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At least 811 records · Page 45

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NASA NTRS1986

Elastic analysis of a mode II fatigue crack test specimen

Elastic displacements and stress intensity measurements for a mode II specimen have been obtained over a range of a/W values between 0.500 and 0.900 using the MARC general purpose finite element program. Stress intensity factors were experimentally determined using load point displacement values. Good general agreement between numerical and experimental results for crack mouth, crack surface, and load point displacements, and for stress intensity factors, demonstrates the accuracy of the present method.

Gross, B.↗

NASA NTRS1987

The formation of current sheets in the solar atmosphere

A simple model of the effect of slow photospheric motions on the coronal magnetic field is examined. In this model, a magnetic field is stretched between two conducting plates and slow tangential displacements of the plates cause the field to evolve quasi-statically. If the field is treated as a continuum, the interior magnetic topology is uniquely specified by the displacement of the boundaries, while the topology of a field made up of discrete flux tubes is not determined by the boundary displacement. Treating the field as a continuum, it is found that small perturbations of a constant vertical or weakly sheared field can come to an equilibrium free of singularities if the separation between the plates is finite.

Zweibel, Ellen G.↗

NASA NTRS1988

The analysis of non-linear dynamic behavior (including snap-through) of postbuckled plates by simple analytical solution

Static postbuckling and nonlinear dynamic analysis of plates are usually accomplished by multimode analyses, although the methods are complicated and do not give straightforward understanding of the nonlinear behavior. Assuming single-mode transverse displacement, a simple formula is derived for the transverse load displacement relationship of a plate under in-plane compression. The formula is used to derive a simple analytical expression for the static postbuckling displacement and nonlinear dynamic responses of postbuckled plates under sinusoidal or random excitation. Regions with softening and hardening spring behavior are identified. Also, the highly nonlinear motion of snap-through and its effects on the overall dynamic response can be easily interpreted using the single-mode formula. Theoretical results are compared with experimental results obtained using a buckled aluminum panel, using discrete frequency and broadband point excitation. Some important effects of the snap-through motion on the dynamic response of the postbuckled plates are found.

Ng, C. F.↗

NASA NTRS1988

Fracture resistance of a TiB2 particle/SiC matrix composite at elevated temperature

The fracture resistance of a commercial TiB 2 particle/SiC matrix composite was evaluated at temperatures ranging from 20 to 1400 °C. A laser interferometric strain gauge (LISG) was used to continuously monitor the crack mouth opening displacement (CMOD) of the chevron-notched and straight-notched, three-point bend specimens used. Crack growth resistance curves (R-curves) were determined from the load versus displacement curves and displacement calibrations. Fracture toughness, work-of-fracture, and R-curve levels were found to decrease with increasing temperature. Microstructure, fracture surface, and oxidation coat were examined to explain the fracture behavior.

Fracture↗

NASA NTRS1987

Quantitative use of satellite SAR imagery of sea ice

An automatic technique to obtain ice motion based on maximizing cross-correlation and employing a strategy of nested calculations has been developed and used to obtain displacements of more than 2000 tie-points on a 2 km mesh grid. The mean displacement error is 250 m and is as low as manual data error of 150 m on rigid pieces. The method allows tie-points to be found on a regular grid in a fully automatic fashion. Full resolution data are not needed to maximize the accuracy of displacement measurements, a result which is significant for SAR images, with their large amounts of data.

Fily, M.↗

NASA NTRS1988

Effect of manipulator and feel system characteristics on pilot performance in roll tracking

The control system features of force vs displacement stick command, feel/force displacement gradient, 'feel' system vs command prefilter dynamic lag, and flight control system effective time delay, have been investigated by a fixed-base simulation employing both joysticks and side-sticks in order to ascertain their effect on pilot opinion, pilot performance, and their hand/arm neuromuscular subsystem interaction. The results obtained indicate that the use of displacement sensing added the feel system dynamic lag to the command path and increased effective system flight delay, thereby lowering pilot opinion and performance but reducing the possibility of roll instability; the use of force sensing as stick command to exclude feel system dynamic lag had the obverse effect.

