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NASA NTRS1978

Computer-aided analysis and design of the shape rolling process for producing turbine engine airfoils

Mild steel (AISI 1018) was selected as model cold-rolling material and Ti-6Al-4V and INCONEL 718 were selected as typical hot-rolling and cold-rolling alloys, respectively. The flow stress and workability of these alloys were characterized and friction factor at the roll/workpiece interface was determined at their respective working conditions by conducting ring tests. Computer-aided mathematical models for predicting metal flow and stresses, and for simulating the shape-rolling process were developed. These models utilize the upper-bound and the slab methods of analysis, and are capable of predicting the lateral spread, roll-separating force, roll torque and local stresses, strains and strain rates. This computer-aided design (CAD) system is also capable of simulating the actual rolling process and thereby designing roll-pass schedule in rolling of an airfoil or similar shape. The predictions from the CAD system were verified with respect to cold rolling of mild steel plates. The system is being applied to cold and hot isothermal rolling of an airfoil shape, and will be verified with respect to laboratory experiments under controlled conditions.

Lahoti, G. D.↗

NASA NTRS1999

Design of Composite Structures for Reliability and Damage Tolerance

A summary of research conducted during the first year is presented. The research objectives were sought by conducting two tasks: (1) investigation of probabilistic design techniques for reliability-based design of composite sandwich panels, and (2) examination of strain energy density failure criterion in conjunction with response surface methodology for global-local design of damage tolerant helicopter fuselage structures. This report primarily discusses the efforts surrounding the first task and provides a discussion of some preliminary work involving the second task.

Rais-Rohani, Masoud↗

NASA NTRS1994

Structurally compliant rocket engine combustion chamber: Experimental and analytical validation

A new, structurally compliant rocket engine combustion chamber design has been validated through analysis and experiment. Subscale, tubular channel chambers have been cyclically tested and analytically evaluated. Cyclic lives were determined to have a potential for 1000 percent increase over those of rectangular channel designs, the current state of the art. Greater structural compliance in the circumferential direction gave rise to lower thermal strains during hot firing, resulting in lower thermal strain ratcheting and longer predicted fatigue lives. Thermal, structural, and durability analyses of the combustion chamber design, involving cyclic temperatures, strains, and low-cycle fatigue lives, have corroborated the experimental observations.

Jankovsky, Robert S.↗

NASA NTRS2003

Liner-less Tanks for Space Application - Design and Manufacturing Considerations

Composite pressure vessels, used extensively for gas and fuel containment in space vehicles, are generally constructed with a metallic liner, while the fiber reinforcement carries the major portion of the pressure-induced load. The design is dominated by the liner s low strain at yield since the reinforcing fibers cannot operate at their potential load-bearing capability without resorting to pre-stressing (or autofrettaging). An ultra high-efficiency pressure vessel, which operates at the optimum strain capability of the fibers, can be potentially achieved with a liner-less construction. This paper discusses the design and manufacturing challenges to be overcome in the development of such a pressure vessel. These include: (1) gas/liquid containment and permeation, (2) design and structural analysis, and (3) manufacturing process development. The paper also presents the development and validation tests on a liner-less pressure vessel developed by Kaiser Compositek Inc. (KCI). It should be noted that KCI s liner-less tank exhibits a highly controlled leak-before-burst mode. This feature results in a structure having the highest level of safety.

Jones, Brian H.↗

NASA NTRS2011

Utilizing Photogrammetry and Strain Gage Measurement to Characterize Pressurization of Inflatable Modules

This paper focuses on integrating a large hatch penetration into inflatable modules of various constructions. This paper also compares load predictions with test measurements. The strain was measured by utilizing photogrammetric methods and strain gages mounted to select clevises that interface with the structural webbings. Bench testing showed good correlation between strain data collected from an extensometer and photogrammetric measurements, even when the material transitioned from the low load to high load strain region of the curve. The full-scale torus design module showed mixed results as well in the lower load and high strain regions. After thorough analysis of photogrammetric measurements, strain gage measurements, and predicted load, the photogrammetric measurements seem to be off by a factor of two.

