Modelling of interfacial viscoplastic slip coupled to damage in a polycrystalline microstructure

The mesomechanics behavior of a polycrystalline microstructure subjected to creep and constant strain rate loadings is investigated. The analysis is based on a Voronoi polygonization strategy for the generation of grains, that are bonded to each other via interfaces along the grain boundaries. A new constitutive model is proposed for the rate-dependent debonding along these interfaces, whereby damage is kinetically coupled to viscoplastic slip and dilatation. The paper may be viewed as generalizing the rate-independent model used in Cannmo et al. (1995).     

Non-linear characterization of PP/Twaron laminates based on a model of plasticity with damage

The non-linear stress–strain response of thermoplastic laminates was investigated for a polypropylene matrix composite reinforced with continuous Twaron fibers. Tensile tests were performed on unidirectional composites with a fiber orientation of [0]₅, [24]₅, [33]₅, [45]₅ and [90]₅ and also on a [±45]_2S laminate. The inelastic strains of these materials were described with a modification made to a model of plasticity with damage, originally proposed by Ladeveze (1992). This approach considers the longitudinal plasticity and stiffening behavior of a thermoplastic composite, and models the damaged elastoplastic stress–strain response of the material. The theoretical stress–strain curves and experimental results from the model composites [0]₅, [90]₅ and [±45]_2S were compared, demonstrating that the model can be used as a designing tool for laminated thermoplastic composites.     

Comparison of Bayesian methods on parameter identification for a viscoplastic model with damage

The state of materials and accordingly the properties of structures are changing over the period of use, which may influence the reliability and quality of the structure during its life-time. Therefore, identification of the model parameters of the system is a topic which has attracted attention in the content of structural health monitoring. The parameters of a constitutive model are usually identified by minimization of the difference between model response and experimental data. However, the measurement errors and differences in the specimens lead to deviations in the determined parameters. In this article, the focus is on the identification of material parameters of a viscoplastic damaging material using a stochastic simulation technique to generate artificial data which exhibit the same stochastic behavior as experimental data. It is proposed to use Bayesian inverse methods for parameter identification and therefore the model and damage parameters are identified by applying the Transitional Markov Chain Monte Carlo Method (TMCMC) and Gauss–Markov–Kalman filter (GMKF) approach. Identified parameters by using these two Bayesian approaches are compared with the true parameters in the simulation and with each other, and the efficiency of the identification methods is discussed. The aim of this study is to observe which one of the mentioned methods is more suitable and efficient to identify the model and damage parameters of a material model, as a highly non-linear model, using a limited surface displacement measurement vector and see how much information is indeed needed to estimate the parameters accurately.     

Effect of strain waveform on creep-fatigue life for Sn–8Zn–3Bi solder

This paper describes the creep‐fatigue life of Sn–8Zn–3Bi under push–pull loading. Creep‐fatigue tests were carried out using Sn–8Zn–3Bi specimens in fast–fast, fast–slow, slow–fast, slow–slow and hold–time strain waveforms. Creep‐fatigue lives in the slow–slow and hold‐time waveforms showed a small reduction from the fast–fast lives but those in the slow–fast and fast–slow waveforms showed a significant reduction from the fast–fast lives. Conventional creep‐fatigue life prediction methods were applied to the experimental data and the applicability of the methods was discussed. Creep‐fatigue characteristics of Sn–8Zn–3Bi were compared with those of Sn–3.5Ag and Sn–37Pb.     

Failure analysis of a cracked plate based on endochronic plastic theory coupled with damage

An anisotropic model of damage mechanics for ductile fracture incorporating the endochronic theory of plasticity is presented in order to take into account material deterioration during plastic deformation. An alternative form of endochronic (internal time) theory which is actually an elasto-plastic damage theory with isotropic-nonlinear kinematic hardening is developed for ease of numerical computation. Based on this new damage model, a finite element algorithm is formulated and then employed to characterize the fracture of thin aluminum plate containing a center crack. A new criterion termed as Y _R-Criterion is proposed to define both the crack initiation angle and load. Experiments have been conducted to verify the validity of the proposed damage model and it is found that the theoretical crack initiation loads correspond closely with the measured values.     

The influence of dynamic strain aging on fatigue and creep-fatigue characterization of nickel-base solid solution strengthened alloys

Abstract

The nickel-base solid solution alloys, Alloy 617 and Alloy 230, have been observed to exhibit serrated yielding or dynamic strain aging (DSA) in a temperature/strain rate regime of interest for intermediate heat exchangers (IHX) of high temperature nuclear reactors. At 800°C, these nickel-base alloys are prone to large serrated yielding events at relatively low strains. The presence of DSA introduces challenges in characterizing the creep-fatigue and low cycle fatigue behaviour. These challenges include inability to control the target strains as a result of DSA induced strain excursions and distorted hysteresis loops. Methods to eliminate or reduce the influence of DSA on creep-fatigue testing have been investigated, including varying the strain rate, stepping to the target strain, and adjusting servo-hydraulic tuning parameters. It has not been possible to eliminate the impact of serrated flow in the temperature range of interest for these alloys without compromising the desired test protocols.     

