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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 343 records · Page 19

All sources NASA NTRS DOE OSTI

NASA NTRS1977

Strain partitioning of three alloys in ultrahigh vacuum using the step stress method

The ultrahigh vacuum strain controlled thermal-mechanical strain cycling fatigue behavior of ASTAR 811C, 304 stainless steel and A-286 was studied. The step stress procedure of experimental strainrange partitioning was utilized to partition the inelastic strainranges for tests duplicating conditionings previously reported by TRW. An analytical method is described in which these partitioned inelastic strainrange data can be used to develop the four basic life relationships for these materials.

Kortovich, C. S.↗

NASA NTRS1982

Materials constitutive models for nonlinear analysis of thermally cycled structures

Effects of inelastic materials models on computed stress-strain solutions for thermally loaded structures were studied by performing nonlinear (elastoplastic creep) and elastic structural analyses on a prismatic, double edge wedge specimen of IN 100 alloy that was subjected to thermal cycling in fluidized beds. Four incremental plasticity creep models (isotropic, kinematic, combined isotropic kinematic, and combined plus transient creep) were exercised for the problem by using the MARC nonlinear, finite element computer program. Maximum total strain ranges computed from the elastic and nonlinear analyses agreed within 5 percent. Mean cyclic stresses, inelastic strain ranges, and inelastic work were significantly affected by the choice of inelastic constitutive model. The computing time per cycle for the nonlinear analyses was more than five times that required for the elastic analysis.

Kaufman, A.↗

NASA NTRS2012

Dwell Notch Low Cycle Fatigue Behavior of a Powder Metallurgy Nickel Disk Alloy

A study was conducted to determine the processes which govern dwell notch low cycle fatigue (NLCF) behavior of a powder metallurgy (P/M) ME3 disk superalloy. The emphasis was placed on the environmentally driven mechanisms which may embrittle the highly stressed notch surface regions and reduce NLCF life. In conjunction with the environmentally driven notch surface degradation processes, the visco-plastic driven mechanisms which can significantly change the notch root stresses were also considered. Dwell notch low cycle fatigue testing was performed in air and vacuum on a ME3 P/M disk alloy specimens heat treated using either a fast or a slow cooling rate from the solutioning treatment. It was shown that dwells at the minimum stress typically produced a greater life debit than the dwells applied at the maximum stress, especially for the slow cooled heat treatment. Two different environmentally driven failure mechanisms were identified as the root cause of early crack initiation in the min dwell tests. Both of these failure mechanisms produced mostly a transgranular crack initiation failure mode and yet still resulted in low NLCF fatigue lives. The lack of stress relaxation during the min dwell tests produced higher notch root stresses which caused early crack initiation and premature failure when combined with the environmentally driven surface degradation mechanisms. The importance of environmental degradation mechanisms was further highlighted by vacuum dwell NLCF tests which resulted in considerably longer NLCF lives, especially for the min dwell tests.

Telesman, J.↗

NASA NTRS2011

Effect of Upper-Cycle Temperature on the Load-Biased, Strain-Temperature Response of NiTi

Over the past decade, interest in shape memory alloy based actuators has increased as the primary benefits of these solid-state devices have become more apparent. However, much is still unknown about the characteristic behavior of these materials when used in actuator applications. Recently we have shown that the maximum temperature reached during thermal cycling under isobaric conditions could significantly affect the observed mechanical response of NiTi (55 wt% Ni), especially the amount of transformation strain available for actuation and thus work output. The investigation we report here extends that original work to ascertain whether further increases in the upper-cycle temperature would produce additional changes in the work output of the material, which has a stress-free austenite finish temperature of 113 C, and to determine the optimum cyclic conditions. Thus, isobaric, thermal-cycle experiments were conducted on the aforementioned alloy at various stresses from 50-300 MPa using upper-cycle temperatures of 165, 200, 230, 260, 290, 320 and 350 C. The data indicated that the amount of applied stress influenced the transformation strain, as would be expected. However, the maximum temperature reached during the thermal excursion also plays an equally significant role in determining the transformation strain, with the maximum transformation strain observed during thermal cycling to 290 C. In situ neutron diffraction at stress and temperature showed that the differences in transformation strain were mostly related to changes in martensite texture when cycling to different upper-cycle temperatures. Hence, understanding this effect is important to optimizing the operation of SMA-based actuators and could lead to new methods for processing and training shape memory alloys for optimal performance.

