bluuebunny/crossref_metadata_2025 huggingface dataset:
Political Ecology and Land Degradation: How Does the Land Lie 21 Years after Blaikie and Brookfield'sLand Degradation and Society?
Authors: Jones Samantha
Published: April 2008

The application of political ecology approaches to the study of land degradation sinceBlaikie and Brookfield's (1987) book are notably scant in the academic literature. This brief review traces the way in which the field of political ecology has evolved towards a tendency to question theextentand ‘narratives’ of degradation. It illustrates how studies that have examined thecausesof degradation have concentrated on single relationships (e.g. between land degradation and poverty or population) albeit recognising the mediating effects of other variables such as institutions; rather than examining the multiple causes operating at different scales as the political ecology approach advocates. The influence of post‐structuralism has been strong and some suggest that philosophical debates seem to be leading towards a ‘dissolution of political ecology’ (perhaps only in its early form). Yet, there are calls for a return to appreciating the influence of political economy on land degradation. Some authors note that it is possible for a revived political ecology approach to benefit from the insights of post‐structural political ecology, while still providing a basis for informed policy recommendations.
Early emergence of entecavir‐resistant hepatitis B virus in a patient with hepatitis B virus/human immunodeficiency virus coinfection
Authors: Kanada Aimi , Takehara Tetsuo , Ohkawa Kazuyoshi , Kato Michio , Tatsumi Tomohide , Miyagi Takuya , Sakamori Ryotaro , Yamaguchi Shinjiro , Uemura Akio , Kohga Keisuke , Sasakawa Akira , Hikita Hayato , Kawamura Kiyomi , Kanto Tatsuya , Hiramatsu Naoki , Hayashi Norio
Published: April 2008

The efficacy of entecavir for patients with hepatitis B virus/human immunodeficiency virus coinfection has not been fully elucidated. Here we examined a patient coinfected with both viruses in whom entecavir‐resistant hepatitis B virus appeared. The 60‐year‐old Japanese male with the coinfection received antiretroviral therapy including lamivudine. The therapy initially suppressed replication of both viruses, followed by reactivation of the hepatitis B virus alone by 2 years of therapy. He subsequently received entecavir therapy in addition to the antiretroviral regimen. After entecavir administration, the hepatitis B virus DNA level was slightly reduced, but then increased after 6 months of entecavir therapy. In the sequencing analysis of hepatitis B virus, no drug resistance‐associated amino acid substitutions were observed in the reverse transcriptase (rt) domain before antiretroviral therapy. The lamivudine‐resistant amino acid substitutions at rt173, rt180 and rt204 were detected before entecavir administration, and further the entecavir‐resistant rt202 substitution was observed after 6 months of entecavir therapy. The full‐length hepatitis B sequences showed that the viral strain derived from the patient belonged to genotype H. In summary, this report describes a patient with hepatitis B virus/human immunodeficiency virus coinfection who received entecavir therapy in addition to an antiretroviral regimen and showed the early emergence of entecavir‐resistance hepatitis B virus. In entecavir therapy for patients infected with both viruses, great care should be taken with respect to the emergence of entecavir‐resistant hepatitis B virus, especially in patients with pre‐existing lamivudine‐resistant virus.
Selective lesion of retrotrapezoid Phox2b‐expressing neurons raises the apnoeic threshold in rats
Authors: Takakura Ana C. , Moreira Thiago S. , Stornetta Ruth L. , West Gavin H. , Gwilt Justin M. , Guyenet Patrice G.
Published: April 2008

Injection of the neurotoxin saporin–substance P (SSP‐SAP) into the retrotrapezoid nucleus (RTN) attenuates the central chemoreflex in rats. Here we ask whether these deficits are caused by the destruction of a specific type of interneuron that expresses the transcription factor Phox2b and is non‐catecholaminergic (Phox2b+TH−). We show that RTN contains around 2100 Phox2b+TH− cells. Injections of SSP‐SAP into RTN destroyed Phox2b+TH− neurons but spared facial motoneurons, catecholaminergic and serotonergic neurons and the ventral respiratory column caudal to the facial motor nucleus. Two weeks after SSP‐SAP, the apnoeic threshold measured under anaesthesia was unchanged when fewer than 57% of the Phox2b+TH− neurons were destroyed. However, destruction of 70 ± 3.5% of these cells was associated with a dramatic rise of the apnoeic threshold (from 5.6 to 7.9% end‐expiratory P). In anaesthetized rats with unilateral lesions of around 70% of the Phox2b+TH− neurons, acute inhibition of the contralateral intact RTN with muscimol instantly eliminated phrenic nerve discharge (PND) but normal PND could usually be elicited by strong peripheral chemoreceptor stimulation (8/12 rats). Muscimol had no effect in rats with an intact contralateral RTN. In conclusion, the destruction of the Phox2b+TH− neurons is a plausible cause of the respiratory deficits caused by injection of SSP‐SAP into RTN. Two weeks after toxin injection, 70% of these cells must be killed to cause a severe attenuation of the central chemoreflex under anaesthesia. The loss of an even greater percentage of these cells would presumably be required to produce significant breathing deficits in the awake state.
Nuclear domains during muscle atrophy: nuclei lost or paradigm lost?
Authors: Gundersen Kristian , Bruusgaard Jo C.
Published: April 2008

