bluuebunny/crossref_metadata_2025 huggingface dataset:
Strain Concentration Calculation of Fatigue
Authors: Lin Y. , Lin E. , Ling M.
Published: February 2008

On the basis of the authors’ rule of sliding to transform elastic solution to plastic in a previous paper, a new method of calculating fatigue is developed by using ordinary relaxation methods in conjunction to take care of the gradual enlarging of the plastic zone. The results of the calculations agree with the experimental data by the fine grid method. The calculation method developed here with the authors’ crack propagation prediction method in another previous paper lead to an interpretation of fatigue mechanism.
Fracture Plane of Cruciform Specimen in Biaxial Low Cycle Fatigue—Estimate by Variance Method and Experimental Verification
Authors: Bedkowski Włodzimierz , Macha Ewald , Ohnami Masateru , Sakane Masao
Published: February 2008

This paper presents the variance method of determining the fracture plane under random multiaxial stress states. The fracture plane was estimated analytically by the variance method with the three fatigue criteria. The estimated fracture planes were compared with experimental results using type SUS 304 and 1Cr-1Mo-1/4V steel cruciform specimens. The variance method with the maximum normal strain criterion, by neglecting the strain in direction in which no external forces act, could estimate the actual fracture planes of cruciform specimens in high temperature biaxial low cycle fatigue.
Stresses Induced in Fiber-Reinforced Tubes Due to Changes in Temperature and Moisture
Authors: Kim Byung S. , Lehnhoff Terry F.
Published: February 2008

An orthotropic hygrothermal stress solution has been formulated for an infinitely long fiber-reinforced tube of any number of plies of any material, with arbitrary orientation of each of the plies. The solution allows the determination of stresses when the tube is subjected to changes in temperature and moisture content. In the formulation, a temperature gradient through the thickness has been taken into account such that the formulation can be applied when the temperature inside the tube is not the same as that outside. This set of equations, together with the prescribed boundary conditions, has been solved numerically. It is shown that the stresses induced due to moisture absorbed from the environment are significant.
Polymeric Composite Tube Fabrication
Authors: Kim Byung Sun , Lehnhoff Terry F.
Published: February 2008

An aluminum mold was designed for fabrication of research quality composite tubes on a hot press (miniclave). This mold allows the pressure to be applied uniformly to the inner surface of a composite (prepreg tape) tube while a constant vacuum is also applied independently to the composite throughout the course of fabrication. With proper placement of stoppers and spacers inside the mold, various types of high quality unidirectional composites with fibers oriented either in the zero or 90 degree directions, respectively, were fabricated on a hot press. The mold design had less resin loss throughout fabrication compared to a conventional tubular mold. Physical and material tests have characterized the superior qualities of the resulting composite tubes.
Modeling and Numerical Analysis of Compression Molding of Three-Dimensional Thin Parts With Curing Process
Authors: Kwon T. H. , Kim C. S.
Published: February 2008

A numerical modeling is proposed for the simulation of flow, heat transfer, and reaction kinetics during the compression molding of three-dimensional thin parts. A nonisothermal, non-Newtonian model including the kinetic equation for a curing mechanism of thermosetting materials is implemented in a computer program, and a finite element method is used to simulate a preheating, a filling, and a post-heating stage during the entire compression molding process. As a more rigorous approach, a moving boundary condition due to the drag motion of an upper mold of a nonplanar shape or due to an apparent slip phenomena of particle filled materials is introduced into the present modeling, resulting in a new governing equation and the corresponding finite element formulation. Verifications of the analysis program were performed with a simple geometry for the Newtonian and non-Newtonian isothermal cases, in which the numerical results are found to be in good agreement with theoretical results. Effects of the moving boundary condition and processing conditions, such as thickness of compression molded parts, mold closing velocity and the preheating stage on overall compression molding processing, are numerically investigated. Numerical results for a car fender are also presented as an example of industrial applications.
An Investigation of Transient Creep by Means of Endochronic Viscoplasticity and Experiment
Authors: Wu Han C. , Ho Chin C.
Published: February 2008

Creep of metals has been investigated by means of the endochronic constitutive equation. This is a unified approach. Transient creep tests have been conducted on 304 stainless-steel specimens with carefully monitored precreep loading stage, either loaded at a prescribed constant strain-rate or at a constant stress-rate. It has been found that, for the same hold stress, the creep strain is larger for test with a constant stress-rate preloading than that for a constant strain-rate preloading. This is an effect of plasticity-creep interaction. In all cases, the initial creep strain rate is a continuation of the preloading strain rate. The theory satisfactorily describes the experimental results.
Corrosion Fatigue of AZ91E-T6 Cast Magnesium Alloy in a 3.5 Percent NaCl Aqueous Environment
Authors: Stephens R. I. , Schrader C. D. , Lease K. B.
Published: February 2008

