bluuebunny/crossref_metadata_2025 huggingface dataset:
A Crack-Tip Opening Displacement Method for the Analysis of Ductile Materials
Authors: Garwood SJ
Published: February 2009

Although crack-tip opening displacement (CTOD) analysis procedures are well established for the prediction of tolerable crack sizes in ferritic steels, no specific procedures exist which incorporate a resistance (R) curve to facilitate estimation of the ductile instability conditions for a fully ductile material.

This paper discusses a modification to the CTOD procedure, based on the strip yield model, to enable the prediction of maximum nominal stress and the extent of stable ductile crack growth, using a CTOD R-curve as input.

The effectiveness of this procedure is assessed by predicting the behavior of wide plate tension tests and burst tests in API 5LX56 and A533B Class I ferritic parent steels and of wide plates in stainless steel plate and weldments. The predictions are then compared with physical data obtained from the tests.

Reasonable predictions of maximum applied stress are achieved. However, the accuracy is very dependent on the plastic collapse estimations. This is particularly true for the high work-hardening stainless steel plate where, to avoid an overestimate of collapse, the effective flow stress must be taken to be the stress at 3% strain in a uniaxial tension test.

Predictions of the test results are also made using maximum load toughness values, measured using CTOD test specimens complying with BS 5762, with either the CTOD design curve or the modified strip yield model approach. Predictions give safe estimates of nominal stress.

It is concluded that maximum load values provide the most practical general approach when detailed resistance curve information is not available.
Elastic-Plastic R-Curves for Large Amounts of Crack Growth
Authors: Etemad MR , John SJ , Turner CE
Published: February 2009

Several J-estimation procedures for use in the presence of stable crack growth are reviewed to show the distinction between those based on internal energy or on work done and on the choice of “incremental” or “sequential” analysis. These are then related and compared theoretically and experimentally to show very little differences between them for the J-controlled regime with small amounts of growth. For large amounts of growth, rising and drooping R-curves are discussed using data from open literature. A deformation theory J is shown to give a drooping R-curve shortly after ω < f(η). Two recent expressions for J, Jm, and Ja, formulated to satisfy the Rice, Drugan, and Sham criterion of ˙Jpl ≠ f(˙a), are discussed in light of these arguments. Finally, a reinterpretation of R-curves is discussed for large amounts of growth suggesting Δa/c for the R-curve abscissa instead of Δa with the view to obtaining a geometry-independent R-curve.
Influence of Optical Components for Laser Processing
Authors: Giesen A , Borik S , Schreiner U
Published: February 2009

In order to make full use of the potential of today's high-power lasers it is desirable for numerous applications in material processing to achieve a diffraction limited intensity distribution in the focal plane. Therefore, in addition to the mode of the laser beam the properties of the optical components used for beam guiding and beam shaping are essential. Due to partial absorption of the high-power laser's radiation these components have transient optical properties. In order to investigate this behavior different experiments to evaluate the surface quality, the absorption and the optical deformation under irradiation have been performed.
Materials for Aerospace/Large Optics
Authors: Ernst WW
Published: February 2009

For use in large mirrors in outer space, where low mass is a critical need, 3 D-CFK developments are under our investigation since 8 years. Precision replication future technologys taken from a negative form are our candidates for the ESO, VLT monolithic primarys. In our option we use a Nasmyth Focus, because of mirror stability.
Production and Properties of Perrhenate-Doped Alkali Halide Crystals
Authors: Nestor OH , Figueira JF
Published: February 2009

The growth and selected properties of single crystals of KCl doped with ReO4- is described. The crystals have been used as saturable absorbers to modulate and control CO2 laser radiation. ReO4- ion concentrations in excess of 1017 cm-3 were achieved in KCl with good optical quality. The room temperature absorption of the ReO4- ion in KCl was centered at 936.8 cm-1 with 1.5 cm-1 linewidth and with absorption cross-section determined to be (0.46 + 0.02) × 10-16 cm2. The addition of Li+ as a second dopant resulted in a splitting of the ReO4- resonance into two components at 957.5 cm-1 and 900.7 cm-1.

The characteristically sharp resonance of ReO4- was not detected in NaCl grown with NaReO4 additions to the melt. Only with addition of Ca++ as a co-dopant was the ReO4- resonance observed. The absorption, detected as a very weak resonance through a 92 mm path length, was centered at 946.0 cm-1with linewidth of 6 cm-1, over-lapping the P(20) transition in the 10 micron CO2 band.
Laser Damage in Silicon Avalanche Photodiode
Authors: Acharekar MA
Published: February 2009

Silicon Avalanche photodiodes (APDs) are widely used for detection of visible and near infrared laser radiation. The laser damage of the APD investigated using a Q-switched Nd:YAG laser pulse of 15 ns duration is reported in this paper. Two types of laser damage threshold measurements were performed on the APDs. The minimum flux level of 1064 nm laser radiation resulting in degraded signal-to-noise ration (SNR) is considered the primary type of detector damage. This type of detector damage can be defined in terms of a rearrangement of electrical charge in the semiconductor resulting in increased noise current and therefore poor SNR. The second type of laser damage threshold measurements were performed where the laser power on the detector was increased to the level producing not only a decrease in SNR, but also resulting in visible deformation on the surface of the detector. This type of detector damage can be defined in terms of a rearrangement of some of the atoms of the crystal due to the thermal effects of the laser pulses.
Radiation-Induced Absorption in Fused Silica
Authors: Miley GH , Chapman R , Nadler J , Williams W
Published: February 2009

The transient radiation-induced absorption in fused silica due to neutron and γ-ray irradiation has been investigated. Transient absorption coefficients at 325 nm have been measured during and after pulsed reactor (TRIGA) irradiation. Radiation-induced absorption results from the generation of color centers.

