bluuebunny/crossref_metadata_2025 huggingface dataset:
A Study of the Dynamic Behavior of Elastomeric Materials Using Finite Elements
Authors: Vallee G. E. , Shukla Arun
Published: February 2008

A numerical method is described for determining a dynamic finite element material model for elastomeric materials loaded primarily in compression. The method employs data obtained using the Split Hopkinson Pressure Bar (SHPB) technique to define a molecular constitutive model for elastomers. The molecular theory is then used to predict dynamic material behavior in several additional deformation modes used by the ABAQUS/Explicit (Hibbitt, Karlsson, and Sorenson, 1993a) commercial finite element program to define hyperelastic material behavior. The resulting dynamic material models are used to create a finite element model of the SHPB system, yielding insights into both the accuracy of the material models and the SHPB technique itself when used to determine the dynamic behavior of elastomeric materials. Impact loading of larger elastomeric specimens whose size prohibits examination by the SHPB technique are examined and compared to the results of dynamic load-deflection experiments to further verify the dynamic material models.
An Experimental Study Evaluating Macroscopic Cavity Interactions as a Model of Microvoid Ductility
Authors: Geltmacher A. B. , Matic P. , Harvey D. P.
Published: February 2008

An experimental study of the interactions between macroscopic cavities was conducted. The goal was to determine simple cavity arrangements producing deformations similar to commonly observed features of ductile fracture. The production of cavity-centered fracture surface features and ligament thinning, representative of the type observed around and between microvoids on ductile fracture surfaces, were the specific objectives of this macroscopic study. The specimens employed were cylindrical tension specimens containing from one to six coplanar radial holes, with spherical end profiles, located at the center of the specimen and either zero or one cylindrical holelocated on the longitudinal axis of the specimen. The results of this parametric study identified the two specimens that best produced the desired features. The load-displacement results of these experiments also illustrate the effect of cavity geometry on the net strength and ductility of the array system. Specimens of these designs provide a future basis for simple and economical model studies of void interactions.
Design Curve to Characterize Fatigue Strength
Authors: Shen C. L. , Wirsching P. H. , Cashman G. T.
Published: February 2008

Fatigue (S-N) data exhibit relatively large scatter. For design purposes, an S-N curve that characterizes fatigue strength is required. This curve should lie on the lower, or safe, side of the data. A method based on a modification of the Owen tolerance interval is proposed. The method is general and can be applied to nonlinear heteroscedastic data having runouts.
Nonlinear Elastic Behavior of Unidirectional Composites With Fiber Waviness Under Compressive Loading
Authors: Hsiao H. M. , Daniel I. M.
Published: February 2008

Nonlinear elastic behavior of unidirectional composites with fiber waviness under compressive loading was investigated theoretically and experimentally. Unidirectional carbon/epoxy composites with uniform, graded, and localized fiber waviness were studied. Complementary strain energy was used to derive the material nonlinear stress-strain relations. Nonlinear material properties obtained from shear and longitudinal and transverse compression tests were incorporated into the analysis. Compression tests of specimens with known fiber waviness were conducted to verify the constitutive relations. Experimental results were in good agreement with predictions based on the constitutive model.
Measuring Method for Residual Stresses in Explosively Clad Plates and a Method of Residual Stress Reduction
Authors: Ueda Y. , Murakawa H. , Ma N. X.
Published: February 2008

In this paper, a new layer removal method for measurement of local residual stress through the thickness of clad plates is developed using inherent strain which is considered a source of residual stress. Using the new method, residual stresses in explosively clad plates of Ni/SUS304 and Zr/Zr/Ta/SUS304 are measured. Then, residual stresses remaining in a pipe joint piece cut from the explosively clad plate are estimated by an elastic analysis using inherent strain. Furthermore, the validity of a mechanical stress reduction method by applying axial compressive load to the pipe joint piece, is demonstrated by elastic-plastic FEM analysis.
Role of Welding Parameters in Determining the Geometrical Appearance of Weld Pool
Authors: Kovacevic R. , Cao Z. N. , Zhang Y. M.
Published: February 2008

A three-dimensional numerical model is developed to describe the fluid flow and heat transfer in weld pools. Both full penetration and free deformation of the top and bottom weld pool surfaces are considered. Temperature distribution and fluid flow field are obtained. In order to analyze the influence of welding parameters on the geometrical appearance of weld pools, a normalized model is developed to characterize the geometrical appearance of weld pools. It is found that welding current can significantly affect the geometrical shape. When welding current increases, the curvature of the pool boundary at the trailing end increases. The effect of the welding speed on the geometrical appearance is slight, although its influence on the pool size is great. In the interest range of arc length (from 1 mm to 4 mm), the arc length can affect both the size and the shape of the weld pool. However, compared with the welding current and speed, its influences are much weaker. GTA welding experiments are performed to verify the validity of the numerical models. The appearance of weld pools was obtained by using machine vision and a high-shutter speed camera. It is found that the calculated results have a good agreement with the experimental ones.
Constitutive Description of Temperature-Dependent Nonproportional Cyclic Viscoplasticity
Authors: Yang Xian Jie
Published: February 2008