According to the current paradigm, muscle nuclei serve a certain cytoplasmic domain. To preserve the domain size, it is believed that nuclei are injected from satellite cells fusing to fibres undergoing hypertrophy, and lost by apoptosis during atrophy. Based on single fibre observations in and ex vivo we suggest that nuclear domains are not as constant as is often indicated. Moreover, recent time lapse in vivo imaging of single fibres suggests that at least for the first few weeks, atrophy is not accompanied by any loss of nuclei. Apoptosis is abundant in muscle tissue during atrophy conditions, but in our opinion it has not been unequivocally demonstrated that such nuclei are myonuclei. As we see it, the preponderance of current evidence suggests that disuse atrophy is not accompanied by loss of nuclei, at least not for the first 2 months. Moreover, it has not been proven that myonuclear apoptosis does occur in permanent fibres undergoing atrophy; it seems more likely that it is confined to stromal cells and satellite cells. If muscle atrophy is not related to loss of nuclei, design of intervention therapies should focus on protein metabolism rather than regeneration from stem cells.
Effect of Misalignment on the Characteristics of Journal Bearings
Authors: Ahmed A. M. , El-Shafei A.
Published: April 2008

Due to the renewed interest in the effect of misalignment on the dynamic characteristics of journal bearings, this paper presents a study of the effect of misalignment on the dynamic characteristics of journal bearings. A 2D finite difference program is developed and used for this study. The results show that there are a total of 16 stiffness and 16 damping coefficients considering horizontal and vertical displacements and rotations. The variation of these coefficients with journal tilting (i.e., angular misalignment) is illustrated. It is shown that misalignment can have a significant effect on the bearings’ dynamic characteristics.
Probabilistic Engine Performance Scatter and Deterioration Modeling
Authors: Spieler Stefan , Staudacher Stephan , Fiola Roland , Sahm Peter , Weißschuh Matthias
Published: April 2008

The change of performance parameters over time due to engine deterioration and production scatter plays an important role to ensure safe and economical engine operation. A tool has been developed which is able to model production scatter and engine deterioration on the basis of elementary changes of numerous construction features. In order to consider the characteristics of an engine fleet as well as random environmental influences, a probabilistic approach using Monte Carlo simulation (MCS) was chosen. To quantify the impact of feature deviations on performance relevant metrics, nonlinear sensitivity functions are used to obtain scalars and offsets on turbomachinery maps, which reflect module behavior during operation. Probability density functions (PDFs) of user-defined performance parameters of an engine fleet are then calculated by performing a MCS in a performance synthesis program. For the validation of the developed methodology pass-off test data, endurance engine test data, as well as data from engine maintenance, incoming tests have been used. For this purpose, measured engine fleet performance data have been corrected by statistically eliminating the influence of measuring errors. The validation process showed the model’s ability to predict more than 90% of the measured performance variance. Furthermore, predicted performance trends correspond well to performance data from engines in operation. Two model enhancements are presented, the first of which is intended for maintenance cost prediction. It is able to generate PDFs of failure times for different features. The second enhancement correlates feature change and operating conditions and thus connects airline operation and maintenance costs. Subsequently, it is shown that the model developed is a powerful tool to assist in aircraft engine design and production processes, thanks to its ability to identify and quantitatively assess main drivers for performance variance and trends.
Performance of a Novel Combined Cooling and Power Gas Turbine With Water Harvesting
Authors: Khan J. R. , Lear W. E. , Sherif S. A. , Crittenden John F.
Published: April 2008

A thermodynamic design-point performance analysis is performed on a novel cooling and power cycle that combines a semiclosed cycle gas turbine called the high-pressure regenerative turbine engine (HPRTE) with a vapor absorption refrigeration system (VARS). Waste heat from the recirculated combustion gas of the HPRTE is used to power the VARS. Water produced as a product of combustion is intentionally condensed and harvested. A part of the VARS cooling is used to chill the gas entering the high-pressure compressor, allowing water condensation and extraction as well as large efficiency gains. The remaining cooling capacity is provided to an external refrigeration load. The cycle is modeled using zero-dimensional steady-state thermodynamics, considering conservative values of polytropic efficiencies, a conservative model for turbine blade cooling, conservative values of pressure drops for the turbomachinery, including heat exchangers, and accurate correlations for the properties of the LiBr–H2O mixture and the combustion products. The cycle is shown to operate with a thermal efficiency greater than 40% for parameters appropriate to medium sized engines, while producing about 1.5kg of water per kilogram of fuel (propane) consumed. This thermal efficiency is in addition to the large cooling effect generated in the evaporator of VARS, equivalent to 3–4% increased efficiency. The efficiency would be greater than 51% without turbine cooling bleed. The refrigeration ratio, defined as the ratio of external refrigeration load to the net work output, is found to be 0.38 for the base case. The water extracted is found to be a strong function of the recirculation ratio and low pressure compressor ratio PRc1. Based on these and prior results, which showed that the HPRTE is very compact and has inherently low emissions, it appears that this cycle would be well suited for distributed power and some vehicle applications, especially ones with associated air conditioning loads.
Oxidation of Natural Gas, Natural Gas/Syngas Mixtures, and Effect of Burnt Gas Recirculation: Experimental and Detailed Kinetic Modeling
Authors: Le Cong T. , Dagaut P. , Dayma G.
Published: April 2008