Most of the previous efforts have been concerned with the permanent effects due to low dose rate steady-state irradiations. This study focuses on the transient effects due to pulsed irradiations and their dependence on dose rate and total dose. The transient absorption coefficient increases very rapidly to a maximum value during the pulse, then decreases over a time period >1000 seconds to a permanent value after the pulse. In general, the absorption coefficient is found to increase with increasing dose rate and total dose.
Radiation Effects in Amorphous SiO2 for Windows and Mirror Substrates
Authors: Friebele EJ , Higby PL
Published: February 2009

Amorphous SiO2 is material with unique physical properties, including excellent optical transmission in the uv spectral range and a low coefficient of thermal expansion at cryogenic temperature. These properties make it an attractive candidate for excimer laser windows and as a surveillance mirror substrate. However, in both of these applications the silica may be exposed to substantial doses of ionizing radiation, either from the scattered electrons and x-rays from the laser pump or from the natural and/or enhanced on-orbit space radiation environment. This paper reports the effects of ionizing radiation on the uv transmission and density of SiO2. Current mirror substrates are fabricated from fused natural quartz. However, the greater purity of synthetic silica leads to increased uv transmission and greater radiation resistance. Therefore, measurements have been made on a large suite of silica samples, including fused quartz, synthetic silica fabricated by flame hydrolysis, plasma fusion, or new techniques. In some cases the observed radiation sensitivity has been related to fabrication technique and impurity contents.
Recent Progress in Understanding Fundamental Mechanisms of Laser Induced Damage in Optical Polymers
Authors: Dyumaev KM , Manenkov AA , Maslyukov AP , Matyushin GA , Nechitailo VS , Prokhorov AM
Published: February 2009

Damage studies in polymers of acrylic and methacrylic type with low-molecular additives like water, alcohols, and similar compounds are presented. It was established that some additives significantly (by a factor of 10 to 100) increases multi-shot laser damage resistance of polymers. Vibration energy transfer (vibrational cross-relaxation) from polymer macromolecule to additive molecules which suppress the laser created absorbing radicals was shown to be attributed to such damage resistance increase. A mechano-chemical mechanism of laser damage in polymers is proposed which includes laser excitation of vibration states and creation of absorbing “hot radicals”. Basing on this mechanism we proposed methods of physical modification of polymers with high laser damage resistance comparable with that of silicate glasses and crystals.
Large Scale Damage Testing in a Production Environment
Authors: Weinzapfel CL , Greiner GJ , Walmer CD , Kimmons JF , Wallerstein EP , Marchi FT , Campbell JH , Hayden JS , Komiya K , Kitayama T
Published: February 2009

We are using laser damage test systems on a production scale to scan large castings of laser glass for the presence of damage-causing platinum inclusions. These systems support glass melting production lines at two plants; one is in the U.S. (Schott Glass Technologies, Inc.) and the other is in Japan (Hoya Corporation).

The damage test systems are designed to scan an entire glass casting using the pulsed output from a commercial Nd:YAG laser. The system is fully automated and operates unattended. Following testing, the glass casting is removed from the system and visually inspected for the presence of Pt-damage sites. We routinely test polygonally-shaped castings that are about 0.5 meter in size, weigh approximately 30 kg and contain about 7 liters of glass. It takes roughly 6 to 8 hours to test a piece of this size. To date the systems have been in use about 12 hours-per-day, up to 5 days-a-week for a period of about 15 months. Of the approximately 300 disks that have passed the damage test so far, sixty-two percent of the disks have no platinum inclusions at all and ninety-two percent have an inclusion density of less than 0.2 per liter.
Developments in Beam Profiling with a CCD Area Array Detector
Authors: O'Connell RM , Ferreira RJ , Stewart AF
Published: February 2009

Measurements and calculations of the performance of a frame-transfer CCD area array sensor in a beam profiling system are presented. The measured dark current and photosensitivity pattern noise of the sensor are shown to be better than those of previously studied self-scanned imaging devices. Calculated modulation transfer functions agree with previous measurements, verifying that the CCD sensor should be able to adequately resolve spatial frequencies up to 50 mm-1. A logarithmic video amplifier was successfully interfaced with the CCD camera and an 8-bit image processor for the acquisition of laser beam images with improved contrast.
Mapping of Surface Defects of Optical Components by a High Speed Surface Analysis System
Authors: Lalezari R , Knollenberg R
Published: February 2009

A scanning darkfield laser scatterometer is used to produce maps of surface defects on optical component surfaces. This instrument was originally developed for the examination of silicon wafers. The short depth of filed, high resolution darkfield imaging design of the scattering measurement head, enables the instrument to examine the cleanliness and surface condition of transparent as well as opaque optical surfaces, including coated surfaces.

