bluuebunny/crossref_metadata_2025 huggingface dataset:
Design and Offshore Experience with an In-Situ Vane
Authors: Geise JM , Hoope Jt , May RE
Published: February 2009

The design, development, and use offshore of a commercial in-situ vane, operated from a carrier tool downhole or from a seabed jacking system, is described. This vane is inserted into soil at a rate of 20 mm/s for both deployment modes. The vane is rotated at a constant speed of either 0.1 or 0.2°/s in cohesive soils having a maximum shear strength of 200 kPa. Using the hard-tie drill string heave compensating system, the first downhole vane test may be within about 0.75 m of the borehole base in very soft soils; if the hard-tie is not used, then the first test should be about 1.5 m below the borehole base to avoid soil disturbed by drilling. Standards developed for onshore vane testing can be used for offshore vane testing with few modifications.
The Effect of Elevated Temperature on Shot Peened 403 Stainless Steel
Authors: Childs WH
Published: February 2009

This paper presents the results of an experiment to determine the effect of elevated temperature on residual stresses present in, and the fatigue life of, shot peened 403 stainless steel. The first step involved testing simulated Almen A strips made from strip quench and tempered to 229–235 HB. The strips were shot peened to 8–10 A and subjected to temperatures ranging from 150 to 595°C (300 to 1100°F) for up to 1000 h. The amount of relaxation of the height of each sample was then measured at several intervals during the test. The data revealed that the change at 150°C (300°F) after 50 h was equal to that at 260°C (500°F). This drop was unexpected, as a review of available literature indicated residual stresses in carbon steel would not start to drop until 260°C (500°F), with stainless steels resistant to a much higher temperature. Measurement via x-ray diffraction showed the residual stress dropped from -529 MPa (-76.7 ksi) at room temperature to -269 MPa (-39 ksi) after exposure to 150°C (300°F) and to -112 MPa (-16.3 ksi) after exposure to 595°C (1100°F).

The second part of the paper presents the results of testing shot peened 403 stainless steel rotating beam fatigue test bars at room temperature, 260, and 400°C (500°F and 750°F). The data shows the endurance limit increases by 16 percentage points at room temperature and 6 percentage points at 260 and 400°C (500 and 750°F).
On Fatigue Life Calculation in Thick-Walled Cylinders with Complex Residual Stresses
Authors: Pu S-L , Chen PCT
Published: February 2009

The large variation in stress intensity factors corresponding to various material models for a single, radial, straight-fronted crack in a pressurized, partially autofrettaged cylinder leads to a drastic difference in the fatigue life calculations. None of the calculated lives agree with the experimental results. Possible explanations of the discrepancy are given and corresponding correction factors are introduced. The calculated lives based on the corrected stress intensity factor ranges are reasonably close to a set of well-documented experimental results arrived at by Throop and Fujczak.
Estimating Residual Stresses and Their Effect in Welded Aluminum Components in Fatigue
Authors: Kosteas D
Published: February 2009

Observations and assumptions on residual stresses and their effect on the fatigue behavior of welded aluminum structures are reviewed along with attempts to explain and evaluate experimental component fatigue strength data. Measurements from a recent research program on welded aluminum beams are reported.
Mass Spectrometric Studies of Laser Damage in Calcium Fluoride
Authors: Nogar NS , Apel EC , Estler RC
Published: February 2009

Both electron impact- and laser ionization-mass spectrometry have been used to probe the material spalled from CaF2 surfaces by the fundamental(1.06μ) and harmonic (355 nm and 266 nm) outputs from a Q-switched Nd:YAG laser. We have identified a number of gas phase products, including Ca, CaF and tentatively, CaF2, and have measured their translational and internal energy distributions.

Our results suggest that while the 1.06μ experiments can be adequately modeled in terms of a single damage mechanism (likely avalanche breakdown), the short-wavelength results suggest the onset of second mechanism, perhaps multiphoton absorption. This is consistent with both the observed bimodal velocity distribution following short-wavelength damage, and with the decrease in CaF vibrational and rotational excitation.
CaF2: An ESCA Study
Authors: Raj T , Law M
Published: February 2009

This paper describes two observations made on CaF2 during electron spectroscopy for chemical analysis (ESCA) studies. The first observation is that an ion-etched CaF2 surface adsorbs several layers of water even in an ultrahigh vacuum (UHV) chamber (P ∼ 2×10-10 torr). This water at the sputter-etched surface is responsible for the presence of oxygen peaks in the ESCA data. The presence of oxygen in the ESCA data can easily be misinterpreted as an impurity in the CaF2 bulk. This observation is detailed in Part I of the paper.

The second observation is that CaF2 (whether substrate, source material or coating) in a UHV environment releases a small amount of fluorine-containing gas. This is discussed in Part II of the paper.
Production Oriented Laser Damage Testing at Hughes Aircraft Company
Authors: Mordaunt DW , Arenberg JW , Frink ME
Published: February 2009

This paper describes the laser damage test station used by Hughes Aircraft Company for production support and incoming inspection of optics. It is emphasized that this is a high volume production station which is utilized almost full time for lot sample testing and related production support activities.

