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At least 1,693 records · Page 94

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NASA NTRS1992

Reliability of stiffened structural panels: Two examples

The reliability of two graphite-epoxy stiffened panels that contain uncertainties is examined. For one panel, the effect of an overall bow-type initial imperfection is studied. The size of the bow is assumed to be a random variable. The failure mode is buckling. The benefits of quality control are explored by using truncated distributions. For the other panel, the effect of uncertainties in a strain-based failure criterion is studied. The allowable strains are assumed to be random variables. A geometrically nonlinear analysis is used to calculate a detailed strain distribution near an elliptical access hole in a wing panel that was tested to failure. Calculated strains are used to predict failure. Results are compared with the experimental failure load of the panel.

Stroud, W. Jefferson↗

NASA NTRS1992

Microstructure-failure mode correlations in braided composites

Explication of the fracture processes of braided composites is needed for modeling their behavior. Described is a systematic exploration of the relationship between microstructure, loading mode, and micro-failure mechanisms in carbon/epoxy braided composites. The study involved compression and fracture toughness tests and optical and scanning electron fractography, including dynamic in-situ testing. Principal failure mechanisms of low sliding, buckling, and unstable crack growth are correlated to microstructural parameters and loading modes; these are used for defining those microstructural conditions which are strength limiting.

Filatovs, G. J.↗

NASA NTRS1992

Relationship between mechanical-property and energy-absorption trends for composite tubes

U.S. Army helicopters are designed to dissipate prescribed levels of crash impact kinetic energy without compromising the integrity of the fuselage. Because of the complexity of the energy-absorption process it is imperative for designers of energy-absorbing structures to develop an in-depth understanding of how and why composite structures absorb energy. A description of the crushing modes and mechanisms of energy absorption for composite tubes and beams is presented. Three primary crushing modes of composite structures including transverse shearing, lamina bending, and local buckling are described. The experimental data presented show that fiber and matrix mechanical properties and laminate stiffness and strength mechanical properties cannot reliably predict the energy-absorption response of composite tubes.

Farley, Gary L.↗

NASA NTRS1993

Propulsion Systems Panel

Topics addressed are: (1) cryogenic tankage; (2) launch vehicle TPS/insulation; (3) durable passive thermal control devices and/or coatings; (4) development and characterization of processing methods to reduce anisotropy of material properties in Al-Li; (5) durable thermal protection system (TPS); (6) unpressurized Al-Li structures (interstages, thrust structures); (7) near net shape sections; (8) pressurized structures; (9) welding and joining; (10) micrometeoroid and debris hypervelocity shields; (11) state-of-the-art shell buckling structure optimizer program to serve as a rapid design tool; (12) test philosophy; (13) reduced load cycle time; (14) structural analysis methods; (15) optimization of structural criteria; and (16) develop an engineering approach to properly trade material and structural concepts selection, fabrication, facilities, and cost.

Bianca, Carmelo J.↗

NASA NTRS1993

Preliminary analysis techniques for ring and stringer stiffened cylindrical shells

This report outlines methods of analysis for the buckling of thin-walled circumferentially and longitudinally stiffened cylindrical shells. Methods of analysis for the various failure modes are presented in one cohesive package. Where applicable, more than one method of analysis for a failure mode is presented along with standard practices. The results of this report are primarily intended for use in launch vehicle design in the elastic range. A Microsoft Excel worksheet with accompanying macros has been developed to automate the analysis procedures.

Graham, J.↗

NASA NTRS1993

High Temperature Composite Analyzer (HITCAN) demonstration manual, version 1.0

This manual comprises a variety of demonstration cases for the HITCAN (HIgh Temperature Composite ANalyzer) code. HITCAN is a general purpose computer program for predicting nonlinear global structural and local stress-strain response of arbitrarily oriented, multilayered high temperature metal matrix composite structures. HITCAN is written in FORTRAN 77 computer language and has been configured and executed on the NASA Lewis Research Center CRAY XMP and YMP computers. Detailed description of all program variables and terms used in this manual may be found in the User's Manual. The demonstration includes various cases to illustrate the features and analysis capabilities of the HITCAN computer code. These cases include: (1) static analysis, (2) nonlinear quasi-static (incremental) analysis, (3) modal analysis, (4) buckling analysis, (5) fiber degradation effects, (6) fabrication-induced stresses for a variety of structures; namely, beam, plate, ring, shell, and built-up structures. A brief discussion of each demonstration case with the associated input data file is provided. Sample results taken from the actual computer output are also included.

