An experimental and numerical investigation of the behaviour of AA5083 aluminium alloy in presence of the Portevin–Le Chatelier effect

An experimental investigation of the Portevin–Le Chatelier (PLC) effect in the aluminium alloy AA5083-H116 is undertaken in this study through five different tests involving round, prismatic and flat notched specimen geometries. Measurements based on strain gages and digital image correlation (DIC) are used to capture and characterize the spatio-temporal features of the PLC behaviour. Inhomogeneous deformation with various localization bands caused by the PLC effect is observed in all tests, and the band characteristics are measured. The McCormick elastic–viscoplastic constitutive relation, developed for metals exhibiting this type of dynamic strain aging, is then described in detail, before the various parameters required by the model are determined based on available material tests. The model is finally used in full-scale 3D numerical simulations of the physical tests using the explicit solver of the non-linear finite element code LS-DYNA. It will be shown that the numerical results are able to reproduce most of the experimentally observed phenomena with reasonable accuracy. However, if the model is used to study the micromechanical mechanisms controlling the macromechanical behaviour of materials exhibiting PLC effects (such as the band morphology), more advanced constitutive relations may be required.     

Grain-scale processes governing shear band initiation and evolution in sands

A variety of experimental techniques have been used to advance understanding of strain localization phenomena in sands. However, all of these methods have fallen short in characterizing the evolution of the grain-scale processes that necessarily control shear band formation and growth in sands. This paper presents results of application of the non-destructive displacement measurement technique of digital image correlation (DIC) to measure two- and three-dimensional surface displacements on plane strain and axisymmetric sand specimens over short time steps. The abundance of local displacement data, high level of accuracy, and nearly continuous (spatially and temporally) record of displacement evolution afforded by the DIC technique has finally enabled a means to quantify local displacements to particulate-scale intensity. The data have been used to evaluate the local displacement mechanisms leading to the triggering of the formation of persistent shear bands, the timing of shear band formation with regard to the achievement of peak stress, and the character of displacements within fully formed shear bands. Insights are offered regarding the relation between strain localization and global stress-strain behavior, and the ensuing interpretations of shear banding as a hardening or softening phenomenon. Comparison of behavior between plane strain and triaxial tests offer additional perspective on the influences of three-dimensional stresses and boundary conditions on shear banding. The results further shed light on the micro-deformation mechanisms (i.e. buckling columns) responsible for the observed local strain non-uniformities that characterize “steady-state” shear band evolution.     

A simple method for measuring surface strains around cracks

A simple system has been developed to measure surface strains that occur during in situ deformation of mechanical test specimens. The system uses photolithographically deposited displacement markers and computer image recognition routines to determine in-plane displacements and strains from digital images. The strain calculating routines are integrated into a simple mouse-driven software package that facilitates the transformation from digital images to useful strain field information. Additional routines have been developed to determine crack tip stress fields and J integrals. Crack tip stress intensities have been calculated from strain maps obtained for traction-free cracks in stainless steel. The J integrals were found to be independent of contour and consistent with applied stress intensities. Crack tip stress intensities were calculated for bridged cracks in lamellar TiAl. The toughening effect of the bridging zones was determined by including the bridged region in the contours. Resistance curves generated from strain maps were consistent with those measured during mechanical testing.     

On the influence of stress triaxiality and strain rate on the behaviour of a structural steel. Part I. Experiments

Notched axisymmetric specimens of the structural steel Weldox 460 E have been tested at high strain rates in a Split Hopkinson Tension Bar. The aim was to study the combined effects of strain rate and stress triaxiality on the material's behaviour. It is further considered important to obtain experimental data that can be used in validation of constitutive relations and fracture criteria. The force and elongation of the specimens were measured by strain gauges during the high strain rate tests, while the fracture strain was calculated based on measurements of the fracture area in the microscope. Optical recordings of the notch deformation were obtained using a digital high-speed camera system. Using image processing of the digital images, it was possible to estimate the true strain against time at minimum cross-section. The strength of the material was found to increase with increasing strain rate, while for the ductility no significant effect of strain rate could be ascertained from the notched specimen tests. The ductility of the material was found to depend considerably on the stress triaxiality.     

Full-field characterization of mechanical behavior of polyurethane foams

The foam material of interest in this investigation is a rigid closed-cell polyurethane foam PMDI with a nominal density of 20 pcf (320 kg/m³). Three separate types of compression experiments were conducted on foam specimens. The heterogeneous deformation of foam specimens and strain concentration at the foam–steel interface were obtained using the 3-dimensional digital image correlation (3D-DIC) technique. These experiments demonstrated that the 3D-DIC technique is able to obtain accurate and full-field large deformation of foam specimens, including strain concentrations. The experiments also showed the effects of loading configurations on deformation and strain concentration in foam specimens. These DIC results provided experimental data to validate the previously developed viscoplastic foam model (VFM). In the first experiment, cubic foam specimens were compressed uniaxially up to 60%. The full-field surface displacement and strain distributions obtained using the 3D-DIC technique provided detailed information about the inhomogeneous deformation over the area of interest during compression. In the second experiment, compression tests were conducted for cubic foam specimens with a steel cylinder inclusion, which imitate the deformation of foam components in a package under crush conditions. The strain concentration at the interface between the steel cylinder and the foam specimen was studied in detail. In the third experiment, the foam specimens were loaded by a steel cylinder passing through the center of the specimens rather than from its end surface, which created a loading condition of the foam components similar to a package that has been dropped. To study the effects of confinement, the strain concentration and displacement distribution over the defined sections were compared for cases with and without a confinement fixture.     

