岩石变形演化诱致灾变破坏过程的同步实验观测

将试样变形场演化特征与试样的宏观载荷位移曲线的演化特征结合起来研究是揭示非均匀脆性介质变形演化诱致灾变破坏的一个重要途径.本文发展了一套实验系统,通过对试样表面变形场的演化、宏观载荷和位移信号的同步观测,对单轴加载下岩石试样变形演化和灾变破坏的过程进行了实验研究.揭示了试样变形场由加载初期的随机涨落到灾变破坏前出现明显的变形局部化的演化特征现象,试样最终在变形局部化区内形成宏观破裂面.     

Damage evaluation and damage localization of rock

Knowledge of damage accumulation and corresponding failure evolution are prerequisite for effective maintenance of civil engineering so as to avoid disaster. Based on statistical mesoscopic damage mechanics, it was revealed that there are three stages in the process of deformation, damage and failure of multiscale heterogeneous elastic–brittle medium. These are uniformly distributed damage, localized damage and catastrophic failure. In order to identify the transitions from scattering damage to macroscopically localized one, a condition for damage localization was given. The experiments of rock under uniaxial compression with the aid of observations of acoustic emission and speckle correlation do support the concept of localization. This provides a potential approach to properly evaluate damage accumulation in practice. In addition, it is found in the experiments that catastrophic failure displays critical sensitivity. This gives a helpful clue to the prediction of catastrophic failure.     

含孔复合材料层合板静拉伸损伤演化的实验和表征方法

为了深入探究复合材料层合板结构的损伤机理和损伤演化,应用声发射技术和图像相关技术同步实时监测含孔碳纤维复合材料层合板试样在静拉伸过程中的损伤演化。实验结果表明,试样表面应变场呈现局部化特征。对应变集中带在加载方向的应变值进行了统计分析,获得了应变场的特征统计量(标准差)随加载的演化模型。层合板损伤时产生声发射信号的峰值频率大小能够有效区分复合材料的损伤模式,由此,建立了基于损伤模式累积声发射数的损伤演化模型。通过对应变场演化模型和声发射损伤演化模型的分析,可以将复合材料的损伤演化分为损伤初始阶段、损伤平稳扩展期、损伤严重阶段三个部分。统计分析结果表明:在损伤严重阶段,基于声发射事件数的各种损伤的损伤变量和局部应变场标准差快速增长,因此局部应变场统计标准差可以作为后期局部损伤严重程度的识别指标。     

Localization properties of strain-softening gradient plasticity models. Part I: Strain-gradient theories

This paper compares and evaluates strain-gradient extensions of the conventional plasticity theory. Attention is focused on the ability of individual formulations to act as localization limiters, i.e., to regularize the boundary value problem in the presence of softening and to prevent localization of plastic strain increments into a set of zero measure. To keep the presentation simple and to highlight the essential properties of the investigated models, only the static, rate-independent response in the small-strain range and in the one-dimensional setting is considered. These restrictions permit an analytical or semi-analytical treatment of the problem, while the basic characteristics of the solutions remain valid in the general, multi-dimensional case. The onset of localization is characterized as a bifurcation from a uniform state. The subsequent evolution of the localized process zone and of the shape of the strain profile is studied numerically. It is shown that certain pathologies, e.g., expansion of the plastic region accompanied by stress locking, may arise at later stages of localization. A similar analysis of models with gradients of internal variables is presented in a companion paper.     

Instabilities in power law gradient hardening materials

Tension and compression instabilities are investigated for specimens with dimensions in the micron range. A finite strain generalization of a higher order strain gradient plasticity theory is implemented in a finite element scheme capable of modeling power law hardening materials. Effects of gradient hardening are found to delay the onset of localization under plane strain tension, and significantly reduce strain gradients in the localized zone. For plane strain compression gradient hardening is found to increase the load-carrying capacity significantly.     

