bluuebunny/crossref_metadata_2025 huggingface dataset:
Laser Damage Studies of Ion Beam Milled Fused Silica
Authors: Wilson SR , Reicher DW , McNeil JR , McNally JJ , Stowell K , Milam D , Gonzales R , Rainer F
Published: February 2009

Ion beam figuring is a promising optical fabrication technique in which substrate material is removed from an optical component by selectively ion beam milling the surface. The effects of ion beam milling on the properties of the optical surface are not completely understood. One area of investigation is the influence of ion beam milling on the laser damage threshold of the substrate. The ion milling process removes part of the damage layer at the surface of the optic formed during fabrication, and this might result in modification of laser damage behavior.

As part of an investigation of the ion beam figuring process, a study was undertaken to determine the effect of ion milling on the laser damage threshold of fused silica. Fused silica substrates were ion milled to depths up to ten microns using a 1500 eV, 40mA Argon ion beam from a Kaufman ion source. The milled substrates were then subjected to laser damage testing using a 1064 nanometer wavelength laser. The tests were performed using one nanosecond pulse width single shot pulses and sixteen nanosecond pulse width thirty Hz pulses. The results indicate that ion beam milling has no significant influence on the damage threshold.
Minimizing Scattering in Multilayers: Technique for Searching Optimal Realization Conditions
Authors: Amra C
Published: February 2009

Reducing losses in optical coatings implies a search for optimal realization conditions of multidielectric coatings to obtain minimal absorption and scattering. Numerous works have been devoted to the absorption problem. Here we deal with scattering minimization. Scattering from a coated substrate is a function of numerous factors that can be successively measured: substrate roughness, interfaces roughness, cross-correlation state between interfaces. First, the substrate roughness is determined by scattering measurement before coating. Then, the interfaces roughness depends on the grain size of the material in thin film form. For a given material, we find that the grain is, in a large extent, function of the preparation conditions of the layer. And mainly, when a whole layer stack is concerned, the spatial distribution of the scattered light depends on the cross-correlation state between successive interfaces. With a scattering vector theory and an apparatus for measuring scattering curves at our disposal, we can show how it is possible to determine the interface roughness and the cross-correlation state, by a systematic comparison between theory and experiment. In order to eliminate any ambiguity in interpreting the experimental results, we must work with multilayer stacks the design of which is chosen so that the scattering curves have an aspect very different according to the cross-correlation state.
Ion Assisted Deposition of Optical Thin Films at Reduced Substrate Temperature
Authors: Williams FL , McNeil JR , McNally JJ , Exarhos GJ
Published: February 2009

Ion assisted deposition (IAD) involves bombarding a thin film with low energy ions (Ei <1 keV) while it is being deposited. We have investigated the application of this technique to deposit optical thin films onto substrates at low temperature (T < 150°C). Since conventional deposition methods require heating the substrate to temperatures of −300°C, IAD is important for coating substrate materials that cannot be subjected to elevated temperature.

In this study, we have observed that optical coatings of A12O3, Ta2O5 and TiO2 deposited at low temperature using IAD have higher values of refractive index compared to similar films produced without IAD. In addition, the optical absorption of Al203 and Ta2O5 thin films can be decreased by applying IAD to deposit these materials. We were unable to detect visible absorption of TiO2 thin films produced in this study. We also present results correlating Raman spectra and optical scatter of these films to IAD conditions. Which suggests that IAD TiO2 at low temperature can promote anatase crystallites in the film, with a corresponding increase in optical scatter.
Optical Properties of Low Energy Ion Assisted Deposited TiO2 Films
Authors: Krishna MG , Rao KN , Murthy MA , Mohan S
Published: February 2009

Absorption in TiO2 films limits their efficiency and stability against high power laser radiation. The optical absorption in single layer TiO2 films is greatly influenced by deposition parameters such as the rate of deposition, oxygen partial pressure, substrate temperature and the starting material. It is also observed that low energy ion assisted deposition enhances the reactivity and results in reduction of absorption.

In this investigation, single layers of TiO2 films have been prepared using ionized oxygen (produced using a Heitmann source) and TiO as the starting material. The optical properties (refractive index n, and extinction coefficient k) and the laser induced damage threshold as a function of deposition parameters has been studied. The discharge currents were varied between 0 and 400 mA with the rate of deposition being 9 and 18 nm/min. The oxygen pressure was maintained at 2×10−4torr. At low deposition rates, in situ optical monitoring of transmission has shown absorption free single layers for moderately low discharge currents (200 mA) whereas at higher deposition rates absorption free films were obtained at higher currents. The damage threshold of these films was found to be directly dependent on the absorption.