This paper is concerned with the constitutive modeling of the temperature history dependent behavior of metallic materials under uniaxial and nonproportional cyclic loadings. In the study, a class of kinematic hardening rules characterized by a decomposition of the total kinematic hardening variable is discussed. A new nonproportionality is defined. In order to consider the influence of complex cyclic loading and temperature histories on materials behavior, an apparent isotropic deformation resistance parameter Qasm is proposed and the evolution equations of the isotropic deformation resistance Q are offered to correlate the memory effect of previous loading history on material behavior. The proposed model is applied to the description of complex cyclic deformation behavior of 1Cr18Ni9Ti stainless steel, and this model gives good results for the prediction of complex tests under complex loading history and at stepwise temperature changes.
Biaxial Creep Strains at Notch Roots—Measurement and Modeling
Authors: Zeng H. , Sharpe W. N.
Published: February 2008

Biaxial strains were measured at the roots of single notches in zirconium alloy specimens under loading and creep conditions (up to 1200 hours) at 250°C. A laser-based interferometric technique was used; details of the measurement procedures are given in another paper. Here, the Bodner model is incorporated into the ABAQUS finite element code and used to predict the biaxial strains at the notch root during loading and for a creep duration of 100 hours. The strains during loading agree quite well with the measured ones while the predicted creep strains are smaller. This discrepancy is attributed to the lack of precision in determining the constants for the Bodner model from smooth-specimen tests. Both the measurement technique and the Bodner model are well-suited for studies of notch response.
Automated Fatigue Data Fitting Using Structural Shift Detection
Authors: Vogel J. H. , Yan X. , Cordes T. S. , Dindinger P. M.
Published: February 2008

A set of data fitting algorithms are presented for determining the cyclic material properties used in the local strain approach to fatigue life prediction. These properties are obtained by fitting power-law curves to the elastic and plastic components of uniaxial strain-life fatigue data obtained under constant amplitude testing. It is well known that these power-law relationships may be expected to be valid only within a limited range of strain amplitude; it is not uncommon, however, to have significant amounts of test data outside of this range. The algorithms presented here address this problem by applying a structural shift detection scheme to the elastic and plastic component life data sets to identify the limits of the valid range. The curve fits obtained are then the fits of only the data lying within this range. The fitting approach is illustrated for two sets of SAE 1045 steel data. In the examples shown, proper fitting of the curves is seen to improve the quality of life prediction by as much as a factor three.
Residual Stress Evaluation by the Hole-Drilling Method With Off-Center Hole: An Extension of the Integral Method
Authors: Vangi Dario
Published: February 2008

With the incremental hole-drilling method, it is possible to evaluate residual stress variations with depth, acquiring strain values for each step by strain gages. These data can be processed by the integral method. Usually strain gages are not spaced equidistant from the hole center nor arranged in the radial direction, due to practical difficulties in drilling a centered hole. In this study we present a development of the integral method for evaluating stress variations with depth in the specimen for eccentric blind hole cases. The results can be extended and adopted for use with a non-conventional strain gage array.
Determination of Sub-Surface Distributions of Residual Stresses by a Holographic-Hole Drilling Technique
Authors: Makino A. , Nelson D. V.
Published: February 2008

A method is described for combining incremental hole drilling with holographic interferometry to determine the distribution of in-plane residual stresses as a function of depth below the surface. The computational foundation of the method is summarized. Results of experiments to check the ability of the method to determine two different sub-surface stress distributions are given. Utilizing the same hologram, the feasibility of obtaining results at greater depths by drilling a large hole directly over a preceding smaller one is demonstrated.
On the Prediction of Stress Relaxation From Known Creep of Nonlinear Materials
Authors: Touati D. , Cederbaum G.
Published: February 2008

A method to predict the nonlinear relaxation behavior from creep experiments of nonlinear viscoelastic materials is presented. It is shown that for given nonlinear creep properties, and creep compliance represented by the Prony series, the Schapery creep model can be transformed into a set of first order nonlinear equations. The solution of these equations enables the obtaining of the nonlinear stress relaxation curves. The strain-dependent constitutive equation can then be constructed for a given nonlinear viscoelastic model, as needed for engineering applications. A comparison example of the calculated stress relaxation curves, with test data for polyurethane demonstrates the very good accuracy of the proposed method.
In Situ Measurement of the Young’s Modulus of an Embedded Inclusion by Acoustic Microscopy
Authors: Canumalla S. , Gordon G. A. , Pangborn R. N.
Published: February 2008

