bluuebunny/crossref_metadata_2025 huggingface dataset:
Elastic Anisotropy of Directionally Solidified Superalloy
Authors: Hasebe Tadashi , Sakane Masao , Ohnami Masateru
Published: April 2008

This paper describes the elastic compliance matrix and Young’s modulus of directionally solidified (DS) plates which are composed of five columnar grains. Threedimensional finite element method analyses (FEM), taking account of the anisotropy of a crystal, were made to determine the elastic compliance matrix of the DS plate. The elastic compliance matrix calculated in the FEM analysis was reduced to an orthotropic matrix after giving an appropriate rotation of coordinates. The elastic compliance matrix calculated by Reuss and Voigt averages were compared with the FEM results. The two averages precisely estimated the FEM results but the Reuss model had a better accuracy. The Young’s modulus in any direction was discussed in both the coaxis and non-coaxis models.
Melt Processing of Bulk YBaCuO Superconductors With High Jc
Authors: Murakami M. , Kondoh A. , Fujimoto H. , Sakai N. , Takaichi H. , Yamaguchi K. , Takata T. , Takamuku K. , Ogawa N. , Hirabayashi I. , Koshizuka N. , Tanaka S.
Published: April 2008

There are several melt processes which promote high Jc in bulk YBaCuO. A comparison of various melt processes was performed in this study. It was found that size control of the Y2BaCuO5 inclusions is the key to the improvement in Jc values.
Strain Hardening Response of Sintered Porous Iron Tubes With Various Initial Porosities Under Combined Tension and Torsion
Authors: Kim K. T. , Kwon Y. S.
Published: April 2008

Elastic-plastic strain hardening responses of sintered porous iron are investigated. By using the yield function of Kim, two sets of constitutive equations are obtained from the constitutive theories by Kim and Suh and by Gurson. Theoretical predictions from these constitutive equations are compared with experimental data for sintered porous iron tubes with various initial porosities under combined tension and torsion.
Corrosion-Fatigue (Circumferential) Cracking in Cr-Mo Low Alloy Boiler Tube Steels: Part 1—Initiation and Propagation
Authors: Smith B. J. , Marder A. R.
Published: April 2008

The metallurgical mechanism responsible for the initiation and propagation of circumferentially oriented, fireside cracks on waterwall boiler tubes was investigated. The materials studied were Cr-Mo low alloy steels in the as-received and post-service (exhibited cracking) condition. It was determined that cracks were initiated by a corrosion/fatigue interactive mechanism at the fireside tube surface. The corrosion mechanism is sulfidation while the material fatigue degradation occurs due to thermal stress cycles. Crack propagation occurs by an environmentally assisted thermal fatigue crack propagation mechanism.
On the Decrease of Fatigue Limit Due to Small Prestrain
Authors: Nagase Y. , Suzuki S.
Published: April 2008

Fatigue behavior of plain specimens of low carbon steel subjected to small tensile prestrain is investigated through rotating bending tests and the mechanism of the decrease of fatigue limit due to the prestrain is discussed. It is found that 3 percent prestraining causes the acceleration of both slip and crack initiations, and increases the growth rate of a small surface crack of less than 0.3 mm. It also decreases the fatigue limit. If prestrained material is aged, the fatigue limit increases. These effects of the small prestrain are explained based on the unpinning of locked dislocations due to the prestrain.
A State Variable Modeling of Plasticity and Necking Under Uniaxial Tension
Authors: Ferron G. , Karmaoui Idrissi H. , Zeghloul A.
Published: April 2008

Constitutive equations based on a state variable modeling of the thermo-viscoplastic behavior of metals are discussed, and incorporated in an exact, long-wavelength analysis of the neck-growth process in uniaxial tension. The general formalism is specialized to the case of f.c.c. metals in the range of intragranular, diffusion controlled plastic flow. The model is shown to provide a consistent account of aluminum behavior both under constant strain-rate and creep. Calculated uniaxial tensile ductilities and rupture lives in creep are also compared with experiments.
Microstructure and Phase Selection in Laser Treatment of Materials
Authors: Kurz W. , Trivedi R.
Published: April 2008

Microstructure formation during the laser treatment of materials is discussed so as to enable one to design appropriate alloys and laser processing conditions which will produce a microstructure that has optimum properties. In order to predict microstructures for different laser processing conditions, the theoretical models for single phase and two-phase eutectic growth under a wide range of solidification rates have been developed. Specific emphasis is placed on the new physics that become important under rapid solidification conditions of laser processing. Based on these models, the selection of a specific microstructure under given laser processing conditions has been established. These results are then used to construct microstructure/ processing condition maps for the laser treatment of materials. These diagrams theoretically predict the variation in the microstructure along the depth of the solidified pool as a function of laser scanning velocity.
A Comparative Study of the Creep Rupture Properties of Type 316 Stainless Steel Base and Weld Metals
Authors: Mathew M. D. , Sasikala G. , Mannan S. L. , Rodriguez P.
Published: April 2008