The evolution of damage testing at Hughes over a 15 year period is described from early efforts with ruby lasers to the current facility used in support of our Nd:YAG laser programs. The test station and procedures are described in detail including the use of a reference optic as a standard of comparison. Also discussed is the long term success of this test in providing usable components which are acceptable in actual hardware. Plans are presented for the development of the next generation of production related damage test station which incorporates more sophisticated diagnostics and the potential for almost complete automation of the actual damage test.
Microstructural and Physico-Chemical Investigations of Dielectric Multilayers
Authors: Guenther KH , Pawlewicz WT
Published: February 2009

Dielectric multilayer coatings of SiO2 and Si3N4 prepared by RF diode sputtering show very good environmental stability which surpasses that of conventional vapor deposited coatings. It is generally assumed that sputtered films have a higher packing density which makes them less susceptible to water penetration. Transmission electron micrographs of cross-section replicas of these multilayer structures show a columnar microstructure of the individual Si3N4 films which is qualitatively denser than that of other high index films made by electron beam deposition. Hence, some porosity may be still assumed for the sputtered films, and therefore other mechanisms to make them less humidity susceptible than just desiccation. For instance, it has been found that the wetting properties of dielectric thin films not only depend on their material but also, to a minor extent though, on the method of how they were deposited. Another possible explanation for improved humidity stability is that the plasma discharge in the sputtering process may polymerize residual hydrocarbons or silicone in the vacuum chamber to form hydrophobic thin films in between the dielectric layers and also on their inner surfaces, so that the voids between the columns in the films become less able to absorb water. First results on both contact angle measurements to assess the wettability of coating surfaces and on electron spectroscopic imaging for detecting presumed hydrocarbon contamination are presented.
Amorphous Silicon for CO2 Laser Mirror Coatings
Authors: Dutois F , Mouchart J , Pointu B , Villela G
Published: February 2009

High power CO2 laser require mirrors with high reflectivity, high laser damage threshold and efficient mechanical and chemical protection. Enhanced metallic coatings are generally used.

Amorphous silicon thin films are hard, with low porosity and have a high refractive index: unfortunately the low absorption at 10.6 μm is however too high for quarterwave layer at that wavelength.

When associated with a classical transparent but softer high index material such as ZnSe or ZnS, amorphous silicon can nevertheless be used in order to design the last composite high index quaterwave. Theoretical calculations predict an optimum reflectance for a fixed a : Si : H thickness.

The amorphous silicon thickness is thin enough to obtain a high reflectivity coating although its absorption, but thick enough to provide a good chemical and mechanical protection. Besides, experimentally determined that such enhanced coatings have a high laser damage threshold level.
Admixture of SiO2 to Suppress TiO2 Crystallization
Authors: Sites JR , Postek JS , Robinson RS , Schemmel TD , She CY
Published: February 2009

Crystallization of glassy TiO2 films used in optical coatings can be a problem in high power applications and may be a major limitation to the laser damage threshold. An attempt to suppress such crystallization involved mixing small amounts of SiO2 into TiO2 during the ion-beam sputter deposition process. An admixture of 5% SiO2 increased the crystallization onset temperature, determined by Raman spectroscopy, by approximately 150°C. The refractive index was relatively unaffected by the addition of 5% SiO2, dropping from 2.42 to 2.38. Optical loss, which shows a sharp increase near 350°C for pure TiO2, did not increase measurably for the films with SiO2 admixture of 5% or greater.
Chemical Vapor Deposition of TiO2 Thin Films at Room Temperature
Authors: Wilder J , Thomas I
Published: February 2009

Titania coatings were prepared using a chemical vapor deposition (CVD) apparatus at room temperature. Coating were deposited by the reaction of gaseous TiCl4 and H2O at the substrate surface. High refractive indices were obtained with damage thresholds to 12.9 J/cm2, at 1.06 μm, 1 ns.
Nondestructive Depth Profiling of Optically Transparent Films by Spectroscopic Ellipsometry
Authors: Vedam K , Kim SY , D'Aries L , Guenther AH
Published: February 2009

Spectroscopic ellipsometric (SE) measurements followed by linear regression analysis of the SE data obtained on optically transparent thin films of ZnS and MgO on vitreous silica substrates, reveal the distribution of voids (or low density regions) in these thin films.
Properties of IAD Single- and Multi-Layer Oxide Coatings
Authors: McNally JJ , Williams FL , Wilson SR , McNeil JR
Published: February 2009

Properties of oxide thin films deposited using oxygen-ion assisted deposition (IAD) were studied. Previously, we reported on the properties of Al2O3/SiO2 and Ta2O5/SiO2 anti-reflection coatings fabricated using IAD. In this report we present results illustrating the effects of ion bombardment during the deposition of single-layer films of Ta2O5, Al2O3 and SiO2. The optical constants and stress measurements of these films are reported. Oxygen ion bombardment during deposition produced Ta2O5 and Al2O3 coatings with larger values of refractive index (n). No increase in n was observed for IAD SiO2. Laser damage results at 351 nm for Al2O3/SiO2 and Ta2O5/SiO2 AR coatings are presented. It appears that the IAD coatings did not have higher damage threshold values than the coatings deposited with no ion bombardment. A number of coatings were exposed to fluorine gas tests. The results from these tests are reported.
Damage Thresholds of Antireflection Coatings at 790 nm
Authors: Gallegos G , Foltyn S , Boyer J
Published: February 2009

