bluuebunny/crossref_metadata_2025 huggingface dataset:
Effect of an External Stress on Moisture Diffusion and Degradation in a Graphite-Reinforced Epoxy Laminate
Authors: Gillat O , Broutman LJ
Published: March 2009

The effect of external stress on moisture diffusion and mechanical properties degradation of a graphite/epoxy composite, Scotchply SP-313, has been investigated. Moisture gain as a function of time is determined for the composite subjected to four tensile stress levels in the range of 0 to 65 percent of its ultimate tensile strength (UTS).

Experimental techniques to determine moisture gain in a composite subjected to external stress have been developed and discussed. In addition, the residual (after drying) and the current (wet) short-beam shear strength and the flexural strength as a function of moisture content and external stress level have been determined for the composite.

The formation of environmental damage by the combined stress and humid environment conditions as well as the mode of failure in the short-beam test was investigated by scanning electron microscope (SEM) studies. Results indicated that the moisture absorption process into the graphite/epoxy composite subjected to external stress, at the levels used in this study, can be predicted effectively by Fick's second law of diffusion. Increasing the external stress level produces higher effective diffusivities, and higher initial absorption rates. A significant increase was observed in laminates subjected to more than 45 percent of their UTS and containing more than 1 percent moisture. This was attributed to early formation of cracks within the 90-deg plies caused by moisture degradation of the matrix or interface strength.

The current and residual short-beam shear strength was reduced with increase in moisture content. By enhancing the moisture penetration into the composite, the external stress accelerates the mechanical degradation process.

The flexural strength, which is a fiber-dominated property, showed a very low sensitivity to application of external stress for the levels used in the present study.
Environmental Synergism and Simulation in Resin Matrix Composites
Authors: McKague L
Published: March 2009

Recent investigations have shown that absorbed moisture lowers glass transition of epoxy resin more than indicated by currently accepted models. The resulting glass transition value limits supersonic service conditions. This limitation results because exceeding glass transition causes the “thermal spike effect” and greatly reduces the strength of graphite-epoxy. These circumstances are reviewed and used to show the importance of realistically simulating airfield humidity and flight temperatures during testing. A new, economical approach is described for simulating realistic humidities for large structures.
Influence of Moisture Absorption and Elevated Temperature on the Dynamic Behavior of Resin Matrix Composites: Preliminary Results
Authors: Maymon G , Briley RP , Rehfield LW
Published: March 2009

The objective of this research is to experimentally determine the effects of moisture absorption and elevated temperature on the dynamic flexural behavior of resin matrix composite beams. Trend information obtained under two limiting environmental conditions—dry at room temperature [25°C, 77°F] and fully moisture-saturated at 93°C (200°F)—is presented for graphite/epoxy beams of three distinct layups. The data have been obtained by transient excitation of the fundamental vibration mode of cantilever specimens. The results indicate that stiffness is reversibly altered in matrix-controlled modes of deformation and that damping characteristics are altered substantially in both fiber and matrix-controlled modes due to environmental conditioning.
Oxidative Resistance of Carbon Fibers and Their Composites
Authors: McMahon PE
Published: March 2009

Relative oxidative resistances of various commercial carbon fibers have been determined by measuring weight loss as a function of time at fixed temperature in air [typically 24 h and 588 K (600°F)]. Trace metal analyses of the same fibers have been determined with an emission spectrograph. When these results are interrelated, it is shown that the more stable polyacrylonitrile (PAN)-based fibers have lower sodium and potassium contents. It can also be seen that stability increases with increasing process temperature (that is, degree of carbonization). Since fiber stability is of significance only when the composites also have similar oxidative resistance, good composite property retention after 500 h at 588 K (600°F) has been demonstrated for Celion 6000/NR150B2 polyimide composites.
Estimating Aufwuchs Biomass
Authors: Clark JR , Dickson KL , Cairns J
Published: March 2009

Estimates of Aufwuchs biomass derived from gravimetric, phytopigment, and adenosine triphosphate (ATP) analyses are evaluated and compared with respect to each method's specificities and biases. Samples of Aufwuchs from glass slides in artificial streams treated with chlorine, copper, dichromate, sucrose, dextrose, phosphate, or nitrogen were used to compare these nontaxonomic methods as water quality assessment techniques. The correlations and limitations of the methods are discussed. ATP analysis compared very favorably with the other techniques and added another important dimension in water quality assessment. When corrected for extraction efficiency, ATP analyses provide a reliable estimate of Aufwuchs biomass. This estimate can be used in combination with other estimates of biomass to partition the community into autotrophic-heterotrophic components or viable organic carbon and non- viable organic carbon. Ratios of organic carbon estimates such as chlorophyll a organic carbon/-ash-free dry weight organic carbon (Trophic Index), chlorophyll a organic carbon/ATP organic carbon (Functional Trophic Index), and ATP organic carbon/- ash-free dry weight organic carbon (Viability Index) were used to further assess the impact of perturbations on the biomass of the Aufwuchs community.
A Review and Analysis of Some Methods Used to Measure Functional Aspects of Periphyton
Authors: Rodgers JH , Dickson KL , Cairns J
Published: March 2009