The measurement system consists of a laser illuminated darkfield optical head mounted on a linear translation stage, a spinning chuck, and a data acquisition and display system. The instrument is capable of resolving the ac scattered light component to two parts in 109 for scattering sites smaller than the pixel size (50 microns). The dc sensitivity of the instrument is roughly one part in 106 (1 ppm).

The instrument has been successfully used in inspecting mirror substrates before and after coating and in developing cleaning techniques. Operation of the system and some data will be presented.
The Growth and Stability of Ag Layers on Cu(110) Monitored by Second-Harmonic Generation
Authors: Muenchausen RE , Hoffbauer MA , Taylor TN
Published: February 2009

Understanding the fundamental limits for laser/optical mirror fabrication is directly relevant to the development of mirrors with improved durability and damage thresholds. Ag layers deposited in ultra-high vacuum on a well characterized, single-crystal Cu(llO) substrate as a function of substrate temperature have been analyzed using Auger electron spectroscopy (AES) and low-energy electron diffraction (LEED) in order to answer questions concerning substrate overlayer formation and film degradation at the early stages of growth (̃ 10 monolayers).

Another aspect of this work has centered on developing diagnostics transferable from UHV to “real-world” environments. Optical second-harmonic generation (SHG) was chosen due to its surface specificity and experimental simplicity. The observed SH intensity as a function of Ag deposition even at submonolayer coverages. A comparison between the observed SH intensity and polarization dependence with the AES and LEED analysis indicates that optics SHG provides a fingerprint of the nucleation of Ag clusters.
Super-Polished Silicon Carbide Mirror for XUV Radiation
Authors: Yoshida K , Kato Y , Nakai S , Kurosawa K , Sasaki W
Published: February 2009

We have coated a large silicon carbide (SiC) thin film on the graphite substrate by thermal CVD method. The surface of the SiC is first lapped and then polished with a precision ring-polisher. The roughness of super-polished SiC surface was about 5 Arms. This super-polished SiC surface is expected to be useful as a grazing incidence mirror for XUV radiation.

We have applied this SiC as a high reflectance mirror for the argon excimer laser (λ=126 nm), and obtained peak output power of more than 14 MW (MgF2 output mirror and 3 mm in beam diameter) without any damage on the SiC mirror surface. The output power and power density on the cavity mirror are the highest in comparison to the previous results obtained with other mirrors.
UV Light Cleaning of Silica Surfaces for Improved Laser Damage of AR Coatings
Authors: Yoshida K , Yoshida H , Kato Y , Nakai S , Ohtani M
Published: February 2009

The glass substrates for the optical coatings have many contaminations on the surface which can not be removed by standard optical cleaning techniques. Especially, it is very difficult to remove the polishing compound which is buried into the silica substrates.

In this report, the improvement of laser damage threshold due to UV light cleaning is presented. The excimer laser, Xe flashlamps, and Xe arc-lamp have been used as UV light sources. The damage threshold of the AR coating on the silica surface which was cleaned with KrF laser light has increased by 50% at 355 nm.
A Chemical Precursor to Optical Damage? Studies by Laser Ionization Mass Spectrometry
Authors: Nogar NS , Estler RC
Published: February 2009

Mass spectrometry has been used in conjunction with Nomarski microscopy to characterize the initiation of optical damage in selected commercial optics. For a sample with a Al2O3/SiO2 multilayer coating (351 nm) on a Si substrate, our results suggest layer by layer removal of the coating material with low-fluence irradiation at 1.06 μ. Inaddition, carbon impurities were observed in the low-damage threshold sample. For the Sc2O3/SiO2 multilayer coated (351 nm) 7940 substrates, transient iron signals were observed at each increasing fluence level, with concomitant appearance of small circular (10μ) pits in the surface. These pits were also associated with macroscopic damage features due to threshold testing.
Multiphoton Absorption Near Surface Damage Thresholds of Ionic Crystals and Metals
Authors: Matthias E , Petzoldt S , Elg AP , West PJ , Reif J
Published: February 2009

By means of acoustic probe beam deflection we have measured the energy of acoustic pulses released during the interaction of intense laser pulses with solid surfaces. This technique is well suited to determine surface damage thresholds. We find that for a sufficiently high defect density at the surface the primary energy absorption is by multiphoton ionization, even within a certain energy interval above the damage threshold. The order of the multiphoton process corresponds to the bulk band gap of ionic materials or, for metals, to the work function. “Inert” surfaces with a low density of surface defects, like MgF2 and CaF2, on the other hand, show evidence for dielectric breakdown.
Laser Fluorescence Spectroscopy of Zinc Neutrals Originating from Laser-Irradiated and Ion-Bombarded Zinc Sulfide and Zinc Surfaces
Authors: Arlinghaus HF , Calaway WF , Young CE , Pellin MJ , Gruen DM , Chase LL
Published: February 2009

