Extended tables of stiffness and carry-over factor for structural members under axial load
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Software Performs Complex Design Analysis
Designers use computational fluid dynamics (CFD) to gain greater understanding of the fluid flow phenomena involved in components being designed. They also use finite element analysis (FEA) as a tool to help gain greater understanding of the structural response of components to loads, stresses and strains, and the prediction of failure modes. Automated CFD and FEA engineering design has centered on shape optimization, which has been hindered by two major problems: 1) inadequate shape parameterization algorithms, and 2) inadequate algorithms for CFD and FEA grid modification. Working with software engineers at Stennis Space Center, a NASA commercial partner, Optimal Solutions Software LLC, was able to utilize its revolutionary, one-of-a-kind arbitrary shape deformation (ASD) capability-a major advancement in solving these two aforementioned problems-to optimize the shapes of complex pipe components that transport highly sensitive fluids. The ASD technology solves the problem of inadequate shape parameterization algorithms by allowing the CFD designers to freely create their own shape parameters, therefore eliminating the restriction of only being able to use the computer-aided design (CAD) parameters. The problem of inadequate algorithms for CFD grid modification is solved by the fact that the new software performs a smooth volumetric deformation. This eliminates the extremely costly process of having to remesh the grid for every shape change desired. The program can perform a design change in a markedly reduced amount of time, a process that would traditionally involve the designer returning to the CAD model to reshape and then remesh the shapes, something that has been known to take hours, days-even weeks or months-depending upon the size of the model.
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Non-Linear Analysis of the NASA Super Pressure Balloons: Whole Flight Simulations
Tensys have a long-established background in the shape generation and load analysis of architectural stressed membrane structures. Founded upon their inTENS finite element analysis suite, these activities have broadened to encompass 'lighter than air' structures such as aerostats, airships, hybrid air vehicles and stratospheric balloons. Tensys have acted as consultants to the NASA Super Pressure Balloon (SPB) Program since 2004. Previous papers have focussed upon the application of inTENS to the overall structural and stability analysis of pumpkin type balloons as used in the SPB Program. Particular emphasis has been placed upon the ability to study both stress and stability at all stages of a flight. As the program has developed, increasing modelling fidelity has been introduced as design refinement has moved emphasis from the overall shape to the performance of individual details. Examples include the introduction of a contact capability into inTENS to better represent the separate cap layer of film. Localised investigations have considered the consequence of local geometric anomalies introduced during fabrication and the effects of debonding of the PBO tendon within its sleeve. Analysis to date has used a material model for the polyethylene shell film developed by Dr Rand of Winzen Engineering, supported by a program of fine resolution material tests at the Balloon Research and Development Laboratory (BRDL) at the GSFC Wallops Island Facility. Based upon work first reported by Schapery, this model has been incorporated into so-called 'snapshot' analyses within inTENS. For a given elapsed time point and temperature distribution, non-linear material properties are iteratively updated for the current stress state in each individual element until changes in those material properties are insignificant. This process has to be iterative as the film stresses and material properties are interdependent. Attempts to derive a time-stepping incremental viscoelastic capability using the Schapery Rand model encountered problems when dealing with the varying temperatures associated with balloon deployment, pressurisation and diurnal behaviour. An alternative approach has been developed by Pellegrino et al at Caltech, again with support from the NASA Balloon Program Office. This is a large strain non-linear viscoelastic model that includes film out-of-plane mechanical and thermal effects. Working from a unified base, this model can be utilised for time-stepping analyses in both a modulus or compliance mode, or a combination of both. This new capability is intended to enhance the current NASA balloon design process. This paper presents the implementation of the Caltech model into a specialist finite element suite, inTENS and its application to whole flight simulations.
Wakefield, David↗
On the optimization of discrete structures with aeroelastic constraints
The paper deals with the problem of dynamic structural optimization where constraints relating to flutter of a wing (or other dynamic aeroelastic performance) are imposed along with conditions of a more conventional nature such as those relating to stress under load, deflection, minimum dimensions of structural elements, etc. The discussion is limited to a flutter problem for a linear system with a finite number of degrees of freedom and a single constraint involving aeroelastic stability, and the structure motion is assumed to be a simple harmonic time function. Three search schemes are applied to the minimum-weight redesign of a particular wing: the first scheme relies on the method of feasible directions, while the other two are derived from necessary conditions for a local optimum so that they can be referred to as optimality-criteria schemes. The results suggest that a heuristic redesign algorithm involving an optimality criterion may be best suited for treating multiple constraints with large numbers of design variables.
Mcintosh, S. C., Jr.↗
Derivation of improved load transformation matrices for launchers-spacecraft coupled analysis, and direct computation of margins of safety
Load and stress recovery from transient dynamic studies are improved upon using an extended acceleration vector in the modal acceleration technique applied to structural analysis. Extension of the normal LTM (load transformation matrices) stress recovery to automatically compute margins of safety is presented with an application to the Hubble space telescope.