The objective of this research was to obtain and compare constant and variable amplitude fatigue behavior of AZ91E-T6 cast magnesium alloy in both an air and 3.5 percent NaCl aqueous corrosive environment. An additional objective was to determine if commonly used models that describe fatigue behavior and fatigue life are applicable to this material and test environment. Fatigue tests included constant amplitude strain-controlled low cycle fatigue with strain ratio, R, equal to 0, −1 and −2, Region II constant amplitude fatigue crack growth with load ratio, R, equal to 0.05 and 0.5 and variable amplitude fatigue tests using keyhole notched specimens. In all fatigue tests, the corrosion environment was significantly detrimental relative to the air environment. Mean strains influenced fatigue life only if accompanied by significant mean stress. The Morrow and Smith, Watson, and Topper mean stress models provided both accurate and inaccurate fatigue life calculations. Likewise, variable amplitude fatigue life calculations using the local strain approach and based upon the formation ofal mm crack at the keyhole notch were both accurate and fairly inaccurate depending on the specific model used.
Characterization of Elevated Temperature Crack Growth in Hastelloy-X Using Integral Parameters
Authors: Kim K. S. , Van Stone R. H.
Published: February 2008

Linear elastic fracture mechanics approaches are not suitable for prediction of fatigue crack growth in the nonlinear regime at elevated temperatures. The objective of this paper is to investigate the ability of the integral parameters by Blackburn (J*), by Kishimoto et al. (Jˆ), and by Atluri et al. (ΔTp*, ΔTp) to correlate crack growth data of Hastelloy-X at elevated temperatures under nominally elastic and nominally plastic loading. Crack growth is analyzed using a finite element method, and the integral parameters are computed from the results of analysis. The experimental crack growth rates are correlated with these parameters. It is found that J*, Jˆ, and ΔTp* can correlate crack growth data within an acceptable accuracy.
Compressive Fatigue of a Plasma Sprayed Zr02-8 wt%Y203 and Zr02-10wt%NiCrAlCoY TTBC
Authors: Johnsen B. P. , Cruse T. A. , Miller R. A. , Brindley W. J.
Published: February 2008

High cycle (>106 cycles) fatigue (HCF) behavior of thick thermal barrier coatings (TTBC’s) was examined for applied stresses near the compressive strength of the material. Test data were obtained on four coating systems: two base materials in the unsealed and CrO2 dip sealed conditions. Free standing tubular specimens were evaluated. The data show that compressive fatigue limits exist for the four coating systems at room temperature (RT) and sealing with CrO2 improves RT fatigue life. Test results also show the ratio of the peak cyclic stress, at 105 cycles, to ultimate compressive strength (UCS) of the Yttria Stabilized Zirconia (YSZ) tubular specimens approaches 0.90 at R = 0.6 (R = minimum/maximum compressive stress). Residual UCS increases were observed after fatigue run-outs at elevated temperatures (5 at 700°C and 1 at 400°C) for both YSZ and the cermet. Ratchetting with cyclic hardening was observed during fatigue with deformation occurring primarily in the initial portion of the high cycle fatigue life of the specimens.
Shearographic and Holographic Assessment of Defective Laminates With Bond-Lines of Different Elasticities
Authors: Shang H. M. , Tham L. M. , Chau F. S.
Published: February 2008

While defects in laminated plates are readily revealed using double-exposure shearography and holography, when an incremental vacuum pressure is applied between the exposures, the apparent defect size, measured from the boundary of the perturbed fringes, may over-estimate the actual defect size depending on the elasticity of the bond-line. In this paper, the perturbed fringes were described by a mathematical model derived from the theory of thin plates supported by a Winkler type elastic foundation which takes into consideration the elasticity of the bond-line. An iterative technique is subsequently presented for estimating the size and depth of the defect from the fringe pattern without the need to pre-determine the current elasticity of the bond-line. Laminates with a thick and highly elastic bond-line, found in foam-adhesive bonded metal laminates, and those with a thin and brittle bond-line, found in multi-layered glassflbre reinforced plastic plates, were tested. Results have shown that the defects in these laminates were accurately assessed.
Experimental Study of Interfacial Contacting Process Controlled by Power Law Creep
Authors: Takahashi Y. , Tanimoto M.
Published: February 2008

Interfacial contacting processes under a high temperature and a high bonding pressure (T = 973 K, P = 30 MPa) are experimentally studied, using oxygen free copper. The faying surfaces were machined by lathe, resulting in controlled regular surface asperities. The asperity angle of surface ridges was changed from 10 to 60 deg. The change in the interfacial deformation mode with the asperity angle has been investigated. Results show the interfacial contact process is strongly influenced by the asperity angle (shape of surface ridge). The bonding tests were carried out in high vacuum atmosphere (10−4 Pa) so that the surface oxide film need not be considered. Experimental results are in good agreement with the results calculated by a finite element model, in which the interfacial contact is assumed to be produced by power law creep alone. It was thus suggested that void coalescence is governed by power law creep under the present test conditions (T = 973 K and P = 30 MPa) except for the final stage of bonding. Experimental results also suggest that the elementary rate process of interfacial contact due to power law creep is classified into two types; surface folding and interfacial expansion. Here, the surface folding is the phenomenon that two faying surfaces are overlapped to each other and the interfacial expansion means that the bonded interface area is extended along the bond-interface.
An Investigation of the Effect of Speeds of Work Rolls on Rolling Strip
Authors: Lin Zone-Ching , Cheng Y. C.
Published: February 2008