Creep rupture properties of a modified grade of type E316-15 stainless steel weld metal have been studied in the temperature range 823 to 923 K at different stress levels. The results are compared with the creep properties of the base metal under identical test conditions. Rupture lives of the weld metal were found to be lower than those of the base metal by a factor of five to ten. Minimum creep rates of the weld metal were usually higher than those of the base metal whereas the rupture ductility of the weld metal was generally higher at 823 and 873 K and lower at 923 K. Differences in the microstructural changes, creep damage and fracture behaviour between the weld and the base metals have also been investigated.
A Comparison of the Strains Due to Edge Cracks and Cuts of Finite Width With Applications to Residual Stress Measurement
Authors: Cheng W. , Finnie I.
Published: April 2008

The strains on the surface near a loaded crack or cut are compared for cuts of different width to depth ratios. General solutions are obtained for an arbitrary distribution of normal and shear tractions on the faces of the crack or cut. Numerical results are presented for normal stress distributions which vary with distance from the surface as power functions of zero to third order. The results are useful for measurement of near surface residual stresses and should also be of value for crack compliance measurements of stress intensity factors.
The Influence of Grain Size on the Toughness of Monolithic Ceramics
Authors: Bower A. F. , Ortiz M.
Published: April 2008

Experiments have shown that there may be an optimal grain size which maximizes the toughness of polycrystalline ceramics. In this paper, we attempt to develop a theoretical model which can predict the effect of grain size on the toughness of ceramics. We assume that three principal mechanisms affect the toughness of the material: distributed microcracking; crack trapping by tough grains; and frictional energy dissipation as grains are pulled out in the wake of the crack. The grain size influences these mechanisms in several ways. The energy dissipated due to frictional crack bridging increases with the size of the bridging grains, tending to improve toughness. However, as the grain size increases, the density of microcracks in the solid also increases, which eventually weakens the material. In addition, the level of inter-granular residual stress is also reduced by microcracking, which as a detrimental effect on the toughening due to bridging. We have developed a simple model to quantify these effects. However, the model does not predict the dramatic loss of strength which has been observed to occur beyond a critical grain size. We have therefore proposed an alternative explanation for the apparent decrease in toughness in coarse grained ceramics. Calculations indicate that in a coarse grained material, the main contribution to toughness is due to frictional crack bridging. However, to produce this toughening, the bridging zone must be over 500 grains long. In practice, the length of the bridging zone in a coarse grained solid may be comparable to the dimensions of the specimen used to measure its toughness. Under these conditions, it is not appropriate to use the concept of a geometry independent toughness to characterize the strength of the specimen. We have therefore developed a simple model of a double cantilever beam fracture specimen, which accounts for the effects of large scale bridging. Using this model, we are able to predict the apparent decrease in toughness measured in coarse grained specimens.
Cyclic Fatigue-Crack Growth in Grain Bridging Ceramics
Authors: Dauskardt Reinhold H.
Published: April 2008

Cyclic fatigue degradation is contemplated in polycrystalline ceramics exhibiting grain bridging as the principle toughening mechanism. A frictional-wear mechanism based on repetitive sliding wear degradation of frictional grain bridges is discussed. A micromechanical model is developed which provides an insight into the effects of salient mechanical and microstructural variables such as wear rate at the grain/matrix interface and initial residual stress of bridging grains, on fatigue-crack growth behavior. Results of the analysis are compared to experimentally observed fatigue-crack growth rates in coarse grained alumina and silicon nitride.
Stable Crack Growth in Rate-Dependent Materials With Damage
Authors: Fager Leif-Olof , Bassani J. L.
Published: April 2008

A cohesive zone model of the Dugdale-Barenblatt type is used to investigate crack growth under small-scale-creep/damage conditions. The material inside the cohesive zone is described by a power-law viscous overstress relation modified by a one-parameter damage function of the Kachanov type. The stress and displacement profiles in the cohesive zone and the velocity dependence of the fracture toughness are investigated. It is seen that the fracture toughness increases rapidly with the velocity and asymptotically approaches the case that neglects damage.
Stochastic Aspects of Matrix Cracking in Brittle Matrix Composites
Authors: Spearing S. M. , Zok F. W.
Published: April 2008

A computer simulation of multiple cracking in fiber-reinforced brittle matrix composites has been conducted, with emphasis on the role of the matrix flaw distribution. The simulations incorporate the effect of bridging fibers on the stress required for cracking. Both short and long (steady-state) flaws are considered. Furthermore, the effects of crack interactions (through the overlap of interface slip lengths) are incorporated. The influence of the crack distribution on the tensile response of such composites is also examined.
Notch Ductile-to-Brittle Transition Due to Localized Inelastic Band
Authors: Suo Z. , Ho S. , Gong X.
Published: April 2008

