Results for “critical loads” — Page 14

NASA NTRS1983

Analyses of large quasistatic deformations of inelastic bodies by a new hybrid-stress finite element algorithm - Applications

A new hybrid-stress finite element algorithm suitable for analyzing large quasistatic deformations of inelastic solids is presented and its feasibility and performance are demonstrated with examples. The algorithm provides extremely accurate bifurcation analysis which is stable with respect to variation in the finite element mesh, so long as the same type of element is used in every mesh. When the mesh element is varied, the result changes in a predictable manner. The method does not necessarily lead to an upper or lower bound for the critical load. An explicit forward gradient scheme is used to improve stability and is shown to be useful also for elongation-dominated deformations. The application of the method to the onset of necking in plane extension and to deformation and stress in plane extension of an elasticoviscous fluid with an array of cylindrical voids is given in detail.

Reed, K. W.↗

NASA NTRS1986

Hygrothermal effects on the mechanical behaviour of graphite fibre-reinforced epoxy laminates beyond initial failure

The critical load levels and associated cracking beyond which a multidirectional laminate can be considered as structurally failed has been determined by loading graphite fiber-reinforced epoxy laminates to different strain levels up to ultimate failure. Transverse matrix cracking was monitored by acoustic and optical methods. The residual stiffness and strength parallel and perpendicular to the cracks were determined and related to the environmental/loading history. Within the range of experimental conditions studied, it is concluded that the transverse cracking process does not have a crucial effect on the structural performance of multidirectional composite laminates.

Ishai, O.↗

NASA NTRS1986

Inclusion of elastically connected members in exact buckling and frequency calculations

A standard stiffness matrix procedure which permits any combination of rigid, elastic, pinned or sliding connections of the degrees of freedom at the ends of a member to the nodes of its parent structure is described, in order to show how easily it can be extended to allow an existing algorithm to be used to ensure that no eigenvalues of the parent structure can be missed even when 'exact' member theory is used. The eigenvalues are the natural frequencies of undamped free vibration analyses or the critical load factors of buckling problems. The method preserves the exactness of the member theory and an efficient method for computer application is indicated. The theory also permits any combination of rigid, elastic, pinned or sliding connections between the freedoms of a substructure and those of its parent structure.

Williams, F. W.↗

NASA NTRS1984

Practical considerations in aeroelastic design

The structural design process for large transport aircraft is described. Critical loads must be determined from a large number of load cases within the flight maneuver envelope. The structural design is also constrained by considerations of producibility, reliability, maintainability, durability, and damage tolerance, as well as impact dynamics and multiple constraints due to flutter and aeroelasticity. Aircraft aeroelastic design considerations in three distinct areas of product development (preliminary design, advanced design, and detailed design) are presented and contrasted. The present state of the art is challenged to solve the practical difficulties associated with design, analysis, and redesign within cost and schedule constraints. The current practice consists of largely independent engineering disciplines operating with unorganized data interfaces. The need is then demonstrated for a well-planned computerized aeroelastic structural design optimization system operating with a common interdisciplinary data base. This system must incorporate automated interfaces between modular programs. In each phase of the design process, a common finite-element model for static and dynamic optimization is required to reduce errors due to modeling discrepancies. As the design proceeds from the simple models in preliminary design to the more complex models in advanced and detailed design, a means of retrieving design data from the previous models must be established.

Rommel, B. A.↗

NASA NTRS1988

Adhesion, friction and micromechanical properties of ceramics

The adhesion, friction, and micromechanical properties of ceramics, both in monolithic and coating form, are reviewed. Ceramics are examined in contact with themselves, other harder materials, and metals. For the simplicity of discussion, the tribological properties of concern in the processes are separated into two parts. The first part discusses the pull-off force (adhesion) and the shear force required to break the interfacial junctions between contacting surfaces. The role of chemical bonding in adhesion and friction, and the effects of surface contaminant films and temperature on tribological response with respect to adhesion and friction are discussed. The second part deals with abrasion of ceramics. Elastic, plastic, and fracture behavior of ceramics in solid state contact is discussed. The scratch technique of determining the critical load needed to fracture interfacial adhesive bonds of ceramic deposited on substrates is also addressed.

Miyoshi, Kazuhisa↗

NASA NTRS1988

Fracture analysis of local delaminations in laminated composites

A shear deformation model was developed to analyze local delaminations growing from transverse cracks in 90 degree plies located around the mid plane of symmetric laminates. The predictions of the model agree reasonably with experimental data from T300/934 graphite epoxy laminates. The predicted behavior is such that, in combination with an edge delamination model, the critical loads can be predicted accurately in the range of n from .5 to 8.