Alumina-silicate inclusions (shot) have been found to adversely affect the mechanical properties of a short alumina-silicate fiber reinforced aluminum alloy (A356). To better understand the differences between the responses of the shot and fibers to applied loads, the Young’s modulus of the shot is measured and compared to that of the fibers. The Rayleigh wave speed in the shot particle (cross-sectional area of 200 μm × 150 μm), measured in situ to be 4041 m/s using a scanning acoustic microscope, was used to calculate the Young’s modulus of the shot particle (132 GPa). The accuracy of the technique and the experimental arrangement used was verified to be better than four percent by independent measurements of the Rayleigh wave speeds in the aluminum alloy matrix and an embedded sapphire fiber. The fiber modulus was estimated to be 225 GPa based on a comparison of previously measured composite modulus with micromechanical predictions. Thus, shot was found to have a Young’s modulus 40 percent lower than that of the fibers. The applicability of the V(z) technique has been demonstrated for measuring the elastic properties over a microscopic area, even when the target material is an embedded inclusion.
Initiation of Surface and Interface Edge Cracks in Functionally Graded Ceramic Thermal Barrier Coatings
Authors: Kokini Klod , Case Michael
Published: February 2008

The initiation of surface and interface edge cracks in functionally graded ceramic thermal barrier coatings resulting from the application of a thermal load is studied. For a given specimen configuration, the singular behavior of the thermal stresses is analytically determined in terms of a singularity power β and pseudo-stress intensity factors K1 and K2. Crack initiation at the surface at the interface is related to the surface temperature and the temperature difference between the surface of the coating and the substrate. The experimental results are used to provide Weibull curves for the probability of survival of plasma sprayed mullite and CoCrAlY coating systems.
An Investigation of Cyclic Transient Behavior and Implications on Fatigue Life Estimates
Authors: Jiang Yanyao , Kurath Peter
Published: February 2008

Current research focuses on proportional cyclic hardening and non-Massing behaviors. The interaction of these two hardenings can result in the traditionally observed overall softening, hardening or mixed behavior exhibited for fully reversed strain controlled fatigue tests. Proportional experiments were conducted with five materials, 304 stainless steel, normalized 1070 and 1045 steels, and 7075-T6 and 6061-T6 aluminum alloys. All the materials display similar trends, but the 304 stainless steel shows the most pronounced transient behavior and will be discussed in detail. Existing algorithms for this behavior are evaluated in light of the recent experiments, and refinements to the Armstrong-Frederick class of incremental plasticity models are proposed. Modifications implemented are more extensive than the traditional variation of yield stress, and a traditional strain based memory surface is utilized to track deformation history. Implications of the deformation characteristics with regard to fatigue life estimation, especially variable amplitude loading, will be examined. The high-low step loading is utilized to illustrate the effect of transient deformation on fatigue life estimation procedures, and their relationship to the observed and modeled deformation.
A Methodology for Predicting Variability in Microstructurally Short Fatigue Crack Growth Rates
Authors: Gall Ken , Sehitoglu Huseyin , Kadioglu Yavuz
Published: February 2008

A finite element model, which implements single crystal constitutive relationships, was used to simulate fatigue cracks growing at the microstructural level. Plastic deformation (slip) was allowed along two specified microscopic crystallographic planes. As the orientations of the slip systems were changed several crucial fatigue crack growth parameters, measured over all possible orientations, were found to vary: (1) crack tip forward slip band size, rp, 0.03 ≤ rp/(Kmax/λo)2 ≤ 0.31 where λo is the critical resolved shear stress on a slip system, (2) crack opening displacement, δ, 1.2 ≤ δ/(Kmax2/Emσo) ≤ 7.8 where Em and σo are the elastic modulus and yield stress of a polycrystalline material with many randomly oriented double slip crystals, and(3) crack closure level, Sopen/Smax, 0.02 ≤ Sopen/Smax ≤ 0.35. Corresponding to these differences in crack growth parameters, crack growth laws were used to estimate the expected changes in crack growth rates when microstructurally short cracks grow through grains with different crystallographic orientations. The resulting predictions form approximate upper and lower bounds on crack growth rates for microstructurally short cracks. For several different materials, the crack growth rate variability predictions were in the range 7 ≤ (da/dN)(max)/(da/dN)(min) ≤ 37, which is consistent with experimentally measured variations.
Response of SiCf/Si3N4 Composites Under Static and Cyclic Loading—An Experimental and Statistical Analysis
Authors: Mahfuz Hassan , Maniruzzaman Md. , Vaidya Uday , Brown Timothy , Jeelani Shaik
Published: February 2008

Monotonic tensile and fatigue response of continuous silicon carbide fiber reinforced silicon nitride (SiCf/Si3N4) composites has been investigated. The monotonic tensile tests have been performed at room and elevated temperatures. Fatigue tests have been conducted at room temperature (RT), at a stress ratio, R = 0.1 and a frequency of 5 Hz. It is observed during the monotonic tests that the composites retain only 30 percent of its room temperature strength at 1600°C suggesting a substantial chemical degradation of the matrix at that temperature. The softening of the matrix at elevated temperature also causes reduction in tensile modulus, and the total reduction in modulus is around 45 percent. Fatigue data have been generated at three load levels and the fatigue strength of the composite has been found to be considerably high; about 75 percent of its ultimate room temperature strength. Extensive statistical analysis has been performed to understand the degree of scatter in the fatigue as well as in the static test data. Weibull shape factors and characteristic values have been determined for each set of tests and their relationship with the response of the composites has been discussed. A statistical fatigue life prediction method developed from the Weibull distribution is also presented. Maximum Likelihood Estimator with censoring techniques and data pooling schemes has been employed to determine the distribution parameters for the statistical analysis. These parameters have been used to generate the S-N diagram with desired level of reliability. Details of the statistical analysis and the discussion of the static and fatigue behavior of the composites are presented in this paper.
Automated Evaluation of Material Parameters of Viscoplastic Constitutive Equations
Authors: Schwertel J. , Schinke B.
Published: February 2008

