K.-C. Liao

Approximate yield criteria for anisotropic porous ductile sheet metals

Abstract An approximate macroscopic yield criterion for anisotropic porous sheet metals is developed under plane stress conditions in this paper. The metal matrices are assumed to be rigid perfectly plastic and incompressible. The Hill quadratic and non-quadratic anisotropic yield criteria are used to describe the matrix normal anisotropy and planar isotropy. The voids in sheet metals are assumed in the form of through-thickness holes. Under axisymmetric loading, a closed-form upper bound macroscopic yield criterion is derived as a function of the anisotropy parameter R , defined as the ratio of the transverse plastic strain rate to the through-thickness plastic strain rate under in-plane uniaxial loading conditions. The plane stress upper bound solutions for different in-plane strain ratios can be fitted well by the closed-form macroscopic yield criterion.

Modification of Hydrophilic Property of Polypropylene Films by a Parallel-Plate Nitrogen-Based Dielectric Barrier Discharge Jet

Modification of the hydrophilic properties of polypropylene (PP) films has been investigated using the postdischarge region of a pulsed nitrogen-based dielectric barrier discharge under atmospheric-pressure conditions. Results show that, for the stationary PP films, the contact angle (CA) decreases dramatically from 103° (untreated) to less than 30° (treated) with a wide range of O2/N2 ratios (<; 1%) and treating distances (<; 10 mm). In addition, the CA can still be maintained at ~ 40° after 24 h of the aging test. For the nonstationary PP films, a highly hydrophilic surface can only be obtained when the PP film is placed near the jet exit with O2/N2 ratios of 0.06%-0.2%. Additionally, the CA can only be maintained at ~80°-90° when the moving speed is ~ 1 cm/s after 24 h of the aging test. These observations are explained through measured optical emission spectra and ozone concentration data, in which the metastable nitrogen plays a key role in breaking the surface chemical bonds and the UV emission (200-300) participates in the process of converting the ozone into oxygen radical. Furthermore, X-ray photoelectron spectroscopy analysis showed that the O/C ratio increases dramatically for improved hydrophilic surface and the incorporation of functional groups containing oxygen (e.g., C-O and C = O ) is critical in increasing the aging time after plasma treatment.

Texture development and plastic anisotropy of B.C.C. strain hardening sheet metals

Abstract A Taylor-like polycrystal model is adopted here to investigate the plastic behavior of body centered cubic (b.c.c.) sheet metals under plane-strain compression and the subsequent in-plane biaxial stretching conditions. The pencil glide system is chosen for the slip mechanism for b.c.c. sheet metals. The {110} and {112} slip systems are also considered. Plane-strain compression is used to simulate the cold rolling processes of a low-carbon steel sheet. Based on the polycrystal model, pole figures for the sheet metal after plane-strain compression are obtained and compared with the corresponding experimental results. Also, the simulated plane-strain stress—strain relations are compared with the corresponding experimental results. For the sheet metal subjected to the subsequent in-plane biaxial stretching and shear, plastic potential surfaces are determined at a given small amount of plastic work. With the assumption of the equivalence of the plastic potential and the yield function with normality flow, the yield surfaces based on the simulations for the sheet metal are compared with those based on several phenomenological planar anisotropic yield criteria. The effects of the slip system and the magnitude of plastic work on the shape and size of the yield surfaces are shown. The plastic anisotropy of the sheet metal is investigated in terms of the uniaxial yield stresses in different planar orientations and the corresponding values of the anisotropy parameter R, defined as the ratio of the width plastic strain rate to the through-thickness plastic strain rate under in-plane uniaxial tensile loading. The uniaxial yield stresses and the values of R at different planar orientations from the polycrystal model can be fitted well by a yield function recently proposed by Barlat et al. (1997b).

The first order load-balanced algorithm with static fixing scheme for centralized WSN system in outdoor environmental monitoring

The power of nodes in the first layer of a wireless sensor network (WSN) cannot last due to their frequent relay tasks. One possible solution to this disadvantage is to adopt the first order load-balanced algorithm (LBA) to redistribute the loading of the first layer node uniformly. The balanced low-latency convergecast tree (BLLCT) algorithm, a LBA, is generally utilized in route planning in ecological surveillance. Given that the balance between layers does not ensure the balance of the whole WSN, we propose a LBA combining with a static fixing scheme to shift the node groups from the subtree with the largest loading to the one with smaller loading. This procedures repeats until the loading of each subtree is approximately identical. Such a modified LBA successfully overcomes many complicated problems when a WSN is implemented, based on the simulation results using grid and random topologies. In addition, the performance of the static fixing on random node distribution is better than that on the grid node distribution. In this regards, the static fixing is an adequate method to apply to non-uniform loading situations.

