bluuebunny/crossref_metadata_2025 huggingface dataset:
Correlation Between Local HeNe Scatter and Defect-Initiated Laser Damage at 2.7 μm
Authors: Porteus J , Spiker CJ , Franck JB
Published: February 2009

Evidence of a correlation between defect-initiated pulsed laser damage and local predamage scatter in multilayer infrared mirror coatings was reported last year at this conference. Analysis of a much larger data base confirms this result on certain sample types over a substantial range of energy densities above the damage onset. Scatter signals from undamaged test sites were obtained using an HeNe scatter-probe damage monitor with a focal spot that nearly coincides with the 150-μm-diameter (D1/e2) focal spot of the damaging beam. Subsequent damage frequency measurements (1-on-1) were made near normal or at 45-deg incidence with 100-ns pulses at 2.7-μm wavelength. The correlation is characterized by an increase in damage frequency with increasing predamage scatter signal. Characteristics of the correlation are compared with a simple model based on focal spot intensity profiles. Conditions that limit correlation are discussed, including variable scatter from defects and background scatter from diamond-turned substrates. Results have implication for nondestructive defect detection and coating quality control.
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.
Multicomponent Detection and Determination of Polycyclic Aromatic Hydrocarbons Using HPLC and a Phase-Modulation Spectrofluorometer
Authors: Cobb WT , Nithipatikom K , McGown LB
Published: February 2009

The combination of HPLC with phase-modulation spectrofluorometry provides fluorescence spectral and lifetime selectivity for the qualitative and quantitative analysis of Chromatographic peak components. The phase-modulation spectrofluorometer can be used for on-line HPLC detection and for batch analysis of Chromatographic fractions, as discussed in this paper for the analysis of mixtures of polycyclic aromatic hydrocarbons. In the case of on-line detection, fluorescence lifetime heterogeneity of Chromatographic peaks and peak portions is used as an indicator of Chromatographic peak purity. Fluorescence lifetimes and fractional intensity contributions can be determined from the observed phase and modulation lifetimes at an appropriate number of excitation modulation frequencies. Batch analysis of Chromatographic fractions can be performed by phase-resolved fluorescence spectroscopy, and involves the use of multidimensional data formats in which phase-resolved fluorescence intensity is plotted as a function of Chromatographic and fluorimetric experimental parameters. Advantages of the combined HPLC/phase-modulation spectrofluorometer system are discussed.
Photochemical Reaction Detection in HPLC
Authors: Birks JW , Poulsen JR , Shellum CL
Published: February 2009

Post-column photochemical reactions may be used to enhance the sensitivity or selectivity of detection in high-performance liquid chromatography (HPLC). Several new photochemical reaction detection schemes are discussed. Anthraquinones may be detected with extremely high selectivity based on a photocatalytic reaction in the presence of oxygen to produce hydrogen peroxide. The hydrogen peroxide is subsequently detected by the highly sensitive peroxyoxalate chemiluminescence reaction. Catalytic factors ranging from 40 to 70 are achieved for many quinone analytes. Use of an immobilized fluorophore and solid-bed reagent addition makes this detection scheme particularly easy to implement. In the absence of oxygen, quinones are photoreduced in the presence of certain hydrogen-atom-donating substrates to produce highly fluorescent dihydroquinones. Thus, quinone derivatives may be used as photochemical reagents for the detection of analytes such as aliphatic alcohols, ethers, aldehydes, and amines. This post-column photochemical reaction is particularly useful for the detection of compounds having no chromophore such as saccharides and steroids. The fast reaction of singlet oxygen with either 2,5-diphenylfuran or 2,5-dimethylfuran forms the basis of a photochemical amplifier for UV-absorbing compounds. This method is photocatalytic and leads to improved detection limits in UV absorption by factors of one to two orders of magnitude. This photochemical reaction allows non-fluorescent compounds to be detected in HPLC using a UV detector with sensitivities rivaling those of fluorescence detection (typically 1 to 10 pg of compound).
Resolution of DL-Amino Acids by Capillary Zone Electrophoresis Using Chiral Electrolytes
Authors: Gozel P , Zare RN
Published: February 2009

