bluuebunny/crossref_metadata_2025 huggingface dataset:
On the Solid-Fluid Transition Zone in Welding Analysis
Authors: Chen Y. , Sheng I. C.
Published: April 2008

Equilibrium and energy equations have been developed in describing the solid-fluid transition zone during the melting and solidification of the binary alloys. Due to the existence of the transition region exhibiting both solid and fluid phases at the same material point from continuum point of view, mixture theory was utilized to analyze the region. Unlike the Stefan problem, the latent heat due to the phase change appears as a source term in the heat equation. The molten fluid is treated as a thermoviscous and incompressible fluid, whereas the solid is thermoviscoplastic described by the Bodner-Partom/Walker type of constitutive equations. Thermal mechanical behaviors of the solid and the fluid phases are determined separately because of insignificant mechanical interactions between them. Volume fractions of the phases are obtained according to the equilibrium phase diagram. The simulation process of the transition zone and the welding process was carried out by FEM. The molten fluid motion, the sizes, and the contours of the transition zone were presented.
Flow Effects on the Solidification Environment in a GTA Spot Weld
Authors: Bertram L. A.
Published: April 2008

The ultimate aim in the simulation of weld pools is to depict the final state of the solidified metal. Solidification history determines the metallurgical state, and can be partially derived from macroscale simulation. This requires realistic initial conditions—weld pool temperatures and flows at power off time—in order to produce accurate solidification histories for the pool points. Numerical simulation of the heat and mass tranfer is carried out here with a finite volume finite difference scheme on a flat surfaced pool (appropriate for currents of present interest). Suface tension, buoyancy and Lorentz forces are included in the flow model. When the arc heating and Lorentz forces are shut off, continuation of the calculation allows examination of the solidification thermal environment. Welds on stainless steel are simulated with fluid flow driven by an “effective” surface tension coefficient of ∂γ/∂T = −0.01 dyne/cm K. Solidification events consist of an initial phase which smooths the fusion zone boundary and removes the superheat from the pool, followed by a quasisteady stage, and end with a terminal boundary layer with time dependence similar to the singular spherical Stefan solution. Introduction of “numerical macrographs” allows convenient comparison of simulated conditions with actual weld macrographs.
Freezing Characteristics Along a Horizontal Cooled Tube Immersed in Aqueous Binary Solution With Main Flow
Authors: Yamada M. , Fukusako S. , Tago M. , Horibe A.
Published: April 2008

An experimental study has been performed to investigate the freezing characteristics of an aqueous binary solution along a horizontal cooled tube. A copper coaxial cooling tube was set horizontally in a flow duct which has cross-sectional dimensions of 120 mm × 200 mm. Ethylene-glycol solution was utilized as a test solution. Observation of frozen layer, measurement of heat-transfer coefficient, and visualization of flow pattern were extensively carried out under a variety of concentration of the solution, initial temperature, cooling temperature, and flow velocity as parameters. It was found that the characteristics of the frozen layer could be well grouped using both the Reynolds number and the cooling temperature ratio, and that the flow field had a considerable effect on the characteristics of the frozen layer. The correlations of the averaged frozen-layer thickness at the steady state were determined.
The Mechanics of the Interface Crack Using the Finite Element Method
Authors: Reynolds R. R. , Kokini K. , Chen G.
Published: April 2008

The interface crack between two dissimilar materials subjected to a tension load was analyzed using the finite element method in order to study the “open” and “closed” crack tip formulations of this problem. It was shown that these two formulations can be modelled by the absence or presence of interface elements along the crack faces with a very fine mesh. The fracture parameters (k and G) were calculated using crack flank displacements and extrapolation of the interfacial stresses to the crack tip. It was shown that even if the mesh is not fine enough to detect crack closing, the fracture parameters can be determined with a reasonable accuracy.
Processing and Characterization of Laser-Cladded Coating Materials
Authors: Komvopoulos K. , Nagarathnam K.
Published: April 2008

The feasibility of laser cladding as a surface modification process was experimentally investigated. Emphasis was placed on identification of the effects of independent critical process parameters such as laser power, process speed (interaction time), and feed rate of cladding powder mixture on the microstructure, compositional homogeneity, geometry (e.g., thickness and width), and mechanical properties of the developed coatings. Rapidly solidified coatings metallurgically bonded to AISI 1018 steel substrates were formed in situ by using a 10 kW continuous wave CO2 laser to melt a thin layer of the substrate as well as a powder mixture consisting of Fe, Cr, C, and W with a weight ratio of 10:5:1:1 delivered to the substrate by means of blown shielding gas and a pneumatic screw feed system. Various diagnostic methods, indentation hardness measurements, and scratch-resistance testing of the laser-cladded coating materials revealed a high degree of grain refinement, increased solid solubility and uniform distribution of alloying elements, high hardness, and appreciable resistance against plastic shear deformation when the important process parameters were optimized. Microstructure studies demonstrated that coatings with very fine or relatively coarse dendritic, feathery, and particulate type microstructures were obtained, depending on the processing conditions. This investigation verified that due to the inherent rapid solidification and high concentration of key elements in the surface, hard coating materials of novel microstructures and physical properties which can be tailored to the surface requirements of the application can be produced with minimum dilution and thermal distortion. Implications of the laser cladding process in tribological applications are also interpreted qualitatively in light of the obtained results.
Processing and Characterization of Multi-Layered Wear-Resistant Ceramic Coatings
Authors: Murthy A. K. , Komvopoulos K. , Brown S. D.
Published: April 2008

