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NASA NTRS1985

Exploiting parallel computing with limited program changes using a network of microcomputers

Network computing and multiprocessor computers are two discernible trends in parallel processing. The computational behavior of an iterative distributed process in which some subtasks are completed later than others because of an imbalance in computational requirements is of significant interest. The effects of asynchronus processing was studied. A small existing program was converted to perform finite element analysis by distributing substructure analysis over a network of four Apple IIe microcomputers connected to a shared disk, simulating a parallel computer. The substructure analysis uses an iterative, fully stressed, structural resizing procedure. A framework of beams divided into three substructures is used as the finite element model. The effects of asynchronous processing on the convergence of the design variables are determined by not resizing particular substructures on various iterations.

Rogers, J. L., Jr.↗

NASA NTRS1985

Housing flexibility effects on rotor stability

Preliminary rotordynamic evaluations are performed with a housing stiffness assumption that is typically determined only after the hardware is built. In addressing rotor stability, a rigid housing assumption was shown to predict an instability at a lower spin speed than a comparable flexible housing analysis. This rigid housing assumption therefore provides a conservative estimate of the stability threshold speed. A flexible housing appears to act as an energy absorber and dissipated some of the destabilizing force. The fact that a flexible housing is usually asymmetric and considerably heavier than the rotor was related to this apparent increase in rotor stability. Rigid housing analysis is proposed as a valuable screening criteria and may save time and money in construction of elaborate housing finite element models for linear stability analyses.

Davis, L. B.↗

NASA NTRS1985

Selecting step sizes in sensitivity analysis by finite differences

This paper deals with methods for obtaining near-optimum step sizes for finite difference approximations to first derivatives with particular application to sensitivity analysis. A technique denoted the finite difference (FD) algorithm, previously described in the literature and applicable to one derivative at a time, is extended to the calculation of several simultaneously. Both the original and extended FD algorithms are applied to sensitivity analysis for a data-fitting problem in which derivatives of the coefficients of an interpolation polynomial are calculated with respect to uncertainties in the data. The methods are also applied to sensitivity analysis of the structural response of a finite-element-modeled swept wing. In a previous study, this sensitivity analysis of the swept wing required a time-consuming trial-and-error effort to obtain a suitable step size, but it proved to be a routine application for the extended FD algorithm herein.

Iott, J.↗

NASA NTRS1985

Stress-intensity factors for circumferential surface cracks in pipes and rods under tension and bending loads

The purpose of this paper is to present stress-intensity factors for a wide range of nearly semi-elliptical surface cracks in pipes and rods. The configurations were subjected to either remote tension or bending loads. For pipes, the ratio of crack depth to crack length (a/c) ranged from 0.6 to 1; the ratio of crack depth to wall thickness (a/t) ranged from 0.2 to 0.8; and the ratio of internal radius to wall thickness (R/t) ranged from 1 to 10. For rods, the ratio of crack depth to crack length also ranged from 0.6 to 1; and the ratio of crack depth to rod diameter (a/D) ranged from 0.05 to 0.35. These particular crack configurations were chosen to cover the range of crack shapes (a/c) that have been observed in experiments conducted on pipes and rods under tension and bending fatigue loads. The stress-intensity factors were calculated by a three-dimensional finite-element method. The finite-element models employed singularity elements along the crack front and linear-strain elements elsewhere. The models had about 6500 degrees of freedom. The stress-intensity factors were evaluated using a nodal-force method.

Raju, I. S.↗

NASA NTRS1984

A finite element method for nonlinear forced vibrations of beams

Techniques for defining a finite element model (FEM) for analysis of nonlinear vibrations in beam structures subjected to harmonic excitation are presented. The resulting model covers longitudinal deformation and inertial effects. The nonlinear oscillations of a beam element under forced excitation are modeled by a harmonic force matrix based on first order approximations of the Jacobian elliptic forcing function. Harmonic force and nonlinear stiffness matrices are derived and the nonlinear forced responses of beams are calculated under various boundary conditions. The results of FEM computations for simply-supported and clamped beams show that midplane stretching caused by large deflections increases the nonlinearity. Axially-restrained beams experience only hardening nonlinearity, while axially-free beams have reduced nonlinearity in deformation and inertia and an increase in linearity due to large deflection.

Mei, C.↗

NASA NTRS1984

Environmental effects on the dynamics and control of an orbiting large flexible antenna system

Solar radiation pressure on the vibrating antenna structure, temperature gradients induced by solar heating, and stabilizing gravity-gradient torques were considered when the linear regulator theory was used to obtain orientation and shape control of a hoop/column antenna system being considered for the land mobile satellite system. A finite element model of the antenna system which includes all six rigid modes and seven flexible modes was used. Results show that the environmental disturbances affect only the rigid modes of the structure. The effect of solar radiation pressure interacting with the vibrating system is smaller than the solar radiation pressure acting on the thermally deformed structure. To reduce control effect in order to maintain shape and orientation, thermal deformations must be minimized in the preliminary design of the system.

