bluuebunny/crossref_metadata_2025_split huggingface dataset:
Infra-Red Absorption Spectroscopy of Nd:YAG and Nd:GSGG Surface Contaminants
Authors: Acharekar MA 
Published: February 2009

Neodymium doped yttrium aluminum garnet (Nd:YAG) is a commonly used laser material for solid-state high power, high PRF lasers. Gadolinium scandium gallium garnett doubled doped with neodymium and chromium (Nd:Cr:GSGG) shows higher efficiency than the Nd:YAG laser. We previously reported the bulk and surface damage thresholds of the Nd:Cr:GSGG crystal. It was shown that the bulk damage threshold of the Nd:Cr:GSGG was as high as the Nd:YAG crystal.
The surface damage threshold measurements for the Nd:YAG and the Nd:Cr:GSGG are reported in this paper. Infrared absorption spectroscopy was used to analyze surface contaminants. The data show that the organic contaminants, CHn, derived from hydrocarbon (skin oil) associated with improper parts handling, signifantly reduce the surface damage threshold.
Excimer Radiation Colouration Processes and Surface Damage to Single Crystal CaF2 Substrates
Authors: Laidler I , Emmony DC 
Published: February 2009

This paper considers the effects of a 249nm excimer pulse on CaF2 substrates and is concerned with both surface damage and bulk colouration processes.
The effect of different polishing procedures on the surface damage threshold of CaF2 has been investigated.
CaF2 has been shown to develop colour centres under prolonged U.V. irradiation. A technique has been developed to monitor the transient excited states created in the bulk material when irradiated with a 25ns excimer pulse. This involves a simultaneously pumped dye laser pulse probe operating at the absorption wavelengths of the excited species. From the results information about the photon absorption cross sections can be derived, the concentration of defect states at any time during the excimer pulse may be estimated and a knowledge of the maximum safe fluence levels established.
Glancing Incidence Absorption Measurements of Metal Mirrors
Authors: Kimura WD , Ford DH 
Published: February 2009

Glancing incidence absorption measurements are performed on bare Cu, Mo, Au, Al, and Ag mirrors, and on Al and Ag mirrors with a protective dielectric overcoat. Data is obtained for angles of incidence from 0° to ⩾89°. Wavelengths studied are 0.5 μm and 10.6 μm at both s and p polarizations. The absorption is measured using a photoacoustic calorimetry technique in which the transducer is attached directly to the back of the test mirror. Absolute calibration of the absorption data is obtained by using a ratiometer to independently measure the absorption at near normal incidence.The results are compared with the absorption predicted by Fresnel's equations for bare metals using values of n and k of the complex index of refraction measured by other sources. The absorption of a mirror at glancing incidence appears to be a sensitive function of its surface condition. New mirrors tend to agree with Fresnel's equations; while “old” mirrors have much higher absorption probably due to a layer of tarnish. The overcoated Al data deviates greatly from Fresnel's equations indicating that bare metal theory cannot be used and a thin film theory should be used instead. Representative results are given.
Thermal Conductivity of Enhanced High Reflectors
Authors: McGuirk M , Poirier RD 
Published: February 2009

Perkin-Elmer has designed and fabricated a simple thermal conductivity tester for thin film coatings. The work is motivated by the results Don Decker, et al., reported at this conference last year. 1 Our measurements confirm his finding that the thermal conductivity of thin films is substantially less than the thermal conductivity of the corresponding bulk material. In one case the measured thermal conductivity of an enhanced high reflectance coating is a factor of 30 less than the conductivity predicted from bulk values.
Perkin-Elmer has considered the theoretical reasons for the low conductivity, and methods for increasing the conductivity. Increased thermal conductivity should increase the laser damage threshold for CW applications.
Time-Resolved Temperature Determinations From Raman Scattering of TiO2 Coatings During Pulsed Laser Irradiation
Authors: Exarhos GJ 
Published: February 2009