Damage thresholds of antireflection coatings on sapphire and alexandrite substrates have recently been measured. Optimization of AR coatings was necessary to improve damage thresholds on alexandrite laser rods, over that observed on the bare material. Eight different coatings supplied by seven different vendors plus uncoated sapphire and alexandrite were tested. Thresholds from 7 J/cm2 to 80 J/cm2 were observed. Sapphire was used because it has an index of refraction similar to alexandrite and is more readily available in sizes suitable for testing. An alexandrite laser at a wavelength of 790 nm with a pulse duration of 200 ns at 30 Hz for two seconds was used. The mean spot diameter of the nearly Gaussian beam was 0.380 mm. All of the damage testing in the AR screening survey was done at normal incidence.
Layer-Dependent Laser Sputtering of BaF2 (111)
Authors: Reif J , Fallgren H , Nielsen HB , Matthias E
Published: February 2009

Laser-induced sputtering from the (111) surface of BaF2 was investigated under ultrahigh vacuum conditions, applying fluences well below the macroscopic damage threshold. Measurement of the wavelength dependent desorption of Ba+ indicates that Ba+ is emitted from two chemically different surroundings at the surface. For a fixed wavelength, the emission rate of Ba+ and F+ as functions of time show a distinct anticorrelation, confirming the existence of two different chemical states of the surface, and suggesting that the sputtering takes place layer by layer.
An Anomalous Absorption Model to Account for Accumulation in N-on-1 Damage in Si and GaAs
Authors: Shetty NR , Becker MF , Walser RM
Published: February 2009

In this work we have investigated a model for anomalous optical absorption and laser damage in Si and GaAs. The model determines the increase in charge density in the vicinity of small, compared to the optical wavelength, clusters of defects in the material. The aim is to determine if increases in defect density in these regions can account for the experimentally observed decrease in damage threshold or accumulation. We found that 10% to 20% reductions in damage threshold could be accounted for by this model, but that this is only sufficient to account for some of the experimental observations.
Model Comparisons with Glancing Incidence Measurements of Overcoated Metal Mirrors
Authors: Kimura WD , Woodberry FJ , DeSandre LF
Published: February 2009

The absorptance characteristics of metal mirrors, with thin dielectric overcoats on their surface, have been measured at angles of incidence from 0 to 89°, for both s and p-polarizations. The mirrors tested are silver overcoated with ThF4, and aluminum overcoated with MgF2. To understand the experimental results requires the use of models incorporating thin film theory. Estimations of parameters, such as film thickness, can be made by varying parameters within the model until the best fit with the experimental data is obtained. This paper presents the results of model comparisons with the glancing incidence data. In addition, thin film designs for overcoated mirrors with enhanced reflectivity at glancing incidence are discussed.
Improvement of The Bulk Laser Damage Threshold of Potassium Dihydrogen Phosphate Crystals by Ultraviolet Irradiation
Authors: Yokotani A , Sasaki T , Yoshida K , Yamanaka T , Yamanaka C
Published: February 2009

Potassium dihydrogen phosphate (KDP)crystals were grown under the irradiation of ultraviolet light. The bulk laser damage threshold was improved two to three times (15 – 20 J/cm2) compared to the case of crystals grown by conventional methods. Microbes such as germs and bacteria are frequently generated in the KDP solution with the usual growth method, causing a low damage threshold. A main reason of the low damage threshold is due to the organic materials such as microbes or their carcass which are incorporated in the growing KDP crystals.
Study of Mo, Au, and Ni Implanted Molybdenum Laser Mirrors By Spectroscopic Ellipsometry
Authors: Snyder PG , Bu-Abbud GH , Oh J , Woollam JA , Poker D , Aspnes DE , Ingram D , Pronko P
Published: February 2009

The implantation of 150 kev molybdenum ions into polished molybdenum laser mirrors is found to increase the complex dielectric constant in the visible spectrum. Analysis using the Bruggeman effective medium approximation demonstrates that the increase is due to surface smoothing and that the surface is made nearly atomically smooth by a fluence of 5 × 1015/cm2. Implantation of Au at 1Mev caused considerable microscopic roughening, as well as a change in the bulk optical properties. 3 MeV Ni ion implantation caused only a slight surface roughening. A thin dielectric film (probably a hydrocarbon) is found to condense in a laboratory atmosphere, reducing the reflectivity, and is removable by rinsing with methanol and distilled water.
Laser Induced Mo Mirror Damage for High Power CO2 Laser
Authors: Okamoto H , Matsusue N , Kitazima K , Yoshida K , Ichikawa Y , Yamanaka M , Yamanaka T , Tsunawaki Y
Published: February 2009

We have developed a high-purity molybdenum (Mo) mirror by electron-beam melting method (e.b.m. Mo mirror). Its high laser-damage threshold has been demonstrated for the high-power TEA CO2 laser system. The e.b.m. Mo mirror has a damage threshold of 2 to 4 times higher than that of an Au coated glass mirror and had a life time of 3 times longer than that of a powder metallurgy Mo mirror (p.m.Mo mirror) when the laser energy density of higher than 60 J/cm2 was irradiated. We found that on the laser-damage threshold the difference between the e.b.m. Mo mirror and the p.m.Mo mirror was due to the fine surface without voids and the small amount of impurities.
Phase Control Design Analysis
Authors: McGuirk M , Poirier RD
Published: February 2009