A new concept of a multi-layered ceramic coating consisting of a porcelain enamel bond interlayer with thermal expansion characteristics tailored to match those of a cast iron substrate and a plasma-sprayed wear-resistant surface layer of chromium oxide, chromium carbide, or partially stabilized zirconia was investigated. Flat substrates of grit-blasted and surface-treated cast iron were slurry sprayed with a finely ground glass frit mixed with isopropyl alcohol and water. Subsequently, the specimens were dried in an oven before firing at 1023 K to produce a smooth, dense, and strongly adherent enamel coating 75 to 100 μm thick. Wear-resistant coatings of oxides and carbides with varying particle sizes and powder densities were then thermally sprayed, by means of a plasma spray gun, at controlled power ratings and gas flow rates. The produced ceramic coatings were subsequently ground to a thickness of about 120 μm. Adherence testing by the tensile pull-off technique revealed high interfacial strength between the enamel coating and the substrate resulting from enhanced chemical interdiffusion and mechanical interlocking. Unlubricated sliding wear experiments using a ball-on-flat tribotester and tungsten carbide balls as sliders demonstrated low initial and moderately high steady-state friction coefficients. Optical and scanning electron microscopy and surface profilometry of the tested ceramic-coated specimens verified that surface damage and wear rate were negligibly small. The important role of the main process parameters on the interfacial adherence and uniformity of the enamel and ceramic layers and the potentiality of the developed processes in the deposition of relatively low-friction and wear-resistant multi-layered thick ceramic coatings are discussed in the context of the obtained results.
Prediction of Fatigue Life With and Without Hold Times Using the Chaboche Viscoplastic Constitutive Theory
Authors: Chiu S. S. , Eftis J. , Jones D. L.
Published: April 2008

The titanium alloy Ti-6Al-4V is known to exhibit creep behavior at temperatures as low as room temperature. Consequently, for cyclic loading with hold times it is possible that the rate dependent behavior of Ti-6Al-4V can have negative bearing upon the low cycle fatigue life. If this effect is shown to be present at room temperatures, then it will certainly be magnified and, therefore, very important at elevated temperatures. In order to account for the effects of strain rate dependent deformation in fatigue life prediction methodology, it was considered necessary to incorporate a viscoplastic constitutive equation into the fatigue life calculational algorithm. After critical evaluation of a score of recently proposed viscoplastic constitutive theories, the Chaboche theory, which employs a yield condition, was considered to offer the most promise for description of a wide range of inelastic material behavior characteristics. The six viscoplastic material parameters that are required for nonelevated temperature applications were determined from data of uniaxial tests, conducted elsewhere and made avialable to this study. The fatigue life testing of smooth round bar specimens included load cycles with load hold times. Fatigue life predictions were performed using the equivalent fully reversed symmetric cycle, and the Smith-Watson-Topper parameter, for load cycles having varying stress amplitudes and varying hold times. The predicted fatigue life results indicate that: (i) For a given stress level above the initial yield stress, shorter load hold time periods result in longer fatigue lives. (ii) The higher the stress level (above the initial yield stress) the more pronounced becomes the effect of the load hold time on the fatigue life prediction. (iii) The rate of loading also has an effect on fatigue life. Analysis indicates that the slower the rate of loading, the higher the rate dependent (primary creep) deformation, and consequently, the lower the resulting fatigue life.
Creep Analysis of Beams and Arches Based on a Hereditary Visco-Elastic-Plastic Constitutive Law
Authors: Stubstad J. M. , Simitses G. J.
Published: April 2008

An analytic study of planar beams and arches subjected to significant thermal cycling from ambient temperatures up to 800°C is presented. The study employs a recently unified nonlinear hereditary type of viscoelastoplastic constitutive law to characterize the time- and temperature-dependent properties of Hastelloy X, a typical aerospace alloy. The results demonstrate a strong interaction between the backstress variable of the constitutive law and the time-dependent stress distribution produced by the deformation. This interaction tends to control, in a highly nonlinear manner, the creep ratchetting response of the beam or arch. Moreover, temperature gradients in the thickness direction tend to exert an important influence during thermal cycling.
Investigation of Residual Stresses in Drawn Wire by the Finite Element Method
Authors: Sawamiphakdi K. , Kropp P. K. , Lahoti G. D.
Published: April 2008