Variation in damage threshold values is explained on the basis of film composition, structure and optical properties.
Basic Studies of Optical Coating Thermal Properties
Authors: Swimm RT
Published: February 2009

Characteristics of a technique to measure the thermal transport properties of coatings are discussed. The measurement technique is based on interferometry using thermal diffusion waves. Initial measurements of thermal transport in single-layer metal coatings are described.
Damage Threshold of Oscillator Mirrors in Mark III FEL
Authors: Deacon DAG , Benson SV , Madey JMJ , Schultz J
Published: February 2009

We have observed damage of gold resonator mirrors in the Mark III FEL with 1.2 microsecond macropulses at a fluence threshold of 21 J/cm2. This number agrees with YAG laser damage measurements scaled by the square root of the ratio of the pulse lengths. Silver coated mirrors have not shown damage up to an equivalent threshold of 40 J/cm2. Work aimed at increasing the pulse energy on the silver mirrors is continuing; we expect to extend these measurements in the near future.
The Discovery of Laser-Induced Intrinsic Optical Damage in Wide-Gap Materials at Visible Wavelength
Authors: Braunlich P , Jones SC , Shen XA , Casper RT , Kelly P
Published: February 2009

After two decades of research, the question “What is the nature of intrinsic breakdown of wide-gap optical materials exposed to short laser pulses at visible wavelength?” has recently been answered unambiguously for two cases: 532 nm, picosecond pulses tightly focused in NaCl and KBr. This remarkable achievement warrants some reflection on its impact on the field of high laser power optical materials.

Clearly, the problem of extrinsic damage to coatings and surfaces remains unsolved and formidable, and the quest for higher damage thresholds will continue without any realistic hope to achieve intrinsic behavior in the foreseeable future. However, such pessimistic realism is no longer justified for bulk breakdown, because the possibility of fabricating intrinsic high-power optical materials—not just alkali halides, but also technologically important alkaline earth halides and metal oxides—must now be taken seriously.

In light of this new development we will re-examine the results of some classic damage experiments and the conclusions on the laser damage mechanisms that were based on erroneous assumptions of intrinsic material behavior. Extrinsic damage, presumed to be intrinsic for the purpose of data interpretation, will always appear to be avalanche formation via electron impact ionization if only damage thresholds are measured.

With this in mind, we will look again at self-focusing and defocusing effects, the traditional exclusive reliance on subjectively defined damage criteria and, thus, uncertain thresholds, and the role of primary laser-generated photo-chemical defects in the historical “workhorse” materials, the alkali halides.
The Laser Damage Mechanism for NaCl and KBr at 532 nm — Theoretical Predictions and Experimental Tests
Authors: Casper RT , Jones SC , Shen XA , Braunlich P , Kelly P
Published: February 2009

Recent work by Jones et al. and Shen et al. established the detailed intrinsic damage mechanisms in NaCl and KBr exposed to short laser pulses at 532 nm. Predictions of the dependence on experimental and material parameters of energy deposition from the laser pulse to the wide-gap optical solid can now be made with confidence. We examine the effects of initial temperature and of pulse length on the damage threshold. We also discuss the role of defect accumulation in multiple shot damage in KBr.
Reverse Thermal Wave Approximation for Temperature Transients in Optical Thin Fitms and Substrates Reflective Optics for High Power Continuous Wave Lasers
Authors: Palmer JR
Published: February 2009

It is the intent for the purpose of this paper to continue with the application of the Reverse Thermal Wave Model to problems associated with the damage thresholds and temperature gradients for optical components which are provided with optical thin films. This problem is particularly acute for those optical components which are part of a high power Continuous Wave laser system.

In this paper are descriptions of how the approximations provide techniques for evaluating the maximum threshold for optical thin films deposited on components with very thick and very thin optical faceplates. The Reverse Thermal Wave Model provides some rather dramatic results for those optical systems that are used for imaging in the infrared portion of the spectrum, in as much as these optical elements are generally very thin faceplates and may be subject to damage from very high power lasers.

Both mathematical descriptions and figures are provided to show the damage thresholds predicted by the model and correlated to one dimensional semi-infinite plate experimental data For the most part, authors have treated with infinite and semi-infinite walls. The attention for this article is to treat with those conditions that do not meet the boundary requirements for the semi-infinite plate. Given this boundary, the use of what we describe as the Reverse Thermal Wave Approximations will be implemented to allow the calculation of the transient temperature rise in thin optical faceplates. Equations are described and the regimes of their application are outlined Additionally, curves are generated to provide the reader examples of how to implement the equations.
Effects of Testing Method on Cyclic Hardening Behavior in Face-Centered-Cubic Alloys
Authors: Pickard AC , Knott JF
Published: February 2009

Several types of test methods are employed to evaluate the cyclic hardening behavior of engineering alloys. Three such tests are the plastic-strain limit (PSL) test, the multiple-step test (MST), and the incremental-step test (IST). In each type of test, the material experiences a different plastic-strain history prior to measurement of the saturation stress. The present paper examines effects of these different histories on cyclic hardening behavior in a number of face-centered-cubic (fcc) alloys, using all three types of test. For 316 stainless steel and a weldable Al-4.5Zn-2.5Mg alloy, behavior in the IST is similar to that in the MST. In a series of experiments carried out on Cu and Cu-Al alloys, significant differences between the different tests are observed; these are explained in terms of the dislocation substructures generated by the strain histories. Differences in behavior are also observed for Al-4.5Zn-2.5Mg in under-aged, peak-aged, and over-aged conditions; these are most pronounced for the under-aged alloy, where slip is planar.
Fracture of Pearlite under Conditions of High Deformation Fatigue
Authors: Schwartz M , Crespi JC
Published: February 2009