Sriram, P.↗

NASA NTRS1989

Fracture analysis of local delaminations in laminated composites

A shear deformation model was developed to analyze local delaminations growing from transverse cracks in 90 degree plies located around the mid plane of symmetric laminates. The predictions of the model agree reasonably with experimental data from T300/934 graphite epoxy laminates. The predicted behavior is such that, in combination with an edge delamination model, the critical loads can be predicted accurately in the range of n from .5 to 8.

Armanios, E. A.↗

NASA NTRS1988

Adhesion, friction and micromechanical properties of ceramics

The adhesion, friction, and micromechanical properties of ceramics, both in monolithic and coating form, are reviewed. Ceramics are examined in contact with themselves, other harder materials, and metals. For the simplicity of discussion, the tribological properties of concern in the processes are separated into two parts. The first part discusses the pull-off force (adhesion) and the shear force required to break the interfacial junctions between contacting surfaces. The role of chemical bonding in adhesion and friction, and the effects of surface contaminant films and temperature on tribological response with respect to adhesion and friction are discussed. The second part deals with abrasion of ceramics. Elastic, plastic, and fracture behavior of ceramics in solid state contact is discussed. The scratch technique of determining the critical load needed to fracture interfacial adhesive bonds of ceramic deposited on substrates is also addressed.

Miyoshi, Kazuhisa↗

NASA NTRS1989

Fundamental tribological properties of ion-beam-deposited boron nitride films

The adhesion, friction, and micromechanical properties of ion-beam-deposited boron nitride (BN) films are reviewed. The BN films are examined in contact with BN metals and other harder materials. For simplicity of discussion, the tribological properties of concern in the processes are separated into two parts. First, the pull-off force (adhesion) and the shear force required to break the interfacial junctions between contacting surfaces are discussed. The effects of surface films, hardness of metals, and temperature on tribological response with respect to adhesion and friction are considered. The second part deals with the abrasion of the BN films. Elastic, plastic, and fracture behavior of the BN films in solid-state contact are discussed. The scratch technique of determining the critical load needed to fracture interfacial adhesive bonds of BN films deposited on substrates is also addressed.

Miyoshi, Kazuhisa↗

NASA NTRS1989

A probabilistic approach to crack instability

A probabilistic model of brittle fracture is examined with reference to two-dimensional problems. The model is illustrated by using experimental data obtained for 25 macroscopically identical specimens made of short-fiber-reinforced composites. It is shown that the model proposed here provides a predictive formalism for the probability distributions of critical crack depth, critical loads, and crack arrest depths. It also provides similarity criteria for small-scale testing.

Chudnovsky, A.↗

NASA NTRS1990

Fundamental tribological properties of ion-beam-deposited boron nitride films

The adhesion, friction, and micromechanical properties of ion-beam-deposited boron nitride (BN) films are reviewed. The BN films are examined in contact with BN metals and other harder materials. For simplicity of discussion, the tribological properties of concern in the processes are separated into two parts. First, the pull-off force (adhesion) and the shear force required to break the interfacial junctions between contacting surfaces are discussed. The effects of surface films, hardness of metals, and temperature on tribological response with respect to adhesion and friction are considered. The second part deals with the abrasion of the BN films. Elastic, plastic, and fracture behavior of the BN films in solid-state contact are discussed. The scratch technique of determining the critical load needed to fracture interfacial adhesive bonds of BN films deposited on substrates is also addressed.

Miyoshi, K.↗

NASA NTRS1991

Adaptive finite element analysis of hypersonic laminar flows for aerothermal load predictions

The use of an adaptive mesh refinement procedure for analyzing hypersonic laminar flows with application to aerothermal load predictions is described. The adaptation procedure, which uses both quadrilateral and triangular elements, is implemented with the multistep Galerkin-Runge-Kutta scheme. Elements that lie in regions of strong gradients are refined based on indicators to obtain better definition of flow features. The effectiveness of the adaptive procedure is demonstrated by modeling Mach 11.7 flow over a 15-deg ramp. Numerical results are compared with predictions of strong interaction theories and experimental data. Surface quantities such as heating rates and pressure loads, critical for the effective design of high-speed vehicles, are found to be in good agreement with experimental values.

Ramakrishnan, R.↗

NASA NTRS1991

Fracture analysis of transverse crack-tip and free-edge delamination in laminated composites

A shear deformation model including hygrothermal effects is developed for the analysis of local delaminations originating from transverse cracks in 90-deg plies located in and around the laminate midplane. A sublaminate approach is used and the model is applied to (+/- 25/90n)s T300/934 graphite/epoxy laminates for n values between 0.5 and 8, along with previously developed edge-delamination shear-deformation models. Critical loads and delamination modes are identified and compared with experimental results. Hygrothermal effects are included in all the models to make the comparisons realistic.

