基于数字图像相关法的应变场测量系统校准技术研究综述

介绍了基于数字图像相关法（Digital Image Correlation, DIC）的应变场测量系统的原理、应用场合及发展现状,阐述了对该系统进行校准的必要性。介绍了四点弯曲校准法、单轴拉伸校准法、垂直加载校准法、散斑变换校准法等具有代表性的DIC应变场测量系统校准方法的原理,从校准维度、校准范围、装置成本、装置复杂性、计量溯源性、测量不确定度等方面对比分析了各校准方法的优势与不足,指出可通过改进传感器性能、优化测量光路、提升散斑质量等手段提升校准准确性。展望了基于DIC的应变场测量系统校准技术的发展方向,指出未来应针对大变形场的可靠产生与复现、散斑图样的三维立体化、多种校准数据的有机融合等开展研究,以进一步拓展应变场校准范围并降低测量不确定度。     

3D Strain Field Measurement by Correlation of Volume Images Using Scattered Light: Recording of Images and Choice of Marks

Abstract:  We have extended the Digital Image Correlation technique to the case in three dimensions. This new technique, allowing the full three-dimensional (3D) strain measurement in the bulk of a solid, needs volume images containing a 3D variation of the grey levels. Generally, volume images are obtained by X-ray computed tomography. In this paper, we present a procedure that is easier to implement and enables to generate volume image in transparent materials. The principle consists in the optical slicing of the specimen. To obtain a random distribution of grey levels within the volume image, we use the scattered light phenomenon induced by particles included in the specimen. The recording of 3D images by optical slicing is presented and the influence of different kinds of particles on the scattered light and on the accuracy of measurement is described. Through several tests involving rigid body displacements and a tensile test we show the performance of this technique and we evaluate the measurement error of displacement and strain components.     

Delamination tests on CFRP-reinforced masonry pillars: Optical monitoring and mechanical modeling

In this methodological study, delamination phenomena of Carbon Fiber Reinforced Polymer (CFRP) strips from masonry pillars were investigated on the basis of single-lap shear tests. The masonry pillar considered in this case was constituted of three historical bricks, derived from a XVII century rural building. The bricks were bonded by a high strength mortar recently proposed for the restoration process. To follow the specimen response up to collapse, the test was controlled by a clip gauge located at the anchorage of the reinforcement strip. The conventional experimental information, concerning the overall reaction force and relative displacements provided by point sensors (LVDTs and clip gauge), were herein enriched with no-contact, kinematic full-field measurements provided by 2D Digital Image Correlation (2D DIC). Special care was devoted to improve the precision of the optical measurement, which included correction of the effects owing to the lens distortion and to lack of coplanarity between the camera sensor and the monitored flat surface. The overall accuracy was assessed through laboratory benchmarking tests. Moreover, an advanced three-dimensional mechanical model based on nonlinear finite elements was developed under the simplifying assumption of perfect adhesion. This model was adopted to describe bulk damage inside the heterogeneous specimen and to correlate it to the macroscopic response and surface measurements. Coherence between the above mentioned sources of information and the predictions provided by the advanced finite element model were critically discussed.     

An experimental method for measurement of strain distribution between wood the flour particles and polymer matrix on micro-mechanical level

Wood plastic composites (WPCs) are comprised of wood flour and thermoplastic polymer. The matrix is typically high-density polyethylene, poly (vinyl chloride), or polypropylene. The effect of morphology and micromechanics of wood flour particles on the mechanical performance of the bulk composite is a relatively unexplored area. The knowledge about the in situ properties of wood particles and the interfacial properties between the wood particles and the polymer matrix in the bio-composites is very limited. The objective of this work is to characterize the full-field deformation and strain distribution in and around wood particles embedded in polymer matrix. The mechanical tests are performed in small-scale tensile loading stage on thin composite samples containing 1-3 wood particles orientated at various angles. The deformations and strains is measured using optical measurement system based on Digital Image Correlation (DIC) principle.     

Mechanical behavior of the alumina-filled silicone rubber under pure shear at finite strain

The mechanical behavior of filler-reinforced rubber was investigated under pure shear at large deformation. Nanocomposites were composed of silicone rubber with different volume fractions of aluminum oxide nanoparticles, ranging from 0% to 5.0%. Thin sheet specimens were manufactured and submitted to monotonic tensile loading. The Digital Image Correlation method was employed to obtain displacement fields and consequently the stress–stretch responses were achieved. The experimentally measured stress–stretch curves were fitted to Yeoh and Lopez-Pamies models in order to estimate material parameters of the neat silicone rubber. Using this information, Mullins–Tobin and Bergstrom–Boyce approaches were employed to evaluate the strain amplification factor of the nanocomposites with different volume fractions of nanoparticles. The results indicate that a significant increase in the stiffness of filled rubber is obtained by small concentrations of nanoparticles. The effect of nanoparticles on mechanical behavior of reinforced rubber can be described using Yeoh and Lopez-Pamies models with Mullins–Tobin and Bergstrom–Boyce approaches. However, the strain amplification factor expression proposed by Bergstrom–Boyce was not suitable to describe the obtained results. In the case, the Guth model was more indicated.     