The oxidation of methane-based fuels was studied experimentally in a fused-silica jet-stirred reactor (JSR) operating at 1–10atm, over the temperature range of 900–1450K, from fuel-lean to fuel-rich conditions. Similar experiments were performed in the presence of carbon dioxide or syngas (CO∕H2). A previously proposed kinetic reaction mechanism updated for modeling the oxidation of hydrogen, CO, methane, methanol, formaldehyde, and natural gas over a wide range of conditions including JSR, flame, shock tube, and plug flow reactor was used. A detailed chemical kinetic modeling of the present experiments was performed yielding a good agreement between the modeling, the present data and literature burning velocities, and ignition data. Reaction path analyses were used to delineate the important reactions influencing the kinetic of oxidation of the fuels in the presence of variable amounts of CO2. The kinetic reaction scheme proposed helps understand the effect of the additives on the oxidation of methane.
Compression Fatigue Response for Carbon Fiber With Conventional and Toughened Epoxy Matrices With Damage
Authors: Swanson S. R. , Cairns D. S. , Guyll M. E. , Johnson D.
Published: April 2008

This paper compares the open hole compression, compression after impact, compression fatigue of open hole specimens, and compression fatigue after impact response of quasi-isotropic laminates with IM7 carbon fiber and 3501-6 and 8551-7 epoxy matrices. These matrices can be considered to be a relatively brittle and a high-toughness resin, respectively. The objective was to establish whether the improved compression after impact response associated with high toughness matrices also held after fatigue loading. The results of impact and compression fatigue tests show that residual strengths of the toughened epoxy matrix system were approximately twice that of the brittle matrix system, and that fatigue resistance after impact and of open hole specimens was generally improved.
Boundary-Value Problem for Viscoelastic Creep in Deeply Imbedded Salt Vaults
Authors: Xu Wei , Genin Joseph
Published: April 2008

In this paper we develop a mathematical model and associated boundary-value problem for the encapsulating mechanism of a deeply imbedded salt vault at the Department of Energy’s Waste Isolation Pilot Plant in Carlsbad, New Mexico. The solution of the resulting boundary-value problem, which is the subject of a follow-on paper, provides significant, accurate information for predicting the time history encapsulating mechanism of salt vaults.
Historical Aspects in Our Understanding of the Ductile-Brittle Transition in Steels
Authors: Finnie I. , Mayville R. A.
Published: April 2008

The historical development of present understanding of the ductile-brittle transition in steel is traced from the earliest studies over 150 years ago to the present day. Although understanding of this topic is still incomplete, recent work has confirmed the physical insight of early investigators.
Thermo-Mechanical Fatigue of Mar-M247: Part 1—Experiments
Authors: Boismier D. A. , Sehitoglu Huseyin
Published: April 2008

Isothermal fatigue tests, out-of-phase and in-phase thermo-mechanical fatigue tests were performed on Mar-M247 nickel-based superalloy. The experiments were conducted in the temperature range 500°C to 871°C. Results indicate that the lives differ with strain-temperature phasing and with strain rate. The results of out-of-phase thermo-mechanical tests correspond well with strain-life data of isothermal tests conducted at the peak temperature (871°C). However, the in-phase thermo-mechanical results differed depending on the strain amplitude. Significant surface and crack tip oxidation and gamma prime depletion has been observed based on metallographic and Auger Spectroscopic analyses. These changes were measured as a function of time. The environment induced changes significantly influenced the fatigue lives in isothermal and out-of-phase thermo-mechanical fatigue cases. In these cases transgranular cracking was observed. Grain boundary crack nucleation and grain boundary crack growth dominated the in-phase thermo-mechanical fatigue cases. Based on these observations the requirements for a life prediction model are outlined. The life prediction model and the predictions are given in Part 2 of this paper.
High Temperature Inelastic Deformation of the B1900 + Hf Alloy Under Multiaxial Loading: Theory and Experiment
Authors: Chan K. S. , Lindholm U. S. , Bodner S. R. , Nagy A.
Published: April 2008

The multiaxial deformation behavior of the Ni-based alloy B1900 + Hf has been studied at elevated temperatures in the range of 649–982°C. Combined tension/ torsion cyclic tests were performed on thin-wall tubular specimens under both in-phase and out-of-phase strain-controlled loading cycles. Both straining conditions resulted in stress loci of comparable magnitude, exhibiting no difference in cyclic hardening response. A phase angle was observed between the deviatoric stress and the incremental plastic strain vectors during 90° out-of-phase strain cycling, and nonproportional stress relaxation occurred under biaxial strain hold. The overall results have been used to assess the flow law, the hardening equations, and the applicability of the J2-based, elastic-viscoplastic model of Bodner-Partom for multiaxial loading conditions. The overall agreement between theory and experiment is good and discrepancies are discussed in relation to micromechanical considerations.
External Pressure Loading of Spiral Paper Tubes: Theory and Experiment
Authors: Gerhardt T. D.
Published: April 2008

