bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurement of Electrical Conductivity of Ceramic Insulators in a Fission Reactor
Authors: Shikama T , Narui M , Kayano H , Sagawa T , Endo Y
Published: March 2009

The electrical conductivity of two ceramic insulators, alumina, and silicon nitride was measured in a fission reactor under a dynamic vacuum condition. The instrumented irradiation rig was dynamically evacuated during fission reactor operation. The uppermost vacuum level attained was better than 10-2 Torr. In this experiment, we attempted to reveal effects of the gaseous environment on the measurement of electrical conductivity under ionizing irradiation. The results showed that the gaseous environment has a most hazardous effect in a certain gas pressure range.
Radiation Induced Mechanical Stresses in Polycrystalline Alumina Insulators
Authors: Wu WY , Chin BA , Zee RH
Published: March 2009

Anisotropic growth and volumetric expansion of individual alumina crystal have been demonstrated to occur as a result of neutron irradiation. In the case of a simple alumina polycrystal, the anisotropic growth has been found to generate internal or micro stresses in polycrystalline alumina leading to microcrackings. When alumina is incorporated into complex composite structures, such as niobium and alumina in a trilayer concentric cylinder, the differential swelling of the components may lead to complex configurational or macro stresses in the system. Both the micro and macro stress generation processes were modeled using a stress compliance method based on the ANSYS finite element code. The effects of irradiation conditions and operation temperature were examined. Additional relaxation due to long term thermal creep and effect of irradiation enhanced creep were also included. The overall governing equation of the internal stress generation process was solved by numerical integration. The critical failure modes of the sheath insulator were illustrated based on the numerical calculation results.
The Effect of Neutron Irradiation on the Structure and Properties of Carbon-Carbon Composite Materials
Authors: Burchell TD , Eatherly WP , Strizak JP
Published: March 2009

Carbon-carbon composite materials are used for plasma-facing applications in fusion energy devices. Next generation fusion reactors will produce high energy neutrons which damage the plasma facing materials and degrade their properties. Here the results of two irradiation experiments, HTFC-1 and -2, each containing specimens of carbon-carbon composites are described. Data are reported for the dimensional changes of the materials as a function of fluence in the range 0.5-5 dpa for an irradiation temperature of 600°C. The observed dimensional changes are analyzed in terms of the composite's architecture, fiber precursor, and graphitization temperature. Dimensional change “turnaround” behavior is observed for several of the materials. Strength is shown to increase with increasing neutron fluence for most of the carbon-carbon composites. High temperature thermal conductivity is reported for two 3D carbon-carbon composites, before and after irradiation. Irradiation reduces thermal conductivity by 60%. However, after thermal annealing at 1600°C the reduction in thermal conductivity is as little as 20%.
Irradiation Behavior of Carbon-Boron-Titanium Materials
Authors: Tanabe T , Fujitsuka M , Shinno H , Shikama T
Published: March 2009

Various compounds of carbon-boron-titanium for improving the hydrogen-retention properties and erosion resistances of the carbon materials under the high heat load from the plasma were synthesized. Thermal diffusivity of the compounds were measured. The materials were neutron irradiated in a fission reactor. Effects of neutron irradiation on the swelling of the materials were determined and presented. Thermal diffusivity data on the irradiated materials are not presented
The Role of Phase-Boundary Fission-Gas Bubbles in the Cavitational Swelling of Irradiated U-Pu-Zr Fuel
Authors: Rest J
Published: March 2009

Cavitational void swelling, the bias-driven growth of voids, has been identified as a potential swelling mechanism in the alpha-uranium phase of irradiated U-Pu-Zr metal fuels for the Integral Fast Reactor being developed at Argonne National Laboratory. The trends in U-Pu-Zr swelling data prior to fuel cladding contact can be interpreted in terms of unrestrained cavitational-driven void swelling. It is theorized that the swelling mechanisms at work in the alpha-uranium phase can be modeled by single-vacancy and single-interstitial kinetics, with phase-boundary gas bubbles providing the void nuclei, thus avoiding the use of complicated defect-interaction terms required for the calculation of void nucleation. The focus of the examination of the kinetics of fission-gas evolution, as it relates to cavitational void swelling, is on the period prior to the formation of significant interconnected porosity and on the development of small phase-boundary gas bubbles that can act as void nuclei. Calculations for the evolution of phaseboundary fission-gas bubbles show that such bubbles provide critical cavity sizes (i.e., the size above which the cavity will grow by bias-driven vacancy flux) consistent with the observed incubation dose for the onset of rapid swelling and gas release.
Thermomechanical Treatments and Hydrogen Embrittlement of Ferritic Stainless Steels with Different Interstitial Contents
Authors: Iyer RN , Hehemann RF , Troiano AR
Published: March 2009

