bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental Techniques for Dynamic Characterization of Composite Materials
Authors: Greif Robert , Hebert Benjamin
Published: February 2008

This research combines theoretical and experimental approaches for dynamic material characterization of composite materials. The samples studied include continuous fiber graphite/epoxy beams with various symmetric lay-up configurations. Included are laminated beams with the following lay-ups: [08/908]s, [908/08]s, [(45/0/−45)5]s and [(0/45/0/−45)3/90/0/01/2]s. The resonant dwell technique is used to determine the material damping and the real part of the dynamic flexural modulus of double cantilever beam specimens in the first mode of vibration over the frequency range 25 Hz to 300 Hz. The dynamic properties are determined as a function of the frequency of oscillation at room temperature. In addition, the Metravib Viscoanalyzer, based on off-resonance tests, is also used to provide another source of experimental data for comparison. Although the Viscoanalyzer was originally intended for testing viscoelastic polymers, the present research establishes the limits of applicability for composite materials, with particular emphasis on the three point bending test. Comparisons and limitations of both techniques are critically discussed.
A Constitutive Model for Commercially Pure Titanium
Authors: Sheikh-Ahmad J. Y. , Bailey J. A.
Published: February 2008

Flow stress data for CP titanium were obtained using the torsion test in the temperature range from ambient to 750°C and at shear strain-rates in the range from 0.192 to 122.0 s−1. The temperature rise because of deformation heating was calculated numerically. It was found that the temperature rise is insignificant for strain rates below 0.192 s−1 where the deformation conditions are essentially isothermal while for strain rates above 27.0 s−1 deformation conditions are adiabatic. Large temperature gradients exist in the longitudinal direction of the specimen with the maximum temperature occurring at the middle of the gage length. The temperature gradients in the radial direction are much smaller and can be neglected for most practical applications. Isothermal shear stress-shear strain curves for CP titanium were obtained from the experimental torsion test data by accounting for the temperature rise produced by deformation. It was found that the data exhibits a good fit to a Johnson-Cook relationship with exponential thermal softening. The work emphasizes the importance of the need to consider the role of deformation heating when interpreting data on the effect of strain rate and temperature on the mechanical properties of materials.
A Nonlinear Kinematic Hardening Model for Elastoplastic Deformations in Grey Cast Iron
Authors: Josefson B. L. , Stigh U. , Hjelm H. E.
Published: February 2008

A kinematic hardening model including an associated flow rule is proposed for elastoplastic deformations in graphitic grey cast iron. Quantitatively good results are obtained when comparing with previously performed biaxial experiments. Use of a nonassociated flow rule is found to result in an undesirable weakening behavior that can be explained as a deficiency with the combination of kinematic hardening and the present choice of yield potential. The model proposed is also extended to include multilinear kinematic hardening. With this model qualitatively good agreement with experimental cyclic results from the literature is obtained. A three-dimensional FE-analysis of a cylinder head for a heavy duty Diesel engine is performed as an application. To predict initiation of thermal fatigue cracks, it is essential to use an elastoplastic material model.
Fatigue Design Model Based on Damage Mechanisms Revealed by Acoustic Emission Measurements
Authors: Fang D. , Berkovits A.
Published: February 2008

Integration of the microcomputer into acoustic emission instrumentation has brought AE monitoring of fatigue tests into the realm of practicality. On-line processing makes available a selection of software tools, enhancing classical techniques for eliminating the background noise which usually blanked out the desired data. Fatigue tests monitored for acoustic emission were carried out at room temperature on Incoloy 901 material specimens, over a stress-ratio range of −1 ≤ R ≤ .2. Valid AE data were obtained even when the load cycle passed through zero. The AE data permitted specific identification of the various phenomena occurring on the way to final failure. These included initial plasticity, crack nucleation and propagation phases. The AE findings were supported by microscopic examination. Based on the experimental data, a preliminary damage-prediction model was formulated.
Analysis of Mass Transfer Process in the Pore Free Technique
Authors: Bar-Meir Genick
Published: February 2008

Air entrainment is one of the most significant problems in pressure die casting. A possible solution is to use the Pore Free technique. In this technique an oxygen is introduced into the die to react with the liquid metal. The vacuum created by the reaction reduces the porosity. The life span of the mold is augmented by the reduction of the pressure during the process. In some cases the Pore Free technique yields acceptable results, i.e., low porosity, while in other cases the results are not satisfactory. These differing results can be explained by an analysis of the mechanisms involved. A simple model is proposed based on conservation laws that describes the relationship between mass transfer mechanisms and pressure deviation. The model indicates that there is a critical dimensionless parameter above which the pressure is decreased and below which the pressure is increased. In the first case, when the parameter exceeds the critical value, the technique is useful. In the second case, when the parameter is less than the critical value, introducing oxygen does not greatly reduce the porosity. The analysis demonstrates that the Pore Free and vacuum venting techniques account for two different extremes. The first case is when the flow is orderly throughout the chamber in which vacuum venting can produce acceptable results. The second case is when the flow is turbulent and the Pore Free technique is the better choice.
The Distribution of Temperatures in a Hot/Cold Die Set: The Effect of the Pressure, Temperature, and Material
Authors: Lenard J. G. , Davies M. E.
Published: February 2008

