SEARCH · Search NASA

Results for “FAILURES”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 505 records · Page 28

All sources NASA NTRS DOE OSTI

NASA NTRS1985

Evaluation of failure criterion for graphite/epoxy fabric laminates

The development and application of the tensor polynomial failure criterion for composite laminate analysis is described. Emphasis is given to the fabrication and testing of Narmco Rigidite 5208-WT300, a plain weave fabric of Thornel 300 Graphite fibers impregnated with Narmco 5208 Resin. The quadratic-failure criterion with F sub 12=0 provides accurate estimates of failure stresses for the graphite/epoxy investigated. The cubic failure criterion was recast into an operationally easier form, providing design curves that can be applied to laminates fabricated from orthotropic woven fabric prepregs. In the form presented, no interaction strength tests are required, although recourse to the quadratic model and the principal strength parameters is necessary. However, insufficient test data exist at present to generalize this approach for all prepreg constructions, and its use must be restricted to the generic materials and configurations investigated to date.

Tennyson, R. C.↗

NASA NTRS1985

Reliability bounds for fault-tolerant systems with competing responses to component failures

Bounds are established on the probability of system failure for fault-tolerant systems of the type used, for example, in aviation control. Event series leading to system failure are assumed to follow a semi-Markov model in which the potential sojourn times associated with component failures have exponential distributions and those associated with system responses have distributions with unspecified form. A product form of the bounds is derived by using a model that provides for multiple competing system responses to component failures.

Lee, L. D.↗

NASA NTRS1985

Nonlinear response and failure characteristics of internally pressurized composite cylindrical panels

Results of an experimental and analytical study of the nonlinear response and failure characteristics of internally pressurized 4- to 16-ply-thick graphite-epoxy cylindrical panels are presented. Specimens with clamped boundaries simulating the skin between two frames and two stringers of a typical transport fuselage were tested to failure. Failure results of aluminum specimens are compared with the graphite-epoxy test results. The specimens failed at their edges where the local bending gradients and interlaminar stresses are maximum. STAGS nonlinear two-dimensional shell analysis computer code results are used to identify regions of the panels where the response is independent of the axial coordinate. A geometrically nonlinear one-dimensional cylindrical panel analysis was derived and used to determine panel response and interlaminar stresses. Inclusion of the geometric nonlinearity was essential for accurate prediction of panel response. The maximum stress failure criterion applied to the predicted tensile stress in the fiber direction agreed best with the experimentally determined first damage pressures.

Boitnott, R. L.↗

NASA NTRS1985

Techniques for accommodating control effector failures on a mildly statically unstable airplane

Commercial airplanes are becoming increasingly more sophisticated, placing an increasing burden on pilots to detect and resolve the exhaustive set of possible control effector failures. Automatic techniques are needed to either reconfigure an existing control law or restructure a new control law after failure. A discrete control law has been designed for the longitudinal channel of a mildly statically unstable commercial airplane, to track the glideslope during the approach to landing phase of flight. Single effector failures with time delays for failure detection and identification are analyzed for both the reconfigured and restructured control laws, and results are compared with those from previous research using a statically stable airplane. Strategies considered include reconfiguration and restructuring with new flight conditions. Validation of all cases is made using a 6 DOF nonlinear airplane simulation.

Ostroff, A. J.↗

NASA NTRS1985

Recent materials problems relating to catastrophic balloon failures

Balloons fabricated of thin polyethylene materials have provided relatively inexpensive and reliable scientific research platforms for approximately three decades. Reliability of the modern day balloon, as launched by the U.S.A. National Scientific Balloon Facility (NSBF), has been approximately 85 percent. Recent balloon failures, coupled with an increased ocurrence of catastrophic failures, created grave concern over the integrity of the present balloon inventory of the U.S.A. National Aeronautics and Space Administration (NASA). An investigative team was established by NASA to review the circumstances surrounding the catastrophic balloon failures, determine the cause and to make recommendations to correct the problem and to prevent its reoccurrence. The most probable cause of failure as determined by the investigation was the polyethylene balloon film, although the film had passsed the established standard quality control measures of the film manufacturer. The approach, findings, and conclusions of the investigation are presented along with planned procedures to assure future quality balloon film for NASA balloons.

Smith, I. S.↗

NASA NTRS1986

Compression-Failure Mechanisms in Composite Laminates

Failure mechanisms observed using transparent fiberglass/epoxy birefringent materials. Technique based on use of transparent fiberglass/epoxy birefringent material. Transparency allows visual observation of location of initial laminate failure and of subsequent failure propagation; birefringence allows laminate stress distribution to be observed during test and also after test if permanent residual stresses occur. Nondestructive technique developed to observe failure as it develops and as propagates within laminate.