The theory of Kettunen and Kocks allows us to obtain a description of the relationship between the saturation stress amplitude, the saturation plastic strain range, and the number of cycles to fracture that fit the experimental data for fully annealed SAE 1038 steel with at least as comparable agreement as that obtained by the Coffin-Manson law. The parameters C and p of the Kettunen and Kocks relationship can be predicted fairly well from two types of tests: the monotonic stress-strain test and the incremental step test. The method of analysis through the linear multiple regression proposed by Bucher and Grozier for ferritic-pearlitic steels satisfactorily predicts the monotonic properties of the material, and the resistance to total strain cycling can be reliably predicted by the Feltner and Landgraf prediction criterion. When controlled strain amplitudes are greater than ±0.37% instantaneous strain hardening is observed during cyclic straining. For smaller strain amplitudes, cyclic softening occurs.
Viscoplasticity Based on Overstress Applied to Creep-Fatigue Interaction
Authors: Krempl E , Lu H , Satoh M , Yao D
Published: February 2009

The uniaxial version of the viscoplasticity theory based on overstress is applied to the deformation behavior of Type 304 stainless steel at 650°C. The effects of strain rate, waveform, and position of hold-time are computed for a cyclic steady-state as well as the ratchetting behavior under zero-to-tension load control.

The computations of deformation behavior are used in correlating and predicting the low-cycle fatigue life at 593°C using the strainrange partitioning method as well as an incremental damage accumulation law developed previously. The results of the computations exhibit the appropriate trends, but can show deviations from actual life exceeding a factor of two. Considering all the uncertainties, the results are deemed acceptable and promising.

The incremental damage accumulation law permits application to arbitrary stress or strain histories. The effect of position of hold-time on the hysteresis loop is computed and shown to exhibit the appropriate trend.
Application of Small Specimen Crack Growth Data to Engineering Components at High Temperature: A Review
Authors: Skelton RP
Published: February 2009

This review considers the standard low-cycle fatigue (LCF) specimen, the area of the component it represents, the empirical crack growth relations measured on that specimen, and their ranges of validity. It also presents some recent observations on the effects of crack shape, strain rate, weld material, notches, and aging upon cyclic propagation rates at high temperature.

In applying LCF growth results to estimate crack behavior in service, size effects must be considered. From crack growth tests at constant plastic strain on LCF specimens of differing width, it is shown in both cases that the cracks accelerated, then decelerated after a critical depth which occurred sooner in the smaller specimens. Accompanying variations in the shape of the hysteresis loop and crack closure effects (all of which have implications for service loading) are also reported.

Beyond a certain depth in the specimen, crack growth is no longer describable by the bulk plastic strain, so other parameters (cyclic J integral and equivalent stress intensity) are compared and their ranges of validity discussed.

The survey next considers a constant-load test where, despite equal tension and compression ranges, ratchetting of the specimen occurred, leading to quite different conditions for crack closure. The crack growth data nevertheless agree with LCF data when the correct stress range is taken into account.

Finally, a high temperature lifetime assessment route is briefly described, where the user is permitted to enter a permissible defect path which utilizes many of the experimental short crack data mentioned in this review.
Life of Crack Initiation, Propagation, and Final Fracture under High-Temperature Creep, Fatigue, and Creep-Fatigue Multiplication Conditions
Authors: Yokobori AT , Yokobori T , Kuriyama T
Published: February 2009

Studies were carried out on the crack initiation and propagation laws under high-temperature creep, fatigue, and creep-fatigue multiplication conditions for notched specimens of SUS 304 stainless steel.

For crack initiation life, it was found that relative notch opening displacement (RNOD) can be used. Furthermore, the remaining part of crack initiation life can be predicted by RNOD criterion; that is, by recording RNOD with respect to the suitable time within the service term and extrapolating the data with a straight line until the line intersects the critical value of RNOD.

The crack propagation life was expressed by a thermally activated type equation in explicit terms of independent parameters such as applied stress and temperature. The equation was approximately expressed in terms of the time-temperature equivalent parameter T(ℓn tp + C), where T = absolute temperature, tp = crack propagation life, and C = a nearly constant numerical factor. The experimental data are well expressed by this parameter.
Comparison of Creep-Fatigue Life Prediction by Life Fraction Rule and Strainrange Partitioning Methods
Authors: Yamauchi M , Igari T , Setoguchi K , Yamanouchi H
Published: February 2009