Simultaneous measurement of Stokes/anti-Stokes Raman intensitities for a particular vibrational mode allows instantaneous surface temperatures to be determined. Time-resolved Raman spectra were measured on a specially constructed spectrometer using pulsed laser excitation and gated detection techniques. Measurements on equilibrated coatings and uncoated substrates at several temperatures served to confirm the utility of the technique for temperature determination as well as calibrate the intensity response of the instrument.
Pulsed irradiated SiO2 and Y(PO3)3 glass substrates yield no temperature increase at laser energies where surface damage is not apparent. At higher pulse energies, an increase in anti-Stokes scattered radiation is observed, signifying a temperature rise. Analogous results are seen in Raman spectra of low energy pulse-irradiated TiO2 coatings recorded at delays of 0 and 72 nsec from the excitation pulse. At higher energies, increased coating temperatures are observed at these times as well as at short times preceding the peak maximum of the excitation pulse. Raman results are used to model the thermal response of pulsed laser-irradiated dielectric coatings.
Influence of Deposition and Post Deposition Conditions on Laser Induced Damage in Electron Beam Evaporated TiO2 and SiO2 Films
Authors: Rao KN , Murthy MA , Mohan S , Rao MR 
Published: February 2009

Composition and structure of oxide films vary with different deposition techniques, conditions and starting materials, resulting in different damage thresholds. In this investigation, single and multi layer films of TiO2 and SiO2 were deposited with TiO and SiO2 as starting materials. The substrate temperatures were maintained 25, 75, 125 and 175 ° C and post heat treatment was carried out at 75, 125, 175 and 225° C in air. These films were subjected to laser damage threshold studies, using Q-switched Nd:YAG Laser of 8 to 9ns. pulse width.
Maximum threshold values obtained were 19 J/cm2 for TiO2 films deposited at 25 °C and post heated at 75 °C; 44J/cm2 for SiO2 film deposited at 25 °C and 18J/cm2 for multilayer stacks of TiO2/SiO2 deposited at 25 °C. Variation in damage threshold values is explained on the basis of film composition, structure and optical properties.
Anomalous Absorption in Optical Coatings
Authors: Lange MR , McIver JK , Guenther AH 
Published: February 2009

The observation of isolated damage sites in optical thin films subjected to pulsed laser irradiation has lead to a model based upon localized absorbing regions. This “inclusion initiated” model does not adequately treat several details of the radiation absorption process, but relies principally on thermal conduction as the sole energy transport process. Despite these rather severe shortcomings, qualitative predictions of this theory agree remarkably well with much of the quality experimental data. On the other hand, quantitative agreement can only be achieved if absorption coefficients several orders of magnitude higher than those measured for todays thin films are employed, which themselves are already very high compared to the materials value in bulk form.
In order to explain this apparent discrepancy a detailed theory of the spatial and temporal evolution of the highly absorbing regions must be conceived. In this paper we present our initial steps in this direction. The absorption/thermal diffusion process that leads to damage is modeled by two equations; the evolution equation for the free carrier density and the thermal diffusion equation. In practice these equations are strongly coupled. We decouple the free carrier equation by assuming that all rate constants are independent of the lattice temperature. The solutions of these simplified equations are presented along with their sensitivity to the magnitude of several of the unknown rate constants. Those regions of parameter space in which the original thermal diffusion model is valid are pointed out, as well as the potential implications for more appropriate modeling of the absorption process as functions of experimental and material variables.
Modeling Laser Damage Caused by Platinum Inclusions in Laser Glass
Authors: Pitts JH 
Published: February 2009