Shuart, M. J.↗

NASA NTRS1985

Failure mechanisms of laminates transversely loaded by bolt push-through

Stiffened composite panels proposed for fuselage and wing design utilize a variety of stiffener-to-skin attachment concepts including mechanical fasteners. The attachment concept is an important factor influencing the panel's strength and can govern its performance following local damage. Mechanical fasteners can be an effective method for preventing stiffener-skin separation. One potential failure mode for bolted panels occurs when the bolts pull through the stiffener attachment flange or skin. The resulting loss of support by the skin to the stiffener and by the stiffener to the skin can result in local buckling and subsequent panel collapse. The characteristic failure modes associated with bolt push-through failure are described and the results of a parametric study of the effects that different material systems, boundary conditions, and laminates have on the forces and displacements required to cause damage and bolt pushthrough failure are presented.

Waters, W. A., Jr.↗

NASA NTRS1986

The effect of resin toughness and modulus on compressive failure modes of quasi-isotropic graphite/epoxy laminates

Compressive failure mechanisms in quasi-isotropic graphite/epoxy laminates were characterized for both unnotched and notched specimens and also following damage by impact. Two types of fibers (Thornel 300 and 700) and four resin systems (Narmco 5208, American Cyanamid BP907, and Union Carbide 4901/MDA and 4901/mPDA) were studied. For all material combinations, failure of unnotched specimens was initiated by kinking of fibers in the 0-degree plies. A major difference was observed, however, in the mode of failure propagation after the 0-degree ply failure. The strength of quasi-isotropic laminates in general increased with increasing resin tensile modulus. The laminates made with Thornel 700 fibers exhibited slightly lower compressive strengths than did the laminates made with Thornel 300 fibers. The notch sensitivity as measured by the hole strength was lowest for the BP907 resin and highest for the 5208 resin. For the materials studied, however, the type of fiber had no effect on the notch sensitivity.

Sohi, M. M.↗

NASA NTRS1986

Compression failure mechanisms of composite structures

An experimental and analytical study was conducted to delineate the compression failure mechanisms of composite structures. The present report summarizes further results on kink band formation in unidirectional composites. In order to assess the compressive strengths and failure modes of fibers them selves, a fiber bundle was embedded in epoxy casting and tested in compression. A total of six different fibers were used together with two resins of different stiffnesses. The failure of highly anisotropic fibers such as Kevlar 49 and P-75 graphite was due to kinking of fibrils. However, the remaining fibers--T300 and T700 graphite, E-glass, and alumina--failed by localized microbuckling. Compressive strengths of the latter group of fibers were not fully utilized in their respective composite. In addition, acoustic emission monitoring revealed that fiber-matrix debonding did not occur gradually but suddenly at final failure. The kink band formation in unidirectional composites under compression was studied analytically and through microscopy. The material combinations selected include seven graphite/epoxy composites, two graphite/thermoplastic resin composites, one Kevlar 49/epoxy composite and one S-glass/epoxy composite.

Hahn, H. T.↗

NASA NTRS1986

Robust detection, isolation and accommodation for sensor failures

The objective is to extend the recent advances in robust control system design of multivariable systems to sensor failure detection, isolation, and accommodation (DIA), and estimator design. This effort provides analysis tools to quantify the trade-off between performance robustness and DIA sensitivity, which are to be used to achieve higher levels of performance robustness for given levels of DIA sensitivity. An innovations-based DIA scheme is used. Estimators, which depend upon a model of the process and process inputs and outputs, are used to generate these innovations. Thresholds used to determine failure detection are computed based on bounds on modeling errors, noise properties, and the class of failures. The applicability of the newly developed tools are demonstrated on a multivariable aircraft turbojet engine example. A new concept call the threshold selector was developed. It represents a significant and innovative tool for the analysis and synthesis of DiA algorithms. The estimators were made robust by introduction of an internal model and by frequency shaping. The internal mode provides asymptotically unbiased filter estimates.The incorporation of frequency shaping of the Linear Quadratic Gaussian cost functional modifies the estimator design to make it suitable for sensor failure DIA. The results are compared with previous studies which used thresholds that were selcted empirically. Comparison of these two techniques on a nonlinear dynamic engine simulation shows improved performance of the new method compared to previous techniques

Emami-Naeini, A.↗

NASA NTRS1986

A Sensor Failure Simulator for Control System Reliability Studies

A real-time Sensor Failure Simulator (SFS) was designed and assembled for the Advanced Detection, Isolation, and Accommodation (ADIA) program. Various designs were considered. The design chosen features an IBM-PC/XT. The PC is used to drive analog circuitry for simulating sensor failures in real-time. A user defined scenario describes the failure simulation for each of the five incoming sensor signals. Capabilities exist for editing, saving, and retrieving the failure scenarios. The SFS has been tested closed-loop with the Controls Interface and Monitoring (CIM) unit, the ADIA control, and a real-time F100 hybrid simulation. From a productivity viewpoint, the menu driven user interface has proven to be efficient and easy to use. From a real-time viewpoint, the software controlling the simulation loop executes at greater than 100 cycles/sec.