The paper is an investigation of strip curvature caused by the different speeds between the upper work roll and the lower work roll in the rolling process for an aluminum strip. At the same time, we analyzed the variations in the temperature field and strain field, and used a method of speeds variation of the upper and lower work rolls to calibrate the deformation curvature caused by the coolant condition in the hot rolling process. Based on the large deformation-large strain theory, and by means of the Updated Lagrangean Formulation (ULF) and increment theory, a coupled thermoelastic-plastic analysis model for hot rolling process is thus constructed. At the same time the finite difference method was also used to solve the transient heat transfer equation. Finally, the numerical analysis method developed in this study was employed to analyze the changes in the aluminum strip’s temperature and other changes during rolling. In addition, the average rolling force obtained from the simulation was compared with that from the experiments. It verified that the model in this study is reasonable.
Effect of State of Stress on Compression Failure in Carbon/Epoxy Laminates
Authors: Jee Yong Ju , Swanson Stephen R.
Published: February 2008

Compression loadings in thick composites can in some cases lead to three-dimensional states of stress with a compressive mean stress. A model is developed in the present work that attempts to incorporate this compressive mean stress into a prediction for compression strength of carbon/epoxy fiber composites. The model assumes that the fibers have an initial misalignment, and that composite failure occurs when the fiber-matrix bond strength is exceeded. Literature values for the effect of pressure on bond strength are included. Comparisons with experimental data support the predicted increase in compression strength with pressure.
Scaling of Quasi-Brittle Fracture and the Fractal Question
Authors: Bazˇant Zdeneˇk P.
Published: February 2008

The paper represents an extended text of a lecture presenting a review of recent results on scaling of failure in structures made of quasibrittle materials, characterized by a large fracture process zone, and examining the question of possible role of the fractal nature of crack surfaces in the scaling. The problem of scaling is approached through dimensional analysis, the laws of thermodynamics and asymptotic matching. Large-size and small-size asymptotic expansions of the size effect on the nominal strength of structures are given, for specimens with large notches (or traction-free cracks) as well as zero notches, and simple size effect formulas matching the required asymptotic properties are reported. The asymptotic analysis is carried out, in general, for fractal cracks, and the practically important case ofnonfractal crack propagation is acquired as a special case. Regarding the fractal nature of crack surfaces in quasibrittle materials, the conclusion is that it cannot play a signification role in fracture propagation and the observed size effect. The reason why Weibull statistical theory of random material strength does not explain the size effect in quasibrittle failures is explained. Finally, some recent applications to fracture simulation by particle models (discrete element method) and to the determination of size effect and fracture characteristics of carbon-epoxy composite laminates are briefly reviewed.
From Instability and Time Dependence on the Microscale to Stability and Time Independence on the Macroscale
Authors: Drucker Daniel C.
Published: February 2008

The quasistatic inelastic deformation of ductile structural metals observed on the macroscale reflects a diversity of dynamic inelastic effects on the microscale. The generation, motion, and immobilization of dislocations are primary among them, but a host of other activities such as the opening and growth of cracks and voids, also may contribute. Dynamic activity on the microscale is strongly time-dependent on the time scales of importance to the microscopic processes. Also, the atomic configurations of single dislocations and groups of dislocations are highly unstable over a significant portion of each path of rapid motion. Nevertheless, engineers continue to design structures and machines with a reasonable factor of safety against failure on the basis of conventional plasticity theory with its assumption of both time-independence and stability (normality and convexity). This discussion of the validity of these simplifying assumptions for macroscopic constitutive relations despite instability and time-dependence on the atomic- and micro-scale expands upon a recent paper with Ming Li.
Residual Stresses and Fracture Mechanics
Authors: Finnie lain , Cheng Weili
Published: February 2008

The first part of the paper discusses two problems in which the role of residual stresses may be treated using the procedures of fracture mechanics. One is the significant degradation of the tensile strength of glass due to the residual stresses produced by surface damage. The other is the effect of surface compressive stresses in inhibiting the detection of surface flaws. The second part of the paper discusses the inverse problem of using fracture mechanics solutions to measure residual stresses. This approach which has only been developed in the past decade leads to an experimental procedure which is easy to implement and is also more versatile than conventional methods.
Mixed Mode Fracture Toughness of Engineering Materials
Authors: Kamat S. V. , Hirth J. P.
Published: February 2008

