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NASA NTRS1990

On the nature of the interaction between structures and proof-mass actuators

This paper presents an analysis of the interaction between a structure, an actuator used to control the vibration of the structure, and the control law to be implemented by the actuator. The control hardware used is a proof-mass actuator with experimentally verified dynamics capable of being used in a space structure configuration. A local rate-feedback control law is used. The control of two different structures is presented. The first structure is a cantilevered beam constructed of a quasi-isotropic composite material that is controlled by a single actuator forming the experimental component of the investigation. The second structure is a finite-element model of a truss system controlled by a single actuator. Models of both structures predict the presence of potential instabilities in system performance if proper consideration is not given to interactions between the control law, the structure, and the actuator.

Zimmerman, David C.↗

NASA NTRS1989

Interlaminar fracture toughness of thermoplastic composites

Edge delamination tension and double cantilever beam tests were used to characterize the interlaminar fracture toughness of continuous graphite-fiber composites made from experimental thermoplastic polyimides and a model thermoplastic. Residual thermal stresses, known to be significant in materials processed at high temperatures, were included in the edge delamination calculations. In the model thermoplastic system (polycarbonate matrix), surface properties of the graphite fiber were shown to be significant. Cricital strain energy release rates for two different fibers having similar nominal tensile properties differed by 30 to 60 percent. The reason for the difference is not clear. Interlaminar toughness values for the thermoplastic polyimide composites (LARC-TPI and polyimidesulfone) were 3 to 4 in-lb/sq in. Scanning electron micrographs of the EDT fracture surfaces suggest poor fiber/matrix bonding. Residual thermal stresses account for up to 32 percent of the strain energy release in composites made from these high-temperature resins.

Hinkley, Jeffrey A.↗

NASA NTRS1990

Measurements of radiated elastic wave energy from dynamic tensile cracks

The role of fracture-velocity, microstructure, and fracture-energy barriers in elastic wave radiation during a dynamic fracture was investigated in experiments in which dynamic tensile cracks of two fracture cofigurations of double cantilever beam geometry were propagating in glass samples. The first, referred to as primary fracture, consisted of fractures of intact glass specimens; the second configuration, referred to as secondary fracture, consisted of a refracture of primary fracture specimens which were rebonded with an intermittent pattern of adhesive to produce variations in fracture surface energy along the crack path. For primary fracture cases, measurable elastic waves were generated in 31 percent of the 16 fracture events observed; the condition for radiation of measurable waves appears to be a local abrupt change in the fracture path direction, such as occurs when the fracture intersects a surface flaw. For secondary fractures, 100 percent of events showed measurable elastic waves; in these fractures, the ratio of radiated elastic wave energy in the measured component to fracture surface energy was 10 times greater than for primary fracture.

Boler, Frances M.↗

NASA NTRS1990

Differential equation based method for accurate approximations in optimization

This paper describes a method to efficiently and accurately approximate the effect of design changes on structural response. The key to this new method is to interpret sensitivity equations as differential equations that may be solved explicitly for closed form approximations, hence, the method is denoted the Differential Equation Based (DEB) method. Approximations were developed for vibration frequencies, mode shapes and static displacements. The DEB approximation method was applied to a cantilever beam and results compared with the commonly-used linear Taylor series approximations and exact solutions. The test calculations involved perturbing the height, width, cross-sectional area, tip mass, and bending inertia of the beam. The DEB method proved to be very accurate, and in msot cases, was more accurate than the linear Taylor series approximation. The method is applicable to simultaneous perturbation of several design variables. Also, the approximations may be used to calculate other system response quantities. For example, the approximations for displacement are used to approximate bending stresses.

Pritchard, Jocelyn I.↗

NASA NTRS1990

A technique for predicting mode I energy release rates using a first-order shear deformable plate theory

Utilizing a first order shear deformable plate theory, a technique is described for predicting the distribution of the energy release rate along a curved or straight mode I planar crack in the plane of a plate (such as a delamination crack). Accuracy of the technique is assessed by comparing the distributions of energy release rate with those predicted by two- and three-dimensional finite element analyses of double cantilever beam specimens with straight crack fronts.