Holes are often drilled in a panel for cooling or fastening. For a panel made of a monolithic ceramic, such a hole concentrates stress, reducing load-carrying capacity of the panel by a factor of 3. By contrast, for a ductile alloy panel, plastic flow relieves stress concentration so that the small hole does not reduce load-carrying capacity. A panel made of ceramic-matrix composite behaves in the middle: matrix cracks permit unbroken fibers to slide against friction, leading to inelastic deformation which partially relieves stress concentration. Load-carrying capacity is studied in this paper as an outcome of the competition between stress concentration due to the notch, and stress relaxation due to inelastic deformation. The inelastic deformation is assumed to be localized as a planar band normal to the applied load, extending like a bridged crack. The basic model is large-scale bridging. A material length, δ0E/σ0, scales the size of the inelastic band, where σ0 is the unnotched strength, δ0 the inelastic stretch at the onset of rupture, and E Young’s modulus. Load-carrying capacity is shown to depend on notch size a, measured in units of δ0E/σ0. Calculations presented here define the regime of notch ductile-to-brittle transition, where ceramic-matrix composites with typical notch sizes would lie. Both sharp notches and circular holes are considered. The shape of the bridging law, as well as matrix toughness, is shown to be unimportant to load-carrying capacity.
Fatigue Damage in Cross-Ply Titanium Metal Matrix Composites Containing Center Holes
Authors: Bakuckas J. G. , Johnson W. S. , Bigelow C. A.
Published: April 2008

The development of fatigue damage in four [0/90]s SCS-6/Ti-15-3 laminates containing center holes was investigated. A methodology to predict damage initiation based on an effective strain parameter was used to determine the stress levels and the number of cycles required for matrix crack initiation. Damage progression was monitored at various stages of fatigue loading. In general, a saturated state of damage consisting of matrix cracks and fiber-matrix debonding was obtained which reduced the composite modulus. Matrix cracks were bridged by the 0° fibers. The fatigue limit (stress causing catastrophic fracture of the laminates) was also determined. The static and post-fatigue residual strengths were accurately predicted using a three dimensional elastic-plastic finite element analysis. The matrix damage that occurred during fatigue loading significantly reduced the notched strength.
How Affine is the Entanglement Network of Molten Low-Density Polyethylene in Large Amplitude Oscillatory Shear?
Authors: Giacomin A. J. , Jeyaseelan R. S.
Published: April 2008

The nonlinear viscoelastic behavior of molten plastics is often attributed to interactions between long-chain molecules. Entanglement theories model this behavior with a network of junctions. Whether this network deforms differently than the liquid is a question of considerable interest to polymer engineers since nonlinear viscoelasticity governs many plastics processing operations. In this paper, large amplitude oscillatory shear flow is used to measure how affine the network deformation is relative to the deforming melt. For a low-density polyethylene melt, a small amount of nonaffine network deformation is observed when the large amplitude oscillations are analyzed with a nonaffine structural network theory. Inaccurate predictions were obtained from both the nonaffine network theory due to Phan-Thien, and the nonaffine network strand theory due to Larson.
On the Propagation of a Shear Band in a Steel Tube
Authors: Batra R. C. , Zhang Xiangtong
Published: April 2008

Marchand and Duffy tested thin-walled steel tubes in a split Hopkinson torsion bar at a nominal strain-rate of approximately 1600/s and could not determine conclusively whether a shear band initiating at a point in the tube propagated around the circumference in one direction or in both directions. They estimated the speed of propagation to be 520 m/s in the former case and 260 m/s in the latter. Here we simulate their test numerically, and find that the shear band propagates in both directions around the circumference of the tube. When the tube is twisted at a nominal strain-rate of 5000/s, the band speed varies from 180 m/s at the site of the initiation to approximately 1000 m/s at the nearly diametrically opposite point. The band speed increases with an increase in the nominal strain-rate. The material defect is modeled by assuming that a small region near the center of the tubular surface is made of a material weaker than that of the rest of the tube.
A Fatigue Crack Growth Model for Single Overload Tests
Authors: Goel H. S. , Chand Satish
Published: April 2008

Crack growth rate experiments are conducted by applying an intermediate single overload cycle in constant amplitude load (CAL) tests. For a particular overload ratio, three to four tests are conducted by applying the overload cycle at different crack lengths. The loads are selected in such a way that for a given overload ratio, the size of the overload and CAL monotonic plastic zones are the same at each crack length. A functional form for the crack growth during the retardation was developed that accurately describes all the tests. For comparison, the corresponding CA-load tests are also conducted separately. Finally, a crack growth rate model is developed.
The Effect of Matrix Microstructure on Thermally Induced Residual Stresses in SiC/Titanium Aluminide Composites
Authors: Pindera Marek-Jerzy , Freed Alan D.
Published: April 2008