数字图像相关法测量局域变形场中形函数和模板尺寸的影响

数字图像相关法测量均匀变形场已被普遍接受, 其测量结果可与应变片测量结果比较. 然而, 在工程测量中, 针对局域变形场(应变高度集中, 如波特文-勒夏特利埃带、试件缺口附近和裂纹尖端等), 应变片受限于其尺寸, 其测量结果是接触面内的平均应变值. 此时, 采用数字图像相关法能够测量这些局域变形场. 但形函数和模板尺寸等计算参数对计算结果影响很大, 这也导致使用者很难判断计算结果的可靠性. 论文通过对合金拉伸实验获得的不同应变梯度的波特文-勒夏特利埃带和模拟生成的带的计算分析, 发掘了形函数和模板尺寸作用于计算结果的深层机制, 证明了二阶形函数比一阶形函数更适用于高度非均匀的局域变形场. 提出了在局域应变场测量中, 当一阶和二阶形函数计算结果的相对误差小于10% 时, 二阶形函数的结果是可靠的判据.     

UHPFRC圆盘动态劈裂试验及基于μXCT图像的破坏机理研究

采用分离式霍普金森压杆对钢纤维体积分数为0～3%的超高性能纤维增强混凝土（ultra high performance fibre reinforced concrete, UHPFRC）圆盘试件进行应变率为1.72～7.42 s-1的动态劈裂试验，使用高速摄像机结合数字图像相关（digital image correlation, DIC）法获得试件表面裂缝扩展全过程图像和应变演化过程，并对冲击前后试件进行微观X射线计算断层扫描（micro X-ray computed tomography,μXCT），获得分辨率为56.7μm的三维内部图像，并进行统计和破坏机理分析。结果表明：(1)相比无纤维试件，掺入1%～3%的钢纤维，静、动劈裂强度分别提高84%～131%和47%～87%，动劈裂强度增强因子（即动静强度比值）为1.07～1.72;(2) DIC应变图像分析表明，无纤维试件裂缝集中、破坏快、能耗低；含纤维试件裂缝弥散程度大、能耗高、延性好，且随着纤维含量的提高而提升；(3)μXCT图像分析表明，试件中钢纤维体积分数为1.04%～2.47%，与设计基本一致，孔洞体积分数为0.98%...     

Modelling and experimental characterisation of the Lüders strain in complex loaded ferritic steel compact tension specimens

The use of 3D digital image correlation (DIC) has been used to capture the Lüders strains in a low carbon ferritic steel. Results were used to calibrate and compare with finite element (FE) results based on a constitutive plasticity model, capable of yield drop behaviour and therefore Lüders strains, by Zhang et al. (2001). Tensile tests were carried out at several strain rates to characterise the material behaviour. The results of these tests were used to fit parameters in the constitutive plasticity model. The FE model was then tested on a complex loading situation of in-plane compression of a compact tension (CT) specimen. The FE model predicts the shape and formation of the Lüders bands well. This FE model, using Zhang’s constitutive plasticity model, was used to predict the residual stress profile to compare with standard elastic–plastic isotropic hardening models with no yield point. The yield point reduced both the predicted peak tensile stress, at the notch root, and the amount of plastic strain. In regions where the plastic strain was of a similar size to the Lüders strain the stress profiles were perturbed from flat profiles predicted by the standard elastic–plastic hardening models.     

基于数字图像的分离式霍普金森压杆实验中试件应变及两端应力的同步测量法

本文提出一种基于高速摄像和数字图像相关方法(DIC)的分离式Hopkinson压杆(SHPB)测量技术,从而实现试件应变和两端应力的同步测量。即在与试件接触的输入输出杆两端制作散斑,通过高速摄像获取SHPB实验过程中的散斑变形图像,由DIC测得各时刻试件的应变、输入输出杆端的应变(可直接换算为试件两端的应力)。由于试件和杆端的应变都是从同一张高速摄影的图像上分析得到的,因此它们是同步的。应用该方法对钢纤维混凝土试件的SHPB试验进行了测量,测量结果与传统应变片测量结果吻合,验证了该方法的可行性。该技术不仅实现了SHPB实验中试件应变和应力的同步测量,还将有助于直接检验各材料在SHPB实验中试件两端的力在实验过程中是否平衡。     

基于图像分析法的水稻茎秆力学性能测量与分析

水稻茎秆力学性能的准确描述是水稻抗倒伏力学分析的关键。由于茎秆是由生物活性材料组成的形状不规则柔性结构,传统的测试方法难以对其力学性能进行准确测量。以南方水稻为研究对象,采用拉伸试验机对水稻茎秆进行了拉伸试验。首先采用传统的应变片方法测量应变,继而探索采用图像分析法实现非接触、高精度地测量水稻茎秆的截面尺寸、拉伸变形。用数字摄像机记录不同荷载水平下试件表面的数字图像,采用数字图像相关分析法(DIC)进行分析,通过两种方法计算茎秆的轴向应变;将完成拉伸实验的试件切片,采集其横截面的数字图像,通过数字图像分析技术得到其几何特性,并对大量实验数据进行了统计分析,实现了对水稻茎秆弹性模量、抗弯刚度等的合理描述。实验结果表明,采用传统的应变片测量求得的水稻茎秆的弹性模量的离散性极大,超出样本差异性的范围,而采用数字图像分析方法得到的弹性模量、抗弯刚度的结果有一定的离散性,但分布比较合理,比较准确地反映了水稻茎秆相关的力学性能。