Klein, M.↗
MSC/NASTRAN Stress Analysis of Complete Models Subjected to Random and Quasi-Static Loads
Space payloads, such as those which fly on the Space Shuttle in Spacelab, are designed to withstand dynamic loads which consist of combined acoustic random loads and quasi-static acceleration loads. Methods for computing the payload stresses due to these loads are well known and appear in texts and NASA documents, but typically involve approximations such as the Miles' equation, as well as possible adjustments based on "modal participation factors." Alternatively, an existing capability in MSC/NASTRAN may be used to output exact root mean square [rms] stresses due to the random loads for any specified elements in the Finite Element Model. However, it is time consuming to use this methodology to obtain the rms stresses for the complete structural model and then combine them with the quasi-static loading induced stresses. Special processing was developed as described here to perform the stress analysis of all elements in the model using existing MSC/NASTRAN and MSC/PATRAN and UNIX utilities. Fail-safe and buckling analyses applications are also described.
Hampton, Roy W.↗
Tubular space truss structure for SKITTER 2 robot
The Skitter 2 is a three legged transport vehicle designed to demonstrate the principle of a tripod walker in a multitude of environments. The tubular truss model of Skitter 2 is a proof of principal design. The model will replicate the operational capabilities of Skitter 2 including its ability to self-right itself. The project's focus was on the use of light weight tubular members in the final structural design. A strong design for the body was required as it will undergo the most intense loading. Triangular geometry was used extensively in the body, providing the required structural integrity and eliminating the need for cumbersome shear panels. Both the basic femur and tibia designs also relied on the strong geometry of the triangle. An intense literature search aided in the development of the most suitable weld techniques, joints, linkages, and materials required for a durable design. The hinge design features the use of spherical rod end bearings. In order to obtain a greater range of mobility in the tibia, a four-bar linkage was designed which attaches both to the femur and the tibia. All component designs, specifically the body, femur, and the tibia were optimized using the software package IDEAS 3.8A Supertab. The package provided essential deformation and stress analysis information on each component's design. The final structure incurred only a 0.0544 inch deflection in a maximum (worst case) loading situation. The highest stress experienced by any AL6061-T6 tubular member was 1920 psi. The structural integrity of the final design facilitated the use of Aluminum 6061-T6 tubing. The tubular truss structure of Skitter 2 is an effective and highly durable design. All facets of the design are structurally sound and cost effective.
Beecham, Richard↗
Probabilistic Evaluation of Advanced Ceramic Matrix Composite Structures
The objective of this report is to summarize the deterministic and probabilistic structural evaluation results of two structures made with advanced ceramic composites (CMC): internally pressurized tube and uniformly loaded flange. The deterministic structural evaluation includes stress, displacement, and buckling analyses. It is carried out using the finite element code MHOST, developed for the 3-D inelastic analysis of structures that are made with advanced materials. The probabilistic evaluation is performed using the integrated probabilistic assessment of composite structures computer code IPACS. The affects of uncertainties in primitive variables related to the material, fabrication process, and loadings on the material property and structural response behavior are quantified. The primitive variables considered are: thermo-mechanical properties of fiber and matrix, fiber and void volume ratios, use temperature, and pressure. The probabilistic structural analysis and probabilistic strength results are used by IPACS to perform reliability and risk evaluation of the two structures. The results will show that the sensitivity information obtained for the two composite structures from the computational simulation can be used to alter the design process to meet desired service requirements. In addition to detailed probabilistic analysis of the two structures, the following were performed specifically on the CMC tube: (1) predicted the failure load and the buckling load, (2) performed coupled non-deterministic multi-disciplinary structural analysis, and (3) demonstrated that probabilistic sensitivities can be used to select a reduced set of design variables for optimization.