A novel graphical design technique was developed for two-wavelength achromatization of phase control coatings. The technique makes it possible to rapidly identify regions of approximate solutions for exact computer optimization. The technique gives an indication of the sensitivity of the design to thickness errors. The technique has been applied to design coatings yielding an average phase error of 0.3 phase degrees per mirror pair over a ±5% bandwidth.
Fundamental Mechanisms Of Optical Damage In Short-Wavelength High-Power Lasers
Authors: Haglund RF , Tolk NH , York GW
Published: February 2009

Evidence has been accumulating for many years that the physical mechanisms responsible for damage to optical materials in and from high-power, short-wave-length lasers (SWLs) differ in fundamental ways from the thermal processes identified in infrared and visible-wavelength laser damage problems. We propose that this difference stems primarily from the electronic nature of the absorption and excitation processes which occur when SWL photons strike an optical surface, and that electrons, ions and uv photons generated in the laser excitation cycle also contribute to optical damage. In this paper, we present recent experimental results which have pinpointed specific electronic excitation mechanisms which can operate in the high-power laser environment. In many optical materials of interest for SWLs, the deposition of electronic energy creates self-trapped excitons which decay through the energetic expulsion of atoms and molecules from the surface of the material. This erosion process is accompanied by the creation of permanent electronic defects which become nucleation sites for further damage. The relationship between these microscopic mechanisms and observed macroscopic damage phenomenology is discussed, along with evidence for the existence of a surface overlayer which may point the way to radically new techniques for protecting SWL optical elements from laser damage.
Temporal Distribution of Electron and Ion Emission Caused by Laser Excitation of Optical Surfaces in Ultra-high Vacuum
Authors: Siekhaus WJ , Chase LL , Milam D
Published: February 2009

We have measured electron and ion emission under high vacuum conditions from several semiconductor and insulator surfaces excited by the first and third harmonics of a 1.06 μm Nd laser. The dependence of this emission on laser fluence and wavelength is consistent with a multiphoton excitation process. The dependence on laser pulse length implies, however, that other processes, either diffusive, thermal or thermomechanical, are also involved. Additional information is provided by the temporal and spatial distribution of the charge emission. The most extensive results have been obtained with single crystals of ZnS, for which nearly equal magnitudes of negative and positive charge emission are observed. The results suggest the possibility that the observed charge emission results primarily from the ejection of neutral atoms and molecules from the surface, followed by selective multiphoton ionization of one or more of the neutral species. The irreversible surface modification implied by the observed emission may be the precursor to observable surface damage. Detailed studies of the properties of the emitted particles may be very useful in developing a basic understanding of the damage mechanisms.
Optimization of Fluorescence Measurements
Authors: Warner IM , Patonay G , Rollie ME , Thomas M , Nelson G
Published: February 2009

Various components of a conventional fluorometer are discussed. Several approaches to optimization of some of these components are discussed by citing specific research examples. The four areas of research that are described include: stabilization of a xenon arc lamp, improvements in sample chemistry, increased detector selectivity, and development of data reduction strategies for complex data. Improvements in the fluorescence measurement process are cited with each example.
Use of Analytical Fluorescence with Fiber Optics
Authors: Burgess LW , Fuh M-RS , Christian G
Published: February 2009

The development of fiber-optic based sensors is of growing interest in analytical chemistry. The ability to produce miniature sensors that are immune to electrical interference, that do not require a reference electrode, and that can be operated remotely are attractive advantages of this technology. Fiber optics provide a mechanism for continuous optical monitoring of process, biological, or hazardous environments that previously were difficult or impossible to access. Fluorescence spectroscopy is particularly suited for fiber optic sensing because of its sensitivity, selectivity, and versatility. In recent years, a number of sensing configurations have been described that take advantage of either the intrinsic fluorescence of the analyte or incorporate a fluorescent reagent at or near the sensor tip. Analyte concentrations can be determined from the enhancement, quenching, changing lifetime, or colorimetric modulation of the induced reagent fluorescence. These individual sensors integrate technology of fiber optics, chemistry, optics, and spectroscopy into their designs. The operating principles and optical configurations of typical fiber optic fluorescent sensors are described.
Change of Dislocation Structures and Macroscopic Conditions from Initial State to Fatigue Crack Nucleation
Authors: Murakami Y , Mura T , Kobayashi M
Published: February 2009