Hydrogen embrittlement of 26Cr-lMo ferritic stainless steels, with low and high concentrations of interstitial elements of carbon and nitrogen and with high-temperature an nealing or prestraining treatments or both, was investigated. Tests involved cathodic charging of the specimens in sulfuric acid solution at room temperature, with simultaneous tensile loading using a uniaxial constant load fixture. The steel with high interstitial contents (26-1S) hydrogen embrittled intergranularly, when either heated to 1050°C and subsequently water quenched, or plastically prestrained by 5% elongation; but the low interstitial alloy (E-Brite) hydrogen embrittled transgranularly only when both of these treatments were given in this order. The cracks originated at the surface grain boundaries in 26-1S and at interior precipitate regions in E-Brite. Based on interrupted tests and fractography, the inferred hydrogen embrittlement mechanism has been stress-induced niobium hydride formation in E-Brite; whereas, this mechanism has been hydrogen trapping and absorption by nitrogen and faceted titanium carbo-nitrides in the vicinity of grain boundaries in 26-1S. Stress corrosion crack propagation of these alloys in boiling chloride solutions can be analyzed from these mechanisms and invoking potential drop concepts.
Role of the Oxide Film in the Transgranular Stress Corrosion Cracking of Copper
Authors: Cassagne TB , Kruger J , Pugh EN
Published: March 2009

Cyclic voltammetry, ellipsometry, and slow strain rate tests have been carried out on copper monocrystals in 0.1 M sodium acetate and 1 M sodium nitrite. These studies demonstrate that the presence of an oxide film is necessary but not sufficient to induce transgranular stress corrosion cracking, and that the growth rate and other characteristics of the oxide(s) are important factors. The results are discussed in terms of the film-induced cleavage model for cracking.
Role of Selective Dissolution in Transgranular Stress-Corrosion Cracking: Studies of Transient and Steady-State Dealloying in Copper-Gold Alloys
Authors: Flanagan WF , Lee JB , Massinon D , Zhu M , Lichter BD
Published: March 2009

Transgranular stress-corrosion cracking (T-SCC) was studied for disordered single crystals of Cu-25Au in 0.6 M sodium chloride in the potential range 0 to 600 mV (sce),over which exclusive selective dissolution of copper occurs and hydrogen generation is thermodynamically excluded. Under these conditions the strain-free anodic dissolution displays “passive” behavior below a critical (“transpassive”) potential, Ec. Massive dealloying occurs above this potential, leading to the formation of a gold sponge surface layer. On the other hand, scratching experiments established that transient dealloying occurs well below the critical potential, Ec = 430 mV, and straining electrode tests show that T-SCC also occurs in this same passive potential domain. In the passive domain, transient currents resulting from crack advance during T-SCC are shown to be analogous to transients observed during surface scratching in the same corrosive environment. It is also shown that dynamic straining enhances steadystate dissolution below Ec, thus indicating that crack-related deformation could lead to enhanced dissolution in the newly-formed crack. This has implications that affect our understanding of the renucleation process in TSCC.
Interactive Effects of Cold Work, Yield Strength, and Temperature on Sulfide Stress Cracking
Authors: Joosten MW , Murali JJ , Hess JL
Published: March 2009

In an effort to better define the interactions among cold work, yield strength, and temperature with respect to sulfide stress cracking (SSC), a statistically designed experiment was conducted using a 90 grade, sour service, low-alloy tubular steel. The SSC tests were conducted using stressed, notched, bent beam specimens exposed to the National Association of Corrosion Engineers (NACE) solution at various temperatures. The experimental strategy was based on a face-centered cube design, and regression analyses were used for interpretation of the data. Results indicated that the primary influences on SSC were those of yield strength and temperature and that cold work had little effect up to a prestrain of 5%. The threshold stress for a given yield strength was found to change with temperature.
Sensitivity to Sulfide-Stress Cracking at Welds in Line-Pipe Steels
Authors: Cialone HJ , Williams DN
Published: March 2009

A metallurgical investigation was undertaken to identify the factors that are responsible for sensitivity to sulfide-stress cracking (SSC) in the heat-affected zone (HAZ) of microalloyed high-strength line-pipe steels, when the HAZ does not contain regions of high hardness. Sustained-load tests were conducted in the National Association of Corrosion Engineers (NACE) laboratory sour-gas environment to determine the regions in which SSC initiated, and thereby to identify which regions were most sensitive to SSC. In every steel studied, SSC initiated in a specific region of the HAZ, usually in the region that was heated into the intercritical temperature range during welding. One steel exhibited SSC sensitivity in the region adjacent to the weld metal, which was heated above the upper critical temperature during welding, and another steel exhibited SSC sensitivity in the base metal following a laboratory normalizing heat treatment. In all of the steels that were studied, the region that exhibited sensitivity to SSC contained columbium carbonitride precipitates that were coarser than those that were present in the pipe body, which was immune to SSC as fabricated. The only steel that did not fail in the SSC experiments was not alloyed with columbium. These observations suggest that sensitivityo to SSC is related to the growth of columbium carbonitride precipitates from a size below 50 Å to a size ranging from 100 to 200 Å during welding or during some heat treatments. Thus, it should be possible to reduce sensitivity to SSC by modifying the chemical composition or the cooling rate from welding or both to suppress precipitate growth.
Factors Affecting the Susceptibility of Carbon-Manganese Steel Welds to Cracking in Sour Environments
Authors: Pargeter RJ
Published: March 2009

Weldments in structural and pipeline steels have been characterized in terms of hardness and microstructure (using light and electron microscopy) and resistance to stress corrosion cracking (SCC) in NACE TM-01-77 test solution. In addition, the propensities of the steels for pitting and hydrogen pickup in NACE solution have been determined, and stress corrosion fracture surfaces have been examined in the scanning electron microscope.