Modern hot metalforming operations require a predictive-adaptive control program in which the predictive component precalculates the necessary parameters—such as temperatures and loads—and the adaptive component sets up, monitors and adjusts the forming equipment. One of the needs of a predictive model of the process is for an understanding of the boundary conditions at the surface of contact. For high temperature operations, one of these conditions is described by the heat transfer coefficient, relating the heat flux and the difference of the temperatures of the contacting surfaces, at the die-workpiece interface. The surface temperatures are, of course, dependent on the distribution of the temperatures within the body of the dies. The determination of these distributions is the topic of the present project. Thermocouples embedded in a cold/hot die set as well as thermocouples placed on the surface of contact are used to monitor the temperature fields, within the body as well as on the surfaces, for various interface pressures, initial temperatures and materials. The results indicate that direct measurements of the temperatures of the contacting surfaces is very difficult and may lead to significant errors. It is recommended that a mathematical model, which uses the measured internal temperatures as initial conditions, should be employed to calculate the contact surface temperatures.
Flow Instability in the Torsional Straining of Commercially Pure Titanium
Authors: Sheikh-Ahmad J. Y. , Bailey J. A.
Published: February 2008

Analysis of the phenomenon of adiabatic flow instability during the torsional straining of CP titanium has been made and the instability shear strain predicted. The constitutive equation for isothermal deformation of CP titanium was integrated over the total shear strain received by the material. Thermal softening caused by adiabatic heating was calculated and used to derive the adiabatic shear stress-shear strain curves. The instability shear strain was determined as the value that corresponds to zero slope of these curves. The effect of strain rate, temperature, and specific heat on the instability shear strain was also discussed. Comparison of the experimental results with predictions showed that flow instability in the torsion test occurred at strains approximately equal to the theoretical thermo-plastic instability strains for a temperature of 150°C but at lower strains at higher temperatures. The difference between the theoretical and experimental instability strains increases with an increase in temperature. A discussion of this difference is also presented.
Compressive Stress-Strain Relations and Shock Hugoniot Curves of Flexible Foams
Authors: Zaretsky E. , Ben-Dor G.
Published: February 2008

Based on the general shape of the curves describing experimental compressive stress-strain relations of flexible porous materials (e.g., flexible foams) which are known to depend on their relative density, a general mathematical functional dependence of the stress on the strain is proposed. The general function includes two constants. Using experimental and empirical information the values of these constants are determined, so that a final compressive stress-strain relation, in which the relative density is a parameter, is obtained. Good agreement is found when the presently developed empirical stress-strain relations are compared to experimental ones for a wide range of relative densities. The proposed compressive stress-strain relations are then used to derive shock Hugoniot relations for flexible porous materials. With the aid of these relations one can investigate the dynamic behavior of foams struck head-on by shock waves. The finally obtained empirical shock Hugoniot relations are found to be similar to experimental relations which are proposed in the literature. In addition, a mathematical investigation of the asymptotic behavior of the shock Hugoniot relations is conducted. The results of this investigation are found to agree excellently with actual experimental data.
A Phenomenological Model for Intergranular Failure by r-type and Wedge-Type Cavitation
Authors: Lee Soon-Bok , Miller Alan K.
Published: February 2008

Equations to predict local intergranular failure (by “r”-type and “wedge”-type cavitation and the coupling between them) have been developed. The derivation has utilized physically based concepts such as thermal activation of the controlling processes, wedge cracking driven by grain boundary sliding, and cavity growth driven by diffusion. It has also been based upon phenomenological observations such as the variation in the steady-state creep rate with stress and temperature, incomplete healing of cavities under compression, and differences in life under “slow-fast” and “fast-slow” cycling. The model has been tested against data on the low-cycle fatigue life of 304 stainless steel under unequal ramp rates. The new equations simulate, for example, the differences in life produced by slow-fast, fast-slow, and equal ramp rate cycling in terms of their effects on internal cavitation. Simulations have also been generated concerning creep crack advance by cavitation. Together with the new equations’ ability to treat monotonic creep rupture, these comparisons demonstrate that the intergranular failure equations are capable of simulating a number of phenomena of importance in life prediction for high-temperature structures.
Effect of Surface Asperity on Interfacial Contact Process Controlled by Power Law Creep—Numerical Study of Viscoplastic Adhering Process
Authors: Takahashi Y. , Tanimoto M.
Published: February 2008