Time-of-flight (TOF) measurements, employing high-resolution laser-induced fluorescence spectroscopy (LFS) as a probe, have been used to measure the yield and velocity distribution of Zn atoms ejected from a ZnS single crystal under irradiation by 308 nm photons. By comparison with the known io: sputtering yield for pure zinc, the absolute yield was determined to be 109 atoms/pulse at a laser fluence of 30 mJ/cm2. The velocity distribution of the Zn atoms could be fitted by a Maxwell-Boltzmann distribution, having a characteristic temperature of 2360 K. In addition, Doppler-shift techniques have been combined with TOF measurements in order to separate prompt from delayed emission of ablated atoms, as well as to probe possible molecular or cluster fragmentation. The results obtained suggest the possibility of molecular or cluster emission from ZnS.
Wavelength Dependence of Edge Filter Absorption
Authors: Basegio LJ , von Gunten MK
Published: February 2009

Laser isotope separation and free electron laser programs have driven the need for optical coatings which function in environments of high average power density. Low absorption is the key requirement for these coatings to prevent catastrophic laser damage and to minimize wavefront deformation caused by localized heating of the coating and substrate. Dichroic edge filters have been utilized in wavelength separation applications; however, the intrinsic absorption of these designs at the edge limits their power handling capability for closely spaced wavelengths. In order to fully understand the absorption behavior and to optimize the performance of these designs, measurement of absorption as a function of wavelength has been undertaken, with particular emphasis on the transition region.

The absorption measurements have been performed with a calorimeter which utilizes a matched thermistor bridge and an ultra-precision instrumentation amplifier in conjunction with both discrete and tunable laser sources. Several types of edge filter coatings have been measured. These measurements have been performed over the wavelength region from the stopband through the edge region into the passband. This empirical data is compared to theoretical predictions.
A Comparison of Various Rugate Filter Designs
Authors: Carniglia CK
Published: February 2009

A rugate filter is a multilayer reflector with a continuously varying refractive index profile. The performance desired from such a filter is the reflection of light at a single band centered at some wavelength λ0 without the associated reflectance bands at the harmonics λ0/3, λ0/5,.... Three index profiles are compared: sinusoidal variation of refractive index, sinusoidal variation of dielectric constant and sinusoidal variation of the natural log of the refractive index. The latter profile gives complete suppression of the reflectance bands at the harmonics.
HR Coatings Prepared from Colloidal Suspensions
Authors: Thomas IM , Wilder JG , Gonzales RP
Published: February 2009

We have now established that sol-gel HR coatings prepared from colloidal oxide suspensions are superior to those prepared from solutions of precursor materials. Our investigation of single coatings of Zr02, Hf02 and A1203. H20 prepared from suspensions has indicated that those from Al203. H20 have the highest laser damage threshold at 1064 nm. We have prepared HR coatings by stacking Al203. H20 and Si02, also prepared from suspension, in the conventional manner achieving reflectivity of 99% with 35 layers. The damage threshold of these materials has however only been about half that of single coats of the component oxides.
1064 nm and 350nm Radiation Stability of Low Density ThO2-SiO2 High-Reflective Coatings Deposited from Sols
Authors: Floch H , Priotton JJ , Mengue JF , Cordillot C
Published: February 2009

We maintain our effort together with the Lawrence Livermore National Laboratory in the research for high damage threshold optical coatings prepared from colloidal oxide suspensions, we have recently observed that single layers of ThO2 exhibits up to 12–13 J/cm2 under single pulse laser irradiation at 1064nm wavelength and Ins pulse duration. Associating this thoria with the silica reported earlier/1,2/, multilayer HR-coatings have been also elaborated by laying down quaterwave thick alternating coats, at room temperature. In this manner, we have achieved reflectivity adjacent to 97–98% with 21 layers. The laser-induced breakdown of these mirrors has unfortunately only been about 8–9 J/cm2, lower than the one of the high index thoria component.
Oxide Optical Coatings Prepared by Metal-Organic Vapor Deposition
Authors: Brusasco R
Published: February 2009

We prepared films of alumina, zirconia and hafnia by Chemical Vapor Deposition (CVD) for evaluation as high refractive index optical components for high power laser applications. Films with high index of refraction (1.89-1.63) but with rather low single shot laser damage thresholds (4–10 J/cm-2) were prepared by high temperature (400%C) thermal decomposition. Lower temperature deposition with chemical assist improves some optical properties and laser damage thresholds at the expense of a lower index of refraction.
Deposition with Ultra-Fine Particles
Authors: Day AC
Published: February 2009

A study has been initiated into the possible uses of deposition with ultra-fine particles and/or clusters. The starting point was the Ionized Cluster Beam technique reported by Takagi et. al. Experimental studies in this laboratory strongly suggest that clusters cannot, in general, be formed in an adiabatic expansion of a pure metal vapor as had been claimed, although other mechanisms cannot be ruled out. Numerical simulations have also been carried out which support our conclusion.

