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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Stress-intensity factors for a wedge-loaded edge crack in a semi-infinite strip

The problem of a semi infinite strip containing an edge crack is considered. It is assumed that the strip is loaded by a frictionless rigid wedge pressed into the crack. The resulting crack contact problem is formulated in terms of a system of singular integral equations. The behavior of the solution near the singular points is studied in detail. A series of numerical examples are given and the results are compared with those obtained by the method of boundary collocation and by the simple beam theory.

Erodogan, F.↗
NASA NTRS1995

Investigation of piloting aids for manual control of hypersonic maneuvers

An investigation of piloting aids designed to provide precise maneuver control for an air-breathing hypersonic vehicle is described. Stringent constraints and nonintuitive high-speed flight effects associated with maneuvering in the hypersonic regime raise the question of whether manual control of such a vehicle should even be considered. The objectives of this research were to determine the extent of manual control that is desirable for a vehicle maneuvering in this regime and to identify the form of aids that must be supplied to the pilot to make such control feasible. A piloted real-time motion-based simulation of a hypersonic vehicle concept was used for this study, and the investigation focused on a single representative cruise turn maneuver. Piloting aids, which consisted of an auto throttle, throttle director, autopilot, flight director, and two head-up display configurations, were developed and evaluated. Two longitudinal control response types consisting of a rate-command/attitude-hold system and a load factor-rate/load-factor-hold system were also compared. The complete set of piloting aids, which consisted of the autothrottle, throttle director, and flight director, improved the average Cooper-Harper flying qualities ratings from 8 to 2.6, even though identical inner-loop stability and control augmentation was provided in all cases. The flight director was determined to be the most critical of these aids, and the cruise turn maneuver was unachievable to adequate performance specifications in the absence of this flight director.

Raney, David L.↗
NASA NTRS2000

Airframe Design and Integration

The objectives of the project this viewgraph presentation summarizes include the following: (1) Decompose operational, safety, and cost requirements into a comprehensive and consistent set of design criteria for different structural and material concepts for Reusable Launch Vehicles (RLVs); (2) Develop compliance methods to ensure that different structural and material concepts are assessed at a consistent and adequate level of fidelity and safety; (3) Develop and assess weight reduction potential of integrated airframe concepts for RLVs, e.g., Thermal Protection System (TPS)/TPS Support/Cryogenic Tank System; (4) Compare performance and weight of various airframe structural and material concepts and structural arrangements and identify technology development needs; and (5) Develop high fidelity parametric models that include airframe structural interactions and major design drivers. The approaches taken to complete these objectives include the definition of vehicle requirements, airframe structural design requirements, load conditions, factors of safety, and integrated concepts.

Scotti, Stephen↗
NASA NTRS2008

Probabilistic Analysis and Design of a Raked Wing Tip for a Commercial Transport

An approach for conducting reliability-based design and optimization (RBDO) of a Boeing 767 raked wing tip (RWT) is presented. The goal is to evaluate the benefits of RBDO for design of an aircraft substructure. A finite-element (FE) model that includes eight critical static load cases is used to evaluate the response of the wing tip. Thirteen design variables that describe the thickness of the composite skins and stiffeners are selected to minimize the weight of the wing tip. A strain-based margin of safety is used to evaluate the performance of the structure. The randomness in the load scale factor and in the strain limits is considered. Of the 13 variables, the wing-tip design was controlled primarily by the thickness of the thickest plies in the upper skins. The report includes an analysis of the optimization results and recommendations for future reliability-based studies.

Mason Brian H.↗
NASA NTRS1977

Stress-intensity factors for cracks emanating from the loaded fastener hole

Using a series approach and the Muskhelishvili formulation in the two-dimensional theory of elasticity, stress-intensity factors K are derived for problems in which cracks emanate radially from the boundary of an arbitrarily loaded internal circular hole in an infinite plate. Numerical values are obtained for K(I) and K(II) for radial cracks from a hole containing a loose-fitted pin or rivet that is pulled perpendicular to the crack direction in the plane of the plate. The method is a general one for determining K for a set of symmetrically emanating radial cracks for a variety of concentrated or distributed tractions on the circular hole.

Shivakumar, V.↗
NASA NTRS1984

Three-dimensional finite-element analysis of chevron-notched fracture specimens

Stress-intensity factors and load-line displacements were calculated for chevron-notched bar and rod fracture specimens using a three-dimensional finite-element analysis. Both specimens were subjected to simulated wedge loading (either uniform applied displacement or uniform applied load). The chevron-notch sides and crack front were assumed to be straight. Crack-length-to-specimen width ratios (a/w) ranged from 0.4 to 0.7. The width-to-thickness ratio (w/B) was 1.45 or 2. The bar specimens had a height-to-width ratio of 0.435 or 0.5. Finite-element models were composed of singularity elements around the crack front and 8-noded isoparametric elements elsewhere. The models had about 11,000 degrees of freedom. Stress-intensity factors were calculated by using a nodal-force method for distribution along the crack front and by using a compliance method for average values. The stress intensity factors and load-line displacements are presented and compared with experimental solutions from the literature. The stress intensity factors and load-line displacements were about 2.5 and 5 percent lower than the reported experimental values, respectively.