A closed-form elasticity solution is developed to predict stresses and strains in spiral paper tubes loaded axisymmetrically. No assumptions are made on stress distributions through the tube wall. Thus, the solution is valid for thick-walled tubes. The validity of this solution is established by comparison with experimental results. Measured strains in tubes subjected to external pressure showed remarkable agreement with the elasticity solution. After experimental verification, the elasticity solution is used to examine stress distributions in paper tubes loaded in external pressure. In both paper and isotropic tubes, the hoop stress dominates the other three stresses. However, the hoop stress distribution in paper tubes was radically different from the isotropic case. In paper tubes: (1) hoop stress was concentrated at the outer wall, especially for thicker tubes and (2) maximum hoop stress remained constant as tube thickness was increased. These differences can be attributed to the extremely small modulus in the radial direction of a paper tube. The hoop stress distributions indicate that isotropic, thick-walled cylinder theory is inapplicable for modeling paper tubes.
Neutron Diffraction NDE for Advanced Composites
Authors: Kupperman D. S. , Majumdar S. , Singh J. P.
Published: April 2008

The strength and toughness of whisker- and fiber-reinforced composites are controlled to a large extent by the nature of the bonding between the fibers/whiskers and the matrix. An important problem in the development of metal matrix and ceramic-ceramic matrix composites is the simultaneous measurement of whisker and matrix residual stress. In certain systems, chemical bonding should be nonexistent and frictional forces at the interfaces provide the necessary link between the fiber and the matrix. In metal matrix composites, stresses in the fibers can lead to thermal-cyclic fatigue damage. Strain measurements have been acquired nondestructively for alumina/silicon carbide and titanium/silicon carbide composites by means of neutron diffraction techniques at the Argonne National Laboratory Intense Pulsed Neutron Source. Analysis of the experimental results shows that the in situ measurement of stress in the laboratory by neutron diffraction techniques is practical. The accumulated strain data could be used to calculate residual stresses in the fibers and the matrix (including stresses at the interface) simultaneously, or to determine stresses resulting from external loading.
A Constitutive Model for the Inelastic Multiaxial Response of Rene’ 80 at 871C and 982C
Authors: Stouffer D. C. , Ramaswamy V. G. , Laflen J. H. , Van Stone R. H. , Williams R.
Published: April 2008

This paper contains an extension of the uniaxial state variable constitutive model of Ramaswamy et al. (1988) to the case of multiaxial loading. The correlation between uniaxial and multiaxial loading conditions is achieved through the assumptions of material isotropy and conservation of inelastic volume. The multiaxial extension is based only on the material parameters evaluated from uniaxial loading. The research is accompanied by a multiaxial experimental program to evaluate the response of Rene’ 80 at 871°C and 982° C. Experiments in the program include torsion, proportional axial and torsion, and nonproportional loading. It was shown experimentally that there is no extra hardening from the multiaxial loading than results from uniaxial loading. Further, it is shown that the multiaxial model is successful in predicting the experimental results using only the parameters determined from the uniaxial experiments.
Acoustoelasticity in Layered Composites
Authors: Mase G. T. , Wong T. E. , Johnson G. C.
Published: April 2008

Acoustoelasticity is a nondestructive technique used for determining applied and residual stresses in structural materials. It is based on the fact that as a material undergoes deformation, the longitudinal and shear wave speeds change. This technique has been successfully used in slightly anisotropic materials such as rolled aluminum plates. The purpose of this study is to examine the acoustoelastic response in layered composites: materials which are extremely anisotropic. The acoustoelastic response of these polymers is highly nonlinear and very different than that of metals.
Quantification of Numerical Stiffness for a Unified Viscoplastic Constitutive Model
Authors: Arnold S. M.
Published: April 2008

Here the question of numerical stiffness pertaining to a unified viscoplastic constitutive model is examined. The viewpoint maintained throughout this study is the state variable approach. Stiffness is quantified by examining, analytically, the eigenvalues of the associated Jacobian matrix. Specific results, in the form of stiffness contours, for the material parameters characterizing the copper alloy NARloy-Z are presented in the associated uniaxial state space. The results indicate that the potential for numerical stiffness does exist, however the severity is highly dependent upon the location of the state point within the state space. Finally a qualitative analogy between the maximum difference in stiffness indicating eigenvalues and the G vectors of the corresponding state space is suggested.
Bodner-Partom Constitutive Model and Nonlinear Finite Element Analysis
Authors: Kolkailah F. A. , McPhate A. J.
Published: April 2008