Historically, assessments of water quality using periphytic communities have centered around indexes of structure such as biomass estimates, diversity indices, cluster analyses, and species abundances and distributions. However, perturbation of a community's structural attributes may not be concomitant with functional changes, and it follows that protection of community structural integrity would not signify protection of ecosystem function. A series of model streams provided semicontrolled experimental conditions for study of nontaxonomic attributes of periphytic community structure and function (photosynthesis) in response to a variety of perturbants. Over all of the model stream experiments, measured functional values were statistically less variable (P = 0.05) than measured structural values. No significant reduction in variability of structural or functional measurements was found for the treated streams relative to untreated or reference streams. A unified effort incorporating both structural and functional measurements should yield information adequate for accurate evaluation of impacts of perturbants on periphytic communities and aquatic ecosystems.
Methods for Determining the Elastic and Viscoelastic Response of Composite Materials
Authors: Sims DF , Halpin JC
Published: March 2009

Two test methods for determining the longitudinal or in-plane shear stress-strain response of a unidirectional composite material are presented. These two methods are the uniaxial tension test on a ±45 deg laminate and the rail shear test on a 0/90 deg laminate. Although these tests are conducted on laminates, the laminates are special cases of geometric construction. It is shown that unidirectional shear properties can be found from the laminate test data. The stress-strain response for both methods agrees very well for glass-epoxy and graphite-epoxy composites. The nonisothermal viscoelastic shear response of a unidrectional composite is determined from tension creep tests on a ±45 deg laminate at temperatures of 81°, 125°, and 150° F. A master shear creep compliance curve is constructed by using the principle of time-temperature superposition. The other unidirectional creep compliances necessary for characterization of an orthotropic material under a state of plane stress are calculated from equations of micromechanics. Agreement is good for a comparison of the shear creep compliance calculated from micromechanics and the ±45 deg laminate. It was found that the behavior of the tensile relaxation modulus of the epoxy resin used in a typical graphite-epoxy composite is similar to the behavior of the epoxy resin used in a typical glass-epoxy. The creep behavior of any laminate can be predicted from laminated plate theory once the unidirectional material is characterized. Viscoelastic interconversion formulas are derived which relate the creep test, constant loading rate test, and dynamic test when the creep compliance can be approximated by a power law in time and linear viscoelasticity theory is valid.
Analysis, Testing, and Design of Filament Wound, Carbon-Carbon Burst Tubes
Authors: Reuter RC , Guess TR
Published: March 2009

Filament wound, carbon-carbon, composite tubular specimens were tested to failure by internal pressurization of a compliant rubber bladder. When the specimen length exceeded the bladder length, significant strength increases were measured relative to those measured when the specimen length was equal to or less than the bladder length. A stress analysis is performed to study the effects of this material overhang on strength measurement variations using Laplace transform techniques. An explanation for and discussion of the strength variation is included. Optimum specimen design criteria are presented which minimize data falsification and material consumption.
Elastic Torsional Buckling of Thin-Walled Composite Cylinders
Authors: Marlowe DE , Sushinsky GF , Dexter HB
Published: March 2009

The elastic torsional buckling strength has been determined experimentally for thin-walled cylinders fabricated with glass/epoxy, boron/epoxy, and graphite/epoxy composite materials and composite-reinforced aluminum and titanium. Cylinders have been tested with several unidirectional-ply orientations and several cross-ply layups. Specimens were designed with diameter-to-thickness (D/t) ratios of approximately 150 and 300 and in two lengths (L), 10 in. (25.4 cm) and 20 in. (50.8 cm).

The results of these tests were compared with the buckling strengths predicted by the torsional buckling analysis of Chao. In this analysis, the instability loads of heterogeneous anisotropic cylinders are calculated with Timoshenko's equilibrium equations. The computer program associated with the analysis seeks the solution with the lowest buckling strength by iterating on the number of circumferential buckling waves. For the cylinders considered (L/r = 3.3 and L/r = 6.1), the experimental buckling torques were approximately 85 percent of the torques predicted by the Chao analysis. In the cross-ply laminate cylinders, the stacking sequence of the plies was found to have a marked effect on the elastic buckling torque.

Reversal of the stacking sequence of the cross-plied cylinders resulted in buckling torques which differed by a factor of 2. Similar results were obtained by reversal of the direction of twist on the original stacking sequence. This observation may be important in applications where reversal of loading can occur. The size effect of linear scaling was investigated for cylinders whose dimensions were different but whose L/r and D/t ratios were the same. As expected from the analysis, the experimental buckling stress remained constant.
A Correlation Study of Finite-Element Modeling for Vibrations of Composite Material Panels
Authors: Thornton EA , Clary RR
Published: March 2009

A correlation study has been made to evaluate the effectiveness of finite-element modeling for vibrations of composite material panels. The three types of composite material panels studied were: (1) Symmetrically laminated cross-ply panels, (2) unsymmetrically laminated cross-ply panels, and (3) a 1/6-scale sandwich wing model. Correlations of finite-element predictions for frequencies and vibration modes were made with Rayleigh-Ritz analyses and experimental data.

