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NASA NTRS1988

Tharsis block tectonics on Mars

The concept of block tectonics provides a framework for understanding many aspects of Tharsis and adjoining structures. This Tharsis block tectonics on Mars is manifested partly by mantle-related doming and partly by response to loading by subsequent volcanic construction. Although the origin of the volcanism from beneath Tharsis is a subject of controversy explanations have to include inhomogeneities in Martian internal structure, energy distribution, magma accumulation and motion below the lithosphere. Thermal convection can be seen as a necessary consequence for transient initial phase of Martian cooling. This produced part of the elevated topography with tensional stresses and graben systems radial to the main bulge. The linear grabens, radial to the Tharsis center, can be interpreted to indicate rift zones that define the crustal block boundaries. The load-induced stresses may then have contributed on further graben and ridge formation over an extended period of time.

Raitala, Jouko T.↗

NASA NTRS1989

Integrated aerodynamic/dynamic optimization of helicopter rotor blades

An integrated aerodynamic/dynamic optimization procedure is used to minimize blade weight and 4 per rev vertical hub shear for a rotor blade in forward flight. The coupling of aerodynamics and dynamics is accomplished through the inclusion of airloads which vary with the design variables during the optimization process. Both single and multiple objective functions are used in the optimization formulation. The Global Criteria Approach is used to formulate the multiple objective optimization and results are compared with those obtained by using single objective function formulations. Constraints are imposed on natural frequencies, autorotational inertia, and centrifugal stress. The program CAMRAD is used for the blade aerodynamic and dynamic analyses, and the program CONMIN is used for the optimization. Since the spanwise and the azimuthal variations of loading are responsible for most rotor vibration and noise, the vertical airload distributions on the blade, before and after optimization, are compared. The total power required by the rotor to produce the same amount of thrust for a given area is also calculated before and after optimization. Results indicate that integrated optimization can significantly reduce the blade weight, the hub shear and the amplitude of the vertical airload distributions on the blade and the total power required by the rotor.

Chattopadhyay, Aditi↗

NASA NTRS1989

Efficient sensitivity analysis and optimization of a helicopter rotor

Aeroelastic optimization of a system essentially consists of the determination of the optimum values of design variables which minimize the objective function and satisfy certain aeroelastic and geometric constraints. The process of aeroelastic optimization analysis is illustrated. To carry out aeroelastic optimization effectively, one needs a reliable analysis procedure to determine steady response and stability of a rotor system in forward flight. The rotor dynamic analysis used in the present study developed inhouse at the University of Maryland is based on finite elements in space and time. The analysis consists of two major phases: vehicle trim and rotor steady response (coupled trim analysis), and aeroelastic stability of the blade. For a reduction of helicopter vibration, the optimization process requires the sensitivity derivatives of the objective function and aeroelastic stability constraints. For this, the derivatives of steady response, hub loads and blade stability roots are calculated using a direct analytical approach. An automated optimization procedure is developed by coupling the rotor dynamic analysis, design sensitivity analysis and constrained optimization code CONMIN.

Lim, Joon W.↗

NASA NTRS1990

Dissipation Of Energy In Extension Of Cracks

Dissipation in elastic-plastic material calculated by two-dimensional finite-element method. Analytical procedure developed to calculate various components of dissipation of energy during extension of crack and to relate them to total dissipation of energy computed from global load-displacement response. Observations in study valid for other elastic-plastic fracture situations, including growth of cracks in pressure vessels in nuclear powerplants.

Shivakumar, Kunigal N.↗

NASA NTRS1990

Postbuckling response of long thick plates loaded in compression including higher order transverse shearing effects

Buckling and postbuckling results are presented for compression-loaded simply-supported aluminum plates and composite plates with a symmetric lay-up of thin + or - 45 deg plies composed of many layers. Buckling results for aluminum plates of finite length are given for various length-to-width ratios. Asymptotes to the curves based on buckling results give N(sub xcr) for plates of infinite length. Postbuckling results for plates with transverse shearing flexibility are compared to results from classical theory for various width-to-thickness ratios. Characteristic curves indicating the average longitudinal direct stress resultant as a function of the applied displacements are calculated based on four different theories: Classical von Karman theory using the Kirchoff assumptions, first-order shear deformation theory, higher-order shear deformation theory, and 3-D flexibility theory. Present results indicate that the 3-D flexibility theory gives the lowest buckling loads. The higher-order shear deformation theory has fewer unknowns than the 3-D flexibility theory but does not take into account through-the-thickness effects. The figures presented show that small differences occur in the average longitudinal direct stress resultants from the four theories that are functions of applied end-shortening displacement.