In this paper, results from an elastic-plastic finite-element model incorporating the Bodner-Partom model of nonlinear time-dependent material behavior are presented. The parameters in the constitutive model are computed from a leastsquare fit to experimental data obtained from uniaxial stress-strain and creep tests at 650°C. The finite element model of a double-notched specimen is employed to determine the value of the elastic-plastic strain and is compared to experimental data. The constitutive model parameters evaluated in this paper are found to be in good agreement with those obtained by the other investigators. However, the parameters determined by the numerical technique tend to give response that agree with the data better than do graphically determined parameters previously used. The calculated elastic-plastic strain from the model agreed well with the experimental strain.
Physical Modeling of Ductile Fracture During Metalforming Process
Authors: Kao A. S. , Kuhn H. A.
Published: April 2008

The process of ductile fracture during metalforming was modeled by deforming plasticine specimens which contained steel-wire segments to simulate the inclusions in a fully plastic matrix. Due to the strain-rate-sensitive characteristics of plasticine, slant shearing from inclusion was found to be the dominant mechanism in fracture initiation at deformation rate of 10−4 s−1. As strain rate increased to 10−1 s−1, this shearing crack transformed into tearing cracks normal to the maximum tensile stress. These two basic modes of fracture, slant shearing and normal tearing, were further substantiated by the process of void coalescence, depending on inclusion morphology and matrix characteristics. These fracture behaviors were explained by a predictive model based on plastic instability of the deforming matrix and were evidenced by real metallic material. A qualitative criterion is proposed to depict the mode transition in ductile fracture as a function of inclusion morphology and matrix material characteristics.
Unrecoverable Strain Hardening in Torsionally Strained OFHC Copper
Authors: Field D. P. , Adams B. L.
Published: April 2008

This paper investigates the recoverable and unrecoverable components of strain hardening in OFHC copper tubing subjected to torsional strain. Individual hardening components are classified and the magnitude of each is experimentally determined. Recoverable strain hardening is defined to be the difference between the final shear stress and the yield stress measured after recovery annealing. The recoverable hardening, due primarily to dislocation pileups, accounts for about 95.5 percent of the measured strain hardening at a shear strain of 1.9. Crystal lattice rotation during shear strain accounts for a portion of the unrecoverable hardening at shear strains less than .25, but becomes a strain softening effect at shear strains above .5. The evolution of the texture is measured experimentally and analyzed using both Taylor’s and Kochendorfer’s models. Texture evolution is also simulated up to a shear strain of 2.0 using Taylor’s model. This simulation yields similar results to the measured texture in determining strain hardening caused by rotation of the crystal lattice. The softening effect of crystalline reorientation accounts for a decrease in the observable hardening of 1.5 percent at a shear strain of 1.9.
Creep After Cyclic-Plasticity Under Multiaxial Conditions for Type 316 Stainless Steel at Elevated Temperature
Authors: Murakami S. , Kawai M. , Yamada Y.
Published: April 2008

History effects of cyclic-plasticity on subsequent creep have been elucidated for type 316 stainless steel at 600°C under multiaxial states of stress. Tension-compression and circular strain paths were specified for the prior cyclic plasticity. Constant stress creep experiments under simple tension, simple torsion, and combined tensiontorsion were first performed after uniaxial tension-compression cycles stabilized under a constant total strain amplitude. Then, in order to elucidate the path shape effects of prior strain cycles, the subsequent creep curves under uniaxial tension were compared for the uniaxial tension-compression and the non-proportional circular strain cycles which stabilized at identical stress amplitudes. The experimental results showed that the prior tension-compression cycles induced the anisotropy in creep behavior; creep resistance which was initially isotropic was enhanced in torsional direction, while it was decreased in tensile one. Another significant observation was that the circular strain cycles showed much larger hardening effect on creep than the tension-compression cycle. Regarding the creep flow direction, the effect of the prior cycles was negligible.
Cumulative Damage in Composites
Authors: Whitworth H. A.
Published: April 2008

The problem of cumulative fatigue damage in composites is analyzed based upon the development of a phenomenological damage model. In this modeling, a damage function is defined based on the degradation of the residual stiffness and used to predict the remaining life of composite specimens subjected to dual stress level fatigue loading. Available experimental data for graphite/epoxy composites are compared with the predictions of the present model.
High Temperature Creep Damage Under Biaxial Loading—Part I: Experiments
Authors: Trivaudey F. , Delobelle P.
Published: April 2008

The respective influences of the Von-Mises equivalent stress and of the maximum principal stress on the high temperature creep damage of two industrial alloys (INCO 718 and 17–21 SPH stainless steel) are pointed out in a quantitative way through tensile-torsion biaxial tests. Through inversions of the shear component, the important part taken by the principal direction corresponding to the maximum principal stress is also shown. Opposite results are observed according to whether the alloy suffers cyclic hardening as 17-12 SPH does or cyclic softening which is the case of INCO 718. These results are supported by metallographic observations. They demand an anisotropic form for the damage variable D, while besides a time dependence kinetics equation must include the part taken by the strain. The bases of the formulation are described in Part II.
Dislocation Substructures and Nonproportional Hardening
Authors: Doong Shiing-Hwa , Socie D. F. , Robertson I. M.
Published: April 2008