The principal factor affecting susceptibility to SCC was hardness, after which the presence of martensite was the only factor that clearly increased risk of cracking. While there were marked differences in hydrogen pickup and pitting tendency, no direct correlation with risk of cracking was observed.

It is concluded that the use of hardness limits is applicable to carbon-manganese steel weldments operating in sour environments, but that compositional ranges over which such limits are used need to be determined. Attention is also drawn to the fact that the inter/sub critical heat affected zone (HAZ) could be the most susceptible region for cracking in some steels.
Role of Environment in Elevated Temperature Crack Growth Behavior of Renè N4 Single Crystal
Authors: Khobaib M , Nicholas T , Ram SV
Published: March 2009

Sustained load creep and fatigue crack growth tests have been conducted over a temperature range of 870 to 1038°C for single-crystal (SC) Renè N4 nickel-base superalloy.Tests were conducted in both laboratory air and an inert environment of a vacuum of 10-5 torr under similar conditions to understand the role of environment in elevated temperature crack growth behavior of this material. Mini-compact tension type specimens were fabricated with orientation [001] parallel to the loading axis and [100] direction parallel to the crack growth direction. The creep crack growth rate in laboratory air at 870°C was found to be nearly an order of magnitude higher as compared to tests conducted in a vacuum of 10-5 torr. However, in fatigue at a stress ratio of 0.1 and a frequency of 1.0 Hz, the crack growth rate was found to be somewhat higher in vacuum as compared to growth rates obtained from tests in laboratory air. A number of constant K tests were conducted with different frequencies of 0.1 and 0.01 Hz and several hold-times to investigate the time-dependent effects. Fractographic studies of the fatigue crack growth specimens tested in laboratory air and vacuum did not show any distinct differences in failure mechanism. However, interdendritic failure was observed for creep specimens tested in laboratory air, while the fracture surface obtained from vacuum tests showed crystallographic features.
Environmental Influence on the Effect of a Single Overload on the Fatigue Crack Growth Behavior on a High-Strength Aluminum Alloy
Authors: Ranganathan N , Quintard M , Petit J , de Fouquet J
Published: March 2009

The delaying effect of a single overload on the 2024 T351 aluminum alloy covering a wide ΔK range has been studied in ambient air, secondary vacuum, and high purity nitrogen (N2). It is shown that for a given environment and an overload ratio, the delay behavior can be rationalized in terms of the peak stress intensity factor corresponding to the overload for the studied test conditions. At low ΔK values, the overload effect is confined to a grain. The results obtained show that while the size of the overload affected zone is not affected by the environment, the number of delay cycles at an R ratio of 0.5 in vacuum can be ten times as high as that obtained in the two other environments. At an R ratio of 0.1, at low ΔK levels, the delaying effects can be similar in vacuum and in air. The observed effects are explained on the basis of mechanisms governing constant-amplitude crack growth behavior of the studied alloy.
Keyhole Compact Tension Specimen Fatigue of Selected High-Strength Steels in Seawater
Authors: Rajpathak SS , Hartt WH
Published: March 2009

Fatigue experiments have been performed in flowing natural seawater on nine different steels with yield stress in the range 370 to 990 MPa (54 to 143 ksi) employing keyhole compact tension specimens. All experiments were at a frequency of 1.0 Hz and R = 0.5. Corrosion state was either freely corroding (FC) or cathodically polarized (CP) to ∼1.10 V saturated calomel electrode (SCE). Limited experiments were also performed in air. Seven of the steels represent relatively new technology where strengthening involves either (1) precipitation hardening, (2) microalloying and quenching and tempering, (3) control rolling, or (4) thermomechanical control processing (TMCP). The results are represented in a ΔK(ρ)-1/2 versus cycles to initiation format and are evaluated with regard to the effects of environment and cathodic polarization. Endurance limit (CP tests only) was related to tensile strength and fracture behavior.
Fatigue Crack Growth Behavior of Different Stainless Steels in Pressurized Water Reactor Environments
Authors: Amzallag C , Maillard J-L
Published: March 2009

An experimental program has been conducted in order to determine the fatigue crack growth rate (FCGR) curves of different stainless steels for nuclear pressure vessels and pipings in pressurized water reactor environments and to define reference fatigue crack growth rate curves for these materials. The parameters studied are: the steel structure, the load ratio (R), and the water chemistry. A slight, but clear effect of environment was observed for the five steels studied. The paper presents the FCGR curves and the fractographic examinations for the different testing conditions and stresses the differences between FCGR behavior of stainless steels and low-alloy steels.
Corrosion Fatigue Cracking of Chromium-Containing Steels
Authors: Harty BD , Noël REJ
Published: March 2009

