1,721,227 research outputs found
Adaptive mesh refinement procedure for shear localization problems
No full textThe present work is concerned with the development of a procedure for adaptive computations of shear localization problems. The maximum jump of equivalent strain rates across element boundaries is proposed as a simple error indicator based on interpolation errors, and successfully implemented in the adaptive mesh refinement scheme. The time step is controlled by using a parameter related to the Lipschitz constant, and state variables in target elements for refinements are transferred by L-2-projection. Consistent tangent moduli with a proper updating scheme for state variables are used to improve the numerical stability in the formation of shear bands. It is observed that the present adaptive mesh refinement procedure shows an excellent performance in the simulation of shear localization problems
An investigation on collapse behavior of shear localization in elasto-thermo-viscoplastic materials
No full textThe stress collapse in the formation of shear bands in elasto-thermo-viscoplatic materials is systematically studied within the framework of one-dimensional formulation via analytical and numerical methods. The elastic energy released in a domain is found to play an important role in the collapse behavior of shear localization. A non-dimensional parameter named the stability indicator is introduced to characterize the collapse behavior, with approximate forms of the incremental governing equations. The stability indicator offers useful information regarding the degree of severity of an abrupt change of deformations during the stress collapse. Numerical experiments are carried out to verify the stability indicator by varying material properties
Approximate analysis of a shear band in a thermoviscoplastic material
No full textShear localization is systematically examined with the aid of an approximate model for the three stages of deformation in the formation and development of a shear band, which comprise: the growth of inhomogeneous deformation, the stress collapse, and the postcollapse state. A basic model of one-dimensional simple shear deformation for a thermoviscoplastic material is employed with a linear thermal softening in the absence of strain hardening. The evolution of plastic strain rare at the center of the shear band is explicitly obtained and an approximate scaling law for the critical strain, at which the stress collapse begins, is proposed in terms of material parameters and boundary velocity. All state variables including stress, plastic strain rate, and temperature after the stress collapse are evaluated and the overshoot of the plastic strain rate during stress collapse is explained in terms of a nondimensional parameter and elastic unloading. Finally, numerical experiments confirm the analytical results
Characterization of highly preferred Pb(Zr,Ti)O3 thin films on La0.5Sr0.5CoO3 and LaNi0.6Co0.4O3 electrodes prepared at low temperature
No full textLa0.5Sr0.5CoO3 (LSCO) and LaNi0.6Co0.4O3 (LNCO) thin films were deposited on Pt/Ti/SiO2/Si substrates by DC reactive sputtering at 450 degreesC and were annealed at temperatures ranging from 550 degreesC to 750 degreesC for 30 min in an O-2 ambient to improve the crystallinity of the films and to reduce their resistivity. LSCO and LNCO thin films were successfully prepared at temperatures as low as 450 degreesC. Pb(Zr0.48Ti0.52)O-3 (PZT) thin films of 150 nm thickness were deposited on the LSCO and LNCO electrodes by DC reactive sputtering at a substrate temperature of 550 degreesC. PZT films grown on LSCO and LNCO electrodes showed a (001) preferred orientation and had a uniform matrix of densely packed round grains. The leakage current density remained on the order of 10(-7)-10(-9) A/cm(2) at an applied voltage below 5 V. PZT thin films grown on LSCO/Pt showed a remanent polarization (2P(r)) of about 46-52 muC/cm(2), and a coercive voltage of about 1 V. PZT thin films grown on LNCO/Pt electrodes showed a lower coercive voltage (<0.6 V) and a smaller remanent polarization (2P(r)) of about 28.8 muC/cm(2) than those of PZT films grown on LSCO/Pt. LSCO/Pt and LNCO/Pt electrodes were essential for lowering the crystallization temperature as well as for obtaining good electrical properties of PZT capacitors
Non-isothermal in situ dielectric cure monitoring for thermosetting matrix composites