A comparison of two kinds of creep-fatigue life prediction methods, Life Fraction Rule and Strainrange Partitioning, was performed for the cases of creep-fatigue tests under uniaxial strain-controlled conditions at 500 to 600°C in air, using a normalized and tempered 2 1/4Cr-1Mo steel. The main focus was on the influence of stress-strain behavior on life prediction. Two kinds of stress-strain values (i.e., experimental values and calculated values by inelastic analysis) were used for the life prediction of creep-fatigue tests. The results show that life prediction by the Life Fraction Rule was very sensitive to the stress-strain values, indicating a large discrepancy between predicted and observed lives in some cases, and that life prediction by the Strainrange Partitioning method was not so sensitive to the stress-strain values, indicating a good agreement between predicted and observed lives in all cases.
Creep and Creep-Fatigue Intergranular Damage in Austenitic Stainless Steels: Discussion of the Creep-Dominated Regime
Authors: Levaillant C , Grattier J , Mottot M , Pineau A
Published: February 2009

Creep and strain controlled low-cycle fatigue tests with an imposed hold time at maximum tensile strain were carried out, mainly at 600°C, on two heats of 316L stainless steel. Very long times to failure (≃ 104 h) corresponding to low applied strain and long dwell period (24 h) were investigated. Both fatigue and creep damage were measured by quantitative metallography.

Creep intergranular surface cracking shortens the initiation stage of fatigue cracks, while creep intergranular bulk cracking accelerates the average fatigue crack growth rate. These observations are the basis for a creep-fatigue interaction model previously proposed using intergranular damage as a life correlating parameter. The applicability of this model is tested with results of 104 h creep-fatigue tests showing a saturation effect. The saturation in creep-fatigue life observed for long dwell periods is explained by an improvement in creep ductility.

This intergranular damage approach is discussed in the light of the creep dominated regime concept. It is shown that pure creep failure data cannot provide reliable prediction of creep fatigue life. The need to use a model that takes into account the effect of creep intergranular damage on fatigue crack propagation is emphasized.
Elevated Temperature Fatigue-Creep Behavior of Nickel-Base Superalloy IN 625
Authors: Bui-Quoc T , Gomuc R , Biron A , Nguyen HL , Masounave J
Published: February 2009

A research program has been carried out to establish the low-cycle fatigue and creep-fatigue behaviors of Inconel 625 at elevated temperatures (650 and 815°C). The main observations were related to the effect of temperature and of hold times. Under continuous cycling, a temperature increase from 650 to 815°C caused a reduction in the fatigue life by a factor of 2 at a high strain and by a factor of 3 at a low strain.

Tension hold times had little detrimental effect on cyclic life at 650°C concerning the life reduction with respect to continuous cycling; at 815°C, on the other hand, this became more significant. Compressive hold times also had a damaging effect on fatigue life, and this was larger than that associated with tensile hold times; in particular, it was very pronounced for low strain levels at 815°C.

An analysis of data using a life prediction method previously suggested for creep-fatigue combination loadings has also been carried out. The method takes into account the damaging effect due to a compression hold time separately from that due to a tensile hold time in interspersed creep-fatigue loadings. The overall correlation between theoretical calculations and experimental results is reasonably good, for strain levels ranging from 1.2 to 0.4%.
Effect of Nitrogen on Creep-Fatigue Interaction in Austenitic Stainless Steels at 600°C
Authors: Nilsson J-O
Published: February 2009

Three austenitic stainless steels (AISI 316, AISI 316LN, and 253MA) have been investigated at 600°C with respect to fatigue and creep-fatigue behavior. Transmission electron microscopy showed that cross-slip and climb of dislocations are inhibited in AISI 316LN and 253MA due to the presence of nitrogen in solid solution. This results in a planar slip character and a concomitant increase in fatigue strength. However, during creep-fatigue deformation, nitrogen has an adverse effect on strength. This can be explained as an enhanced interaction between creep and fatigue due to the generation of high stresses in the grain boundary regions, caused by inhibited recovery.
A Micromechanics Model of the Initiation of Grain-Boundary Crack in High-Temperature Fatigue
Authors: Tanaka M , Iizuka H
Published: February 2009

A continuum mechanics model which incorporated the recovery effect by diffusion of atoms is presented to explain the initiation of grain-boundary wedge-type crack in high-temperature fatigue. The direction of deformation is periodically changed in fatigue, and the crack initiation depends on the deformation history of the grain boundary. A theoretical calculation based on the present model was made on the grain-boundary crack initiation in high-temperature fatigue where the deformation is path-dependent. The calculation results satisfactorily explained the experimental observations in polycrystalline metallic materials.
Effect of Temperature, Microstructure, and Stress State on the Low Cycle Fatigue Behavior of Waspaloy
Authors: Stahl DR , Mirdamadi M , Zamrik SY , Antolovich SD
Published: February 2009

The low-cycle fatigue (LCF) behavior of Waspaloy was studied in uniaxial and torsional loading for two heat treatments at 24 and 649°C. It was shown that for both heat treatments deformation and failure mechanisms were independent of stress state at 24°C. Deformation occurred by precipitate shearing and the formation of intense shear bands for the coarse-grain/small-precipitate (CG-SP) condition and by precipitate looping and loosely defined shear bands for the fine-grain/large-precipitate (FG-LP) condition. Failure in both microstructures was associated with the formation of shear cracks.

At 649°C deformation and failure mechanisms for the FG-LP condition were independent of stress state, and the mechanisms were similar to those observed at 24°C. For the CG-SP condition, the situation was different. Failure occurred on principal planes when tested in torsion and on shear planes when tested in uniaxial tension. The mechanism transition is interpreted in terms of deformation mode and microstructural instability.