Abumeri, Galib H.↗
Damage tolerance of pressurized graphite/epoxy tape cylinders under uniaxial and biaxial loading
The damage tolerance behavior of internally pressurized, axially slit, graphite/epoxy tape cylinders was investigated. Specifically, the effects of axial stress, structural anisotropy, and subcritical damage were considered. In addition, the limitations of a methodology which uses coupon fracture data to predict cylinder failure were explored. This predictive methodology was previously shown to be valid for quasi-isotropic fabric and tape cylinders but invalid for structurally anisotropic (+/-45/90)(sub s) and (+/-45/0)(sub s) cylinders. The effects of axial stress and structural anisotropy were assessed by testing tape cylinders with (90/0/+/-45)(sub s), (+/-45/90)(sub s), and (+/-45/0)(sub s) layups in a uniaxial test apparatus, specially designed and built for this work, and comparing the results to previous tests conducted in biaxial loading. Structural anisotropy effects were also investigated by testing cylinders with the quasi-isotropic (0/+/-45/90)(sub s) layup which is a stacking sequence variation of the previously tested (90/0/+/-45)(sub s) layup with higher D(sub 16) and D(sub 26) terms but comparable D(sub 16) and D(sub 26) to D(sub 11) ratios. All cylinders tested and used for comparison are made from AS4/3501-6 graphite/epoxy tape and have a diameter of 305 mm. Cylinder slit lengths range from 12.7 to 50.8 mm. Failure pressures are lower for the uniaxially loaded cylinders in all cases. The smallest percent failure pressure decreases are observed for the (+/-45/90)(sub s) cylinders, while the greatest such decreases are observed for the (+/-45/0)(sub s) cylinders. The relative effects of the axial stress on the cylinder failure pressures do not correlate with the degree of structural coupling. The predictive methodology is not applicable for uniaxially loaded (+/-45/90)(sub s) and (+/-45/0)(sub s) cylinders, may be applicable for uniaxially loaded (90/0/+/-45)(sub s) cylinders, and is applicable for the biaxially loaded (90/0/+/-45)(sub s) and (0/+/-45/90)(sub s) cylinders. This indicates that the ratios of D(sub 16) and D(sub 26) to D(sub 11), as opposed to the absolute magnitudes of D(sub 16) and D(sub 26), may be important in the failure of these cylinders and in the applicability of the methodology. Discontinuities observed in the slit tip hoop strains for all the cylinders tested indicate that subcritical damage can play an important role in the failure of tape cylinders. This role varies with layup and loading condition and is likely coupled to the effects of structural anisotropy. Biaxial failure pressures may exceed the uniaxial values because the axial stress contributes to the formation of 0 deg ply splitting (accompanied by delamination) or similar stress-mitigating subcritical damage. The failure behavior of similar cylinders can also vary as a result of differences in the role of subcritical damage as observed for the case of a biaxially loaded (90/0/+/-45)(sub s) cylinder with a 12.7 mm slit. For this case, the methodology is valid when the initial coupon and cylinder fracture modes agree. However, the methodology underpredicts the failure pressure of the cylinder when a circumferential fracture path, suggestive of a 0 deg ply split, occurs at one slit tip. Thus, the failure behavior of some tape cylinders may be highly sensitive to the initial subcritical damage mechanism. Finite element analyses are recommended to determine how structural anisotropy and axial stress modify the slit tip stress states in cylinders from those found in flat plates since similarity of these stress states is a fundamental assumption of the current predictive methodology.
Priest, Stacy Marie↗
Study made of procedures for externally loading and corrosion testing stress corrosion specimens
Study was initiated to determine methods or test specimens for evaluating stress corrosion cracking characteristics of common structural materials. It was found that the methods of externally loading and corrosion testing were reliable in yielding reproducible results for stress corrosion evaluation.
Humphries, T. S.↗
Unified constitutive models for high-temperature structural applications
Unified constitutive models are characterized by the use of a single inelastic strain rate term for treating all aspects of inelastic deformation, including plasticity, creep, and stress relaxation under monotonic or cyclic loading. The structure of this class of constitutive theory pertinent for high temperature structural applications is first outlined and discussed. The effectiveness of the unified approach for representing high temperature deformation of Ni-base alloys is then evaluated by extensive comparison of experimental data and predictions of the Bodner-Partom and the Walker models. The use of the unified approach for hot section structural component analyses is demonstrated by applying the Walker model in finite element analyses of a benchmark notch problem and a turbine blade problem.
Lindholm, U. S.↗
Computer analysis of two-dimensional fatigue flaw-growth problems
A description is given of a computer program for analyzing the crack propagation in cyclic loaded structures. By using equations for the stress intensity factors for surface and embedded flaws, the capability of ascertaining lengthwise crack growth as well as crack growth through the thickness of the structure was determined. Equations used are written in general form so that many other types of crack growth problems, such as through the thickness type cracks, can be analyzed. Provisions have been made in the computer program to analyze crack growth for repeated blocks of variable-amplitude fatigue load spectra. Additional features of the computer program include a variable print interval and a variable integration interval to minimize computer printout and computer run time. A source listing and instructions for using the computer program are included.