However, it is well known that small particles or clusters can be condensed in a low pressure quenching gas. Work is now being directed toward exploiting these particles to produce materials composed of nanometer-sized “splats” somewhat analogous to those in plasma spray coatings. It is hoped this radical departure from the usual columnar structures of deposited materials can lead to improved damage thresholds in many types of optical coatings. The feasibility of this new approach will be discussed along with its potential merits.
Cluster Beam Deposition for Optical Thin Films
Authors: Waddell EM , Monachan BC , Lewis KL , Wyatt-Davies T , Pitt AM
Published: February 2009

A study has been carried out of the deposition of films of barium fluoride using cluster beam techniques. The effect of different aspects of the deposition process has been assessed including deposition temperature, cluster source geometry and acceleration voltage. X-ray measurements suggest that a transition temperature exists between 200 and 300°C above which dense oriented films of excellent crystallinity are produced. At lower temperatures the films are of a different orientation, are less than theoretical density and are optically inhomogeneous. However an enhancement of laser damage threshold at 1.06μm has been observed for films produced at 200°C. The transition temperature itself can be influenced by cluster source geometry and acceleration voltage used during deposition. The films have been used for anti-reflection of ZnSe substrates and, under optimum conditions, have laser damage thresholds at 10.6μm significantly in excess of the uncoated substrate material.
Measurement of HR Coatings Absorptance at 10.6 Microns by Mirage Effect
Authors: Baron PJ , Culoma A , Boccara AC , Fournier D
Published: February 2009

To get a high optical quality beam from a High Power Laser (HPL), optical distorsions must be minimized. For a CW CO2 HPL, the key parameter is absorptance. Generally, High Reflective (HR) coated mirrors exhibit absorptance less than one percent. A photothermal measurement method of absorptance (Mirage Effect) seems to be adequate to reach the required resolution. In this paper we present a method of absolute measurement of HR coatings absorptance at 10.6 microns. The measurement key point is calibration. The linearity of the deflection angle versus absorptance predicted by the theory led us to calibrate our experimental set up with a strongly absorbing sample which could be measured by reflection. (scattering neglected). The first step is to verify the signal linearity versus absorbed power and its evolution as a function of chopping frequency in order to neglect convective transfers. Measurements performed on different HR coated mirrors are presented.
Raman Studies of Inherent and Applied Stress in Thin Optical Films
Authors: Exarhos GJ , Frydrych WS , Ferris KF , Hess NJ
Published: February 2009

Vibrational mode frequency shifts have been shown to correlate with strain in optical coatings the extent of which is dependent upon film deposition conditions. The presence of strain and strain inhomogeneity in thin films as well as crystallite phase and phase homogeneity affects the optical properties, environmental durability, and laser damage resistance. The influence of these variables on the response and stability of optical thin films can be effectively probed using Raman spectroscopy. Molecular vibrational spectra for several sol-gel and sputter-deposited TiO2 films have been recorded at variable pressures approaching 100 kbar using a diamond anvil cell. Results demonstrate the presence of reversible pressure-induced phase transformations and suggest that introduction of voids in the film microstructure can influence the pressure at which such transformations are observed.
Operational Limits for ZnSe/ThF4 Multilayer Mirrors in the Los Alamos Free-Electron Laser Oscillator
Authors: Newnam BE , Bender SC
Published: February 2009

The Los Alamos free-electron oscillator is driven by an rf linear accelerator at an energy of ̃20 MeV. This L-band linac delivers electrons in a 100-μs-long train of micropulses (̃10-ps width) each separated by 46 ns. A semi-confocal resonator (g1g2 = 0.87 to 0.95) is used to maximize the optical intensity within the magnetic undulator by which extraction of energy from the electrons and optical gain are maximized.

For operation in the spectral range of 8 to 12 μm. multilayer dielectric reflectors of ZnSe/ThF4 on ZnSe substrates have been used for the end mirrors. At moderate laser power, these reflectors have performed without evidence of deterioration in the multiple environment of laser photons, gamma rays and neutrons [1]. However, when the time-average optical intensity on the mirrors exceeded the range of 500 to 700 kW/cm2, the resonator losses increased and the output power dropped [2]. Post inspection revealed coating damage in the form of a collection of many elongated pits plus one or two larger (1 × 1 mm2) pits penetrating to the ZnSe substrate. The damage mechanism most consistent with the observed morphology is heating of microscopic absorbing defects leading to film rupture by excessive stress.
Laser Induced Damage Measurements of Free Electron Laser Optical Components
Authors: Jolin LJ , Sanders VE , Salazar SJ
Published: February 2009

A study of free electron laser (FEL) related optical damage is currently under way at Los Alamos National Laboratory. A single burst of the FEL has been simulated using a Nd:YAG laser system operating at 1064 nm. Thfe laser is a modelocked, 100 microsecond, pulse selected system operating at 10 pulses per second.

Damage thresholds for conventionally deposited dielectric high reflectors as well as bare metals have been studied and the results are reported. Some high reflectors have demonstrated laser induced damage thresholds of greater than 5 megawatts/cm2 average pulse power.