Three chromium-containing steels, 3CR12, Alloy 825, and AISI 431, have been subjected to cyclic loading in air, distilled water, and low chloride containing waters. The 3CR12 steel showed significant fatigue crack growth rate (FCGR) enhancement when subjected to the aqueous environments. Fractography revealed the presence of isolated regions of intergranular and cleavage cracking that were correlated with plateaus observed on the FCGR curves. The Alloy 825 steel showed a FCGR enhancement in the aqueous environments but exhibited a decrease in FCGR with an increase in chloride content. The fracture was observed to be a mixed mode, that is, intergranular separation and transgranular cleavage. The AISI 431 stainless steel exhibited a FCGR enhancement and a low density of widely distributed intergranular features when tested in distilled water; in the chloride-containing environments, the FCGRs decreased progressively with respect to the air data. The corrosion fatigue behavior of the three steels has been discussed in terms of dissolution and hydrogen-assisted cracking.
Assessing Transverse Fillet Weld Fatigue Behavior in Aluminum from Full-Size and Small-Specimen Data
Authors: Erickson D , Kosteas D
Published: March 2009

Differences between full-size and small specimens in the fatigue performance of transverse welded aluminum joints are investigated. Residual stresses and their possible effect on fatigue strength are also discussed. In addition, comparative methods of evaluating and presenting test data are introduced.
Fatigue Crack Initiation and Growth in Tensile-Shear Spot Weldments
Authors: McMahon JC , Smith GA , Lawrence FV
Published: March 2009

Fatigue crack initiation and growth in SAE 960X steel tensile-shear spot welds were studied by sectioning companion specimens and by replicating the exposed site of crack initiation in a presectioned weldment. Constant-amplitude R = 0 and R = -1 tests, as well as variable-load history tests were performed on as-welded weldments and weldments peened (“coined”) after welding.

Approximately 50% of the total fatigue life was devoted to developing a 0.25-mm-depth crack under constant-amplitude loading in the life range 104 to 106 cycles. At lives greater than 106 cycles, this percentage appeared to increase. Similar results were found under a variable load history, although, in this case, only 40% of the life was devoted to developing a 0.25-mm-depth crack. Postweld coining increased the fatigue life by over an order of magnitude. Several analytical models for predicting the fatigue life of the tensile-shear spot weldments studied were compared.
Wide-Plate Testing of Weldments: Part I—Wide-Plate Testing in Perspective
Authors: Denys RM
Published: March 2009

Depending on the ultimate purpose of the test, many wide-plate test specimen designs have been developed to provide a quantitative framework for evaluating both the fracture initiation and the fracture propagation or arrest conditions. This paper focuses on historical developments in wide-plate testing with respect to fracture initiation behavior problems only. This paper also addresses the role, as well as the limitations, of wide-plate testing in fracture research.
Wide-Plate Testing of Weldments: Part II—Wide-Plate Evaluation of Notch Toughness
Authors: Denys RM
Published: March 2009

This paper, which is Part II of this series on wide-plate testing of weldments, reviews various wide-plate test specimen designs in relation to their intended uses. In addition, some specimen designs are discussed which are recommended for particular structural details that cannot be modeled by means of a flat uniaxial wide-plate test. Further, the preparation and testing of wide-plate specimens are reviewed. Finally, consideration is given to certain aspects of the weldment test procedure and to the interpretation and significance of wide-plate test results.
Wide-Plate Testing of Weldments: Part III—Heat-Affected Zone Wide-Plate Studies
Authors: Denys RM
Published: March 2009

This paper, Part III in this series on wide-plate testing of weldments, reviews the latest developments in wide-plate test procedures used for evaluation of the heat-affected zone (HAZ) toughness in steel weldments. In particular, emphasis is placed on the transversely loaded wide-plate test specimen in which the orientation of both the crack and the weld is transverse to the loading direction. This wide-plate test configuration is often conducted to evaluate the structural significance of low HAZ CTOD values. To this end, fatigue-precracked surface-notched wide-plate specimen tests are employed to assess this specific local brittle zone (LBZ) problem. The need for sectioning the wide-plate specimen after testing to identify whether the intended microstructures have been sampled is discussed, and examples are given to illustrate the type of information which is currently produced.
Stress Effect on Post-Weld Heat Treatment Embrittlement
Authors: Lim J-K , Chung S-H
Published: March 2009