Dislocation structures in fatigue were investigated with special consideration given to the compatibility of the macroscopic observation by an optical microscope and the microscopic observation by a transmission electron microscope (TEM). First, previous works are reviewed from this point of view, that is, the compatability of macro- and microscopic observations. It is postulated that this viewpoint is very important for both the elucidation of fatigue mechanism and the construction of crack nucleation models. Second, the experiments are planned and carried out, and the results are discussed in this direction. Copper polycrystalline sheets with a thickness of 0.1 mm are used for the preparation of specimens for fatigue testing. The specimen contains an artificial hole of 200 μm diameter. The fatigue processes at the edge of the hole were observed successively with an optical microscope. In the early stage of the fatigue test, slipbands initiated at the hole edge and subsequently cracks nucleated after stress cycles of N ≅ 8 × 105. The foil specimens for TEM observation were prepared from the sheet specimens that were tested for various numbers of cycles, say, N = 2 × 105, 4 × 105, and 8 × 105. Every effort was made to show the correspondence between the site of TEM observation and that of macroscopic observation. Since it was very difficult to thin the edge of a hole sufficiently, only a few specimens were available for TEM observation though more than 100 sheet specimens were tested. The first part of the paper poses five fundamental questions; especially considered in the direction described above are (1) the relationship between vein structures and persistent slipbands (PSBs), (2) the relationship between crack nucleation and PSBs, and (3) dislocation structure at the stress concentration or stress gradient.

Conclusions may be summarized as follow: (1) Vein structures and ladder structures in PSBs are closely related. (2) In the fatigued copper polycrystal specimens with no hole, PSBs with ladder structure can develop in certain grains of the specimen. (3) At the edge of a hole of the specimens which contain a hole, however, PSBs with ladder structure are unlikely to be formed. (4) The crack nucleation sites at the edge of a hole are PSBs without ladder structure.
Crystallographic Study of the Fatigue Crack Nucleation Mechanism in Pure Iron
Authors: Tanaka T , Kosugi M
Published: February 2009

The fundamental question posed in this study is, What crystallographic conditions are satisfied at fatigue crack nucleation sites in polycrystalline iron? To answer this question it is vital to establish an effective technique to determine the crystallographic orientation of each grain under observation. This problem was solved by developing an etch pit method of grain orientation analysis. Stereographic projection geometry was successfully applied in determining the orientation of each grain with great accuracy from the measured angles between ridge lines of sharp-edged etch pit patterns on the film. The critical experiment, then, is to examine the geometrical relationship between the nucleated fine cracks and the orientations of surrounding grains on fatigued specimen surfaces. The final step is to extract such crystallographic conditions as are commonly satisfied at the sites of crack nucleation.

Electrolytically polished commercial base pure iron plate specimens were tested under completely reversed plane bending stress in the high-cycle fatigue range. Tests were interrupted at about half of the fracture life, and the crystallographic configuration of microcracks with respect to the grain orientation was thoroughly examined by making use of the newly developed technique. Cracks were observed to nucleate only along grain boundaries on the specimen surface. And it was found that there are five conditions satisfied at those cracked sites. These conditions concern the favorable configuration of slip systems, Schmid factor, grain size, and the direction of grain boundaries. Three more conditions were added from information pertaining to the crystallographic structure below the surface gained by observation after removal of the surface layer and of cross sections containing a crack. A brief discussion is also given on some statistical aspects of important parameters.
The Significance of Sliding Mode Crack Closure on Mode III Fatigue Crack Growth
Authors: Tschegg EK , Stanzl SE
Published: February 2009

During Mode III loading, the surfaces of a crack move parallel to each other; friction, abrasion, and mutual support of the contacting parts of the microscopically rough crack faces result. These energy-dissipative processes reduce the effective load at the crack tip, a phenomenon known as “sliding mode crack closure” (SMCC). This effect has been studied by performing crack growth experiments on circumferentially notched specimens of high-strength (AISI 4340) and mild steel (AISI C1018) under pure cyclic torsion and under combined loading (cyclic Mode III + static Mode I). Crack growth rates without the influence of sliding mode crack closure (“true” crack growth rates) can be determined using an extrapolation procedure. These crack growth rates are independent of crack depth, specimen diameter, and loading level and are therefore a material characteristic for Mode III fatigue crack growth. With the aid of the “true” crack growth curve the extent of sliding mode crack closure can be quantitatively determined and the changes in fracture mode explained. By superimposing a static tensile load the sliding mode crack closure is reduced whereas the “true” crack growth rates are only slightly influenced.
Use of Nondestructive Evaluation Techniques in Studies of Small Fatigue Cracks
Authors: Resch MT , Nelson DV , Yuce HH , London BD
Published: February 2009

This paper reports progress to date in the development of an ultrasonic surface acoustic wave (SAW) technique to monitor in situ the depth of small cracks as they grow, as well as the stresses required to cause the cracks to begin to open and to fully open.

Two SAW transducers (transmitter and receiver) are used to monitor crack reflection signals. The surface length is measured by conventional methods. Based on new acoustic theory, the reflection signal and length data are used to infer crack depth below the surface. Acoustically determined depth is compared with that obtained by destructive examination of specimens, and agreement is good.