For the symmetrically laminated panels the finite-element representation predicted vibration modes very accurately. For the unsymmetrically laminated panels it was found that predictions using uncoupled finite-element plate bending elements did not give good agreement with experimental data. The constant-chord, swept-wing model was effectively analyzed by finite-element presentations using either sandwich elements or ribs and spars.
Probabilistic Concepts in Modeling the Tensile Strength Behavior of Fiber Bundles and Unidirectional Fiber/Matrix Composites
Authors: Phoenix SL
Published: March 2009

A brief review is presented of several current probabilistic models for explaining the random tensile strength behavior of fibers, generalized fiber bundles, and fiber/matrix composites. The emphasis is on assumptions, mathematical structure, methods of analysis, and physical implications with regard to the performance of such structures.
On the Determination of Physical Properties of Composite Materials by a Three-Dimensional Finite-Element Procedure
Authors: Kang SI , Rentzepis GM
Published: March 2009

In this work the effective moduli of a composite material have been found by employing the finite-element method. The correspondence between the actual composite and its equivalent homogeneous anisotropic material is established on the requirement of equality of the total strain energies for the two media. The method is applied to the unidirectional fiber composites, the short unidirectional fiber composite, and the lamellar composite, and the results compare satisfactorily with available analytical data.
Laminate Strength—A Direct Characterization Procedure
Authors: Wu EM , Scheublein JK
Published: March 2009

Laminate failure surface is represented by a third-order tensor polynominal. Experimental determination of the strength coefficients (the failure tensors) is aided by a hybrid approach which utilizes an estimate of failure surface determined from a lamina failure criterion and laminated plate theory. The estimated laminate failure surface in turn identifies the necessary critical experiments with the consequence of minimizing experimental permutation and maximizing the reliability of laminate failure thus determined.
Effect of Temperature and Strain Rate on the Tensile Properties of Boron-Aluminum and Boron-Epoxy Composites
Authors: Meyn DA
Published: March 2009

The effects of strain rates from 8 × 10-5 to 0.4 s-1 on tensile strengths and moduli of 6-ply 6061 aluminum-46.5 percent boron at temperatures up to 1000°F (537°C) and of 10-ply unidirectional and cross-ply epoxy-50 percent boron composites at temperatures up to 500°F (260°C) have been measured. Longitudinal properties were reduced moderately at high temperatures, and were only slightly higher at high strain rates than at low strain rates. Transverse properties decreased rapidly with temperature, and were either unaffected by high strain rates, particularly at low temperatures, or were substantially increased, especially at high temperatures, in both aluminum and epoxy systems.
Effects of Moisture on the Properties of High-Performance Structural Resins and Composites
Authors: Browning CE , Hartness JT
Published: March 2009

Graphite, boron, S-glass, and DuPont's PRD-49-III fiber reinforced composites, as well as castings of current epoxy resin systems, were evaluated to determine the effects of moisture and high humidity on their physical properties and their room- and elevated-temperature mechanical properties. All of the neat resin castings were found to absorb moisture and swell. Associated with moisture absorption is a loss in elevated-temperature tensile strength. All of the composite systems showed weight gains and thickness increases when subjected to a high-humidity environment. However, the effect of absorbed moisture on the elevated-temperature mechanical properties of composites is determined principally by fiber orientation and test method employed. Unidirectional composites may show a significant reduction of 350°F (177°C) flexural strength due to absorbed moisture, whereas a multidirectional lay-up may show only a minor loss of 350°F tensile strength after equivalent moisture absorption. Fiber-controlled composite properties are relatively unaffected by absorbed moisture whereas matrix-controlled properties are adversely affected.
Geometric and Loading Effects on Strength of Composite Plates with Cutouts
Authors: Rowlands RE , Daniel IM , Whiteside JB
Published: March 2009

Stress concentration and strength of uniaxially loaded composite plates containing central circular, elliptical, and square holes are presented. Both smooth and rough holes are considered, as are the effects of relative plate width and thickness. The plates are subjected to static, cyclic, and controlled strain-rate loading. Fracture strength associated with rough holes is slightly lower than with smooth holes, while broad variations in loading history have minimal influence on strength. The tensile stress concentration factor increases and strength decreases as the hole-diameter to plate-width ratio increases. Strength decreases with decreasing ratio of laminate thickness to hole diameter.
Effect of Filament-Matrix Interdiffusion on the Fatigue Resistance of Boron-Aluminum Composite
Authors: Hancock JR , Shaw GG
Published: March 2009

The effects of diffusion across filament-matrix interfaces and of weak interfacial bonds on the axial fatigue resistance of boron-6061 aluminum composites were determined. Interfacial microstructures and composite fatigue resistance were compared with those previously determined for current, state-of-the-art boron-6061 aluminum composites. It was concluded that (a) the fracture behavior of the silicon carbide (SiC) coating on coated boron filaments preferentially degrades the low-cycle fatigue resistance of composites, (b) weak interfacial bonds extend the fatigue life of boron-6061 aluminum composites, (c) increased diffusion penetration across boron-6061 aluminum interfaces shortens composite fatigue life by a detectable but minimal amount, and (d) the fatigue resistance of current, state-of-the-art boron-6061 aluminum composites can be improved substantially
Analysis of Filament-Reinforced Spherical Pressure Vessels
Authors: Gerstle FP
Published: March 2009

