bluuebunny/crossref_metadata_2025 huggingface dataset:
Is the Concept of a Fatigue Threshold Meaningful in the Presence of Compression Cycles?
Authors: Ritchie RO , Zaiken E , Blom AF
Published: February 2009

The fatigue threshold ΔKTH, represents the nominal stress intensity range below which fatigue cracks are presumed not to propagate. Its magnitude, however, has been linked to the degree of crack closure. Since the extent of closure may vary, at fixed nominal ΔK, with such variables as crack length and loading history, the concept of a unique threshold as a material constant for a specific material/microstructure/environment system may become questionable. Of particular importance here for aerospace applications is whether crack growth remains dormant below ΔKTH in the presence of variable-amplitude loading, specifically during the occurrence of single compression cycles. In the present paper, a series of critical experiments on the effect of single compression overloads on the behavior of fatigue cracks arrested at ΔKTH is examined. Based on tests on I/M 7150-T6, 7150-T7, and 7475-T7351 aluminum alloys and 17-4 PH stainless steel, it was found that the occurrence of a single compressive cycle, of magnitude five times the maximum tensile load, caused immediate propagation of previously arrested threshold cracks at ΔKTH. Such observations are interpreted in terms of the degree of compressive residual stress in the cyclic plastic zone ahead of the crack and a measured reduction in crack closure following the compression cycle. The results serve to confirm the dependence of the threshold on crack closure and furthermore to illustrate the danger of utilizing nominal ΔKTH threshold values in damage-tolerant designs to predict fatigue life.
Crack Closure and Variable-Amplitude Fatigue Crack Growth
Authors: McEvily AJ , Minakawa K
Published: February 2009

The prediction of service lifetime and the selection of alloys for maximum fatigue crack growth resistance are made complicated by the effect of prior loading histories on the rate of crack growth. Some research indicates that crack closure can be an important factor in accounting for this history effect. This paper presents a review of the lifetime on load-interaction effects on fatigue crack growth and indicates some of the future research needed to better understand the factors giving rise to history effects in the growth of long and short cracks.
High-Temperature Crack-Arrest Toughness Measurements Using Compact Specimens
Authors: Rosenfield AR , Mincer PN , Marschall CW
Published: February 2009

The ASTM Crack-Arrest Test, which is under development, is generally limited to providing data up to 20 to 40°C above the ductile-to-brittle transition temperature (expressed as RTNDT in the ASME Boiler and Pressure Vessel Code). For this test, the geometry is a wedgeloaded compact specimen containing a brittle weld starting notch, which appears incapable of reliably generating a rapid crack above the stated temperature range.

Two alternatives have been used for obtaining data at higher temperatures: very large specimens combined with soft loading systems and modified compact specimens. This paper describes compact-specimen modifications which have extended the successful temperature range to the onset of the Charpy upper shelf, the highest relative temperature reached being RTNDT + 100°C.

Specific modifications utilized are: a high-strength steel crack-starter section (duplex specimen), a low-friction loading arrangement (inverted split pins), and contoured side grooves. The high-temperature data generated using these modifications compare favorably with largespecimen results.
The Failure of Aluminum Compact Shear Specimens Under Mixed-Mode Loading
Authors: Riddle RA , Streit RD , Finnie I
Published: February 2009

The failure behavior of 7075-T6 aluminum under mixed-mode loading was investigated using the compact shear specimen. This specimen is capable of providing a range of Mode I and Mode II loading conditions at the crack tip, depending upon the boundary conditions provided by the loading fixtures.

Finite-element analyses of the specimen were made which included the effects of loading fixture constraints and the material's elastic-plastic properties. The symmetric and antisymmetric components of the stress and deformation fields were used to calculate the applied J-integral values, JI and JII, corresponding to the Mode I and Mode II loading at the crack tip. Methods to obtain the critical values of the J-integral at crack initiation from the load displacement curves of the specimen are discussed.

The failure behavior of the material varied greatly depending upon the relative amounts of Mode I and Mode II loading at the crack tip. For near Mode I loading, the specimen behavior at failure loads was nearly elastic, and the failure processes appeared to be typical of those for Mode I plane-strain fracture toughness testing. A fairly significant amount of shear loading seemed to have little effect on the failure load. For near Mode II loading, the amount of plastic deformation is much more significant, with the ligament ahead of the crack tip nearly fully plastic at the failure load. The failure processes in the material, as evidenced by the fracture surface morphology, are different as compared to Mode I type failure. There is evidence of slow stable crack growth occurring at loads near the departure from linearity of the specimen load-displacement curve.

For a mixed-mode case in which Mode II dominates with a smaller amount of positive Mode I loading, the critical J-integral value is substantially larger than the value of the critical J value for Mode I loading. When it is transformed into the corresponding critical stress intensity factor values, KIIc and KIc, it is found that KIIc is approximately twice KIc. However, there is some question as to whether the specimen failure is controlled by a critical stress intensity value or by the applied shear stress.