Post-weld heat treatment (PWHT) is carried out to improve fracture toughness and to remove residual stress in the heat-affected zone (HAZ). There are some problems, such as a toughness decrease and stress-relief cracking (SRC) in the coarse-grained HAZ subject to the effect of the tempering treatment. Therefore, in this paper, the effect of the heating rate and heat input on PWHT embrittlement under applied stresses of 0, 98, 196, and 294 MPa (0, 10, 20, and 30 kg/mm2), applied to simulate residual stress in the welded HAZ of chromium-molybdenum (Cr-Mo) steel was evaluated using the crack-opening displacement (COD) fracture toughness test and observation of the fracture surfaces. The fracture toughness of welded HAZ decreased with an increase in the heating rate under no stress, but it improved with an increase in the heating rate under stress. Applied stress in welded HAZ during PWHT assisted precipitation of oversaturated alloying elements in the structure, so grain boundary failure from the welding heat input was barely evident at a heat input of 10 kJ/cm and a heating rate of 600°C/h, but it appeared at an applied stress of 294 MPa at 30 kJ/cm and 220°C/h and of 196 MPa at 40 kJ/cm and 60°C/h.
Certification Testing Methodology for Fighter Hybrid Structure
Authors: Sanger KB , Dill HD , Kautz EF
Published: March 2009

A methodology for certification testing of bolted hybrid aircraft structures was developed that accounts for inherent differences between composite and metal behavior. The effects of scatter and environment on static strength and fatigue life certification were characterized based on analytic correlation with test results of over 8000 carbon/epoxy specimens.
Impact Damage and Residual Strength Analysis of Composite Panels with Bonded Stiffeners
Authors: Madan RC , Shuart MJ
Published: March 2009

Blade-stiffened, compression-loaded cover panels were designed, manufactured, analyzed, and tested. All panels were fabricated from IM6/1808I interleafed graphite-epoxy. An orthotropic blade stiffener and an orthotropic skin were selected to satisfy the design requirements for an advanced aircraft configuration. All specimens were impact damaged prior to testing. Experimental results were obtained for three- and five-stiffener panels. Analytical results described interlaminar forces caused by impact and predicted specimen residual strength. The analytical results compared reasonably with the experimental results for residual strength of the specimens.
Failure Analysis of Bolted Joints in Composite Laminates
Authors: Jurf RA , Vinson JR
Published: March 2009

An investigation into the bolted joint strength of Kevlar/epoxy and graphite/ epoxy [0/45/90/-45]2s composite laminates is presented. The fundamental problem of a single bolt hole loaded symmetrically in tension is considered. The first objective of the study was to generalize the relationships between width, edge distance, hole size, thickness, washer diameter, and degree of lateral constraint with bolted joint strength based on experimental observations. The second objective, which is the focus of this paper, was to numerically predict the same observation using only the composite's lamina properties. The effective laminate behavior is calculated using a nonlinear laminate analysis based on lamina properties and ply orientations. The effective laminate properties are then input into a nonlinear finite-element analysis computer code to model the bolted joint. The predicted bolted joint failure regions and strengths were consistent with the experimental data, although improved nonlinear constitutive finite-element models are needed. The analysis is useful in predicting critical width, edge distance, and washer sizes needed to optimize bolted joint strength.
Matrix Toughness, Long-Term Behavior, and Damage Tolerance of Notched Graphite Fiber-Reinforced Composite Materials
Authors: Bakis CE , Simonds RA , Vick LW , Stinchcomb WW
Published: March 2009

The emergence of new toughened-matrix composites with improved durability has resulted in the need to understand the influence of increased toughness on long-term damage tolerance. Several graphite fiber-reinforced composite material systems with epoxy and thermoplastic matrices and unidirectional tape and woven cloth fiber architectures are compared in the present investigation of the fatigue response of notched laminates. Through the evaluation of damage and measurement of residual strength during fatigue damage development, it was concluded that long-term behavior and damage tolerance are controlled by a number of interacting factors such as matrix toughness, fiber architecture, loading levels, and damage types and distributions. Deficiencies associated with the prediction of long-term behavior from common measures of damage tolerance are discussed.
Investigation of Damage in Composite Laminates Using the Incremental Strain Test
Authors: Swain RE , Reifsnider KL , Vittoser J
Published: March 2009

A new test method, named the incremental strain test (IST), is introduced. In the IST a composite laminate is subjected to an “interrupted-ramp strain input,” allowing for load relaxation to occur at each increment of applied constant strain. The magnitude of this relaxation reflects the change of laminate compliance caused by the inception of time-dependent damage or viscoelastic creep. A variable quantifying the load relaxation, when plotted as a discrete function of applied strain, gives a unique indicationof damage onset, duration, and degree. Such plots are obtained for a quasi-isotropic graphite/epoxy laminate with and without interlayers. IST results are positively correlated with the results of X-ray radiography and acoustic emission. Ultimately, the IST is seen as a potential tool in the investigation of damage development in composite laminates.
Damage Development and Shear Modulus Degradation in Graphite/Epoxy Laminates
Authors: Yaniv G , Lee JW , Daniel IM
Published: March 2009

