Numerical analysis of localization using a viscoplastic regularization: influence of stochastic material defects

The finite element reliability method is applied to evaluate the effects of stochastic imperfections on the localization behaviour of elasto‐viscoplastic softening solids. Material properties as the Young's modulus, the softening modulus and the initial yield stress are considered to be random fields. Failure criteria are defined for the peak load and the dissipated energy. Likely configurations of imperfections which lead to failure are obtained and the nature and relative importance of the corresponding localization patterns are analyzed. Copyright © 1999 John Wiley & Sons, Ltd.     

Development of non-iterative self correcting solution (NONSS) method for the viscoplastic analysis with the Chaboche model

In a finite element program for the viscoplastic analysis with Chaboche model, the non-iterative and self-correcting solution (NONSS) method, proposed by Tanaka and Miller has been implemented. Towards this, the necessary Jocobian coefficients have been derived analytically for the sopisticated 23-parameter Chaboche viscoplastic model and finally the tangent modulus matrix D^* and the residual viscoplastic stress vector ξ^* have been obtained in order to determine the global tangent stiffness matrix as well as pseudo-load vector. The computational efficiency of this method is demonstrated by solving a variety of bench mark problems over a wide range of rate sensitivity domain under complex monotonic and cyclic loading histories relevant to fast breeder reactor applications. An improved Self-Adaptive Eulers Forward (SAEF) method is taken as the reference for comparision. © 1998 John Wiley & Sons, Ltd.     

Least‐squares mixed finite elements for small strain elasto‐viscoplasticity

The main aim of this contribution is to provide a mixed finite element for small strain elasto‐viscoplastic material behavior based on the least‐squares method. The L₂‐norm minimization of the residuals of the given first‐order system of differential equations leads to a two‐field functional with displacements and stresses as process variables. For the continuous approximation of the stresses, lowest‐order Raviart–Thomas elements are used, whereas for the displacements, standard conforming elements are employed. It is shown that the non‐linear least‐squares functional provides an a posteriori error estimator, which establishes ellipticity of the proposed variational approach. Further on, details about the implementation of the least‐squares mixed finite elements are given and some numerical examples are presented. Copyright © 2008 John Wiley & Sons, Ltd.     

Multiaxial creep–fatigue under anisothermal conditions

Because creep–fatigue is mainly studied in uniaxial tension, it is shown here how to proceed to perform both experiments and calculations under multiaxial loading and when the temperature varies both in time and space. The constitutive equations used are those of elasto‐visco‐plasticity coupled or not, to damage, with isotropic and kinematic hardening. It is shown that the unified damage law first proposed for ductile failure and then for fatigue may also be applied to multiaxial creep–fatigue interactions with a new expression for the damage threshold. The procedure for the identification of material parameters is described in detail. Finally, it is shown that the uncoupled calculation procedure, where damage is calculated as a post‐processing of an elasto‐visco‐plastic computation, gives satisfactory results in comparison to the fully coupled analysis; the latter being more accurate but very expensive in computer time.     

含瓦斯煤卸围压蠕变试验及其理论模型研究

以重庆松藻煤电公司打通一矿突出煤层取得的煤样为研究对象,运用自制的含瓦斯煤三轴蠕变加载渗流试验系统,进行了含瓦斯煤卸围压蠕变与渗流试验研究,分析了其黏塑性本构关系与考虑Klinkenberg效应的卸压瓦斯渗流规律。结果表明：通过卸围压可使含瓦斯煤加速破坏,对于脆性煤岩体则容易引起冲击地压或煤与瓦斯突出。由于煤样在加压再卸压的过程中产生大量的宏观裂隙,优化了煤体中孔隙的连通性,会引起瓦斯流动速度显著增加。改进了Chaboche黏塑性本构模型,可用以描述含瓦斯煤的卸压短期蠕变破坏。通过实验数据可以获得该本构模型中的系列参数,并使之与含瓦斯煤的卸压变形吻合。通过试验得出了考虑Klinkenberg效应的卸围压过程中瓦斯流量的变化规律、视渗透率与滑脱系数。     

Creep/Creep-Recovery Response of Fibredux 920C-TS-5-42 Composite under Flexural Loading

The flexural behavior of unidirectional composites at 90° to the axis was investigated. The experimental data and finite element method results indicate that beam specimens undergo large deformations. An analytical model was developed to calculate the correct maximum stress level obtained during the flexural tests. Models have been developed to describe the time-dependent viscoelastic-viscoplastic stress-strain relationship of unidirectional fibre reinforced thermoset matrix composites. A procedure that extends the classical lamination theory to include time related response of composite materials for membrane and flexural loading resulted in a FORTRAN program, LAMFLU. For Fibredux 920C-TS-5-42 graphite-epoxy composite the constants in the stress-strain relationships were determined from data obtained in four-point bending tests. The model characteristics for tensile loading have been determined by Qin [1] and were adapted to the LAMFLU procedure. The maximum strain history calculated with both models was compared with experimental results from two different flexural loading programs.     

Determination of HgCdTe elasto-plastic properties using nanoindentation

Depth sensing indentation has been used to investigate the elasto-plastic behavior of Hg_0.7Cd_0.3Te prepared by molecular beam epitaxy, liquid phase epitaxy, as well as of bulk Hg_0.7Cd_0.3Te prepared by the modified Bridgman method. It was found that Hg_0.7Cd_0.3Te was characterized by a modulus of elasticity of E _avg ∼ 50 GPa and hardness of H _avg ∼ 0.66 GPa, independent of growth technology. The measured hardness was observed to increase with decreasing size of indentation owing to the nucleation of dislocations within the plastic zone. The elasto-plastic response of the samples to nanoindentation was observed to be purely elastic at low indentation depths and developed into ∼10% elastic and ∼90% plastic response, with an increase in penetration contact depth to above 100 nm exhibiting significant amounts of creep. The transition from purely elastic to elasto-plastic behavior has been observed to be marked by discontinuities, or “pop-in” events in the indenter load-penetration curves, with the indentation zone maximum sheer stress varying with HgCdTe growth method in the range 1.1–1.8 GPa. This onset and subsequent flow of plasticity is postulated to be associated with the spontaneous nucleation and propagation of dislocations.