The use of the J-integral as a failure criterion and as a means to predict the angle of crack growth for elastic-plastic failure is discussed.
Investigation of Ductile Fracture Properties of Welded Type 304 Stainless Steel Pipe and Large Plan Compact Specimens
Authors: Hays RA , Vassilaros MG , Gudas JP
Published: February 2009

An experimental investigation was performed to evaluate the J-integral resistance curve behavior of 10-cm (4-in.) nominal pipe size Schedule 80 welded Type 304 stainless steel pipe. Tests were performed in four-point bending at 288°C (550°F) on 1219-mm (48-in.)-long pipe specimens with machined flaws contained in circumferential welds. Two flaw geometries were investigated. The first was a through-wall crack growing circumferentially (simple) and the second was a 360-deg part-through radial crack superimposed on a through-wall circumferential crack (complex). During the tests load, load line displacement, crack mouth opening displacement, and electrical potential drop across the crack mouth were monitored. Elastic compliance and direct current potential-drop (DCPD) techniques were used simultaneously to estimate crack extension during the tests. Pipes containing simple cracks had an average J-initiation of 1120 kJ/m2 (6400 in. · lb/in.2). The presence of the complex crack reduced the J-initiation level by approximately a factor of four and also reduced the apparent tearing modulus significantly as compared to pipes containing simple cracks.

A second part of this investigation concerned the use of laboratory-sized specimens of the same thickness as the wall of the pipe to model the J-R curve behavior of the simple through-wall cracked pipe. Large plan compact specimens were cut from a welded plate and tested at 288°C (550°F) using both elastic compliance and DCPD crack length estimation techniques. J-initiation values from the two types of specimens appear comparable, but the small amount of crack extension from the compact specimens limited comparison of the far-field J-R curve.
A Study of Variability, Size, and Temperature Effects on the Fracture Toughness of an Arctic-Grade Steel Plate
Authors: Faucher B , Tyson WR
Published: February 2009

The fracture toughness of a 75-mm-thick plate of an LT60 grade steel developed by Algoma Steel has been evaluated by three-point bend tests on specimens 10, 20, and 50 mm thick. The fracture toughness measured by the value of the J-integral at onset of cleavage fracture is higher with thin specimens than with thick ones. The statistical model of Tyson and Marandet describes the temperature dependence, the experimental scatter, and, to a considerable extent, the size dependence of fracture toughness. Loss of constraint in thin specimens may be significant when the size requirements of the ASTM Test Method for J1c, a Measure of Fracture Toughness (E 813-81) are not met.
Propagating Shear Fracture in Natural Gas Transmission Pipelines
Authors: Sugie E , Matsuoka M , Akiyama T , Tanaka K , Tsukamoto M
Published: February 2009

The propagating shear fracture in natural gas transmission pipelines can be theoretically analyzed, and the crack velocity and propagation distance can now be calculated. The propagation theory has a good consistency with the experimental results for 1219 mm (48 in.) outside diameter (OD) × 18.3 mm (0.72 in.) wall thickness (WT) line pipes, and it predicts the notch ductilities of line pipes required for arresting the fracture according to the type of the gas, the acceptable fracture length, the pipe size, and the design stress of the pipeline.
Determination of Mixed Mode Stress-Intensity Factors Using Explicit Weight Functions
Authors: Sha GT , Yang C-T
Published: February 2009

An efficient finite-element methodology for evaluating the stress-intensity factors (KI, KII) and the weight functions (hI, hII) of a self-similar propagating mixed-mode crack has been established with a single colinear virtual crack extension. This is accomplished through the use of a symmetric mesh in the crack-tip neighborhood that provides the decoupling characteristics for the field parameters of stresses, strains, displacements, tractions, and strain energy release rates into Mode I and Mode II components within the symmetric mesh zone. The application of this newly established technique to the three-point bend specimen has been studied with the mixed fracture mode introduced by deviations from normal test conditions of either loading or support eccentricity. These eccentricity effects on KI and KII values for three-point bend specimens are detailed in this paper.

The angular dependence of the strain energy release rate, G(θ), as a function of eccentricity, is also provided herein to account for the testing irregularities associated with three-point bend tests, which have span-to-width ratio s/W = 4 and different crack lengths 0.1 ≤ a/W ≤ 0.8. By assuming the crack propagation taking place at the angle (θcr), with maximum strain energy release rate (Gmax), the predictions are also provided for expressing θcr angles as a function of both loading and support eccentricities. It is found that the load eccentricity has a more severe effect than that of the support eccentricity as indicated in our numerical results for three-point bend specimen with s/W = 4.

Also presented explicitly are the characteristics of the Mode I and Mode II weight functions for three-point bend specimens (equivalent to edge crack geometry) with s/W = 4 and 0.1 ≤ a/W ≤ 0.8 at the crack faces and other key traction application surfaces. The explicit weight function information at these key traction application boundaries is very useful to the fracture mechanics practitioner and designer for guiding traction application locations and directions that produce the least stress-intensity factor.