Armanios, Erian A.↗

NASA NTRS1990

Modeling of the flexural behavior of ceramic-matrix composites

This paper examines the effects of matrix cracking and fiber breakage on the flexural behavior of ceramic composite beams. A model has been proposed to represent the damage evolution of the beam, of which the matrix fracture strain is smaller than that of the fibers. Close form solutions of the critical loads for the initiation of matrix cracking and fiber breakage in the tension side of the beam have been found. The effects of thermal residual stresses and fiber/matrix debonding have been taken into account. The initial deviation of the load-deflection curve from linearity is due to matrix cracking, while fiber breakages are responsible for the drop in the load carrying capacity of the beam. The proportional limit as well as the nonlinear behavior of the beam deflection have been identified. The growth of the damaged zone has also been predicted. A three-point bending case is given as a numerical example.

Kuo, Wen-Shyong↗

NASA NTRS1939

Experimental Study of Torsional Column Failure

Thirty-three 24ST aluminum-alloy 2- by 2- by 0.10-inch channels, with lengths ranging from 10 to 90 inches were tested at Stanford University in compression to obtain an experimental verification of the theoretical formulas for torsional failure developed by Eugene E. Lundquist of the N.A.C.A. The observed critical loads and twist-axis locations were sufficiently close to the values obtained from the formulas to establish the substantial validity of the latter. The differences between observed and computed results were small enough to be accounted for by small and mostly unavoidable differences between actual test conditions and those assumed in deriving the formulas. Some data were obtained from the shorter specimens regarding the growth of the buckles that resulted in local buckling failure.

Nile, Alfred S↗

NASA NTRS1943

A method for determining the column curve from tests of columns with equal restraints against rotation on the ends

The results are presented of a theoretical study for the determination of the column curve from tests of column specimens having ends equally restrained against rotation. The theory of this problem is studied and a curve is shown relating the fixity coefficient c to the critical load, the length of the column, and the magnitude of the elastic restraint. A method of using this curve for the determination of the column curve for columns with pin ends from tests of columns with elastically restrained ends is presented. The results of the method as applied to a series of tests on thin-strip columns of stainless steel are also given.

Lundquist, Eugene E↗

NASA NTRS2003

Addendum to the User Manual for NASGRO Elastic-Plastic Fracture Mechanics Software Module

The elastic-plastic fracture mechanics modules in NASGRO have been enhanced by the addition of of the following: new J-integral solutions based on the reference stress method and finite element solutions; the extension of the critical crack and critical load modules for cracks with two degrees of freedom that tear and failure by ductile instability; the addition of a proof test analysis module that includes safe life analysis, calculates proof loads, and determines the flaw screening 1 capability for a given proof load; the addition of a tear-fatigue module for ductile materials that simultaneously tear and extend by fatigue; and a multiple cycle proof test module for estimating service reliability following a proof test.

Gregg, M. Wayne↗

NASA NTRS2005

AC/DC Power Systems with Applications for future Lunar/Mars base and Crew Exploration Vehicle

ABSTRACT The Power Systems branch at JSC faces a number of complex issues as it readies itself for the President's initiative on future space exploration beyond low earth orbit. Some of these preliminary issues - those dealing with electric power generation and distribution on board Mars-bound vehicle and that on Lunar and Martian surface may be summarized as follows: Type of prime mover - Because solar power may not be readily available on parts of the Lunar/Mars surface and also during the long duration flight to Mars, the primary source of power will most likely be nuclear power (Uranium fuel rods) with a secondary source of fuel cell (Hydrogen supply). The electric power generation source - With nuclear power being the main prime mover, the electric power generation source will most likely be an ac generator at a yet to be determined frequency. Thus, a critical issue is whether the generator should generate at constant or variable frequency. This will decide what type of generator to use - whether it is a synchronous machine, an asynchronous induction machine or a switched reluctance machine. The type of power distribution system - the distribution frequency, number of wires (3- wire, 4-wire or higher), and ac/dc hybridization. Building redundancy and fault tolerance in the generation and distribution sub-systems so that the system is safe; provides 100% availability to critical loads; continues to operate even with faulted sub-systems; and requires minimal maintenance. This report descril_es results of a summer faculty fellowship spent in the Power Systems Branch with the specific aim of investigating some of the lessons learned in electric power generation and usage from the terrestrial power systems industry, the aerospace industry as well as NASA's on-going missions so as to recommend novel surface and vehicle-based power systems architectures in support of future space exploration initiatives. A hybrid ac/dc architecture with source side and load side redundancies and including emergency generators on both ac and dc sides is proposed. The generation frequency is 400 Hz mostly because of the technology maturity at this frequency in the aerospace industry. Power will be distributed to several ac load distribution buses through solid state variable speed, constant frequency converters on the ac side. A segmented dc ring bus supplied from ac/dc converters and with the capability of connecting/disconnecting the segments will supply power to multiple de load distribution buses. The system will have the capability of reverse flow from dc to ac side in the case of an extreme emergency on the main ac generation side.

Chowdhury, Badrul H.↗