Aponso, Bimal L.↗

NASA NTRS1987

Fatigue crack growth with single overload - Measurement and modeling

This paper compares experiments with an analytical model of fatigue crack growth under variable amplitude. The stereoimaging technique was used to measure displacements near the tips of fatigue cracks undergoing simple variations in load amplitude-single overloads and overload/underload combinations. Measured displacements were used to compute strains, and stresses were determined from the strains. Local values of crack driving force (Delta-K effective) were determined using both locally measured opening loads and crack tip opening displacements. Experimental results were compared with simulations made for the same load variation conditions using Newman's FAST-2 model. Residual stresses caused by overloads, crack opening loads, and growth retardation periods were compared.

Davidson, D. L.↗

NASA NTRS1989

Effect of adhesive interleaving and discontinuous plies on failure of composite laminates subject to transverse normal loads

Results of a series of tests to determine the effects of adhesive interleaving and discontinuous plies (plies with end-to-end gaps) on the displacements, failure loads and failure modes of graphite-epoxy laminates subjected to transverse normal loads are presented. Adhesive interleaving can be used to contain local damage within a group of plies, i.e., to arrest crack propagation on the interlaminate level, and it can increase the amount of normal displacement the laminate can withstand before failure. However, the addition of adhesive interleaving to a laminate does not significantly increase its load carrying capability. A few discontinuous plies in a laminate can reduce the normal displacement and load at failure by 10 to 40 percent compared to a laminate with no discontinuous plies, but the presence of the ply discontinuities does not generally change the failure location or the failure mode of the laminate.

Jegley, Dawn C.↗

NASA NTRS1989

Dynamics of articulated structures

An articulated structure is defined as an assembly of flexible bodies that may be coupled by kinematic connections and force elements that permit large relative displacement and rotation. Kinematics of such systems is defined using one reference frame for each body in the system and deformation modal coordinates that define displacement fields within flexible bodies. Deformation kinematics are defined by both elastic vibration and static correction deformation modes. Linear elastic deformation is presumed; i.e., a linear stress-strain relation is valid and relative displacements within each elastic component are small enough so that the theory of linear elasticity applies. Coupling of reference and modal coordinates leads to a system of nonlinear equations of motion. Methods of automatically generating and solving these equations of motion are outlined.

Haug, Edward J.↗

NASA NTRS1989

A weak Hamiltonian finite element method for optimal control problems

A temporal finite element method based on a mixed form of the Hamiltonian weak principle is developed for dynamics and optimal control problems. The mixed form of Hamilton's weak principle contains both displacements and momenta as primary variables that are expanded in terms of nodal values and simple polynomial shape functions. Unlike other forms of Hamilton's principle, however, time derivatives of the momenta and displacements do not appear therein; instead, only the virtual momenta and virtual displacements are differentiated with respect to time. Based on the duality that is observed to exist between the mixed form of Hamilton's weak principle and variational principles governing classical optimal control problems, a temporal finite element formulation of the latter can be developed in a rather straightforward manner. Several well-known problems in dynamics and optimal control are illustrated. The example dynamics problem involves a time-marching problem. As optimal control examples, elementary trajectory optimization problems are treated.

Hodges, Dewey H.↗

NASA NTRS1988

Design considerations of manipulator and feel system characteristics in roll tracking

A fixed-base simulation was performed to identify and quantify interactions between the pilot's hand/arm neuromuscular subsystem and such control system features of typical modern fighter aircraft roll rate command mechanizations as: (1) force versus displacement sensing side-stick type manipulator, (2) feel force/displacement gradient, (3) feel system versus command prefilter dynamic lag, and (4) flight control system effective time delay. The experiment encompassed some 48 manipulator/filter/aircraft configurations. Displacement side-stick experiment results are given and compared with the previous force sidestick experiment results. Attention is focused on control bandwidth, excitement (peaking) of the neuromuscular mode, feel force/displacement gradient effects, time delay effects, etc. Section 5 is devoted to experiments with a center-stick in which force versus displacement sensing, feel system lag, and command prefilter lag influences on tracking performance and pilot preference are investigated.