A method will be presented that allows for a special class of viscoplastic constitutive models a fast and safe determination of the material parameters. All parameters are determined simultaneously from creep, tensile, and strain-controlled cyclic tests. It is not necessary to solve the model differential system numerically; approximations allow for almost all of these loading cases an analytical integration with sufficient precision. The procedure for the solving of the model differential equations as well as the fit procedure will be described. The computer time for the parameter evaluation lies in the range of only some minutes even for a large number of experimental data. Results of fits for the material AISI 316L at 700°C will be presented for the viscoplastic models of Robinson, Chaboche and Walker. In addition to the fits, the models are subjected to a statistical investigation which makes evident their structures with respect to the convergence behaviour of the fit procedure and the sensitivity of the models to parameter variations.
The Influence of Thermoplastic Film Interleaving on the Interlaminar Shear Strength and Mode I Fracture of Laminated Composites
Authors: Li L. , Lee-Sullivan P. , Liew K. M.
Published: February 2008

Results are presented on the interlaminar shear strength (ILSS) and Mode I fracture toughness (GIC) of glass/epoxy laminates interleaved with thermoplastic polyester films using four-point bending and double cantilever beam tests, respectively. The ILSS equation from classical beam theory was modified to account for the increased film thickness. It was found that the ultimate failure load and ILSS could be doubled if a thermoplastic film of high ductility but low glass transition temperature is used. Good film/composite ply adhesion is necessary. Mode I fracture toughness is influenced by film thickness and interleaving with a 0.2 mm thick film increased the GIC by 40 percent. Interleaving with a thinner film (0.1 mm), however, resulted in adhesive failure and reduced fibre bridging.
Behavior of a Unidirectional Metal-Matrix Composite Under Thermomechanical Loading
Authors: Chun Heoung-Jae , Daniel Isaac M.
Published: February 2008

The thermoelastoplastic behavior of a unidirectional metal matrix composite (SiC/Al) under thermomechanical loading was studied with a micromechanical model based on the average field theory. The silicon carbide fiber is considered elastic and temperature independent whereas the matrix is thermoviscoplastic and fitted into a series of power law strain hardening models. The thermoelastoplastic analysis of the composite was carried out by introducing the concept of secant properties of the matrix. Analytical predictions were compared with experimental results. Under longitudinal tensile loading the predicted stress-strain curves were in good agreement with experimental curves at three temperatures (24, 288, and 399°C). Under transverse tensile loading the secant properties of the matrix and the average stresses in the matrix and fiber (at room temperature) were obtained as a function of applied stress. The predicted stress-strain curves under transverse loading were in satisfactory agreement with experimental ones at temperatures up to 399°C. Longitudinal and transverse thermal strains as a function of temperature were also predicted and compared favorably with experimental measurements.
Size Effect and Fracture Characteristics of Composite Laminates
Authors: Bazˇant Zdeneˇk P. , Daniel Isaac M. , Li Zhengzhi
Published: February 2008

Measurements of the size effect on the nominal strength of notched specimens of fiber composite laminates are reported. Tests were conducted on graphite/epoxy crossply and quasi-isotropic laminates. The specimens were rectangular strips of widths 6.4, 12.7, 25.4 and 50.8 mm (0.25, 0.50, 1.00 and 2.00 in.) geometrically similar in two dimensions. The gage lengths were 25, 51, 102 and 203 mm (1.0, 2.0, 4.0 and 8.0 in.). One set of specimens had double-edge notches and a [0/922]s crossply layup, and another set had a single-sided edge notch and a [0/±45/90]s, quasi-isotropic layup. It has been found that there is a significant size effect on the nominal strength. It approximately agrees with the size effect law proposed by Bazˇant, according to which the curve of the logarithm of the nominal strength versus the logarithm of size represents a smooth transition from a horizontal asymptote, corresponding to the strength criterion (plastic limit analysis), to an inclined asymptote of −0.5 slope, corresponding to linear elastic fracture mechanics. Optimum fits of the test results by the size effect law are obtained, and the size effect law parameters are then used to identify the material fracture characteristics, particularly the fracture energy and the effective length of the fracture process zone. Finally, the R-curves are also identified on the basis of the maximum load data. The results show that in design situations with notches or large initial traction-free cracks the size effect on the nominal strength of fiber composite laminates must be taken into account.
Symmetric Branching of Mode II and Mixed-Mode Fatigue Crack Growth in a Stainless Steel
Authors: Qian Cai-Fu , Wang Ming-O , Wu Bao-Juan , Dai Shu-Ho , Li J. C. M.
Published: February 2008