Stein, Manuel↗

NASA NTRS1988

Analysis of nonuniform subsonic flows about a row of moving blades

The motion of a nonuniform flow about a row of moving blades is modeled as that of a three-dimensional rotational disturbance convected by the mean flow. The aerodynamic theories for such flows are reviewed for both linearly propagating disturbances and disturbances distorted by the mean flow. Linear theories results are examined to assess the effects of the gust variables, the Mach number, and the blade spacing, on the unsteady blade response. For loaded blades, the upstream disturbances are distorted by the blade mean flow. For a potential mean flow, the governing equations can be reduced to a single inhomogeneous nonconstant coefficient convective wave equation. A numerical procedure is developed using an elliptic grid system. The accuracy of the solutions strongly depends on the far-field radiation condition and the optimum determination of the grid system. Solutions are presently obtained for thick symmetric airfoils in three-dimensional gusts.

Atassi, H. M.↗

NASA NTRS1990

Postbuckling response of long thick plates loaded in compression including higher order transverse shearing effects

This paper presents buckling and postbuckling results for compression-loaded simply-supported aluminum plates and composite plates with a symmetric lay-up of thin +/-45-deg plies composed of many layers. Buckling results for aluminum plates of finite length are given for various length-to-width ratios. Asymptotes to the curves based on the buckling results give N(XCr) for plates of infinite length. Postbuckling results for plates with transverse shearing flexibility are compared to results from classical theory for various width-to-thickness ratios. Characteristic curves indicating the average longitudinal direct stress resultant as a function of the applied displacements are calculated based on four different theories: classical von Karman theory using the Kirchhoff assumptions, first-order shear deformation theory, higher-order shear deformation theory, and three-dimensional flexibility theory. Present results indicate that the three-dimensional flexibility theory gives the lowest buckling loads. The higher-order shear deformation theory has fewer unknowns than the three-dimensional flexibility theory but does not take into account through-the-thickness effects. The figures presented show that small differences occur in the average longitudinal direct stress resultants from the four theories that are functions of applied end-shortening displacement.

Stein, Manuel↗

NASA NTRS1990

Regolith-structure modeling of lunar facilities

The mechanical properties of lunar regolith are significantly different from terrestrial soils; at low stress levels, the ultimate strength envelope is highly nonlinear, and the material is essentially dilatant. The effects of platen friction, membrane confinement, and material self weight in triaxial tests have been shown to be of great significance in low stress level tests. An inverse identification technique has been devised to account for these effects where the load-displacement response of the entire specimen, including the boundary conditions and the self-weight, was analyzed by nonlinear finite element techniques so as to obtain true material parameters.

Perkins, Steven W.↗

NASA NTRS1992

Crushing characteristics of continuous fiber-reinforced composite tubes

Composite tubes can be reinforced with continuous fibers. When such tubes are subjected to crushing loads, the response is complex and depends on interaction between the different mechanisms that control the crushing process. The modes of crushing and their controlling mechanisms are described. Also, the resulting crushing process and its efficiency are addressed.

Farley, Gary L.↗

NASA NTRS1992

Structurally adaptive space crane concept for assembling space systems on orbit

Many future human space exploration missions will probably require large vehicles that must be assembled on orbit. Thus, a device that can move, position, and assemble large and massive spacecraft components on orbit becomes essential for these missions. A concept is described for such a device: a space crane concept that uses erectable truss hardware to achieve high-stiffness and low-mass booms and uses articulating truss joints that can be assembled on orbit. The hardware has been tested and shown to have linear load-deflection response and to be structurally predictable. The hardware also permits the crane to be reconfigured into different geometries to satisfy future assembly requirements. A number of articulating and rotary joint concepts have been sized and analyzed, and the results are discussed. Two strategies were proposed to suppress motion-induced vibration: placing viscous dampers in selected truss struts and preshaping motion commands. Preliminary analyses indicate that these techniques have the potential to greatly enhance structural damping.