The dislocation substructures created in 1100 aluminum, OFHC copper, and type 304 and 310 stainless steels by in-phase (proportional) and 90 deg out-of-phase (nonproportional) tension-torsion cyclic loading were examined with a transmission electron microscope. Multislip structures (cells and subgrains) are observed in aluminum under both in-phase and 90 deg out-of-phase tension-torsion loading. For copper and stainless steel, single-slip structures (planar dislocations, matrix veins, and ladders) are observed after proportional loading, whereas multislip structures (cells and labyrinths) are observed after nonproportional loading. The increased cyclic hardening of copper and stainless steels under nonproportional loading is attributed to the change of dislocation substructures. Based on these observations, formulation of a nonproportionality parameter for constitutive modeling is discussed.
On the Calculations of the Stored Energy of Cold Work
Authors: Aravas N. , Kim K-S. , Leckie F. A.
Published: April 2008

When a metal deforms plastically, most of the mechanical energy expended in the deformation process is converted into heat and the remainder is stored in the material. A method for the calculation of the stored energy from an experimentally determined load-displacement curve of an elastic-plastic structure is presented. The method is applied to the problem of simple tension of a polycrystalline metal and a simple technique for the calculation of the stored energy from the uniaxial stress-strain curve is presented.
In-Phase and Out-of-Phase Multiaxial Fatigue
Authors: Ellyin F. , Golos K. , Xia Z.
Published: April 2008

In this investigation, thin-walled circular cylindrical specimens fabricated from a low alloy pressure vessel steel (ASTM A-516 Gr. 70) were subjected to various multiaxial loading conditions. The tests were conducted under strain-controlled condition, and loading was provided through an axial actuator and internal and external pressure across the specimen wall. Four in-plane strain ratios (ρ = Δεt/Δεa) were tested, and the most damaging case was the equi-biaxial in-plane straining, ρ = 1. For the latter condition, 90 deg out-of-phase loading was also investigated. These tests indicated a dramatic decrease in the number of cycles to failure, Nf, as a result of out-of-phase loading. The influence of the plastic strain path on life is thus clearly demonstrated. It is shown that the total strain energy density, ΔWt = ΔWe+ + ΔWp, correlates with both the in-phase and out-of-phase cyclic tests, and therefore is a proper damage parameter to be used for life predictions. A brief description of how ΔWt can be calculated is given for the case of proportional loading. The predicted results are compared with the experimental data, and the agreement is found to be very good indeed.
Stress Intensity Factors for a Cracked Stiffened Sheet With Cracked Stiffeners
Authors: Nishimura T.
Published: April 2008

The stress intensity factors are calculated for a cracked sheet and cracked stiffeners attached with rivets. The approach adopted is based on compatibility of displacements among the sheet, fasteners, and stiffeners. Displacements in the cracked stiffener are determined by adding the induced displacements from cracks to intact stiffener displacements, which are obtained using empirical stress intensity equations. In addition, displacements in the cracked sheet are calculated using the basic solution of a single stiffener for analyzing any combinations of stiffener (either intact, broken, or cracked). Bending flexibilities of the sheet and stiffeners are ignored, and the analysis is conducted within the limitations of elasticity theory. The proposed method is verified by comparing it to numerically computed results from finite element analysis.
Modeling of Plane Strain Fatigue Crack Closure
Authors: Sehitoglu Huseyin , Sun Wei
Published: April 2008

Mechanisms and models proposed for plane strain fatigue crack closure are evaluated. A mechanism based on out-of-plane plastic strain component, εzp, in plane strain is shown not to be adequate in explaining closure over a wide range of applied load levels. In the second model, partial relief of compressive stresses in front of the crack tip upon crack advance is forwarded as responsible for crack closure in plane strain. It is argued that this model would hold only if the crack advanced into a compressive stress zone which is highly improbable. A third model based on compressive strain accumulation in the x-direction, εxp, (transverse or crack growth direction) is studied. Material ahead of the crack tip contracts in the transverse direction and this mechanism provides residual material for crack surfaces as the crack advances. Stress-strain history and material displacements as crack advances are presented for plane strain conditions that lend further support to the third model. The results are obtained with a specialized finite element analysis with provisions for crack advance and crack closure. The crack opening load corresponding to relief of compressive residual stresses behind the crack tip is determined for plane stress and plane strain cases under R= − 1, 0 and 0.3 loading. The load at which stresses ahead of the crack tip become tensile, Pt, is also determined for plane stress and plane strain conditions and is found to exceed the crack opening load in all cases. The relevance of this parameter on fatigue crack growth behavior is discussed.
Study of the Effect of Deformation on the Axes of Anisotropy
Authors: Kumar A. , Samanta Shyam K. , Mallick K.
Published: April 2008