In general, there was a more pronounced decrease in life with increasing temperature for uniaxial specimens than for torsional specimens. This result is interpreted in terms of an environmental interaction that is accelerated by a comparatively significant dilatational strain component.
Effect of HIP on Elevated-Temperature Low Cycle Fatigue Properties of an Equiaxed Cast Superalloy
Authors: DL Anton , LH Favrow
Published: February 2009

A typical high strength equiaxed cast superalloy, B-1900, was tested in the fully heat-treated condition, preceded by an optional hot isostatic pressing (HIP) cycle. Fully reversed constant total strain amplitude testing was carried out at temperatures ranging from 760 to 982°C in air. The HIP cycle successfully eliminated all non-surface connected porosity, and specimens, machined from the center of test bars, were totally pore free. Changes in grain boundary carbide morphology and γ′ size also ensued from the HIP treatment. Fatigue crack initiation sites were identified in scanning electron microscopy (SEM) fractographic analysis along with modes of crack propagation.

An enhancement of fatigue life by a factor of 6 to 8 was realized in the HIP alloy at temperatures of 871°C and below. Above this temperature no effect of HIP was observed. Cracks of HIP specimens initiated at grain boundary-surface intersections. Fatigue crack initiation was enhanced by local oxidation of surface-connected carbides, with fatigue crack propagation being predominantly intergranular. The non-HIP specimens typically contained surface cracks which propagated through near surface clusters of microporosity which in turn were formed adjacent to the interdendritic carbide networks. The HIP treating of equiaxed cast superalloys for turbine blade applications completely closed interdendritic and intergranular microporosity and caused grain growth to occur. This led to enhanced fatigue life at low temperatures where oxidation of grain boundary carbides was minimized. Fatigue life enhancement due to HIP was separated into microstructural modifications effecting yield stress and grain boundary structure.
Effect of Microstructures on Low Cycle Fatigue Behavior in a TC6 (Ti-6AI-2.5Mo-2Cr-0.5Fe-0.3Si) Titanium Alloy
Authors: SQ Zhang , CH Tao , MG Yan
Published: February 2009

The results of an investigation on the low cycle fatigue (LCF) behavior, at room temperature and 400°C for four conventional microstructures (Widmannstatten, basket-weave, equiaxed, and duplex) in a TC6 titanium alloy are presented. The fatigue crack nucleation and propagation in fatigue-tested specimens have been observed by scanning electron microscopy (SEM). The duplex microstructure is associated with the longest LCF life at room temperature and 400°C, while the Widmannstatten microstructure has the shortest. The crack initiation sites and propagation paths were examined and discussed. The cracks primarily initiated along slip bands on the specimen surface for all four microstructures. In addition, many voids appeared along slip bands for the equiaxed microstructure. By linking-up these voids, the formation of microcracks is realized. The propagation of interior cracks in specimens with Widmannstatten structure proceeded by cross-cutting Wα platelets by way of a plastic blunting mechanism, whereas for the equiaxed microstructure interior cracks grew by the linking-up of voids by way of a renucleation mechanism.
Effect of Superimposed Stresses at High Frequency on Low Cycle Fatigue
Authors: J-Y Guedou , J-M Rongvaux
Published: February 2009

The effects of superimposed stresses at high frequency on low cycle fatigue behavior were studied on two aeronautical alloys (Ti 6-4 and Inconel 718) in terms of crack initiation, crack propagation, and failure. It is shown that, if the minor cycles have limited amplitudes, not more than 10 to 15% of low cycle fatigue stresses, the life is monitored by the maximum stress of the cycles. This is correlated with the fact that, in this case, the endurance limit of the material is not exceeded. The same criterion is suitable for crack initiation of Inconel 718, but not for Ti 6-4 which is, in all cases, much more sensitive to mixed fatigue than Inconel 718.

The crack propagation regime is ruled by minor cycles only beyond an observed transition ΔK value. For both materials, linear cumulation rules are inadequate for predicting the number of cycles to failure and for calculating the crack growth rates in mixed fatigue.
Fatigue Life and Short Crack Propagation of Turbine Rotor Steels under Variable Strain Loading
Authors: T Mori , H Toyoda , S Ohta
Published: February 2009

Fatigue behavior of steam turbine rotor steels with and without defects has been investigated at 550°C with emphasis on the effects of service loadings. It is shown that the life of defect-free materials is reduced remarkably by a factor of 10 to 100 due to the accumulation of the plastic strain range, which becomes conspicuous under a programmed fatigue test which simulates service loadings. A linear damage cumulative law in terms of the plastic strain range to estimate the fatigue life is presented. It is also shown that in order to evaluate the effect of defects, the conventional low-cycle fatigue test is not appropriate but a variable strain test is necessary. The fatigue life of the material with a defect is discussed in light of crack initiation and growth periods.
The Improved Neuber's Rule and Low Cycle Fatigue Life Estimation
Authors: Y-S Wu
Published: February 2009