The effect of mixed mode loading on the fracture toughness of engineering materials is rationalized in terms of the operative fracture mechanism as well as the deformation field ahead of the crack tip. It was found that mixed mode fracture is of utmost importance in materials which fail by stable ductile fracture caused by second phase particles, since in such materials there is a significant reduction in fracture toughness on mixed-mode loading. An autocatalytic shear localization-void formation mechanistic model was found to qualitatively explain the mixed mode fracture behavior in such materials.
Effective Elastic Constants of Superlattice Films Measured by Line-Focus Acoustic Microscopy
Authors: Kim Jin O. , Achenbach Jan D. , Shinn Meenam , Barnett Scott A.
Published: February 2008

The effective elastic constants of single-crystal nitride superlattice films have been determined by calculation and by measurement methods. The calculation method uses formulas to calculate the effective elastic constants of superlattices from the measured elastic constants of the constituent layers. The calculated effective elastic constants are tested by comparing the corresponding surface acoustic wave (SAW) velocities calculated for thin-film/substrate systems with the corresponding SAW velocities measured by line-focus acoustic microscopy (LFAM). The measurement method determines the effective elastic constants of the superlattices directly from the SAW velocity dispersion data measured by LFAM. Two kinds of superlattice films are considered: one has relatively flat and sharp interfaces between layers, and the other has rough interfaces with interdiffusion. The calculation method has yielded very good results for the superlattices with flat and sharp interfaces but not for the superlattices with rough interfaces. The measurement method yields results for both kinds, with the restriction that the constituent layers have similar crystal symmetries.
Elastic Properties of Particle-Occlusion Composites: Measurements and Modeling
Authors: Ledbetter Hassel , Datta Subhendu , Dunn Martin
Published: February 2008

We review some of our recent studies on the effective elastic constants of composites where the occluded phase is spherical or spheroidal (oblate or prolate). From constituent properties and phase geometry, we estimated the composites’ effective macroscopic elastic constants using two models: the Ledbetter-Datta scattered-plane-wave ensemble-average model and the Mori-Tanaka effective-field model. We measured elastic constants by three principal methods: resonance, pulse-echo, and acoustic-resonance spectroscopy. We show microstructures, measurements, and model calculations for five representative composites: SiCp/Al, Al2O3-mullitep/Al, Al2O3p/ mullite, graphitep/ferrite (cast iron), voids/Ti.
Modeling of High Homologous Temperature Deformation Behavior Using the Viscoplasticity Theory Based on Overstress (VBO): Part I— Creep and Tensile Behavior
Authors: Tachibana Yukio , Krempl Erhard
Published: February 2008

The viscoplasticity theory based on overstress (VBO) is a state variable theory without a yield surface and without loading/unloading conditions. It contains two tensor valued state variables, the equilibrium (back) stress and the kinematic stress that is a repository for work hardening (softening). The scalar valued isotropic or time (rate)-independent stress models cyclic hardening (softening). For application to high homologous temperature, the effects of diffusion which counteracts the hardening of inelastic deformation has to be accounted for. Recovery of state terms are introduced in the growth laws for the state variables. A high homologous temperature VBO model is introduced and applied to the creep and tensile tests of Alloy 800 H between 750°C and 1050°C. Primary, secondary and tertiary creep as well as tensile behavior were well reproduced. It is shown that the transition to fluid state can be modeled with VBO.
Micromechanics Modeling of Electronic Composites
Authors: Taya Minoru
Published: February 2008

The microstructure-macroproperty relation of electronic composites is studied by use of the Eshelby’s method which has been extended to treat the inclusion problem of thermal and electromagnetic behavior. Eshelby’s model has further been extended to the case of coupling behavior such as piezoelectric or pyroelectric behavior. When fillers embedded in the matrix are overlapped, the Eshelby’s model may not be applicable. Then one must use a percolation model, which is also discussed in this paper. Finally, the predictions based on these models are compared with experimental data to check the validity of the analytical modeling.
Experimental Evidence of Temperature Path Independence in the Polycrystalline Alloy Ni3AI
Authors: Webb G. , Antolovich S. D.
Published: February 2008

The results from a series of elevated temperature prestrain experiments on a hypostoichiometric polycrstalline Ni3Al are presented. Experiments were conducted to examine the deformation characteristic of “thermal reversibility” or temperature path history independence (TPHI). Temperature path history independence was experimentally observed from prestraining experiments (also known as Cottrell-Stokes experiments) in which the specimen was deformed at different temperatures; the results were compared to those obtained from tests conducted at constant temperature. The purpose of such experiments was to macroscopically evaluate the effects of intrinsic dislocation mobility and dislocation substructure on deformation. These experiments provide a framework in which to evaluate fundamental characteristics of thermally activated deformation processes. The results for polycrystalline Ni3Al alloys indicate that the mechanisms responsible for thermal strengthening is independent of prior deformation history. This observation implies that the mechanism of anomalous strengthening in such alloys is fully reversible and independent of the development of a dislocation “substructure”.
Which Elasticity Tensors are Realizable?
Authors: Milton Graeme W. , Cherkaev Andrej V.
Published: February 2008