The crack opening stress determined acoustically is compared with opening stress determined by scanning electron microscope (SEM) measurements of crack mouth opening displacement versus applied stress. This comparison requires specimens small enough to fit inside an SEM chamber. Preliminary results demonstrate the ability of the acoustic measurements of crack opening to detect behavior below the surface not detected by SEM measurements. Acoustically determined changes in crack opening behavior as a function of constant-amplitude and variable-amplitude stress histories are also presented.
The Effect of Loading History on Closure Behavior in Rene' 95
Authors: Zawada LP , Nicholas T
Published: February 2009

Experimental observations of closure in fatigue crack growth as a function of loading history are presented for a high-strength nickel-base superalloy, Rene' 95, at 649°C. Fatigue crack propagation (FCP) tests were conducted under either constant load amplitude (increasing ΔK) or under threshold load shedding (decreasing ΔK) test conditions. Closure loads were determined from load-offset displacement traces that were recorded with the aid of a crack mouth opening displacement extensometer. Plots of Kcl versus Kmax revealed that Kcl remained constant for both the constant load and decreasing load FCG tests, indicating that Kcl was not a consequence of the loading history. Similar plots for side-grooved specimens and smooth specimens revealed identical behavior, thus ruling out plasticity-induced closure as the operating closure mechanism.
Crack Arrest Fracture Toughness of 7075-T6 Aluminum Alloy
Authors: Mukherjee B , McCluskey D
Published: February 2009

The plane-strain fracture toughness KIc represents the resistance of a material against crack initiation. The material toughness in the case of a run-arrest event involves a special “material property” called the crack arrest toughness KIa, which within certain limitations, is a measure of the ability of a material to arrest a fast running crack under linear elastic conditions. It has been indicated that KIa is a material property independent of specimen geometry, but this is not a universal belief. Whether or not KIa is a material property is still being debated. The objectives of this project were to measure the crack arrest fracture toughness of 7075-T6 aluminum alloy and to determine the effect of specimen size and thickness on KIa. The tests were performed in accordance with a proposed ASTM procedure developed for high-toughness and mediumstrength steels. Specimens that were used in this program exceeded the proposed minimum linear elastic size requirement of ferritic steel specimens by a factor of 4 between 2 and 10. KIa showed a slight dependence on test parameters such as stress intensity at crack initiation, specimen thickness, and crack jump lengths. However, KIa was not a strong function of the stress intensity at crack initiation K0. The mean value of KIa was an adequate representation of the crack arrest toughness value within the test matrix. The average KIa was only 10% lower than the average plane-strain fracture toughness KIc.
Stress-Intensity Factors for an Edge Crack in a Stiffened Sheet
Authors: Yu JC
Published: February 2009

This paper presents the two-dimensional fracture mechanics solutions of a through-the-thickness edge crack in a stiffened sheet. The sheet is assumed to be under the plane-stress condition, and the stiffeners are taken as one-dimensional rods discretely attached to the sheet. The fastener forces are determined by enforcing the displacement compatibility at fastener locations and by the equilibrium condition for broken stiffeners. The problem is then reduced to the solution of a set of two simultaneous integral equations. The solution procedure employed allows direct evaluation of the unknowns, leading to efficient determination of the fastener forces as well as the stress intensity factors. Numerical examples are presented.
Development of a Criterion for the Effect on the J-R Curve of Elastic Unloadings
Authors: Joyce JA
Published: February 2009

A criterion is proposed which allows estimation of the fraction of cyclic damage which would be expected from a given number of partial or complete unloadings applied during the course of a J-R curve fracture toughness test. The criterion requires both geometry and material inputs, but for the special case of a steel compact specimen, an upper-bound criterion is developed and presented graphically.

Data from a series of compact specimens of a 3% nickel alloy steel are used to verify both the model used and the resulting fractional damage criterion developed.
Growth of Parabolic Fatigue Cracks from Flaws at Fastener Holes
Authors: Berkovits A , Prinz D
Published: February 2009

A crack of parabolic shape, emanating from corner and embedded flaws at a hole in a plate of finite width, was analyzed by the slice synthesis. In contrast with other crack shapes, equilibrium solutions for a parabolic crack are finite and continuous as the crack passes the bottom of the hole and starts to propagate along the rear surface. Fracture parameters derived were KI for predicting fatigue crack growth along the plate surfaces and down the bore of the hole, and GI for residual strength. The two cases studied were a flaw emanating from the corner of the hole and a flaw embedded in the side of the hole at the mid-depth, each case being either remotely loaded, or loaded at the hole. Fatigue crack curves were calculated with the aid of the derived fracture parameters, and fatigue crack curves were compared with experimental data and with calculated results obtained assuming an elliptically shaped crack. The parabolic crack analysis resulted in superior correspondence with measured data. Tables of constants for evaluating the fracture parameters are provided.
Effect of Residual Organic Carbon in Mother Solution of Potassium Dihydrogen Phosphate on Damage Threshold of the Crystals for High Power Lasers
Authors: Yokotani A , Sasaki T , Yoshida K , Yamanaka T , Nakai S , Yamanaka C
Published: February 2009

We obtained the damage-resistant crystals of potassium dihydrogen phosphate (KDP) by reducing organic impurities in the growth solution. To prevent organic impurity exuded from the wall of growth vessel, Pyrex glass was used. The residual organic impurities which mainly derived from KDP raw powder were reduced by photo-chemical dissociation using the combination of ultraviolet light and an oxidant (hydrogen peroxide). The resultant total organic carbon (TOC) of the solution was less than 0.1ppm. The damage threshold of the crystals grown from this solution were always high (20–22 J/cm2), which is about three times higher than that of crystals grown by conventional methods.
Laser Damage on Zinc Selenide and Cadmium Telluride Using the Stanford Mark III Infrared Free Electron Laser
Authors: Benson SV , Szarmes EB , Hooper BA , Dottery EL , Madey JMJ
Published: February 2009