Symmetric branching was found in all fatigue experiments carried out in a cruciform specimen in which the initial crack was in the middle of the specimen and at 45 deg with respect to the loading axes. The loading was biaxial, sinusoidal along each axis and out-of-phase from each other. Stress intensity factor calculations using finite element methods showed ΔKII was almost zero along each branch for all four branches so that all branches were Mode I cracks. The propagation of branched Mode I cracks in the initial Mode II damaged zone was faster than in their own Mode I zone created afterwards. The two dominantly branched cracks shielded the other small cracks appeared during the initiation of branching.
Thermal Phase Transformation and Residual Stress Estimation of Multi-Layer Materials for Cylindrical or Flat Shape
Authors: Li Ya Yun , Sheng I-Ching , Liang Shawn S.
Published: February 2008

The estimation of the thermal, phase transformation, and residual stresses in multilayer materials during arbitrary loading is a complex problem, which is important in many manufacturing processes. This paper describes an analytical method to predict these stresses using classic elasticity/plasticity theories under cylindrical coordinates. The method is based on the well-established analysis of an inclusion embedded into a matrix. The multi-layer problem is treated in increments of two layers at a time in a straightforward manner. The computational time required for two sample quenching problems and one sample surface grinding problem was less than half minute each on a PC. Both the thermal stress and residual stress estimation results agree well with the FEM results, especially in the limited depth of the surface. This program is not limited to quenching and grinding problems. It can be applied to many manufacturing processes for predicting in-process stress and residual stress of work-pieces in order to optimize the processes.
Nondestructive Detection of Damage Produced by a Sharp Indenter in Ceramics
Authors: Ahn Hyo-Sok , Wei Lanhua , Jahanmir Said
Published: February 2008

The feasibility of an ultrasonic technique using normal-incident compressional waves and a thermal wave measurement technique was evaluated for their sensitivity to surface and subsurface damage in ceramics. Well-defined damage in the form of surface and subsurface cracks was introduced by Vickers indentation in soda-lime glass and silicon nitride. The indentation impressions were first examined by optical microscopy to identify the types of cracks and to measure the size of the indents and cracks. As expected, indentation produced median/radial cracks emanating from the indent corners and subsurface lateral cracks. The ultrasonic technique was successful in detecting the subsurface lateral cracks in both materials. The signals obtained by focusing the transducer into the material (i.e., defocusing) was used to estimate the depth of subsurface cracks. The lateral cracks and the median/radial cracks were detected by the thermal wave measurement technique using the optical beam deflection method. The lateral cracks and the median/radial cracks were identified separately by using two deflection components of the probe beam. The transverse deflection component of the probe beam was used for the detection of the median/radial cracks, whereas the normal deflection component was used for the detection of the lateral cracks. The results are discussed in terms of the applicability of these two techniques as nondestructive methods for the detection of machining-induced damage in ceramics.
Accommodation of Springback Error in Channel Forming Using Active Binder Force Control: Numerical Simulations and Experiments
Authors: Sunseri M. , Cao J. , Karafillis A. P. , Boyce M. C.
Published: February 2008

Springback in a forming process is due to the elastic deformation of the part during unloading. This manufacturing defect can be accounted for through proper tooling design or through proper design and control of the magnitude and history of restraining force. Using finite element analyses of the process: (1) the effects of restraining force on the springback phenomena when stamping channels from aluminum sheet are investigated; (2) a strategy to control the binder force during the forming operation in order to reduce springback and simultaneously avoid tearing failure is described; and (3) a binder force control strategy which provides robustness in the presence of process parameter uncertainty is implemented. The process history and controller designed using finite element analyses is then experimentally verified: excellent agreement between simulation and experiments is obtained. A binder force history, which leads to a significant reduction in the amount of springback incurred by the formed part without reaching critical stretching conditions, was proposed. Although an optimal forming history was found, in order to ensure that part shape error remained minimized even in the event of variations in processing parameters such as friction, a closed-loop control algorithm was developed whereby the binder force is altered during the process in order to provide a robust, repeatable stretching history. Experiments were performed using a double-action servo-controlled process and were found to produce the desired results demonstrating both the accuracy of the numerical simulation and the success of the proposed active-binder force control method to obtain net shape.
Tension-Torsion Multiaxial Low Cycle Fatigue of Mar-M247LC Directionally Solidified Superalloy at Elevated Temperature
Authors: Shirafuji Nakao , Shimomizuki Kenji , Sakane Masao , Ohnami Masateru
Published: February 2008