Padula, Santo, II↗

NASA NTRS1976

The role of fracture mechanics in the design of fuel tanks in space vehicles

With special reference to design of fuel tanks in space vehicles, the principles of fracture mechanics are reviewed. An approximate but extremely simple relationship is derived among the operating stress level, the length of crack, and the number of cycles of failure. Any one of the variables can be computed approximately from the knowledge of the other two, if the loading schedule (mission of the tank) is not greatly altered. Two sample examples illustrating the procedures of determining the allowable safe operating stress corresponding to a set of assumed loading schedule are included. The selection of sample examples is limited by the relatively meager available data on the candidate material for various stress ratios in the cycling.

Denton, S. J.↗

NASA NTRS1972

Mechanical component screening for scanner

The critical mechanical components of the scan mirror mechanism are described and their evaluation and screening procedures are discussed. A bumper/damper unit is used in the design to reverse motion of the mirror and effect scan and retrace cycles. A wear evaluation was conducted on the bumper impact surfaces that established nylon 6-10 as an acceptable material. The elastomeric dampers were subjected to thermal vacuum tests for condensables and outgassing as well as parametric life tests. The flexure pivots that support the mirror were tested to establish a curve of stress plotted as a function of cycles to failure for rotational operation. The pivots met the life requirement of 150,000,000 cycles at a + or - 2.9 deg amplitude during fatigue testing. Screening procedures were established for dampers and flexure pivots to obtain flight quality components.

Olson, J. L.↗

NASA NTRS1977

Design of rotors for improved structural life

Major rotor design criteria are discussed with particular emphasis on those aspects of rotor design that ensure long life component integrity. Dynamic considerations, that necessitate tuning of bladed disk and seal assemblies to avoid excessive vibratory stress at both design and off-design conditions are reviewed as well as low cycle fatigue considerations, which have resulted in detailed analysis procedures to establish part temperature and stress variation throughout an operating cycle and extensive specimen and component fatigue testing to establish safe cyclic operating limits. The frequency, size, and behavior of intrinsic material defects were investigated. Manufacturing process improvements, including the application of increasingly sophisticated inspection techniques and quality control procedures are reviewed in light of their impact on component durability.

Hill, J. T.↗

NASA NTRS1995

Low Noise Research Fan Stage Design

This report describes the design of a Low Noise ADP Research Fan stage. The fan is a variable pitch design which is designed at the cruise pitch condition. Relative to the cruise setting, the blade is closed at takeoff and opened for reverse thrust operation. The fan stage is a split flow design with fan exit guide vanes and core stators. This fan stage design was combined with a nacelle and engine core duct to form a powered fan/nacelle, subscale model. This model is intended for use in aerodynamic performance, acoustic and structural testing in a wind tunnel. The model has a 22-inch outer fan diameter and a hub-to-top ratio of 0.426 which permits the use of existing NASA fan and cowl force balance designs and rig drive system. The design parameters were selected to permit valid acoustic and aerodynamic comparisons with the PW 17-inch rig previously tested under NASA contract. The fan stage design is described in detail. The results of the design axisymmetric analysis at aerodynamic design condition are included. The structural analysis of the fan rotor and attachment is described including the material selections and stress analysis. The blade and attachment are predicted to have adequate low cycle fatigue life, and an acceptable operating range without resonant stress or flutter. The stage was acoustically designed with airfoil counts in the fan exit guide vane and core stator to minimize noise. A fan-FEGV tone analysis developed separately under NASA contract was used to determine these airfoil counts. The fan stage design was matched to a nacelle design to form a fan/nacelle model for wind tunnel testing. The nacelle design was developed under a separate NASA contract. The nacelle was designed with an axisymmetric inlet, cowl and nozzle for convenience in testing and fabrication. Aerodynamic analysis of the nacelle confirmed the required performance at various aircraft operating conditions.