No full textCure monitoring is important for the quality and productivity of composite structures during the manufacturing process of thermosetting resin matrix composites. Since isothermal cure cycle was used in the previous researches for the dielectric cure monitoring, temperature effect could not be included effectively. In this study, dynamic cure cycle was adopted for the investigation of the composite cure process under non-isothermal conditions. Experimental results obtained from both differential scanning calorimetry (DSC) and dielectrometry under the same cure curing conditions were compared to predict the degree of cure of thermosetting resin matrix composites during the curing process. The sensitivity of dielectrometry was improved by separating the effect of sensor external on the measured signal. A new algorithm to obtain the degree of cure during non-isothermal dielectric cure monitoring for glass/polyester and glass/epoxy composites was developed by employing a function of both temperature and dissipation factor, in which five cure monitoring parameters were used to calculate the degree of cure. The decreasing pattern of dissipation factor was compared with the relationships between the degree of cure and the resin viscosity. The developed algorithm might be employed for the non-isothermal in situ cure monitoring of thermosetting resin composites
Dielectric cure monitoring for glass/polyester prepreg composites
No full textThe on-line cure monitoring of fiber reinforced thermosetting resin matrix composite material has been performed for improving quality and productivity during manufacturing. Since the dissipation factor measured by dielectrometry method is dependent on the degree of cure and temperature of resin. in this study, a new method to obtain the degree of cure during on-fine cure monitoring for S-glass/polyester composites without temperature effect was developed by employing a combination function of the temperature and the dissipation factor. The temperature signal was measured with a K-type thermocouple and the dissipation factor signal was measured with an interdigital dielectric sensor during curing process. Then the calculated degree of cure using the measured data from dielectrometry was compared to the measured value from differential scanning calorimetry. The developed on-line cure monitoring method was applied to a 2-step cure cycle for the verification of the developed procedure. (C) 2002 Elsevier Science Ltd. All rights reserved
Numerical implementation of a thermo-elastic-plastic constitutive equation in consideration of transformation plasticity in welding
No full textFinite element analysis of welding processes, which entail phase evolution, heat transfer and deformations, is considered in this paper. Attention focuses on numerical implementation of the thermo-elastic-plastic constitutive equation proposed by Leblond et al. [J. Mech. Phys. Solids 34(4) (1986a) 395; J. Mech. Phys. Solids 34(4) (1986b) 411] in consideration of the transformation plasticity. Based upon the multiplicative decomposition of deformation gradient, hyperelastoplastic formulation is borrowed for efficient numerical implementation, and the algorithmic consistent moduli for elastic-plastic deformations including transformation plasticity are obtained in the closed form. The convergence behavior of the present implementation is demonstrated via a couple of numerical examples. (c) 2004 Published by Elsevier Ltd
PRESSURIZED WATER-REACTOR CORE PARAMETER PREDICTION USING AN ARTIFICIAL NEURAL NETWORK
No full textIn pressurized water reactors, the fuel reloading problem has significant meaning in terms of both safety and economics. The local power peaking factor must be kept lower than a predetermined value during a cycle, and the effective multiplication factor must be maximized to extract the maximum energy. If these core parameters could be obtained in a very short time, the optimal fuel reloading patterns would be found more effectively and quickly. A very fast core parameter prediction system is developed using the back propagation neural network. This system predicts the core parameters several hundred times as fast as the reference numerical code, within an error of a few percent. The effects of the variation of the training rate coefficients, the momentum, and the hidden layer units are also discussed
On the application of a meshless method to shear band problems
No full textThe multiple-scale Reproducing Kernel Particle Method (RKPM), one of the meshless methods, is applied to the analysis of a strain localization problem. The Lagrangian RKPM formulation and explicit time integration are employed for the simulation of shear band formation in a viscoplastic material. The multiresolution study is also performed using the multiple-scale RKPM and then, an efficient high-scale component from the viewpoint of meshless method is proposed for mesh-free adaptive procedures that will be accomplished in the future
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