Full-field measurement technique and its application to the analysis of materials behaviour under plane strain mode

The purpose of the paper is to provide a comprehensive experimental and numerical analysis of one of the encountered and critical state modes in sheet metal forming processes. The study is carried out with the help of the full-field measurement techniques. In order to confer some generality to the proposed work, several materials and different specimen shapes are considered that exhibit more or less homogeneous strain field. The proposed experimental study of the plane strain test is completed by a preliminary identification of the material parameters for non-linear behaviour at finite strains, using heterogeneous strain field.     

Dynamic Deformation of Clamped Circular Plates Subjected to Confined Blast Loading

In this paper, the dynamic deformation of thin metal circular plates subjected to confined blast loading was studied using high‐speed three‐dimensional Digital Image Correlation (3D DIC). A small‐scale confined cylinder vessel was designed for applying blast loading, in which an explosive charge was ignited to generate blast loading acting on a thin metal circular plate clamped on the end of the vessel by a cover flange. The images of the metal plates during the dynamic response were recorded by two high‐speed cameras. The 3D transient displacement fields, velocity fields, strain fields and residual deformation profiles were calculated by using 3D DIC. Some feature deformation parameters including maximum out‐of‐plane displacement, final deflection, maximum principal strain and residual principal strain were extracted, and the result was in good agreement with that simulated by AUTODYN. A dimensionless displacement was introduced to analyse the effects of plate thickness, material types and charge mass on the deflection of metal plates. DIC is also proven to be a powerful technique to measure dynamic deformation under blast loading.     

Image‐based Continuous Displacement Measurements Using an Improved Spectral Approach

Digital Image Correlation algorithms capable of determining continuous displacement fields are receiving growing attention in the field of mechanical properties identification. In this paper, we develop an Improved Spectral Approach (ISA) to reconstruct continuous displacements based on their Fourier decomposition. This approach leads to a time and memory‐efficient algorithm, thanks to the fast Fourier transform. Moreover, the Fourier‐based decomposition enables accurate heterogeneous measurements. Improvements consist in increasing the accuracy and convergence rate as well as dealing with non‐periodic displacements and images. Furthermore, a theoretical framework is presented to quantify the noise sensitivity of the ISA from which useful information is retrieved. The approach is evaluated using synthetic images deformed by heterogeneous displacement fields. Comparisons show that the introduced modifications lead to lower uncertainties by one order of magnitude in the case of non‐periodic images and displacement field studied. Moreover, first‐order (SO1) and second‐order (SO2) subset‐based Digital Image Correlation algorithms are compared with the ISA. The comparisons herein reveal that the uncertainties of the ISA are 6–9 times smaller than those of the SO1 due to insufficiency of the first‐order shape function for the estimation of heterogeneous displacements, while being slightly smaller than those of the SO2. Moreover, as the image smoothness decreases, the uncertainties of the SO2 deviate from those of the ISA and the exact displacements. The presented approach shows great potentials for challenging applications such as strain measurements at microstructural levels.     

Experimental study of heterogeneities in strain and temperature fields at the microstructural level of polycrystalline metals through fully-coupled full-field measurements by Digital Image Correlation and Infrared Thermography

In this paper, we investigate and quantify the thermal effects induced by plastic deformation at the level of the microstructure of a polycrystalline metallic sample. For the first time, this investigation is conducted on a specimen containing hundred of grains. We use a unique experimental setup to access—simultaneously in-situ and in real time—strain and temperature fields of an austenitic stainless steel under tensile loading. We show that strain fields are directly linked to the expression of plasticity at the grain scale. We show, on the other hand, that thermal fields at the last increment of deformation are linked to the microstructural expression of plasticity on a larger lengthscale corresponding, instead, to grain clusters. Hence strain fields exhibit stronger localization features than the temperature fields in terms of both values and space. For a mean temperature rise of 0.75 °C and a global deformation of 2.4% in the fastest quasi-static regime investigated in this paper, the maximum local temperature rise is measured to be 0.88 °C even though local strain in grains can reach up to 6.7%. These fully-coupled measurements also provide the first experimental evidence that an instantaneous coupling takes place within grains between strain gradients and thermal dissipation. Finally, an estimation of a grain-scale field of the fraction of plastic work converted into heat is conducted and shown to be not only heterogeneous but also to be related to the microstructural features of deformation at the surface of the material, namely to the absence or presence of slip bands. The results obtained support the relevance of establishing energy balances and acquiring stored energy data at the microstructural scale where damage localization takes place.     