This paper examines the effect of varying the microstructural composition of titanium aluminide on the evolution of residual stresses in titanium aluminide matrix composites. An analytical model is developed to determine residual stresses in fiber and matrix phases of unidirectional, SiC/Ti-Al composites subjected to axisymmetric thermal loading. The model uses elements of the concentric cylinder model and the method of cells to calculate residual thermal stresses in the presence of temperature-dependent and inelastic behavior of the fiber and matrix phases. The concentric cylinder model is employed as a geometric model for the unidirectional composite, whereas the method of cells is employed in modeling the microstructure of the titanium aluminide matrix phase. The titanium aluminide matrix consists of distinct brittle and ductile α and β phases whose volume content is varied in the present scheme to understand how the resulting residual stresses can be altered. Both spatially uniform and nonuniform variations of the α and β phases are considered. The results explain the occurrence of radial microcracks in SiC/Ti-Al composites in the presence of a β-depleted region at the fiber/matrix interface, and validate the potential of engineering the matrix phase to reduce residual stresses in these composites.
Dynamic Modeling and Control of Packing Pressure in Injection Molding
Authors: Hu J. , Vogel J. H.
Published: April 2008

A dynamic model of injection molding developed from physical considerations is used to select PID gains for pressure control during the packing phase of thermo-plastic injection molding. The relative importance of various aspects of the model and values for particular physical parameters were identified experimentally. The controller gains were chosen by pole-zero cancellation and root-locus methods, resulting in good control performance. Both open and closed-loop system responses were predicted and verified, with good overall agreement.
Micromechanical Approach of the Coated Inclusion Problem and Applications to Composite Materials
Authors: Cherkaoui M. , Sabar H. , Berveiller M.
Published: April 2008

A micromechanical model using simultaneously Green’s function techniques and interfacial operators is proposed in order to solve the elastic inhomogeneous coated inclusion problem. For a composite material made of a non dilute concentration of coated inclusions and a homogeneous matrix, the interactions between the reinforcements are solved by a self-consistent scheme. The theoretical results for a composite of hollow spheres of glass in a polyester matrix are in good agreement with experimental measurements of Huang and Gibson.
Dynamic Damage Evolution of Solids in Compression: Microcracking, Plastic Flow, and Brittle-Ductile Transition
Authors: Deng H. , Nemat-Nasser S.
Published: April 2008

Dynamic compressive damage evolution in solids, associated with brittle microcracking and ductile plastic flow, is modeled through plastic flow and tensile microcracking, which are induced by the deformation of preexisting microflaws at grain boundaries, slip bands, and microcavities. The micromechanical aspect of this model is discussed in terms of the dominance of microcracking or plastic flow, and possible transition from microcracking to plastic flow is investigated. The effect of lateral confinement on the dynamic damage evolution is investigated, emphasizing the brittle-ductile transition.
Influence of Random Geometry on Effective Properties and Damage Formation In Composite Materials
Authors: Ostoja-Starzewski M. , Sheng P. Y. , Jasiuk I.
Published: April 2008

We study the effective moduli and damage formation in out-of-plane elasticity (i.e., two-dimensional conductivity) of matrix-inclusion composite materials with either randomly or periodically distributed inclusions (fibers). In this paper, we focus our attention on composites with isotropic phases, both of which have elastic-brittle response in damage. The elastic-brittle behavior is modeled with the help of a fine mesh finite-difference system, whereby damage evolution is simulated by sequentially removing/breaking bonds in this lattice in accordance with the state of stress/strain concentrations. The composite systems are specified by two parameters: stiffness ratio and strength ratio of both phases. In particular, we investigate the following aspects: basic classification of effective constitutive responses, geometric patterns of damage, varying degrees of randomness of the inclusions’ arrangements, and mesh resolutions of continuum phases.
Matrix Crack Extension at a Frictionally Constrained Fiber
Authors: Selvadurai A. P. S.
Published: April 2008

The paper presents the application of a boundary element scheme to the study of the behavior of a penny-shaped matrix crack which occurs at an isolated fiber which is frictionally constrained. An incremental technique is used to examine the progression of self similar extension of the matrix crack due to the axial straining of the composite region. The extension of the crack occurs at the attainment of the critical stress intensity factor in the crack opening mode. Iterative techniques are used to determine the extent to crack enlargement and the occurrence of slip and locked regions in the factional fiber-matrix interface. The studies illustrate the role of fiber-matrix interface friction on the development of stable cracks in such frictionally constrained zones. The methodologies are applied to typical isolated fiber configurations of interest to fragmentation tests.
Experimental Characterization of the Tensile Behavior of Microcellular Polycarbonate Foams
Authors: Kumar Vipin , VanderWel Michael , Weller John , Seeler Karl A.
Published: April 2008

Novel polycarbonate (PC) foams with bubbles on the order of 10 μm and cell nucleation densities between 1 and 10 billion cells per cubic centimeter of foam have been produced using carbon dioxide as the blowing agent. The size and number of bubbles can be controlled to produce a wide range of foam densities. This paper presents the results of an experimental study of the tensile behavior of these unique microcellular foams. It was found that the tensile strength of microcellular PC foams is proportional to the foam density. The strength is less than that predicted by the rule of mixtures, suggesting that the microcellular structure is inefficient in carrying the tensile load. The saturation of PC by CO2 was found to reduce the tensile strength of the virgin material by approximately 20 percent. This showed that the sorption of a very high concentration of gas molecules by the polymer must be considered when characterizing and modelling the microcellular foam mechanical properties. The relative tensile modulus of microcellular foam was found to increase as the square of the foam’s relative density over the range of densities explored.
Effect of CO2 Saturation and Desorption on the Fatigue Life of Polycarbonate
Authors: Seeler K. A. , Kumar Vipin
Published: April 2008