Based on a finite element modeling of the wire drawing process, a parametric study was conducted to investigate the effects of die geometry and area reduction factor on the magnitude and distribution of residual stresses through the wire cross section at the die exit. Two major variables of die geometry were considered in the study: the die radius and the die inlet angle. Three different die inlet angles of 12.5, 16.0 and 22.0 degrees were used in the analyses while the die radius was fixed at 25.4 mm. The die inlet angle was then set at 16.0 degrees and the die radius was varied from 12.7 mm through 38.1 mm in 12.7 mm increments. For each of the above cases, the area reduction factor was considered for 16.0 and 20.0 percents. In addition, the effect of initial stresses in wire was also investigated. The calculated results were compared to the analytical results published in the literature and an excellent agreement was obtained. The parametric study indicated that the die inlet angle has significant effect on the residual stresses at the surface of drawn wire. Specifically, smaller die inlet angle causes less tensile stresses at the surface and more compressive stresses at the center. The larger die radius reduces the level of residual stresses, but this reduction is only marginal. No significant change in either magnitude or distribution on patterns of residual stresses due to the initial stresses were found.
Texture Development During Wire Drawing
Authors: Mathur K. K. , Dawson P. R.
Published: April 2008

The development of deformation induced anisotropy in wire drawing operations is modeled with a finite element formulation that incorporates a polycrystalline plasticity model to describe the metal anisotropy. An aggregrate of grains is assumed to underly every material point on the continuum scale. The averaged response of an aggregate defines the macroscopic behavior of the corresponding material point, which is used in the finite element formulation to solve the boundary value problem corresponding to wire drawing. Macroscopic deformations imposed on the aggregate cause changes in the relative orientation of grains, thereby producing texture and associated anisotropy. Textures predicted from the drawing of aluminum wire are presented with special attention to the variations in texture from redundant deformations.
Three-Dimensional Simulation of Open-Die Press Forging
Authors: Kiefer B. V. , Shah K. N.
Published: April 2008

A three-dimensional, large deformation elastic-plastic finite element model is used to study the distribution of internal stresses and strains which occur during the forging process. Simulations are carried out on a rectangular ingot, using conventional dies which are flat and symmetrical top and bottom, to study the effects of die width and amount of reduction. A comparison is then carried out between the conventional dies and FML dies, which are also flat but of differing size on top and bottom. Also, a brief study is made of the effects of temperature gradients from ingot surface to core when forged with both types of dies.
Evaluation of a Modified Monotonic Neuber Relation
Authors: Sharpe W. N. , Wang K. C.
Published: April 2008

It has been proposed in the literature that the Neuber relation be modified to read Kε/Kt×(Kσ/Kt)m=1 in order to improve its predictive capability when plane strain loading conditions exist. Kε, Kσ, and Kt are respectively the strain, stress, and elastic concentration factors. The exponent m is proposed to be 1 for plane stress and 0 for plane strain. This paper reports the results of biaxial notch root strain measurements on three sets of double-notched aluminum specimens that have different thicknesses and root radiuses. Elastoplastic strains are measured over gage lengths as short as 150 micrometers with a laser-based in-plane interferometric technique. The measured strains are used to compute Kε directly and Kσ using the uniaxial stress-strain curve. The exponent m can then be determined for each amount of constraint. The amount of constraint is defined as the negative ratio of lateral to longitudinal strain at the notch root and determined from elastic finite element analyses. As this ratio decreases for the three cases, the values of m are found to be 0.65, 0.48, and 0.36. The modified Neuber relation is an improvement, but discrepancies still exist when plastic yielding begins at the notch root.
Friction Models for Metal Forming in the Boundary Lubrication Regime
Authors: Wilson William R. D.
Published: April 2008

Wilson and Shen’s semi-empirical equation for the effective hardness of a plastically deforming workpiece is used to develop models for tool-workpiece friction in metal-forming processes operating in the boundary lubrication regime. The models treat both steady and unsteady conditions and include the influence of tooling and workpiece topography, sliding speed, interface pressure, and workpiece strain rate on friction. Simplified models are suggested for conditions typical of bulk forming and sheet-metal forming processes.
Stress Intensity Factors of Multiple Cracked Sheet With Riveted Stiffeners
Authors: Nishimura T.
Published: April 2008

A new method is proposed for analyzing the stress intensity factors of multiple cracks in a sheet reinforced with riveted stiffeners. Using the basic solution of a single crack and taking unknown density of surface tractions and fastener forces, Fredholm integral equations and compatibility equations of displacements among the sheet, fasteners, and stiffeners are formulated. After solving the unknown density, the stress intensity factors of multiple cracks in the sheet are determined. Some numerical examples are analyzed.
Characteristics of Surfaces Machined With Abrasive-Waterjets
Authors: Hashish Mohamed
Published: April 2008