A method developed recently for nondestructive monitoring of in-plane shear modulus of composite specimens was used to detect longitudinal fatigue cracks in cross-ply composite laminates. Cross-ply graphite/epoxy specimens were subjected to tension-tension fatigue (R = 0.1) at five different cyclic stress levels: 85. 66, 53, 35, and 28% of the static strength of the specimen. In-plane shear modulus was measured periodically, and its reduction due to transverse and longitudinal cracks was compared with the reduction of the longitudinal modulus. It was found that the sensitivity of the longitudinal modulus to longitudinal cracks is very low, whereas the sensitivity of the in-plane shear modulus is approximately four times higher. The onset of longitudinal cracking can be detected clearly. The results were compared with X-radiographs of the tested specimens. Good correlation was found between the initiation and accumulation of longitudinal cracks sensed by the in-plane shear modulus and the actual cracks observed in the corresponding X-radiographs. The relation between the residual life of the cross-ply specimen and the shear modulus reduction is also discussed.
Analytical and Experimental Investigations of Composites Using Vibrothermography
Authors: Lin SS , Henneke EG
Published: March 2009

Vibrothermography uses real-time video thermography for nondestructive evaluation of a structure or component excited with mechanical vibration. During vibrothermo-graphic testing of composite materials, it has been found that conditions for the generation of a thermal pattern are strongly dependent on the frequency of excitation. A local resonance model was proposed to describe this behavior, and several experiments were performed to verify this model and to qualify the heat generation mechanism. A significant conclusion of the results is that the local resonance model is indeed the mechanics model for the frequency-dependent heat generation behavior. The experimental results also show that the majority of heat generation during vibrothermographic testing results from higher stresses and strains due to local resonance. The heat generation was affected by the combination of the principal strains and shear strain for the lower modes of resonant vibration, and it was dominated by the shear strain for the higher modes of resonant vibration.
Loading Rate Sensitivity of Open-Hole Composite Specimens in Compression
Authors: Lubowinski SJ , Guynn EG , Elber W , Whitcomb JD
Published: March 2009

This paper reports the results of an experimental study on the compressive, time-dependent behavior of fiber-reinforced polymer composite laminates with open holes. The effect of loading rate on compressive strength was determined for six graphite/matrix material systems ranging from brittle epoxies to thermoplastic, at both room temperature and 104°C (220°F). Tests were conducted by loading specimens to failure using different loading rates. The slope of the strength versus elapsed-time-to-failure curve was used to rank the material's loading rate sensitivity. All of the materials had greater strength at room temperature compared to the same material at the higher temperature. All of the materials showed loading rate effects in the form of reduced failure strength for longer elapsed time to failure. When the temperature was increased to 104°C (220°F), loading rate sensitivity was less than the same material at 21°C (70°F). However, C12000/ULTEM and IM7/8551-7 were more sensitive to loading rate than the other materials at the higher temperature. AS4/APC2 laminates with 24, 32, and 48 plies and 1.59 and 6.35-mm (1/16 and 1/4-in.) diameter holes were tested. The sensitivity to loading rate was less for either increasing number of plies or larger hole size. The failure of the specimens made from brittle resins was accompanied by extensive delaminations, while the failure of the toughened systems was by shear crippling with small amounts of delaminations. Fewer delamination failures were observed at the higher temperature. A new fixture developed for this study permitted the testing of smaller specimens while producing failures typical in large panels.
Thermoviscoelastic Characterization and Prediction of Kevlar/Epoxy Composite Laminates
Authors: Gramoll KC , Dillard DA , Brinson HF
Published: March 2009

Unidirectional fiber-reinforced plastic (FRP) composite lamina, made from Kevlar 49 fibers and Fiberite 7714A epoxy, were characterized for nonlinear thermoviscoelastic response using the time-temperature superposition principle (TTSP). Mechanical static creep and creep recovery tests were performed using strain gages on 0°, 10°, and 90° unidirectional specimens at numerous temperature and stress levels. Master curves and shift factor functions for all four orthotropic material properties (fiber direction S11, fiber/transverse coupling direction S12, transverse direction S22, and shear S66 compliances) were obtained. Results were used in a numerical procedure to predict the viscoelastic response of general laminates. Predictions are compared with actual test results on selected two, three, and four fiber direction laminates made from Kevlar/epoxy.
Influence of Dimensional Parameter of an Impact Test Machine on the Results of a Test
Authors: Revise G
Published: March 2009

The calibration of impact test machines is done by two methods. A direct method which consists to verify and control the dimensions of the machines; an indirect method which consists to compare the results of a test done with reference test pieces, between a reference machine and a machine which is to verify. The values of the geometrical parameters of the machines, have an influence on the results of a test. The present work quantifies the parameters variations and compares the results obtained with bending specimens and with charpy specimens. As a matter of fact, the influence of the dimensional parameters of machines can be finally expressed in different values of energy obtained with the test pieces, and allow the comparison of different types of specimens. The choice of reference specimen is now done by ISO and by ECISS. This present work has brought some constructive element before the choice.
Factors Influencing Accuracy of Charpy Impact Test Data
Authors: Lowe AL
Published: March 2009

