction models.

Dynamic effects of member failure on response of truss-type space structures

Besides the usual causes of structural failure such as material defects and fabrication and construction errors, damage to space structures can come from such adverse events as impact by foreign objects, docking, drastic temperature changes, and exposure to radiation and charged particles. This paper presents a methodology to determine effects of member failure on the dynamic response of a truss-type space structure. Emphasis has been given to effects of the dynamic nature of member failure on the structural response. Two types of member failure are considered: one, the sudden brittle-type damage or failure of a member, and the other, member snap or dynamic jump due to buckling. The study is specially directed toward potential progressive member failure in the structure. It includes the postbuckling regime of member behavior. A three-dimensional truss-type structure is analyzed. Results are presented to delineate dynamic effects of member failure on the overall structural response measured in terms of deformations and stresses.

Modeling of Chill Down in Cryogenic Transfer Lines

A numerical model to predict chill down in cryogenic transfer lines has been developed. Three chill down cases using hydrogen as the working fluid are solved: 1) a simplified model amenable to analytical solution, 2) a realistic model of superheated vapor flow, and 3) a realistic model of initially subcooled liquid flow. The first case compares a numerical model with an analytical solution with very good agreement between the two. Additionally, the analytical solution provides a convenient way to look at parametric effects on the chill down. The second and third cases are numerical models that provide temperature histories of the fluid and solid tube wall during chill down, as well as several other quantities of interest such as pressure and mass flow rate. Of great interest in chill down applications is the ability to predict accurate values of chill down time (the time required to achieve steady-state cryogenic flow). The models predict that a 66.04-cm-long, 0.48-cm-internal-diameter aluminum tube has a shorter chill down time (100 s) and uses less hydrogen with superheated vapor flow than with initially subcooled liquid flow (>200 s for chill down).

Dynamic Member Failure in Trusses by Pseudo-Force Method

The paper presents a systematic technique based on “pseudo-force” concept to determine dynamic response of truss structures during sudden progressive member failure. Emphasis has been given to the dynamic nature of the member failure and effects of such dynamic member failure on the overall structural behaviour. Responses of two-dimensional cantilever truss and a three-dimensional, corner supported, double-layer grid truss structure were investigated during sudden failure of some compressive members. In the method the sudden reduction in the load carrying capacity of a member is represented by external forces applied suddenly at the end joints of the member. Results obtained delineate the dynamic effects of member failure on joint displacements and member forces/stresses. In the present study the method is established for linear analysis. However, the pseudo-force approach can be extended for non-linear analysis and to represent the member snap-through and dynamic jump due to inelastic member buckling/post buckling in a truss-type structure.

Dynamic Response of a Pressurized Frame-Membrane Lunar Structure with Regolith Cover Subjected to Impact Load

AbstractA three-dimensional internally pressurized frame-membrane structure covered with regolith shielding has been proposed as a possible lunar habitat. This paper presents results from the static, frequency, and dynamic impact analysis of the structure using a nonlinear (large deformation) finite-element technique. The results are presented, taking into account the effects of the added mass of the regolith and stress stiffening because of the applied internal pressurization load. The impact loading is analytically derived by considering the impact of a moving object/debris/projectile hitting the midpoint of one of the frame members at the crest of the structure. For the frequency analysis, the results show that both pressurization and added regolith mass affect the frequency and mode-shape characteristics, where the effects of the added mass of the regolith are observed to be substantially larger than the effects of pressurization. For the static and dynamic analyses, the midspan impact results show th...

Temperature Aging, Compression Recovery, Creep, and Weathering of a Foam Silicone Sealant for Bridge Expansion Joints

Silicone foam was investigated as a sealant for small movement expansion joints in bridge decks. This paper presents results from the laboratory assessment of a model foam sealant subjected to thermal aging (exposure to high and low temperatures and temperature cycling), fatigue conditions, and outdoor weathering. Parallel tests were performed on a commercial solid-silicone bridge-joint sealant. Test results showed that the solid sealant recovered faster than the foam sealant after being subjected to prolonged compression at elevated temperature. When subjected to a constant tensile force, both the foam and solid sealants exhibited high initial creep rates (rate of elongation), but appeared to reach an equilibrium level of elongation at longer times. The foam sealant creeps at a slower rate and takes more time to get to the equilibrium elongation, whereas the solid sealant creeps much faster initially and reaches equilibrium faster. Thermal aging was found to have significant effects on the sealant modulus (increases with temperature); however, the effects on ultimate stress and strain were not apparent for both types of sealants. Temperature cycling between 24 and -29°C was observed to diminish the ultimate stress and strain of both the sealants by roughly 25%; however, no significant changes in modulus were found. Results from the tests on a limited number of outdoor-weathered sealant specimens showed that weathering appeared to produce an increase in sealant tensile and shear moduli (i.e., hardening effects because of weathering) and a decrease in ultimate strain. The weathered samples show tension loading-unloading behavior similar to the unaged samples. The tensile stress relaxation rate of the outdoor-weathered sealants could not be distinguished from those laboratory-cured (unweathered) counterparts.