数字相关方法在S195型柴油机气缸内径测量中的应用

提出一种新的非接触式、在线、高精度的测量方法,用来测量柴油机气缸内径。对数字相关方法在测量应用中所涉及到的测量原理、相关函数等进行了分析研究,得到这种测试系统在测量气缸内径的测量准确度理论值一般可达±1μm,在实用中能满足检测S195型柴油机气缸内径在线测量±4μm的精度要求。     

Driving simulators for robust comparisons: A case study evaluating road safety engineering treatments

Road authorities considering the implementation of speed management interventions should have access to the results of scientifically robust evaluations on which to base their decisions. However, studies that evaluate a diverse range of interventions with comparable metrics are rare, with most focussing on one type, for example, types of signage, perceptual countermeasures or physical traffic calming. This paper describes a driving simulator study designed to overcome these constraints. Twenty diverse speed-reducing treatments were developed and tested in urban and rural road environments. Forty participants encountered all the treatments allowing a comparison to be made with their driving behaviour when the treatment was not present. A number of speed parameters were developed to encapsulate the range of effects of the treatments. The results suggest that whilst straight sections of road are difficult to treat, speed reductions can be obtained by increasing risk perception. In contrast, alerting treatments had more effect at junctions, particularly in an urban environment; drivers approaching curves demonstrated improved speed adaptation if the curve radius was highlighted (either implicitly or explicitly). The study highlights how driving simulators can be used to overcome methodological constraints encountered in real-world evaluations of this type.     

Optimal design and testing of laminated light-weight composite structures with local reinforcements considering strength constraints: Part II: Testing

The present paper explains details of the measurement methods applied in the testing of strength-optimized locally reinforced laminates whose design has been explained in Part I. The measurement methods are an optical surface method, based on Digital Image Correlation (DIC), and measurements of Acoustic Emission (AE) which can originate from anywhere within the test specimen. Both methods are intended to identify early damage events or accumulation and the corresponding loads are compared to predicted first-ply-failure loads. The DIC method identifies non-linearities in the displacement path which infer that damage events have occurred. The AE method utilizes the signal energy rate per time to identify damage events. The results of the particular specimens show no correlation between the two measurement methods. The averaged values for the different type specimens show a significant dependency.     

Surface stress and strain fields on compressed panels of corrugated board boxes. An experimental analysis by using Digital Image Stereocorrelation

The complex behaviour of corrugated board packages under compression loading is investigated in this work. Original experimental data are obtained by using a Digital Image Stereocorrelation technique for measuring the displacement and strain fields of the panels’ outer liner of the tested boxes. The stress field is also estimated by accounting for the anisotropic mechanical behaviour of the outer liner, its residual stress state induced by the processing of the corrugated board and the effects of box manufacturing operations and compression. Results show that these fields are extremely heterogeneous on the panels’ surface. Most stressed areas are located along the panels’ edges. The elastic limit of the outer liner is reached quite soon during compression. Box geometry and panel flaps are of primary importance on the observed phenomena. This approach delivers useful information to improve kinematic and constitutive assumptions for buckling and post-buckling models of boxes or thin-walled sandwich structures.     

Experimental and numerical investigation of thickness effects in plasticity-induced fatigue crack closure

The understanding of fatigue crack closure has been proved to be a challenging and controversial topic among the fatigue community over the last three decades. The effect of the specimen (or component) thickness has been shown to have a significant effect on closure behaviour and this seems to be related to the relative size of the plastic zone. Real cracks are inherently three-dimensional; plane stress-like behaviour is found close to the region where the crack front intersects the free surface, whereas most of the crack front will experience something close to plane strain. The aim of the present work is to investigate the influence of specimen thickness on closure behaviour (both close to and remote from the surface) and on fatigue crack propagation. The paper will present results from a simple experimental program, which consists of fatigue testing CT specimens with different thicknesses. Fatigue crack propagation is measured optically. Crack closure is assessed using traditional compliance techniques (clip gauge and back face strain gauge) and Digital Image Correlation methods. Experimental results are compared with two and three-dimensional simulations of plasticity-induced fatigue crack closure. The implications of thickness effects for predicting the propagation of three-dimensional fatigue cracks are discussed.     