Hobbs, David E.↗

NASA NTRS2013

Understanding How Kurtosis Is Transferred from Input Acceleration to Stress Response and Its Influence on Fatigue Llife

High cycle fatigue of metals typically occurs through long term exposure to time varying loads which, although modest in amplitude, give rise to microscopic cracks that can ultimately propagate to failure. The fatigue life of a component is primarily dependent on the stress amplitude response at critical failure locations. For most vibration tests, it is common to assume a Gaussian distribution of both the input acceleration and stress response. In real life, however, it is common to experience non-Gaussian acceleration input, and this can cause the response to be non-Gaussian. Examples of non-Gaussian loads include road irregularities such as potholes in the automotive world or turbulent boundary layer pressure fluctuations for the aerospace sector or more generally wind, wave or high amplitude acoustic loads. The paper first reviews some of the methods used to generate non-Gaussian excitation signals with a given power spectral density and kurtosis. The kurtosis of the response is examined once the signal is passed through a linear time invariant system. Finally an algorithm is presented that determines the output kurtosis based upon the input kurtosis, the input power spectral density and the frequency response function of the system. The algorithm is validated using numerical simulations. Direct applications of these results include improved fatigue life estimations and a method to accelerate shaker tests by generating high kurtosis, non-Gaussian drive signals.

Kihm, Frederic↗

NASA NTRS1975

Effect of thermal cycling in a Mach 0.3 burner rig on properties and structure of directionally solidified gamma/gamma prime - delta eutectic

Tensile and stress rupture properties at 1040 C of a thermally cycled gamma/gamma prime - delta eutectic were essentially equivalent to the as-grown properties. Tensile strength and rupture life at 760 C appeared to decrease slightly by thermal cycling. Thermal cycling resulted in gamma prime coarsening and Widmanstatten delta precipitation in the gamma phase. An unidentified precipitate, presumably gamma prime, was observed within the delta phase. The eutectic alloy exhibited a high rate of oxidation-erosion weight loss during thermal cycling in the Mach 0.3 burner rig.

Gray, H. R.↗

NASA NTRS1979

Investigation of reliability attributes and accelerated stress factors on terrestrial solar cells

The results of accelerated stress testing of four different types of silicon terrestrial solar cells are discussed. The accelerated stress tests used included bias-temperature tests, bias-temperature-humidity tests, thermal cycle and thermal shock tests, and power cycle tests. Characterization of the cells was performed before stress testing and at periodic down-times, using electrical measurement, visual inspection, and metal adherence pull tests. Electrical parameters measured included short-circuit current, open circuit voltage, and output power, voltage, and current at the maximum power point. Incorporated in the report are the distributions of the prestress electrical data for all cell types. Data were also obtained on cell series and shunt resistance.

Prince, J. L.↗

NASA NTRS2001

X-33 Tank Failure During Autoclave Fabrication

During a repair cure cycle on tank #1 of the X-33 liquid hydrogen tanks, a skin to core disbond occurred. Both the inner skin and outer skin of the lobe #1 sandwich panel was noted to have been disbonded and cracked- An investigation was undertaken to determine the cause of this failure. The investigation consisted of reviewing all of the processing data and performing testing on the failed lobe #1, as well as the other lobes, which did not fail during the cure cycle. The tests consisted of residual stress measurements in one of the intact lobes and "plug-pulls" to assess skin to core strength on all of the remaining lobes. Results showed an extremely low bondline strength due to lack of proper filleting of the adhesive, in addition, tests showed a very rapid decrease in strength with increasing temperature, as well as a further decrease in strength with a larger number of cycles. Also, the honeycomb used was not vented so pressure could build up within the cells. All of these factors appeared to be contributors to the failure.

Nettles, Alan T.↗

NASA NTRS1989

A finite element method for nonlinear panel flutter

Panel flutter is the self excited oscillations of a plate in supersonic flow. Linear theory gives no information about the panel's deflections and stresses. Hence, the service life of the panel cannot be predicted by linear methods. Nonlinear structural theory determines the limit cycle oscillating frequency, and also panel deflections and stresses. Panel fatigue life, therefore, can be predicted. For a more thorough understanding of panel flutter behavior, the geometric nonlinearity effects due to large deflections must be considered in the formulation. The Galerkin's method was used in the investigation of limit cycle oscillations of panels in supersonic flow in the spatial domain, and the panel deflections. All nonlinear flutter methods and results presently available were limited to a linear or linearized aerodynamic theory. Extension of the finite element flutter formulation to include nonlinear hypersonic aerodynamic loading and finite element nonlinear panel flutter results are presented.