An interfacial contact process due to power law creep is studied using a finite element technique. The contact process is assumed to be produced by power law creep alone after initial intimate contact by instantaneous plastic deformation, i.e., no diffusional mechanisms for void shrinkage are taken into account. Also, the surface oxide film is not considered. If the bonded material is deformed, then the deformation is influenced by the initial faying surface wauiness with the asperity angle αo, and the contact process is achieved by two modes; surface folding at the bond-interface (type I) and interfacial expansion (type II), where the surface folding is the phenomenon that two faying surfaces are overlapped to each other. The surface folding phenomenon occurs preferentially when αo is less than 30 deg (as the surface wauiness height decreases). On the other hand, the interfacial expansion is dominant at αo > 45 deg. This can be explained in terms of the distribution of equivalent strain (stress) in the vicinity of the bond-interface.
From Micro- to Macro-Plasticity: The Scale Invariance Approach
Authors: Aifantis E. C.
Published: February 2008

An account of recent contributions to the theory of plasticity advanced by the author and his co-workers is provided with emphasis on deformation-induced anisotropy and texture formation. For the description of these effects, the so-called scale invariance approach is adopted which allows information and constitutive relations pertaining to single slip to be cast in the form of macroscopic constitutive equations. Various phenomenological plasticity models are derived this way with the extra dividend of deducing explicit expressions for the phenomenological coefficients. The method is particularly suited for large deformation anisotropic plasticity, as it reveals the inherent coupling between the evolution of back stress and the plastic spin. Extended models of macroscopic plastic behavior accounting for vertex and texture phenomena can also be obtained by slightly generalizing the kinematics and kinetics of the microscopic configuration to include rate, non-Schmid, secondary slip and grain orientation effects.
Mechanistic Approach for Creep-Fatigue Evaluation of 9Cr-1Mo-V-Nb Steel
Authors: Asada Yasuhide , Yaguchi Masatsugu
Published: February 2008

A mechanistic model was developed to describe the creep-fatigue interaction of 9Cr-1Mo-V-Nb steel based on a damage mechanics approach. The model assumes that each of the fatigue and creep damage consists of basic damages with a size and density. The mechanism of the creep-fatigue interaction is considered to be an early growth of a size of fatigue basic damage from a creep basic damage formed prior to fatigue loading. The model interprets well the experimental creep-fatigue life under complex strain histories.
Natural Strain
Authors: Freed Alan D.
Published: February 2008

The purpose of this paper is to present a consistent and thorough development of the strain and strain-rate measures affiliated with Hencky. Natural measures for strain and strain-rate, as I refer to them, are first expressed in terms of the fundamental body-metric tensors of Lodge. These strain and strain-rate measures are mixed tensor fields. They are mapped from the body to space in both the Eulerian and Lagrangian configurations, and then transformed from general to Cartesian fields. There they are compared with the various strain and strain-rate measures found in the literature. A simple Cartesian description for Hencky strain-rate in the Lagrangian state is obtained.
Laminate Delamination Due to Thermal Gradients
Authors: Hutchinson J. W. , Lu T. J.
Published: February 2008

Flaw-induced delamination of orthotropic laminates subject to through-thickness temperature gradients is analyzed. A crack-like flaw impedes heat flow through the laminate, producing thermal stresses and crack tip stress intensities. The focus is on delamination cracks which propagate under steady-state conditions. The steady-state analysis becomes accurate for a crack whose length is about one laminate thickness. Moreover, the analysis provides realistic fail-safe criteria for excluding delamination.
Fatigue-Life Prediction Methodology Using a Crack-Closure Model
Authors: Newman J. C.
Published: February 2008

This paper reviews the capabilities of a plasticity-induced crack-closure model and life-prediction code, FASTRAN, to predict fatigue lives of metallic materials using small-crack theory. Crack-tip constraint factors, to account for three-dimensional state-of-stress effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (ΔKeff) under constant-amplitude loading. Some modifications to the ΔKeff-rate relations were needed in the near-threshold regime to fit small-crack growth rate behavior and endurance limits. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched specimens made of several aluminum alloys and a titanium alloy under constant-amplitude and spectrum loading. Fatigue lives were calculated using the crack-growth relations and microstructural features like those that initiated cracks for the aluminum alloys. An equivalent-initial-flaw-size concept was used to bound the fatigue lives for the titanium alloy. Results from the tests and analyses agreed well.
Fatigue Life Prediction in Titanium Matrix Composites
Authors: Nicholas T.
Published: February 2008

Methods used for life prediction of titanium matrix composites under isothermal and thermomechanical (TMF) fatigue are reviewed. Models containing a single parameter are shown to have applicability only under limited conditions. Two models, a dominant damage and a life fraction model, demonstrate predictive capabilities over a broad range of loads, frequencies, temperatures, and TMF parameters. Relationships between the underlying fatigue mechanisms and the individual terms in the models are illustrated.
Thermoplastics as Engineering Materials: The Mechanics, Materials, Design, Processing Link
Authors: Stokes Vijay K.
Published: February 2008