Numerical–experimental assessment of roughness-induced metal–polymer interface failure

A numerical–experimental method is presented to study the initiation and growth of interface damage in polymer–steel interfaces subjected to deformation-induced steel surface roughening. The experimentally determined displacement field of an evolving steel surface is applied to a numerical model consisting of a polymer coating and interface layer. The measured displacement field is obtained with a Finite Element based Digital Image Correlation method.The resulting simulations provide novel insights into the mechanical behaviour of the polymer–steel interface during interface roughening. The appearance of local hills and valleys on the evolving steel surface results in local bands of intensified stress in the polymer layer. These localized deformation bands trigger interface damage, which grows as the surface deformation increases. Polymer ageing initially delays the initiation of interface damage. However, for increased polymer ages the average interface damage increases. Likewise, the critical strain, at which the interface integrity is locally compromised, decreases.The presented method allows for a detailed study of the interface integrity during deformation-induced steel surface roughening. With properly identified material parameters, it becomes possible to tailor the polymer–steel material properties to minimize interface damage during production and storage of cans or canisters, e.g. for food and beverages.     

Assessment of Digital Image Correlation Measurement Accuracy in the Ultimate Error Regime: Main Results of a Collaborative Benchmark

We report on the main results of a collaborative work devoted to the study of the uncertainties associated with Digital image correlation techniques (DIC). More specifically, the dependence of displacement measurement uncertainties with both image characteristics and DIC parameters is emphasised. A previous work [Bornert et al. (2009) Assessment of digital image correlation measurement errors: methodology and results. Exp. Mech. 49, 353–370] dedicated to situations with spatially fluctuating displacement fields demonstrated the existence of an ‘ultimate error’ regime, insensitive to the mismatch between the shape function and the real displacement field. The present work is focused on this ultimate error. To ensure that there is no mismatch error, synthetic images of in‐plane rigid body translation have been analysed. Several DIC softwares developed by or in use in the French community have been used to explore the effects of a large number of settings. The discrepancies between DIC evaluated displacements and prescribed ones have been statistically analysed in terms of random errors and systematic bias, in correlation with the fractional part τ of the displacement component expressed in pixels. Main results are as follows: (i) bias amplitude is almost always insensitive to subset size, (ii) standard deviation of random error increases with noise level and decreases with subset size and (iii) DIC formulations can be split up into two main families regarding bias sensitivity to noise. For the first one, bias amplitude increases with noise while it remains nearly constant for the second one. In addition, for the first family, a strong dependence of random error with τ is observed for noisy images.     

冻融条件下玄武岩纤维混凝土断裂韧度研究

该文使用数字图像相关方法实时观测三点弯试验中切口混凝土梁全场变形,分析混凝土梁断裂破坏过程中水平位移和应变的变化规律,基于切口处水平位移和应变变化规律确定起裂荷载,并研究冻融循环次数和纤维掺量对混凝土起裂韧度、失稳韧度、临界开口位移的影响,结果表明：对于C30混凝土而言,起裂韧度和失稳韧度随冻融次数的增加而减小,降幅约为0.6 MPa·m^1/2~0.80 MPa·m^1/2,两种韧度随玄武岩纤维掺量增加有小幅增加,最大增幅分别约为0.1 MPa·m^1/2和0.2 MPa·m^1/2,说明玄武岩纤维能提高C30混凝土的抗冻性,但提高程度是一定的,玄武岩纤维不能完全抑制冻融对混凝土的损伤;纤维掺量对失稳韧度的提高幅度要比起裂韧度大;起裂韧度增益比和失稳韧度增益比都随纤维掺量增加先增大后减小,两种韧度增益比均在纤维掺量为2.0 kg/m³时最大,因此,在混凝土强度等级为C30时,2.0 kg/m³可作为最佳纤维掺量;临界开口位移随冻融次数增加而增大,随纤维掺量变化效果不明显,与纤维增强作用相比冻融损伤是影响断裂过程变形性能的主要因素;最后在试验结果的基础上,建立了起裂韧度和失稳韧度随纤维掺量和冻融次数的拟合模型。     

Thermo-mechanical fatigue behaviour of welded tubular parts made of ferritic stainless steel

Car exhaust manifolds are critical components subjected to cyclic thermo-mechanical fatigue (TMF) during function. To reduce design costs, robust numerical design tools are required to assess their behaviour and lifetime. Manifolds are constructed by welding several ferritic stainless steel tubular parts together. TMF behaviour of a 1.4509 steel in welded and unwelded conditions is assessed under various loading conditions. Unified elasto-viscoplastic constitutive laws are developed. The specific thermo-mechanical behaviour of the heat-affected zone (HAZ) is also taken into account for welded steel. The reliability of the proposed models in predicting the mechanical response, in particular in the welded zone, is investigated. The local strains of the welded area are measured using a digital image correlation technique. Hence, several numerical models are implemented in ABAQUS and different areas are analysed to reproduce the mechanical behaviour of the heat-affected zone. Results are discussed and compared with experiments to validate the proposed model of the mechanical response of a welded component.