Numerical simulation of plastic-flow localization for simple shear

An algorithm was developed to numerically simulate plastic-flow localization for simple shear of a thermally plastic and viscoplastic material. The algorithm is based on solving the partial differential equations describing continuum flow. The closing equation is the constitutive relation known in the literature as the power law linking the plastic-strain rate to the flow stress, temperature, and accumulated plastic strain. Calculated relations for the time evolution of the shear-band width and the temperature and plastic strains localized in it agree satisfactorily with experimental relations. Good agreement with experimental results is also obtained for the sample temperature distribution at the developed stage of the localization process.Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 46, No. 1, pp. 173–180, January–February, 2005     

Strain Localization in Mild (Low Carbon) Steel Observed by Acoustic Emission - ESPI Coupling during Tensile Test

Two strain localization modes: the Piobert-Lüders band propagation and the development of necking, were investigated in uniaxial tensile tests for a low alloyed and low carbon steel. These two macroscopic localization phenomena were simultaneously monitored by speckle interferometry (ESPI) and acoustic emission (AE). The coupling of these two experimental techniques gives complementary information about strain localization features and mechanisms. For Lüders bands, it was found that the acoustic activity heard during the travel of the Piobert-Lüders band varies in closely correlated to the tensile force fluctuations, the relations between strain rate, band velocity, band width and plastic strain were investigated. Although the strain rate in the wake of the wave front is not always zero, the acoustic activity remains concentrated in the wave front itself. For necking, the acoustic activity is found to decrease regularly through the homogeneous plasticity stage and the diffuse necking stage and then increases when the localized necking starts, while ESPI patterns show a gradual strain concentration.     

巴西圆盘泥岩试件裂纹扩展及应变演化实验研究

为了研究准静态加载条件下岩石试件巴西劈裂裂纹扩展规律,采用MTS试验机进行准静态加载,同时用高速摄像机记录裂纹扩展过程。采用白光数字散斑处理软件对摄像机记录的照片进行处理,得到试件裂纹扩展过程中应变场的演化情况。通过实验和分析可以看出,由于端部效应及加载方式的原因,因此裂纹起裂点在底部加载部位;泥岩试件表面裂纹的平均扩展速度为252m/s;岩石的非均质性即内部微缺陷、微裂纹使得泥岩试样的开裂并不是沿着中心直径方向,而是偏离一定的角度,初始偏离角度约为17°。裂纹扩展过程可以划分为三个阶段:泥岩试件宏观变形阶段(宏观无裂纹)、宏观裂纹稳定扩展阶段、宏观裂纹动态张裂阶段。同时,在裂纹扩展过程中,表面第一主应变场、水平位移场等变化明显,在开裂部位第一主应变最大。通过对圆盘泥岩试件裂纹扩展实验研究,可为研究岩石破裂及其演化规律提供依据。     

3D Strain Field Evolution and Failure Mechanisms in Anisotropic Paperboard

Background

Experimental analyses of the 3D strain field evolution during loading allows for better understanding of deformation and failure mechanisms at the meso- and microscale in different materials. In order to understand the auxetic behaviour and delamination process in paperboard materials during tensile deformation, it is essential to study the out-of-plane component of the strain tensor that is, in contrast to previous 2D studies, only achievable in 3D.

Objective

The main objective of this study is to obtain a better understanding of the influence of different out-of-plane structures and in-plane material directions on the deformation and failure mechanisms at the meso- and microscale in paperboard samples.

Methods

X-ray tomography imaging during in-situ uniaxial tensile testing and Digital Volume Correlation analysis was performed to investigate the 3D strain field evolution and microscale mechanical behaviour in two different types of commercial paperboards and in two material directions. The evolution of sample properties such as the spatial variation in sample thickness, solid fraction and fibre orientation distribution were also obtained from the images. A comprehensive analysis of the full strain tensor in paperboards is lacking in previous research, and the influence of material directions and out-of-plane structures on 3D strain field patterns as well as the spatial and temporal quantification of the auxetic behaviour in paperboard are novel contributions.