While the use of plastics has been growing at a significant pace because of weight reduction, ease of fabrication of complex shapes, and cost reduction resulting from function integration, the engineering applications of plastics have only become important in the past fifteen years. An inadequate understanding of the mechanics issues underlying the close coupling among the design, the processing (fabrication), and the assembly with these materials is a barrier to their use in structural applications. Recent progress on some issues relating to the engineering uses of plastics is surveyed, highlighting the need for a better understanding of plastics and how processing affects the performance of plastic parts. Topics addressed include the large deformation behavior of ductile resins, fiber orientation in chopped-fiber filled materials, structural foams, random glass mat composites, modeling of thickness distributions in blow-molded and thermoformed parts, dimensional stability (shrinkage, warpage, and residual stresses) in injection-molded parts, and welding of thermoplastics.
Elastic Anisotropy Effect of Crystals on Polycrystal Fatigue Crack Initiation
Authors: Teng N. J. , Lin T. H.
Published: February 2008

Fatigue bands have been observed in both monocrystalline and polycrystalline metals. Extrusions and intrusions at the free surface of fatigued specimens are favorable sites for fatigue crack nucleation. Previous studies (Lin and Ito, 1969; Lin, 1992) mainly concerned the fatigue crack initiation in aluminum and its alloys. The elastic anisotropy of individual crystals of these metals is insignificant and was accordingly neglected. However, the anisotropy of the elastic constants of some other metallic crystals, such as titanium and some intermetallic compounds, is not negligible. In this paper, the effect of crystal anisotropy is considered by using Eshelby’s equivalent inclusion method. The polycrystal analyzed is Ni3Al intermetallic compound. The plastic shear strain distributions and the cumulative surface plastic strain in the fatigue band versus the number of loading cycles were calculated, and the effect of crystal anisotropy on the growth of the extrusions was examined.
A State Variable Constitutive Model for Superplastic Ti-6Al-4V Based on Grain Size
Authors: Mosher D. A. , Dawson P. R.
Published: February 2008

In this work, the superplastic material behavior of Ti-6Al-4V has been correlated with the grain size of the a phase. Because this microstructural state variable is directly observable, the viscoplastic flow kinetics could be determined separately from the state evolution (grain growth). The grain growth rate is the sum of static and deformation enhanced components which have been expressed in terms of current (state) variables rather than the historical quantities of plastic strain or time. In this manner, the grain growth relations are valid for arbitrary deformation rate and temperature paths. The deformation of a tapered rod has been measured and compared with simulation to evaluate the constitutive model.
A Finite-Element Work-Hardening Plasticity Model of the Uniaxial Compression and Subsequent Failure of Porous Cylinders Including Effects of Void Nucleation and Growth—Part II: Localization and Fracture Criteria
Authors: Lee J. H. , Zhang Y.
Published: February 2008

In Part I [1] of this paper, Gurson’s mixed hardening plasticity model with strain and stress-controlled nucleations, was used in a large deformation finite element program to study the plastic flow and damage in the uniaxial compression of cylinders under sticking friction. Due to low stress triaxiality at the bulge of the cylinders, it was found that localization may occur before void coalescence. In this paper, necessary conditions of localizations are analyzed for the axial compression of porous cylinders under sticking friction. Shear band type of localization with a normal mode of fracture has been predicted for the majority of the cases studied. Various existing localization conditions and fracture criteria are assessed using the results from the simulation. The maximum shear stress at failure is approximately constant and a constant critical damage can not be found.
Verification of a Cohesive Zone Model for Ductile Fracture
Authors: Yuan Huang , Lin Guoyu , Cornec Alfred
Published: February 2008

In the present paper, ductile crack growth in an aluminium alloy is numerically simulated using a cohesive zone model under both plane stress and plane strain conditions for two different fracture types, shear and normal modes. The cohesive law for ductile fracture consists of two parts—a specific material’s separation traction and energy. Both are assumed to be constant during ductile fracture (stable crack growth). In order to verify the assumed cohesive law to be suitable for ductile fracture processes, experimental records are used as control curves for the numerical simulations. For a constant separation traction, determined experimentally from tension test data, the corresponding cohesive energy was determined by finite element calculations. It is confirmed that the cohesive zone model can be used to characterize a single ductile fracture mode and is roughly independent of stable crack extention. Both the cohesive traction and the cohesive fracture energy should be material specific parameters. The extension of the cohesive zone is restricted to a very small region near the crack tip and is in the order of the physical fracture process. Based on the present observations, the cohesive zone model is a promising criterion to characterize ductile fracture.
Burr/Breakout Model Development and Experimental Verification
Authors: Chern Gwo-Lianq , Dornfeld David A.
Published: February 2008