An analysis of the response of a filament-wound spherical vessel to internal pressurization is presented. Based on a structural model of an elastic-plastic bladder inside a transversely isotropic elastic composite, analytic expressions for the stress, strain, and displacement fields are derived for pressures preceding, during, and after bladder yield. Equations for short-term composite failure pressures are predicted using maximum stress, maximum strain, and Norris-Ashkenazi failure criteria. Using the transversely isotropic model, laminate stiffnesses and strengths are computed from the micromechanically predicted lamina properties. The results are evaluated numerically for the PRD-49/epoxy, Thornel 400/epoxy, and Thornel 75S/epoxy systems, for a range of wall thicknesses. The Norris-Ashkenazi criterion is seen to give the most conservative failure predictions for the structure. The analysis emphasizes the need for reasonable radial compressive strengths and high filament volume fraction in obtaining a high-burst-pressure design.
Developments of a Unique Graphite/Epoxy Antenna Subreflector*
Authors: Robinson EY , Stonier RA , Lofgren CL
Published: March 2009

Advanced concepts for large, furlable space antennas have led to an extensive development program at the Jet Propulsion Laboratory (JPL) with configurations utilizing conical main reflectors. The antenna subreflectors for these conical configurations have unusual geometries and new structural requirements. Structural efficiency of the conical antennas is improved through the use of fiber composites and, as part of JPL's development program, a subreflector was designed and fabricated with graphite/epoxy material. This was sized to fit a subscale, 6-ft-diameter model. The subreflector is a cylindrical paraboloid with demanding criteria for contour surface precision, high thermal stability, and sufficient structural capacity for inertial launch loads in axial and transverse directions (12 to 18 g). In addition, dynamic launch environments impose constraints on allowable natural frequencies. This application presents broad design requirements and novel fabrication constraints. The paper describes the design, analysis, and fabrication of the subreflector.
Low-Weight, Impact-Resistant Helicopter Drive Shafts
Authors: Figge IE , Henshaw J , Roy PA , Olster EF
Published: March 2009

The advantages and disadvantages of all composite helicopter drive shafts are discussed. Previous research has shown that all graphite/epoxy torque tubes severely delaminate under low-velocity, low-energy impact tests simulating accidental “tool drop” conditions.

In response to the need for weight savings without the sensitivity to low-energy impact, two composite metal hybird drive shafts have been designed and tested. These are constructed with unidirectional (parallel to the tube axis) boron epoxy composite, for increasing the flexural stiffness, sheathed with an inner and outer metal tube, for torsional strength and damage resistance.

A limited number of ballistic and low-energy impact tests were conducted. Damage from a 12 ft-lb simulated “tool drop” test was minimal. The same tube while sustaining 1/3 of its undamaged design torque was perforated by a fully tumbled 30-caliber projectile and subsequently failed at 2/3 of its undamaged ultimate load.

The hybird drive shaft is considered to have a significant weight savings potential. As compared to an all-aluminum drive shaft, its specific flexural stiffness is increased by a factor of 2.6 and its specific critical buckling stress by a factor of 1.5. As compared to an all-boron epoxy drive shaft, its specific flexural stiffness is approximately equal and the critical buckling stress is reduced by 40 percent. In addition to the advantage of impact resistance, the hybrid designs offers the advantage of metal-to-metal attachment with conventional splicers and nuts and bolts.
Fracture Mechanics in the Elastic-Plastic Regime
Authors: Paris PC
Published: March 2009

The objective of this paper is to present, as simply as possible, an explanation of the J-integral methods of elastic-plastic fracture mechanics. Its rationale as an extension of the linear-elastic fracture mechanics is emphasized. Other methods, such as crack-opening displacement and equivalent-energy methods, are contrasted with the J-integral methods for both analysis and applications to material characterization. Finally, the broad applicability and usefulness of the J-integral methods are also emphasized.
Comparison of Compliance and Estimation Procedures for Calculating J-Integral Values
Authors: Hickerson JP
Published: March 2009

Methods for J-integral fracture toughness testing are new and, therefore, not well established. Compliance methods developed by Begley and Landes provide accurate data but require extensive specimen testing and analysis. Estimation procedures that allow the calculation of Jc from a single test are convenient to use but are of unknown accuracy. In order to compare these two techniques, compact tension specimen test data from seven alloys were analyzed by both methods. A comparison of the results showed that estimated values of Jc by the Rice, Paris, and Merkle equation agreed best with compliance data when a/w > 0.75 but always tended to underestimate Jc. An equation derived from an improved analysis by Merkle and Corten tended to overestimate Jc.
Ductile Rupture Blunt-Notch Fracture Criterion
Authors: Begley JA , Logsdon WA , Landes JD
Published: March 2009

A blunt-notch fracture criterion is proposed. The criterion is shown to be applicable for the initiation of ductile tearing in the plane-strain regime. Elastic to fully plastic behavior can be treated. Experiments have shown ductile crack initiation to occur at a J-integral value which is only a function of notch root radius. Two specimen geometries have been tested using a nickel-chromium-molybdenum-vanadium-rotor steel. The onset of ductile tearing from the notch tip was measured using the multiple specimen heat tinting resistance curve technique. Notch-root radii for blunt-notch compact-toughness specimens and center-notched panels varied from 0.10 in. (0.25 cm) to that of a fatigue precrack. Results are also presented for blunt-notch toughness tests of aluminum alloy 6061-T651 using compact-toughness specimens.
Fatigue-Crack Growth Rate Testing at High Stress Intensities
Authors: Dowling NE
Published: March 2009