By combining the separation power of high-voltage capillary zone elctrophoresis with the detection sensitivity of laser-induced fluorescence, subfemtomole amounts of racemic mixtures of labelled amino acids can be completely resolved in short periods of time. The separation is based on the diastereomeric interaction between the amino acids and a chiral Cu(II)-aspartame complex present in the support electrolyte. Effects of electrolyte composition, pH, and temperature are described and discussed as well as linearity and sensitivity of response.
Applications of Fluorescence Techniques in the Analysis of Polycyclic Aromatic Carcinogen—DNA Adduct
Authors: Geacintov NE
Published: February 2009

Some typical applications of fluorescence techniques in chemical carcinogenesis research are reviewed. These applications include: the kinetics of metabolism, fluorometric identification of the metabolically generated reactive diol epoxide derivatives which bind to cellular macromolecules, quantitative determination of the level of binding of these carcinogens to cellular DNA, and physico-chemical studies of the conformational properties of the carcinogenic moieties bound to DNA, and the relationships between the properties of the adducts and biological activity. It is shown that in many of these fluorometric applications it is important to first establish the intrinsic fluorescence properties of the bound polycyclic fluorophores, as well as the chemical stabilities of the adducts in the presence of light.
Kinetics of Proton Transfer of Extremely Weak Bases in the Lowest Excited Singlet State in Concentrated Mineral Acid Solutions
Authors: Kelly RN , Schulman SG
Published: February 2009

Recently, a modified form of the steady state treatment for the determination of excited state proton transfer rate constants was successfully applied to a variety of oxygen-and nitrogen-center aromatic excited-state acids. These compounds shared the characteristic of being more acidic in the first excited state, S1, than in the ground state, S0, and of requiring a concentrated mineral acid medium for protonation. The quantitation of excited-state proton transfer kinetics is presently extended to several protonated N-heterocyclic amines that are weak enough bases in the excited state to require concentrated mineral acids for protonation.
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 Critical Comparison of Proposed Parameters for High-Strain Fatigue Crack Growth
Authors: Brown MW , de los Rios ER , Miller KJ
Published: February 2009

A number of elastic-plastic fracture mechanics parameters have been suggested for the description of fatigue crack growth, of which three are considered in this paper: the J-integral, crack-tip opening displacement, and plastic zone size. A critical comparison of the effectiveness of each parameter may be devised from a study of Mode I fatigue crack growth rates under various states of biaxial stress. Data for one heat of AISI 316 austenitic stainless steel at 20°C and 550°C showed that, if crack closure is taken into account, the cyclic plastic zone size is the most suitable correlating parameter for stress levels up to the yield value. A second heat of 316 stainless steel showed a preference for crack opening displacement control of crack growth. The analysis is based on the Dugdale model for crack-tip plasticity.
Toward an Understanding of Mode II Fatigue Crack Growth
Authors: Smith MC , Smith RA
Published: February 2009

The Mode II crack flank displacement and crack growth responses of three precracked specimens made from structural steel were measured, using plastic replicas and a crack-tip compliance gage. Crack surface interaction was found to dominate behavior: at low stress intensity range (ΔKIInom = 9 MPa √m) the precracks did not suffer reversed slip to their tips and no crack growth occurred, while at high stress intensity range (ΔKIInom = 19 MPa √m) the effective stress intensity range was less than half that nominally applied. Three sources of crack flank frictional attenuation were identified: compressive residual stresses due to precracking, Mode I wedging over asperities, and gross plastic deformation of interlocking asperities. The measured unlocking response was modeled successfully by assuming that crack flank frictional stresses obeyed a constant interfacial shear stress friction law.
The Propagation of Short Fatigue Cracks at Notches
Authors: Tanaka K , Akiniwa Y
Published: February 2009

