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TECHNICAL PAPERS

J. Eng. Mater. Technol. 1998;120(4):265-274. doi:10.1115/1.2807011.

This paper used large deformation finite element theory, updated Lagrangian formulation, finite difference method, and incremental theory to develop a three-dimensional thermo-elastic-plastic simulation model for a tool with chip breaker. Both the critical strain energy density theory and the tool feed geometrical location were introduced as the chip separation criterion for cutting. The algorithm of tool movement geometrical limitations was used to examine and correctly the node so as to conform to real cutting conditions. In this model, the tool moved step by step in the simulation, which ran from the initial contact between tool and workpiece to the formation of steady cutting force. Finally, the numerical simulation model proposed in this paper was used to analyze the changes in workpiece and chip shapes, stress, strain rate, residual stress, temperature and cutting force of mild steel workpiece under different chip breaker lengths. The results were also compared with those from tools without chip breaker. The findings indicate that the chip breaker length affects the shorter the chip breaker length, the better the effects of chip breaker, and the lower the values of the aforementioned physical properties.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1998;120(4):275-279. doi:10.1115/1.2807012.

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

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1998;120(4):280-283. doi:10.1115/1.2807013.

Mechanical degradation in fibrous composite electrodes in use in NiMH(nickel metal hydride) batteries is being studied. NiMH cells exhibit failure in the positive plate due to swelling induced during the electrochemical reaction, which leads to gradual breakdown of the connectivity of the substrate microstructure. Particularly, loss of electrical conductivity as related to losses in mechanical properties upon cell cycling (constant current cycles with substantial overcharge) are being investigated experimentally. These properties are measured for the dry substrate material before and after battery cycling, and the losses observed suggest key tradeoffs between use of higher density nickel substrates and improved energy densities using mechanical and resistivity proof tests.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1998;120(4):284-286. doi:10.1115/1.2807014.

Numerical Green's function for steady-state heat conduction problems is derived in a finite-sized medium that may contain inclusions (fibers) in the matrix phase. Green's function is approximated by employing the Galerkin method that uses permissible functions which satisfy the homogeneous boundary condition for the given geometry. The present approach allows physical fields in a medium that contain multiple inclusions to be expressed through isolated integrals semi-analytically while retaining all the relevant material parameters.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1998;120(4):287-290. doi:10.1115/1.2807015.

This paper presents the experimental result on the response and stability of thin-walled tubes subjected to cyclic bending with different curvature-rates. The curvature-ovalization measurement apparatus, designed by Pan et al. (1998), was used for conducting the present curvature-controlled experiments on thin-walled tubular specimens of 304 stainless steel. It is observed that the higher the applied curvature-rate, the greater is the degree of hardening of metal tube. However, the ovalization of tube cross-section increases when the applied curvature-rate increases. Furthermore, due to the higher degree of the ovalization of tube cross-section for higher curvature-rates under cyclic bending, the number of cycles to produce buckling is correspondingly reduced.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1998;120(4):291-296. doi:10.1115/1.2807016.

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

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1998;120(4):297-299. doi:10.1115/1.2807017.

Residual stresses introduced during quenching process in aluminium gas cylinders contribute to the development of cracks. This may result in leakage or fracture of the cylinders. Finite element studies were conducted to evaluate the effect of the quenching process on through thickness inelastic strain and residual stress distributions in the neck area of the gas cylinder. The numerical modeling and experimental studies confirmed that high level of tensile residual stresses exists at the internal surface of the neck of the aluminium gas cylinders which is susceptible to cracking.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1998;120(4):300-303. doi:10.1115/1.2807018.

It was experimentally shown that transverse stitching can provide an over 20 percent improvement in the strength of composite single-lap joints manufactured using the Resin Transfer Moulding (RTM) process. In this study, an analytical model is presented to predict the strength of the RTM single-lap joints with and without transverse stitching. Simple formulas are used to calculate the energy release rates of mode I and II using longitudinal membrane force and bending moment in a continuous adherent at an overlap end. The strengths of single-lap joints are determined using an interactive mixed-mode based failure criterion. A good correlation between the measured and the predicted failure loads validates the present analytical model.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1998;120(4):304-312. doi:10.1115/1.2807019.