Many metal forming operations, such as rolling and tube drawing, are known to induce orthotropic anisotropy. The change of axes of orthotropy with subsequent deformation has been studied in this paper. The change in the orthotropy directions is of great importance for understanding and interpreting the subsequent yield behavior of metals. Based on Hill’s hypothesis that the orthotropy axes coincides with the principal directions of stretch, the change in orthotropy directions has been studied theoretically and experimentally. Since the grain shape and its direction of elongation is a good indicator of the principal stretches and its directions, it has been used as an experimental means of determining, not only the directions of principal stretches in an as received material, but also to determine approximately the deformation it has undergone so far from a reference state. A fully annealed isotropic state is chosen as the reference state. The directions of the axes of anisotropy, induced as a result of finite deformation applied to this reference state, are characterized in terms of the principal directions of the Cauchy’s deformation tensor. An experimental scheme has been developed to determine the varying directions of orthotropy for comparison with the theoretical model.
On the Fatigue at Very High Frequency — Part I: Theoretical and Variational Formulation
Authors: Xiangan Kong , Saanouni K. , Bathias C.
Published: April 2008

This first part of our study is concerned with the theoretical and variational formulations of the problem of elastic cyclic loading at very high frequency (or acoustic fatigue). The problem is treated by using the theory of longitudinal thermoelastic wave motion in a finite medium with and without running crack. Two methods are used to formulate the evolution problem: the first one deals with the use of classical time integration schema, and the second uses the Fourier transformation to solve the evolution problem in the frequency space. Comparison of our results with some closed form solutions of some classical problem is presented. In a second paper this method is used to calculate thermo-mechanical fields in specimens used in ultrasonic fatigue tests (endurance tests and crack growth tests).
Fatigue Crack Growth in Mixed Mode Loading
Authors: Chen Wei-Ren , Keer L. M.
Published: April 2008

In this paper, a direct approach based on the mixed mode Dugdale model, the accumulated plastic displacement criterion for crack propagation and the cyclic J-integral concept is used to develop equations to predict the mixed mode fatigue crack growth. A fourth-power stress intensity factor crack growth equation and a second-power cyclic J-integral equation are developed under conditions of mixed mode loading and small-scale yielding. By comparing the present analysis with the experimental results, the proposed fatigue crack growth rate equations lead to results that appear to yield reasonable magnitudes for various loading conditions.
Biaxial Low Cycle Fatigue of Unaged and Aged 1Cr-1Mo-1/4V Steels at Elevated Temperature
Authors: Sakane M. , Ohnami M. , Sawada M.
Published: April 2008

This paper describes the biaxial low cycle fatigue of unaged and aged 1Cr-1Mo-1/4V rotor steels at 823 K in air. Combined push-pull and reversed torsion tests were carried out on these steels and the biaxial fatigue data were obtained. Aging significantly reduced the hardness of the steel but had no effect on the crack direction in biaxial low cycle fatigue. Aging also had no effect on the data correlation using the biaxial strain parameters, but had a significant effect on the data correlation using the biaxial stress parameters. The change in the effective stress parameter in correlating the biaxial fatigue data due to aging is discussed in connection with the material softening.
A Rate-Dependent Inelastic Constitutive Model—Part II: Creep Deformation Including Prior Plastic Strain Effects
Authors: Xia Z. , Ellyin F.
Published: April 2008

In this part, a constitutive model for creep deformation is presented, which includes the effect of prior plastic straining. The model is capable of predicting the influence of plastic or creep prestrain and step loading on the subsequent creep deformation as well as cross effect between prestrain and creep strain directions in a multiaxial stress state. The predictions of the model are compared with the experimental data and the agreement is found to be very good. This model, in conjunction with that presented in Part I, would enable one to predict material response to complex loading sequences in the creep-plasticity range.
Transient and Residual Thermal Strain-Stress Analysis of GMAW
Authors: Tekriwal P. , Mazumder J.
Published: April 2008

A three-dimensional transient thermomechanical analysis has been performed for the Gas Metal Arc Welding process using the finite element method. Because the heat generated due to elasto-visco-plastic straining in welding is negligible in comparison to the arc heat input, the thermomechanical analysis is uncoupled into two parts. The first part performs a three-dimensional transient heat transfer analysis and computes entire thermal history of the weldment. The second part then uses results of the first part and performs a three-dimensional transient thermo-elastoplastic analysis to compute transient and residual distortions, strains and stresses in the weld. The thermomechanical model incorporates all the thermophysical and mechanical properties of the material as functions of temperature. Boundary conditions used in the numerical simulation are quite general and are matched with the experiment carried out to measure transient strains in the mild steel (0.22 percent carbon steel) weld. Good qualitative agreement was achieved between calculated and measured transient strains.
Elastic Wave Propagation in Laminated Composite Plates
Authors: Karunasena W. M. , Bratton R. L. , Datta S. K. , Shah A. H.
Published: April 2008

A stiffness method and an analytical method have been used to study the dispersion characteristics of guided waves in laminated composite plates. Both cross-ply and angle-ply plates have been considered in the analysis. The objective of the study is to analyze the effect of fiber orientation, ply layout configuration, and number of layers on the dispersion characteristics. A Rayleigh-Ritz type of approximation of the through-thickness variation of the displacements that maintain continuity of displacements and tractions at the interfaces between the layers has been used in the stiffness method. The analytical method solves the exact dispersion relation of the laminated plate by using the Muller’s method with initial guesses obtained through the stiffness method. Both methods are applicable to plates with arbitrary number of layers having distinct mechanical properties. Numerical results presented show strong influence of anisotropy on the guided waves.
A Constitutive Model for Metals Subjected to Cyclic Creep
Authors: Gong Z. L. , Hsu T. R.
Published: April 2008