Center-notched plate specimens and single-edge-notched specimens of a low-carbon steel were fatigued under axial and bending loads. The growth behavior of a short fatigue crack near the notch tip was analyzed based on the measurement of crack closure. The crack closure develops as a crack grows; the effective fraction of the applied stress decreases. This accounts for the decrease of the crack growth rate with increasing crack length. The relation between growth rate and the effective stress intensity range is unique, and nearly identical to that for a long crack, meaning that the effective stress intensity range is a good measure of the crack driving force. The results on the threshold of crack growth are compared with the model proposed by Tanaka and Nakai. The variations of the threshold values of the stress intensity range and the effective stress intensity range, and the crack closure at the threshold with the length of nonpropagating cracks, agree well with the model prediction.
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.
Metallurgical Aspects of Crack-Tip Failure Processes
Authors: Gerberich WW
Published: February 2009

Process zone models for fracture of fiber-reinforced concrete, metal-particle reinforced glass, and polymer crazes have good experimental verification. This is because the microfracture process zone is often macroscopic in size or easily identifiable or both. Similar models have been proposed for metals and ceramic microstructures. This paper addresses how the microfracture process zone develops in such microstructures. Specifically, the microcrack evolution process may be controlled by chemistry and microstructure as well as localized stress distributions. The importance of this is that the microcrack distribution and the energy dissipation process in the remaining ligaments behind the advancing crack front control the R-curve and final fracture instability. Examples of R-curve behavior in ductile fracture and semibrittle composites are shown, and a model for brittle fracture of steel is proposed. The latter shows that a semi-cohesive process zone of variable size and strength may represent an approach to brittle fracture where weakest-link models are not applicable. Predictions of fracture toughness for ferrite/pearlite steels as a function of test temperature and grain size are obtained with such an approach.
Cleavage Toughness Variability and Inclusion Size Distribution of a Weld Metal
Authors: Tyson WR , Marandet B
Published: February 2009

The cleavage toughness of a carbon-manganese electroslag weld metal has been measured in the transition region and related to measurements of the inclusion size distribution. Results were interpreted using the “weakest link” theory, based on the hypothesis that fracture occurs when the local stresses acting on a microcracked inclusion become large enough to propagate the microcrack.
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.
Crack Arrest Testing, Results, and Applications
Authors: Machida S , Yajima H , Kanazawa T
Published: February 2009

This paper reviews two experimental and analytical studies made in Japan on predicting the brittle crack propagation and arrest behavior using wide plate testings and the dynamic or simplified dynamic analysis technique. Also reviewed is an analysis, though statical in technique employed, made of an upper deck plating brittle failure experienced by a 10 379 gross ton bulk carrier.
The Stress Field Surrounding a Rapidly Propagating Curving Crack
Authors: Shukla A , Chona R
Published: February 2009

Results are presented from experiments conducted with a cross-shaped, biaxial fracture specimen which was eccentrically loaded to obtain rapid crack propagation along a curved path. High-speed photographs of the crack-tip isochromatic patterns were analyzed to obtain the leading coefficients of the dynamic, mixed-mode, crack-tip stress field expressed in series form. Values of KII associated with the propagating crack were found to be essentially zero when compared with the dominant opening mode stress intensity factor, KI.
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.
Prediction of Failure Loads of Adhesive Joints Using the Singular Intensity Factor
Authors: Groth HL
Published: February 2009

A method to predict failure of adhesive joints is described. The fracture criterion is based on material-induced singularities that appear at the terminus of the joint. The criterion can be interpreted physically in terms of the elastic strain energy density. The criterion is formulated in terms of a singular intensity factor Q that is analogous to the stress intensity factor for the case when cracks are present. The strengths of the singularities are dependent on the elastic properties of the adhesive and the adherends and on the geometry of the bond terminus, but are postulated to be independent of loading and global geometry.

For the case of an adhesive joint, the different materials inducing the singularities may be the adherend and the adhesive. The criterion assumes that no cracks are present in the vicinity of the material-induced singularity and that fracture will occur as a cohesive fracture in the adhesive layer. It has been shown earlier that the criterion predicts with good accuracy the fracture load of test specimens loaded in combined tension and shear. The fracture load of the single-lap joint is predicted and compared with experimentally obtained values.