Among various abrasives investigated for the chemo-mechanical polishing (CMP) of Si3 N4 balls (Jiang, 1998), cerium oxide (CeO2 ) was found to be the most effective polishing medium (even superior to Cr2 03 , Bhagavatula and Komanduri, 1996), yielding an extremely smooth and damage-free surface with a finish Ra of ≈4 nm and Rt of ≈40 nm. In this investigation, the underlying reasons for the superior finish with CeO2 were investigated. Various chemical reactions involved in CMP of Si3 N4 balls with CeO2 were investigated (Gibbs free energy minimization) and a mechanism for the CMP is proposed. The two important functions that CeO2 performs in the CMP of Si3 N4 are: 1. It participates directly in the chemical reaction (oxidization-reduction reaction) with Si3 N4 workmaterial leading to the formation of a thin SiO2 layer, 2. The hardness of CeO2 is closer to that of the thin SiO2 layer formed on Si3 N4 but significantly lower than Si3 N4 workmaterial (≈1/3). It can thus remove the brittle SiO2 reaction product effectively without damaging the Si3 N4 substrate as no abrasion can take place by CeO2 on Si3 N4 . The kinetic action, which involves the removal of the reaction products from the interface by subsequent mechanical action of flowing water and CeO2 is critical to CMP. The chemical reaction could proceed on a continuing basis so long as the passivation layers are removed by the mechanical action at the same time. CeO2 is found to be very effective in a water environment (hydrolysis) leading to the formation of additional SiO2 by reacting with Si3 N4 thereby enhancing the CMP of Si3 N4 . Several similarities between polishing of Si3 N4 and glass (SiO2 ) (Cook, 1990), including the polishing environment (CeO2 plus the magnetic fluid, pH value ≈6) and the mechanism of polishing were observed. Also, after investigating various reaction species in the CMP of Si3 N4 with CeO2 and Cr2 03 , the former is found to be much safer from an environmental point of view (Reddy and Komanduri, 1998).

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1998;120(4):313-320. doi:10.1115/1.2807020.

A three-dimensional weld pool model has been developed to study the fluid flow and heat transfer process during gas metal arc (GMA) welding. Both droplet heat content and impact force were considered in analyzing the effect of droplets on the formation of weld pool. The fluid flow in the weld pool was induced by the presence of surface tension, electromagnetic and buoyancy force. The surface deformation of weld pool was calculated by considering arc pressure and droplet impact force. Computational results under partial and full penetration welding conditions were obtained. The effect of heat flow and fluid flow characteristics on weld pool geometry was discussed, particularly with respect to the presence of droplet heat input and impact force.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1998;120(4):321-327. doi:10.1115/1.2807021.

Laser has been widely used in various industrial applications including machining. However, in shaping operation of composite material after curing, thermal damage associated with laser energy can be produced. It leads to poor assembly tolerance and long-term performance deterioration. The current research investigates the anisotropic formation of the heat affected zone (HAZ) in unidirectional fiber-reinforced plastics induced by laser grooving. Preliminary analytical and experimental analysis reveal that the laser energy per unit length and fiber orientation-dependent thermal conductivity primarliy determine the induced thermal damage. The extent of HAZ is estimated by the isotherm of the matrix char temperature. Heat conduction is maximum along the fibers, and the HAZ shape is thus affected by the beam scanning direction relative to fiber orientation. The study investigates the grooving of laminated unidirectional carbon/epoxy, which demonstrates clear thermal damage in 90 degree (i.e., perpendicular grooving), 60 degree, 30 degree, and 0 degree (i.e., parallel grooving) relative to the fiber axis. A theoretical analysis based on moving point heat source is adopted to determine the extent of thermal damage in correlation with process parameters and material properties. Mirror Image Method is used for specimen of finite thickness. Considerations of temperature-dependence of thermal conductivity and the emmerged heat source further improve the prediction of HAZ. While HAZ in grooving along the principal material axes can be solved analytically, conductivity ellipsoid and finite difference can calculate the extent of HAZ induced by grooving in any direction relative to fiber axis.

Commentary by Dr. Valentin Fuster
J. Eng. Mater. Technol. 1998;120(4):328-337. doi:10.1115/1.2807022.

This work examines the fracture behavior of ferritic steel welds in the transition temperature regime, where failure can occur either by ductile tearing or cleavage fracture. A computational and probabilistic-based mechanistic approach to cleavage fracture and ductile crack growth is adopted to model the fracture processes. The softening effect of ductile damage close to the crack tip is described by a Gurson-type material model. A statistical approach linked to both the Weibull stress and the initial void volume fraction is employed to determine the probability of cleavage fracture and the coupling between both fracture mechanisms. Finite element results are relied upon to interpret experimental fracture toughness data for the welds and to examine the effects of near crack tip damage and crack growth on the cleavage failure probabilities and cleavage and ductile fracture toughness distributions. The scatter in the weld experimental fracture toughness data is well reproduced by the proposed cleavage and ductile tearing models.

Commentary by Dr. Valentin Fuster

BOOK REVIEW

J. Eng. Mater. Technol. 1998;120(4):338-339. doi:10.1115/1.2807023.
FREE TO VIEW
Abstract
Commentary by Dr. Valentin Fuster

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