An elastic-plastic finite element (FE) analysis was performed on the Society for Automotive Engineers (SAE) keyhole specimen in order to evaluate the prediction capability of Neuber's rule. Twenty different material properties were considered in the FE analyses. It was found that the accuracy of Neuber's rule is dependent on the material properties. In order to improve the prediction capability a new parameter concerning the material properties was introduced into Neuber's rule. The further improved Neuber's rule exhibits the format, Kt=KδmKε(1−m) The relation between m (exponent) and the material properties was studied. For the RQC-100 SAE keyhole specimen, stress-strain analyses and low cycle fatigue (LCF) life estimations were performed by both the conventional Neuber's rule and the extended rule.
Evaluation of Life Prediction Methods in High Temperature Fatigue
Authors: L Rémy , F Rezai-Aria , R Danzer , W Hoffelner
Published: February 2009

A collaborative study supported by the European Community COST 50 program has been conducted on life prediction methods under high-temperature low-cycle fatigue and thermal fatigue cycling in the case of MAR-M509, a cast cobalt-base superalloy. Low-cycle fatigue tests were carried out by the Ecole des Mines de Paris at 900°C in air under longitudinal strain control and included sawtooth triangular shape cycles at three strain rates (frequency range 10−3 to 20 Hz), tensile strain hold cycles, and compressive strain hold cycles. Life to initiation data deduced from a-c potential drop measurements were used to evaluate three types of predictive models: strainrange partitioning, studied by BBC-Baden; creep damage models, studied by the University of Leoben; and an oxidation-fatigue interaction model, studied by the Ecole des Mines de Paris. All the methods have been found to correlate experimental data within a factor of three. However, they range from simple correlating methods with adjustable parameters to be fitted to fully predictive methods with no fitting parameter. Thermal fatigue data on wedge-type specimens (i.e., components instead of laboratory specimens) have been used to check the prediction capability of the various methods and have shown the superiority of oxidation-fatigue interaction models for this material.
Fatigue Life Estimations for Cast Steels Using Low Cycle Fatigue Behavior
Authors: SG Lee , RI Stephens
Published: February 2009

Fatigue notch factors for compact keyhole specimens of five common cast steels were estimated using an elastic stress distribution around the notches and the fatigue tests of simple specimens. The values were used with modified Neuber's rule to calculate the stress and strain at an equivalent characteristic length under variable loadings. Another method, which uses the load-strain relation obtained from elastic-plastic finite element method (FEM) analysis, was also used to calculate the strain at the equivalent characteristic length. Both methods were used to calculate the fatigue damage at the keyhole notch using the Palmgren-Miner linear damage rule and low cycle fatigue properties obtained from smooth specimens. The results showed that both methods could be used for the crack initiation calculation within a reasonable accuracy.
Low Cycle Fatigue Characterization of a HP-IP Steam Turbine Rotor
Authors: V Bicego , C Fossati , S Ragazzoni
Published: February 2009

Results of an extensive low cycle fatigue characterization of HP and IP stages of a 1Cr-Mo-V steam turbine rotor are presented. Uniaxial strain controlled fatigue tests were carried out at 480 and 540°C, with hold times up to 24 h and strain rates down to 3 × 10−6 s−1. Moderate life reductions due to long hold periods were found only in tests with strain ranges less than 0.8%, though microstructural observations revealed evidence of creep damage morphologies in a few tests with 24 h hold times carried out at high strain ranges. Life reductions caused by the lowest strain rate were more significant. Generally, inferior fatigue resistance was provided by the rotor bore material, particularly in the more heavily segregated zones of the intermediate pressure stage.

Application of the linear damage summation (LDS) rule to hold time data was not satisfactory. Better results were generally obtained by the strainrange partitioning (SRP) method, with a few unconservative life predictions mainly due to inclusions that dominated early failures. A life correlation was attempted by an energy-based criterion suitable to provide rationalization of creep and fatigue damage in terms of thermodynamic concepts.
Thermal-Mechanical Fatigue Test Apparatus for Metal Matrix Composites and Joint Attachments
Authors: Westfall LJ , Petrasek DW
Published: February 2009

Two thermal-mechanical fatigue (TMF) test facilities were designed and developed, one to test tungsten fiber reinforced metal matrix composite specimens at temperatures up to 1700 K (2600°F) and another to test composite/metal attachment bond joints at temperatures up to 1030 K (1400°F). The TMF facility designed for testing tungsten fiber reinforced metal matrix composites permits test specimen temperature excursions from room temperature to 1700 K (2600°F) with controlled heating and loading rates. A strain-measuring device measures the strain in the test section of the specimen during each heating and cooling cycle with superimposed loads. Data are collected and recorded by a computer. The second facility was designed to test composite/metal attachment bond joints and to permit heating to a maximum temperature of 1030 K (1400°F) within 600 s and cooling to 420 K (300°F) within 180 s. A computer controls specimen temperature and load cycling.
Statistical Strength Comparison of Metal-Matrix and Polymeric-Matrix Composites
Authors: Wu EM , Chou SC
Published: February 2009