The presented method of predicting fracture in adhesive joints may be applicable to general joint geometries.
Single-Specimen Test Measurement of Ji and J-Δa with a Pulsed D-C Potential-Drop Technique
Authors: Thaulow C , Hauge M , Gunleiksrud Å , Paauw AJ
Published: February 2009

Fracture mechanics testing [KIc, J-integral, and crack-tip opening displacement (CTOD)] of a QT-steel (AISI 4137 H) has been carried out in order to compare the testing methods. The tests were performed at room temperature, and the material toughness was varied by applying annealing temperatures from 300 to 650°C.

The J-initiation values were successfully determined with a specially developed d-c potential-drop (PD) method. The technique is based upon pulsed current and reference electrodes. Only the potential-drop difference caused by a current difference is measured; this eliminates the thermo-electric effect and offset voltage associated with PD equipment.

On the basis of the PD measurements both the J-initiation value and the R-curve (J-Δa) could be determined from a single-specimen test.

The R-curves could be fitted to a third-order polynomials, but even if the initiation values were nearly constant for replicate specimens, the slope of the R-curve would vary significantly. The mechanisms of crack initiation and growth are different, and the observed results indicate that the crack growth properties, as characterized with the R-curve, of the present steel quality vary from one specimen to the next.

The J-initiation values determined with the multispecimen technique recommended in ASTM E 813 showed considerably more scatter in comparison with the present PD technique. The results suggest that the J-initiation value should be determined on each specimen and not calculated with extrapolation techniques.
Determination of the Fracture Toughness of Irradiated Reactor Pressure Tubes Using Curved Compact Specimens
Authors: (Peter) Chow CK , Simpson LA
Published: February 2009

This paper describes a case in which nonstandard test configurations had to be used to assess fracture toughness, and an experimental, analytical program to support the validity of the results.

Zirconium alloy pressure tubes (103 mm inside diameter, 5 mm thick) contain the primary coolant in CANDU™ reactors. Because irradiated materials lacks ductility, the pressure tube cannot be flattened to produce flat specimens to determine fracture toughness. Therefore, a method was devised for spark eroding 17-mm compact specimens from sections of pressure tubing. These specimens retained the normal in-plane dimensions of the standard compact specimens, but possessed the curvature of the tube. By comparing results from flat and curved specimens, this paper shows experimentally that flat-plate equations can be applied to both specimen types to calculate the fracture toughness parameters with little error. A simple analytical calculation estimates that the maximum curvature-induced stress should be less than 10% of the in-plane tensile stress at the crack tip.
Developments in the Application of the CTOD Approach to Fracture Assessment of Welded Construction
Authors: Harrison JD , Anderson TL
Published: February 2009

The crack-tip opening displacement (CTOD) approach to the assessment of the risk of fracture, which has been incorporated into the British Standard's Published Document PD 6493, has served the United Kingdom welding fabrication industry well for the past 15 years. However, application of the approach and further research have drawn attention to several aspects which need improvement. For example, the PD 6493 document gives no guidance on the derivation of the critical CTOD value to be used in the analysis. Also, the fracture assessment method in PD 6493 suffers from a number of limitations.

This paper describes proposed improvements to the CTOD test and CTOD-based design approaches. A recently drafted appendix for weldment testing, which is proposed to be incorporated into the British CTOD test standard, is discussed briefly. Also, proposals for dealing with stable tearing, pop-in, and scatter in CTOD data are described. Finally, a three-tier fracture assessment procedure is proposed which allows flaws to be assessed at various levels of complexity, depending on the application. The three levels of assessment, in increasing order of complexity and accuracy, are 1. Modified version of the present PD 6493 method. 2. Strip yield model. 3. Reference stress approach.

All three levels utilize CTOD as the fracture toughness input. Some guidelines for choosing the appropriate assessment level are given.
Effect of Crack and Plate Geometry on the Fracture Resistance of Wide Plates in Bending
Authors: Lereim J
Published: February 2009

The study included examination of the influence of variation in the geometry of the structural component as well as the variation in surface crack geometry. Strong dependency on geometry was experienced on the instability fracture toughness, which is directly a function of the triaxial state of stress, whereas the fracture toughness at initiation of stable crack growth confirmed that the crack-tip opening displacement (CTODi) essentially is a pure material parameter.
Unloading-Compliance and Load-Drop Analysis of JIc Tests of Irradiated 348 Stainless Steel
Authors: Underwood JH , Haggag FM , Reuter WG
Published: February 2009

Procedures for determining J-integral fracture toughness were applied to load versus load-line displacement data from a series of three-point bend specimens of irradiated 348 stainless steel tested at 23 and 427°C. The ASTM Test Method for JIc, a Measure of Fracture Toughness (E 813-81) was emphasized, using both unloading-compliance and load-drop measures of crack growth.