Results

The results show that volumetric and deviatoric strain, dominated by the out-of-plane normal strain component of the strain tensor, localize in the out-of-plane centre already in the initial linear stress-strain regime. In-plane strain field patterns differ between samples loaded in the Machine Direction (MD) and Cross Direction (CD); in MD, strain localizes in a more well-defined zone close to the notches and the failure occurs abruptly at peak load, resulting in angular fracture paths extending through the stiffer surface planes of the samples. In CD, strain localizes in more horizontal and continuous bands between the notches and at peak load, fractures are not clearly visible at the surfaces of CD-tested samples that appear to fail internally through more well-distributed delamination.

Conclusions

In-plane strain localization preceded a local increase of sample thickness, i.e. the initiation of the delamination process, and at peak load, a dramatic increase in average sample thickening occurred. Different in-plane material directions affected the angles and continuity of the in-plane strain patterns as well as the sample and fracture properties at failure, while the out-of-plane structure affected how the strain fields distributed within the samples.

     

A Finite Element Analysis of a Tapered Flat Sheet Tensile Specimen

A uniaxial tension sheet metal coupon with a tapered instead of a straight gage section has been used for centering the location of diffuse neck and for measuring sheet stretchability in a non-uniform strain field. A finite element analysis of such a tensile coupon made of automotive steel sheet metals has been carried out to assess the effect of the tapered gage section geometry and material plastic strain hardening characteristics on the development of local plastic deformation pattern and local stress state, especially beyond the onset of diffuse necking but before localized necking. In particular, the finite element analysis was used in this study to evaluate the accuracy and reliability of an experimental data analysis method for estimating the post-necking effective plastic stress-strain curve based on the direct local surface axial plastic strain measurements for base metal, heat-affected zone, and weld metals of a dual-phase steel DP600. It is concluded that the estimated lower and upper bounds of the effective stress-strain curve at large strains are not satisfactory for low strain-hardening materials such as heat-affected zone and weld metals with the tapered tension coupons. A simple correction method utilizing only the additional local surface strain measurement in the transverse direction is proposed and it is shown to be effective in correcting the estimated effective stress-strain curve of dual-phase steel weld metals obtained for two tapered gage section geometries.     

Evolution of strain localization autowaves in a zirconium alloy and evaluation of plasticity margin in a rolling area

The stress-strain state of a zirconium alloy in a cold rolling area is studied by considering the evolution of strain localization autowaves and changes in the ultrasound velocity. It is found that in the region of transition from the upsetting zone to the reduction zone, there is a significant exhaustion of the plasticity reserve of the material, so that fracture is most likely in exactly this region. It is shown that the traditional methods of estimating the plasticity margin from mechanical characteristics cannot reveal this region; an integrated analysis of plastic deformation macrolocalization patterns and acoustic measurement results is required.     

Effects of spatial grain orientation distribution and initial surface topography on sheet metal necking

The finite element method is used to numerically simulate localized necking in AA6111-T4 under stretching. The measured EBSD data (grain orientations and their spatial distributions) are directly incorporated into the finite element model and the constitutive response at an integration point is described by the single crystal plasticity theory. We assume that localized necking is associated with surface instability, the onset of unstable growth in surface roughening. It is demonstrated that such a surface instability/necking is the natural outcome of the present approach, and the artificial initial imperfection necessitated by the macroscopic M–K approach [Marciniak and Kuczynski (1967). Int. J. Mech. Sci. 9, 609–620] is not relevant in the present analysis. The effects of spatial orientation distribution, material strain rate sensitivity, texture evolution, and initial surface topography on necking are discussed. It is found that localized necking depends strongly on both the initial texture and its spatial orientation distribution. It is also demonstrated that the initial surface topography has only a small influence on necking.     

Experimental and Computational Investigation of Low-Impact Velocity and Quasi-Static Failure of PMMA

An investigation of the low velocity impact and quasi-static failure of polymethylmethacrylate (PMMA) based on global and local post-impact strain measurements was conducted. Local strains were obtained from surface mounted Fiber Bragg Grating (FBG) sensors, and they were combined with global measurements from quasi-static indentation and low-velocity impact experiments, and finite-element analyses to obtain detailed maps of how failure spatially initiates and evolves. For both loading regimes, the interactions between the host PMMA specimens and the sensors played a crucial role in the evolution of residual strains. A mapping of the strains clearly shows that strains decrease radially, from high values near the point of impact to far-field values. Sensors located in critical locations, which are near the impact region, had the highest residual strains prior to PMMA fracture. Furthermore, it was determined that strain transfer to the sensor is strongly influenced by the bonding conditions at the specimen’s surface. Due to the debonding of the sensor and the frictional effects associated with the bonding agent, compressive residual strains occurred on the rear-surface. Hence, a detailed understanding of how strain evolves due to sensor-host interactions and catastrophic fracture can be obtained, which can potentially be used to mitigate damage in PMMA for a range of strain-rates.     

