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FOREWORD

J. Eng. Mater. Technol. 1993;115(1):1. doi:10.1115/1.2902151.
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Abstract
Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Papers on Heat and Mass Transfer in Solidification Processing

J. Eng. Mater. Technol. 1993;115(1):2-7. doi:10.1115/1.2902153.

One of the major objectives of solidification modeling is to determine, prior to pouring, whether porosity, such as massive cavities and dispersed pores, are likely to appear in the casting. The numerical solutions of solidification heat transfer alone, however, cannot provide such information. In order to predetermine the presence of porosity, various criteria functions have been proposed by a number of investigators. These criteria functions are associated with cooling rate, thermal gradient, solidus velocity and local solidification time, etc. Since these parameters can be derived from numerical solutions, the reliability of porosity prediction largely depends on the accuracy of the numerical solutions employed. Thermal contact and phase change affect the numerical solutions significantly, and hence the local values of the predicted parameters. Consequently, these phenomena must be given special attention. This paper addresses some important aspects of thermal contact and phase change in determining the values of criteria functions. The free thermal contraction method is used to describe the variation and distribution of the heat transfer coefficient at the casting/mold interface. The phase change problem is treated by the heat source/sink algorithm. The sensitivity of criteria functions and the role of computational error are also discussed.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):8-16. doi:10.1115/1.2902163.

For many continuous ingot casting processes, turbulent heat transfer in the molten pool plays a critical role which, along with buoyancy and surface tension, is responsible for the quality of the end products. Based on a modified low Reynolds number k-ε two-equation closure, accounting for the phase change and mushy zone formation, the effect of turbulent heat transfer on the solidification characteristics during titanium alloy ingot casting in an electron beam melting process is investigated. The overall heat transfer rate is enhanced by turbulent transport via two sources, one through the correlated velocity and temperature fluctuations present for both single- and multi-phase flows, and the other through the correlated velocity and release of latent heat fluctuations which are unique to the flows with phase change. The roles played by both mechanisms are identified and assessed. The present turbulence model predicts that although the mushy zone defined by the mean temperature field is generally of substantial thickness as a result of the convection effect, the actual instantaneous zone thickness varies substantially due to turbulence effect. This finding is in contrast to the traditionally held viewpoint, based on the conduction analysis, of a generally thin mushy zone. The impact of turbulent heat transfer on local dendrite formation and remelting is illustrated and the issues involved in model development highlighted.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):17-23. doi:10.1115/1.2902150.

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.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):24-29. doi:10.1115/1.2902152.

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.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):30-36. doi:10.1115/1.2902154.

The build-up of the residual stresses in the plastic parts are directly affected by the transient temperature distribution during the solidification stage of the semicrystalline polymers. Hence for crystallizable polymers, it is important to couple the crystallization kinetics with the transport of heat flow to determine the thermal history during solidification. In this paper, an experimental study of solidification of two semi-crystalline polymers under quiescent conditions is carried out to examine the influence of crystallization kinetics on the thermal history. The selected model materials are Nylon-66 and Poly Ethylene Terephalate (PET). The temperature profiles and the location of the solidification front are recorded during the cooling process. The experimental results are compared with the classical heat diffusion model and also with a numerical approach which couples the crystallization kinetics to the heat diffusion equation by modifying the phase change temperature based on the overall cooling rate in the crystalline domain. The front movement and the solidification temperature predictions of the coupled model are in good agreement with the experimental observations.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):37-47. doi:10.1115/1.2902155.