Raju, I. S.↗
NASA NTRS1983

Digital Single-Phase Power-Factor Controller

Digital circuit has faster response to load changes. Digital power-factor controller senses changing motor-load torques by sampling open-circuit voltage across gate-controlled silicon switch. Circuit responds more rapidly to hanging loads than analog power-factor controllers because no low-pass filter is in feedback loop.

Dabney, R. W.↗
NASA NTRS1924

The Influence of the Form of a Wooden Beam on Its Stiffness and Strength II : Form Factors of Beams Subjected to Transverse Loading Only

The general aim of the investigation described in this report is the achievement of efficient design in wing beams. The purpose of the tests was to determine factors to apply to the usual beam formula in order that the properties of wood based on tests of rectangular sections might be used as a basis of design for beams of any sections and if practical to develop formulas for determining such factors and to verify them by experiment. Such factors for various sections have been determined from test by comparing properties of the beam in question to similar properties of matched beams 2 by 2 inches in section. Furthermore, formulas were worked out, more or less empirical in character, which check all of these test values remarkably well.

Newlin, J A↗
NASA NTRS2001

Probabilistic Structural Analysis and Reliability Using NESSUS With Implemented Material Strength Degradation Model

This project included both research and education objectives. The goal of this project was to advance innovative research and education objectives in theoretical and computational probabilistic structural analysis, reliability, and life prediction for improved reliability and safety of structural components of aerospace and aircraft propulsion systems. Research and education partners included Glenn Research Center (GRC) and Southwest Research Institute (SwRI) along with the University of Texas at San Antonio (UTSA). SwRI enhanced the NESSUS (Numerical Evaluation of Stochastic Structures Under Stress) code and provided consulting support for NESSUS-related activities at UTSA. NASA funding supported three undergraduate students, two graduate students, a summer course instructor and the Principal Investigator. Matching funds from UTSA provided for the purchase of additional equipment for the enhancement of the Advanced Interactive Computational SGI Lab established during the first year of this Partnership Award to conduct the probabilistic finite element summer courses. The research portion of this report presents the cumulation of work performed through the use of the probabilistic finite element program, NESSUS, Numerical Evaluation and Structures Under Stress, and an embedded Material Strength Degradation (MSD) model. Probabilistic structural analysis provided for quantification of uncertainties associated with the design, thus enabling increased system performance and reliability. The structure examined was a Space Shuttle Main Engine (SSME) fuel turbopump blade. The blade material analyzed was Inconel 718, since the MSD model was previously calibrated for this material. Reliability analysis encompassing the effects of high temperature and high cycle fatigue, yielded a reliability value of 0.99978 using a fully correlated random field for the blade thickness. The reliability did not change significantly for a change in distribution type except for a change in distribution from Gaussian to Weibull for the centrifugal load. The sensitivity factors determined to be most dominant were the centrifugal loading and the initial strength of the material. These two sensitivity factors were influenced most by a change in distribution type from Gaussian to Weibull. The education portion of this report describes short-term and long-term educational objectives. Such objectives serve to integrate research and education components of this project resulting in opportunities for ethnic minority students, principally Hispanic. The primary vehicle to facilitate such integration was the teaching of two probabilistic finite element method courses to undergraduate engineering students in the summers of 1998 and 1999.

Bast, Callie C.↗
NASA NTRS1973

Plastic intensity factors for cracked plates subjected to biaxial loading

An elastic-plastic analysis is performed for an infinite plate under plane stress conditions which contains a finite line crack and is subjected to biaxial loading. In the large scale yielding range, the amplitude of the dominant singularity at the crack tip, the plastic stress (or strain) intensity factor, is found to depend on the magnitude of loading parallel to the crack direction. The predicted value for the fracture initiation stress which is based on a plastic intensity factor fracture criterion, increases under lateral, tensile loading. Compressive loading parallel to the crack has the opposite effect on the fracture initiation stress.

Hilton, P. D.↗
NASA NTRS1994

Improved load distribution in parallel sparse Cholesky factorization

Compared to the customary column-oriented approaches, block-oriented, distributed-memory sparse Cholesky factorization benefits from an asymptotic reduction in interprocessor communication volume and an asymptotic increase in the amount of concurrency that is exposed in the problem. Unfortunately, block-oriented approaches (specifically, the block fan-out method) have suffered from poor balance of the computational load. As a result, achieved performance can be quite low. This paper investigates the reasons for this load imbalance and proposes simple block mapping heuristics that dramatically improve it. The result is a roughly 20% increase in realized parallel factorization performance, as demonstrated by performance results from an Intel Paragon system. We have achieved performance of nearly 3.2 billion floating point operations per second with this technique on a 196-node Paragon system.