This paper presents a constitutive model which can be used to predict creep strains and rupture lives of engineering materials undergoing entire creep process, including primary, secondary, and tertiary stages. The model is constructed on the basis of two evolution rules: (1) the continuum damage evolution, and (2) the evolution of internal stresses. It can be used for materials subjected to either static or cyclic creep loadings. Excellent correlations of computed results from the proposed model and those measured from specimens made of 316L stainless steel and 2024-T4 aluminium alloy were obtained.
Machining of Graphite/Epoxy Composite Materials With Polycrystalline Diamond (PCD) Tools
Authors: Ramulu M. , Faridnia M. , Garbini J. L. , Jorgensen J. E.
Published: April 2008

Machining of graphite/epoxy composite material is investigated by turning (cutoff) tests using different grades of polycrystalline diamond (PCD) inserts. The wear behavior of the PCD cutting edge is characterized by small cracks, rounded edges, and flank wear. The flank wear growth rate was found to depend on the microstructure of the tool and machining time. The coarser the PCD grade was, the better the wear resistance. The machined surface characteristics were evaluated by analyzing the surface roughness data and by scanning electron microscopy inspection of machined surface textures. Surface quality was found to get better with the cutting time.
The Effect of Proof Loading on the Fatigue Behavior of Open Link Chain
Authors: Tipton S. M. , Shoup G. J.
Published: April 2008

Open link lifting chain is routinely proof loaded during manufacture. However, the effect of residual stresses imposed by this operation on the fatigue strength of the chain has not been quantitatively investigated. This paper discusses the results of constant amplitude fatigue tests on open link chain segments which have received proof loading at various levels. The chain was initially heat treated to relieve manufacturing residual stresses and then proof loaded at levels ranging from 0 to 82 percent of its break strength. Tests were performed at two different mean loads and four different load amplitudes. Failure site trends are noted as a function of applied loading and are correlated with results of a finite element stress analysis. Residual stresses are estimated using strains measured from strain gages placed at critical locations on individual links during the proof load operation. Residual stress estimates are used with standard fatigue damage parameters to estimate the fatigue life of the chain and predictions are compared to data. Proof loading was shown to substantially increase the fatigue life of the chain. Residual stresses can explain the increase in fatigue life. Neuber’s rule demonstrated the ability to model the data trends.
SQUID NDE Apparatus and Magnetic Calibration to Detect Degradation in Duplex Stainless Steel
Authors: Evanson S. , Otaka M. , Hasegawa K.
Published: April 2008

We report on apparatus for the magnetic material characterization of a wide range of steel grades of high and low magnetic permeability. The instrument has been used to detect thermal aging in Duplex steel by the measurement of the residual magnetization (Br) of test specimens. A method to estimate the value of Br by remote measurements up to 200 mm from a specimen is described. The results are compared with data from conventional hysteresis loop measurements.
A Micro-Mechanics Model for In-Situ Compression Strength of Fiber Composite Laminates
Authors: Swanson S. R.
Published: April 2008

Compression failure in fiber composites is of much interest, and is often a limiting factor in applications because of the lower compressive strength relative to tensile strength. In the present work a model for compression failure of fiber composite laminates is developed. This model is based on the assumption of fiber microbuckling similar to other models in the literature, but in addition includes the support offered by the plies in the overall laminate that are adjacent to the critical axial plies. The result is that the apparent strength of the axial plies depends on the total laminate layup. The model predictions are seen to be in reasonable agreement with recent experimental data. Initial fiber waviness is included and seen to be a dominant factor in the results.
Ultrasonic Evaluation of Thermal Fatigue of Composites
Authors: Kasap S. O. , Yannacopoulos S. , Mirchandani V. , Hildebrandt J. R.
Published: April 2008

Results are presented on the evaluation of thermally fatigued E-glass composite specimens using ultrasonic techniques. The changes of the acoustic velocity and the attenuation coefficient in the ultrasonic range have been examined as a function of accumulated damage in various thermally fatigued composites. Three types of E-glass fiber reinforced composites have been used in this study having the following fiberglass weaves: (a) randomly oriented short fiber, (b) uniaxial fiber mat with continuous fiber bundles, (c) bidirectional continuous glass fiber mat. Representative specimens were subjected to thermal fatigue by thermal cycling between 25°C and 75°C. The structural integrity of these specimens was evaluated by determining the flexural strength after a number of thermal cycles via three point bending tests. It was observed that both the acoustic velocity and the flexural strength decreased whereas the ultrasonic attenuation increased with the number of thermal cycles which have been attributed to an accumulation of defects in the material and especially to debonding at the fiber-matrix interface. It has been observed that the use of ultrasonics may be very appropriate in evaluating composites in service since a good correlation has been noted between changes in the flexural strength, acoustic velocity and the attenuation coefficient in thermally fatigued composites.