This work employs a coupled analysis of the fluid flow and heat transfer in the polymer melt during the filling and post-filling stages of the injection-molding process and of mold cooling/heating which occurs during the entire process. Polymer melt analysis (PMA) has been carried out through a unified theoretical model implemented using a hybrid finite-element/finite-difference/control-volume numerical solution of the generalized Hele-Shaw flow of a compressible viscous fluid under non-isothermal conditions. Further, mold-cooling analysis (MCA) has been carried out utilizing a periodic heat conduction model implemented using a modified three-dimensional boundary-element method. To faithfully accommodate the effects of mold cooling on the fluid flow and heat transfer in the polymer melt, PMA and MCA have been coupled for appropriate data exchange and iterations carried out until a convergent solution for mold temperatures and for flow, pressure and temperatures within the polymer melt is obtained. The results obtained from this integrated simulation for different test cases have been compared with experimental data and a favorable agreement has been noticed. Using an illustrative example, the results are discussed in detail.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):48-53. doi:10.1115/1.2902156.

Coinjection molding comprises sequential or concurrent injection of two different but compatible polymer melts into a cavity in which the materials laminate and solidify. This innovative process offers the inherent flexibility of using the optimal properties of each material or modifying the properties of the molded part. The lack of previous experience and engineering know-how has made numerical analysis a useful tool for enhancing the engineers’ capability to handle this special process. This paper presents the methodology for analyzing the flow of two different polymer melts injected sequentially into a three-dimensional thin cavity. This study is distinct from numerous previous works dealing with single polymer melts typically used in the conventional injection molding process. As an illustration, a comparison between the predictions and experimental data for a co-injected part is presented, together with other relevant output showing the effect of different material properties on the outcome of the coinjection molding process.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):54-62. doi:10.1115/1.2902157.

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.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):63-67. doi:10.1115/1.2902158.

The paper presents initial results of a modified planar flow casting process with the aim of producing a multilayer composite material with an AlPb-alloy-based coating. The monotectic AlPb system is characterized by rapid separation behavior of the components in the liquid phase. The coarsening of Pb precipitates due to diffusion-and coagulation-effects is estimated and with regard to the required microstructure, Al with finely dispersed Pb, the necessary cooling rate and the temperature gradient perpendicular to the substrate are deduced. Based upon these assumptions the numerical simulation aided design of the thermal process is described. Experimental results, achieved on a pilot plant, are shown and the microstructure of the produced multilayer composite is discussed.

Commentary by Dr. Valentin Fuster

RESEARCH PAPERS: Additional Research Papers

J. Eng. Mater. Technol. 1993;115(1):68-76. doi:10.1115/1.2902159.

The results of experiments performed on an austenitic stainless steel of the type 316L at a temperature of 600°C are presented. The tests were made under both unidirectional (1D) and bidimensional (2D) cyclic tension-torsion loading, both in and out of phase with one (case of 2D ratchet) or two cyclic components. For the 2D loadings, it is shown that a weak supplementary hardening ΔH+ appears which is mostly a function of the degree of phase difference φ between the strain components and the ratio R between the maximum amplitudes of these components. These observations conform qualitatively to those already reported for ambient temperature but quantitatively it is shown that the maximum amplitude of this supplementary hardening is a strongly decreasing function of the temperature. A simple phenomenological formulation is proposed which, when integrated into a unified viscoplastic model developed elsewhere, leads to a correct representation of the experimental results.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):77-82. doi:10.1115/1.2902160.

By means of combining Drucker’s yield function with Hill’s quadratic yield function, an anisotropic yield function of the sixth degree is proposed. It is able to include the effects of the third deviatoric stress invariant and initial anisotropy. Experiments are carried out on fully annealed 1050 aluminum tubes under multiaxial stress states. By applying proportional loadings of axial load, internal pressure and torsion to the specimens, the change in yield stress with a rotation of the principal stress axes and the difference between the directions of the principal stress and principal strain increment are examined. The yield surface in the tension-internal pressure stress field reveals orthotropic anisotropy. The yield surface in the tension-torsion stress field lies outside von Mises’ yield surface. Such behavioral characteristics can be expressed precisely by the proposed yield function. In addition, it is experimentally verified that the normality rule is obeyed in strain behavior.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):83-88. doi:10.1115/1.2902161.

