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NASA NTRS1975

Life analysis of helical gear sets using Lundberg-Palmgren theory

A mathematical model is developed for surface fatigue life of helical gears. The expected fatigue life of a pinion, gear, or gear set may be calculated from the model. An equation for the dynamic capacity of a gear set was also derived. Dynamic capacity is the transmitted tangential load which gives a 90 percent probability of survival of the gear set for one million pinion revolutions. The equations, when simplified by setting the helix angle to zero, reduce to the results which were previously developed for spur gears. A sample calculation is given which illustrates the use of the new fatigue life model.

Coy, J. J.↗

NASA NTRS2000

A Study of Single Crystal Fatigue Failure Criteria

This paper presents the results of a study whose objective was to study the applicability of different failure equations in modeling low cycle fatigue (LCF) test data for single crystal test specimens. A total of four failure criteria were considered in this study. One of the failure equations was developed by Pratt & Whitney and is based on normal and shear strains on the primary crystallographic slip planes of the single crystal material. Other failure equations considered are based on isotropic criteria. Because these failure equations were originally developed for isotropic materials such as structural steel, they were modified to be applicable to the single crystal slip systems of the LCF specimen material. By observing how closely the various equations were able to reduce the scatter in the LCF test data, the applicability of those equations in modeling the LCF test data was assessed. It is desired to subsequently use the failure equation with the highest correlation in the development of a new single crystal failure criterion for the Alternative Turbopump Development (ATD) for the space shuttle main engine (SSME) High Pressure Fuel Turbopump (HPFTP).

Sayyah, Tarek↗

NASA NTRS1975

Fatigue of notched fiber composite laminates. Part 1: Analytical model

A description is given of a semi-empirical, deterministic analysis for prediction and correlation of fatigue crack growth, residual strength, and fatigue lifetime for fiber composite laminates containing notches (holes). The failure model used for the analysis is based upon composite heterogeneous behavior and experimentally observed failure modes under both static and fatigue loading. The analysis is consistent with the wearout philosophy. Axial cracking and transverse cracking failure modes are treated together in the analysis. Cracking off-axis is handled by making a modification to the axial cracking analysis. The analysis predicts notched laminate failure from unidirectional material fatique properties using constant strain laminate analysis techniques. For multidirectional laminates, it is necessary to know lamina fatique behavior under axial normal stress, transverse normal stress and axial shear stress. Examples of the analysis method are given.

Mclaughlin, P. V., Jr.↗

NASA NTRS1995

Gear Crack Propagation Investigation

Reduced weight is a major design goal in aircraft power transmissions. Some gear designs incorporate thin rims to help meet this goal. Thin rims, however, may lead to bending fatigue cracks. These cracks may propagate through a gear tooth or into the gear rim. A crack that propagates through a tooth would probably not be catastrophic, and ample warning of a failure could be possible. On the other hand, a crack that propagates through the rim would be catastrophic. Such cracks could lead to disengagement of a rotor or propeller from an engine, loss of an aircraft, and fatalities. To help create and validate tools for the gear designer, the NASA Lewis Research Center performed in-house analytical and experimental studies to investigate the effect of rim thickness on gear-tooth crack propagation. Our goal was to determine whether cracks grew through gear teeth (benign failure mode) or through gear rims (catastrophic failure mode) for various rim thicknesses. In addition, we investigated the effect of rim thickness on crack propagation life. A finite-element-based computer program simulated gear-tooth crack propagation. The analysis used principles of linear elastic fracture mechanics, and quarter-point, triangular elements were used at the crack tip to represent the stress singularity. The program had an automated crack propagation option in which cracks were grown numerically via an automated remeshing scheme. Crack-tip stress-intensity factors were estimated to determine crack-propagation direction. Also, various fatigue crack growth models were used to estimate crack-propagation life. Experiments were performed in Lewis' Spur Gear Fatigue Rig to validate predicted crack propagation results. Gears with various backup ratios were tested to validate crack-path predictions. Also, test gears were installed with special crack-propagation gages in the tooth fillet region to measure bending-fatigue crack growth. From both predictions and tests, gears with backup ratios (rim thickness divided by tooth height) of 3.3 and 1.0 produced tooth fractures, whereas a backup ratio of 0.3 produced rim fractures. For a backup ratio of 0.5, the experiments produced rim fractures and the predictions produced both rim and tooth fractures, depending on the initial geometry of the crack. Good correlation between predicted and measured crack growth was achieved when the fatigue crack-closure concept was introduced into the analysis. As the gear rim thickness decreased, the compressive cyclic stress in the gear-tooth fillet region increased. This retarded crack growth and increased the number of crack-propagation cycles to failure.