Mei, Chuh↗

NASA NTRS1982

Structural analysis

Hot section components of aircraft gas turbine engines are subjected to severe thermal-structural loading conditions, especially during the start-up and take-off portions of the engine cycle. The most severe and damaging stresses and strains are those induced by the steep thermal gradients induced during the start-up transient. These transient stresses and strains are also the most difficult to predict, in part because the temperature gradients and distributions are not well known or predictable, and also because the cyclic elasto-viscoplastic behavior of the materials at these extremes of temperature and strain are not well known or predictable. One element of the structures program will develop improved time-varying thermal-mechanical load models for the entire engine mission cycle from start-up to shutdown. The thermal model refinements will be consistent with those required by the structural code including considerations of mesh-point density, strain concentrations, and thermal gradients. Models will be developed for the burner liner, turbine vane and turbine blade.

Johns, R. H.↗

NASA NTRS2020

Residual Stress Measurements of Ni-base Superalloys through Neutron Diffraction

Linear friction welding (LFW) is being considered for joining single crystal to polycrystalline nickel-base alloys for advanced high temperature gas turbine disks. Unfortunately, the transient thermal cycle during the welding generates undesirable residual stresses. Additionally, upon thermal treatment to relieve these residual stresses the weld joints often crack. To understand this issue, state-of-the-art neutron diffraction was performed near weld joints to quantify the residual stresses created by varying linear friction weld conditions. The residual stresses were measured along three orthogonal axes in both the polycrystalline LSHR and single crystal SC-180 alloys. Residual stress measurements using a contour method were used to compare the calculations obtained by neutron diffraction. The results suggest high residual stresses can be successfully reduced using both pre- and post-weld processing steps. Moreover, the effect of grain size and pre-heat temperatures on the LFW process were explored.

Superalloys↗

NASA NTRS1973

High temperature, low cycle fatigue of copper-base alloys in argon. Part 3: Zirconium-copper; thermal-mechanical strain cycling, hold-time and notch fatigue results

The low-cycle fatigue characteristics of smooth bar and notched bar specimens (hourglass shape) of zirconium-copper, 1/2 Hard, material (R-2 Series) were evaluated at room temperature in axial strain control. Over the fatigue life range from about 300 to 3000 cycles the ratio of fatigue life for smooth bar to fatigue life for notched bar remained constant at a value of about 6.0. Some additional hold-time data for the R-2 alloy tested in argon at 538 C are reported. An analysis of the relaxation data obtained in these hold-time tests is also reported and it is shown that these data yield a fairly consistent correlation in terms of instantaneous stress rate divided by instantaneous stress. Two thermal-mechanical strain cycling tests were also performed using a cyclic frequency of 4.5 cycles per hour and a temperature cycling interval from 260 to 538 C. The fatigue life values in these tests were noticeably lower than that observed in isothermal tests at 538 C.

Conway, J. B.↗

NASA NTRS1978

Low-cycle fatigue testing methods

The good design of highly stressed mechanical components requires accurate knowledge of the service behavior of materials. The main methods for solving the problems of designers are: determination of the mechanical properties of the material after cyclic stabilization; plotting of resistance to plastic deformation curves; effect of temperature on the life on low cycle fatigue; and simulation of notched parts behavior.

Lieurade, H. P.↗

NASA NTRS2008

Effect of Upper-Cycle Temperature on the Load-Biased, Strain-Temperature Response of NiTi

Over the past decade, interest in shape memory alloy based actuators has increased as the primary benefits of these solid-state devices have become more apparent. However, much is still unknown about the characteristic behavior of these materials when used in actuator applications. Recently we have shown that the maximum temperature reached during thermal cycling under isobaric conditions could significantly affect the observed mechanical response of NiTi (55 wt% Ni), especially the amount of transformation strain available for actuation and thus work output. This investigation extends that original work to ascertain whether further increases in the upper-cycle temperature would produce additional improvement in the work output of the material, which has a stress-free Af of 113 oC, and to determine the optimum cyclic conditions. Thus, isobaric, thermal-cycle experiments were conducted in the aforementioned alloy at various stress levels from 50-300 MPa using upper-cycle temperatures of 165, 200, 230, 260, 290, 320 and 350 oC. The data indicated that the amount of applied stress influenced the transformation strain available in the system, as would be expected. However, the maximum temperature reached during the thermal excursion also plays a role in determining the transformation strain, with the maximum transformation strain being developed by thermal cycling to 290 oC. In situ, neutron diffraction showed that the differences in transformation strain were related to differences in martensite texture within the microstructure when cycling to different upper-cycle temperatures. Hence, understanding this effect is important to optimizing the operation of SMA-based actuators and could lead to new methods for processing and training shape memory alloys for optimal performance.

Padula, Santo, II↗