Melcher, K. J.↗

NASA NTRS1986

Reliability bounds for fault-tolerant systems with competing responses to component failures

Lee, L. D.↗

NASA NTRS1985

Failure detection and accommodation in structural dynamics systems using analytic redundancy

The concept of analytic redundancy is extended to identify nonzero constant output failures in large spacecraft active control sensors and actuators, using a set of distributed sensors which do not nominally produce equivalent signals but are related through the structural dynamics of the system. The suboptimal technique uses a Sequential Probability Ratio Test on the residual sequence of a Kalman filter based on a modal structure model, to determine failure of a component based on an assumed failure hypothesis. The specific failure is identified and then the Kalman filter gains are reconfigured for the identified remaining working sensor set. Experimental data using an apparatus whose dynamics are representative of a large spacecraft show the nominal filter performance under failed and unfailed conditions.

Williams, J. P.↗

NASA NTRS1986

Optimally robust redundancy relations for failure detection in uncertain systems

All failure detection methods are based, either explicitly or implicitly, on the use of redundancy, i.e. on (possibly dynamic) relations among the measured variables. The robustness of the failure detection process consequently depends to a great degree on the reliability of the redundancy relations, which in turn is affected by the inevitable presence of model uncertainties. In this paper the problem of determining redundancy relations that are optimally robust is addressed in a sense that includes several major issues of importance in practical failure detection and that provides a significant amount of intuition concerning the geometry of robust failure detection. A procedure is given involving the construction of a single matrix and its singular value decomposition for the determination of a complete sequence of redundancy relations, ordered in terms of their level of robustness. This procedure also provides the basis for comparing levels of robustness in redundancy provided by different sets of sensors.

Lou, X.-C.↗

NASA NTRS1987

A micrographic study of bending failure in five thermoplastic/carbon fiber composite laminates

The local deformation and failure sequences of five thermoplastic matrix composites were microscopically observed while bending the samples in a small fixture attached to a microscope stage. The themoplastics are polycarbonate, polysulfone, polyphenylsulfide, polyethersulfone, and polyetheretherketone. Comparison was made to an epoxy matrix composite, 5208/T-300. Laminates tested are (0/90) sub 2S, with outer ply fibers parallel to the beam axis. Four point bending was used at a typical span-to-thickness ratio of 39:1. It was found that all of the thermoplastic composites failed by abrupt longitudinal compression buckling of the outer ply. Very little precursory damage was observed. Micrographs reveal typical fiber kinking associated with longitudinal compression failure. Curved fracture surfaces on the fibers suggest they failed in bending rather than direct compression. Delamination was suppressed in the thermoplastic composites, and the delamination that did occur was found to be the result of compression buckling, rather than visa-versa. Microbuckling also caused other subsequent damage such as ply splitting, transverse ply shear failure, fiber tensile failure, and transverse ply cracking.

Yurgartis, S. W.↗

NASA NTRS1987

The effect of resin toughness and modulus on compressive failure modes of quasi-isotropic graphite/epoxy laminates

Hahn, H. Thomas↗

NASA NTRS1987

A systems engineering approach to automated failure cause diagnosis in space power systems

Automatic failure-cause diagnosis is a key element in autonomous operation of space power systems such as Space Station's. A rule-based diagnostic system has been developed for determining the cause of degraded performance. The knowledge required for such diagnosis is elicited from the system engineering process by using traditional failure analysis techniques. Symptoms, failures, causes, and detector information are represented with structured data; and diagnostic procedural knowledge is represented with rules. Detected symptoms instantiate failure modes and possible causes consistent with currently held beliefs about the likelihood of the cause. A diagnosis concludes with an explanation of the observed symptoms in terms of a chain of possible causes and subcauses.

Dolce, James L.↗

NASA NTRS1987

Efficient 3-D finite element failure analysis of compression loaded angle-ply plates with holes

Finite element stress analysis and the tensor polynomial failure criterion predict that failure always initiates at the interface between layers on the hole edge for notched angle-ply laminates loaded in compression. The angular location of initial failure is a function of the fiber orientation in the laminate. The dominant stress components initiating failure are shear. It is shown that approximate symmetry can be used to reduce the computer resources required for the case of unaxial loading.

Burns, S. W.↗