动力应变局部化传播及尺寸效应数值模拟

采用 FLAC-3 D模拟了应变软化岩土材料局部化剪切带的发展、扩容对剪切带倾角的影响及试件的宽度效应。随着加载时步的增加 ,彼此孤立的两个应变场逐渐靠近、叠加 ,最终形成了剪切带网络。无论端面约束强或弱 ,剪切带的倾角的数值模拟结果都与罗斯科倾角比较接近。增加试样宽度 ,剪切带的宽度增加 ,剪切带变得不平直 ;宽度越大 ,岩样上端面中点的压应力 -加载时步曲线的峰值强度越大 ,宽度对弹性阶段没有大的影响     

Shear band formation in thermo-viscoplastic solids under plane strain compression

The plane strain compression of a rectangular block is numerically investigated for the study of dynamic shear band development in thermo-elasto-viscoplastic materials from an internal inhomogeneity. As expected, it plays an important role in triggering the onset of shear, localization as well as thermal softening. And the competition between the strain, strain-rate hardening and thermal softening exists throughout the process. It is found that shear band develops at a 45-degree angle to the compression axis. In the light of given patterns of deformation and temperature, shear band evolution accelerated by thermal softening is retarded by the inertial effects. Interestingly, a similar temperature band is also formed along the trajectory of the localized deformation band. The calculations also show the energy evolution during the coupled thermo-mechanical process of shear band propagation. Finally, the mesh effect is discussed in terms of the numerical results from two different meshes. The project is supported by the National Natural Sciences Foundation of China.     

Properties of dynamic rupture and energy partition in a solid with a frictional interface

We study properties of dynamic ruptures and the partition of energy between radiation and dissipative mechanisms using two-dimensional in-plane calculations with the finite element method. The model consists of two identical isotropic elastic media separated by an interface governed by rate- and state-dependent friction. Rupture is initiated by gradually overstressing a localized nucleation zone. Different values of parameters controlling the velocity dependence of friction, the strength excess parameter and the length of the nucleation zone, lead to the following four rupture modes: supershear crack-like rupture, subshear crack-like rupture, subshear single pulse and supershear train of pulses. High initial shear stress and weak velocity dependence of friction favor crack-like ruptures, while the opposite conditions favor the pulse mode. The rupture mode can switch from a subshear single pulse to a supershear train of pulses when the width of the nucleation zone increases. The elastic strain energy released over the same propagation distance by the different rupture modes has the following order: supershear crack, subshear crack, supershear train of pulses and subshear single pulse. The same order applies also to the ratio of kinetic energy (radiation) to total change of elastic energy for the different rupture modes. Decreasing the dynamic coefficient of friction increases the fraction of stored energy that is converted to kinetic energy. General considerations and observations suggest that the subshear pulse and supershear crack are, respectively, the most and least common modes of earthquake ruptures.     

Influence of damage on properties of strain localization in geomaterials at plane stress and plane strain

Summary By regarding geomaterials under loading as a mixture of intact and damaged parts, we investigate the influence of damage on the properties of strain localization in elastoplastic geomaterials at plane stress and plane strain. Conditions for the onset of strain localization including the effects of damage are derived for the cases of plane strain and plane stress. Discussed are the inclination of the localized band and the hardening modulus corresponding to the onset of strain localization. It is shown that the properties of the strain localization are dependent on the damage and the capacity of bearing hydrostatic pressure by the damaged part, and that damage may induce an earlier onset of strain localization and lead to instability of a geomaterial.accepted for publication 11 March 2004