We have encountered intracavity optical damage, and have estimated fluence damage thresholds, in samples of CVD zinc selenide which have been used as intracavity Brewster plate output couplers for the Stanford Mark III infrared free electron laser. For spot radii of approximately 1.4 mm and wavelengths in the range of 3 μn to 4 μm, we have determined damage thresholds ranging from 8.2 J/cm2 to 11.3 J/cm2 for optical macropulses of 2 μs duration and 15 Hz repetition rate, with corresponding circulating micropulse intensities on the order of 5 GW/cm2; in most cases catastrophic damage seemed to occur due to the coalescence of tiny pits in the surface. We also report on damage threshold measurements for uncoated cadmium telluride at 3.2 μm, and observe a fluence dependent damage threshold of approximately 6.3 J/cm2 for macropulses scaled (via τ1/2) to a 1 μs duration. Finally, we observe a lowering of the damage threshold of uncoated CdTe in the presence of the laser harmonics, presumably due to free carrier absorption of the fundamental.
Multiphoton Photoconductivity and Laser Induced Damage in Alkali-Halide Crystals Under Picosecond Pulses from YAG:Nd Laser and its Harmonics
Authors: Garnov SV , Epifanov AS , Klimentov SM , Manenkov AA , Prokhorov AM
Published: February 2009

The results are presented of studying photoconductivity induced by picosecond pulses of the second, third, and fourth harmonics of YAG:Nd-laser in KCl, NaCl, KBr, KI crystals. The processes of multiphoton photoexcitation of nonequilibrium carriers are revealed up to the 4-th order and the corresponding multiphoton absorption coefficients are measured. Drift mobility of thermalized nonequilibrium electrons is measured. The role of multiphoton absorption in laser-induced damage in crystals is discussed. With a view to elucidating the role of impurities (inclusions) in the laser induced damage in alkali-halide crystals in the picosecond region, impurity photoconductivity is studies depending on thermal treatment of crystals.
Ultrafast Imaging of Optical Damage in PMMA
Authors: Zyung T , Kim H , Postlewaite JC , Dlott DD
Published: February 2009

A new imaging system with ultrafast (75 ps) time resolution, the “picosecond microscope” is used to study the damage dynamics of PMMA irradiated at 532 nm. A small diameter (50 μm), intense laser pulse is used to damage the PMMA, and a larger diameter, delayed probe pulse is imaged onto a video camera detector. The resultant time-dependent images are a “motion picture” of the damage process.
Ultrafine Polishing of Tungsten and Molybdenum Mirrors for Co 2 Laser
Authors: Yamashita M , Hara S , Matsunaga H
Published: February 2009

A study has been performed experimentally to determine the polishing methods for high-power laser mirrors. This paper presents ultrafine polishing of tungsten (W) and molybdenum (Mo) which cannot be turned by a diamond tool.

Samples were prepared by five methods: lapping with SiC abrasives of 4000-grit size; lapping with 3 μm diamond powder; float polishing with SiO2 powder; Ar ion beam etching; and electrolytic polishing. The finished surfaces are characterized using several techniques, including profilometry, electron probe microanalysis, X-ray diffraction, calorimetric technique with cw CO2 laser and so on.

The surfaces lapped with 3 μm diamond powder have damaged layers of about 2 μm deep in the case of W and about 3 μm deep in the case of Mo. In either case, however, if about 1 μm thickness of the damaged layer is removed by float polishing from the lapped surface, the layer which increases absorptance at 10.6 μm wavelength will be almost removed.
Laser Induced Surface Emission of Neutral Species and Its Relationship to Optical Surface Damage Processes
Authors: Chase LL , Smith LK
Published: February 2009

The laser-induced emission of neutral constituents and impurities from surfaces of several optical materials is shown to be correlated with optical surface damage thresholds. The characteristics of the emission can be utilized to investigate physical processes involved in the absorption of laser energy at the surface. Examples are given of neutral emission correlated with catastrophic surface heating, changes in surface stoichiometry, and thermally-induced cracking.
Laser Induced Particle Emission as a Precursor to Laser Damage
Authors: Domann FE , Stewart AF , Guenther AH
Published: February 2009

Mass spectrometry was used to investigate particle emission, specifically contaminant species, as a possible precursor to laser damage. Experiments were carried out in a clean ultra-high vacuum chamber with a background pressure of 4×10-8Pa. Both pulsed (8 ns) and CW Nd:YAG lasers operating at 1.06 microns were used to irradiate a variety of samples which included uncoated Si, fused silica coated with films of Al2O3, MgF2, or ZrO2, and Si coated with a SiO2/ZrO2 stack. In the pulsed laser experiments, both 1-on-1 and N-on-1 irradiations were investigated. Emitted particles were mass analyzed with a quadruple mass spectrometer which recorded the partial pressure of a given species as a function of time.