Dorsey, John T.↗

NASA NTRS1992

A structurally adaptive space crane concept for assembling space systems on orbit

A space crane concept is presented which is based on erectable truss hardware to achieve high stiffness and low mass booms and articulating-truss joints which can be assembled on orbit. The hardware is characterized by linear load-deflection response and is structurally predictable. The crane can be reconfigured into different geometries to meet future assembly requirements. Articulating-truss joint concepts with significantly different geometries are analyzed and found to have similar static and dynamic performance, which indicates that criteria other than structural and kinematic performance can be used to select a joint. Passive damping and an open-loop preshaped command input technique greatly enhance the structural damping in the space crane and may preclude the need for an active vibrations suppression system.

Dorsey, John T.↗

NASA NTRS1991

Deformation and failure mechanisms in metal matrix composites

An investigation was undertaken to determine the key deformation mechanisms and their interaction leading to failure of both 0 degree and 90 degree Ti 15-3/SCS-6 laminae under monotonic loading. The experimental results suggest that inelastic deformation in the 0-degree lamina is dominated by plastic deformation and that in the 90-degree lamina is dominated by both fiber-matrix debonding and plasticity. The loading-unloading response, monitoring of Poisson's ratio and microscopy were utilized to identify the key deformation mechanisms. The sequence of deformation mechanisms leading to failure are identified for both the 0 and the 90-degree specimens. The threshold strains for plasticity or damage which are referred to as 'microdeformation' in the 0 deg and 90 deg laminae are approximately 0.004 and 0.002, respectively, at room temperature. These strain levels may be considered critical in initiation based structural design with these composites.

Newaz, G.↗

NASA NTRS1992

Structural characterization of an articulated-truss joint

A first generation space crane articulated-truss joint was statically and dynamically characterized in a configuration which approximated an operational environment. The articulated-truss joint was integrated into a test bed for structural characterization. Static characterization was performed by applying known loads and measuring the corresponding deflections to obtain load-deflection curves. Dynamic characterization was performed using modal testing to experimentally determine the first six mode shapes, frequencies, and modal damping values. The static and dynamic characteristics were also determined for a reference truss, which served as a characterization baseline. Load-deflection curves are presented for the reference truss, and the articulated-truss joint mounted in the test bed. The static and dynamic experimental results are compared to analytical predictions obtained from finite element analyses. The load-deflection response is also presented for one of the linear actuators used in the articulated-truss joint. Finally, an assessment is presented for the predictability of the truss hardware used in the reference truss and articulated-truss joint based upon hardware stiffness properties which were previously obtained during the Precision Segmented Reflector (PSR) Technology Development Program.

Sutter, Thomas R.↗

NASA NTRS1993

Improvements to a method for the geometrically nonlinear analysis of compressively loaded stiffened composite panels

The NLPAN computer code uses a finite-strip approach to the analysis of thin-walled prismatic composite structures such as stiffened panels. The code can model in-plane axial loading, transverse pressure loading, and constant through-the-thickness thermal loading, and can account for shape imperfections. The NLPAN code represents an attempt to extend the buckling analysis of the VIPASA computer code into the geometrically nonlinear regime. Buckling mode shapes generated using VIPASA are used in NLPAN as global functions for representing displacements in the nonlinear regime. While the NLPAN analysis is approximate in nature, it is computationally economical in comparison with finite-element analysis, and is thus suitable for use in preliminary design and design optimization. A comprehensive description of the theoretical approach of NLPAN is provided. A discussion of some operational considerations for the NLPAN code is included. NLPAN is applied to several test problems in order to demonstrate new program capabilities, and to assess the accuracy of the code in modeling various types of loading and response. User instructions for the NLPAN computer program are provided, including a detailed description of the input requirements and example input files for two stiffened-panel configurations.