The general characteristics of surfaces machined with abrasive-waterjets (AWJs) are discussed in terms of surface texture and surface integrity. Data are presented from several studies involving different machining operations, including cutting, milling, turning, and drilling, and various advanced materials. The effects of AWJ parameters on surface texture effects, such as waviness, kerf taper, and burr height, in thin sheet metals typically used in the aerospace industry are presented and discussed. Other surface texture parameters, including surface finish and lay, are also addressed for selected machining applications such as milling and turning. Abrasive particle size is found to be the dominant parameter affecting the surface finish. The surface integrity effects are defined as particle deposition, delamination, gouging, microstructural changes, cracking, chipping, work hardening, and heataffected zones. Particle deposition occurs in the machining of ductile materials, but deposited particles can easily be removed by cleaning. Minor work hardening may result but can be totally eliminated if the dominant material removal mechanism is the cutting wear mode, which prevails under certain jet and traverse conditions. Thus, although the machining of advanced materials with AWJs may result in minor but controllable surface texture effects, it does not generally affect the integrity of the material.
Modeling of Delayed Hydride Crack Initiation
Authors: Shalabi A. F. , Meneley D. A.
Published: April 2008

This paper presents a solution to the one-dimensional time (transient condition) and temperature dependent diffusion problem adjacent to a crack-tip/flaw within the plastic zone region. The solution is used in addressing the problem of delayed hydride crack initiation in zirconium-2.5 wt. percent niobium. The mathematical solution predicts the critical hydride length at a given stress level and temperature for crack initiation.
Evaluation of the Vanishing Fiber Diameter Model for Composite Plasticity
Authors: Hsu F. J.-K. , Fahrenthold E. P.
Published: April 2008

The Vanishing Fiber Diameter (VFD) model for composite plasticity offers a computationally efficient approach to composite structural analysis and design problems. However this material model provides rather inaccurate predictions of composite deformation in a direction transverse to the fiber reinforcement. Empirical modification of the model is generally required to account for mesomechanical effects not present in the basic theory. Although systematic empirical modification of the VFD approach can provide accurate models of unidirectional composites, similarly accurate models of more complex laminates do not necessarily follow. The results suggest that laminate material models based on orthotropic lamina substructures do not in general provide a suitable basis for elastic-plastic structural design.
Simulation of Slump in Plastic Pipe Extrusion
Authors: Githuku D. N. , Giacomin A. J.
Published: April 2008

Extruded plastic pipe leaving an annular die is solidified in a long cooling tank by spraying the outer surface with cold water. The inside surface can take a long time to solidify as the solidification progresses radially inward. This results in flow of molten polymer down the inside of the pipe. This gravity flow of molten extrudate is called slumping, and it can cause serious nonuniformity in pipe wall thickness particularly in large diameter, thick walled pipes. It can also lead to another phenomenon known as “knuckle” formation where melt accumulates at specific locations. A simple numerical scheme to model this flow has been developed. Three-dimensional graphical illustrations of the slumping phenomenon based on this simulation are presented in this paper. The model predictions have been compared with commerical pipe data and they are in qualitative agreement.
Dynamic Response to Normal Stresses in a Transversely Isotropic Material Containing an External Circular Crack
Authors: Tsai Y. M.
Published: April 2008

The dynamic response of an external circular crack to a harmonic longitudinal wave in a transversely isotropic material is investigated using the techniques of Hankel transform. The wave impinges normally onto the crack surfaces. The inversion integral is evaluated and simplified through a complete contour integration. An exact expression for the dynamic stress intensity factor is obtained in terms of the wave frequency and the anisotropic material constants. The maximum value of the normalized dynamic stress-intensity factor is shown to occur at different wave frequencies for different sample composite and metallic materials. The dynamic effect on the crack surface displacement is also shown to be a function of the wave frequency and the material anisotropy.
Corrosion-Fatigue (Circumferential) Cracking in Cr-Mo Low Alloy Boiler Tube Steels: Part 2—Laboratory Simulation
Authors: Smith B. J. , Erskine C. P. , Hartranft R. J. , Marder A. R.
Published: April 2008

A laboratory procedure which causes cracking similar to that found in the waterwalls of coal fired boilers was developed. The procedure was designed to simulate the mechanism of crack initiation by a combination of intergranular corrosion and thermal stress cycling. It can also be used to study the subsequent environmentally assisted fatigue crack propagation. A finite element analysis of the transient thermal stresses induced by the laboratory simulation procedure was also performed in order to understand and document the nature and magnitude of the thermal stresses in the specimen. The result is that, although the magnitude of the thermal stress cycle in the simulation was greater than that calculated for in-service tubes, its nature is the same. This simulation procedure has potential as a tool to evaluate the relative resistance of materials to circumferential cracking and as a life estimation procedure for new boiler tube surface modifications.
Enhancement of Thermal Conductivities in Polymeric Fiber Reinforced Composite Materials
Authors: Gordaninejad F.
Published: April 2008

In this study it is demonstrated that thermal conductivities of polymeric fiber-reinforced composite materials can be enhanced by using coated fibers and by adding thermally conductive microspheres to the resin. Two and three-dimensional finite element unit cell models are developed to predict the directional thermal conductivities. The analyses are based on the flash pulse method. It is found that the thermal conductivities in the longitudinal and the transverse directions are highly dependent on the fiber and microsphere volume fractions as well as on the thermal conductivities of fiber, microsphere, and coatings. It is shown that the 2-D analysis is a good approximation for the 3-D model. Close agreements among analytical, finite element and experimental results are obtained.
The Variable Threshold Rod Experiment: A Comparison of Measured and Computed Deformations
Authors: Dawson P. R. , Follansbee P. S.
Published: April 2008