Effects of yield surface shape on sheet metal forming simulations

Three phenomenological yield criteria are adopted to describe the plastic behaviour of sheet metals with normal plastic anisotropy. The sheet metals are assumed to be elastic-plastic, rate-sensitive and incompressible. A rate-sensitive thin shell nite element formulation based on the virtual work principle is derived for the three yield criteria. The eects of the yield surface shapes based on the three yield criteria with the same value of the plastic anisotropy parameter R on the strain distribution and localization are investigated under a hemispherical punch stretching operation and a plane strain drawing operation. The results of the simulations show that the yield surface shape, in addition to the plastic anisotropy parameter R, controls the punch force, strain distribution and strain localization for the punch stretching operation. However, the yield surface shape does not aect the punch force and the strain distribution signicantly for the plane strain drawing operation. ? 1998 John Wiley & Sons, Ltd. Int. J. Numer. Meth. Engng., 41, 559{584 (1998)

Texture development and hardening characteristics of steel sheets under plane-strain compression

Crystallographic texture development and hardening characteristics of a hot-rolled, low-carbon steel sheet due to cold rolling were investigated by idealizing the cold rolling process as plane-strain compression. The starting anisotropy of the test material was characterized by examination of the grain structure by optical microscopy and the preferred crystal orientation distribution by x-ray diffraction. Various heat treatments were used in an effort to remove the initial deformation texture resulting from hot rolling. The plastic anisotropy of the starting material was investigated with tensile tests on samples with the tensile axis parallel, 45°, and perpendicular to the rolling direction. The grain structure after plane-strain compression was studied by optical microscopy, and the new deformation texture was characterized by x-ray diffraction pole figures. These figures are compared with the theoretical pole figures produced from a Taylor-like polycrystal model based on a pencil-glide slip system. The uniaxial tensile stress-strain curve and the plane-strain, compressive stress-strain curve of the sheet were used to calibrate the material parameters in the model. The experimental pole figures were consistent with the findings in the theoretical study. The experimental and theoretical results suggest that the initial texture due to hot rolling was insignificant as compared with the texture induced by large strains under plane-strain compression.

A Gurson Yield Function for Anisotropic Porous Sheet Metals and its Applications to Failure Prediction of Aluminum Sheets

An approximate anisotropic yield function is presented for anisotropic sheet metals containing spherical voids. Hills quadratic anisotropic yield function is used to describe the anisotropy of the matrix. The proposed yield function is validated using a three-dimensional finite element analysis of a unit cell model under different straining paths. The results of the finite element computations are shown in good agreement with those based on the yield function with three fitting parameters. For demonstration of applicability, the anisotropic Gurson yield function is adopted in a combined necking and shear localization analysis to model the failure of AA6111 aluminum sheets under biaxial stretching conditions.

Ar/O 2 Argon-Based Round Atmospheric-Pressure Plasma Jet on Sterilizing Bacteria and Endospores

In this paper, we want to develop and characterize a round argon-based nonthermal atmospheric-pressure plasma jet (APPJ) with an application of sterilization. We measured the basic properties of APPJ by an alcohol thermometer, an optical emission spectrometer (OES), a high-voltage probe and a Rogowski coil. The plasma jet temperature at the tube exit is lower than 37°C, which will not induce damage to living tissue. The plasma absorbed power is about 2.7 W. The OES data indicate that this APPJ has a relatively high intensity of hydroxyl radicals (OH) in postdischarge region. Furthermore, we applied this device to inactivate the bacteria (E. coli and B. subtilis) and endospore (B. subtilis endospore). In addition, the trace amount of oxygen addition (0.04%) into working gas can not only enhance the OH radicals but also speed up the killing of bacteria and endospore. The results of D value and scanning electron microscope images show its great potential for the use of general sterilization in the future.

A novel coverage-preserving algorithm with energy efficiency

Deploying wireless sensor networks (WSNs) has been an area of interest in recent years. Adoption of WSNs with energy efficiency is a critical issue since the energy of sensor nodes is usually rather limited. Energy-efficient coverage control is regarded as an essential factor influencing the performance of WSNs. Coverage might gradually degrade after the activation of WSNs. Thus, a novel coverage-preserving algorithm (CPA) that is able to prolong the lifetime and to maximize the coverage is proposed in this study. The new scheme turns redundant nodes off according to the optimal arrangement based on a Memetic Algorithm (MA), and awakens some inactive nodes at proper time to recover the lost coverage due to the node failure or energy exhaustion. Simulation results via a cluster-based WSN (CWSN) show that the performance of the suggested algorithm is promising in terms of energy efficiency and coverage preservation.

Investigation of Relaxation Behavior for Terminals of CPU Socket Connectors

Terminals of a CPU socket connector dominate signal transmission between the CPU and the motherboard. Contact forces between the terminal of an electronic connector and the corresponding counterparts generally play an important role in signal transmissions. The contact forces of these terminals, however, could decay gradually due to the stress relaxation effect under the high temperature conditions. A finite element analysis is carried out to examine the relaxation phenomenon of the terminal in the present study. The strain-hardening form of the power-law creep model is adopted to account for the relaxation behavior of the terminal material. According to the field life acceleration test specifications established by the Electronic Industry Association (EIA), relationships between the calculated contact force and duration period are then constructed. Moreover, the contact force of the terminal subjected to thermal cycling test is also investigated. Calculations show that the decreased amount of the contact force is rather limited. Associated electrical resistances of the current terminal are further measured to validate the suitability of the contact force as an index of the signal transmission quality.