A fracture-mechanic theory suggests that crack-propagation rates are proportional to the square root of the inclusion radius and to the three-quarters power of the fluence. The theory does not yet account for pressure loss or convection heat transfer as platinum vapor flows into the crack, and needs to be developed further before accurate propagation rates can be predicted. However, a simple estimate obtained by multiplying the time that the platinum is in the vapor state by the sonic velocity in glass gives a conservative but reasonable result. At fluence levels of 2 J/cm2, crack propagation of 4 μm per shot are predicted.
A crack-development hypothesis, based on shock-wave propagation, successfully predicts the shape of typical damage sites. The sites have an annular crack extending from the equator of the inclusion and planar crack lobes extending from the poles. The lobe at the pole exposed to laser light is larger than the lobe at the unexposed pole.
A numerical heat-transport model predicts temperature profiles in and around a metallic platinum inclusion imbedded in a phosphate laser glass matrix when illuminated with 1-μm laser light. The thermal model predicts that glass damage will occur if the platinum temperature exceeds the boiling point. Predicted fluence-damage limits of 1.4 J/cm2 for a 1-ns pulse and 4.3 J/cm2 for a 10-ns pulse agree with experimental data. The damage limit is independent of inclusion size above about 1 μm in diameter.
Substructural Developments During Strain Cycling of Wavy Slip Mode Metals
Authors: Plumtree A , Pawlus LD 
Published: February 2009

Substructural developments have been monitored during strain-controlled cycling of aluminum at room temperature. This has allowed a relationship between dislocation substructure and stress response to be developed as a function of the number of cycles.
At the onset of saturation, the dislocations were arranged in a loose cellular network for the particular strain range investigated. Continued cycling resulted in a slight but significant decrease in stress response, associated with the formation of a stable structure. With further strain cycling, the stress response and cell size remained relatively constant. The misorientation between the cells also remained constant, although the boundary width decreased due to dynamic recovery. These results indicate that the cyclic stress response is directly related to the dislocation density, and that at saturation the energy absorbed per cycle causes important changes in cell morphology to take place.
Comparison of these results with those of aluminum, copper and iron obtained under both monotonic and cyclic conditions show that the stress-cell size relationship is characteristic for all these wavy slip mode metals.
Crack Growth Studies in Biaxial Fatigue
Authors: Sehitoglu H , Socie DF , Worthem D 
Published: February 2009

The influence of strain state on the nucleation and early growth of fatigue cracks was investigated. Thin-walled tubular specimens of Inconel 718 were subjected to combinations of axial and torsional loading in the low-cycle fatigue regime.
The changes in crack growth rates as a function of crack size have been examined at effective strain amplitudes of 0.01 and 0.005. The crack growth rates under pure torsion, combined loading, and axial loading were found to be similar for the same crack size. Crack growth rates were interpreted based on an elastoplastic fracture mechanics characterization. A strain intensity range parameter was defined based on the shear and normal strain calculated on the crack plane. For mixed-mode loading conditions, similar crack growth rates were observed at a given range of strain intensity.
Extensive crack surface rubbing has been observed due to facets formed along the crack. Results indicate that relative changes in the ratio of Mode II/Mode I strain intensity components did not alter the crack growth rates notably at an effective strain amplitude of 0.005. However, at higher strain levels a relative decrease in Mode II/Mode I ratio promoted higher crack growth rates. If the magnitude of Mode I component of loading is high enough to result in crack opening beyond the size of the facets, then Mode II and Mode III components may be fully effective in propagating the crack.
Influence of Stress State on Crack Growth Retardation
Authors: Fleck NA 
Published: February 2009

Several mechanisms have been proposed in the literature to account for crack growth retardation following a single peak overload.
In order to determine the dominant mechanism, overload tests were performed on thick and thin specimens made from BS4360 50B structural steel. The baseline stress intensity range was 25 MPa √m and the load ratio (= minimum load/maximum load of fatigue cycle) was 0.05, while the overload was of stress intensity range 50 MPa √m.
It was observed that the crack growth and closure responses were different at the surface and in the bulk of the thick specimen; no such variations in behavior occurred along the crack front for the thin specimen. For both thicknesses of material, the crack growth rate predicted by measurements of the crack opening load was in agreement with the observed crack growth rate, except for the period when crack growth rates were recovering from the slowest transient growth rate to the post-overload stabilized value. This discrepancy was due to the phenomenon of discontinuous closure—the crack first closed at a location far from its tip. Fracture surface profiles showed that the crack path deviated by only a small amount after application of the overload; hence the retarded growth cannot be due to crack branching.
It is concluded from these tests and from a critical examination of the literature that, at high baseline ΔK levels, retardation is due to plasticity-induced crack closure. At low baseline ΔK levels approaching the threshold value, retardation may be due to plasticity-induced crack closure or to irregularities of the crack front.
Fatigue Damage Accumulation During Cycles of Nonproportional Straining
Authors: de los Rios ER , Brown MW , Miller KJ , Pei HX 
Published: February 2009