A novel experiment has been developed to induce inhomogeneous deformations under simple extension boundary conditions. By appropriate prior cold working of stock material and fabrication of test specimens with tailored dimensions, the location of the deformation zones along the gage length of tensile specimens was controlled. By design, the deformations were chosen to concentrate either toward the harder or softer ends of the specimens. The experiment provides a sensitive test to judge the ability of constitutive models to replicate the behavior of metals undergoing inhomogeneous deformation. A comparison of experimental results with simulated response using an isotropic state variable model is presented.
Time Dependent Fracture and Cohesive Zones
Authors: Knauss W. G.
Published: April 2008

This presentation is concerned with the fracture response of materials which develop cohesive or bridging zones at crack tips. Of special interest are concerns regarding crack stability as a function of the law which governs the interrelation between the displacement(s) or strain across these zones and the corresponding holding tractions. It is found that for some materials unstable crack growth can occur, even before the crack tip has experienced a critical COD or strain across the crack, while for others a critical COD will guarantee the onset of fracture. Also shown are results for a rate dependent nonlinear material model for the region inside of a craze for exploring time dependent crack propagation of rate sensitive materials.
Constitutive Equations for the Thermomechanical Response of Rene´ 80: Part 2—Effects of Temperature History
Authors: Bhattachar V. S. , Stouffer D. C.
Published: April 2008

The unified constitutive equations for Rene´ 80 developed by Bhattachar and Stouffer (1992) are used to predict the thermomechanical fatigue (TMF) response of a Nickel base superalloy Rene´ 80 between 649°C and 1093°C. Predictions using these equations suggest that temperature history effects are significant during TMF, and that the TMF response of Rene´ 80 cannot be predicted completely using only isothermal parameters. It is postulated without metallurgical observations that the two deformation mechanisms in Rene´ 80, planar slip at low temperatures and dislocation climb at high temperatures, produce characteristic microstructures which interact under nonisothermal conditions to produce extra hardening that is not present during isothermal deformation. A state variable approach has been used to model this interaction. The nonisothermal model with temperature history effects could successfully predict the initial and saturated TMF response, and block isothermal response of Rene´ 80 from several tests between 649°C and 1093°C.
Mixed-Mode Fatigue Crack Propagation of Penny-Shaped Cracks
Authors: Chen W. R. , Keer L. M.
Published: April 2008

A three-dimensional penny-shaped crack under combined tensile and shear loadings is analyzed. The assumptions of Dugdale are applied to estimate the effects of plasticity around the edge of the crack. The solution for mode I tensile loading is well established within the context of the Dugdale assumptions, and for the case of shear loading, approximate results are derived for the yield ring width and crack sliding displacements, with the assumptions similar in form to the mode I case. By superposing the results of the tensile and shear loading, the solutions for a penny-shaped Dugdale crack under mixed mode static loading and modified for the analysis of fatigue crack growth. Based on the mixed mode Dugdale model and the accumulated plastic displacement criterion for crack growth, a fatigue crack growth equation with four-power effective stress intensity factor dependence is developed for a penny-shaped crack under conditions of mixed mode loading and small-scale yielding.
Material Property Studies of Two High-Pressure Turbine Rotors for Remaining Life Assessment
Authors: Swaminathan V. P. , Viswanathan R. , Clark C. P.
Published: April 2008

A remaining life assessment study was performed on two no-bore HP rotors after 169,000 hours of operation. These rotors were manufactured from 1CrMoV (ASTM A470, Class 8) steel. Southwest Research Institute conducted a materials evaluation program to generate pertinent material property data necessary to perform a SAFER analysis by Lehigh University. Two ring samples were removed from periphery locations of a rotor. One of the rings was from a packing location where the operating metal temperature was 860°F (460°C), and the other ring from the cold coupling end. Evaluation tests included chemical analysis, metallography, Auger analysis, tension and hardness, Charpy impact, fracture toughness, and stress rupture. A small sliver sample was available from the second rotor. Limited metallurgical analysis was conducted on this sample. Results indicated that the hot end embrittled in service with a shift in FATT of 240°F (116°C) when compared to the cold end. Fracture toughness versus temperature data were developed using various correlations available from the literature. Good correlation was found among the various procedures and lower bound curves were established for the hot and cold ends. Hot end toughness properties were significantly lower than those of the cold end. Auger results indicated segregation of phosphorus on the grain boundaries at the hot location. After service FATT can be predicted using the bulk phosphorus content and correlations available from the literature.
Yield Surface for Grey Cast Iron Under Biaxial Stress
Authors: Hjelm H. E.
Published: April 2008