Krishna, R.↗

NASA NTRS1984

Evolution of oceanic lithosphere - A driving force study of the Nazca plate

The role of the evolution of oceanic lithosphere in driving oceanic plates is tested with elastic and viscous finite element modelling of the Nazca plate. The results indicate that ridge-push forces, when distributed over that portion of the oceanic lithosphere giving rise to the force, predict a stress state that varies more spatially than does previous modelling with ridge forces concentrated at the ridge crest. In particular, the component of stress normal to the ridge vanishes near the ridge for the new modelling. Stress states have also been calculated for a variety of slab-pull models, which include possible dependence on age, rate, and angle of subduction. The predicted stresses vary substantially near the trench among the models with less variation farther from the trench.

Richardson, R. M.↗

NASA NTRS1984

The effects of epoxy shrinkage on the Advanced X-ray Astrophysics Facility Technology Mirror Assembly

A method is shown analytically which reduces the effects of epoxy shrinkage for an ultra-high precision X-ray telescope to within the system error budget. The three-dimensional shrinkage effects are discussed with reference to this telescope. The results of the analysis point to the use of an interrupted rather than continuous bond line as the best solution. Discussion of the finite element modelling techniques is included.

Cohen, L. M.↗

NASA NTRS1985

Structural shape optimization - A survey

This paper is a survey of structural shape optimization with an emphasis on techniques dealing with shape optimization of the boundaries of two and three dimensional bodies. Attention is focused on the special problems of structural shape optimization which are due to a finite element model which must change during the optimization process. These problems include the requirement for sophisticated automated mesh generation techniques and careful choice of design variables. They also include special problems in obtaining sufficiently accurate sensitivity derivatives.

Haftka, R. T.↗

NASA NTRS1985

Direct finite element equation solving algorithms

This paper presents and examines direct solution algorithms for the linear simultaneous equations that arise when finite element models represent an engineering system. It identifies the mathematical processing of four solution methods and assesses their data processing implications using concurrent processing.

Melosh, R. J.↗

NASA NTRS1985

A sequential linear optimization approach for controller design

A linear optimization approach with a simple real arithmetic algorithm is presented for reliable controller design and vibration suppression of flexible structures. Using first order sensitivity of the system eigenvalues with respect to the design parameters in conjunction with a continuation procedure, the method converts a nonlinear optimization problem into a maximization problem with linear inequality constraints. The method of linear programming is then applied to solve the converted linear optimization problem. The general efficiency of the linear programming approach allows the method to handle structural optimization problems with a large number of inequality constraints on the design vector. The method is demonstrated using a truss beam finite element model for the optimal sizing and placement of active/passive-structural members for damping augmentation. Results using both the sequential linear optimization approach and nonlinear optimization are presented and compared. The insensitivity to initial conditions of the linear optimization approach is also demonstrated.

Horta, L. G.↗

NASA NTRS1985

Evaluation of a pulse control law for flexible spacecraft

The following analytical and experimental studies were conducted: (1) A simple algorithm was developed to suppress the structural vibrations of 3-dimensional distributed parameter systems, subjected to interface motion and/or directly applied forces. The algorithm is designed to cope with structural oscillations superposed on top of rigid-body motion: a situation identical to that encountered by the SCOLE components. A significant feature of the method is that only local measurements of the structural displacements and velocities relative to the moving frame of reference are needed. (2) A numerical simulation study was conducted on a simple linear finite element model of a cantilevered plate which was subjected to test excitations consisting of impulsive base motion and of nonstationary wide-band random excitation applied at its root. In each situation, the aim was to suppress the vibrations of the plate relative to the moving base. (3) A small mechanical model resembling an aircraft wing was designed and fabricated to investigate the control algorithm under realistic laboratory conditions.