The mechanisms of burr formation and breakout in orthogonal cutting are analyzed. A burr formation/breakout model for orthogonal cutting is proposed, based on the observation of SEM micro-machining tests. It is found that a negative deformation plane begins to form when the steady-state chip formation stops as the tool approaches the end of the cut. Plastic bending and shearing of the negative deformation plane contributes to the burr formation, while crack propagation along the plane causes the breakout. Simulated orthogonal cutting experiments using copper, Al 2024-T4, and Al 6061-T6 were performed with a modified impact machine to verify the model. The tests showed excellent agreement with the model predictions.
Measurement of Residual Stresses in a Circular Ring Using the Successive Cracking Method
Authors: Kang K. J. , Seol S. Y.
Published: February 2008

A method which we describe as the “successive cracking method” for measuring residual stresses in a circular ring is presented. In this method, the residual stresses are evaluated using a fracture mechanics approach. The strains measured at a point on the outer edge of the ring as a crack is introduced and extended from the edge are used to deduce the residual stress distribution in the uncracked ring. Finite element analysis is carried out to examine the validity of the theoretical derivation. Experiments to measure the residual stresses in a steel ring specimen are done by the successive cracking method. For comparison purposes, the experimental results using the sectioning method are presented as well. The successive cracking method is shown to be valid, simple, and effective for measuring the two-dimensional residual stress distribution in an axisymmetric member.
Determination of Nonuniform Residual Stresses Using the Ring-Core Method
Authors: Ajovalasit A. , Petrucci G. , Zuccarello B.
Published: February 2008

This paper considers residual stress analysis using the ring-core method. In particular, the so-called integral equation method is applied to evaluate nonuniform residual stress fields. The proposed method overcomes typical drawbacks of the incremental strain method which lead to incorrect results for strongly varying stress fields. The experimental results obtained with a specimen subjected to a bending load confirm the theoretical predictions.
Fatigue Crack Growth Properties of a Cryogenic Structural Steel at Liquid Helium Temperature
Authors: Konosu Shinji , Kishiro Tomohiro , Ivano Ogi , Nunoya Yoshihiko , Nakajima Hideo , Tsuji Hiroshi
Published: February 2008

The structural materials of the coils of superconducting magnets utilized in thermonuclear fusion reactors are used at liquid helium (4.2 K) temperatures and are subjected to repeated thermal stresses and electromagnetic forces. A high strength, high toughness austenitic stainless steel (12Cr-12Ni-10Mn-5Mo-0.2N) has recently been developed for large, thick-walled components used in such environments. This material is non-magnetic even when subjected to processing and, because it is a forging material, it is advantageous as a structural material for large components. In the current research, a large forging of 12Cr-12Ni-10Mn-5Mo-0.2N austenitic stainless steel, was fabricated to a thickness of 250 mm, which is typical of section thicknesses encountered in actual equipment. The tensile fatigue crack growth properties of the forging were examined at liquid helium temperature as function of specimen location across the thickness of the forging. There was virtually no evidence of variation in tensile strength or fatigue crack growth properties attributable to different sampling locations in the thickness direction and no effect of thickness due to the forging or solution treatment associated with large forgings was observed. It has been clarified that there are cases in which small scale yielding (SSY) conditions are not fulfilled when stress ratios are large. ΔJ was introduced in order to achieve unified expression inclusive of these regions and, by expressing crack growth rate accordingly, the following formula was obtained at the second stage (middle range). da/dN = CJ ΔJmJ, CJ = AJ/(ΔJ0)mJ, where, AJ = 1.47 × 10−5 mm/cycle, ΔJ0 = 2.42 × 103N/m.
On the Constitutive Response of 63/37 Sn/Pb Eutectic Solder
Authors: Skipor A. F. , Harren S. V. , Botsis J.
Published: February 2008

The constitutive response of 63/37 Sn/Pb eutectic solder is studied under uniaxial tension. Stress-strain tests were performed under a wide range of applied strain rate, and creep tests were performed at several levels of applied load. Temperature dependence was ascertained by running the stress-strain and creep tests at various temperatures. With the goal of providing a constitutive description of the eutectic suitable for use in electronics packaging engineering finite element analyses, the material parameters of the phenomenological viscoplasticity model of Bodner and Partom were fit to these experiments as functions of temperature. The experiments described herein also yield important information concerning how many microstructural units must be present in a sample in order for repeatable, isotropic response to be obtained.
The Effect of Stress-State on the Large Strain Inelastic Deformation Behavior of 304L Stainless Steel
Authors: Miller M. P. , McDowell D. L.
Published: February 2008