This paper studies the high temperature multiaxial low cycle fatigue of Mar-M247LC directionally solidified superalloy. Tension-torsion multiaxial low cycle fatigue tests were carried out using Mar-M247LC tubular specimens at 1173K in air. Several multiaxial strain and stress parameters were applied to the experimental data to examine the suitability of the parameters. All the multiaxial strain parameters proposed so far could not successfully correlate the multiaxial low cycle fatigue data of the directionally solidified superalloy, but the maximum principal stress and the equivalent stress based on crack opening displacement could correlate the data within a small scatter. This paper proposes a new multiaxial strain parameter which takes account of the anisotropy of elastic constant of directionally solidified superalloys. The proposed strain parameter correlates the multiaxial low cycle fatigue data within a factor of two scatter band. This paper also describes the crack mode and cyclic constitutive relation of the superalloy in connection with the anisotropy of the elastic constant.
Matrix Cracking With Irregular Fracture Fronts as Observed in Fiber Reinforced Ceramic Composites
Authors: Hu Kai X. , Yeh Chao-pin , Wyatt Karl W.
Published: February 2008

As a result of matrix cracking in fiber reinforced composites, fracture planforms assume a wide variation of profiles due to the fact that fiber bridging strongly affects the behavior of local crack fronts. This observation raises the question on the legitimacy of commonly used penny-shaped crack solutions when applied to fiber reinforced composites. Accordingly, investigation of the effects of fracture front profiles on mechanical responses is the thrust of this paper. We start with the solution of a penny-shaped crack in a unidirectional, fiber reinforced composite, which demonstrates necessarity of considering wavy fracture fronts in fiber reinforced composites. A theoretical framework for fiber reinforced composites with irregular fracture fronts due to matrix cracking is then established via a micromechanics model. The difference between small crack-size matrix cracking and large crack-size matrix cracking is investigated in detail. It is shown that the bridging effect is insignificant when matrix crack size is small and solution of effective property are obtained using Mori-Tanaka’s method by treating cracks and reinforcing fibers as distinct, but interacting phases. When the crack size becomes large, the bridging effects has to be taken into consideration. With bridging tractions obtained in consistency with the micromechanics solution, and corresponding crack energy backed out, the effective properties are obtained through a modification of standard Mori-Tanaka’s treatment of multiphase composites. Analytical solutions show that the generalization of a crack density of a penny-shaped planform is insufficient in describing the effective responses of fiber-reinforced composites with matrix cracking. Approximate solutions that account for the effects of the irregularity of crack planforms are given in closed forms for several irregular crack planforms, including cracks of cross rectangle, polygon and rhombus.
Local Variations of Strain and Strain Rate in Roll Bite Region During Rolling of Steels
Authors: Tseng Ampere A. , Wang Shi R. , Lau A. C. W.
Published: February 2008

A combined experimental-numerical approach has been developed to quantify the strain rate variation of the workpiece in the roll bite region. In this approach, cold rolling experiments at a production mill were conducted first. Then tensile and microhardness tests were performed on workpieces before and after cold rolling to establish the relationship between the microhardness and plastic strain of the material. Microhardness measurements were also conducted in the roll bite region on a partially cold rolled workpiece. A finite element rolling simulation was performed to predict the spatial variations of the strain and strain rate. Through microhardness matching, it was found that the finite-element predicted strains agree very well with those actually existing in the rolled workpiece. Consequently, the finite-element predicted strain rates, whose time-accumulation directly gave strains which matched the actual strains, were verified. Finally, a finite-element simulation of both cold and hot rolling was conducted to assess the effect of several major rolling parameters on the strain rate variation in the bite region. Results show that the spatial variations of strain rate in the roll bite region are extremely nonuniform for both cold and hot rolling.
Oxidation Induced Stress-Rupture of Fiber Bundles
Authors: Lara-Curzio Edgar
Published: February 2008

The effect of oxidation on the stress-rupture behavior of fiber bundles was modeled. It is shown that oxidation-induced fiber strength degradation results in the delayed failure of the associated fiber bundle and that the fiber bundle strength decreases with time as t-1/4. It is also shown that the temperature dependence of the bundle loss of strength reflects the thermal dependence of the mechanism controlling the oxidation of the fibers. The effect of gauge length on the fiber bundle strength was also analyzed. Numerical examples are presented for the special case of Nicalon™ fibers.
Evaluation of Creep-Fatigue Life Prediction Methods for Low-Carbon Nitrogen-Added 316 Stainless Steel
Authors: Takahashi Yukio
Published: February 2008

Low-carbon, medium-nitrogen 316 stainless steel is a principal candidate for a main structural material of a demonstration fast breeder reactor plant in Japan. A number of long-term creep tests and creep-fatigue tests have been conducting for two heats of the steel. Two representative creep-fatigue life prediction methods, i.e., time fraction rule and ductility exhaustion method were applied. An introduction of a simple viscous strain term improved the description of stress relaxation behavior and only the conventional (primary plus secondary) creep strain was assumed to contribute to creep damage in the ductility exhaustion method. The present ductility exhaustion approach was found to have very good accuracy in creep-fatigue life prediction, while the time fraction rule overpredicted failure life as large as a factor of 30.
The Burdened Area as a Structural Indicator of Interface Bonding Strength
Authors: Luo Y. F.
Published: February 2008