Simulation of perforation and penetration in metal matrix composite materials using coupled viscoplastic damage model

In the first part of the two companion papers, theoretical formulation of the multiscale micromechanical constitutive model that couples the anisotropic damage mechanism with the viscoplastic deformation is presented. In the second part of these companion papers the numerical simulation of the computational aspects of the theory are elaborated. The perforation and penetration problem of metal matrix composites (MMCs) due to high impact loading is simulated. In this sense, the computational aspects of the developed theory are elaborated here. First, the verification of the developed model is performed through its numerical implementation in order to test the model predictions of the material characteristic tests. This encompasses uniaxial monotonic loading and unloading under different strain rates, uniaxial cyclic loading, and uniaxial loading and relaxation. The verified material routine of the developed model is then implemented in the explicit finite element code ABAQUS via the user defined subroutine VUMAT at each integration point in order to analyze the projectile impact and penetration into laminated composite plates.     

Thermodynamically consistent coupled viscoplastic damage model for perforation and penetration in metal matrix composite materials

Accurate modeling and efficient analysis of the metal matrix composite materials failure mechanism during high velocity impact conditions is still the ultimate goal for many researchers. The objective is to develop a micromechanical constitutive model that can effectively simulate the high impact damage problem of the metal matrix composite materials. Therefore in this paper, a multiscale micromechanical constitutive model that couples the anisotropic damage mechanism with the viscoplastic deformation is presented here as a solution to this situation. This coupled viscoplastic damage model is formulated based on thermodynamic laws. Nonlinear continuum mechanics is used for this heterogeneous media that assesses a strong coupling between viscoplasticity and anisotropic damage. It includes the strong directional effect of the fiber on the evolution of the back stress and the development of the viscoplastic strain in the material behavior for high velocity impact damage related problems.     

Nonlinear viscoelastic and viscoplastic constitutive equations with growing damage

Nonequilibrium thermodynamics, rate-process theory, viscoelastic fracture mechanics and various experimentally-motivated simplifications are used to develop constitutive equations that account for effects of viscoelasticity, viscoplasticity, growing damage and aging. Their form is more general than previously developed by the author, and allows for relatively general tensorial effects of damage. Some important special cases are then covered, with emphasis on viscoelasticity. Evolution equations for the damage expressed in terms of internal state variables (ISVs) are discussed, comparing formulations using scalar ISVs and tensor ISVs. Finally, some experimental support for the theory is described. An Appendix illustrates the theory for an aging, linear viscoelastic material with growing cracks. This revised version was published online in July 2006 with corrections to the Cover Date.     

Non-linear coupled multi-field mechanics and finite element for active multi-stable thermal piezoelectric shells

In this paper, a coupled multi-field mechanics framework is presented for analyzing the non-linear response of shallow doubly curved adaptive laminated piezoelectric shells undergoing large displacements and rotations in thermal environments. The mechanics incorporate coupling between mechanical, electric and thermal fields and encompass geometric non-linearity effects due to large displacements and rotations. The governing equations are formulated explicitly in orthogonal curvilinear coordinates and are combined with the kinematic assumptions of a mixed-field shear-layerwise shell laminate theory. A finite element methodology and an eight-node coupled non-linear shell element are developed. The discrete coupled non-linear equations of motion are linearized and solved, using an extended cylindrical arc-length method together with a Newton–Raphson technique, to enable robust numerical predictions of non-linear active shells transitioning between multiple stable equilibrium paths. Validation and evaluation cases on laminated cylindrical strips and cylindrical panels demonstrate the accuracy of the method and its robust capability to predict non-linear response under thermal and piezoelectric actuator loads. Moreover, the results illustrate the capability of the method to model piezoelectric shells undergoing large shape changes by actively jumping between stable equilibrium states and quantify the strong relationship between shell curvature, applied electric potential, applied temperature differential and induced shape change. Copyright © 2008 John Wiley & Sons, Ltd.     

立方非线性包装系统在基础振动激励下的响应

对振动环境中立方非线性包装系统进行了计算机仿真及分析,通过对系统响应的时间历程、相图、Poincare映射及功率谱等的分析,讨论了在确定性振动环境中包装系统的动力学行为,并讨论了控制响应的措施.     