For a given crack geometry, the explicit weight functions depend on the constraint conditions. The method of obtaining the required stress-intensity factors of an asymmetric crack geometry under the constraint conditions, which are different from the available weight function for the same asymmetric crack geometry, is also made available here. This is accomplished by combining the predetermined explicit weight functions with self-equilibrium forces, which include both the applied surface tractions and the reaction forces induced by constraints.
An Evaluation of J-R Curve Testing Using Three-Point Bend Specimens
Authors: Kirk MT , Hackett EM
Published: February 2009

This investigation concerns the accuracy of load-line deflection measurement techniques used in J-integral/R-curve testing of three-point bend specimens. Four methods of load-line deflection measurement were investigated. One method employed a calibrated transducer that gave a direct reading of load-line deflection. The other three methods mathematically related load-line deflection to some other displacement (that is, crack mouth opening displacement, crosshead displacement) measured during testing. Photographs taken at regular intervals during loading provided an absolute measure of load-line deflection. In comparison with this reference, the other methods had equal accuracy with varying degrees of precision. In no case, however, did the measurements from the laboratory techniques deviate from the photographic reference by more than 0.052 mm (0.002 in.) on average. The small differences between the load-line deflections measured by the various laboratory techniques cause less scatter in the J-R curves and JIc values than attributable to material variability. Further, J-R curves over the first 1.75 mm (0.068 in.) of crack extension from both bendbars and compacts were shown to be coincident for the alloys tested.
Modifications of ASTM E 813-81 Standard Test Method for an Improved Definition of JIc Using New Blunting-Line Equation
Authors: Heerens J , Schwalbe K-H , Cornec A
Published: February 2009

A test procedure for determining the fracture resistance of ductile metallic materials under stable crack growth conditions is described. The procedure consists of the determination of a J-Δa curve with particular emphasis on the near-initiation region. In our proposal, JIc is defined as the fracture resistance at 0.2-mm stable ductile tearing. It is determined by the intersection of the J-Δa curve with an intercept line which is parallel to and 0.2 mm off the blunting line. The blunting-line formula is improved by accounting for the strain hardening behavior of the material. A simple determination of the blunting-line equation is shown, and its applicability is checked using a broad range of materials.
Precracking and Computerized Single-Specimen JIc Determination for Irradiated Three-Point Bend Specimens
Authors: Haggag FM , Richardson AK
Published: February 2009

An experimental procedure is described for precracking and computerized single-specimen JIc testing of irradiated three-point bend specimens. The specimens were automatically precracked in cantilever bending at constant deflection. The JIc values determined by the single-specimen unloading compliance technique were found to be in close agreement with multiple-specimen values. The JIc testing system comprised a three-point bend fixture and a furnace mounted on a screw-driven Instron testing machine, a Hewlett-Packard computer, and a data acquisition system. This procedure was found to be successful and cost effective in precracking and JIc testing of the irradiated specimens.
Evaluation of the Fracture Toughness of Ductile Iron Using Fatigue Precracked Charpy, Dynamic Tear, and Compact Tension Specimens
Authors: Tanner GM , Bradley WL
Published: February 2009

The fracture toughness of ductile iron has been studied using dynamic tear, fatigue precracked (FPC) Charpy, and compact tension (CT) specimens. Specimens were fractured both quasi-statically and dynamically. Initial crack extension was found to occur at maximum load for both the compact tension specimens and the FPC Charpy specimens. However, initial crack extension occurred prior to maximum load for the dynamic tear specimens. Upper-shelf KJIc values determined from the compact tension specimens and the FPC Charpy specimens were similar (83 and 76 MPa √m) while the J-value at maximum load for the dynamic tear specimens was found to fit nicely on the J-R curve from the CT specimens. The shift in ductile-to-brittle transition temperature with strain rate (comparing quasi-static with impact loading) was similar to what has been observed for structural steel with a similar yield strength.
Fracture Mechanics in Failure Analysis
Authors: Barsom JM , Rolfe ST
Published: February 2009

Fracture mechanics is the best available technology for preventing and analyzing failures of structural and equipment components that are caused by the initiation and propagation of cracks. This paper presents a few observations on the failure and fracture of metals and on the use of correlations to obtain estimates of fracture toughness. The usefulness of these observations in failure analyses is illustrated by analyzing actual structural failures. The paper also presents a brief discussion of some common misconceptions that can lead to incorrect conclusions in a failure investigation.
The Significance of CTOD in Transversely Loaded Weldments with Weld Metal Overmatching in Strength
Authors: Denys RM
Published: February 2009

The purpose of this paper is to review the effects of weld metal overmatching on weld joint behavior. The notch toughness parameters crack-tip opening displacement (CTOD) and crack opening displacement (COD) are used to evaluate the degree of weld metal overmatching on weld joint performance. Exploratory experimental work on wide-plate specimens loaded in tension is presented to support the theoretical observations.