The effect of a cycle of CO2 saturation and desorption on the yield strength, Young’s modulus, and fatigue life of polycarbonate was investigated. The fatigue life of the saturation-cycled polycarbonate exceeded that of unprocessed polycarbonate by up to a factor of thirty. The fatigue life of these rough-surfaced saturation-cycled specimens was approximately equal to or greater than that of polished smooth bar specimens. The increase in fatigue life is a function of the CO2 saturation pressure, reaching a maximum at the same saturation pressure at which there is no further reduction in static yield strength. The increased fatigue life is hypothesized to result from one or more of following mechanisms: (1) reduction of shear strength promoting slow-growing shear fatigue cracks: (2) “healing” or blunting of pre-existing flaws or microcracks by swelling strain or by stress relief strain resulting from plasticization by CO2 during saturation.
Numerical Simulation of Fracture in Coated Brittle Materials Subjected to Tribo-Contact
Authors: ONeil David A. , Wayne Steven F.
Published: April 2008

Manufacturing processes used to deposit hard coatings often produce cracks which reside within the coating, at the interface, or in the substrate. These cracks originate from material defects and thermal expansion mismatch. When subjected to stress (from solid body contact) the cracks can act as flaws which initiate and/or propagate subsequent fracture contained entirely in the coating, substrate, interface, or combinations thereof. The finite element method has been used in conjunction with a numerical interface fracture mechanics model to investigate the structural response of coated brittle materials subjected to normal and shear loads (tribo-contact). Residual stresses from depositing TiC onto a WC-TiC-TaC-Co substrate were superimposed with loads that simulate a single point scratch test. Experimental observations of metallographic cross sections, taken through scratched TiC films, were used for verification and guidance in modeling. This study has examined how flaw orientation affects crack propagation through the coating, interface, and substrate. The importance of interface fracture toughness and anisotropy in coating mechanical properties are discussed in light of wear particle formation.
Laser Cutting of Fibrous Quartz Insulation Materials
Authors: Dobbs Robert , Bishop Patricia , Minardi Antonio
Published: April 2008

An experimental investigation was conducted using a CO2 laser to cut fibrous insulation blankets for the Space Shuttle and high speed aircraft. This investigation was unique in identifying important factors for controlling the cut depth, thus, allowing the laser to be used for making partial cuts in a composite quilt. The investigation determined an algorithm for essential physical parameters used in automating the laser machining of these materials.
The Compliance Method for Measurement of Near Surface Residual Stresses—Analytical Background
Authors: Gremaud M. , Cheng W. , Finnie I. , Prime M. B.
Published: April 2008

Introducing a thin cut from the surface of a part containing residual stresses produces a change in strain on the surface. When the strains are measured as a function of the depth of the cut, residual stresses near the surface can be estimated using the compliance method. In previous work, the unknown residual stress field was represented by a series of continuous polynomials. The present paper shows that for stress states with steep gradients, superior predictions are obtained by using “overlapping piecewise functions” to represent the stresses. The stability of the method under the influence of random errors and a zero shift is demonstrated by numerical simulation.
The Compliance Method for Measurement of Near Surface Residual Stresses—Application and Validation for Surface Treatment by Laser and Shot-Peening
Authors: Cheng W. , Finnie I. , Gremaud M. , Rosselet A. , Streit R. D.
Published: April 2008

Residual stresses due to surface treatment are measured using the compliance method. The method makes use of the strains measured on the surface while a cut is extended progressively along a plane of interest. The experimental results for a shot peened specimen show good agreement with those obtained by the X-ray method. This experiment demonstrates that the compliance method is accurate and capable of measuring residual stresses which vary rapidly over a depth of less than 50 μm. Good general agreement with results by the X-ray method is also obtained for a laser treated specimen. Some advantages and disadvantages of the present method relative to hole-drilling, layer removal and X-ray methods are discussed.
The Impact of Real Gas Properties on Predictions of Static and Rotordynamic Properties of the Annular Gas Seals for Injection Compressors
Authors: Shin Yoon-Shik , Childs Dara W.
Published: April 2008

Predictions are presented for an annular gas seal that is representative of the division-wall seal of a back-to-back compressor or the balance-piston seal of an in-line compressor. A two-control-volume bulk-flow model is used including the axial and circumferential momentum equations and the continuity equations. The basic model uses a constant-temperature prediction (ISOT) and an ideal gas law as an equation of state. Two variations are used: adding the energy equation with an ideal gas law (IDEAL), and adding the energy equation with real gas properties (REAL). The energy equations assume adiabatic flow. The ISOT model has been used for prior calculations. Concerning predictions of static characteristics, the calculated mass leakage rates were, respectively, 9.46 kg∕s, 9.55 kg∕s, and 7.87 kg∕s for ISOT, IDEAL, and REAL. For rotordynamic coefficients, predicted effective stiffness coefficients are comparable for the models at low excitation frequencies. At running speed, REAL predictions are roughly 40% lower than ISOT, which could result in lower predicted critical speeds. Predicted effective damping coefficients are also generally comparable. REAL and IDEAL predictions for the crossover frequency are approximately 20% lower than ISOT. REAL predictions for effective damping are modestly lower in the frequency range of 40–50% of running speed where higher damping values are desired.
A Bonded Joint Analysis for Surface Mount Components
Authors: Yamada S. E.
Published: April 2008