An effective modulus procedure was included in the unloading—compliance method using recently published load-line displacement results. Two procedures were used for calculating JIc from the J versus unloading-compliance crack growth plots: the linear fit procedure from ASTM Method E 813 and a power-law fit with a 0.2-mm offset value of JIc.

Based on the results of these various procedures, conclusions were reached regarding the most consistent measures of fracture toughness from these data. Suggestions were given regarding the use of effective modulus with unloading-compliance measurement of crack growth and the limitations of the load-drop method for measuring crack growth.
Effects of Monotonic and Cyclic Prestrain on Fracture Toughness: A Summary
Authors: Liaw PK , Landes JD
Published: February 2009

The effects of monotonic and cyclic prestrain on fracture toughness properties of 4340 steel, 4335 steel, and 316 stainless steel were investigated. Monotonic prestrain lowered the toughness levels of the JR-curves and J–T diagrams for these three steels, relative to the virgin material. While cyclic prestrain increased the JR and J–T toughness of 4340 and 4335 steels, it decreased that of 316 stainless steel. The effects of monotonic and cyclic prestrain on JR and J–T curves can be rationalized by material strength. Furthermore, the influence of cyclic prestrain was attributed to the cyclic softening or cyclic hardening characteristics of the virgin material. The transition behavior of the 4340 steel was also altered by monotonic and cyclic prestrain. Moreover, material strength appears to provide a rationale to account for the effects of prestrain history on the transition behavior.
Application of Direct Measurement of J-Integral on a Pressure Vessel with Axial Notch
Authors: Sedmak S , Petrovski B
Published: February 2009

The J-contour integral value was determined by the method of direct evaluation on a pressure vessel with an axial crack in a welded specimen. The specimen, cut from a weldment made by manual-arc welding, was machine-notched and fatigue precracked. It was mounted by welding on the pressure vessel, positioned in the opening cut on the cylindrical shell. The test was performed by gradually increasing water pressure. Crack mouth opening displacement (CMOD) was measured by clip gage, and strains were measured by strain gages, positioned along the J-integral path. J-integral, CMOD, and hoop stress are presented as a function of applied pressure. The fatigue crack length was extended by approximately 10 mm during pressurization. The development of an experiment to determine the JR curve in a full-scale test and to predict the residual strength of a cracked pressure vessel is considered.
A Comparison of J-Integral and CTOD as Fracture Toughness Parameters
Authors: Anderson TL
Published: February 2009

The J-integral and the crack-tip opening displacement (CTOD) are widely used throughout the world to measure the fracture toughness of structural materials. There are many similarities between these two parameters, as well as a number of important differences. Because of these differences, the CTOD parameter tends to be more conservative when applied to structures.

The J-integral and CTOD were compared qualitatively and analytically. This work differs from previous comparisons in that the present study compared engineering estimates of J and CTOD, which are not necessarily equal to the true values of J and CTOD. However, the fracture toughness of a material is nearly always characterized in terms of the engineering approximations of J or CTOD.

The main difference between J and CTOD as measures of toughness is that J is directly proportional to the area under the load-displacement curve whereas CTOD is proportional to the clip gage displacement. This means that the relationship between the two parameters depends on strain hardening and constraint.
Elevated Temperature Fatigue Crack Growth of Single Crystal René N4 Superalloy
Authors: Venkataraman S , Nicolas T , Khobaib M
Published: February 2009