Also, reported are damage data for a non-modelocked laser configuration.
Database of Average-Power Damage Thresholds at 1064 nm
Authors: Rainer F , Hildum EA , Milam D
Published: February 2009

We have completed a database of average-power, laser-induced, damage thresholds at 1064 nm on a variety of materials. Measurements were made with a newly constructed laser at the Lawrence Livermore National Laboratory (LLNL) to provide design input for moderate and high average-power laser projects. The measurements were conducted with 16-ns pulses at pulse-repetition frequencies ranging from 6 to 120 Hz. Samples were typically irradiated for times ranging from a fraction of a second up to 5 minutes (36,000 shots). We tested seven categories of samples which included antireflective coatings, high reflectors, polarizers, single and multiple layers of the same material, bare and overcoated metal surfaces, bare polished surfaces, and bulk materials. The measured damage thresholds ranged from < 1 J/cm2 for some metals to > 46 J/cm2 for a bare polished glass substrate.
Causes of Damage in Multilayer Dielectric Coatings Exposed to High Average Power Visible Laser Radiation
Authors: Aikens DM , Taylor JR
Published: February 2009

Causes of damage in multilayer dielectric coatings exposed to high average power visible lasers have been examined. Damage has been seen to be related to coating defects, surface contamination, and coating absorption. Defects and contamination, particularly those larger than 4um, have been observed to significantly lower the damage threshold for high reflective coatings. Attempts have been made to visually evaluate defects and surface contamination prior to damage in order to find correlation with the damage threshold. The methods used to evaluate coating defects and conduct high average power damage testing as well as test results are presented. This work demonstrates the value of controlling sources of defects in manufacture and contamination for use in high average power visible laser systems.
Pulse Length Scaling Results at 248 nm
Authors: Boyer J
Published: February 2009

Damage thresholds were measured on a variety of optics at 248 run for both 11 and 36 ns pulse lengths. Test at both pulse lengths were run on each optic. If a power law scaling is assumed, the mean exponent is 0.3 in agreement with results for other wavelengths. The exponents for individual optics range from 0.20 to 0.47. The experimental precision is such that variation in exponents is significant.
Measurements of Ultra-wide Pulse Damage Thresholds of Anti-reflection Coated IR Materials at 10.6 μn
Authors: Eng RS , Grimm JG , Greene J , Daley JA , Harris NW
Published: February 2009

A method has been developed to generate ultra-wide (10–100 μs regime) CO2 laser pulses using a laser master oscillator-power amplifier configuration. These laser pulses have been used to conduct laser induced damage threshold measurements of anti-reflection coated cadmium telluride samples. The question of damage threshold dependence on the laser pulse width is discussed and compared with the results of other workers. The paper also addresses the effects of different types and physical properties of the AR-coating materials, and the effects of laser beam polarization on the damage thresholds of the materials.
Intrinsic Optical Damage in Potassium Bromide at 532 nm
Authors: Shen XA , Braunlich P , Jones SC , Kelly P
Published: February 2009

The nonlinear interaction of intense 100 ps laser pulses at 532 nm with KBr single crystals is monitored up to damage by measuring the resulting a component of the self-trapped exciton recombination luminescence as a function of incident photon flux. Information on the temperature rise in the interaction volume is obtained from the σluminescence vs flux relation with the aid of the known temperature dependence of the luminous efficiency. We show that the mechanism of energy deposition is four-photon free carrier generation and ensuing free carrier heating with small contributions by self-trapped holes. Single-pulse damage occurs without electron avalanche formation at a temperature very close to the melting point.
Observation of Two Photon Absorption Prior to Laser-Induced Damage in ZrO2
Authors: Mansour N , Mansour K , Soileau MJ , Van Stryland EW
Published: February 2009

We observed substantial dispersion in the breakdown electric field for yttria stabilized cubic zirconia (Zr02). The measured breakdown field at 1064 run is a factor of 5 larger than that at 532 nm for 18 nsec (FWHM) pulses. For 532 run pulses, we measured a significant decrease in transmission prior to the observation of catastrophic damage (i.e., damage accompanied by a spark and fracture of the sample). The observed nonlinear transmission is attributed to absorption by color centers generated by two-photon absorption (2PA). Presence of the color centers were directly monitored by transmission measurements. A permanent change in transmission was measured by irradiating the samples at high intensities (300 MW/cm2). These induced defects are long lived (on the order of hours) but can be bleached by repeated irradiation with relatively low intensities (20 MW/cm2). In a separate experiment, using picosecond pulses at 532 nm, the 2PA coefficient was determined to be 0.015 cm/GW.
Behavior Studies on Dirt Spikes in Laser Discharge and Their Effects on Thyratron Operations
Authors: Lin CE , Yang CY , Huang CL
Published: February 2009