In laminate structures, the interface debonding failures usually result from very complex fracture mechanisms which lead to different strengthening approaches. The bonding strength of two dissimilar bodies depends, to a great extent, on the bulk materials and the bonding structure in addition to the intrinsic interface adhesion. The structural effect on bonding strength will be emphasized in this investigation. The analysis will exhibit the relationship of materials, structures, and interface adhesion which is essential in understanding the failure mechanisms. This paper proposes that the burdened area on an interface is, according to experimental and analytical results, a structural indicator for bonding strength. Such a comprehensive parameter makes it easy to determine the complex contributions of so many structural parameters to the bonding strength. Although there are many microscopic observations of separated surfaces which support the concept of burdened area, a direct measurement of the burdened area is difficult in practice. Therefore an analytical or numerical evaluation is necessary. The solutions from the classical equation of elastic plate will be used to determine the distribution of interface stress as well as the size of burdened area. The bonding strength is considered as fracture toughness which is directly related with the burdened area. As a useful indicator for evaluating the bonding strength, the burdened area includes the effects of many structural parameters and mechanical properties, such as: elastic modulus, Poisson’s ratio, layer thickness, slanting angel, and corner radius. Burdened area is a property of structure because it is independent of the peeling loads, the material’s yielding strength and interface adhesion. As far as energy release rate is concerned, the boundary load is virtually distributed on the whole burdened area. The concept of burdened area will facilitate mechanical design of bonding strength and leads to a better understanding of various debonding failures.
Damage Accumulation Mechanisms in Thermal Barrier Coatings
Authors: Newaz G. M. , Nusier S. Q. , Chaudhury Z. A.
Published: February 2008

Progressive damage evolution leading to spallation was investigated in Electron Beam—Plasma Vapor Deposition (EB-PVD) partially stabilized zirconia thermal barrier coating (TBC) applied to Nickel-based single crystal superalloy, Rene N5 with PtAl bondcoat. Thermal cycles were between 200-1177C. Progressive damage evolution was monitored using microscopy on samples subjected to a series of thermal cycles. Fick’s law can describe the thermally grown oxide (TGO) thickness for early cycles. However, at higher number of thermal cycles, damage in the form of microcracks and their link-up results in the development of a larger delamination crack through the TGO layer and monitoring oxide thickness becomes difficult. Thus, both oxidation kinetics and damage appears to play significant roles as they relate to spallation. As the early microcracks coalesce to form a major delamination crack, the susceptibility for TBC buckling is increased. The damage thickness continues to increase with number of thermal cycles indicating a progressive buckling condition. Estimation shows that a delamination crack length of about sixteen times the TBC thickness is needed for the current material system to cause buckling. Progressive microcrack linking to form a large delamination crack followed by progressive buckling of the TBC layer appear to promote spallation. Physical evidence of microcrack link-up and progressive buckling was found in specimens prior to complete spallation.
An Investigation of Thin-Film Coating/Substrate Systems by Nanoindentation
Authors: Li Jackie , Thostenson Erik T. , Chou Tsu-Wei , Riester Laura
Published: February 2008

The indentation load-displacement behavior of three material systems tested with a Berkovich indenter has been examined. The materials studied were the substrate materials—silicon and polycarbonate, and the coating/substrate systems—diamond-like carbon (DLC) coating on silicon, and DLC coating on polycarbonate. They represent three material systems, namely, bulk, soft-coating on hard-substrate, and hard-coating on soft-substrate. Delaminations in the soft-coating/hard-substrate (DLC/Si) system and cracking in the hard-coating/soft-substrate system (DLC/Polycarbonate) were observed. Parallel to the experimental work, an elastic analytical effort has been made to examine the influence of the film thickness and the properties of the coating/substrate systems. Comparisons between the experimental data and analytical solutions of the load-displacement curves during unloading show good agreement. The analytical solution also suggests that the Young’s modulus and hardness of the thin film can not be measured accurately using Sneddon’s solution for bulk materials when the thickness of the film is comparable to the loading contact radius of the indenter. The elastic stress field analysis provides a basis for understanding the experimentally observed delaminations and cracking of the coating/substrate systems.
The Single Slice Method for Measurement of Axisymmetric Residual Stresses in Solid Rods or Hollow Cylinders in the Region of Plane Strain
Authors: Cheng Weili , Finnie lain
Published: February 2008

A new method is presented for the measurement of axisymmetric residual stresses in a solid rod or hollow cylinder in the region which is in a state of plane strain. The experimental procedure involves, first, making a complete transverse cut, normal to the axis of the part in the region which is initially in plane strain. Next, a number of strain gages are mounted on one of the faces produced by the first cut. A second transverse cut, parallel to the first, is then made to remove a thin slice containing the strain gages. It is shown that the change in strain gage readings due to removal of the slice may be used to deduce the axial stress in plane strain at the location of the first cut. The hoop stresses in the thin slice, which is in a state of plane stress, are readily obtained from the “crack compliance” procedure which we have described in earlier work. From this information and the initial axial stress in plane strain it is shown that the initial hoop and radial stresses in plane strain may be determined. The method is validated by obtaining the stresses in a water quenched rod which are shown to be in excellent agreement with those measured using a different experimental approach. Since the method does not require the measurement to be carried out on the original rod or cylinder, it allows the original residual stresses to be measured from a fractured shaft by making a slice near the plane of the fracture if the fracture is dominated by elastic deformation.
A Consistent Criterion for Diffuse Necking in Sheet Metals Using Hill’s 1979 Yield Surface
Authors: Esche S. K. , Shivpuri R.
Published: February 2008