Source record↗

NASA NTRS1983

HOST liner cyclic facilities

The HOST Liner Cyclic Program is utilizing two types of test apparatus, rectangular box rigs and a full annular rig. To date two quartz lamp cyclic box rigs have been tested and a third is to begin testing in late October 1983. The box rigs are used to evaluate 5x8 inch rectangular linear samples. A 21 inch diameter outer liner simulator is also being built up for testing beginning in April 1984. All rigs are atmospheric rigs. The first box rig, a three 6-kVA lamp installation, was operated under adverse conditions to determine feasibility of using quartz lamps for cyclic testing. This work was done in December 1981 and looked promising. The second box rig, again using three 6-kVA lamps, was operated to obtain instrumentation durability information and initial data input to a Finite Element Model. This limited test program was conducted in August 1983. Five test plates were run. Instrumentation consisted of strain gages, thermocouples and thermal paint. The strain gages were found to fail at 1200 F as expected though plates were heated to 1700 F. The third box rig, containing four 6-kVA lamps, is in build up for testing to begin in late October 1983. In addition to 33 percent greater power input, this rig has provision for 400 F backside line cooling air and a viewing port suitable for IR camera viewing. The casing is also water cooled for extended durability.

Schultz, D.↗

NASA NTRS1985

Lightweight two-stroke cycle aircraft diesel engine technology enablement program, volume 1

An experimental Single Cylinder Test Engine Program is conducted to confirm the analytically projected performance of a two-stroke cycle diesel engine for aircraft applications. The test engine delivered 78kW indicated power from 1007cc displacement, operating at 3500 RPM on Schnuerle loop scavenged two-stroke cycle. Testing confirms the ability of a proposed 4-cylinder version of such an engine to reach the target power at altitude, in a highly turbocharged configuration. The experimental program defines all necessary parameters to permit design of a multicylinder engine for eventual flight applications; including injection system requirement, turbocharging, heat rejection, breathing, scavenging, and structural requirements. The multicylinder engine concept is configured to operate with an augmented turbocharger, but with no primary scavenge blower. The test program is oriented to provide a balanced turbocharger compressor to turbine power balance without an auxiliary scavenging system. Engine cylinder heat rejection to the ambient air has been significantly reduced and the minimum overall turbocharger efficiency required is within the range of commercially available turbochargers. Analytical studies and finite element modeling is made of insulated configurations of the engines - including both ceramic and metallic versions. A second generation test engine is designed based on current test results.

Freen, P. D.↗

NASA NTRS1985

Increasing productivity of the McAuto CAD/CAE system by user-specific applications programming

Significant improvements in the productivity of the McAuto Computer-Aided Design/Computer-Aided Engineering (CAD/CAE) system were achieved by applications programming using the system's own Graphics Interactive Programming language (GRIP) and the interface capabilities with the main computer on which the system resides. The GRIP programs for creating springs, bar charts, finite element model representations and aiding management planning are presented as examples.

Plotrowski, S. M.↗

NASA NTRS1986

Dynamic characteristics of two 300 kW class dual keel space station concepts

Results from an investigation of the dynamic behavior of a 300 kW class solar dynamic powered, dual kell space station are presented. The purpose of the investigation was to determine and assess the influence of space station truss bay size on station controllability during rigid body attitude adjustment and orbit reboost maneuvers. The dual keel space station concept is defined and two finite element models (one which has a truss bay size of 5m and another with a truss bay size of 9 feet) are described. Rigid and flexible body characteristics of the two space station models are also presented. Finally, results from a transient response analysis, where the stations are subjected to an orbit reboost maneuver, are summarized.

Dorsey, J. T.↗

NASA NTRS1985

Composite laminate free edge reinforcement concepts

The presence of a free edge in a laminated composite structure can result in delamination of the composite under certain loading conditions. Linear finite element analysis predicts large or even singular interlaminar stresses near the free edge. Edge reinforcements which will reduce these interlaminar stresses, prevent or delay the onset of delaminations, and thereby increase the strength and life of the structure were studied. Finite element models are used to analyze reinforced laminates which were subsequently fabricated and loaded to failure in order to verify the analysis results.

Howard, W. E.↗

NASA NTRS1986

Stress concentration investigations using NASTRAN

Parametic investigations are performed using several two dimensional finite element formulations to determine their suitability for use in predicting extremum stresses in marine propellers. Comparisons are made of two NASTRAN elements (CTRIM6 and CTRAIA2) wherein elasticity properties have been modified to yield plane strain results. The accuracy of the elements is investigated by comparing finite element stress predictions with experimentally determined stresses in two classical cases: (1) tension in a flat plate with a circular hole; and (2) a filleted flat bar subjected to in-plane bending. The CTRIA2 element is found to provide good results. The displacement field from a three dimensional finite element model of a representative marine propeller is used as the boundary condition for the two dimensional plane strain investigations of stresses in the propeller blade and fillet. Stress predictions from the three dimensional analysis are compared with those from the two dimensional models. The validity of the plane strain modifications to the NASTRAN element is checked by comparing the modified CTRIA2 element stress predictions with those of the ABAQUS plane strain element, CPE4.

Gillcrist, M. C.↗