Stoll, Frederick↗

NASA NTRS1994

The post-pinatubo evolution of stratospheric aerosol surface area density as inferred from SAGE 2

Following the eruption of Mount Pinatubo in June of 1991, the aerosol mass loading of the stratosphere increased from -1 Mt to approximately 30 Mt. This change in aerosol loading was responsible for numerous radiative and chemical changes observed within the stratosphere. As a result, the ability to quantify aerosol properties on a global basis during this period is important. Aerosol surface area density is a critical parameter in governing the rates of heterogeneous reactions, such as ClONO2 plus H2O yields HNO3 plus HOCl, which influence the stratospheric abundance of ozone. Following the eruption of Mt. Pinatubo, measurements by the Stratospheric Aerosol and Gas Experiment (SAGE 2) indicated that the stratospheric aerosol surface area density increased by as much as a factor of 100. Using SAGE 2 multi-wavelength aerosol extinction data, aerosol surface area density as well as mass are derived for the period following the eruption of Mt. Pinatubo through the present.

Poole, L. R.↗

NASA NTRS2000

Comparison of Fatigue Life Estimation Using Equivalent Linearization and Time Domain Simulation Methods

The Monte Carlo simulation method in conjunction with the finite element large deflection modal formulation are used to estimate fatigue life of aircraft panels subjected to stationary Gaussian band-limited white-noise excitations. Ten loading cases varying from 106 dB to 160 dB OASPL with bandwidth 1024 Hz are considered. For each load case, response statistics are obtained from an ensemble of 10 response time histories. The finite element nonlinear modal procedure yields time histories, probability density functions (PDF), power spectral densities and higher statistical moments of the maximum deflection and stress/strain. The method of moments of PSD with Dirlik's approach is employed to estimate the panel fatigue life.

Mei, Chuh↗

NASA NTRS2003

Cryogenic Evaluation of an Advanced DC/DC Converter Module for Deep Space Applications

DC/DC converters are widely used in power management, conditioning, and control of space power systems. Deep space applications require electronics that withstand cryogenic temperature and meet a stringent radiation tolerance. In this work, the performance of an advanced, radiation-hardened (rad-hard) commercial DC/DC converter module was investigated at cryogenic temperatures. The converter was investigated in terms of its steady state and dynamic operations. The output voltage regulation, efficiency, terminal current ripple characteristics, and output voltage response to load changes were determined in the temperature range of 20 to -140 C. These parameters were obtained at various load levels and at different input voltages. The experimental procedures along with the results obtained on the investigated converter are presented and discussed.

Elbuluk, Malik E.↗

NASA NTRS2004

An Experimental Investigation Into the Temperature Profile of a Compliant Foil Air Bearing

A series of tests was performed to determine the internal temperature profile in a compliant bump-type foil journal air bearing operating at room temperature under various speeds and load conditions. The temperature profile was collected by instrumenting a foil bearing with nine, type K thermocouples arranged in the center and along the bearing s edges in order to measure local temperatures and estimate thermal gradients in the axial and circumferential directions. To facilitate the measurement of maximum temperatures from viscous shearing in the air film, the thermocouples were tack welded to the backside of the bumps that were in direct contact with the top foil. The mating journal was coated with a high temperature solid lubricant that, together with the bearing, underwent high temperature start-stop cycles to produce a smooth, steady-state run-in surface. Tests were conducted at speeds from 20 to 50 krpm and loads ranging from 9 to 222 N. The results indicate that, over the conditions tested, both journal rotational speed and radial load are responsible for heat generation with speed playing a more significant role in the magnitude of the temperatures. The temperature distribution was nearly symmetric about the bearing center at 20 and 30 krpm but became slightly skewed toward one side at 40 and 50 krpm. Surprisingly, the maximum temperatures did not occur at the bearing edge where the minimum film thickness is expected but rather in the middle of the bearing where analytical investigations have predicted the air film to be much thicker. Thermal gradients were common during testing and were strongest in the axial direction from the middle of the bearing to its edges, reaching 3.78 8C/mm. The temperature profile indicated the circumferential thermal gradients were negligible.

Radil, Kevin↗