Singhal, S. N↗

NASA NTRS1991

Advanced technology commercial fuselage structure

Boeing's program for Advanced Technology Composite Aircraft Structure (ATCAS) has focused on the manufacturing and performance issues associated with a wide body commercial transport fuselage. The primary goal of ATCAS is to demonstrate cost and weight savings over a 1995 aluminum benchmark. A 31 foot section of fuselage directly behind the wing to body intersection was selected for study purposes. This paper summarizes ATCAS contract plans and review progress to date. The six year ATCAS program will study technical issues for crown, side, and keel areas of the fuselage. All structural details in these areas will be included in design studies that incorporate a design build team (DBT) approach. Manufacturing technologies will be developed for concepts deemed by the DBT to have the greatest potential for cost and weight savings. Assembly issues for large, stiff, quadrant panels will receive special attention. Supporting technologies and mechanical tests will concentrate on the major issues identified for fuselage. These include damage tolerance, pressure containment, splices, load redistribution, post-buckled structure, and durability/life. Progress to date includes DBT selection of baseline fuselage concepts; cost and weight comparisons for crown panel designs; initial panel fabrication for manufacturing and structural mechanics research; and toughened material studies related to keel panels. Initial ATCAS studies have shown that NASA's Advanced Composite Technology program goals for cost and weight savings are attainable for composite fuselage.

Ilcewicz, L. B.↗

NASA NTRS1991

Design and analysis of grid stiffened concepts for aircraft composite primary structural applications

An approach to buckling resistant design of general grid stiffened flat plates based on smeared stiffener theory for combined inplane loading is discussed. Some results from parametric studies performed to assess the validity of smeared stiffener for practical stiffener configurations and to illustrate the benefits of different stiffening concepts are given. Details of a design study are discussed where the present analysis method is used to design a grid stiffened panel for a fuselage application and verified using a finite element analysis results.

Ambur, Damodar R.↗

NASA NTRS1991

Prediction of stiffener-skin separation in composite panels

A methodology was developed to predict the failure of stiffened composite panels by stiffener-skin separation. The methodology is applicable to curved or flat panels under combined uniaxial compression and in-plane shear loads. The analytical crux of the methodology are two different analysis packages: PACL1, which predicts the stress and displacement fields in a stiffened postbuckled panel; and WEBSTER, which predicts the local interfacial (shear and normal) stresses at the skin and stiffener flange junction. PACL1 is a Rayleigh-Ritz analysis of curved composite panels loaded into a post-buckling range. WEBSTER is a two-dimensional elasticity analysis for the skin-stiffener interface. Results of the predictive methodology are given.

Kan, Han-Pin↗

NASA NTRS1992

Vibration of compressively loaded shear deformable flat panels exhibiting initial geometric imperfections

Numerical study results are presented which indicate that, in the prebuckling range, the natural frequency predicted by the shear-deformable plate theory is smaller than that associated with its transverse-shear rigid counterpart. The opposite behavior occurs in the postbuckling/postcritical range. With increasing transverse shear flexibility, greater differences between frequencies predicted by shear deformation and infinitely rigid transverse shear theories emerge. The character of in-plane boundary conditions is a determinant of both increasing/decreasing buckling loads and increasing/decreasing vibratory frequencies.

Librescu, L.↗

NASA NTRS1992

Multidisciplinary design integration system for a supersonic transport aircraft

An aircraft preliminary design system which provides the multidisciplinary communications and couplings between several engineering disciplines is described. A primary benefit of this system is to demonstrate advanced technology multidisciplinary design integration methodologies. The current version includes the disciplines of aerodynamics and structures. Contributing engineering disciplines are coupled using the Global Sensitivity Equation approach to influence the global design optimization problem. A high speed civil transport configuration is used for configuration trade studies. Forty four independent design variables are used to control the cross-sectional areas of wing rib and spar caps and the thicknesses of wingskincover panels. A total of 300 stress, strain, buckling and displacement behavioral constraints and minimum gages on the design variables were used to optimize the idealized wing structure. The goal of the designs to resize the wing cover panels and internal structure for minimum mass.

Dovi, A. R.↗

NASA NTRS1993

Impact damage tolerance of thin wall composite struts

An experimental investigation was made to study the impact damage tolerance of thin wall composite struts made of both brittle epoxy and toughened epoxy based composite materials. Damage parameters such as barely visible surface damage and internal damage represented by the ultrasonic C-scan, and residual compressive strengths were evaluated against impact energy for two impactor sizes. From both a damage resistance (internal damage vs. impact energy) and a damage tolerance (residual compressive strength vs. internal damage) point of view, the toughened IM7/977-2 struts exhibited better performance than the brittle epoxy based T50/934 struts. This is attributed to the toughening mechanism in 977-2 which impedes delamination initiation from impact, and delamination growth and subsequent buckling under a compression loading. At barely visible damage thresholds, regardless of the impactor sizes, a maximum strength reduction of 45-55 percent was observed for the T50/934 struts, and approximately 10 percent for IM7/977-2 struts. This is of great interest for developing a damage tolerance design approach and risk assessment methodology in which the design allowable would be defined by the residual strength at the threshold of barely visible damage.