In metals, large strain inelastic deformation processes such as the formation of a preferred crystallographic orientation (crystallographic texture) and strain hardening processes such as the formation and evolution of dislocation substructures depend on stress-state. Much of the current large strain research has focused on texture. Crystallographic texture development and strainhardening processes each contribute to the overall material behavior, and a complete description of large strain inelastic material response should reflect both. An investigation of the large strain behavior of 304L stainless steel (SS 304L) subjected to compression, torsion, and sequences of compression followed by torsion and torsion followed by tension is reported. This paper focuses on the stress-state dependence of strain-hardening processes as well as the relative effect such processes have on the overall material behavior. To characterize these processes, transmission electron microscopy (TEM) as well as magnetization investigations were conducted at different strain levels and under different deformation modes. The γ → α′ martensitic transformation which occurs in this material was found to be related to both the strain level and stress state. Dislocation substructures in the form of Taylor lattices, dense dislocation walls, and microbands were also present. The ramifications of using a thin-walled tubular torsion specimen were also explored.
Analysis of Layer Waviness in Flat Compression-Loaded Thermoplastic Composite Laminates
Authors: Adams Daniel O’Hare , Hyer Michael W.
Published: February 2008

A finite element analysis was used to investigate layer waviness effects in flat compression-loaded composite laminates. Stress distributions in the vicinity of the layer waves as well as the locations and modes of failure were investigated. Two layer wave geometries were considered, each modeled within an otherwise wave-free thermoplastic composite laminate. These two wave geometries, classified as moderate and severe, corresponded to layer waves fabricated in actual laminates and tested under uniaxial compression loading. Material nonlinearities obtained from intralaminar shear and 0 and 90 deg tension and compression testing were incorporated into the analysis. The nonlinearity observed in the intralaminar shear stress-strain behavior was assumed to be valid for interlaminar shear stress-strain behavior, and the nonlinearity observed in the 90 deg tension and compression stress-strain behavior was assumed to be valid for interlaminar normal stress-strain behavior. Failure was predicted using a maximum stress failure theory. An interlaminar tension failure was predicted for the severe layer wave geometry, producing a large compression strength reduction in comparison to the wave-free laminate. Fiber compression failure was predicted for the moderate layer wave, producing only a slight compression strength reduction. Although significant material nonlinearity was present in the interlaminar compression and shear response of the material, the inclusion of material nonlinearity produced only slight decreases in predicted compression strengths relative to predictions based on linear material behavior.
Thermomechanical and Isothermal Fatigue Behavior of Bare and Coated Superalloys
Authors: Kadioglu Yavuz , Sehitoglu Huseyin
Published: February 2008

Thermomechanical fatigue (TMF) and isothermal fatigue (IF) experiments were performed on bare Mar-M246 as well as bare and coated Mar-M247 nickel based superalloys at strain ranges from 0.335 to 1 percent. The experiments were conducted in air, through a temperature range of 500 to 1038°C at a constant strain rate of 5.0 × 10-5 s-1. Due to the coarse-grained structure of Mar-M246, TMF lives scattered considerably under low strain range conditions. Electron microscopy studies show that significant surface oxidation and gamma prime, γ′, depleted zones occur for Mar-M246 under these conditions. In the surface grain, where the change in γ′ morphology is most pronounced, γ′ rafted along axes ≈ ±45 deg from the loading axis. The influence of a protective coating on the IF and TMF lives of Mar-M247 was also examined. Results indicate that the coating does not significantly affect the fatigue lives of Mar-M247. The oxidation behavior of bare and coated Mar-M247 was investigated via X-ray microprobe and Auger Spectroscopy. Through these analyses, it is evident that the coating was completely degraded when exposed to the test environment for a long time. In both Mar-M246 and Mar-M247, in-phase thermomechanical fatigue (TMF IP) loading resulted in intergranular cracking. TMF IP loading promoted multiple cracks at the coating/substrate interface of coated Mar-M247. Finally, Eshelby techniques were used to calculate stress distributions in the vicinity of a surface oxide and second-phase particle at the coating/substrate interface.
A Dislocation Density Based Constitutive Model for Cyclic Deformation
Authors: Estrin Y. , Braasch H. , Brechet Y.
Published: February 2008

A new constitutive model describing material response to cyclic loading is presented. The model includes dislocation densities as internal variables characterizing the microstructural state of the material. In the formulation of the constitutive equations, the dislocation density evolution resulting from interactions between dislocations in channel-like dislocation patterns is considered. The capabilities of the model are demonstrated for INCONEL 738 LC and Alloy 800H.
High Temperature Densification Forming of Alumina Powder—Constitutive Model and Experiments
Authors: Kwon Y. S. , Kim K. T.
Published: February 2008

Densification and grain growth of alumina powder compacts were investigated under pressureless sintering, sinter forging, and hot pressing. A set of constitutive equations by Kwon et al. was used to predict densification under diffusional creep. A novel grain growth equation is proposed by generalizing the grain growth model of Wilkinson and Ca´ceres to predict grain growth during densification forming process of ceramic powders under general loading states. Material parameters in the constitutive equations were determined from experimental data under pressureless sintering and sinter forging. Theoretical predictions by using the constitutive equations for creep densification and grain growth were compared with experimental data of alumina powder compacts.
Effect of Architecture on the Strength of Braided Tubes Under Biaxial Tension and Compression
Authors: Smith L. V. , Swanson S. R.
Published: February 2008