Forman, R. G.↗
Probabilistic design analysis using Composite Loads Spectra (CLS) coupled with Probabilistic Structural Analysis Methodologies (PSAM)
Composite loads spectra (CLS) were applied to generate probabilistic loads for use in the PSAM nonlinear evaluation of stochastic structures under stress (NESSUS) finite element code. The CLS approach allows for quantifying loads as mean values and distributions around a central value rather than maximum or enveloped values typically used in deterministic analysis. NESSUS uses these loads to determine mean and perturbation responses. These results are probabilistically evaluated with the distributional information from CLS using a fast probabilistic integration (FPI) technique to define response distributions. The main example discussed describes a method of obtaining load descriptions and stress response of the second-stage turbine blade of the Space Shuttle Main Engine (SSME) high-pressure fuel turbopump (HPFTP). Additional information is presented on the on-going analysis of the high pressure oxidizer turbopump discharge duct (HPOTP) where probabilistic dynamic loads have been generated and are in the process of being used for dynamic analysis. Example comparisons of load analysis and engine data are furnished for partial verification and/or justification for the methodology.
Newell, J. F.↗
An Inverse Interpolation Method Utilizing In-Flight Strain Measurements for Determining Loads and Structural Response of Aerospace Vehicles
An important and challenging technology aimed at the next generation of aerospace vehicles is that of structural health monitoring. The key problem is to determine accurately, reliably, and in real time the applied loads, stresses, and displacements experienced in flight, with such data establishing an information database for structural health monitoring. The present effort is aimed at developing a finite element-based methodology involving an inverse formulation that employs measured surface strains to recover the applied loads, stresses, and displacements in an aerospace vehicle in real time. The computational procedure uses a standard finite element model (i.e., "direct analysis") of a given airframe, with the subsequent application of the inverse interpolation approach. The inverse interpolation formulation is based on a parametric approximation of the loading and is further constructed through a least-squares minimization of calculated and measured strains. This procedure results in the governing system of linear algebraic equations, providing the unknown coefficients that accurately define the load approximation. Numerical simulations are carried out for problems involving various levels of structural approximation. These include plate-loading examples and an aircraft wing box. Accuracy and computational efficiency of the proposed method are discussed in detail. The experimental validation of the methodology by way of structural testing of an aircraft wing is also discussed.
Shkarayev, S.↗
Stress analysis and design considerations for Shuttle pointed autonomous research tool for astronomy /SPARTAN/
The Shuttle Pointed Autonomous Research Tool for Astronomy (SPARTAN) family of spacecraft are intended to operate with minimum interfaces with the U.S. Space Shuttle in order to increase flight opportunities. The SPARTAN I Spacecraft was designed to enhance structural capabilities and increase reliability. The approach followed results from work experience which evolved from sounding rocket projects. Structural models were developed to do the analyses necessary to satisfy safety requirements for Shuttle hardware. A loads analysis must also be performed. Stress analysis calculations will be performed on the main structural elements and subcomponents. Attention is given to design considerations and program definition, the schematic representation of a finite element model used for SPARTAN I spacecraft, details of loads analysis, the stress analysis, and fracture mechanics plan implications.
Ferragut, N. J.↗
Giant Electrostriction via Nanodomain Engineering in Relaxor Ferroelectric Polymers
Relaxor ferroelectric (RFE) polymers hold great promise for artificial muscles due to their high actuation strain, high loading stress, and fast response. However, the structural origin underlying their large electrostrictive deformation remains elusive. In this study, we investigate poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)]-based RFE terpolymers, incorporating 1,1-chlorofluoroethylene (CFE) or chlorotrifluoroethylene (CTFE) (the terpolymers are denoted as terP-CFE and terP-CTFE, respectively) as termonomers. Although both terpolymers show similar semicrystalline morphology, drastically different electrostrictive properties are observed. Specifically, the terP-CFE annealed at 100 °C achieves a record-high transverse strain of ∼10.6%, whereas 100 °C-annealed terP-CTFE only shows a much lower actuation strain of ∼4.2% at the same poling field of 190 MV/m. To elucidate the origin of this difference, time-resolved wide-angle X-ray diffraction, small-angle X-ray scattering, and Fourier transform infrared experiments are performed during in situ electric poling. An RFE-to-ferroelectric (FE) crystal phase transition is observed for terP-CFE but is absent for terP-CTFE. Beyond the contribution of the crystalline phase, the oriented amorphous fraction and crystalline defects (e.g., taut-tie molecules) also play significant roles in enhancing electrostriction. In conclusion, this mechanistic insight provides a valuable foundation for the rational design of next-generation RFE polymers with tunable properties through defect engineering of their semicrystalline structures.
36 MATERIALS SCIENCE↗
The Goddard Summer Workshop program in analysis of space data and measurement of space environments Final report
Space data information systems, radiation sensors and solar simulation, structural responses to dynamic loads and thermal stresses, and magnetic fields and plasmas - workshop program
SOLAR SIMULATION↗
Discrete element methods for the plastic analysis of structures subjected to cyclic loading.
Discrete element method for plastic analysis of complex built-up structures subjected to cyclic loading causing membrane stress and stress reversal
STRESS ANALYSIS↗