Controlled load fatigue-crack growth rate tests are conducted using compact specimens of various widths between 1 and 10 in. Two engineering metals are tested: A533B pressure vessel steel and A469 alloy rotor steel. In these high-toughness materials, the upper limit of data interpretation based on linear-elastic analysis appears to be controlled by fully plastic limit load behavior. This resulted in the largest specimens yielding data at higher stress intensities than can be investigated in the smaller specimens usually employed in fatigue-crack growth rate testing. Test methods and limitations relative to plasticity are discussed, and it is recommended that deflections be monitored during load controlled testing. Some observations also are made concerning the relevance of static fracture toughness to failure following fatigue-crack growth.
Fracture Behavior of Bridge Steels
Authors: Roberts R , Krishna GV , Irwin GR
Published: March 2009

The results of a recent study of the dynamic and static fracture toughness behavior, over a wide-temperature range, of eight different bridge steels are presented. These results are discussed in connection with current toughness requirements for bridge steels. Strain rate and temperature effects on fracture toughness are examined.
Fracture Characteristics of Plain and Welded 3-In.-Thick Aluminum Alloy Plate at Various Temperatures
Authors: Nelson FG , Brownhill DJ
Published: March 2009

The tensile, notch tensile, tear, and, in some cases, fracture-toughness properties (KQ and KIc, ksi √in.) have been evaluated for 3-in.-thick 5083-H321, 5086-H32, 6061-T651, and 7005-T6351 plain plate and gas metal arc (GMA) welds in 5083-H321 (5183 filler), 6061-T651 (4043 and 5356 fillers), and 7005-T6351 (5356 filler) with no subsequent thermal treatment. The tests were made in the longitudinal, long-transverse and short-transverse orientations for the plain plate and in the “cross-weld” orientation for the welded panels. The tests were made at room temperature and generally at −100 and −320°F.

The fracture indexes, that is, notch-yield ratio and unit-propagation energy values, rank the plate and welds. The KQ and KIc values have been developed for the 6061-T651 and 7005-T6351 plate.

For the relatively tough materials in this investigation, the notch-yield ratio versus KIc relationship was not useful in estimating KIc values. However, the unit-propagation energy values versus KIc relationship does appear to provide a satisfactory means of estimating KIc values. Estimates of KIc values for all the materials tested in this program have been made from this correlation.
Fracture Toughness of Random Glass Fiber Epoxy Composites: An Experimental Investigation
Authors: Gaggar S , Broutman LJ
Published: March 2009

An experimental investigation to determine the applicability of linearelastic fracture mechanics to a randomly oriented, discontinuous fiber epoxy composite is described. Two epoxy resins differing in ductility were reinforced by a glass fiber mat such that the volume fraction of glass could be varied from 20 to 45 percent. The candidate stress-intensity factor (KQ) was measured using single-edge notched (SEN), double-edge notched (DEN), and notched bend tests (NBT). The effect of specimen thickness, notch-root radius, and a/w ratio was investigated. It is suggested that a conservative value of stress-intensity factor representative of the onset of fiber-matrix debonding (KD) be utilized for design purposes. It is shown that good correlation for KQ exists between the various types of specimens and that KQ is independent of specimen thickness. KQ is also independent of the a/w ratio for DEN and NBT specimens. It is also shown by calculation that the magnitude of the fracture energy is dominated by the fiber-matrix debonding energy and the fiber pull-out energy.
Effect of Cold Working on KIscc in a 4340 Steel
Authors: Clark WG
Published: March 2009

The effects of cold working on the stress-corrosion cracking performance of a 140 ksi (965 MPa) yield strength type 4340 steel are evaluated. Compact-toughness specimens representing 0, 10, 25, and 50 percent cold-worked material were subjected to an accelerated stress-corrosion test in hydrogen sulfide gas and the results expressed in terms of fracture mechanics parameters. The effect of mechanical strenghtening on KIscc is compared with the effect of metallurgical strengthening, and the influence of stress relieving and crack-plane orientation are considered. Results show that mechanical and metallurgical strengthening have a similar influence on KIscc, and stress relieving can improve the stress-corrosion resistance of cold-worked 4340 steel. No significant effect of crack-plane orientation on KIscc was observed.
Effect of Thickness on Retardation Behavior of 7075 and 2024 Aluminum Alloys
Authors: Chanani GR
Published: March 2009