To gain a better understanding of the stress state in surface mount joints and their design considerations, an analysis method of bonded joints is developed and consequently three parameters that govern the stress distribution are identified. The solutions are obtained in terms of trigonometric and hyperbolic functions, and then applied to a simplified surface mount device. Contrary to general belief that the surface mount joints are mainly subjected to shear, it is shown that a peel stress also exists in a joint. Especially in a short joint used to connect stiff members as in direct mounting of chips and chip carriers, the magnitude of the peel stress is far greater than the shear stress.
Laminar-Transitional Convection From Repeated Ribs in a Channel
Authors: Wirtz R. A. , Chen Weiming
Published: April 2008

Velocimetry, heat transfer, and pressure drop experiments are reported for laminar/transitional air flow in a channel containing rectangular transverse ribs located along one channel wall. The geometry is intended to represent an array of low profile electronic packages. At fixed pumping power per unit channel volume, the heat transfer rate per unit volume is independent of rib-to-rib spacing and increases with decreasing wall-to-wall spacing. The fully developed, rib-average heat transfer coefficient is found to be linearly related to the maximum streamwise rms turbulence measured above the rib-tops. Linear correlations, in terms of a descriptor of the rms streamwise turbulence, are shown to unify heat transfer/pressure drop data for channels containing either two-or three-dimensional protrusions.
Transient Natural Convection From a Leadless Chip Carrier in a Liquid Filled Enclosure: A Numerical Study
Authors: Wroblewski D. , Joshi Y.
Published: April 2008

The passive cooling, including the transient start-up process, of a leadless chip carrier package mounted on a vertical substrate in a liquid filled cubic enclosure is investigated numerically. A relatively detailed thermal model of the electronic package is included. The governing three-dimensional unsteady equations for natural convection within the fluid and for the coupled heat conduction in the package are solved numerically using a finite difference method. The transient heat up of the system can be characterized by four stages: 1) initial heat up of the chip with some conduction through the package and the surrounding fluid; 2) increased conduction characterized by symmetric thermal spreading through the substrate, accompanied by the development of the natural convection flow field; 3) increased convection effects, as the buoyant driving force reaches its peak level, leading to asymmetric spreading through the substrate and a strong plume above the package; 4) gradual approach to steady state as the fluid in the enclosure is slowly heated, with reduced buoyant driving force and hence decreasing velocities. Steady-state conditions are characterized by thin boundary layers along the hot and cold surfaces, well stratified temperature in the core, a strong plume above the package, and oval shaped isotherms along the substrate surface. With FC-75 as the coolant, a large fraction of the heat generated in the chip is conducted to the substrate and then transferred to the fluid, due to the high thermal conductivity of the substrate relative to that of the fluid. Even when the upper lid of the package is removed, exposing the chip directly to the fluid, the chip temperature drops only by 5 percent, indicating the importance of substrate conduction. Use of a chip that has twice the cross-sectional area as the base case results in a 25 percent drop in chip temperature, with little effect on the remainder of the flow. When water is used as the coolant instead of FC-75, the nondimensional temperature of the chip increases, due to the lower Rayleigh number and lower ratio of substrate-to-fluid thermal conductivity. In dimensional units, however, the actual chip temperature is 25°C cooler.
A Note on the Implementation of Temperature Dependent Coefficient of Thermal Expansion (CTE) in ABAQUS
Authors: Pao Yi-Hsin , Jih Edward , Artz Bruce E. , Cathey Larry W.
Published: April 2008

When modeling the thermomechanical behavior of electronic packages, engineers often need to include the temperature dependence of coefficients of thermal expansion (CTE) of materials involved in the finite element model. In ABAQUS some input parameters associated with such temperature dependent CTE are not clearly defined, and directions to determine the value of these parameters are not given. Misinterpretation of these variables can result in serious errors in the finite element result. This brief tends to illustrate in detail the implementation steps of the temperature dependent CTE in ABAQUS and presents an error analysis so that a quantitative measure of the error can be obtained. The information presented here is regarded critical to those who are using ABAQUS with temperature dependent CTE.
Characteristic Response of a Production Diesel Oxidation Catalyst Exposed to Lean and Rich PCI Exhaust
Authors: Jacobs Timothy J. , Assanis Dennis N.
Published: April 2008