非线性振动定量分析技术的一些回顾与展望

本文对非线性振动定量分析的主要技术方法的发展现状与趋势作一次较系统的分析与讨论。     

Non-linear switching based on dual-core non-linear optical fiber couplers with XPM and Raman intrapulse applied to femtosecond pulse propagation

In this work, we investigated the optical switching process for three shapes of femtosecond pulses (soliton, Gaussian and super-Gaussian) propagating inside a symmetrical dual-core non-linear directional coupler by simulating their propagation via the coupled non-linear Schrödinger equations. In all simulations, we considered the dispersive effects of second and third order, besides the self-phase modulation and self-steepening non-linear effects. We studied three scenarios for each of the three pulse shapes under investigation. In the first scenario, we added only cross-phase modulation (XPM); in the second approach, we added only Raman scattering; in the third one, we combined both. The study was performed for distinct polarization modes and for different values of the Raman factor, with power range varying from 1 to 300 W. We noted that the XPM non-linear effect results in a decrease in the critical power threshold, whereas the Raman scattering causes an increase. For the first scenario (only XPM effect), the critical power threshold reduced from 113.72 to 104.69 W for the soliton pulse, from 111.49 to 100.77 W for the Gaussian and from 92.79 to 80.47 W for the Super-Gaussian pulse shape. For the second scenario (only Raman scattering), the critical power increased for a Raman factor varying from 1 to 10 fs, and the three pulse shapes reached thresholds above 150 W from a 5 fs factor, reaching more than 200 W for the super-Gaussian pulse as the Raman factor increased. For the third scenario (with both effects combined), we highlight that for a fixed XPM factor of 2, the critical power remained unchanged with the variation of the Raman factor. Hence, we observed that the Super-Gaussian pulse reached lower values for critical power when compared to the other pulse shapes.     

考虑扭转耦合的土-结构动力响应的一种简化分析法

提出了一个利用模态分解和傅里叶变换求解不对称结构扭转耦合动力响应的土-结构相互作用分析的实用简化方法，从而不仅使矩阵计算大大简化，计算结果具有较高准确性。数值参数分析表明当地基土为软土，尤其是地基土剪切波波速低于1000m／s时，基础水平位移和摇摆幅度分别随结构质量与基础质量之比和结构高度与回转半径之比的增加而显著增大。对于地基土不是硬土，而且具有一定的高度和刚度偏心距的结构，必须考虑扭转耦合的土一结构动力相互作用的影响。     

一类具指数函数系数的非线性复差分方程

众所周知,在复微分方程和复差分方程领域中,Malmquist型方程是比Painlev′e方程和Riccati方程形式更一般的非线性方程.在本文中,我们运用Nevanlinna理论的差分模拟结果和微分域理论对一类具指数函数系数的Malmquist型复差分方程进行了研究.当上述Malmquist型复差分方程的有限级超越亚纯解具有较少的零点和极点时,我们得到其增长性和指数函数ez的增长性一致.该结果是对复微分Malmquist定理和复差分Malmquist定理的推广和补充.     

金纳米线-介孔二氧化硅薄膜的制备和非线性光学性能研究

利用热气流法制备的宏观平行取向介孔二氧化硅薄膜为载体, 经氨基硅烷APTS对孔道进行表面修饰后, 在介孔孔道中原位生成了金纳米线。研究结果表明: 由于介孔孔道的限制效应, 在氯金酸的乙醇溶液中浸泡较长时间后, 利用氢气还原可获得长径比较大的金纳米线。Z扫描测试表明, 这种金纳米线-介孔二氧化硅复合薄膜材料具有明显的非线性光学特性。在532 nm的激光下, 它还表现出明显的各向异性, 有望在新型光学器件方面得到应用。     

The Influence of Superimposed Creep Loadings on the Thermal-Mechanical Fatigue Behaviour of the Ni-Base Superalloy IN 792 CC

The effect of superimposed creep loadings on the cyclic deformation andthe lifetime behaviour of the Ni-base superalloy IN 792 CC underout-of-phase TMF loadings is presented and discussed. The mean stressand the stress amplitude are not affected significantly by the creeploading. A slight cyclic hardening is observed even for cycles withsuperimposed creep loadings at a maximum temperature of 920°C whichmeans that the work hardening processes occurring under TMF loading arenot fully compensated by recovery during the creep phase. The plasticstrain amplitude increases with increasing creep stress. Using theManson–Coffin relationship, it is possible to calculate the influence ofthe superimposed creep loadings on the lifetime behaviour in goodaccordance with the experimental results.     

基于损伤累积的爆破振动传播规律试验研究

为进一步揭示基于损伤累积的爆破振动波传播与衰减规律,设计混凝土试块进行损伤累积及爆破振动测试试验。结果表明,测点损伤值随爆破次数增加持续增大,中远区测点损伤有初期缓增及后期突增两阶段;测点单次爆破振动速度随距离增大而减小,近区测点振动速度减小较中、远区测点快;测点爆破振动速度随爆破次数增加总体减小,且爆源近区减小较快,中远区呈小幅度缓慢减小;随爆破次数继续增大各测点振动速度存有突增过程,且衰减系数 增加缓慢, 值减小,两者均存在小幅度畸变过程;多次爆破时确定振动安全阈值及爆破振动波传播衰减规律( 、 值)时应充分考虑损伤累积效应。