Braided preforms of carbon fiber are under consideration for structural applications because of the potential for automated fabrication and possible improvement in through-the-thickness properties, although some loss of in-plane properties is expected. In the present investigation, the biaxial compressive strength properties of triaxial braided cylinders are measured experimentally, and the effects of four different sets of braid architectures are considered. The results show that fiber direction strain can correlate the results of biaxial tensile and compression failure tests. The braid parameters are found to have an effect on measured strengths, primarily through the introduction of defects into the fiber paths. The axial compression strength was found to be sensitive to waviness in the fiber path that was produced by uneven braiding coverage. Braid direction compression and tension strength is significantly lower than axial strength, and is also affected by braid parameters.
Thermodynamic Law of Corresponding Shock States in Flexible Polymeric Foams
Authors: Zaretsky E. , Ben-Dor G.
Published: February 2008

Based on thermodynamic considerations and experimental data on isothermal compression of elastomeric foams, a simple equation of state of flexible polymeric foams was developed. The developed equation of state is, in fact, a simple universal function relating the thermodynamic properties of the material of which the skeleton of the foam is made and the foam porosity. It was shown that the Hugoniot adiabat of foams, whose porosity is less than 0.3 and which are exposed to moderate shocks, could be expressed in a form similar to that of bulk solids, i.e., D = Co + Su, where D is the shock front velocity, Co is the speed of sound, u is the particle velocity and S is the maximum material compressibility.
Architecture of Functionally Graded Ceramic Coatings Against Surface Thermal Fracture
Authors: Choules B. D. , Kokini K.
Published: February 2008

An analytical study was performed to study the effect of architecture on the initiation of surface cracking in multilayer ceramic coatings. Two modes of crack initiation were considered: 1) tension resulting from stress relaxation, and 2) cyclic thermal fatigue. Transient temperature distributions were determined using a finite difference technique, and stress distributions were calculated using a multilayer beam theory. The results showed that as more layers were added and as the ceramic coating became thinner, lower maximum surface stresses resulted during cooling after stress relaxation. Also, a thick eight layer coating had similar thermal fatigue behavior to a thin single layer coating. It was determined that a thick multilayer coating adds a significant amount of thermal protection when compared to a thin single layered coating.
Life Prediction of Notched Specimens Using Multiaxial Surface and Subsurface Strain Analyses
Authors: Shatil G. , Smith D. J.
Published: February 2008

Multiaxial fatigue life prediction analyses are applied to notched specimens subjected to high strain constant amplitude loading. The specimens are made from isotropic and anisotropic batches of the structural steel EN15R. Two sets of experimental data are used for the analyses; data from the biaxial fatigue of thin walled specimens and data from the uniaxial fatigue of hourglass specimens. The maximum strain parameter and two multiaxial fatigue approaches, the Brown-Miller and the Lohr-Ellison theories, are used to predict the fatigue life. A simple subsurface strain model is developed to overcome the geometry difference between the thin walled biaxial specimens and the solid bar notched specimens. Fairly good life prediction is obtained with the multiaxial fatigue parameters and the subsurface model using elastic-plastic finite element simulations.
Correlation of Tensile and Bending Strength of Composites With a Circular Hole
Authors: Zhao Yi , Pang Su-Seng , Yang Chihdar
Published: February 2008

While significant studies have been conducted on composites containing a circular or an elliptic hole under tensile loading, few investigations have been reported on notched composites under bending due to the complex nature of the problem. In this paper, a correlation of strengths of orthotropic plates containing a circular hole under tension and bending has been proposed based on Lekhnitskii’s solutions for infinite width plates. Finite element analysis has been performed to verify the theoretical correlation. Uniaxial tension and four-point bending tests have also been conducted on Scotchply samples with circular holes of various sizes. A good agreement has been observed among the theoretical, FEA, and experimental results.
A Probabilistic Micromechanics Model for Damaged Composites
Authors: Duva J. Mark , Aboudi Jacob , Herakovich Carl T.
Published: February 2008

The generalized method of cells, an interfacial constitutive model and a probabilistic model are combined to yield an approach for modeling the stress-strain response of fibrous composites with fiber/matrix interfacial damage. It has been shown that the limitations of a single repeating cell in representing the response of a composite with debonding interfaces can be overcome through the introduction of a probabilistic modelling approach based upon an effective interfacial strength. This approach is used to recover the transverse tensile and axial shear response of unidirectional silicon-carbide/titanium as measured in the laboratory.
Analysis and Testing of Dynamic Micromechanical Behavior of Composite Materials at Elevated Temperatures
Authors: Pant R. H. , Gibson R. F.
Published: February 2008