It is shown that any given positive definite fourth order tensor satisfying the usual symmetries of elasticity tensors can be realized as the effective elasticity tensor of a two-phase composite comprised of a sufficiently compliant isotropic phase and a sufficiently rigid isotropic phase configured in an suitable microstructure. The building blocks for constructing this composite are what we call extremal materials. These are composites of the two phases which are extremely stiff to a set of arbitrary given stresses and, at the same time, are extremely compliant to any orthogonal stress. An appropriately chosen subset of the extremal materials are layered together to form the composite with elasticity tensor matching the given tensor.
ASME 1995 Nadai Lecture—Plasticity of Porous and Particulate Materials
Authors: Cristescu N. D.
Published: February 2008

The paper discusses the formulation of constitutive equation for those materials for which the irreversible changes of the volume is also to be taken into account. These are mainly geomaterials, cement, powders of various kinds, ceramics, etc. Experimental evidence is first presented showing that the time effects on irreversible volumetric changes and failure are very important. The concept of compressibility/dilatancy boundary is further introduced. The general constitutive equation able to describe instantaneous response, transient and stationary creep, dilatancy and/or compressibility during creep, failure, creep damage and creep failure, is presented. Examples formulated for various materials are given. Comparison between model prediction and experimental data shows a very good agreement. A few examples of applications of the model to mining engineering problems are mentioned.
Elastic Response of Flexible Polyurethane Foams in Uniaxial Tension
Authors: El-Ratal W. H. , Mallick P. K.
Published: February 2008

Flexible polymeric foams are used in many industrial applications, including cushioning, shock absorption, and noise suppression. There have not been many systematic studies on the characterization of their elastic response that can be used in developing their constitutive behavior. This paper presents the uniaxial tensile data of two different flexible polyurethane foams and determines the relationships between their elastic properties and the major stretch ratio. These relationships will be useful in formulating constitutive models for flexible compressible materials.
Hydrogen Embrittlement Characterization by Disk Pressure Tests: Test Analysis and Application to High Chromium Martensitic Steels
Authors: Beghini M. , Benamati G. , Bertini L.
Published: February 2008

The paper reports and discusses the results of an experimental and numerical activity, aimed to the characterization of the influence of hydrogen on the mechanical properties of a few high chromium martensitic steels which are candidates for fusion reactor and chemical applications. Experiments were conducted with the Disk Pressure Test technique, according to which a circular thin specimen is loaded up to rupture by an uniform pressure. As a detailed analysis of the stress/strain distributions in the specimen was not available, this kind of test being mainly used to obtain comparative information about different materials, a nonlinear numerical (Finite Element) model of the specimen was set up, by which stress and strain could be accurately evaluated as a function of the applied pressure. This model was employed both for interpreting experimental results and to achieve a more general understanding of the capabilities of the Disk Pressure Test for the characterization of hydrogen embrittlement effects. The calculated strain at failure showed the typical dependence on hydrogen content, falling to very low levels as a threshold concentration is exceeded.
Tensile Failure of Stainless-Steel Notched Bars Under Hydrogen Charging
Authors: Valiente A. , Toribio J. , Corte´s R. , Caballero L.
Published: February 2008

The influence of hydrogen embrittlement on the tensile failure of 316L stainless-steel notched bars is phenomenologically modeled in this paper. Tensile tests of notched samples suffering hydrogen embrittlement show that hydrogen damage consists in multicracking in the area surrounding the notch, but the macromechanical behavior of the specimens remains ductile. This suggests two different ways for modeling the damage in order to explain its effect on the tensile failure load. The Notch Extension Model (NEM) considers that damage intensity around the notch is high enough to cancel out the mechanical resistance of this multicracked zone, so it assumes that the hydrogen effect is equivalent to a geometric enlargement of the notch. In the Notch Cracking Model (NCM), it is assumed that high intensity damage is concentrated at the notch root and causes this area to behave as a macroscopic crack that extends the original notch. Experimental values from tests and calculated values from models indicate that the notch extension model describes well the influence of hydrogen on the tensile notch behavior of 316 L stainless steel.
A Model of Residual Stress in the Fabrication of Thin Glass Films
Authors: Murray P. E.
Published: February 2008

We investigate the residual stress which occurs during fabrication of thin glass films. This research involves an innovative approach to modeling of residual stress which will be used to resolve important scientific issues related to the manufacturing of thin glass films for semiconductor devices. The validity of our results is confirmed by comparing analytical predictions of stress in glass films and experimental data reported previously.
Prediction of Residual Stresses in Welded T- and I-Joints Using Inherent Strains
Authors: Yuan M. G. , Ueda Y.
Published: February 2008