It is concluded that weld metal overmatching has a beneficial effect on the overall performance of transversely loaded welded joints loaded in tension. The results also indicate that the CTOD bend test gives mainly an idea about the local notch tip ductility and that, if a “fitness for purpose” evaluation is to be based upon the small-scale CTOD test, the benefits which accrue from the use of overmatching weld metal on tolerable defect size cannot be described adequately by the design curve approach.
Wide Plates in Bending: Application of CTOD Design Approach
Authors: Berge S , Ebrahimi F , Read DT
Published: February 2009

Wide plates of A572 Grade 50 steel were tested in four-point bending. The plate thickness was 50 mm. The plates were surface cracked, with two different aspect ratios. The test temperature was varied through the transition range. Critical crack-tip opening displacement (CTOD) was measured by scanning electron microscopy (SEM). CTOD at fracture compared well with data from standard bend bar tests for the same materials. Applied strains were measured using three different methods. For low aspect ratios, the measure of applied strain was a major source of uncertainty in the application of the CTOD design curve. Safe predictions from standard bend bar data could be made only if lower-bound CTOD values were assumed. For larger aspect ratios, the design curve was overconservative due to an underestimation of CTOD for a given applied strain. Scatter in CTOD data was due primarily to manganese sulfide stringers which caused delamination at the crack tip under conditions of ductile crack initiation.
Specimen Thickness Effects for Elastic-Plastic CTOD Toughness of an A36 Steel
Authors: Wellman GW , Sorem WA , Dodds RH , Rolfe ST
Published: February 2009

This paper describes the results of an experimental and analytical study of the effect of specimen size on the fracture toughness behavior of A36 steel, primarily in the lower-transition region. This region is beyond the limits of plane strain as defined by ASTM Method E 399, but in the region where failure occurs prior to any macroscopic fibrous tearing. Experimental results were obtained using crack-tip opening displacement (CTOD) three-point bend specimens, and analytical results were obtained using a 3-D elastic-plastic finite-element analysis. Two specimen sizes, full thickness [31.8 mm (1.25 in.)] and reduced thickness [12.7 mm (0.50 in.)], were investigated using the preferred t × 2t × 8t specimen size.

Testing was conducted according to the procedure outlined in BS 5762:1979, Methods for Crack Opening Displacement Testing. CTOD-temperature transition curves were developed for both specimen sizes. A sufficient number of specimens were tested at two temperatures in the lower-transition region to allow statistical characterization of the CTOD results. Both normal and Weibull distributions were used for that characterization.

Three-dimensional elastic-plastic finite-element analyses were conducted to determine the effect of specimen size on the near-crack-tip opening stresses through the specimen thickness. The near-crack-tip stress fields were compared at various CTOD levels well into the elastic-plastic regime, but prior to any stable crack growth. At each of the applied CTOD levels, the maximum near-crack-tip opening stress occurred at the specimen centerplane and was found to be essentially independent of specimen size.

The results of this study showed that the lower bound of the elastic-plastic CTOD toughness values of A36 steel for small specimens is essentially equal to those of larger specimens in the lower-transition region prior to any stable crack growth. Thus it may be possible to characterize the notch toughness of low-strength structural steels, such as A36 steel in the lower-transition region, using small specimens provided that a sufficient number of specimens are tested. However, additional study is required before any recommendations can be made.
A Study of Crack Initiation in the Ductile-to-Brittle Transition Region of a Weld
Authors: Ebrahimi F
Published: February 2009

The ductile-to-brittle transition behavior of a shielded metal-arc weld was characterized for various specimen geometries by the crack opening displacement (COD) testing method. The transition curve shifted to higher temperatures with increasing the specimen thickness and the crack length to width ratio (a/W).

Emphasis was placed on mechanisms of crack initiation as characterized by detailed fractography. A ductile shear zone, next to the fatigue precrack, preceding the unstable fracture, was revealed on the fracture surface of all specimens tested, except for specimens that broke under small-scale-yielding conditions. It is shown that brittle or ductile microcracks, depending on the test temperature, initially form at a distance ahead of the tip of a blunted crack. Subsequently, the ligament between these microcracks and the crack tip fails by a shearing mechanism. On the basis of this observation it is proposed that the statistical nature of fracture in the ductile-to-brittle transition regime is related to the probability of propagation of an unstable crack from the brittle or ductile microcracks.
JR-Curve Testing of Three-Point Bend Specimens by the Unloading Compliance Method
Authors: Steenkamp PAJM
Published: February 2009

The unloading compliance technique for JR-curve testing is not well established for single-edge notched specimens loaded in three-point bending (SENB) to date. This paper investigates various effects hampering the successful application of the method of SENB specimens. It is shown that the specimen's compliance is significantly affected by rotation of the specimen halves during deformation and stable crack extension. By contrast, the effects of roller indentation, outer roller movements and friction, and of crack front curvature are relatively minor. Based on these findings, a general procedure is proposed for correcting the experimentally measured compliance of SENB specimens for the above effects.