The effect of specimen thickness on retardation behavior was investigated by using 12.7, 6.4, and 1.6-mm thick single-edge notched specimens of 7075 alloy in the T6 and T73 conditions and 2024 alloy in the T3 and T8 conditions. Single overload cycles were used with two different overload ratios, and interferometry was used to measure the surface plastic-zone size. The cyclic hardening exponents for all four materials were determined by obtaining cyclic stress-strain curves using incremental strain-cycling tests on 6.4-mm diameter cylindrical specimens. Test results indicated that the amount of retardation decreases with increasing specimen thickness. However, the relative decrease was more predominant in the 7075 alloy and at the higher overload ratio, 2.0. The effect of thickness on the retardation behavior of the 2024-T8 alloy was not conclusive. It appeared that thickness changes have only a minor effect on the retardation behavior of the 2024-T8 alloy. Fractography was used to study the micromechanisms of crack growth. Well-defined striations and changes in the striation spacings after an overload cycle were not seen in all specimens studied, and no striations were observed immediately following the overload cycle. The lack of discernable striations could be due to: abrasion, difficulty in resolving striations associated with relatively low crack-growth rates, change in crack-propagation mode, or a combination of all of these factors. However, the agreement between the measured da/dN and the striation spacings was good for all the specimens in which striations were discernable.
Spectrum Loading—A Useful Tool to Screen Effects of Microstructure on Fatigue Crack-Growth Resistance
Authors: Bucci RJ
Published: March 2009

The purpose of this paper is to promote consideration of variable amplitude or spectrum fatigue loading as a useful tool for screening effects of alloy microstructure on fatigue crack-propagation resistance. Increased sensitivity to microstructure, practical interpretation, and economy of testing are offered as rationale for advantageous use of this type of test.
Analysis of Stable and Catastrophic Crack Growth Under Rising Load
Authors: Varanasi SR
Published: March 2009

Some metals exhibit slow crack growth prior to instability under rising load. This paper is concerned with an elastic-plastic finite element plane-stress analysis of stable and catastrophic crack growth in a center-cracked panel of a ductile material, subjected to a monotonically increasing applied stress. The stable crack growth phenomenon is modeled by incorporating a local failure criterion in the stress-analysis procedure. An automatic reidealization procedure is developed to refine the finite element mesh at the new crack-tip position before external load is increased. The method is applied to study the crack-growth behavior of geometrically similar panels of different material stress-strain curves, and results are compared with experiment. The effects of some important parameters on crack growth and stability under rising load are discussed.
Rupture Characteristics of Zircaloy-4 Cladding with Internal and External Simulation of Reactor Heating
Authors: Fiveland WA , Barber AR , Lowe AL
Published: March 2009

Rupture characteristics of internally heated Zircaloy-4 nuclear fuel cladding were investigated to develop rupture data that would provide a basis to bench-mark mathematical modeling of clad rupture. Single- and five-tube configurations of pressurized water reactor cladding were ruptured at constant pressures and heating rates with simulated reactor boundary conditions. The results showed that the α + β material property transformation region had a strong effect on the material ductility, and the clad rupture temperature varied linearly with hoop stress in the α + β region. Axial distribution of multitube ruptures was not coplanar. The rupture positions on all specimens were distributed normally about the point of maximum temperature. This paper emphasizes test techniques as well as the rupture characteristics of Zircaloy-4 clad.
Deformation and Rupture Behavior of Zircaloy Cladding under Simulated Loss-of-Coolant Accident Conditions
Authors: Chung HM , Garde AM , Kassner TF
Published: March 2009

Information on the diametral expansion and rupture characteristics of Zircaloy-4 cladding has been obtained in a vacuum environment over a wide range of internal pressures at several heating rates. The effect of axial constraint of the cladding, exerted by a mandrel that simulated the pellets in a fuel rod, on the relationship between the maximum circumferential strain and the burst temperature also was investigated. The circumferential strain for unconstrained cladding was significantly larger than for axially constrained tubes, particularly for burst temperatures below ∼850°C, in which cladding remains essentially in the α-phase. Three superplastic strain peaks have been identified, namely, at rupture temperatures of ∼850, ∼1050, and ∼1220°C. In the case of complete axial restraint, the failure strains were not dependent on heating rate for burst temperatures above ∼920°C; however the low-temperature (∼850°C) strain peak increases and moves to lower temperatures as the heating rate decreases. The deformation data in this investigation also have been used to evaluate instability criteria proposed for thin-wall tubes under a biaxial stress state. The onset of plastic instability or local ballooning in the cladding has been defined in terms of the effective stress and strain during transient-heating conditions.
Influence of Metallurgical Variables on the Performance of Zircaloy Fuel Sheathing
Authors: Hardy DG , Bain AS
Published: March 2009

In 1969, analysis of fuel defects in the Douglas Point power reactor indicated that the power ramp defects tended to occur preferentially in fuel with high strength/low ductility sheathing. The sheathing specification for all CANada Deuterium Uranium (CANDU) reactor fuel was changed to a lower strength/higher ductility Zircaloy which was found to retain a greater proportion of its ductility after irradiation than the high strength/low ductility tubing. Subsequently, more detailed mechanical testing failed to substantiate the relationship between the mechanical properties and the defect behavior.

Experimental fuel irradiations were undertaken to investigate the ability of fuel with high ducitility sheathing to survive power ramps after undergoing significant burnup at low power. Defects occurred in the experimental fuel bundles regardless of the mechanical properties of the sheathing, even in bundles annealed after the low power soak, but before the power ramp. These defects are attributed to stress corrosion cracking at areas of localized stress, not tensile failure.