In order to develop a predicting method of residual stresses in fillet welded T- and I-joints, a concept of inherent strain, being regarded as a source of the residual stresses, was introduced. With the proposed method, the residual stress of an interested weldment may be predicted by performing an elastic analysis, in which the inherent strain is replaced to equivalent distributed loads. The inherent strain distributions in various welded T- and I-joints were investigated by numerical simulations. The results showed that the inherent strains distributing in flange side and in web side of the several joints are almost the same. The inherent strains vary not only with the average temperature rise due to welding, but with the geometric ratio of the joints. Being simplified by a trapezoid curve, the inherent strain distribution in a fillet weld was expressed by formulas, in which heat input, material properties, and geometric dimensions were taken into account. Welding residual stresses in T- and I-joints, predicted by the proposed method employing the derived formulas, were compared with those obtained by thermal elasto-plastic analysis, and good agreement was recognized. The validity of the proposed method was also confirmed by experiments.
Effect of Carbonization Heating Rate, Microstructure, and Lay-Up on the Interlaminar Tensile Properties of a Two-Dimensional Carbon-Carbon Composite
Authors: Roy Ajit K. , Anderson David P.
Published: February 2008

A study of the effect of the carbonization heating rate, initial phenolic microstructure, and lamination lay-up on the interlaminar tensile (ILT) stiffness and strength (matrix-dominated properties) of two-dimensional carbon-carbon was performed. It was found that the heating rate (2-50°C/hr) during first carbonization did not have any appreciable effect on the final ILT strength of the composite. The quasi-isotropic lay-up produced thicker, less dense laminate than warp-aligned due to less efficient packing of the weaves but yield higher interlaminar tensile strength; probably due to formation of better matrix microstructure than warp-aligned laminates. Overall, the initial laminate quality (porosity) was found to be not important on the final properties; the carbonization and densification processes was found to heal significant amounts of original defects.
Stress-Strain Analysis of Single-Lap Composite Joints Under Tension
Authors: Yang Chihdar , Pang Su-Seng
Published: February 2008

Based on the laminated anisotropic plate theory, an analytical model is proposed to determine the stress and strain distributions of adhesive-bonded composite single-lap joints under tension. The laminated anisotropic plate theory is applied in the derivation of the governing equations of the two bonded laminates. The entire coupled system is then obtained through assuming the peel stress between the two laminates. With the Fourier series and appropriate boundary conditions, the solutions of the system are obtained. Based on the proposed model, the stress and strain distributions of the adherends and the adhesive can be predicted. The coupling effect between the external tension and the induced bending due to the asymmetry of composite laminates are also included. The two adherends can also have different materials and properties. An existing FEA code, “ALGOR,” is used as a comparison with this proposed analytical model. Results from this developed model are also compared with Goland and Reissner’s as well as Hart-Smith’s papers.
A Study of Kerf Characteristics in Abrasive Waterjet Machining of Graphite/Epoxy Composite
Authors: Arola D. , Ramulu M.
Published: February 2008

Kerf geometry, kerf wall features, and cutting front characteristics of an Abrasive Waterjet (AWJ) machined Graphite/Epoxy (Gr/Ep) laminate were studied. A macroscopic analyses suggests that geometrical features associated with AWJ machining of Gr/Ep laminates are influenced by three macro regions along the cutting depth. The presence of these regions, including initial damage at jet entry, smooth cutting, and rough cutting near the jet exit, depends on the operating conditions. Design of experiments and analysis of variance were used to determine the effect of cutting parameters on kerf characteristics and to develop empirical models for kerf profile and features of the three distinct macroscopic regions. Cutting front analysis revealed that the mechanisms of material removal in AWJ machining of Gr/Ep do not change over the jet penetration depth. In general, high quality uniform cuts may be obtained by minimizing initial damage at the jet entry and by extending the smooth cutting region beyond the laminate thickness through the appropriate choice of cutting parameters.
Overcoming Limitations of the Conventional Strain-Life Fatigue Damage Model
Authors: Langlais T. E. , Vogel J. H.
Published: February 2008

The strain-based approach to fatigue life prediction usually relies on the conventional strain-life equation which correlates the elastic and plastic strain to the life. The correlation is based on separate log-linear curve fits of the elastic and plastic components of the strain data versus the life. It is well known, however, that these linear relationships may be valid only within a specific interval of stress or strain. When material behavior approaches elastic-perfectly plastic, for instance, it is not uncommon for the test data to deviate from linearity at both very high and very low strains. For such materials a separate fit of each curve is likely to give material constants significantly inconsistent with the fit of the cyclic stress-strain curve, especially if a good local fit over a restricted interval is obtained. In this work, some of the errors that arise as a result of this inconsistency are described, and recommended methods are developed for treating these errors. Numerical concerns are also addressed, and sample results are included.
A Mechanical Material Model for Aluminium Extrusions During On-line Quenching
Authors: Ja¨rvstra˚t Niklas , Tjo̸tta Stig
Published: February 2008

A material model is suggested, suitable for modelling the mechanical behaviour of aluminium sections, from directly after the extrusion and throughout the on-line quenching to room temperature. An experimental procedure is detailed, whereby all material parameters in the model can be determined by a single test. In the test, the specimen is subjected to a carefully prescribed load history in tension and compression during continuous cooling. Material parameters are determined for the AIMgSi alloy AA6060. Finally, the model is compared with conventional plasticity and viscoplasticity, and found to give much better accuracy.
Stress Calculation Error Analysis for Incremental Hole-Drilling Residual Stress Measurements
Authors: Schajer G. S. , Altus E.
Published: February 2008