Chen, G.-S.↗

NASA NTRS1993

Effect of impact damage on the compression fatigue behavior of composite tubes

Experimental results are presented from an exploratory study to assess the effect of barely visible impact damage on the fatigue behavior of thin walled composite tubes. Undamaged composite tubes and tubes containing barely visible impact damage were tested in compression-compression fatigue. All tubes were inspected ultrasonically prior to testing. Tubes were outfitted with strain gages both in the damaged and far-field regions and an extensometer was utilized to measure axial displacements across the damaged region. Periodically, testing was halted and tubes were inspected ultrasonically and statically tested. In general, the impact damaged tubes exhibited a significantly shorter life than the undamaged tubes; however, there were considerable differences in the lives of the individual impact damaged tubes. No damage growth or stiffness loss was observed prior to failure. Failure of the impact damaged tubes was observed to occur in a combination of delamination buckling and shear crippling modes.

Krafchak, T. M.↗

NASA NTRS1993

Probabilistic assessment of composite structures

A general purpose methodology for integrated probabilistic assessment of composite structures is demonstrated using aircraft fuselage type structures with rectangular cutouts. The computational simulation took into account buckling loads, vibration frequencies, global displacements, and local stresses. The procedure was embedded into the computer code IPACS (Integrated Probabilistic Assessment of Composite Structures) that calculates the simulated scatter, the sensitivity of the composite structural behavior to all the primitive variables that affect the structural behavior. The probabilistic assessment for the composite structure with rectangular cutouts indicates that the uncertainty in the longitudinal ply stress is mainly due to uncertainty in the laminate thickness.

Shiao, Michael C.↗

NASA NTRS1993

Optimal design of imperfect, anisotropic, ring-stiffened cylinders under combined loads

Development of an algorithm to perform the optimal sizing of buckling resistant, imperfect, anisotropic ring-stiffened cylinders subjected to axial compression, torsion, and internal pressure is presented. The enforcement of stability constraints is treated in a way that does not require any eigenvalue analysis. Assumption of criticality of these stability constraints during the optimal sizing of the cylinders produced designs that nevertheless satisfied all of the stress constraints as well as the stability constraints. Case studies performed using a combination of penalty function and feasible direction optimization methods indicate that the presence of the axisymmetric initial imperfection in the cylinder wall can significantly affect the optimal designs. Weight savings associated with the addition of two rings to the unstiffened cylinder and/or the addition of internal pressure is substantial when torsion makes up a significant fraction of the combined load state.

Ley, Robert P.↗

NASA NTRS1993

Design of a blade stiffened composite panel by a genetic algorithm

Genetic algorithms (GAs) readily handle discrete problems, and can be made to generate many optima, as is presently illustrated for the case of design for minimum-weight stiffened panels with buckling constraints. The GA discrete design procedure proved superior to extant alternatives for both stiffened panels with cutouts and without cutouts. High computational costs are, however, associated with this discrete design approach at the current level of its development.

Nagendra, S.↗

NASA NTRS1993

A Lanczos algorithm for vibration, suckling and termal analysis

This paper reviews an eigensolver algorithm based on the Lanczos Method for vibration, buckling and thermal analysis. The original code was written for inclusion in the Computational Mechanics Testbed (COMET), a general purpose finite element code. A portable version of the Lanczos code that is optimized for high-performance supercomputers has been developed. Special features of the algorithm include the capability to compute rigid body modes, thermal modes and Lanczos vectors that are derived from the applied load vector. The latter is necessary when using the Lanczos vectors as reduced-basis vectors in transient structural response and transient heat conduction calculations. The modularity of the code allows the user the option of including the most up-to-date utilities, such as the equation solver best suited for the application. The algorithm is discussed in detail and results of several applications are presented. Timing results for a vibration application indicate that the Lanczos algorithm is twenty times faster than the subspace iteration method which has been extensively used in the past.

Bostic, Susan W.↗

NASA NTRS1993

Probabilistically configured adaptive composite structures

A composite wing with spars, bulkheads and built-in sensor/control devices is probabilistically configured with a methodology for the probabilistic assessment of smart composite structures. Structural responses such as changes in angle of attack, vertical displacements and stresses in the non-control and control plies are probabilistically assessed to quantify their respective scatter ranges. Sensitivity factors are evaluated to identify those parameters that have the greatest influence on a specific structural response. Results show that smart composite structures can be configured to control distortions and ply stresses, and to have specified scatter ranges in the frequencies, buckling loads in the presence of defects to satisfy specified design requirements.

Shiao, Michael C.↗