This paper describes the use of a recently developed high temperature impulse-frequency response apparatus to directly measure dynamic modulus and internal damping of high temperature composite materials, matrix materials, and reinforcing fibers as a function of temperature. An extensional vibration test was used for determination of the complex Young’s modulus of fiber specimens as a function of temperature. A flexural vibration test was used for determination of the complex flexural modulus of matrix and unidirectional composite specimens (0 and 90 deg fiber orientations) as a function of temperature. These results were obtained from tests done on two different fiber reinforced composite materials: boron/epoxy (B/E) and Silicon Carbide/Ti-6Al-4V (SiC/Ti). The results from these tests were then used to assess the validity of micromechanics predictions of composite properties at elevated temperatures. Micromechanics predictions of composite moduli and damping at elevated temperatures show good agreement with measured values for the 0 deg case (longitudinal) but only fair agreement for the 90 deg case (transverse). In both cases, the predictions indicate the correct trends in the properties.
A Study on the Distribution of Residual Stress Due to Surface Induction Hardening
Authors: Xu Dong-hui , Kuang Zhen-Bang
Published: February 2008

A finite element method (FEM) is used here to analyse the distribution of residual stresses generated by surface induction hardening. The analysis considers both transformation induced plasticity (TRIP) and latent heat effects. The calculation model considering TRIP is found to improve the agreement with experimental measurements significantly. From a large number of numerical results, a set of simple formulas is derived to predict the residual stress distribution in the workpiece.
Mean Strain Effect on Crack Initiation Lives for Notched Specimens Under Biaxial Nonproportional Loading Paths
Authors: Yip Ming-Chuen , Jen Yi-Ming
Published: February 2008

This paper discusses the mean strain effect on the crack initiation lives for notched specimens under biaxial nonproportional loading paths. Elastic-plastic finite element method was used to evaluate the local stresses and strains. Several prediction models related to the mean stress/strain effect were employed to correlate the experimental results with reference fatigue data for smooth specimens. It is found that Fatemi-Socie model gives good prediction for the present research with the assistance of finite element method. The stress behavior in this deflection-controlled tests is discussed in this study, and the failure surfaces are also examined after tests.
A General Solution to the Two-Surface Plasticity Theory
Authors: Jiang Wei
Published: February 2008

This paper obtains a closed-form general solution to the two-surface plasticity theory for linear stress paths. The simple two-surface model is discussed first. It is shown that according to this model, the response of the material stabilizes immediately during the first loading cycle. That is, the memory surface reaches its maximum size with a radius equal to the maximum effective stress and then remains unchanged thereafter, while the yield center translates along a line parallel to the stress path, thus always leading to a constant plastic strain growth rate. As a result, the model predicts that under any cyclic linear loading conditions, the material response can always be ratchetting, with no possibility of shakedown of any kinds, which violates those aspects of material behavior that are generally deemed essential in constitutive modeling. The general two-surface theory is also discussed in this paper, and some comments are made.
Plastic Zone Size Determination by Temperature Measurement
Authors: Rajic N. , Lam Y. C. , Wong A. K.
Published: February 2008

This paper presents a temperature-based method for the determination of the size of elastoplastic zones. A mathematical model describing the one-dimensional thermal problem of the propagation of a plastic zone is developed enabling the use of nonadiabatic temperature measurements in this role. Validation of the model with numerically generated temperature data has shown the proposed method to yield both accurate and robust estimates for the elastoplastic boundary. An experimental application revealed the method to be efficacious and the estimates for the elastoplastic boundary to agree well with those obtained by finite element analysis.
A New Method for Estimating Nonproportional Notch-Root Stresses and Strains
Authors: Gu Randy J. , Lee Yung-Li
Published: February 2008

This paper presents a generalized two-step endochronic approach for estimating notch stresses and strains based on elastic stress solutions. In the first stress-controlled step, notch root strains are calculated from elastic stresses using a conventional uniaxial method, such as Glinka’s energy density method and Neuber’s rule. In the second strain-controlled step notch root stresses corresponding to the estimated local strains are calculated from the given material properties. Both stress-controlled and strain-controlled algorithms based on endochronic plasticity theory are presented herein. The proposed method is used to calculate multiaxial strains under monotonie and nonproportional loads. Various geometric constraints (plane stress, plane strain, and intermediate level) are also examined. The results are compared with experimental measurements by other researchers and with predictions from other models.
Failure Mechanisms of Composite Plates With a Circular Hole Under Remote Biaxial Planar Compressive Loads
Authors: Khamseh A. R. , Waas A. M.
Published: February 2008

We report the results of an experimental investigation carried out for the analysis of failure mechanisms in fibrous laminated composite plates containing stress raisers, in the form of circular cutouts, under static biaxial planar compressive loading (i.e., compression in the two inplane orthogonal directions). A series of biaxial tests were carried out with 48 ply graphite/epoxy composites of varying fiber orientation. In all cases, the hole diameter to plate width aspect ratio remained in a range suitable for infinite plate assumptions. Fiber microbuckling, fiber kink banding, and fiber/matrix debonding are identified as the dominant failure mechanisms.