The reliability of a composite structure depends on the materials strength variability. Unidirectional composites fail sequentially initiating from the very weakest fiber sites with matrix binder providing local redundancy by transferring load to neighboring fibers until cumulation and clustering of these sites lead to sever stress concentration and ultimate structure failure. As a consequence, the variability of the metal matrix structure is traceable to the strength variability of the constituent fiber, the metal matrix coating process, and the composite wire consolidation process. This report focuses on the partitioning of the first two sources of variability, identification and modeling of the dominant parameters, together with experimental measurement on a current graphite-aluminum composite. The statistical strength of several graphite spools are measured by testing single filament specimens at the beginning and at the end of the spools, thereby characterizing the statistical parameters associated with the strength variability among the spools and within each spool. The graphite-aluminum wire, produced from continuous liquid infiltration process are tested in tension. The metal matrix composite statistical strengths from differ spools are compared with the respective statistical strength of the parent fiber. The results suggest that, given proper interpretation, single filament fiber strength is a sensitive parameter for quality assurance of metal matrix composites.
Minimechanics Analysis and Testing of Short Fiber Composites: Analytical Model and Data Correlation
Authors: Goering J , Buesking K , Awerbuch J
Published: February 2009

This paper describes an analytical modeling effort designed to study the stress-strain behavior of silicon carbide whisker reinforced aluminum. The objective of the study was to develop an approximate method for predicting the stress-strain response of whisker reinforced metals as a function of constituent properties, whisker geometry, volume fraction, and extrusion ratio. The study utilized experimental results (described in a companion paper) for qualitative understanding of the material behavior and quantitative data correlations with the model.
A Unique Set of Micromechanics Equations for High-Temperature Metal Matrix Composites
Authors: Hopkins DA , Chamis CC
Published: February 2009

A unique set of micromechanics equations is presented for high-temperature metal matrix composites. The set includes expressions to predict mechanical properties, thermal properties, and constituent microstresses for the unidirectional fiber reinforced ply. The equations are derived based on a mechanics of materials formulation assuming a square array unit cell model of a single fiber, surrounding matrix, and an interphase to account for the chemical reaction which commonly occurs between fiber and matrix. A preliminary validation of the equations was performed using three-dimensional finite element analysis. The results demonstrate excellent agreement between properties predicted using the micromechanics equations and properties simulated by the finite element analyses. Implementation of the micromechanics equations as part of an integrated computational capability for nonlinear structural analysis of high-temperature multilayered fiber composites is illustrated.
Thermoviscoplastic Nonlinear Constitutive Relationships for Structural Analysis of High-Temperature Metal Matrix Composites
Authors: Chamis CC , Hopkins DA
Published: February 2009

A set of thermoviscoplastic nonlinear constitutive relationships (TVP-NCR) is presented. This set is unique and has been developed mainly for application to high-temperature metal matrix composites (HT-MMC) and is applicable to thermal and mechanical properties. Formulation of the TVP-NCR is based at the micromechanics level. The TVP-NCR are of simple form and readily integrated into nonlinear composite structural analysis. Results show that this unique set of TVP-NCR is computationally effective. It provides a direct means for predicting complex materials behavior at all levels of the composite simulation; that is, from the constituent materials, through the several levels of composite mechanics, and up to the global response of complex HT-MMC structural components.
Development of Design Allowables for Metal Matrix Materials
Authors: Harmsworth CL
Published: February 2009

One of the most critical elements in the transition of a structural material from the development stage to an aerospace application is the existence of reliable design allowables. The Department of Defense (DOD) and the Federal Aviation Administration (FAA) require the use of MIL-HDBK-5 (for metals) and MIL-HDBK-17 (for composites) allowables unless alternate values are approved by these organizations. The major benefit of handbook allowables is not, however, the automatic approval of a material for use. It is the reliability and confidence that comes to the designer in being able to use a material with predictable property values. The end result is a design that is safe yet not overly burdened with excess structure. This article reviews the requirements for the development of allowables and some of the potential problems which may be encountered in developing allowables for metal matrix composites.
Thermal Expansion Measurements of Metal Matrix Composites
Authors: Tompkins SS , Dries GA
Published: February 2009

A high-precision Fizeau type, laser interferometric dilatometer has been developed and used to measure the thermal expansion of metal matrix composites. The strain resolution of this dilatometer system is about 1 × 10−6. The thermal expansion of graphite/magnesium and graphite/aluminum composite laminates were measured over the temperature range of 117 to 394 K. The expansion of a P100 graphite/6061 aluminum (P100 Gr/6061 Al) unidirectional laminate was measured and compared with an approximate nonlinear analysis. The good agreement obtained between analytical and experimental results was further improved when temperature dependent properties were included in the analysis. Data showed that the behavior of the specimen at the end of the first thermal cycle was path dependent during the cycle. Test data on a P100 Gr/AZ91C Mg undirectional laminate indicated a lower coefficient of thermal expansion and smaller residual strain than P100 Gr/6061 Al after thermal cycling between about 117 and 394 K.
Fracture Toughness of Thin-Walled Cylinders Fabricated from Discontinuous Silicon Carbide Whiskers/Aluminum Metal Matrix Composites
Authors: Raymond L , Jennings JA
Published: February 2009

Six cylinders (16 cm diameter with 6 mm wall thickness) were manufactured from two unreinforced aluminum alloy and four silicon carbide (SiC) whisker reinforced aluminum composites. Curved, subthickness, Charpy sized specimens were used to measure the fracture toughness, KIc in accordance with ASTM E 399. For the more ductile, unreinforced alloys, the British crack opening displacement (COD) method was used as an elastic-plastic approximation of the fracture toughness. The results rank the relative toughness of the aluminum alloy metal matrix composite (MMC) materials.