Nonproportional biaxial fatigue tests were performed using cruciform-shaped specimens of AISI 316 stainless steel at room temperature, in order to study the effect of nonproportional straining on crack growth rate. While the fluctuating stress applied perpendicular to the crack was common to all the tests, including the uniaxial test, the stress parallel to the crack was applied for only half the former period and only on either the raising or the downward part of the cycle.
The crack growth rate of the specimen subjected to uniaxial loading was more than five times faster than the growth rate of the cracks in the biaxial specimen, which is contrary to the reduced endurance observed for nonproportional loading of 1% chromium-molybdenumvanadium steel in tension-torsion.
Transmission electron microscopy observations of regions next to the fracture surface showed a very different dislocation substructure relating to uniaxial and biaxial loading. The degree of strain hardening in both conditions was estimated by employing dislocation models suitable for each case. If a “slipping off” mechanism for crack propagation is adopted, then the difference in crack growth rate experienced by the biaxial specimens can be explained by considering the magnitude of the strength of the slipband in each case.
A Study of the Mechanism of Striation Formation and Fatigue Crack Growth in Engineering Alloys
Authors: Ouyang J , Yan M 
Published: February 2009

A large number of fatigue cracks and fractures of test specimens and components of various engineering alloys were investigated by means of the fractographic technique. In particular, critical experiments were performed on the observation of natural profiles of fracture plateaus and mating fracture surfaces using the scanning electron microscope. It was found that the profile of a fatigue crack appears to be a tunnel with numerous steps. In consequence, the striations on the matching surfaces are basically in register other than any kinds of symmetrical configurations such as peaks to peaks and valleys to valleys. Furthermore, the appearance of them on one surface is usually more regular and distinct than that on the matching surface due to their greater slip length and, sometimes, slipband notches or cracks. The process of fatigue crack growth is just like alternatively walking up or down the steps of a path in different areas or grains.
A more general and practical model schematically illustrating the mechanism of fatigue crack growth and striation formation, which is one of the most fundamental questions on fatigue, is proposed. According to this model, the process of fatigue crack growth under a loading cycle includes four steps: (i) crack-tip opening; (ii) shear crack growth in a very short distance by single system slip decohesion; (iii) normal crack growth by alternative conjugate slip decohesion; and (iv) crack-tip closing, resharpening and, sometimes, inducing one or two slipband notches or cracks at the very tip. It differs markedly from the previous models which, in general, describe the crack advancing under conditions of symmetric deformation at the crack tip and growing straight along the crack plane in a loading cycle. Based on the present model, the markings on the fracture surfaces and phenomena associated with the process of crack growth, such as the secondary shear cracks accompanying by the striations, the slipbands between striations, and the simultaneous growth of branching cracks, can properly be explained.
Application of Probabilistic Fracture Mechanics to the Pressurized-Thermal-Shock Issue
Authors: Cheverton RD , Ball DG 
Published: February 2009

As a part of the Nuclear Regulatory Commission (NRC) effort to obtain a resolution to the pressurized water reactor (PWR) pressurized-thermal-shock (PTS) issue, a probabilistic approach has been applied that includes a probabilistic fracture-mechanics (PFM) analysis. The PFM analysis is performed with OCA-P, a computer code that performs thermal, stress, and fracture-mechanics analyses and estimates the conditional probability of vessel failure, P(F|E), using Monte Carlo techniques. The stress intensity factor (K1) is calculated for two- and three-dimensional surface flaws using superposition techniques and influence coefficients. Importance-sampling techniques are used, as necessary, to limit to a reasonable value the number of vessels actually calculated.
Analyses of three PWR plants indicate that (1) the critical initial flaw depth is very small (5 to 15 mm), (2) the benefit of warm prestressing and the role of crack arrest are transient dependent, (3) crack arrest does not occur for the dominant transients, and (4) the single largest uncertainty in the overall probabilistic analysis is the number of surface flaws per vessel.
Application of the VR Resistance Curve Method to Fracture of Various Crack Configurations
Authors: Newman JC , McNeill SR , Sutton MA 
Published: February 2009