The incremental hole-drilling method for measuring non-uniform residual stresses gives stress results that are very sensitive to errors in the measured data. The resulting stress errors can easily become large enough to compromise seriously the usefulness of the calculated stress results. This paper describes a straightforward method for calculating the stress range that has a specified probability of containing the actual residual stresses. Knowledge of this range allows informed interpretations of the stress results to be made. The four measurement error sources considered are: strain errors, hole depth errors, uniform hole diameter errors, and material constant errors. Both the Integral and Power Series stress calculation methods are investigated, and their different responses to measurement errors are described.
Residual Stress Measurement in 304 Stainless Steel Weld Overlay Pipes
Authors: Yen Hung-Ju , Lin Mark Ching-Cheng , Chen Lih-Jin
Published: February 2008

Welding overlay repair (WOR) is commonly employed to rebuild piping systems suffering from intergranular stress corrosion cracking (IGSCC). To understand the effects of this repair, it is necessary to investigate the distribution of residual stresses in the welded pipe. The overlay welding technique must induce compressive residual stress at the inner surface of the welded pipe to prevent of IGSCC. To understand the bulk residual stress distribution, the stress profile as a function of location within wall is examined. In this study the full destructive residual stress measurement technique—a cutting and sectioning method—is used to determine the residual stress distribution. The sample is type 304 stainless steel weld overlay pipe with an outside diameter of 267 mm. A pipe segment is cut from the circular pipe; then a thin layer is removed axially from the inner to the outer surfaces until further sectioning is impractical. The total residual stress is calculated by adding the stress relieved by cutting the section away to the stress relieved by axially sectioning. The axial and hoop residual stresses are compressive at the inner surface of the weld overlay pipe. Compressive stress exists not only at the surface but is also distributed over most of the pipe’s cross section. On the one hand, the maximum compressive hoop residual stress appears at the pipe’s inner surface. The magnitude approaches the yield strength of the material; the compressive stress exists from the inner surface out to 7.6 mm (0.3 in.) radially. On the other hand, compressive axial residual stress begins at depths greater than 2.5 mm (0.1 in.); its maximum value is located at 10.7 mm (0.42 in.) with magnitude close to four-tenths of yield strength. The thermal-mechanical induced crack closure from significant compressive residual stress is discussed. This crack closure can thus prevent IGSCC very effectively.
A Two Surface Plasticity Model With Bounding Surface Softening
Authors: White C. S.
Published: February 2008

Two surface plasticity models have been used increasingly in recent years to model not only uniaxial, cyclic plasticity but also multiaxial and nonproportional histories. A two surface model is presented here which predicts the increased hardening due to out-of-phase cycling in a natural way. It includes a mechanism by which the bounding surface contracts when the yield surface is not in contact. This provides a mechanism that is useful for modeling cycling behavior. Predictions of the model with experiments at moderate strain are presented.
Analytical, Numerical, and Experimental Studies of Effective Elastic Properties of Periodically Perforated Materials
Authors: Meguid S. A. , Kalamkarov A. L. , Yao J. , Zougas A.
Published: February 2008

Comprehensive analytical, finite element analysis and experimental studies are made of the interaction between closely spaced multiple circular holes. The analytical model utilizes the technique of asymptotic homogenization and the complex variable method to obtain explicit expressions for the effective elastic properties of a completely perforated plate under uniaxial loading. The analytical predictions are then compared with a finite element model and the results of both techniques are verified experimentally using uniaxial testing under controlled conditions. The findings of the present work are also compared with earlier solutions and efforts are made to highlight inconsistencies and limitations of the three techniques.
Response of Resin Transfer Molded (RTM) Composites Under Reversed Cyclic Loading
Authors: Mahfuz Hassan , Haque Anwar , Yu Daixu , Jeelani Shaik
Published: February 2008

Compressive behavior and the tension-compression fatigue response of resin transfer molded IM7 PW/PR 500 composite laminate with a circular notch have been studied. Fatigue damage characteristics have been investigated through the changes in the laminate strength and stiffness by gradually incrementing the fatigue cycles at a preselected load level. Progressive damage in the surface of the laminate during fatigue has been investigated using cellulose replicas. Failure mechanisms during static and cyclic tests have been identified and presented in detail. Extensive debonding of filaments and complete fiber bundle fracture accompanied by delamination were found to be responsible for fatigue failures, while fiber buckling, partial fiber fracture and delamination were characterized as the failure modes during static tests. Weibull analysis of the static, cyclic and residual tests have been performed and described in detail. Fractured as well as untested specimens were C-scanned, and the progressive damage growth during fatigue is presented. Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) for the fractured specimen were also performed and the analysis of the failure behavior is presented.
Fluid and Matrix Components of Polyurethane Foam Behavior Under Cyclic Compression
Authors: Rehkopf Jackie D. , McNeice Greg M. , Brodland G. Wayne
Published: February 2008