Source record↗

NASA NTRS1994

Probabilistic Material Strength Degradation Model for Inconel 718 Components Subjected to High Temperature, Mechanical Fatigue, Creep and Thermal Fatigue Effects

This thesis presents the on-going development of methodology for a probabilistic material strength degradation model. The probabilistic model, in the form of a postulated randomized multifactor equation, provides for quantification of uncertainty in the lifetime material strength of aerospace propulsion system components subjected to a number of diverse random effects. This model is embodied in the computer program entitled PROMISS, which can include up to eighteen different effects. Presently, the model includes four effects that typically reduce lifetime strength: high temperature, mechanical fatigue, creep, and thermal fatigue. Statistical analysis was conducted on experimental Inconel 718 data obtained from the open literature. This analysis provided regression parameters for use as the model's empirical material constants, thus calibrating the model specifically for Inconel 718. Model calibration was carried out for four variables, namely, high temperature, mechanical fatigue, creep, and thermal fatigue. Methodology to estimate standard deviations of these material constants for input into the probabilistic material strength model was developed. Using the current version of PROMISS, entitled PROMISS93, a sensitivity study for the combined effects of mechanical fatigue, creep, and thermal fatigue was performed. Results, in the form of cumulative distribution functions, illustrated the sensitivity of lifetime strength to any current value of an effect. In addition, verification studies comparing a combination of mechanical fatigue and high temperature effects by model to the combination by experiment were conducted. Thus, for Inconel 718, the basic model assumption of independence between effects was evaluated. Results from this limited verification study strongly supported this assumption.

Bast, Callie Corinne Scheidt↗

NASA NTRS1977

Development of a plasma sprayed ceramic gas path seal for high pressure turbine applications

The plasma sprayed graded layered yittria stabilized zirconia (ZrO2)/metal(CoCrAlY) seal system for gas turbine blade tip applications up to 1589 K (2400 F) seal temperatures was studied. Abradability, erosion, and thermal fatigue characteristics of the graded layered system were evaluated by rig tests. Satisfactory abradability and erosion resistance was demonstrated. Encouraging thermal fatigue tolerance was shown. Initial properties for the plasma sprayed materials in the graded, layered seal system was obtained, and thermal stress analyses were performed. Sprayed residual stresses were determined. Thermal stability of the sprayed layer materials was evaluated at estimated maximum operating temperatures in each layer. Anisotropic behavior in the layer thickness direction was demonstrated by all layers. Residual stresses and thermal stability effects were not included in the analyses. Analytical results correlated reasonably well with results of the thermal fatigue tests. Analytical application of the seal system to a typical gas turbine engine application predicted performance similar to rig specimen thermal fatigue performance. A model for predicting crack propagation in the sprayed ZrO2/CoCrAlY seal system was proposed, and recommendations for improving thermal fatigue resistance were made. Seal system layer thicknesses were analytically optimized to minimize thermal stresses in the abradability specimen during thermal fatigue testing. Rig tests on the optimized seal configuration demonstrated some improvement in thermal fatigue characteristics.

Shiembob, L. T.↗

NASA NTRS1978

Development of an improved method of consolidating fatigue life data

A fatigue data consolidation model that incorporates recent advances in life prediction methodology was developed. A combined analytic and experimental study of fatigue of notched 2024-T3 aluminum alloy under constant amplitude loading was carried out. Because few systematic and complete data sets for 2024-T3 were available in the program generated data for fatigue crack initiation and separation failure for both zero and nonzero mean stresses. Consolidations of these data are presented.

Leis, B. N.↗

NASA NTRS1990

The growth of short cracks in 4340 steel and aluminum-lithium 2090

The objectives were to investigate crack initiation characteristics and short crack growth behavior for Al-Li 2090 and for 4340 steel and to evaluate the ability of a closure-based crack-growth model to predict fatigue crack growth rates and total fatigue lives for the steel. Single-edge-notched tension specimens of each alloy were used to obtain the short crack growth rate information via an acetate replica technique. In addition to constant amplitude loading, tests on the steel were conducted using the Felix/28 variable amplitude spectrum (a shortened form of a standard loading sequence for fixed or semi-rigid helicopter rotors). The short crack growth rates were compared to those for long cracks grown under similar loading conditions. Metallurgical features associated with crack initiation are discussed. For Al-Li 2090 under R = -1 loading, the short cracks grew well below the long crack threshold and grew at acute angles to the loading axis. For 4340 steel under constant amplitude loading at R = 0.5 and 0 and for the Felix/28 spectrum loading, short-crack growth rates agreed well with long-crack growth rates, even near the long-crack threshold. A slight short-crack effect, growth below the long-crack threshold, was observed at R = -1. Fatigue lives were found to depend on the size and type of initiation site, especially for the Felix/28 loading sequence. A semi-empirical crack-growth model incorporating crack-closure effects was used to predict crack growth rates and total fatigue lives of notched 4340 steel specimens. An initial defect size and shape typical of those identified in this steel was assumed for the life predictions. For all loading conditions, reasonable agreement was found between measured and predicted values for both crack growth rates and fatigue lives.