Mohammed, Anil↗

NASA NTRS1977

Optimization of flexible wing structures subject to strength and induced drag constraints

An optimization procedure for designing wing structures subject to stress, strain, and drag constraints is presented. The optimization method utilizes an extended penalty function formulation for converting the constrained problem into a series of unconstrained ones. Newton's method is used to solve the unconstrained problems. An iterative analysis procedure is used to obtain the displacements of the wing structure including the effects of load redistribution due to the flexibility of the structure. The induced drag is calculated from the lift distribution. Approximate expressions for the constraints used during major portions of the optimization process enhance the efficiency of the procedure. A typical fighter wing is used to demonstrate the procedure. Aluminum and composite material designs are obtained. The tradeoff between weight savings and drag reduction is investigated.

Haftka, R. T.↗

NASA NTRS1992

Laboratory evaluation of footings for lunar telescopes

Presented here are the results of laboratory experiments with diffferent footing shapes for lunar telescopes. These experiments used a variety of soils including some to simulate regolith response. Based on what is known of regolith and regolith-structure interaction, a shallow-multiple-contact points footing foundation can be adequately designed to support lunar telescopes. Plane-strain load-displacement tests were conducted with different footings and different lunar simulants in a deep transparent plexiglass container. The model footings considered include the rectangular, hemispherical, and spudcan designs. Simulants used to reproduce the mechanical properties of the lunar regolith were fly ash, crushed basalt with and without glass, and a processed lunar simulant. Load-displacement curves were obtained for the different footings in Ottawa sand and in the crushed basalt with glass. The spudcan footing was found to be self-digging and yet stiff, thus providing excellent lateral stability in a large variety of soils.

Chua, Koon M.↗

NASA NTRS2003

Hydrostatic Stress Effects Incorporated Into the Analysis of the High-Strain-Rate Deformation of Polymer Matrix Composites

Procedures for modeling the effect of high strain rate on composite materials are needed for designing reliable composite engine cases that are lighter than the metal cases in current use. The types of polymer matrix composites that are likely to be used in such an application have a deformation response that is nonlinear and that varies with strain rate. The nonlinearity and strain rate dependence of the composite response is primarily due to the matrix constituent. Therefore, in developing material models to be used in the design of impact-resistant composite engine cases, the deformation of the polymer matrix must be correctly analyzed. However, unlike in metals, the nonlinear response of polymers depends on the hydrostatic stresses, which must be accounted for within an analytical model. An experimental program has been carried out through a university grant with the Ohio State University to obtain tensile and shear deformation data for a representative polymer for strain rates ranging from quasi-static to high rates of several hundred per second. This information has been used at the NASA Glenn Research Center to develop, characterize, and correlate a material model in which the strain rate dependence and nonlinearity (including hydrostatic stress effects) of the polymer are correctly analyzed. To obtain the material data, Glenn s researchers designed and fabricated test specimens of a representative toughened epoxy resin. Quasi-static tests at low strain rates and split Hopkinson bar tests at high strain rates were then conducted at the Ohio State University. The experimental data confirmed the strong effects of strain rate on both the tensile and shear deformation of the polymer. For the analytical model, Glenn researchers modified state variable constitutive equations previously used for the viscoplastic analysis of metals to allow for the analysis of the nonlinear, strain-rate-dependent polymer deformation. Specifically, we accounted for the effects of hydrostatic stresses. An important discovery in the course of this work was that the hydrostatic stress effects varied during the loading process, which needed to be accounted for within the constitutive equations. The model is characterized primarily by shear data, with tensile data used to characterize the hydrostatic stress effects.

Goldberg, Robert K.↗

NASA NTRS1996

Design, Fabrication, and Calibration of an Embedded Piezoceramic Actuator for Active Control Applications

In this presentation, the authors describe the design and fabrication processes for a PZT strain actuator that evolved during the initial stages of a research effort to synthesize and process intelligent, cost effective structures (SPICES). The actuator performance requirements were similar to those of conventional actuators, e.g., it had to be robust, highly efficient with adequate force and stroke, as lightweight as possible, and most importantly, affordable. Further, since the actuator was to be integrated within a composite structure, it had to be compatible with the host material and easily embeddable during the fabrication process. In control applications employing strain devices as actuators, a good bond between this actuator and host material is critical to their successful operation. This criterion is often difficult to achieve when attempting to join ceramics with metals or polymers with dissimilar properties such as Young's moduli, thermal expansion coefficients, etc. One unique feature of the actuator design that evolved in this project is that the need for direct bonding between the PZT ceramic and polymers was circumvented, i.e. the strain transfer to the host material was achieved via a frame surrounding the ceramic. Consequently, the frame material could be selected (or coated) for compatibility with the host material. A second feature is that the frame enclosed a co-fired, multilayered, PZT stack that was used to minimize the voltage requirements while maximizing the output strain.