In high repetition rate and high di/dt laser discharges, small concentrations of impurities in the laser tube may cause significant degradation to the laser output. More seriously, the presence of impurities in the discharge may cause dirt spikes on the thyratron, and will result damages to its anode. In this paper, the influences of impurities in laser discharge are investigated from the electrical viewpoints. The generation of dirt spikes is explained by the concept of Townsend discharge and electron attachment. Water vapor is found to be the dominant impurity to cause the dirt spike generation. The dirt spike effects on thyratron operation are studied. A new rating factor, termed as the Db factor, is deduced from the πb factor. This factor is proved with accuracy by comparisons with the experimental results. Measurement methods to reduce the damages of thyratron are discussed. Under well-characterized operation, the lifetime of thyratron can certainly be extended by the results of this study.
Thermal Shock: A Contributing Factor to Laser Damage in Optical Thin Films Used for High Power Continuous Wave Laser Optics
Authors: Palmer JR
Published: February 2009

The direction of this paper is to provide a theoretical model and supporting empirical data for evaluation of laser damage to optical thin films subjected to high power Continuous Wave laser irradiation. Basically, there has been a seeming anomaly in the maximum temperature that optical thin films can withstand before they fail to perform their function. On the one hand, a paper was presented to the Laser Boulder Damage Symposium that clearly provided empirical data that a number of oxide coating materials could reach 59°C to 680°C before failure set ia On the other hand, many investigators have found that coatings have failed at temperatures of 200°C to 300°C.

In this paper we set out to demonstrate that both conditions could be correct, rather than just one or the other. The equations for the absorption and subsequent temperature rise in the multilayer stacks are provided. The temperature rise of the substrate is evaluated and the equations for the actual thermal shock resulting from the conditions in the optical thin films, the substrate, and sensitivity of both to the materials that comprise the optical thin films and the substrates combination. Several coating chemistry combinations and three diverse substrate materials are evaluated and plotted based on measurements made and reported some years ago.
Reverse Thermal Wave Approximation for Temperature Transients in Optical Thin Films and Substrates — Reflective Optics for High Power Repetitive Pulsed Lasers
Authors: Palmer JR
Published: February 2009

This is the companion paper to the Reverse Thermal Wave Approximations for High Power Continuous Wave Lasers. The purpose of this paper is to provide equations that will allow the optical designer to evaluate the temperature gradient in optical thin films and substrates under transient conditions. The semi-infinite plate models have been described rather extensively in other publications. However, the problem that is treated in this paper is that set of boundary conditions that do not satisfy the semi-infinite plate condition.

It is not uncommon to have coated reflective optics that have very thin substrates. For those conditions it is necessary to evaluate the temperature transient much differently than one would if the substrates were of sufficient thickness to satisfy the semi-infinite plate boundary. One finds that the temperature rise and gradients are substantially different for very thin substrate conditions. The Reverse Thermal Wave Modelwill provide the ability to evaluate the temperature gradients during the transient with closed form approximations.

The repetitive pulsed case has a substantially different characteristic temperature profile from that of the continuous wave and in this paper the equations and figures are provided to reflect the unique character of the temperature gradients in the optical thin films and substrates as a function of the hertz rate and pulse width. It becomes clear that the duel cycle of heating and cooling coupled with the thermal shock play important roles in the damage thresholds of many optical components in high power laser systems.
Some Perspectives on Future Directions in Low Cycle Fatigue
Authors: Coffin LF
Published: February 2009

This paper on low cycle fatigue (LCF) is divided into three parts. The first provides a brief historical perspective, the second deals with today's scene, and the third speculates on future LCF directions.
Future Directions for Low Cycle Fatigue
Authors: Manson SS
Published: February 2009

The thesis is maintained that the next 30 years will witness a proliferation of publications in the area of fatigue, following as a continuum of today's high productivity rate. The driving forces for such high production are discussed, and a “wish list” is provided suggesting priority areas requiring development. Some current projects at Case Western Reserve University are outlined.
Effect of Elevated Temperatures on the Low Cycle Fatigue of 2.25Cr-1Mo Steel—Part I: Constant Amplitude Straining
Authors: Polák J , Helešic J , Klesnil M
Published: February 2009

A 2.25Cr-1Mo steel was subjected to constant amplitude straining in the temperature interval 22 to 550°C. The conventional low-cycle fatigue properties at 22, 350, 450, and 550°C at constant diametral strain rate ˙ϵd = 5 × 10−3 s−1 were measured. The fatigue hardening-softening curves, cyclic stress-strain curves, and Manson-Coffin curves were obtained at each temperature. A detailed study of the cyclic stress-strain response was performed on a computer-controlled electrohydraulic machine. The temperature dependence of the cyclic yield stress was measured using a multiple step test procedure. The stress-dip procedure in cyclic straining was adopted in order to separate the effective and internal component of the cyclic stress. The dependence of both components on temperature and during fatigue life was measured.

The effective stress component was found to be relatively high in this material and drops only mildly with temperature. The typical temperature dependence of the cyclic stress is due to the internal stress that drops quickly above 450°C and at 550°C is considerably lower than the effective stress. With continued cycling, the decrease of both components contributes to cyclic softening.