Crack extensions predicted for three steels from SENB unloading compliance experiments incorporating the proposed corrections are seen to correlate within ±10% with dye penetrant beachmarks made at several points during the tests.
An Elastic-Plastic Finite-Element Analysis of the J-Resistance Curve Using a CTOD Criterion
Authors: Newman JC , Booth BC , Shivakumar KN
Published: February 2009

The tentative test procedure for determining J-R curves (crack-growth resistance curves based on the J-integral) on compact and three-point bend specimens showed a discrepancy between specimen types: the bend specimen gave a higher J-R curve than the compact specimen. The purpose of this paper is to investigate this discrepancy, numerically, by simulating the fracture process on compact and bend specimens using a two-dimensional finite-element analysis and a critical crack-tip opening displacement (CTOD) criterion.

An elastic-plastic (incremental and small strain) finite-element analysis was used with the critical CTOD criterion to model stable crack growth in compact specimens made of HY-130 steel. A combined analysis between plane-stress and plane-strain conditions was found to simulate the fracture process quite well. In the combined analysis, plane-strain conditions were imposed around the crack tip while plane-stress conditions were imposed elsewhere. The same analysis was then used to predict stable crack growth behavior of bend specimens. J-R curves were then calculated from the numerical results for each specimen type using several different methods. In general, the J-R curves obtained from the bend specimens were higher than those obtained from the compact specimens, especially beyond maximum load. Up to maximum load, however, the modified deformation theory of plasticity J*D and the contour-integral JΓ methods gave essentially the same J-R curves for both specimen types.
A Study of Plasticity Effects on the Crack Geometry Correction Factors Used with the COD-Design Curve
Authors: Mayville RA , Hilton PD
Published: February 2009

A recent application of the COD-Design Curve to fracture data for surface defects in pipeline girth welds, revealed that use of the elastic defect geometry correction factors leads to a nonuniform factor of safety with respect to surface defect aspect ratio. Calculations were performed in this paper which show that the correction factors should be different in the presence of elastic-plastic deformation. This is a possible explanation for the observed nonuniform safety factor. Tabulated solutions based on the elastic-plastic estimation approach for the center cracked panel, single edge notched specimen, and hollow cylinder with internal, complete circumferential crack were used to study the influence of strain on the crack geometry correction factors for long defects. The calculations show that the correction factors must be greater the greater the amount of plasticity. Elastic-plastic finite-element solutions from the literature were used to study the influence of plasticity on the COD-Design Curve correction factors for shorter surface defects.
An Assessment of the Corresponding Stress-Intensity Factor for Accounting for Boundary Effects in Cracked Bodies
Authors: Smith CW , Lloyd WR , Olaosebikan O
Published: February 2009

An integrated frozen stress photoelastic and moiré interferometric method is briefly described and the results of applying the method to study boundary effects for several crack geometries in finite-thickness nearly incompressible bodies are discussed. Results confirm Benthem's analysis at the free surface but suggest a thick transitional zone through the thickness. A linear elastic fracture mechanics quantity called the corresponding stress-intensity factor is found to be a conservative way of interpreting test data near crack tips in elastic nearly incompressible materials.
A Review of Path-Independent Integrals in Elastic-Plastic Fracture Mechanics
Authors: Kim KS , Orange TW
Published: February 2009

The objective of this paper is to review the path-independent (P-I) integrals in elastic-plastic fracture mechanics which have been proposed in recent years to overcome the limitations imposed on the J-integral. The P-I integrals considered herein are the J-integral by Rice, the thermo-elastic P-I integrals by Wilson and Yu, and by Gurtin, the J*-integral by Blackburn, the Jθ-integral by Ainsworth et al., the Ĵ-integral by Kishimoto et al., and the ΔTp- and ΔT*p-integrals by Atluri et al. The theoretical foundation of these P-I integrals is examined with an emphasis on whether or not the path independence is maintained in the presence of nonproportional loading and unloading in the plastic regime, thermal gradients, and material inhomogeneities. The similarities, differences, salient features, and limitations of these P-I integrals are discussed. Comments are also made with regard to the physical meaning, the possibility of experimental measurement, and computational aspects.
Fatigue Crack Growth at Root Radii in Sheets with Variable Thickness
Authors: Ratwani MM , Kan H-P , Hsu SM
Published: February 2009

Fatigue crack growth data have been obtained for cracks at root radii in sheets with variable thickness. These test data were obtained under constant-amplitude as well as spectrum loading. Analytical stress-intensity factors were developed using mathematical techniques. The Mode I stress-intensity factors were approximately 10% lower than that of a plate with uniform thickness. Crack growth predictions for the test specimens were made using the analytical stress-intensity factors and a crack growth equation for a skin with uniform thickness. A good correlation is shown between observed and predicted crack growth behavior.
Evaluation of the KISCC Testing Procedure by Round Robin Tests on Steels
Authors: Yokobori T , Watanabe J , Aoki T , Iwadate T
Published: February 2009

Round robin test programs of the tentative test procedure for the determination of the threshold stress-intensity factor KISCC were conducted under the auspices of the 129th Committee, Japan Society for the Promotion of Science. The test programs used JIS SNCM 439 (AISI 4340) steels. Thirty-one laboratories participated in these programs. Tests were carried out with both 25.4-mm (1-in.)-thick bolt loaded wedge-opening-loading (WOL) specimens and 25.4 by 25.4-mm cantilever beam (CB) specimens in a 3.5% sodium chloride solution of distilled water.