Brazing is used to attach appendages to the fuel sheaths. Defects have been found in the as-received material, but a majority of defects have occurred in the two braze heat affected zones near the element ends. Because nonbrazed bundles have experienced defects in similar locations, it appears the defect location is related primarily to end-flux peaking. The low overall defect rate in CANDU fuel (0.03 percent since 1972) shows that the brazing process is not affecting fuel performance adversely.

We conclude that the initial metallurgical properties of the tubing have little influence on the defect behavior of power ramped fuel; thus the limits on mechanical property requirements of the sheath have become less stringent. Good fuel management to limit maximum powers and power increases, and an interfacial layer (referred to as a CANLUB layer) between fuel and sheath have proved more effective in reducing defects.
Oxidation of Zirconium Alloys in Steam at 1000 to 1850°C
Authors: Urbanic VF
Published: March 2009

Oxygen embrittlement of fuel sheathing resulting from high temperature oxidation in steam is a potential fuel failure mechanism during a loss-of-coolant accident (LOCA). In high temperature steam, zirconium alloys form an outer layer of zirconium oxide (ZrO2) and an inner layer of oxygen-stabilized α-Zr immediately below. The metal/steam reaction for Zr-2.5Nb in the temperature range 1000 to 1600°C was studied and compared with earlier results for Zircaloy-2 and Zircaloy-4.

Reaction kinetics and the rate of growth of the combined (ZrO2 + α-zirconium) layers were measured. The reaction rates for all three alloys were similar and obeyed parabolic rate laws. The reaction rates were consistently less than those given by the Baker and Just correlation for zirconium oxidation in steam. A discontinuity was observed in the temperature dependence of the rate of growth of the combined (ZrO2 + α-zirconium) layer, and this discontinuity was attributed to a change in the oxide crystal structure. The thickness ratio ZrO2/α-zirconium also varies with temperature.

A method for estimating oxidation temperature and time from metallography, based on these observations, is discussed.
Corrosion of Zirconium-Base Alloys—An Overview
Authors: Hillner E
Published: March 2009

The corrosion and hydriding performance of zirconium-base alloys under pressurized water reactor (PWR) and boiling water reactor (BWR) conditions, as gaged by a comprehensive review of the technical literature, has been evaluated. Starting with a brief historical description of the development of zirconium for cladding and structural material in nuclear reactors and the corrosion problems associated with the use of the pure metal, it is shown that the development of zirconium-base alloys proceeded down two major paths. One development involved the zirconium-tin system and led to the development of the Zircaloys, whereas the other concentrated upon zirconium-niobium materials and produced the two major alloys of this system in use today: Zr-1Nb and Zr-2.5Nb. The corrosion data generated for each system, both in- and ex-reactor, are evaluated, and the benefits and potential problems associated with each alloy are discussed for both PWR and BWR applications. Potential areas of concern for the Zircaloy alloys in both applications include exposure temperature limitations and the formation of nonuniform accelerated corrosion products in the oxygenated irradiation environment. The zirconium-niobium alloys are found to be very sensitive to oxygen in the coolant and to prior heat treatment in ex-reactor experiments but show either minimum or negative acceleration due to the presence of neutron irradiation. Alloys that combine these two additives (for example, Zr-3Nb-1Sn and Ozhennite-0.5) do not appear to show promise as possible replacements for the Zircaloys under present-day conditions. The lack of a uniting theory for describing the mechanisms involved in the corrosion of zirconium-base alloys may hamper seriously possible future applications under different design conditions.
Embrittlement of Zircaloy-4 by Liquid Cesium at 300°C
Authors: Syrett BC , Cubicciotti D , Jones RL
Published: March 2009

Previous attempts to induce embrittlement of zirconium alloys by cesium at light water power reactor operating temperatures (300 to 350°C) have been unsuccessful, although embrittlement has been observed in the 30 to 225°C temperature range. Thus, although cesium is an abundant fission product, it has been discounted previously as a possible cause of failures of Zircaloy-clad fuel rods. In the present study, reactor-grade Zircaloy-4 tube specimens suffered embrittlement in molten cesium at 300°C as well as at 40°C. However, it appeared that embrittlement occurred only if the cesium had a low oxygen content and was contaminated with a trace of iron.
Modeling of Localized Deformation in Neutron Irradiated Zircaloy-2
Authors: Lee D , Adamson RB
Published: March 2009

The onset of plastic instability in irradiated materials is known to be governed by deformation processes that occur on a microscopic scale. In order to identify the nature of the controlling mechanisms giving rise to plastic instability, details of the deformation processes were examined for Zircaloy-2 irradiated to fluences of 1.0 to 3.0 × 1021 neutrons (n)/cm2 (E >1 MeV) and tested in inert and 0.3-torr iodine atmospheres at 322 ∼ 335°C. For irradiated materials tested in air, it was shown that plastic strain at the maximum load decreased by more than an order of magnitude relative to unirradiated material, especially under plane-stress conditions. This was due largely to concentrated shear deformation resulting from dislocation channel development. When the similar materials were tested in iodine, the load-carrying capacity was reduced further. Based on the evidence obtained by metallographic examination, a model has been proposed describing deformation behavior of irradiated plane-stress specimens by assuming consecutive formation and strain hardening of new deformation zones containing channels. Crack nucleation under an iodine atmosphere is believed to occur when the local strain at the channel zone becomes sufficiently large to break oxide films and cause intergranular cracking.
Irradiation Response of the Ordered Phase Zr3Al
Authors: Causey AR , Fidleris V , Rosinger HE , Schulson EM , Urbanic VF
Published: March 2009