Swain, M. H.↗

NASA NTRS1992

Probabilistic material degradation model for aerospace materials subjected to high temperature, mechanical and thermal fatigue, and creep

A probabilistic general material strength degradation model has been developed for structural components of aerospace propulsion systems subjected to diverse random effects. The model has been implemented in two FORTRAN programs, PROMISS (Probabilistic Material Strength Simulator) and PROMISC (Probabilistic Material Strength Calibrator). PROMISS calculates the random lifetime strength of an aerospace propulsion component due to as many as eighteen diverse random effects. Results are presented in the form of probability density functions and cumulative distribution functions of lifetime strength. PROMISC calibrates the model by calculating the values of empirical material constants.

Boyce, L.↗

NASA NTRS1986

Grain boundary oxidation and fatigue crack growth at elevated temperatures

Fatigue crack growth rate at elevated temperatures can be accelerated by grain boundary oxidation. Grain boundary oxidation kinetics and the statistical distribution of grain boundary oxide penetration depth were studied. At a constant delta K-level and at a constant test temperature, fatigue crack growth rate, da/dN, is a function of cyclic frequency, nu. A fatigue crack growth model of intermittent micro-ruptures of grain boundary oxide is constructed. The model is consistent with the experimental observations that, in the low frequency region, da/dN is inversely proportional to nu, and fatigue crack growth is intergranular.

Liu, H. W.↗

NASA NTRS1986

Grain boundary oxidation and fatigue crack growth at elevated temperatures

Liu, H. W.↗

NASA NTRS1982

Modeling stiffness loss in boron/aluminum below the fatigue limit

Boron/aluminum can develop significant internal matrix cracking when fatigued. These matrix cracks can result in a 40 percent secant modulus loss in some laminates, even when fatigued below the fatigue limit. It is shown that the same amount of fatigue damage will develop during stress or strain-controlled tests. Stacking sequence has little influence on secant modulus loss. The secant modulus loss in unidirectional composites is small, whereas the losses are substantial in laminates containing off-axis plies. A simple analysis is presented that predicts unnotched laminate secant modulus loss due to fatigue. The analysis is based upon the elastic modulus and Poisson's ratio of the fiber and matrix, fiber volume fraction, fiber orientations, and the cyclic-hardened yield stress of the matrix material. Excellent agreement was achieved between model predictions and experimental results. With this model, designers can project the material stiffness loss for design load or strain levels and assess the feasibility of its use in stiffness-critical parts.

Johnson, W. S.↗

NASA NTRS2017

Environmental Barrier Coating Fracture, Fatigue and High-Heat-Flux Durability Modeling and Stochastic Progressive Damage Simulation

Advanced environmental barrier coatings will play an increasingly important role in future gas turbine engines because of their ability to protect emerging light-weight SiC/SiC ceramic matrix composite (CMC) engine components, further raising engine operating temperatures and performance. Because the environmental barrier coating systems are critical to the performance, reliability and durability of these hot-section ceramic engine components, a prime-reliant coating system along with established life design methodology are required for the hot-section ceramic component insertion into engine service. In this paper, we have first summarized some observations of high temperature, high-heat-flux environmental degradation and failure mechanisms of environmental barrier coating systems in laboratory simulated engine environment tests. In particular, the coating surface cracking morphologies and associated subsequent delamination mechanisms under the engine level high-heat-flux, combustion steam, and mechanical creep and fatigue loading conditions will be discussed. The EBC compostion and archtechture improvements based on advanced high heat flux environmental testing, and the modeling advances based on the integrated Finite Element Analysis Micromechanics Analysis Code/Ceramics Analysis and Reliability Evaluation of Structures (FEAMAC/CARES) program will also be highlighted. The stochastic progressive damage simulation successfully predicts mud flat damage pattern in EBCs on coated 3-D specimens, and a 2-D model of through-the-thickness cross-section. A 2-parameter Weibull distribution was assumed in characterizing the coating layer stochastic strength response and the formation of damage was therefore modeled. The damage initiation and coalescence into progressively smaller mudflat crack cells was demonstrated. A coating life prediction framework may be realized by examining the surface crack initiation and delamination propagation in conjunction with environmental degradation under high-heat-flux and environment load test conditions.