The test program was successful in demonstrating the changes in fracture toughness due to the addition of SiC whiskers, notch acuity, and prior hydrostatic straining. A new procedure was developed by which materials can be successfully fatigue precracked (FPC) by utilizing the notched strength ratio in bending. This procedure eliminates the cost of the excessive waste commonly generated in attempting to fatigue precrack fracture toughness specimens by conventional techniques.
Mechanical Behavior of Silicon Carbide/2014 Aluminum Composite
Authors: Chou S-C , Green JL , Swanson RA
Published: February 2009

The composite material consists of 25 volume percent of silicon carbide whiskers in 2014-T4 aluminum matrix. Specimens were taken from hot-isostatically-pressed cylindrical billets and tested at various loading conditions and strain rates. Tension and compression tests were performed at strain rates of 10−4 and 5 S−1 to determine the effect of strain rate. Bauschinger effect was investigated by precompressing specimens to two strain levels followed by reversing load to tension until fracture occurred. Torsion tests were carried out to determine the relationship between torque and twist angle. The KIc-value was determined by testing short-rod specimens. The tensile properties and KIc were compared with those of 2014-T4 aluminum to demonstrate the effect of silicon carbide whisker reinforcement on the properties of matrix materials.
Ultrasonic Inspection of Silicon Carbide Reinforced Aluminum Metal Matrix Composite Billets and Secondary Fabricated Products
Authors: Blue PL
Published: February 2009

This work describes the ultrasonic inspection of silicon carbide whisker and particulate reinforced aluminum metal matrix composite billets, extrusions, forgings, and rolled plate and sheet. Inspection techniques will be described and results discussed including metallographic analysis of flaws. A correlation will be made between billet attributes and extrusion results.
Influence of Heat Treatments and Working on Mechanical Properties of Silicon Carbide Reinforced Aluminum Alloys
Authors: Heritier J , Balladon P , Rambaud J , Chevet F , De Coquereaumont M
Published: February 2009

Three grades of silicon carbide (SiC) reinforced aluminium alloys are studied: one grade fabricated by powder metallurgy and reinforced by ≈25% volume fraction (vf) SiC particulates; two grades produced by casting and reinforced by ≈15% vf SiC whiskers.

Their behavior during hot working is simulated using unloading and holding times during high temperature tension testing. The three grades are also worked by extrusion and forging and the defects appearing on the wrought pieces are observed.

Mechanical properties of the three grades are represented versus temperature in the as received state, in the annealed state and in some cases after forging and annealing.

Microstructure is observed and its influence on mechanical properties is discussed.
Characterization of Thin-Wall Graphite/Metal Pultruded Tubing
Authors: Francini RB
Published: February 2009

The characterization of graphite/metal tubing will be discussed. Both destructive and nondestructive testing will be addressed. Test procedures to be discussed will include nondestructive (ultrasonic) modulus determination, longitudinal tension testing, short column compression testing, the hydrostatic burst (transverse) test, and thin-wall tube torsion test. The test fixtures and techniques used for these tests will be described, and typical test results for pultruded graphite/metal tubing will be presented. The use of these procedures to determine four of the five elastic constants and failure strengths necessary to characterize a transversely isotropic material in plane stress laminate analysis will be also discussed. Directions for future research in the testing area will be presented.
Lineaments: Significance, Criteria for Determination, and Varied Effects on Ground-Water Systems—A Case History in the Use of Remote Sensing
Authors: Peter KD , Kolm KE , Downey JS , Nichols TC
Published: February 2009

Lineaments are present in western South Dakota as linear segments of trunk streams and tributaries, soil and vegetation tonal anomalies, and topographic or drainage offsets. Because there is evidence for paleotectonic activity associated with certain lineaments, they appear to represent surface expressions of major basement discontinuities. Crustal movements along these discontinuities affected depositional patterns intermittently throughout the past 600 million years. Accordingly, the permeability and thickness of aquifers and confining beds have been affected.

Potentiometric, hydrochemical, geothermal, and seismic anomalies found in at least two aquifers correlate with surface lineaments mapped by remote sensing techniques. The authors hypothesize that (1) the Madison aquifer of Mississippian age is more transmissive along the Cheyenne River lineament and (2) there is upward leakage through the confining layers from the Paleozoicage aquifers to the Lower-Cretaceous-age aquifers along parts of the Kimball Creek, Bad River, White River lineament, and Cheyenne River lineament.