Davidson, B. D.↗

NASA NTRS1990

Mode I delamination of a three-dimensional fabric composite

Finite-element analysis was conducted on a double-cantilever-beam (DCB) specimen of carbon-epoxy (T300/3501-6) orthogonal interlocked fabric composites. The mode I strain-energy release rate G(I) was evaluated to investigate the influence of through-the-thickness (z-axis) fibers on crack driving force as a function of crack length. The analysis showed good agreement with experimental results for both compliance and G(I). The presence of z-axis fibers sharply reduced G(I) in comparison to the traditional two-dimensional laminated material. A parametric study revealed that increasing the z-axis fiber stiffness reduces G(I). Progressive debonding of the z-axis reinforcement was also modeled. G(I) increased as the fiber debonded but was significantly less than in the two-dimensional laminated material in all cases.

Byun, Joon-Hyung↗

NASA NTRS1990

Experimental implementation of the modified independent modal space control method

An experimental realization of a modified independent modal space control (MIMSC) method to control the vibration of a flexible cantilevered beam is presented. In its operation the method relies on the use of one piezoelectric actuator to control several vibration modes through a time-sharing strategy. The effectiveness of the MIMSC method in damping out the beam vibration is demonstrated by comparing the results with those obtained by other modal control methods. Two methods are considered, the independent modal space control method and the pseudo-inverse method. The feasibility of the MIMSC method as a viable alternative for controlling large flexible structures with a very small number of actuators is emphasized.

Baz, A.↗

NASA NTRS1990

Characterization of Mode I and Mode II delamination growth and thresholds in AS4/PEEK composites

Composite materials often fail by delamination. The onset and growth of delamination in AS4/PEEK, a tough thermoplastic matrix composite, was characterized for mode 1 and mode 2 loadings, using the Double Cantilever Beam (DCB) and the End Notched Flexure (ENF) test specimens. Delamination growth per fatigue cycle, da/dN, was related to strain energy release rate, G, by means of a power law. However, the exponents of these power laws were too large for them to be adequately used as a life prediction tool. A small error in the estimated applied loads could lead to large errors in the delamination growth rates. Hence strain energy release rate thresholds, G sub th, below which no delamination would occur were also measured. Mode 1 and 2 threshold G values for no delamination growth were found by monitoring the number of cycles to delamination onset in the DCB and ENF specimens. The maximum applied G for which no delamination growth had occurred until at least 1,000,000 cycles was considered the threshold strain energy release rate. Comments are given on how testing effects, facial interference or delamination front damage, may invalidate the experimental determination of the constants in the expression.

Martin, Roderick H.↗

NASA NTRS1990

Interface effects in interlaminar fracture of thermoplastic composites

Mode I interlaminar fracture toughness was determined on a series of unidirectional poly(phenylene oxide)/carbon fiber composites using the double cantilever beam test. Initial toughness for growth of a delamination from an insert depends on fiber type, and ranges from 190 to 440 J/sq m, with intermediate modulus fibers tending to give lower values than high-strain fibers. Low toughness values are attributed to poor fiber-matrix adhesion. As a delamination progress down the beam, fiber bridging increases the apparent toughness by up to a factor of 7, depending on the fiber.

Hinkley, J. A.↗

NASA NTRS1991

Grasping-Force Sensor For Robot Hand

Grasping-force sensor designed for use on robot hand including interdigitating claws required to remain in alignment. Sensor integral part of grasping claw assembly on robot hand. Base frames deflect elastically, partly in manner of cantilever beam and partly in manner of parallelogram linkage. Strain gauges measure grasping forces by measuring deflections. Operator feels initial contact between claws and object and modifies motions of robot arm, hand and claws accordingly.