Biaxial plane stress experiments have been performed on cruciform specimens made of graphite grey cast iron. Different ratios of tensile and compressive loads were applied in two perpendicular directions. The primary objective of this investigation is to determine the locus of the yield surface (yield curve) under plane stress, and to establish yield functions that could model the elastoplastic behavior of grey cast iron with reasonably good accuracy. The experiments show that a sufficiently accurate description is obtained by using the ordinary von Mises yield function in the compressive-compressive region, and elsewhere, the von Mises yield function modified with a term containing the first stress invariant. It was also found that for tensile loadings nonelastic deformations develop at low stress levels. Use of the above yield function must therefore be accompanied by a very large hardening modulus for tensile loads.
Creep Behavior of a Unidirectional SCS-6/Ti-15-3 Metal Matrix Composite at 450°C
Authors: Ohno N. , Toyoda K. , Okamoto N. , Miyake T. , Nishide S.
Published: April 2008

As the first step to understand the creep behavior of a unidirectional SCS-6/Ti-15-3 metal matrix composite, creep tests were performed at 450°C using specimens reinforced with fibers oriented at 0 deg (longitudinal), 45 deg (off-axis), and 90 deg (transverse) to the specimen axis. Measuring the elongation in the gage section, we found that creep deformation and rupture can occur even in the longitudinal creep at stress levels much lower than the tensile strength although the SCS-6 fiber itself does not creep at all, and that the 45 deg off-axis creep has some ductility, which is greater at lower stress, whereas the transverse creep behavior is relatively brittle. Results of the longitudinal creep tests were discussed on the basis of the relaxation of stress in the matrix as well as the bundle strength of fibers.
Deformation and Failure Behavior of Woven Composite Laminates
Authors: Karayaka M. , Kurath P.
Published: April 2008

Conceptually, fabric composites have some structural advantages over conventional laminates. However, deformation and failure analyses become more complex with the additional anisotropy introduced by the weaving geometry. A micromechanistic deformation model, that could realistically be incorporated into structural finite element codes, is proposed where loading direction and weave parameters are allowed to vary. Comparisons are made to previous models and experimental results for woven materials, indicating that the proposed model provides improved estimates for the linear elastic stiffness. The model further provides predictions for internal stresses in the longitudinal, transverse, and interlace regions of the woven laminate which qualitatively correspond to the experimentally observed failure mechanisms. The experimental program investigates deformations behavior and failure mechanisms of 5-harness 0/90 weave Graphite/Epoxy laminates under tension, compression, and 3-point and 4-point bending loading. Under these conditions the woven laminates exhibit orientation dependent mechanical properties and strength.
Analysis and Compression Testing of 2024 and 8009 Aluminum Alloy Zee-Stiffened Panels
Authors: Friedman R. , Kennedy J. , Royster D.
Published: April 2008

Zee-stiffened compression test panels, fabricated with dispersion-strengthened, high-temperature 8009 aluminum alloy sheet, were evaluated to determine the alloy’s feasibility for compression-critical applications. A compression panel design configuration was obtained using a strength analysis program that predicts the post-skin buckling strength of flat or curved-skinned, metallic-stiffened structure. Three short-column panels were tested to failure at room temperature: (a) a baseline riveted panel fabricated with 2024-T62 aluminum zee stringers and a 2024-T81 aluminum skin, (b) a riveted panel fabricated with 8009 aluminum zee stringers and skin, and (c) a resistance spot-welded panel fabricated with 8009 aluminum zee stringers and skin. The 8009 alloy exhibited pronounced, compressive strength anisotropy, necessitating panel orientation to take advantage of the higher compressive yield in the sheet transverse direction. Compression test results were in good agreement with the predicted compression allowables since they were within 5 percent of the test strength. The 8009 aluminum riveted panel exhibited superior skin buckling resistance and failed in the wrinkling mode, as predicted, at a load approximately 15 percent higher than that of the baseline 2024 panel. The spotwelded 8009 panel did not fail in the wrinkling mode since the spot welds failed in tension shortly after the skin locally buckled. The latter test indicates that the spot welded skin-stringer combinations should not be used above the buckling stress. Due to its excellent microstructural stability at elevated temperatures, high-temperature compression panels of 8009 alloy offer potential weight savings of 25 percent compared with conventional aluminum alloys.
Exact Results Concerning the Local Fields and Effective Properties in Piezoelectric Composites
Authors: Benveniste Y.
Published: April 2008

This paper consists of two parts: (a) a concise summary and discussion is given of the recent contributions of the author in the micromechanics of piezoelectric composites. The underlying theme here is the derivation of exact connections for the local fields and effective moduli of heterogeneous piezoelectric solids. Composites of arbitrary phase geometry as well as fibrous systems are considered. (b) New results are presented on the effective behavior of fibrous piezoelectric systems. Fibrous composites with transversely isotropic constituents and cylindrical microgeometry are considered. The exact connections of the author (Benveniste (1993), Proc. R. Soc., Series A, Vol. 441, pp. 59-81) are extended to include the most generally possible overall symmetry of the composite aggregate. The other category of the new findings concerns exact expressions for the effective thermal terms of fibrous systems which possess the same shear modulus GT.
Aspects of Heterogenization
Authors: Honein E. , Honein T. , Herrmann G.
Published: April 2008