The results of the round robin test programs showed some informations to standardize the KISCC testing procedure. For valid KISCC values to be obtained, the test must last 7000 h for a WOL specimen and 4000 h for a CB specimen. In the WOL tests, the final crack length and width ratio af/W of a specimen must be smaller than 0.8. An elastic bolt analysis of WOL specimens was recommended to obtain the valid KISCC values and the good relationship between da/dt and KI. In the CB tests, the estimated KISCC value coincided well with that in the WOL tests when at least six CB specimens were used. Slight effect of crack front curvature, test temperature, and pH of the solution on the variance of the KIf values was observed.

With these test results, the KISCC testing procedure was standardized by the Committee and entitled “Standard Test Method for Threshold Stress Intensity Factor KISCC in Stress Corrosion Cracking.”
Long-Term Creep-Crack Growth Behavior of Type 316 Stainless Steel
Authors: Jaske CE
Published: February 2009

For most engineering applications, crack-growth rates of 10−9 m/s or less are of practical interest, but this is the regime where little creep-crack growth data currently are available. Long-term creep-crack growth experiments of about 1000 h or more duration are required to develop such data. In the present study, creep-crack growth data were developed at rates between 10−11 and 10−7 m/s for Type 316 stainless steel. Three specimen configurations were used: compact type (CT); center-cracked tension (CCT); and multiple edge-notch tension (MENT). The CT and CCT specimens had long (25 to 35 mm) and the MENT specimens had short (100 to 500 μm) cracks. Creep-crack growth rate was evaluated both in terms of the linear elastic stress intensity factor K and the C*-integral. It was concluded that the C*-integral is useful for characterizing long-term as well as short-term creep-crack growth rate data for both short and long cracks on a consistent basis.
The Behavior of Short and Long Fatigue Cracks at Threshold and Near-Threshold Levels
Authors: Stephens RI , Lee HW , Bu R , Werner GK
Published: February 2009

The fatigue crack growth behavior of physically short cracks (0.2 ≤ a ≤ 1 mm), intermediate cracks (1 < a ≤ 2 mm), and long cracks (2 < a < 25 mm) at near-threshold and threshold levels was investigated with single-edge-notched-type specimens using four materials with stress ratios R of 0.05, −1, and 0.5. The materials were SAE 0030 cast steel, AISI 1020 and 1045 hot-rolled steels, and 7075-T6 aluminum. Short cracks grew much faster than long cracks based upon applied ΔK. The transition crack lengths at which there was similitude with ΔK were between 1 and 2 mm. Mean stress effects were significant at these low da/dN values for both long and short cracks. Crack closure played an important role in this behavior, but appears not to be the entire cause. Scanning electron microscopic fractographic analysis did not indicate a difference in fatigue crack growth morphology between the long and short cracks.
Crack Opening Displacement Measurements on Small Cracks in Fatigue
Authors: Larsen JM , Jira JR , Weerasooriya T
Published: February 2009

A computerized interferometric displacement gage was used to monitor the crack mouth opening behavior of naturally initiated small fatigue cracks in the high-strength alloy Ti6Al-2Sn-4Zr-6Mo. Complete load versus crack opening displacement curves were obtained for naturally initiated semicircular cracks of surface length as small as 50 µm. For a crack of this size, the maximum crack mouth opening displacement was less than 0.5 µm, and the precision of the individual displacement measurements was approximately 0.01 μm. An excellent correlation between crack mouth opening compliance and surface crack length was demonstrated, and the data were shown to agree with an analytical expression for part-through surface cracks. A matrix of experiments was performed for a range of loading conditions, and crack growth data were plotted versus the nominal ΔK and ΔKeff, which was defined using crack closure measurements. Experimental measurements obtained for a small crack indicated that closure often develops discontinuously as the crack initially extends. This behavior was speculated to result from the formation of individual asperities on the fracture surface. In general, the interferometric method proved to be very effective and demonstrated a unique capability to monitor the behavior of very small fatigue cracks.
The Measurement of Crack Closure During Fatigue Crack Growth
Authors: Allison JE
Published: February 2009