Constant-load-amplitude fatigue crack growth tests were conducted at 0.33 Hz over a temperature range of 538 to 871°C for single crystal René N4 nickel base superalloy. Crack growth behavior was found to be similar in the Paris law regime for the entire temperature range tested except for a slight increase in growth rate at 871°C at higher levels of the stress intensity range. Mainly, two distinct modes of fatigue fracture were observed, namely, a flat, featureless, Stage II fracture morphology and a faceted, crystallographic, Stage I cracking mode. The Stage II fatigue fracture occurred over the entire range of ΔK at the three highest temperatures. At 538°C, however, a transition from Stage II to Stage I fatigue fracture occurred with increase in ΔK. The higher crack growth rates at higher temperatures in the high-stress-intensity range were attributed to casting defects present in the material.
Development of the NASA/FLAGRO Computer Program
Authors: Forman RG , Shivakumar V , Newman JC , Piotrowski SM , Williams LC
Published: February 2009

A preliminary version of a new computer program has been developed for calculating crack propagation in cyclically loaded structures. The new program, titled NASA/FLAGRO, has numerous enhanced features over previous programs used for safe life analysis on space systems. The essential features of the new program consist of modular design, promptive input using a cathode ray tube terminal, improved crack growth rate analytical methods, many new crack case solutions, and incorporation of user-defined files for material properties and fatigue spectrums. Also, five options made available in the program are to compute (1) safe life of a part with a preexisting crack, (2) critical crack size for a specified stress level, (3) stress-intensity factor numerical values for making comparisons or checks of solutions, (4) least-squares fit of growth rate equations to crack growth rate data, and (5) to convert a versus N data to ΔK versus da/dN.
The Influence of Cracks on the Buckling Capacity of Axially Compressed Columns
Authors: Evensen JD , Gradin PA , Lereim J
Published: February 2009

When using slender beams as compression load-carrying members, one has to consider the risk of buckling failure. Apart from cross-sectional properties, length of the beam, and elastic properties, the initial lateral deflection and the boundary conditions will be important in determining the critical load, or buckling capacity. The presence of cracks may release the end constraints and drastically change the boundary conditions.

A growing fatigue crack may then lead to a gradual change from a rigid joint to a more flexible one with its lower extreme as moment-free. Thus it may be expected that the buckling strength will decrease with increasing fatigue crack size and therefore be time-dependent. Such a behavior would be of great concern in dynamically loaded structural elements containing growing fatigue cracks. This situation is highly realistic for a variety of offshore structures which are constructed as complex welded frames where high local stress levels in the nodal points in combination with dynamic loads due to current, wind, and waves may cause fatigue cracks to grow.

In order to provide some further insight into this problem, experiments and corresponding numerical analyses were performed with a Euler-III column of rectangular cross section and with surface cracks of varying length at the clamped end. The numerical results were combined with possible crack length versus load cycle relations to show the time dependence of the buckling load.

From the experiments it was concluded that the calculated elastic buckling load versus crack length relation (based on linear material behavior) represents an upper bound for the buckling load in the crack length interval considered.
A Comparison of Fatigue Crack Propagation in Modes I and III
Authors: Ritchie RO
Published: February 2009

The propagation behavior of fatigue cracks in Mode III (anti-plane shear), measured under cyclic torsion, is described and compared with more commonly encountered behavior under Mode I (tensile opening) loads. It is shown that a unique, global characterization of Mode III growth rates, akin to the Paris “law” in Mode I, is only possible if characterizing parameters appropriate to large-scale yielding are employed and allowance is made for crack tip shielding from sliding crack surface interference (that is, friction and abrasion) between mating fracture surfaces. Based on the crack tip stress and deformation fields for Mode III stationary cracks, the cyclic crack tip displacement, (ΔCTD)III, and plastic strain intensity range, ΔΓIII, have been proposed and are found to provide an adequate description of behavior in a range of steels, provided crack surface interference is minimized. The magnitude of this interference, which is somewhat analogous to crack closure in Mode I, is further examined in the light of the complex fractography of torsional fatigue failures and the question of a “fatigue threshold” for Mode III crack growth. Finally, micromechanical models for cyclic crack extension in anti-plane shear are briefly described, and the contrasting behavior between Mode III and Mode I cracks subjected to simple variable amplitude spectra is examined in terms of the differing role of crack tip blunting and closure in influencing shear, as opposed to tensile opening, modes of crack growth.