In this paper, we briefly review some of the recent developments in the methodology of heterogenization. A connection between a group structure on the set (−1, 1) of real numbers t such that −1 < t < 1, and the elastostatics of a multilayered fiber perfectly bonded to an infinite matrix is pointed out. Also, universal formulae, pertaining to the solution of two circular elastic inclusions perfectly bonded to a matrix, of infinite extent, which is subjected to arbitrary loading, are discussed. As a novel illustration of the heterogenization procedure, we study here the case where the inclusions are elastically (i.e., “imperfectly”) embedded in the matrix. Several cases are presented and discussed.
Nonlinear Elastic Properties of Micro-Heterogeneous Media
Authors: Kro¨ner E.
Published: April 2008

Utilizing statistical methods known from linear elasticity it is shown how effective 3rd (and higher) order elastic constants (TOEC) of micro-heterogeneous media can be calculated. Emphasis is put on the self consistent scheme. The ensemble average of the fluctuating TOEC yields a 0th approximation to the rigorous selfconsistent moduli. A first approximation is also given in closed form. The insight that the well-established statistical methods of the linear theory, which uses Green functions, are applicable also to nonlinear problems is considered as the main result of this paper.
The Adiabatic Heat Transfer Coefficient and the Superposition Kernel Function: Part 1—Data for Arrays of Flatpacks for Different Flow Conditions
Authors: Anderson Ann M. , Moffat Robert J.
Published: April 2008

This paper describes an investigation of the forced convection heat transfer and pressure drop characteristics of a regular in-line array of flatpacks for several channel heights and inlet velocities. The work has both practical and theoretical interest since it relates to technical problems now faced by the electronics industry, and it embodies one of the most general heat transfer problems: non-uniform heat release from nonuniform geometries. To predict operating temperatures in situations where the wall temperature distribution is non-uniform, one must use superposition. Both the adiabatic heat transfer coefficient, had, and the superposition kernel functions, g*, are required. The problem can be solved using superposition directly (had and g*) or indirectly (using had and g* to calculate the correct value of hm). Either way the superposition data is required. This work presents the first full set of superposition data for flatpack arrays. Part 1 presents heat transfer and pressure drop results and part 2 presents a model for heat transfer that is based on the maximum turbulence fluctuations in the channel.
Thermal Effects During Infrared Solder Reflow—Part II: A Model of the Reflow Process
Authors: Eftychiou M. , Bergman T. L. , Masada G. Y.
Published: April 2008

A numerical model is developed that describes the infrared reflow soldering of surface mounted components onto prewired circuit cards. The model predicts convective conditions within the reflow oven and uses these conditions, in conjunction with a radiative heat transfer analysis and a two-dimensional transient conduction analysis, to predict the thermal response of a card assembly as its components are soldered. The model is described, and its performance is illustrated through presentation of base case simulations. Parametric simulations are performed to determine the sensitivity of the card assembly thermal response to variations in oven conditions and uncertainties in the convective, as well as radiative heat transfer portions of the model.
Structural Analyses of Spacecraft Electronic Packages
Authors: Kannappan S. , Kunukkassril V.
Published: April 2008

The spacecraft electronic assemblies are subjected to severe environmental conditions during testing, launching and during orbit mission. The success of the mission depends upon the proper functioning of these critical electronic modules. The structural analyses using Finite Element Methods (FEM) assure the integrity of these components. A typical box, Array Drive Electronics (ADE), for TIROS satellites [1] is discussed in this article. The NASTRAN software was used to perform the stress and modal analyses of the box assembly with five circuit boards, covers and mounting feet. The stress analysis was performed for a static solution. As a conservative loading, 3 sigma load factor was used in the calculation of the acceleration values from the random vibration test conditions. Margins of safety were calculated. Design and material changes were recommended. The mode shapes fall in to three groups as explained in the text.
Reliable Ceramic Window Design for Electronic Devices
Authors: Yeh Hsien-Yang
Published: April 2008

The effective operations of a high voltage vacuum electronic device, such as a traveling wave tube, depends on its ability to maintain high vacuum environments. However, during temperature tests, some tubes fail because of vacuum leaks through cracks in the ceramic window. It is believed that these leaks result from RF heating at the center conductor, which caused the ceramic to crack. To obtain a general understanding of the stress field in the window structure, a closed from analytical approach is imperative. However, due to the complex nature of the problem, only the first order engineering approximation is used in this preliminary study. The theory of linear elastic fracture mechanics and the existing solutions from elastic circular plates are useful for understanding the cause of ceramic window cracks. Some simple design references have also been developed for the design of reliable ceramic windows for traveling wave tubes.
Drill Wear Monitoring in Circuit Board Manufacturing: An Experimental Study
Authors: Ramirez Ciro N. , Thornhill R. Joe
Published: April 2008

The empirical development of an indirect drill wear monitoring method in circuit board drilling is presented. Spindle vibration and mean drilling torque were measured using a single spindle drilling machine, and their usefulness in detecting drill wear was evaluated. The sensitivity of these measurements to potential changes in process parameters is examined, and linear discriminant functions are presented for monitoring drill wear with a sensor fusion approach.
Modeling Thermal Stresses in Periodic Structures: Some Observations Regarding the Boundary Conditions
Authors: Cifuentes A. O. , Stiffler S. R.
Published: April 2008