A fracture resistance curve method based on the crack-tip-opening displacement (VR) for a “stationary” crack has been previously developed to characterize stable crack growth and instability behavior of cracked metallic materials. In this study, the VR-curve method has been applied to various crack configurations made of 2024-T351 and 7075-T651 aluminum alloys. Relationships between VR, crack length, specimen width, and tensile properties have been derived from the Dugdale or strip-yield model. For arbitrarily cracked configurations, two methods of analysis, namely, the boundary-integral equation and finite-element methods, have been developed herein to analyze the strip-yield model. These analyses and the VR-curve determined from compact specimens were used to predict stable crack growth and maximum load behavior for several crack configurations made of two aluminum alloys.
The VR-curve method was found to correlate stable crack growth data from compact specimens up to maximum load. After the maximum load was reached in a test, the values of VR remained nearly constant for each specimen but the constant value was dependent upon specimen type, crack length, and width. Predicted stable crack growth and maximum load behavior for compact, middle-crack and single-edge-crack tension specimens agreed within about 5% of experimental failure loads, whereas predictions made on three-hole-crack tension specimens were within about 15% of experimental failure loads.
J-Integral Values for Small Cracks in Steel Panels
Authors: Read DT 
Published: February 2009

In a test program motivated by the need for a quantitative relationship between fracture toughness, flaw size, and applied loading for small flaws, to be used for fitness-for-service assessment, applied J-integral was measured as a function of applied strain in eight 14-mm-thick specimens of ASTM A710 Grade A Class 3 steel plate. All the edge cracks had lengths less than 3% of the specimen width of 82 mm. Six specimens were tested in tension; two were loaded by four-point-bending in the plane of the plate. One single-edge-cracked, transversely oriented specimen was tested at −30°C. Electrical resistance strain gage and clip-gage crack mouth opening displacement measurements were used to obtain quantities inside the J-integral. The J-integral was calculated by trapezoidal rule integration. Unloading crack mouth compliance measurements were used to obtain crack length values so that tearing effects could be observed.
Lüders strains occurring right after yield caused rapid increases in the applied J-integral values for the tension specimens. Except for the Lüders strain effect, the behavior of the applied J-integral in bending was similar to that in tension. Tearing caused a smooth exponential rise in applied J as strain increased beyond the point of initiation. The initiation toughness and tearing resistance of the panels with short cracks were equal to or greater than those of conventional three-point-bend specimens of the same thickness. No effect of temperature on applied J-integral was found from the specimen tested at −30°C. The present data support Wilson's prediction of the dependence of J on strain for a material with a bilinear stress-strain curve.
Boundary Force Method for Analyzing Two-Dimensional Cracked Plates
Authors: Tan PW , Raju IS , Newman JC 
Published: February 2009

The Boundary Force Method (BFM) was formulated for the two-dimensional stress analysis of complex crack configurations. In this method, only the boundaries of the region of interest are modeled. The boundaries are divided into a finite number of straight-line segments; at the center of each segment, concentrated forces and a moment are applied. This set of unknown forces and moments is calculated to satisfy the prescribed boundary conditions of the problem. The elasticity solution for the stress distribution due to concentrated forces and a moment applied at an arbitrary point in a cracked infinite plate is used as the fundamental solution. Thus, the crack need not be modeled as part of the boundary.
The formulation of the BFM is described, and the accuracy of the method is established by analyzing several crack configurations for which accepted stress-intensity factor solutions are known. The crack configurations investigated include Mode I and mixed-mode (Modes I and II) problems. The results obtained are, in general, within ±0.5% of accurate numerical solutions.
The versatility of the method is demonstrated through the analysis of complex crack configurations for which limited or no solutions are known.