Scheinman, Victor D.↗

NASA NTRS1990

A technique for optimal temperature estimation for modeling sunrise/sunset thermal snap disturbance torque

A predictive temperature estimation technique which can be used to drive a model of the Sunrise/Sunset thermal 'snap' disturbance torque experienced by low Earth orbiting spacecraft is described. The twice per orbit impulsive disturbance torque is attributed to vehicle passage in and out of the Earth's shadow cone (umbra), during which large flexible appendages undergo rapidly changing thermal conditions. Flexible members, in particular solar arrays, experience rapid cooling during umbra entrance (Sunset) and rapid heating during exit (Sunrise). The thermal 'snap' phenomena has been observed during normal on-orbit operations of both the LANDSAT-4 satellite and the Communications Technology Satellite (CTS). Thermal 'snap' has also been predicted to be a dominant source of error for the TOPEX satellite. The fundamental equations used to model the Sunrise/Sunset thermal 'snap' disturbance torque for a typical solar array like structure will be described. For this derivation the array is assumed to be a thin, cantilevered beam. The time varying thermal gradient is shown to be the driving force behind predicting the thermal 'snap' disturbance torque and therefore motivates the need for accurate estimates of temperature. The development of a technique to optimally estimate appendage surface temperature is highlighted. The objective analysis method used is structured on the Gauss-Markov Theorem and provides an optimal temperature estimate at a prescribed location given data from a distributed thermal sensor network. The optimally estimated surface temperatures could then be used to compute the thermal gradient across the body. The estimation technique is demonstrated using a typical satellite solar array.

Zimbelman, D. F.↗

NASA NTRS1991

Effect of initial delamination on Mode 1 and Mode 2 interlaminar fracture toughness and fatigue fracture threshold

Static and fatigue double-cantilever beam (DCB) and end-notch flexure (ENF) tests were conducted to determine the effect of the simulated initial delamination in interlaminar fracture toughness, G(sub c), and fatigue fracture threshold, G(sub th). Unidirectional, 24-ply specimens of S2/SP250 glass/epoxy were tested using Kapton inserts of four different thickness - 13, 25, 75, and 130 microns, at the midplane at one end, or with tension or shear precracks, to simulate an initial delamination. To determine G(sub c), the fatigue fracture threshold below which no delamination growth would occur in less than 1 x 10(exp 6) cycles, fatigue tests were conducted by cyclically loading specimens until delamination growth was detected. Consistent values of model 1 fracture toughness, G(sub Ic), were measured from DCB specimens with inserts of thickness 75 microns or thinner, or with shear precracks. The fatigue DCB tests gave similar values of G(sub Ith) for the 13, 25, and 75 microns specimens. Results for the shear precracked specimens were significantly lower that for specimens without precracks. Results for both the static and fatigue ENF tests showed that measured G(IIc) and G(IIth) values decreased with decreasing insert thickness, so that no limiting thickness could be determined. Results for specimens with inserts of 75 microns or thicker were significantly higher than the results for precracked specimens or specimens with 13 or 25 microns inserts.

Murri, Gretchen Bostaph↗

NASA NTRS1991

Damage prediction in cross-plied curved composite laminates

Analytical and experimental work is detailed which is required to predict delamination onset and growth in a curved cross plied composite laminate subjected to static and fatigue loads. The composite used was AS4/3501/6, graphite/epoxy. Analytically, a closed form stress analysis and 2-D and 3-D finite element analyses were conducted to determine the stress distribution in an undamaged curved laminate. The finite element analysis was also used to determine values of strain energy release rate at a delamination emanating from a matrix crack in a 90 deg ply. Experimentally, transverse tensile strength and fatigue life were determined from flat 90 deg coupons. The interlaminar tensile strength and fatigue life were determined from double cantilevered beam specimens. Cross plied curved laminates were tested statically and in fatigue to give a comparison to the analytical predictions. A comparison of the fracture mechanics life prediction technique and the strength based prediction technique is given.