Both the cyclic stress-strain response and fatigue life were affected by the strain rate and hold periods within the cycle. The time-dependent effects were studied in detail at 550°C. The frequency dependence of fatigue life at different strain amplitudes was measured. In addition to continuous straining, three basic types of hold times within the cycle with strain or stress relaxations were applied. The experimental data were analyzed, and a set of parameters to correlate the low-cycle fatigue life of the material with various straining conditions was evaluated.
Low Cycle Fatigue of Some Aluminum Alloys
Authors: Chung YS , Abel A
Published: February 2009

Axial strain-controlled tests were performed on five aluminum alloys to provide information on their cyclic behavior. The alloys chosen for this investigation represent a complete survey of the strengthening mechanisms associated with commercial aluminum products. The strengthening microstructures ranged from subgrain and dislocation strengthening in the 1200 alloy, solid solution strengthening by magnesium in the 5083, and the high density of GP zones and finely dispersed precipitates in the case of the 7005, 6351, and 6061 alloys. The five alloys were plant-fabricated with commercial tempers.

One important aspect of the tests was the detailed measurement of the hysteresis loop shape change during cycling and its possible correlation with the fatigue performance. The fatigue results show the effect of microstructure and confirm the presence of a break in the Coffin-Manson plot.
Cyclic Stress-Strain-Time Response of a 9Cr-1Mo-V-Nb Pressure Vessel Steel at High Temperature
Authors: Swindeman RW
Published: February 2009

Data are presented that describe the behavior of a 9Cr-1Mo-V-Nb steel under conditions of constant and cyclic loading at temperatures from 25 to 600°C. The general features of hardening under monotonic and cyclic strains are presented. Creep data for temperatures in the range 500 to 650°C and times to 10 000 h are correlated by a creep law that is used in conjunction with hardening data to construct isochronous stress versus strain curves applicable to monotonic and cyclic loading. Simple rules for hardening and recovery are postulated and used to estimate the time dependent behavior after mechanically and thermally induced reversal strains. Data from relaxation, partially restrained creep, and two-bar creep ratchetting experiments are presented, and the estimated response is compared with the test data. It is shown that simple concepts of strain hardening and reversed strain softening are adequate for estimating the material behavior under most of the loading conditions that were examined.
A Nonlinear History-Dependent Damage Model for Low Cycle Fatigue
Authors: Leis BN
Published: February 2009

A nonlinear damage postulate that embodies the dependence of the damage rate on cycle-dependent changes in the bulk microstructure and the surface topography is examined. The postulate is analytically formulated in terms of the deformation history dependence of the bulk behavior. This formulation is used in conjunction with baseline data in accordance with the damage postulate to predict the low cycle fatigue resistance of OFE copper. Close comparison of the predictions with experimentally observed behavior suggests that the postulate offers a viable basis for nonlinear damage analysis.
Very Low Cycle Fatigue Life Influenced by Tensile or Compressive Prestrain
Authors: Iida K
Published: February 2009

The effects of excessive prestrain in tension or in compression upon very low-cycle fatigue life were investigated for two mild steels and two high strength steels as an aid in the analysis of a ship failure in service, in which the bow structure was broken off due to local buckling and by low-cycle fatigue in extremely heavy sea conditions. The test series in the present investigation was: (1) uni-directional tension test after prestraining, (2) completely reversed strain cycling tests of non-prestrained material with several strain ranges, (3) tensile or compressive straining directly followed by completely reversed strain cycling, and (4) completely reversed strain cycling tests of specimens which were machined out of previously prestrained large diameter specimens.

The residual static fracture ductility decreases as a function of the amount of prestrain, showing much more loss of ductility in the case of compressive prestrain. Fairly good agreements on strain range and fatigue life were found between experimental data and estimated values calculated after Manson's and Iida's formulae. The failure life of a specimen subjected to prestrain showed a remarkable decrease from the failure life of the original material depending on three parameters: amount of prestrain, direction of prestrain, and specimen surface conditions. The worst case in the range of the present tests is that of tensile prestraining directly followed by strain cycling. In this case the low-cycle fatigue failure life of a prestrained specimen is, as an example, reduced by 93% of the failure life of the original material, when the amount of the tensile prestrain is 60% of the original static fracture ductility based on area reduction to failure.
Effects of Compression and Compressive Overloads on the Fatigue Behavior of a 2024-T351 Aluminum Alloy and a SAE 1045 Steel
Authors: DuQuesnay DL , Pompetzki MA , Topper TH , Yu MT
Published: February 2009

Smooth cylindrical specimens of 2024-T351 aluminum alloy and SAE 1045 steel were tested under constant amplitude cycling to study the effect of compressive stress on fatigue life. The results show a reduction in the fatigue life at constant maximum stress as the compressive portion of the stress cycle is increased.

Tests performed to study the effect of a periodic compressive overload on the order of the yield strength show an increasing reduction in fatigue life at constant maximum stress as the frequency of application of the overloads is increased. The compressive overload cycle significantly increases the damage done by subsequent smaller cycles, including those far below the constant amplitude fatigue limit. The results indicate that the present techniques used for damage summation for variable amplitude service histories may give grossly unconservative fatigue life predictions, especially for histories containing large compressive load cycles accompanied by a relatively large number of small cycles. It is suggested that conservative life predictions for such histories can be made using the stress-life curve from constant amplitude tests with compression on the order of the yield stress accompanying every cycle.