Mechanical creep and fatigue loading condition↗

NASA NTRS2002

Evaluation of Fatigue Crack Growth and Fracture Properties of Cryogenic Model Materials

The criteria used to prevent failure of wind-tunnel models and support hardware were revised as part of a project to enhance the capabilities of cryogenic wind tunnel testing at NASA Langley Research Center. Specifically, damage-tolerance fatigue life prediction methods are now required for critical components, and material selection criteria are more general and based on laboratory test data. The suitability of two candidate model alloys (AerMet 100 and C-250 steel) was investigated by obtaining the fatigue crack growth and fracture data required for a damage-tolerance fatigue life analysis. Finally, an example is presented to illustrate the newly implemented damage tolerance analyses required of wind-tunnel model system components.

Newman, John A.↗

NASA NTRS1993

Environment enhanced fatigue crack propagation in metals: Inputs to fracture mechanics life prediction models

This report is a critical review of both environment-enhanced fatigue crack propagation data and the predictive capabilities of crack growth rate models. This information provides the necessary foundation for incorporating environmental effects in NASA FLAGRO and will better enable predictions of aerospace component fatigue lives. The review presents extensive literature data on 'stress corrosion cracking and corrosion fatigue.' The linear elastic fracture mechanics approach, based on stress intensity range (Delta(K)) similitude with microscopic crack propagation threshold and growth rates, provides a basis for these data. Results are presented showing enhanced growth rates for gases (viz., H2 and H2O) and electrolytes (e.g. NaCl and H2O) in aerospace alloys including: C-Mn and heat treated alloy steels, aluminum alloys, nickel-based superalloys, and titanium alloys. Environment causes purely time-dependent accelerated fatigue crack growth above the monotonic load cracking threshold (KIEAC) and promotes cycle-time dependent cracking below (KIEAC). These phenomenon are discussed in terms of hydrogen embrittlement, dissolution, and film rupture crack tip damage mechanisms.

Gangloff, Richard P.↗

NASA NTRS1999

Analyses of Fatigue and Fatigue-Crack Growth under Constant- and Variable-Amplitude Loading

Studies on the growth of small cracks have led to the observation that fatigue life of many engineering materials is primarily crack growth from micro-structural features, such as inclusion particles, voids, slip-bands or from manufacturing defects. This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using small-crack theory under various loading conditions. Constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective stress-intensity factor range (delta K(sub eff)) under constant-amplitude loading. Modifications to the delta K(sub eff)-rate relations in the near-threshold regime were needed to fit measured small-crack growth rate behavior. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens under constant-amplitude and spectrum loading. Fatigue lives were predicted using crack-growth relations and micro-structural features like those that initiated cracks in the fatigue specimens for most of the materials analyzed. Results from the tests and analyses agreed well.

Newman, J. C., Jr.↗

NASA NTRS1999

Fatigue Analyses Under Constant- and Variable-Amplitude Loading Using Small-Crack Theory

Studies on the growth of small cracks have led to the observation that fatigue life of many engineering materials is primarily "crack growth" from micro-structural features, such as inclusion particles, voids, slip-bands or from manufacturing defects. This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using "small-crack theory" under various loading conditions. Constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective stress-intensity factor range (delta-Keff) under constant-amplitude loading. Modifications to the delta-Keff-rate relations in the near-threshold regime were needed to fit measured small-crack growth rate behavior. The model was then used to calculate small-and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens under constant-amplitude and spectrum loading. Fatigue lives were predicted using crack-growth relations and micro-structural features like those that initiated cracks in the fatigue specimens for most of the materials analyzed. Results from the tests and analyses agreed well.

Newman, J. C., Jr.↗

NASA NTRS1995

Experimental Verification of a Progressive Damage Model for IM7/5260 Laminates Subjected to Tension-Tension Fatigue

The durability and damage tolerance of laminated composites are critical design considerations for airframe composite structures. Therefore, the ability to model damage initiation and growth and predict the life of laminated composites is necessary to achieve structurally efficient and economical designs. The purpose of this research is to experimentally verify the application of a continuum damage model to predict progressive damage development in a toughened material system. Damage due to monotonic and tension-tension fatigue was documented for IM7/5260 graphite/bismaleimide laminates. Crack density and delamination surface area were used to calculate matrix cracking and delamination internal state variables to predict stiffness loss in unnotched laminates. A damage dependent finite element code predicted the stiffness loss for notched laminates with good agreement to experimental data. It was concluded that the continuum damage model can adequately predict matrix damage progression in notched and unnotched laminates as a function of loading history and laminate stacking sequence.

Coats, Timothy W.↗