The ordered face-centered cubic (fcc) zirconium-aluminum phase (Zr3Al, Ll2 type) is being evaluated as a future CANada Deuterium Uranium (CANDU) reactor pressure-tube material on the hypothesis that ordering may lead to a reduction in rates of atomic diffusion and dislocation climb, and, hence, to a reduction of in-reactor creep. Experiments have been performed to measure the effects of fast neutrons on the dimensional stability, tensile properties, and corrosion resistance, in addition to measuring the in-reactor creep rates of a Zr-8.6Al alloy.

Irradiation-induced dimensional changes (in the absence of an applied stress) occurred which differed significantly in nature from those of Zircaloy and Zr-2.5Nb. The changes appear to have been caused by a combination of growth and swelling and decrease with increasing temperature; the dimensional changes appear to level off at 0.3 percent after 2 × 1024 neutrons (n)/m2 at 600 K. The long-term creep resistance at 570 K, deduced from bent-beam stress relaxation, is expected to be better than that of Zr-2.5Nb provided that the dimensional change does in fact level off. The tensile strength and ductility are superior to those of Zr-2.5Nb, while the resistance to aqueous corrosion is as good, provided that the α-zirconium phase content is ⩽1 percent by volume.
Yield and Fracture of Biaxially Stressed Zircaloy-4 Cladding Tubes at Room Temperature and at 400°C
Authors: Dressler G , Matucha KH
Published: March 2009

Zircaloy-4 cladding tubes differing in texture only (Materials I and II were loaded simultaneously by internal pressure and axial forces at different stress ratios, α = axial stress, σa, and tangential stress, σt. Each stress ratio a was kept constant during the whole test, that is, from the beginning of elastic deformation to the fracture of specimens.

At room temperature the axial and tangential strains ϵa and ϵt, respectively, were measured as a function of σa and σt respectively (α = constant). These stress-strain diagrams were used to determine the apparent Young's moduli and the yield loci. At 400°C the yield locus was measured for Material I. Furthermore, for each stress ratio, α, those stresses σa and σt were measured at which fracture occurred. These “fracture” loci were determined for both materials at room temperature and 400°C.

Although the uniaxial data (stress ratio α = ∞) were very similar for both materials, a marked difference in yield and fracture behavior at a α ≠ ∞ was observed. Therefore, the criteria valid for isotropic materials turned out to be unsuitable for a satisfactory description of the measured data and relations.
Fracture of Zircaloy Cladding by Interactions with Uranium-Dioxide Pellets in Water Reactor Fuel Rods
Authors: Smith E
Published: March 2009

The paper summarizes the main features of a detailed theoretical study of Zircaloy cladding fracture that is caused by the growth of stress corrosion cracks which form near fuel pellet cracks as a consequence of a power increase after a sufficiently high burn up. It is shown that unless stress corrosion crack growth in irradiated Zircaloy is able to proceed at very low stress intensifications, fracture is unlikely when there are uniform friction effects at the fuel-cladding interface, when the interfacial friction coefficient is less than unity, when a symmetric distribution of fuel cracks exists, and when symmetric interfacial slippage occurs (that is, “uniform” conditions). Otherwise, the observed fuel rod failures must be due to departures from uniform conditions, and this investigation shows that a very high interfacial friction coefficient, and particularly fuel-cladding bonding, are means of providing sufficient stress intensification at a cladding crack tip to explain the occurrence of cladding fractures.

The results of the investigation focus attention on the necessity for reliable experimental data on the stress corrosion crack growth behavior of irradiated Zircaloy and for further investigations on the correlation between local fuel-cladding bonding and stress corrosion cracking.
Localized Ductility of Irradiated Zircaloy-2 Cladding in Air and Iodine Environments
Authors: Tomalin DS
Published: March 2009

A laboratory testing method which is capable of imposing plane-stress and localized plane-strain loading conditions on tubing specimens was used to examine effects of annealing temperature and test environment on the strength and ductility of neutron irradiated Zircaloy-2 tubing. Tests were performed at 350°C in both air and iodine environments on specimens previously irradiated at 327°C in a helium atmosphere to fluences ranging between 1.26 and 1.60 × 1021 neutrons (n)/cm2 (E > 1 MeV). The tensile ductility under plane-strain loading conditions depended on preirradiation annealing temperature, with recrystallized specimens showing the largest plastic strain at maximum load (6 to 10 percent) in air. By way of comparison, stress-relieved specimens showed the largest plastic strain at maximum load (4 to 5 percent) under conditions of plane-stress loading in air. An iodine environment reduced the load-carrying capacity of the cladding (plane-strain loading) and lowered the strain at maximum load to near the detection limit of the test technique (approximately 1 percent), regardless of prior heat treatment. A change in failure mode from dimpled ductile rupture to quasicleavage and fluting accompanied this loss of ductility.