Koopmann, Gary H.↗

NASA NTRS2008

High-Temperature Strain Sensing for Aerospace Applications

Thermal protection systems (TPS) and hot structures are utilizing advanced materials that operate at temperatures that exceed abilities to measure structural performance. Robust strain sensors that operate accurately and reliably beyond 1800 F are needed but do not exist. These shortcomings hinder the ability to validate analysis and modeling techniques and hinders the ability to optimize structural designs. This presentation examines high-temperature strain sensing for aerospace applications and, more specifically, seeks to provide strain data for validating finite element models and thermal-structural analyses. Efforts have been made to develop sensor attachment techniques for relevant structural materials at the small test specimen level and to perform laboratory tests to characterize sensor and generate corrections to apply to indicated strains. Areas highlighted in this presentation include sensors, sensor attachment techniques, laboratory evaluation/characterization of strain measurement, and sensor use in large-scale structures.

Piazza, Anthony↗

NASA NTRS1982

Method of measuring elastic strain distribution in specimens used for high frequency fatigue testing

A technique using a point contact piezoelectric detector has been developed which permits the displacement amplitude distributions over the entire length of high frequency fatigue specimens to be measured. Elastic strain distributions can be derived with a computer analysis that uses curve smoothing and numerical derivative methods. The equipment and techniques for measuring and processing the data are described, and displacement amplitudes, strain distributions, and magnification factors for straight rods and dumbbell-shaped specimens are obtained and shown to agree with theoretically predicted values. It is shown that the technique presented here can be used to design specimens with unusual displacement amplitudes and strain distributions.

Sirian, C. R.↗

NASA NTRS1985

Iosipescu shear properties of graphite fabric/epoxy composite laminates

The Iosipescu shear test method is used to measure the in-plane and interlaminar shear properties of four T300 graphite fabric/934 epoxy composite materials. Different weave geometries tested include an Oxford weave, a 5-harness satin weave, an 8-harness satin weave, and a plain weave with auxiliary warp yarns. Both orthogonal and quasi-isotropic layup laminates were tested. In-plane and interlaminar shear properties are obtained for laminates of all four fabric types. Overall, little difference in shear properties attributable to the fabric weave pattern is observed. The auxiliary warp material is significantly weaker and less stiff in interlaminar shear parallel to its fill direction. A conventional strain gage extensometer is modified to measure shear strains for use with the Iosipescu shear test. While preliminary results are encouraging, several design iterations failed to produce a reliable shear transducer prototype. Strain gages are still the most reliable shear strain transducers for use with this test method.

Walrath, D. E.↗

NASA NTRS2002

Optical Fiber Strain Instrumentation for High Temperature Aerospace Structural Monitoring

The objective of the program is the development and laboratory demonstration of sensors based on silica optical fibers for measurement of high temperature strain for aerospace materials evaluations. A complete fiber strain sensor system based on white-light interferometry was designed and implemented. An experiment set-up was constructed to permit testing of strain measurement up to 850 C. The strain is created by bending an alumina cantilever beam to which is the fiber sensor is attached. The strain calibration is provided by the application of known beam deflections. To ensure the high temperature operation capability of the sensor, gold-coated single-mode fiber is used. Moreover, a new method of sensor surface attachment which permits accurate sensor gage length determination is also developed. Excellent results were obtained at temperatures up to 800-850 C.

Wang, A.↗

NASA NTRS1961

Investigation of Three Analytical Hypothesis for Determining Material Creep Behavior under Varied Loads, with an Application to 2024-T3 Aluminum-Alloy Sheet in Tension at 400 F

Three existing hypotheses are formulated mathematically to estimate tensile creep strain under varied loads and constant temperature from creep data obtained under constant load and constant temperature. hypotheses investigated include the time-hardening, strain-hardening, and life-fraction rules. Predicted creep behavior is compared with data obtained from tensile creep tests of 2024-T3 aluminum-alloy sheet at 400 F under cyclic-load conditions. creep strain under varied loads is presented on the basis of an equivalent stress, derived from the life-fraction rule, which reduces the varied-load case to a constant-load problem. Creep strain in the region of interest for structural design and rupture times, determined from the hypotheses investigated, are in fair agreement with data in most cases, although calculated values of creep strain are generally greater than the experimental values because creep recovery is neglected in the calculations.