Although low-temperature premixed compression ignition (PCI) combustion in a light-duty diesel engine offers dramatic and simultaneous reductions in nitric oxides (NOx) and soot, associated increases in unburned hydrocarbons (HC) and carbon monoxide (CO) become unacceptable. Production diesel oxidation catalysts (DOCs) are effective in oxidizing the increased levels of HC and CO under lean combustion conditions. However, the low-temperature∕high CO combination under rich PCI conditions, designed as a lean NOx trap (LNT) regeneration mode, generally renders the DOC ineffective. The objectives of this study are to characterize the oxidizing efficiency of a production DOC under lean and rich PCI conditions, and attempt to identify probable causes for the observed ineffectiveness under rich PCI. The study uses several tests to characterize the behavior of the DOC under lean PCI and rich PCI combustion conditions, including (1) steady-state feed gas characterization, (2) transient feed gas characterization, (3) air injection (4) insulated air-fuel sweep, and (5) combustion mode switching. The DOC never becomes effective under rich PCI for any of the tests, suggesting that the platinum-based catalyst may be incorrect for use with rich PCI. Furthermore, combustion mode switching between lean PCI and rich PCI (mimicking LNT loading and regeneration) demonstrates diminishing effectiveness of the DOC during and after continuous mode transitioning.
Towards Multiplexed Fiber Delivered Laser Ignition for Natural Gas Engines
Authors: Yalin Azer P. , Joshi Sachin , DeFoort Morgan , Willson Bryan
Published: April 2008

The use of laser ignition for advanced gas engines may provide benefits including extension of the lean limit and higher efficiency operation at elevated pressures. This contribution provides a short review of efforts to develop a practical laser ignition system for advanced multicylinder gas engines. The approach is to use a single laser source with fiber optic cables delivering the high power pulses from the source to the engine cylinders. The optical requirements for the fiber delivery lead us to use coated hollow core optical fibers. Characterizations and results of spark delivery tests for the fibers are presented. Single-cylinder engine test results using fiber delivered laser ignition are summarized. For multicylinder operation, a multiplexer based on a moving mirror is used to route the laser output pulses to different fiber channels (cylinders). Benchtop testing and initial engine testing of the multiplexed system are presented.
Plastic Deformation Kinetics of Eutectic Pb-Sn Solder Joints in Monotonic Loading and Low-Cycle Fatigue
Authors: Guo Z. , Sprecher A. F. , Conrad H.
Published: April 2008

The deformation kinetics of near-eutectic Pb-Sn alloys in monotonic and cyclic loading in shear at 300K were investigated by determining the stress exponent n in the Dorn equation using several techniques including strain rate cycling. n was found to be a single-value function of the true stress for the present range of test conditions for both the monotonic and cyclic loading. It ranged from ~4 at the lowest stresses to ~30 at the highest stresses. The n-value of ~4 is concluded to represent dislocation glide and climb as the controlling mechanism, while values of n > 10 reflect power low breakdown.
Deformation and Fracture of Pb-Sn-Eutectic Under Tensile and Fatigue Loading
Authors: Lee S.-M. , Stone D. S.
Published: April 2008

Room temperature, displacement-controlled tensile test and displacement-controlled tension/compression fatigue tests were performed on as-cast specimens of eutectic Pb-Sn solder with lamellar microstructure. A range of strain rates were used to identify how failure varies with strain rate. Under tensile loading at high strain rate (6.7 × 10−2/s) failure was observed after about 20 percent strain. There was evidence of shear localization, and failure was transgranular with a dimpled fracture surface. The low strain rate (6.7 × 10−6/s) tensile specimen failed by cavitation. This cavitation coincided with sliding at boundaries between colonies of the eutectic (grain boundaries); there was little evidence of sliding in the high strain rate specimen. Two behaviors were observed in the fatigue tests. At low frequency and high strain range (2 × 10−3/s, 4 percent, Nf ≈ 300−1000) numerous intergranular cracks initiated at the surface, eventually linking up and propagating along grain boundaries to the interior. What appeared to be fatigue striations were found on the crack face. At high frequency and low strains (>0.1/s, <1 percent) transgranular failure dominated.
Measurement of Mechanical Behavior of High Lead Lead-Tin Solder Joints Subjected to Thermal Cycling
Authors: Pao Yi-Hsin , Badgley Scott , Govila Ratan , Baumgartner Linda , Allor Richard , Cooper Ron
Published: April 2008

Failures in electronic packages under thermal fatigue usually result from cracking in solder joints due to creep/fatigue crack growth. Understanding the stress/strain behavior of such solder joints is the first step in characterizing their fracture behavior. A specimen has been developed to determine the stress/strain hysteresis response of 90Pb/10Sn solder joints under cyclic thermal loadings. Simple and special techniques have been developed to fabricate solder joints with relatively high melting points, such as 90Pb/10Sn. Four high-temperature strain gages are mounted on the specimen to measure mechanical strains which provide the basis for determining the shear stress and strain in the solder. A special Wheatstone bridge has been designed to improve the specimen sensitivity, e.g., 20 με/MPa in the test. Shear stresses in the solder as low as 0.5 MPa can be resolved accurately. The specimen was subjected to thermal cycling between 40°C to 140°C, with 10°C/min ramp rate and 10 minute hold times, in a thermal chamber developed in-house. Excellent experimental results have been obtained for 90Pb/10Sn solder joints in that detailed characteristics of stress relaxation and strain creep as a function of temperature were captured. The specimen developed is not only suitable for use in studying the constitutive response of soft solder alloys and other joining materials, such as adhesives, but can serve as a thermal fatigue specimen to study the fracture behavior.
Solder Creep-Fatigue Interactions With Flexible Leaded Parts
Authors: Ross R. G. , Wen L. C. , Mon G. R. , Jetter E.
Published: April 2008