Dynamic Analysis of a 3-D Frame-Membrane Lunar Structure Subjected to Impact

A three-dimensional (3-D) frame-membrane structure covered with regolith shielding and pressurized internally has been proposed as a possible lunar habitat. This paper presents results from the dynamic impact analysis of the structure using nonlinear (large deformation) finite element technique. Results are presented taking into account the effect of the added mass of regolith and stress stiffening due to applied internal pressurization load. The impact load is considered arising from a moving object/debris/projectile hitting a membrane panel of the structure. The results show that both pressurization and added mass affect the displacement and stress response of the structure under the impact load. At the membrane node, the internal pressurization (prestressing) has greater effects to reduce the amplitude of oscillation of the vertical displacement in the case of without the added regolith mass than that in the case of with the added regolith mass. However, for the frame member nodes, when there was no added mass of regolith the amplitude of displacement seem to reduce slightly or remain the same due to pressurization. However, the effect due to pressurization is observed to be just the opposite (i.e. the displacement amplitude increased) when there was added mass of regolith.

Dynamic Impact Force on a Special Drilling Mechanism for Planetary Exploration

AbstractSpecial percussive mechanisms have been used to explore the lunar, Martian, and other planetary subsurface and extract soil (regolith)/rock samples for further study. The special type of percussive mechanisms investigated in this study, Auto-Gopher and Ultrasonic/Sonic Driller/Corer (USDC) developed by National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory and Honeybee Robotics Spacecraft Mechanisms Corporation, consists of an ultrasonic horn, a free mass (hammer), and the drill rod. This paper presents a general methodology to perform the dynamic contact analysis of the longitudinal impact of the free mass on the drill rod including the effects of structural vibration and damping of the rod. The contact force caused by impact is obtained by using Hertz force-indentation relation coupled with the structural vibration of the rod obtained by using mode superposition method and finite-element technique. Numerical solution, with equilibrium iterations, of the equations of motio...

Longitudinal Impact Force on a Special Drill for Planetary Exploration

Special percussive mechanisms, e.g. Auto Gopher and Ultra Sonic/Sonic Driller/Corer (USDC) have been developed by NASA Jet Propulsion Laboratory and Honeybee Robotics Spacecraft Mechanisms Corporation to explore the Lunar, Martian, and other planetary subsurface and extract soil (regolith)/rock samples for further study. The percussive mechanism consists of an ultrasonic horn, a free mass (hammer) and the drill rod. This paper presents an analytical methodology to analyze the dynamic and contact analysis of the longitudinal impact of the free mass and the drill rod including the effects of structural vibration. The contact force due to impact is obtained using Hertz force-indentation relation coupled with the structural vibration obtained using mode superposition method. Numerical solution, with equilibrium iterations, of the equations of motion is implemented. The contact force was observed to follow a sinusoidal like shape, consistent with results available in literature for similar problems.

Stress and Displacement Wave Propagation in a Percussive Tubular Mechanism for Space Applications

This paper presents results from a study of stress and displacement wave propagation in a dynamically loaded hollow cylindrical tube. This is a representation of a percussive dynamic cone penotrometer device proposed for extra planetary drilling including the lunar surface. Two cases have been studied. First is a base-line case where a simple hollow tubular structure is subjected to a short duration rectangular pulse loading. Second case is a fluted hollow tubular rod with a pair of helical augurs subjected to the same loading and boundary conditions as the first case. For the second case, two scenarios were also created – one where the model has been subjected only to the dynamic loading and second where a static preload has been applied at the loading end before the dynamic loading was applied. The difference in behavior of stress and displacement waves for the base line, and , the fluted model with and without the preload have been studied to draw practical conclusion. It was seen that while the wave velocity remained essentially unchanged (as theoretically expected), difference in crosssectional geometry (non-fluted versus fluted) had some effects in the shape of the propagated wave. Due to presence of preload it was seen that the magnitude of stress and displacement were shifted towards compression corresponding to the compressive preload value, but the nature of waves remained the same.