In recent years, the dominant role of fatigue crack closure in determining fatigue crack growth behavior in metals has been recognized. Attention is now turning to more precise quantification of this process and to the application of standardized measurement techniques. This paper reviews the various techniques which have been used to measure the crack opening stress intensity KOP. These include thickness-averaging compliance methods, indirect methods, various techniques which measure crack closure on the surface near the crack tip, and techniques for measuring crack closure in the interior of thick specimens. The practical and fundamental advantages and limitations of these methods are discussed. Of these techniques, the thickness-averaging compliance techniques (back-face strain and crack mouth opening displacement methods) appear to provide reproducible and practical global measurements of the average closure response and are the most logical choices for standardization. The interferometric displacement gage and direct scanning electron microscope measurements although difficult to standardize, allow more detailed studies of local crack-tip closure processes. The increased use of techniques which measure closure in the specimen interior is important for understanding the three-dimensional nature of crack closure. Finally, this paper reviews selected crack closure data measured using thickness-averaging compliance techniques in several different alloy systems. Even when such “standard” techniques are used, significant variations in crack closure are observed, and general patterns for crack closure are difficult to discern. This points to the need for further standardization of measurement techniques and numerical data analysis methods.
Alterations in Crack-Tip Deformation During Variable-Amplitude Fatigue Crack Growth
Authors: Davidson DL , Hudak SJ
Published: February 2009

Fatigue crack growth rate changes caused by single overloads and an overload followed by an underload have been studied in the high-strength aluminum alloy 7091. Crack-tip plasticity parameters were measured at each step in the loading sequence using the stereoimaging technique. Effective stress-intensity factor was measured with high resolution immediately at the crack tip both before an overload and during the subsequent growth rate retardation period. Crack-tip opening displacement and strain were altered in the same way as growth rate following the overload/underload events, and similitude with respect to crack-tip plasticity was preserved during the growth retardation period. Effective stress-intensity factor was found to correlate well with crack growth rate. Strains within the plastic zone were converted to stress at each step in load change. These stresses were then summed to determine the zone of residual stress caused by the overload. The region of compressive residual stress ahead of the crack tip was found to increase with overload severity but was limited to yield stress in magnitude. An underload shrank the spatial extent of the residual stresses caused by the overload.
On the Accuracy of Ductile Failure Assessments
Authors: Hodulak L , Blauel JG
Published: February 2009

Ductile failure assessments of cracked specimens and components performed on the basis of the J-integral and the limit load concepts are discussed with particular interest in their ability and accuracy to predict actual failure loads. Possible sources of uncertainty in the modeling of material behavior and geometry are treated separately, comparing appropriate calculations with each other and with experimental data.

Especially approximate methods for evaluating Japplied are checked against finite-element calculations and are found accurate in all cases where plane models are relevant. The influence of the stress-strain law on the results of failure predictions is examined in detail, and a procedure is proposed for an appropriate evaluation of the Ramberg-Osgood constants in the Electric Power Research Institute estimation scheme. In addition, the influence of a variation of the material J-R curve and of the crack growth model in failure assessments is demonstrated for several examples.
Computation of the Crack-Tip Energy Release Rate for Cyclic Crack Growth
Authors: Wilkening WW
Published: February 2009

The elastic-plastic finite-element method, employing the mechanical sublayer theory of plasticity, was used to compute a local crack-tip parameter which shows significant promise for characterization of the cyclic crack growth process. The proposed cyclic crack growth rate characterizing parameter is the crack tip value of the energy release rate. This crack-tip energy release rate (G0), has been shown to be equivalent to the global energy release rate (G), for the case of monotonic loading of a stationary crack. This work extends the proposed definition of the crack tip energy release rate to cyclic loading, including the important effects of crack growth.

Elastic-plastic incremental finite-element analyses of a model which represents the elastically-bounded crack-tip region for the case of small-scale-yielding (SSY) have been performed, for both stationary and growing cracks. The crack-tip energy release rate G0, computed by the simple technique described here, was compared with the magnitude of the remote elastic K-field loading which was applied at the outer boundary of the SSY model, under plane-strain conditions.

The effects of plasticity-induced crack closure upon G0 were investigated by allowing the crack tip to advance into the prior residual plastic zone in several steps, until a steady-state condition was achieved. The occurrence of crack face closure during the unloading portion of each loading cycle and of the subsequent reopening of the crack during the next loading cycle was accurately determined. From these computational results, a modified definition of G0, embodying the effects of plasticity-induced crack closure, was developed. The results of these investigations show, generally, that the crack-tip energy release rate, when appropriately defined and computed in the context of cyclic loading, crack advance and crack face closure, provides a consistent and rational description of the local driving force acting upon the tip of a growing crack. As such, G0 should prove to be a viable cyclic crack growth characterizing parameter.
Calibration of a Mode II Test Specimen
Authors: Buzzard RJ , Gross B
Published: February 2009

Mode II and Mode I calibrations of the NASA Lewis Research Center Mode II fatigue specimen were performed experimentally over crack length to specimen width ratios (a/W) of 0.5 to 0.9. Mode II displacements were measured both at the specimen notch mouth and at the intersection of the notch with the centerline of the loading pin holes. Mode I displacements were measured across the span of the specimen at the loading pins' centerline. Analytical stress intensity factor coefficients for both Mode II and Mode I are also presented.
Solubility of Pt in Nd Phosphate Laser Glass
Authors: Izumitani T , Matsukawa M , Miyade H
Published: February 2009