Martin, Roderick H.↗

NASA NTRS1989

Independent modal space control with positive position feedback

An independent modal space control (IMSC) algorithm is presented, whose modal control forces are generated from a positive position feedback (PPF) strategy. The proposed algorithm combines the attributes of both the IMSC and the PPF, and maintains the simplicity of the IMSC as it designs the controller of a complex structure at the uncoupled modal level. The effectiveness of the algorithm in damping out the vibration of flexible structures is validated experimentally. A simple cantilevered beam is employed as an example of a flexible structure whose multimodes of vibration are controlled by a single actuator. Performance of the active control system is determined in the frequency and the time domains. The experimental results indicate the potential of the proposed methodology as a viable method for controlling the vibration of large flexible structures.

Baz, A.↗

NASA NTRS1991

Sensitivity of control-augmented structure obtained by a system decomposition method

The verification of a method for computing sensitivity derivatives of a coupled system is presented. The method deals with a system whose analysis can be partitioned into subsets that correspond to disciplines and/or physical subsystems that exchange input-output data with each other. The method uses the partial sensitivity derivatives of the output with respect to input obtained for each subset separately to assemble a set of linear, simultaneous, algebraic equations that are solved for the derivatives of the coupled system response. This sensitivity analysis is verified using an example of a cantilever beam augmented with an active control system to limit the beam's dynamic displacements under an excitation force. The verification shows good agreement of the method with reference data obtained by a finite difference technique involving entire system analysis. The usefulness of a system sensitivity method in optimization applications by employing a piecewise-linear approach to the same numerical example is demonstrated. The method's principal merits are its intrinsically superior accuracy in comparison with the finite difference technique, and its compatibility with the traditional division of work in complex engineering tasks among specialty groups.

Sobieszczanski-Sobieski, Jaroslaw↗

NASA NTRS1990

A silicon micromachined piezoresistive accelerometer for health and condition monitoring

Silicon micromachining etching techniques were utilized to batch-fabricate hundreds of general purpose microaccelerometers on a single silicon substrate. Piezoresistive sensing elements were aligned to the back-side patterns using an IR mask aligner and then diffused into the areas of maximum stress. Capping of the two-arm cantilever beam structure was achieved using a combination of electrostatic bonding and low temperature glass films. Overrange protection, critical damping, and overall protection from the outside environment are achieved by controlling the cavity depths of the top and bottom covers. Temperature compensation, amplification, and filtering are performed by a companion LSI chip that is interfaced to the accelerometer by conventional wire-bonding techniques.

Walsh, Kevin M.↗

NASA NTRS1991

General shape optimization capability

A method is described for calculating shape sensitivities, within MSC/NASTRAN, in a simple manner without resort to external programs. The method uses natural design variables to define the shape changes in a given structure. Once the shape sensitivities are obtained, the shape optimization process is carried out in a manner similar to property optimization processes. The capability of this method is illustrated by two examples: the shape optimization of a cantilever beam with holes, loaded by a point load at the free end (with the shape of the holes and the thickness of the beam selected as the design variables), and the shape optimization of a connecting rod subjected to several different loading and boundary conditions.

Chargin, Mladen K.↗

NASA NTRS1991

Damping of structural vibrations with piezoelectric materials and passive electrical networks

The possibility of dissipating mechanical energy with piezoelectric material shunted with passive electrical circuits is investigated. The effective mechanical impedance for the piezoelectric element shunted by an arbitrary circuit is derived. The shunted piezoelectric is shown to possess frequency dependent stiffness and loss factor which are also dependent on the shunting circuit. The generally shunted model is specialized for two shunting circuits: the case of a resistor alone and that of a resistor and inductor. For resistive shunting, the material properties exhibit frequency dependence similar to viscoelastic materials, but are much stiffer and more independent of temperature. Shunting with a resistor and inductor introduces an electrical resonance, which can be optimally tuned to structural resonances in a manner analogous to a mechanical vibration arsorber. Techniques for analyzing systems which incorporate these shunting cases are presented and applied to a cantilevered beam experiment. The experimental results for both the resistive and resonant shunting circuits validate the shunted piezoelectric damping models.

Hagood, N. W.↗