A review of some existing criteria for diffuse necking in sheet metals is given and their limitations are discussed. The introduction into production of new sheet materials whose plastic deformation is impossible to be modeled using Hill’s 1948 anisotropic yield function necessitates improvements of these existing criteria to accurately describe their necking behavior. In this paper, a generalization of the existing diffuse necking criteria for materials describable by Case IV of Hill’s 1979 anisotropic yield function is presented. The proposed criterion is consistent with the previous criteria. It predicts a significant effect of Hill’s 1979 yield surface shape factor on the critical principal strain in the range of negative minor strains while in the range of biaxial tension this influence is small.
A Constitutive Model for Powder Materials
Authors: Jin Jishan , Cristescu N. D.
Published: February 2008

A new elastic/viscoplastic constitutive model for compaction of power materials has been proposed. The model can be determined from a few conventional compression triaxial tests. In this model the irreversible volumetric strain is taken as work-hardening parameter. The model prediction matches reasonably well the experimental data.
Cyclic Fatigue Experiments and Analysis of Silicon Nitrides—Stepwise Loading
Authors: Chiu Y. P.
Published: February 2008

Rotating beam fatigue test has been performed at room temperature for two types of advanced hot-pressed silicon nitrides. The test completion modes consist of fast fracture, fatigue fracture, and runout (at about 108 cycles). Some specimens tested have been subjected to step-up load testing. Application of Weibull statistical method has been made as the fatigue lives of specimens under a constant bending load are subjected to wide scatter. A maximum likelihood analysis was applied to test data obtained from the single load and step-up load tests to determine the fatigue (or crack growth) exponent. The effect of specimen surface finish on fast and fatigue fracture has also been investigated using the Weibull analysis.
Damage in Composite NiMH Positive Electrodes
Authors: Sastry A. M. , Choi S. B. , Cheng X.
Published: February 2008

Mechanical degradation in fibrous composite electrodes in use in NiMH(nickel metal hydride) batteries is being studied. NiMH cells exhibit failure in the positive plate due to swelling induced during the electrochemical reaction, which leads to gradual breakdown of the connectivity of the substrate microstructure. Particularly, loss of electrical conductivity as related to losses in mechanical properties upon cell cycling (constant current cycles with substantial overcharge) are being investigated experimentally. These properties are measured for the dry substrate material before and after battery cycling, and the losses observed suggest key tradeoffs between use of higher density nickel substrates and improved energy densities using mechanical and resistivity proof tests.
Numerical Green’s Function for Steady-State Heat Conduction in Heterogeneous Media
Authors: Choi Deok-Kee , Nomura Seiichi
Published: February 2008

Numerical Green's function for steady-state heat conduction problems is derived in a finite-sized medium that may contain inclusions (fibers) in the matrix phase. Green's function is approximated by employing the Galerkin method that uses permissible functions which satisfy the homogeneous boundary condition for the given geometry. The present approach allows physical fields in a medium that contain multiple inclusions to be expressed through isolated integrals semi-analytically while retaining all the relevant material parameters.
Viscoplastic Collapse of Thin-Walled Tubes Under Cyclic Bending
Authors: Pan Wen-Fung , Her Yung-Shun
Published: February 2008

This paper presents the experimental result on the response and stability of thin-walled tubes subjected to cyclic bending with different curvature-rates. The curvature-ovalization measurement apparatus, designed by Pan et al. (1998), was used for conducting the present curvature-controlled experiments on thin-walled tubular specimens of 304 stainless steel. It is observed that the higher the applied curvature-rate, the greater is the degree of hardening of metal tube. However, the ovalization of tube cross-section increases when the applied curvature-rate increases. Furthermore, due to the higher degree of the ovalization of tube cross-section for higher curvature-rates under cyclic bending, the number of cycles to produce buckling is correspondingly reduced.
Automatic Simulation of a Sequence of Hot-Former Forging Processes by a Rigid-Thermoviscoplastic Finite Element Method
Authors: Joun M. S. , Moon H. K. , Shivpuri Rajiv
Published: February 2008

A fully automatic forging simulation technique in hot-former forging is presented in this paper. A rigid-thermoviscoplastic finite element method is employed together with automatic simulation techniques. A realistic analysis model of the hot-former forging processes is given with emphasis on thermal analysis and simulation automation. The whole processes including forming, dwelling, ejecting, and transferring are considered in the analysis model and various cooling conditions are embedded in the analysis model. The approach is applied to a sequence of three-stage hot former forging process. Nonisothermal analysis results are compared with isothermal ones and the effect of heat transfer on predicted metal flows is discussed.