Solubility of Pt was measured in order to make Pt inclusion free laser glass. Pt solubility increased with decreasing temperature. The solubilities were measured in the atmosphere of O2, air and N2 and decreased in the order of O2 > air > N2. The Pt solubility of phosphate glass was higher than that of silicate glass and decreased in the order of LHG-5 > LHG-8 > HAP-3 > LHG-10 > LSG-91H. Pt deposition mechanism is considered that Pt dissolve into the melt in ionic state but some of them are easily reduced to atomic state and deposited as metallic inclusions due to supersaturation.
A New Average-Power Damage Test Facility at LLNL
Authors: Hildum EA , Rainer F , Milam D
Published: February 2009

We describe a new facility for studying damage induced by 1064-nm, 16-ns, 0.6-J laser pulses at repetition rates up to 120 Hz. This facility is built around four commercial 30-Hz Nd:YAG lasers fired under computer control. The beams from the four lasers are combined using thin-film polarizers and rotating half-wave plates to obtain a single irradiated spot on a test target. We also describe computer controlled beam diagnostics based on a commercial video frame-grabber.

This facility has been used to compile a database of damage thresholds for a variety of materials under average power conditions. Details of this database are presented in a companion paper.
A Sensor for Production Oriented Damage Testing at 1.06μm
Authors: Arenberg JW , Schoen CL , Mordaunt DW
Published: February 2009

Detection for production laser damage testing at 1.06μm may be accomplished by a single scheme of flash detection. This scheme uses the test wavelength, 1.06/OT, to detect catastrophic damage events and is sufficient to detect the plasma associated with catastrophic damage, the failure morphology typical of production damage testing.

The detection scheme consists of a fiber optic bundle used as a light pipe and a photomultiplier tube as a detector. Uhen introduced into the target area, the collection end of the light pipe is the only element of the detector that need be in proximity to the target area. Since the light pipe has a wide acceptance angle, precise location and orientation of the light pipe end is not required. Therefore, the detector may be moved in and out easily without compromising detector performance. The detector is used in conjunction with electronics that allow the user to define a non-damage level of scatter. When damage occurs and the characteristic plasma plume is formed, a comparator detects the signal exceeding the set threshold and registers a damage event.

The simplicity and low cost aspects of the detection scheme make it a good candidate for all production damage testers at l.06μm. Furthermore, the detection apparatus may be linked electronically to the test laser to shut off the test source upon detection of damage to prevent gratuitous damage to the sample under test. The incorporation of such a binary damage detector is suitable for use in an automated and, therefore, enclosed (Class I) damage tester for use on factory floors.
Scattered Light as a Laser Damage Diagnostic
Authors: Stewart AF , Guenther AH
Published: February 2009

Laser light scattering from thin film coatings was evaluated as a diagnostic indicator for an automated 1-on-l laser damage test facility. While variations in scattered light has been effectively used in bulk damage testing for many years, a dependable diagnostic of similar ilk for threshold phenomena of surfaces and thin film coatings has not been universally accepted. The data collected in this study indicates that measurement of scattered light is effective when surface damage morphologies mimic the larger scale phenomena observed in bulk damage. When individual micron diameter pits form on the surface, as can frequently be the case, this diagnostic is less reliable. In addition, when the overall scatter level from the surface is high, as in poor quality optics, the change in scattered light becomes undetectable with the onset of damage.
Laser Induced Desorption and Second Harmonic Generation from the (111) Surface of Barium Fluoride
Authors: Reif J , Nielsen HB , Semmler O , Tepper P , Matthias E , Fridell E , Westin E , Rosén A
Published: February 2009

The desorption of positive ions from the (111) surface of BaF2 induced by visible laser light with photon energies far below the bandgap (in the green (500–560 nm) and blue (410–440 nm) spectral regions) shows sharp resonances (∼ 0.01 eV). Cluster calculations indicate that there exist occupied and excitonic surface states which facilitate resonant multiphoton photoemission from the surface. This, in turn, leads to a localized positive charge-up which results in the emission of positive ions. The influence of surface states seems to be present also in air judging from experiments on surface second harmonic generation which exhibit both a pronounced surface anisotropy and a wavelength dependent second harmonic yield.
Damage to Fused Silica Windows while Under Simultaneous Exposure to Flowing Solvents and Laser Radiation at 308 nm
Authors: Mauro BR , Folryn SR , Sanders V
Published: February 2009

This paper reports the results of a study to determine the degrading effects of flowing dye solvents on the laser damage threshold of fused-silica windows at 308 nm. Thresholds were measured at the SiO2solvent interface in a test cell. Bare SiO2 tested in air at 308 nm (20 ns) typically exhibits a threshold ranging from 4 to 10 J/cm2; with the solvent cyclohexane in contact, a threshold as low as 0.3 J/cm2 was measured. The damage data indicate that window lifetime (number of shots) is independent of fluence at higher levels, and asymptotically approaches infinity at levels near threshold. Dielectric coatings were tested as possible damage-resistant barriers between the solvent and SiO2; the results show some improvement in damage threshold. When cyclohexane is replaced with the solvent dioxane, thresholds measured for SiO2 windows are within the range cited above for thresholds measured in air.