The Charpy impact test was originally developed as an acceptance test for steel products. However, using the Charpy test to evaluate the integrity of nuclear reactor vessels has caused the test data to be subject to scientific evaluation. The many small errors in parameters affecting the results produced relatively large errors in any analysis of the data. The sources affecting these errors are identified and an effort is made to quantify them. The Charpy impact test is a simple and convenient test but the end result is engineering data. To provide data for scientific evaluations greater control must be placed on all phases of the testing process.
Effects of the Striking Edge Radius on the Charpy Impact Test
Authors: Naniwa T , Shibaike M , Tanaka M , Tani H , Shiota K , Hanawa N , Shiraishi T
Published: March 2009

Effects of the striking edge radius of the Charpy impact test on the absorbed energy and the abrasion of the striker were investigated. The absorbed energy of the 8mmR was higher than that of the 2mmR when the absorbed energy was above 200N-m. The cause of this difference is that the bending deformation of the test specimen and the friction between the anvil and the test specimen are large in case of the 8mmR. The abrasion of the 8mmR striker was extremely large and it is too difficult to maintain the radius dimension according to ASTM E 23.
The Effect of Fatigue Pre-Cracking Versus V-Notching on Impact Testing of Charpy Specimens
Authors: Fields BA , Low SR , Early JG
Published: March 2009

Charpy impact testing was carried out on both V-notch and fatigue pre-cracked specimens of two steels: AAR M128, and ASTM A212-65. Values of total energy absorbed, lateral expansion and shear fracture appearance were found for both types of specimen. The total energies absorbed by the fatigue pre-cracked specimens were markedly less than those by V-notch samples.

Transition temperature ranges were found for both the V-notch and pre-cracked specimens. The energy absorption results show a small increase in transition temperatures for the pre-cracked specimens. The lateral expansion results are the same in both types of specimen for the AAR M128 steel and show only a small change in the ASTM A212-65 steel. Also for the A212 steel the shear fracture appearance results can be represented by a single curve for V-notch and pre-cracked specimens. However, for the M128 steel these curves occur at lower temperatures for the pre-cracked specimens.
Precracking and Strain Rate Effects on HSLA-100 Steel Charpy Specimens
Authors: Mikalac S , Vassilaros MG , Rogers HC
Published: March 2009

Improved predictions of the ductile-to-brittle transition behavior of structures are possible using Charpy specimens that have increased constraint. This report contains the results of an investigation into the combined effects of a change in notch acuity and a change in loading rate on the transition curve of a high-strength, low alloy (HSLA) steel. Specifically, the changes in absorbed energy and ductile-to-brittle transition temperature were noted. Standard Charpy V-notch specimens were fatigue precracked to a total crack depth (notch and precrack) of approximately 0.280 cm. The specimens were subsequently tested using an impact loading rate of 5.1 m/sec over a wide range of temperatures to fully develop the transition temperature curve. The transition temperature was determined using the 50% Fracture Appearance Transition Temperature (FATT). As expected, the level of absorbed energy was lower for precracked specimens as compared to standard specimens. The transition temperature of the precracked specimens was shifted upward by approximately 40°C. Both standard V-notch specimens and fatigue precracked specimens were tested at a slower loading rate of 0.0025 cm/sec to determine the combined effects of the sharper crack tip and the change in loading rate on both the energy absorbed and the transition temperature. At the slow loading rate the absorbed energy was lower for precracked specimens while the transition temperature was shifted upward by approximately 20°C. The results were also compared with those from 1.6-cm thick dynamic tear (DT) tests. It was found that the transition curve developed through fracture appearance for the DT test was identical to that of the precracked CVN tested at the impact loading rate.
Specimen Size Effects in Charpy Impact Testing
Authors: Alexander DJ , Klueh RL
Published: March 2009

Full-size, half-size, and third-size specimens from several different steels have been tested as part of an ongoing alloy development program. The smaller specimens permit more specimens to be made from small trial heats and are much more efficient for irradiation experiments. The results of several comparisons between the different specimen sizes have shown that the smaller specimens show qualitatively similar behavior to large specimens, although the upper-shelf energy level and ductile-to-brittle transition temperature are reduced. The upper-shelf energy levels from different specimen sizes can be compared by using a simple volume normalization method. The effect of specimen size and geometry on the ductile-to-brittle transition temperature is more difficult to predict, although the available data suggest a simple shift in the transition temperature due to specimen size changes. The relatively shallower notch used in smaller specimens alters the deformation pattern, and permits yielding to spread back to the notched surface as well as through to the back. This reduces the constraint and the peak stresses, and thus the initiation of cleavage is more difficult. A better understanding of the stress and strain distributions is needed.
Fatigue Mechanics: An Assessment of a Unified Approach to Life Prediction
Authors: Newman JC , Phillips EP , Swain MH , Everett RA
Published: March 2009

