Showing papers by "Jianguo Lin published in 2007"
TL;DR: In this article, Luo et al. proposed an approach based on the assumption that the instrumented indentation force-depth response of an elastic-plastic material is a linear combination of the corresponding elastic and perfect plastic materials.
80 citations
TL;DR: In this article, a micromechanical finite element model is established and used to model the damage evolution process of a free-cutting steel bloom, where the inclusions are associated with a lead tail and the damage growth is directly related to the inclusion size and spacing.
Abstract: The spatial variations in the inclusion size and distribution are examined for a free-cutting steel bloom. Chemical analysis has been carried out on the inclusions using energy-dispersive X-ray analysis and the main findings are reported. It has been observed that inclusions are often associated with a lead tail. Thus an assumption is made that the lead layer becomes liquid or gaseous at hot-forming temperatures. Based on this assumption, a micromechanical finite element model is established and used to model the damage evolution process. Microstructure examinations of hot-tensile-tested specimens are used to compare experimentally the observed inclusion shape and void growth features with those obtained from the micromechanical finite element method. It is concluded that there is no significant inclusion debonding process for the material deformed under hot-forming conditions. The damage growth is directly related to the inclusion size and spacing.
23 citations
Journal Article•
TL;DR: In this paper, the authors developed a unified viscoplastic-damage constitutive equations, in which the nucleation and growth of both damage types are considered independently. And they used the fitted model is then used to predict damage and failure features of the same material tested using a set of interrupted constant strain rate tests.
Abstract: Damage constitutive equations are formulated to model the evolution of grain boundary
and plasticity-induced damage for free-cutting steels under hot forming conditions. During
high temperature, high strain rate deformation, material degradation has characteristics of
both creep damage at grain boundaries, and ductile damage surrounding hard inclusions. This
has been experimentally observed and is reported in the companion paper. This paper describes
the development of unified viscoplastic-damage constitutive equations, in which the nucleation
and growth of both damage types are considered independently. The effects of deformation rate,
temperature, and material microstructure on damage evolution are modelled. The proposed
damage evolution equations are combined with a viscoplastic constitutive equation set, enabling
the evolution of dislocation hardening, recovery, recrystallisation, grain size, and damage to be
modelled. This set of unified, mechanism-based, viscoplastic damage constitutive equations is
determined from experimental data of a free-machining steel for the temperature range 1173–
1373 K. The fitted model is then used to predict damage and failure features of the same
material tested using a set of interrupted constant strain rate tests. Close agreement between
the predicted and experimental results is obtained for all the cases studied.
20 citations
01 Jan 2007
TL;DR: Based on Voronoi tessellation and the probability theory, a VGRAIN system is created for the generation of grains and grain boundaries for micro-materials in this article.
Abstract: To aid FE simulation for forming micro-components, an integrated approach is proposed to generate virtual microstructure for micro-mechanics modelling. Based on Voronoi tessellation and the probability theory, a VGRAIN system is created for the generation of grains and grain boundaries for micro-materials. The input data of the system are physical parameters of a material, including average, minimum and maximum grain sizes. Numerical procedures have been established to link the physical parameters of a material to the control variable in a gamma distribution equation and a method has been developed to solve the probability equation. These are the basis for the development of the VGRAIN system, which can be used to generate different grain structures and shapes that follow a certain pattern according to the probability theory. Statistical analyses have been carried out to investigate the distribution of generated virtual grains. The generated virtual microstructure is then implemented in the commercial FE code, ABAQUS, for mesh generation and micro-mechanics analysis using crystal plasticity equations for FCC materials. The crystal plasticity model is implemented in the commercial FE code, ABAQUS, through the used-defined subroutine, UMAT. FE analyses have been carried out to investigate size effects and localised necking encountered in micro-forming processes.Copyright © 2007 by ASME
9 citations
TL;DR: In this paper, a novel optimization approach is proposed to extract mechanical properties of a power law material from its given experimental nano-indentation P-h curves, and a set of equations have been established to relate the Ph curve to mechanical properties, E, σ y and n, of a material.
Abstract: A novel optimization approach is proposed to extract mechanical properties of a power law material from its given experimental nano-indentation P-h curves. A set of equations have been established to relate the P-h curve to mechanical properties, E, σ y and n, of a material. Using the proposed optimization approach, convergence studies were carried out for the determination of the mechanical properties of materials. It was found that the mechanical properties of an elastic-plastic material usually cannot be uniquely determined using a single loading and unloading P-h curve. Thus a technique has also been developed to determine the material properties from indentation p-h curves using indenters with two different angles. This enables the mechanical properties of materials to be uniquely determined.
3 citations