Johnston, Donald E.↗

NASA NTRS1989

Fracture resistance of a TiB2 particle/SiC matrix composite at elevated temperature

The fracture resistance of a commercial TiB2 particle/SiC matrix composite was evaluated at temperatures ranging from 20 to 1400 C. A laser interferometric strain gauge (LiSG) was used to continuously monitor the crack mouth opening displacement (CMOD) of the chevron-notched and straight-notched, three-point bend specimens used. Crack growth resistance curves (R-curves) were determined from the load versus displacement curves and displacement calibrations. Fracture toughness, work-of-fracture, and R-curve levels were found to decrease with increasing temperature. Microstructure, fracture surface, and oxidation coat were examined to explain the fracture behavior.

Jenkins, Michael G.↗

NASA NTRS1989

Radiation damage in scientific charge-coupled devices

Radiation damage is reported on CCDs (charge-coupled devices) that have been primarily exposed to 1.25-MeV gamma rays (Co-60 source). Two important classes of radiation damage are discussed, namely, bulk and ionization effects. Bulk damage or displacement damage is a process in which silicon atoms are displaced from their normal lattice positions by high-energy photons or particles. Single atomic displacements or cluster defect damage is produced, depending on the energy and type of radiation experienced by the detector. Bulk damage creates trapping sites within the CCD's signal channel which in turn degrades charge-transfer efficiency.

Janesick, James↗

NASA NTRS1989

Active Control of the Forced and Transient Response of a Finite Beam

When studying structural vibrations resulting from a concentrated source, many structures may be modelled as a finite beam excited by a point source. The theoretical limit on cancelling the resulting beam vibrations by utilizing another point source as an active controller is explored. Three different types of excitation are considered, harmonic, random, and transient. In each case, a cost function is defined and minimized for numerous parameter variations. For the case of harmonic excitation, the cost function is obtained by integrating the mean squared displacement over a region of the beam in which control is desired. A controller is then found to minimize this cost function in the control interval. The control interval and controller location are continuously varied for several frequencies of excitation. The results show that control over the entire beam length is possible only when the excitation frequency is near a resonant frequency of the beam, but control over a subregion may be obtained even between resonant frequencies at the cost of increasing the vibration outside of the control region. For random excitation, the cost function is realized by integrating the expected value of the displacement squared over the interval of the beam in which control is desired. This is shown to yield the identical cost function as obtained by integrating the cost function for harmonic excitation over all excitation frequencies. As a result, it is always possible to reduce the cost function for random excitation whether controlling the entire beam or just a subregion, without ever increasing the vibration outside the region in which control is desired. The last type of excitation considered is a single, transient pulse. A cost function representative of the beam vibration is obtained by integrating the transient displacement squared over a region of the beam and over all time. The form of the controller is chosen a priori as either one or two delayed pulses. Delays constrain the controller to be causal. The best possible control is then examined while varying the region of control and the controller location. It is found that control is always possible using either one or two control pulses. The two pulse controller gives better performance than a single pulse controller, but finding the optimal delay time for the additional controllers increases as the square of the number of control pulses.

Post, John Theodore↗

NASA NTRS1988

Hybrid deployable support truss designs for LDR

Concepts for a 20-meter diameter Large Deployable Reflector (LDR) deployable truss backup structure, and analytical predictions of its structural characteristics are discussed. The concept shown is referred to as the SIXPAC; It is a combination of the PACTRUSS concept and a single-fold beam, which would make up the desired backup structure. One advantage of retaining the PACTRUSS concept is its packaging density and its capability for synchronous deployment. Various 2-meter hexagonal panel arrangements are possible for this Hybrid PACTRUSS structure depending on the panel-to-structure attachment strategies used. Static analyses of the SIXPAC using various assumptions for truss designs and panel masses of 10 kg sq meters were performed to predict the tip displacement of the structure when supported at the center. The tip displacement ranged from 0.20 to 0.44 mm without the panel mass, and from 0.9 to 3.9 mm with the panel mass (in a 1-g field). The data indicate that the structure can be adequately ground tested to validate its required performance in space, assuming the required performance in space is approximately 100 microns. The static displacement at the tip of the structure when subjected to an angular acceleration of 0.001 rad/sec squared were estimated to range from 0.8 to 7.5 microns, depending on the type of truss elements.