Berkovits, Avraham↗

NASA NTRS1986

Optimal output fast feedback in two-time scale control of flexible arms

Control of lightweight flexible arms moving along predefined paths can be successfully synthesized on the basis of a two-time scale approach. A model following control can be designed for the reduced order slow subsystem. The fast subsystem is a linear system in which the slow variables act as parameters. The flexible fast variables which model the deflections of the arm along the trajectory can be sensed through strain gage measurements. For full state feedback design the derivatives of the deflections need to be estimated. The main contribution of this work is the design of an output feedback controller which includes a fixed order dynamic compensator, based on a recent convergent numerical algorithm for calculating LQ optimal gains. The design procedure is tested by means of simulation results for the one link flexible arm prototype in the laboratory.

Siciliano, B.↗

NASA NTRS1983

Strainrange partitioning: A total strain range version

Procedures are presented for expressing the Strainrange Partitioning (SRP) method for creep fatigue life prediction in terms of total strain range. Inelastic and elastic strain-range - life relations are summed to give total strain-range - life relations. The life components due to inelastic strains are dealt with using conventional SRP procedures while the life components due to elastic strains are expressed as families of time-dependent terms for each type of SRP cycle. Cyclic constitutive material behavior plays an important role in establishing the elastic strain-range - life relations as well as the partitioning of the inelastic strains. To apply the approach, however, it is not necessary to have to determine the magnitude of the inelastic strain range. The total strain SRP approach is evaluated and verified using two nickel base superalloys, AF2-1DA and Rene 95. Excellent agreement is demonstrated between observed and predicted cyclic lifetimes with 70 to 80 percent of the predicted lives falling within factors of two of the observed lives. The total strain-range SRP approach should be of considerable practical value to designers who are faced with creep-fatigue problems for which the inelastic strains cannot be calculated with sufficient accuracy to make reliable life predictions by the conventional inelastic strain range SRP approach.

Halford, G. R.↗

NASA NTRS1983

Strainrange partitioning - A total strain range version

Procedures are presented for expressing the Strainrange Partitioning (SRP) method for creep fatigue life prediction in terms of total strain range. Inelastic and elastic strain-range - life relations are summed to give total strain-range - life relations. The life components due to inelastic strains are dealt with using conventional SRP procedures while the life components due to elastic strains are expressed as families of time-dependent terms for each type of SRP cycle. Cyclic constitutive material behavior plays an important role in establishing the elastic strain-range life relations as well as the partitioning of the inelastic strains. To apply the approach, however, it is not necessary to have to determine the magnitude of the inelastic strain range. The total strain SRP approach is evaluated and verified using two nickel base superalloys, AF2-1DA and Rene 95. Excellent agreement is demonstrated between observed and predicted cyclic lifetimes with 70 to 80 percent of the predicted lives falling within factors of two of the observed lives. The total strain-range SRP approach should be of considerable practical value to designers who are faced with creep-fatigue problems for which the inelastic strains cannot be calculated with sufficient accuracy to make reliable life predictions by the conventional inelastic strain range SRP approach.

Halford, G. R.↗

NASA NTRS2023

Ruggedness Evaluation of ASTM International Standard Test Methods for Shape Memory Materials: E3098 Standard Test Method for Mechanical Uniaxial Pre-strain and Thermal Free Recovery of Shape Memory Alloys

This paper evaluates the ruggedness testing of the recently released ASTM International E3098 Standard Test Method for Mechanical Uniaxial Pre-strain and Thermal Free Recovery of Shape Memory Alloys. The ruggedness experiment was designed with eight runs in two replicates, consisting of seven factors: strain rate (ė), heating and cooling rates (Ṫ heat and Ṫ cool , respectively), upper and lower cycle temperatures (UCT and LCT, respectively), hold time (t hold ), and minimum load (F min ) imparted on the samples. The results indicate that the strain rate factor had no effect on any result variable, whereas the minimum load factor had a large effect on several result variables including the austenite start and martensite finish temperatures as well as most of the strain measurements from the thermal cycle. The lower cycle temperature factor had a large effect on the stress at maximum strain during pre-straining (e i ), denoted by S i , at ~7.2 MPa, as well as the A f , at 5.47 °C. Closely following the LCT effect, Ṫ heat was found to have a large effect on the austenite start temperature (A s ), at a value of ~2.72 °C. The testing methodology, analysis techniques, and resulting conclusions on the ruggedness of the test method are presented.

Shape memory alloys↗