Rothberg, Edward↗
NASA NTRS1994

Interaction of Mechanical Load with Growth Hormone (GH) and Insulin-Like Growth Factor I (IGF-I) on Slow-Twitch Skeletal Muscle and Bone

Exogenous humoral growth factors, combined with increased mechanical loading, reportedly induce hypertrophy of fast-, but not slow-twitch skeletal muscles, and have little effect in attenuating atrophy of slow-twitch muscle associated with exposure to microgravity in animals with intact neuroendocrine systems. These observations suggest that anabolic adjuvants and muscle tension do not interact to stimulate growth or maintenance of slow-twitch skeletal muscle. The purpose of the present study was to determine whether a chronic increase in mechanical loading (synergistic ablation) or hindlimb unweighting (hindlimb suspension) interact with exogenous GH and IGF-I (Genentech, So San Francisco, CA) in the slow-twitch soleus muscles of female rats (approx. 250 g). Bilateral ablation of the plantaris and gastrocnemius muscles induced 38% and 40% increases in the absolute (mg/pair) and relative (mg/100 g body weight) weights of the soleus, respectively (p less than or = 0.05), in ambulatory rats. GH and IGF-I interacted with chronic loading to increase absolute soleus mass an additional 20% (p less than or = 0.05), and mixed and myofibrillar protein contents an additional 12% and 7%, respectively (NS). In contrast, hindlimb suspension (HLS) resulted in 20% and 18% decreases in the absolute and relative weights of the soleus, respectively (p less than or = 0.05); GH and IGF-I did not spare loss of soleus mass or protein content in HLS rats. HLS decreased tibial plate thickness approx. 11% (p less than or = 0.05), but not weights of the tibia or femus. GH and IGF-I increased tibial plate thickness approx. 30% (p less than or = 0.05), in ambulatory and HLS rats, and increased femur and tibial weights 12% (p less than or = 0.05) and 8% (NS), respectively, in ambulatory rats, but had no effect in HLS rats. Results of the present investigation suggest that GH and IGF-I can stimulate hypertrophy of slow-twitch skeletal muscle when chronically overloaded, but can also stimulate growth of hindlimb bones in the absence of mechanical load.

Linderman, Jon K.↗
NASA NTRS1999

Fatigue Analyses Under Constant- and Variable-Amplitude Loading Using Small-Crack Theory

Studies on the growth of small cracks have led to the observation that fatigue life of many engineering materials is primarily "crack growth" from micro-structural features, such as inclusion particles, voids, slip-bands or from manufacturing defects. This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using "small-crack theory" under various loading conditions. Constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective stress-intensity factor range (delta-Keff) under constant-amplitude loading. Modifications to the delta-Keff-rate relations in the near-threshold regime were needed to fit measured small-crack growth rate behavior. The model was then used to calculate small-and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens under constant-amplitude and spectrum loading. Fatigue lives were predicted using crack-growth relations and micro-structural features like those that initiated cracks in the fatigue specimens for most of the materials analyzed. Results from the tests and analyses agreed well.

Newman, J. C., Jr.↗
NASA NTRS1999

Analyses of Fatigue and Fatigue-Crack Growth under Constant- and Variable-Amplitude Loading

Studies on the growth of small cracks have led to the observation that fatigue life of many engineering materials is primarily crack growth from micro-structural features, such as inclusion particles, voids, slip-bands or from manufacturing defects. This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using small-crack theory under various loading conditions. Constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective stress-intensity factor range (delta K(sub eff)) under constant-amplitude loading. Modifications to the delta K(sub eff)-rate relations in the near-threshold regime were needed to fit measured small-crack growth rate behavior. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens under constant-amplitude and spectrum loading. Fatigue lives were predicted using crack-growth relations and micro-structural features like those that initiated cracks in the fatigue specimens for most of the materials analyzed. Results from the tests and analyses agreed well.

Newman, J. C., Jr.↗
NASA NTRS1992

Determination of stress intensity factors for interface cracks under mixed-mode loading

A simple technique was developed using conventional finite element analysis to determine stress intensity factors, K1 and K2, for interface cracks under mixed-mode loading. This technique involves the calculation of crack tip stresses using non-singular finite elements. These stresses are then combined and used in a linear regression procedure to calculate K1 and K2. The technique was demonstrated by calculating three different bimaterial combinations. For the normal loading case, the K's were within 2.6 percent of an exact solution. The normalized K's under shear loading were shown to be related to the normalized K's under normal loading. Based on these relations, a simple equation was derived for calculating K1 and K2 for mixed-mode loading from knowledge of the K's under normal loading. The equation was verified by computing the K's for a mixed-mode case with equal and normal shear loading. The correlation between exact and finite element solutions is within 3.7 percent. This study provides a simple procedure to compute K2/K1 ratio which has been used to characterize the stress state at the crack tip for various combinations of materials and loadings. Tests conducted over a range of K2/K1 ratios could be used to fully characterize interface fracture toughness.

Naik, Rajiv A.↗