The separation between fatigue crack initiation and propagation has often been defined as a macrocrack, visible in a low-power microscope. However, in the last decade, research on the small-crack effect in numerous materials has indicated that crack propagation from a microstructural defect (5 to 20 μm in size) consumes a large percentage (50 to 90%) of the total fatigue life of materials, even near the fatigue limit. Thus the initiation of a “fatigue crack” may occur early in life from defects or irregularities, such as inclusions, voids, dislocations, or slip bands. In the present paper, an assessment has been made of a total-life analysis based solely on crack propagation. The analysis is based on observations of defect sizes at initiation sites and on fatigue-crack-growth rate data for small and large cracks. The assessment was based on data from 2024-T3 aluminum alloy, 2090-T8E41 aluminum-lithium alloy, annealed Ti-6Al-4V titanium alloy, and high-strength 4340 steel under either constant-amplitude or spectrum loading. Comparisons made between fatigue lives measured on notched specimens with those computed from the total-life analysis agreed well. The computed lives generally fell within the scatterband of experimental fatigue life data.
A Probabilistic Fracture Mechanics Approach for Structural Reliability Assessment of Space Flight Systems
Authors: Sutharshana S , Creager M , Ebbeler D , Moore N
Published: March 2009

A probabilistic fracture mechanics approach for predicting the failure life distribution due to subcritical crack growth is presented. A state-of-the-art crack propagation method is used in a Monte Carlo simulation to generate a distribution of failure lives. The crack growth failure model expresses failure life as a function of stochastic parameters including environment, loads, material properties, geometry, and model specification errors. A stochastic crack growth rate model that considers the uncertainties due to scatter in the data and model misspecification is proposed. The rationale for choosing a particular type of probability distribution for each stochastic input parameter and for specifying the distribution parameters is presented. The approach is demonstrated through a probabilistic crack growth failure analysis of a welded tube in the Space Shuttle Main Engine. A discussion of the results from this application of the methodology is given.
A Multiaxial Fatigue Life Estimation Technique
Authors: Bannantine JA , Socie DF
Published: March 2009

A method to estimate the fatigue life of a component subjected to variable amplitude multiaxial loading has been developed. The strain-life approach, which achieved success in correlating the fatigue lives of components subjected to uniaxial variable amplitude loading, is used as the foundation of this approach. The method incorporates critical plane multiaxial damage models that relate fatigue damage to remote loading parameters.

A computer model was developed to implement the proposed method. Measured or estimated strain histories are used as input. Corresponding stress histories are calculated using a nonproportional cyclic plasticity model. Damage is calculated using the stress and strain histories and multiaxial damage models. The plane experiencing the maximum damage is identified as the critical plane, and the fatigue life of the component is estimated from the damage calculations on this plane.

Experimental test results were used to evaluate and verify the proposed method. The measured stress-strain response of thin wall tubes loaded in combined tension and torsion were used to verify the nonproportional cyclic plasticity model. Good correlation between predicted and measured responses was observed. Results from tests conducted on SAE 1045 steel components, loaded in bending, proportional bending and torsion, and nonproportional bending and torsion, were used to evaluate the overall method. Good correlation between predicted and actual fatigue lives was achieved.
Life Prediction of Circumferentially Grooved Components under Low-Cycle Fatigue
Authors: Hatanaka K , Fujimitsu T , Shiraishi S , Omori J
Published: March 2009

Stress-strain response was analyzed in the cross section of circumferentially notched components by the finite element method. Then, the cyclic J-integral range, ΔJ was estimated for the semielliptical surface crack and for the circumferential crack. The growth rate of the surface crack was successfully plotted against the estimated J-integral range under low-cycle fatigue loading.

Furthermore, the transition from the semielliptical surface crack to the circumferential crack, which occurred in the fatigue crack growth process in the acutely notched cylindrical component, was reasonably analyzed in terms of elastic-plastic fracture mechanics.

Finally, the low-cycle fatigue lives of the dully and acutely notched cylindrical components were satisfactorily evaluated through the analysis of the growth of the surface and circumferential cracks.
On the Prediction of the Fatigue Propagation of Semi-Elliptical Defects
Authors: Soboyejo WO
Published: March 2009

The implications of the results of recent experimental and numerical investigations of the propagation of single and multiple configurations of semi-elliptical cracks are assessed for the prediction of fatigue life in structures and components in service. Simplified methodologies for the assessment of the effects of crack closure and stress state on the propagation characteristics of semi-elliptical cracks are proposed, and numerical techniques for the analysis of coplanar and non-coplanar configurations of semi-elliptical cracks are also presented. These include the results of finite element and simplified fracture mechanics analyses in which crack growth was computed on a step-by-step basis from the Paris equation.
Analytical and Experimental Investigation of Fatigue in Lap Joints
Authors: Swenson DV , Chih-Chien C , Derber TG
Published: March 2009

A finite element model is presented that can simulate crack growth in layered structures such as lap joints. The layers can be joined either by rivets or adhesives. The crack is represented discretely in the mesh, and automatic remeshing is performed as the crack grows. Because of the connections between the layers, load is transferred to the uncracked layer as the crack grows. This reduces the stress intensity and slows the crack growth rate.

The model is used to analyze tests performed on a section of a wing spanwise lap joint. The crack was initiated at a rivet and grown under constant amplitude cyclic loads. Both experimentally observed crack growth rates and the analysis show the retardation that occurs as a result of load transfer between layers. A good correlation is obtained between predicted and observed crack growth rates for the fully developed through-thickness crack.