With flexible leaded parts, the solder-joint failure process involves a complex interplay of creep and fatigue mechanisms. To better understand the role of creep in typical multi-hour cyclic loading conditions, a specialized non-linear finite-element creep simulation computer program has been formulated. The numerical algorithm includes the complete part-lead-solder-PWB system, accounting for strain-rate dependence of creep on applied stress and temperature, and the role of the part-lead dimensions and flexibility that determine the total creep deflection (solder strain range) during stress relaxation. The computer program has been used to explore the effects of various solder creep-fatigue parameters such as lead height and stiffness, thermal-cycle test profile, and part/board differential thermal expansion properties. One of the most interesting findings is the strong presence of unidirectional creep-ratcheting that occurs during thermal cycling due to temperature dominated strain-rate effects. To corroborate the solder fatigue model predictions, a number of carefully controlled thermal-cycle tests have been conducted using special bimetallic test boards.
Approximate Structural Analysis of Circuit Card Systems Subjected to Torsion
Authors: Engel P. A. , Vogelmann J. T.
Published: April 2008

Engineering analysis of module-populated printed circuit cards subjected to torsion is pursued by approximate engineering analysis, numerical (finite element), and experimental means. The engineering theory utilizes a simplified method of evaluating the torsional stiffness and maximum lead force, the latter found at the module corners. Finite element methods are used to check these values for circuit cards with a wide variety of module configurations, starting from a single-module to sixteen PLCC modules, having 44, 68, and 84 J-leads. An experimental torsion apparatus is used to obtain data for further comparison with the former approaches, and for getting data from the geometrically nonlinear (large deflection) range.
An Experimental Investigation of a Staggered Array of Heatsinks in the Hydrodynamic and Thermal Entrance Regions of a Duct
Authors: Bazydola Sarah M. , Taslim Mohammad E.
Published: April 2008

The need for methods to predict temperatures of electronic packages is becoming more important as time to market decreases and the need to reduce engineering costs increases. Geometric parameters that are commonly found in a card cage computer design are duct height to package height ratio (H/B) and longitudinal to latitudinal spacing (L/D). The velocity of the air flowing through the card cage is also an important parameter. The purpose of this experimental research is to develop correlations that can be used to predict heat transfer coefficients and thus temperatures of electronic packages in a staggered array in the hydrodynamic and thermal entrance regions of a duct. The ten components in the array are mounted on one wall of the duct to simulate a printed circuit board in a card cage. Results are presented for three values of Reynolds number (20250, 50000 and 65000) for a range of H/B from 1 to 5 by increments of 2 while L/D is varied from 1 to 3 by increments of 0.5. Comparisons are made between upstream and downstream packages. It was determined that one correlation could be used to predict the Nusselt number for upstream heatsinks. This correlation is a function of Reynolds number and H/B. The results also indicate that three correlations are necessary to predict the Nusselt number of downstream heatsinks. When H/B = 1, the Nusselt number is only a function of Reynolds number; when H/B is greater than 1 and L/D is less than 2, the Nusselt number is dependent on Reynolds number, H/B and L/D; when H/B is greater than 1 and L/D is greater than or equal to 2, the Nusselt number is a function of Reynolds number and H/B.
Guidelines for the Use of Approximations in Shock Response Analysis of Electronic Assemblies
Authors: Keltie R. F. , Falter K. J.
Published: April 2008

In order to reduce the time and effort to create models and prevent excessive computer run times, approximations and simplifications are often used in the finite element analysis of the shock response of electronic assemblies. Typical approximations which might be used include neglecting components which have small masses and considering highly stiff connections as rigid connections. It is difficult to determine under what conditions approximations may be applied and to what extent they affect a model’s accuracy. Rather than depending only on an analyst’s experience or intuition, guidelines are desirable to prevent the inappropriate use of approximations. To illustrate the methodology for developing guidelines, this paper examines approximations involving a simple structure which is representative of structures found in electronic assemblies. This structure consists of a rigid body attached by a flexible connection to a beam. Approximations considered were: approximating the stiffness of the connection, neglecting the mass of the rigid body, and approximating the boundary conditions of the beam as either simply-supported or clamped. In developing guidelines a large number of individual analyses were necessary. An important aspect of this investigation is our proposal for a concise format for presenting the results of many analyses. The techniques which were used to reduce the amount of data to be presented are discussed.