Hedgepeth, J.↗

NASA NTRS1990

Results of aircraft open-loop tests of an experimental magnetic leader cable system for guidance during roll-out and turnoff

An experimental magnetic leader cable (MLC) system designed to measure aircraft lateral displacement from centerline and heading relative to centerline during rollout, turnoff, and taxi was tested at NASA's Wallops Flight Facility using NASA's Transport System Research Vehicle (TSRV), a modified B-737. The MLC system consisted of ground equipment that produced a magnetic field about a wire along runway centerline and airborne equipment that detected the strength and direction of this field and computed displacement and heading. Results of these tests indicate that estimates of aircraft displacement from centerline produced by the magnetic leader cable system using either of the two algorithms appear to be adequate for use by an automatic control system during rollout, turnoff, and taxi. Estimates of heading, however, are not sufficiently accurate for use, probably because of distortion of the magnetic field by the metal aircraft.

Bundick, W. Thomas↗

NASA NTRS1990

Three-dimensional elastic-plastic analysis of shallow cracks in single-edge-crack-tension specimens

Three dimensional, elastic-plastic, finite element results are presented for single-edge crack-tension specimens with several shallow crack-length-to-width ratios (0.05 less than or equal to a/W less than or equal to 0.5). Results showed the need to model the initial yield plateau in the stress-strain behavior to accurately model deformation of the A36 steel specimens. The crack-tip-opening-displacement was found to be linearly proportional to the crack-mouth-opening displacement. A new deformation dependent plastic-eta factor equation is presented for calculating the J-integral from test load-displacement records. This equation was shown to be accurate for all crack lengths considered.

Shivakumar, Kunigal N.↗

NASA NTRS1990

Elevated temperature crack growth in aluminum alloys: Tensile deformation of 2618 and FVS0812 aluminum alloys

Understanding the damage tolerance of aluminum alloys at elevated temperatures is essential for safe applications of advanced materials. The objective of this project is to investigate the time dependent subcritical cracking behavior of powder metallurgy FVS0812 and ingot metallurgy 2618 aluminum alloys at elevated temperatures. The fracture mechanics approach was applied. Sidegrooved compact tension specimens were tested at 175, 250, and 316 C under constant load. Subcritical crack growth occurred in each alloy at applied stress intensity levels (K) of between about 14 and 25 MPa/m, well below K (sub IC). Measured load, crack opening displacement and displacement rate, and crack length and growth rate (da/dt) were analyzed with several continuum fracture parameters including, the C-integral, C (sub t), and K. Elevated temperature growth rate data suggest that K is a controlling parameter during time dependent cracking. For FVS0812, da/dt is highest at 175 C when rates are expressed as a function of K. While crack growth rate is not controlled by C (sub t) at 175 C, da/dt appears to better correlate with C (sub t) at higher temperatures. Creep brittle cracking at intermediate temperatures, and perhaps related to strain aging, is augmented by time dependent transient creep plasticity at higher temperatures. The C (sub t) analysis is, however, complicated by the necessity to measure small differences in the elastic crack growth and creep contributions to the crack opening displacement rate. A microstructural study indicates that 2618 and FVS0812 are likely to be creep brittle materials, consistent with the results obtained from the fracture mechanics study. Time dependent crack growth of 2618 at 175 C is characterized by mixed transgranular and intergranular fracture. Delamination along the ribbon powder particle boundaries occurs in FVS0812 at all temperatures. The fracture mode of FVS0812 changes with temperature. At 175 C, it is characterized as dimpled rupture, and at 316 C as mixed matrix superplastic rupture and matrix-dipersoid debonding.

Leng, Yang↗