Systematic experiments were conducted on carburized 4320 steel to study thermalinduced transformation of austenite to bainite by measuring the volumetric transformation strains. Up to about 210°C, thermal-induced transformation involved a diffusion-controlled reaction and therefore was time dependent. The mechanism is similar to that described by the second stage of tempering. An Arhennius-type relation was used to model the kinetics of the reaction leading to a volume increase. A positive hydrostatic stress tended to increase the rate of transformation. The anisotropy of the transformation strains were proportional to the deviatoric stress.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):89-94. doi:10.1115/1.2902162.

Elastic-plastic responses of porous iron under uniaxial strain cycling between two fixed values of strain are investigated. A special set of constitutive equations is formulated by including isotropic, kinematic and saturation hardening responses. The theoretical results from the constitutive equations are compared with experimental cyclic data for porous iron, with various porosities.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):95-100. doi:10.1115/1.2902164.

In this paper we develop a mathematical model and associated boundary-value problem for the encapsulating mechanism of a deeply imbedded salt vault at the Department of Energy’s Waste Isolation Pilot Plant in Carlsbad, New Mexico. The solution of the resulting boundary-value problem, which is the subject of a follow-on paper, provides significant, accurate information for predicting the time history encapsulating mechanism of salt vaults.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):101-105. doi:10.1115/1.2902140.

The work of Evans and Hutchinson (1989) on micromechanics modeling of combined mode fracture is used as the basis for proposing an expression for combined Mode I and Mode II fracture toughness of brittle monolithic materials as well as bimaterial interfaces. The results of the proposed expression are compared with experimental data.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):106-108. doi:10.1115/1.2902141.
Abstract
Topics: Fatigue life
Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):109-115. doi:10.1115/1.2902142.

This paper assesses the high-temperature low-cycle fatigue of the Type 304 stainless steel and Alloy 718 superalloy friction-welded joints. Strain controlled low-cycle fatigue tests for 304-304 and 718-718 friction-welded specimens were carried out at 923K in air to obtain the fatigue strength of the joints. These materials were selected as the cyclic hardening and softening materials, respectively. The 304-304 welded specimens showed inferior fatigue strength in comparison with the base metal while the 718-718 specimens exhibited fatigue strength equivalent to that of the base metal. The difference in the fatigue strength between the two materials is discussed from the viewpoint of the cyclic deformation behavior and strain reduction at weld interface.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):116-121. doi:10.1115/1.2902143.

This paper compares the open hole compression, compression after impact, compression fatigue of open hole specimens, and compression fatigue after impact response of quasi-isotropic laminates with IM7 carbon fiber and 3501-6 and 8551-7 epoxy matrices. These matrices can be considered to be a relatively brittle and a high-toughness resin, respectively. The objective was to establish whether the improved compression after impact response associated with high toughness matrices also held after fatigue loading. The results of impact and compression fatigue tests show that residual strengths of the toughened epoxy matrix system were approximately twice that of the brittle matrix system, and that fatigue resistance after impact and of open hole specimens was generally improved.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):122-128. doi:10.1115/1.2902144.

An experimental program has been conducted to evaluate how changes in laminate lay-up can influence the compression behavior of fiber-dominated, resin-based composites. The testing utilized a cylindrical, 3.81 cm (1.5 in.) diameter test specimen because it provides inherent resistance to global buckling failure modes and lack of free edge effects. Seven different laminates representing 0° dominated lay-ups, axial bias lay-ups, and quasi-isotropic lay-ups were tested. The measured macroscopic stress strain failure data showed a strong in-situ dependence of the 0° failure strain on the lay-up. The quasi-isotropic laminate failure strains were nearly twice the 0° dominated laminate failure strains. Photomicrographs of the failure zone from sections of failed specimens showed the presence of fiber kinking in all the laminate failures.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):129-133. doi:10.1115/1.2902145.