Periodic structures, that is, structures that consist of a basic pattern which is repeated multiple times, are commonplace in the electronics industry. This paper is concerned with the determination of thermal stresses in such structures. Specifically, it is demonstrated that thermal stresses can be computed using a reduced model, if one uses the correct boundary conditions. It is also shown that a reduced model can give very misleading results if incorrect boundary conditions are employed.
Two-Level Nonlinear Mixed Discrete-Continuous Optimization-Based Design: An Application to Printed Circuit Board Assemblies
Authors: Praharaj S. , Azarm S.
Published: April 2008

In this paper, a new approach for optimization-based design of nonlinearly mixed discrete-continuous problems has been developed. The approach is based on a two-level decomposition strategy in which the entire domain of variables is partitioned into two levels, one involving the continuous variables and the other involving the discrete variables. Variables in one level are optimized for fixed values of the variable from the other level. A modified penalty function is formed, based on monotonicity analysis, to solve for the discrete variables, and a conventional optimization method is used to solve for the continuous variables. To improve the computational effectiveness of the approach, a constrained derivative relationship is also adopted. The performance of the entire algorithm is then demonstrated through an example involving a simplified model for printed circuit board assemblies. The objective in the example is to maximize assembly reliability by: (1) adding redundant components to the boards, and (2) optimally distributing allocated mass flow to the individual channels of the circuit boards. Number of variables in the example is then varied to investigate the effectiveness and potential of the approach for large-scale problems.
Time-Dependent Ductility of Electro-Deposited Copper
Authors: Oshida Yoshiki , Chen P. C. , Reid J. D. , Nishihara T.
Published: April 2008

Mechanical ductility property of copper deposits is one of the most important factors necessary to provide a reliable component for the flexible printed circuit boards. Copper deposits are usually fabricated by either an electroless technique or an electrode position process. Among many parameters controlling the deposits quality, it is believed that the current density, bath temperature, the film thickness, annealing temperature and time, and the presence of impurities exhibit particularly strong influences on the mechanical ductility property. In our preliminary studies, it was found that the ductility obtained by the mechanical bulge testing showed a three-stage characteristics in terms of a room temperature annealing; namely (i) initial low ductility regime (from the right-after-copper-deposition until 12 hours aged at room temperature), (ii) transitional rapid increasing ductility regime (from 12 hours to 24 hours), and (iii) the high ductility regime (after 24 hours). In this study, X-ray diffraction analyses of the diffracted line width, the microstrain from which the dislocation density was estimated, residual macrostress, and the scanning electron microscopic observations were conducted and results were correlated with the characteristic three-stage ductility change behavior described above. Results of this study indicate that grain size and residual stress changes are strongly associated with improvement in copper ductility during the room temperature annealing.
Fatigue Lives on 60Sn/40Pb Solder Joints Made With Different Cooling Rates
Authors: Mei Z. , Morris J. W.
Published: April 2008

This paper reports the results of a study on the effect of the cooling rate during solidification on the isothermal shear-fatigue life of 60Sn/40Pb solder joints. Solder joints are made with three different initial microstructures by quenching, air-cooling, and furnace-cooling. The test results show that the quench-solidified solder joints have isothermal fatigue lives of about twice long as those of the furnace cooled solder joints tested at 20°C and 65°C with the straining rates of about 10−4 per s. These results are ascribed to the refined grain size and less lamellar phase morphology that results on increasing the cooling rate.
Ramberg-Osgood Parameters for 63–37 Sn-Pb Solder
Authors: Rafanelli A. J.
Published: April 2008

As part of a fracture properties study, the Ramberg-Osgood parameters were evaluated for 63-37 Sn-Pb (tin-lead) solder. This work was a preliminary step in experimentally determining J-integral values via the Hutchinson-Rice-Rosengren (HRR) power law for hardening materials. Consideration was given to both engineering strain and true strain when plotting the curves. Results disclosed little effect of either engineering or true strain at linear stress levels. For a strain-hardening exponent of 1.0, a material constant of 0.9849 was determined.
Thermal Characterization of Heat Sink Adhesive Systems for Spacecraft Electronics by Time-Resolved Infrared Radiometry
Authors: Spicer J. W. M. , Bevan M. G. , Kerns W. D. , Feldmesser H. S.
Published: April 2008

A new thermal characterization technique, time-resolved infrared radiometry (TRIR), is used to investigate three different heat sink adhesive systems used in spacecraft electronics - a filled epoxy, a filled silicone and a tape adhesive. Measurements of heat sink efficiency are presented as a function of thermal cycling and the TRIR results are compared with ultrasonic and X-ray imaging techniques and with destructive analysis. The TRIR technique is shown to provide a measure of the relative heat sink capabilities of the different systems and can detect subsurface delamination in the filled epoxy system resulting from the development of cracks due to thermal cycling.