Single pulse damage thresholds found in this study were consistent with those reported in the literature. Important differences are exhibited in pulse radiation data depending upon whether the laser fluence is above or below the damage threshold as well as upon the type of sample irradiated. In CW laser experiments, fluence was sufficient to cause damage only in the SiO2/ZrO2multilayered coating due to absorption by the Si substrate. CW irradiation also resulted in desorption signals at lower fluences than was possible with pulsed radiation.
Ion Emission Kinetics of Laser Damage to Semiconductor and Dielectric Thin Film Surfaces
Authors: Trisnadi JI , Jee Y , Walser RM , Becker MF
Published: February 2009

In these experiments we used time-of-flight energy analysis to determine the kinetic processes occurring at laser damage on surfaces. A triaxial charge collector of relatively large area was designed for use in the target vacuum chamber. It could be biased in several ways to detect either positive or negative charge, and it could resolve charged particle time-of-flight over a field free region of about 6 cm. The laser system was a Q-switched Nd:YAG laser with 10 ns pulses at 1064 nm. Measurements were made on single crystal germanium wafers and on half-wave delectric thin film samples of ThF2 and MgF2 on fused silica.

On germanium, ion emission was only detected when the surface morphology indicated boiling. This occurred at about 1 J/cm2 or a little over twice the melt or damage onset fluence. For thin film samples, ion emission correlated with surface damage. For all samples, ion velocity distributions and the observed correlation between kinetic energy and ion flux were characteristic of laser heated plasma processes following charge emission.
Diamond, A Potentially New Optical Coating Material
Authors: Feldman A , Farabaugh EN , Sun YN , Etz ES
Published: February 2009

The ability to synthesize diamond films at low pressures offers the possibility of superior optical coatings for severe environments. This is because of diamond's great hardness, high thermal conductivity, and chemical inertness. However, several problems must be overcome before practical diamond optical coatings can be made. They include difficulties in film nucleation, high substrate temperatures, and large film surface roughness leading to high optical scatter.
Comparative Study of Reactively Evaporated vs. Ion-Plated TiO2 Thin Films
Authors: Guenther KH , Loo B , Pulker HK , Saxer A , Seitel SC
Published: February 2009

Single-layer thin films of TiO2 deposited on glass substrates by means of standard reactive evaporation (from a boat) and by means of the novel deposition technique of ion-plating were characterized with a transmission electron microscope, a Raman microprobe, a thermal deflection spectrometer, and a Talystep stylus-type surface profiling instrument. With each deposition technique, single-layer coatings of different thicknesses were produced, from one QWOT up to 12 QWOT @ 633 nm). The thickness dependence of microstructure, surface roughness, and absorption for the two groups of coatings will be presented, together with laser damage threshold measurements at 532 nm.
Studies of the Thermal Stability of Thin Film Structures
Authors: Lewis KL , Miller A , Muirhead IT , Staromlynska J
Published: February 2009

Interest is currently being shown in the role of thermal transport in determining the laser damage threshold of thin film structures. Experimental evidence [1] suggests that the thermal conductivity of materials in thin film form may be significantly different to that in bulk, and that this is probably related to the microstructure of the film. A recent paper by Abraham and Ogilvy [2] presents a linear theory of heat conduction in an etalon filter structure with a weakly absorbing spacer layer. This allows the calculation of temperature distribution at the centre of the spacer for given values of film thermal conductivity. For spacer layers with significant temperature dependences of the refractive index (eg ZnSe near its fundamental absorption edge), the temperature rise induced by laser heating can be followed in principle as the change in transmission produced as the etalon sweeps along the edge of one of its characteristic fringes. When the appropriate cavity conditions are met, optical bistability can occur with incident powers typically in the milliwatt regime with focussed laser beams. This power depends on such parameters as the effective optical nonlinearity of the material, the design of the etalon and wavelength. The heatsinking of the device, normally on a glass substrate, will affect both the switching power and the switching speed. Although the absorption coefficient of the spacer, a, varies rapidly with wavelength near the band gap, the temperature dependence of the refractive index as measured [3] is much less sensitive to wavelength. It may also be noted that thick polycrystalline samples have shown dispersive optical bistability at 476nm, whereby a polished etalon of ZnSe provides feedback [4,5]; bistability due to increasing absorption in which the feedback is inherent in the thermally induced absorption edge shift was also achieved in this type of sample [5].
Optical Measurements of Surface Oxide Layer Formation on Metal Films
Authors: Scott ML
Published: February 2009

We have employed two optical techniques which give complementary indications of the formation of monolayers of oxide on freshly evaporated aluminum and silicon thin films. Visible ellipsometry is utilized to observe the growth of the initial monolayer of oxide on these films. From these data, we deduce the pressure and coverage dependence as well as the growth rate for the initial monolayer arising from these surface reactions. In addition, extreme ultraviolet (XUV) reflectance vs angle of incidence measurements at 58.4 nm wavelength clearly indicate the growth of oxide on the surface of our freshly deposited aluminum and silicon films as well. We have utilized this reflectance data to deduce the optical constants of aluminum and silicon at 58.4 nm. We find that previous XUV measurements of these optical constants were hampered by the presence of oxides. We also determined that the XUV reflectivity performance of aluminum films freshly deposited in our UHV system does not degrade appreciably when stored for four weeks in a helium atmosphere of 2 × 10-9 Torr.