Thermo-elastoplastic finite element methods are used to investigate the thermal cycling of SiC whisker-reinforced aluminum-matrix composites. In a previous investigation, the development of residual stresses and plastic deformation during cooling from a typical solution-treatment or annealing temperature to room temperature was studied. In the present paper we investigate changes in these residual quantities during thermal cycling, including cycling to cryogenic temperatures, and the effect of these changes on subsequent mechanical behavior. The effective coefficients of thermal expansion and convergence to a stable cyclic loop are also examined.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):134-139. doi:10.1115/1.2902146.

A new method is proposed for analyzing stress intensity factors of multiple cracks in an adhesively bonded metallic sandwich sheet. Using a basic solution of a single crack and taking unknown density of surface tractions and adhesive shear stresses, Fredholm integral equations and compatibility equations are formulated based upon stress free condition along each crack and displacement continuity between the sheets and adhesive layers, respectively. These equations are solved simultaneously, and the stress intensity factors of multiple cracks are determined from the derived density of tractions. It is shown that the mutual interaction of multiple cracks in a sandwich sheet is smaller than that in a monolithic sheet. Also, mutual interaction of cracks in the same sheet is smaller than that of cracks in the different sheets.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):140-145. doi:10.1115/1.2902147.

An experimental investigation is conducted to study the interaction of a running crack with embedded fibers. Dynamic photoelasticity is used to evaluate the crack velocity and the instantaneous stress intensity factor, KID , as the crack propagates across the fibers. Fractography is used to explain the interaction of the dynamic crack front with the fiber. The results show that fibers significantly reduce the stress intensity factor and also the crack velocity. The effect of a weak fiber-matrix interface on crack velocity and KID is studied. A weak interface reduces KID but has no effect on the crack velocity. The crack closing forces applied by fibers bridging the crack faces have been determined for both the strong and weak interfaces and their effect on KID is explained.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):146-149. doi:10.1115/1.2902148.

Polymer-based composite materials are used in a variety of industry. Recently, thermoplastic polymer suitable for the resinous matrix in carbon fiber-reinforced composites has been introduced for lower material and processing costs, improved damage tolerance and higher moisture resistance. The successful use of this material requires sophisticated production technology, however little reference of machining of thermoplastics composites can be found. The existing published results are almost exclusively for epoxy-based composite materials showing difficulty in avoiding poor finish, serious tool wear and delamination at hole entrance and exit due to the brittle material response to machining. Thermoplastics-based composite materials possesses better machinability. The current work reveals the machinability of an example of carbon fiber-reinforced ABS (Acrylonitrile Butadiene Styrene) in drilling compared to representative metals and thermoset-based composites. The observation of chips reveals that considerable plastic deformation is involved. Compared to the chip formation of thermoset plastics, it contributes to the improved edge quality in drilling. The edge quality is generally fine except in the case of concentrated heat accumulation at tool lips, which is generated by high cutting speed and low feed rate. Plastics tend to be extruded out of the edge rather than neatly cut. The average surface roughness along hole walls in commonly below one micron for all sets of cutting conditions in the experiment, values between 0.3 and 0.6 microns are typical. The high speed steel drill presents only minor tool wear during the tests. Based on these results, one concludes that the carbon fiber-reinforced ABS demonstrates good machinability in drilling.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1993;115(1):150-155. doi:10.1115/1.2902149.

The intimate contacting of rough surfaces in the solid state bonding of metals is modeled by a finite element method. The finite element method can be applied to the large deformation process of rate sensitive materials. The material used is an oxygen free copper. We treat only the case that the intimate contact is the rate controlling step in the solid state adhering process which can be realized under high vacuum and high temperature conditions for copper at least. The intimate contacting process is assumed to be produced by viscoplastic deformation after the initial local contact is made by instantaneous plastic deformation. The calculated results are in good agreement with the experimental ones. The model can predict the interfacial deformation during the solid state bonding carried out under high pressure conditions.

Commentary by Dr. Valentin Fuster

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