Semi-rigid polyurethane foam specimens were compressed to 25, 50, or 75 percent strain at nominal strain rates of 0.14/s, 1.4/s or 14/s for up to 100 cycles. The loads carried by the fluid and matrix phases of the foam were determined over these deformation histories using an experimental method recently developed by the authors. The fluid phase contributed significantly to the first-cycle stress of specimens compressed to 50 or 75 percent strain. During subsequent cycles, the fluid component decreased more quickly than the matrix component and became negligible by 100 cycles.
Impact Resistance and Fracture Toughness of Vanadium-Based Microalloyed Forging Steel in the As-Forged and Q&T Conditions
Authors: Yang L. , Fatemi A.
Published: February 2008

Microalloyed (MA) steels are a family of steels which are becoming an increasingly important economic alternative to the traditional quenched and tempered (Q&T) steels. Impact resistance and fracture toughness of vanadium-based MA forging steel, which is the most commonly produced MA steel, are investigated in this study. To compare the behavior with the Q&T steel, both the as-forged and the Q&T conditions are evaluated. Experimental results from Charpy V-notch impact and fracture toughness (KR-curve and JIC) tests are presented and discussed. Correlations between fracture resistance properties based on several proposed equations in the literature are also examined.
An Investigation of Necking Instability in Fiber Reinforced Polypropylene
Authors: Lee J. H. , Vogel J. H.
Published: February 2008

Thermoplastic composite materials hold considerable promise for increased use in low cost high volume applications because of the potential for processing by solid phase forming. Unfortunately, because of the wide variety of such materials, inherent variability in properties, and complex temperature and strain rate dependence, large strain behavior of these materials has not been well characterized. Of particular importance is failure during processing due to localized necking instability, and it is this phenomenon that is the primary focus of this study. Large strain behavior under uniaxial tension is characterized over a range of temperatures and strain rates, and a simple linear viscoelastic model is fit to the observed data. The strain rate and temperature dependence is then used to predict limiting tensile strains, based on Marciniak imperfection theory. Excellent correlation was obtained between theory and experiment, and the results are summarized in maps of forming limit as a function of temperature and strain rate. These results suggest that excellent solid phase formability may be obtained for the materials tested under properly chosen conditions.
Viscoplastic Behavior of a Notch Root at 650°C: ISDG Measurement and Finite Element Modeling
Authors: Li Keyu , Sharpe William N.
Published: February 2008

Viscoplastic behavior of the nickel-based superalloy, Inconel 718, at a notch root has been investigated by conducting isothermal tensile, creep and cyclic elastoplastic tests at 650°C which is the working temperature of the material in aircraft engines. A laser-based interferometric technique was extended to measure biaxial strains over a gage length of 150 μm at the notch root; the computer controlled system permits real-time measurements. A thin-film pure silicon coating and an argon atmosphere were used to prevent oxidation of the reflective indentations that serve as the strain gages. The measured strains were used to evaluate the Bodner-Partom constitutive model which was incorporated into the ABAQUS finite element code. Material constants in the model were determined from uniaxial tests on smooth specimens. The predicted strains agree reasonably well with the measured ones for all three kinds of loadings.
Creep in Building Stones Under Tensile Conditions
Authors: Sorace S.
Published: February 2008

Careful structural analyses of ancient stone constructions methodically showed crack openings under tensile stresses notably lower than the corresponding strength limits. Based on this observation, an experimental research was undertaken aimed at determining the mechanical effects related to permanent loading in time. The results of a first series of long-term tension and bending tests conducted on four natural building stones are presented in this paper. A time-delayed response analogous to the creep behavior of several geomaterials as well as of metals and ceramics was found; the response was then suitably described by the same analytical models usually applied to these last material classes. However, the stone building materials are characterized by a notably higher strength decay taking place in short times which confirmed the trend already shown by the structural studies. A thorough damage analysis developed with reference to the tertiary creep phase revealed both the physical and phenomenological correspondence of the adopted Kachanov-Rabotnov rule to the actual mechanical degradation time-progress.
The Prediction of the Flow Stress of an Extra-Low Carbon Steel in the Two-Phase Region Using Continuous Cooling Curves
Authors: Hwu Yhu-Jen , Lenard J. G. , Too J. J. M.
Published: February 2008

Continuous cooling curves of an extra-low carbon steel under three cooling rates are measured. The flow stress of the steel is established in compression tests during which the temperature is continuously decreasing. The phase transformation temperatures are determined from the cooling rate curve. The latent heat during phase transformation is calculated. A new variable, related to the volume fraction of transformation, is defined. Experimental results show that the relationship between the softening ratio of the flow stress due to phase transformation and this new variable may be described by a quadratic relationship. Based on this relationship and the continuous cooling curves, the flow stresses in the two-phase region are successfully predicted.