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.     

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。     

Reverse Engineering and restyling of aesthetic products based on sketches interpretation

In the conceptual design stage, outcomes of industrial designers work are generally represented by set of sketches where curves, notes, shadows, and colors implicitly represent creative ideas. Signs and annotations are used to synthesize and concretize the design intent that, finally, will be transformed into the styling product visual appearance. The loss of the original design intent may be due to the complexity of the design process, and the involvement of different actors. Our aim is to provide a method and relative tools in order to interpret signs on sketches for eliciting the design intent. The analysis result is a set of aesthetic features that can be used for driving CAD modeling, in the case both of Reverse Engineering applications and of product modeling for restyling purposes. Sketches analysis is based on a semiotic interpretation driven by the formalization of the cognitive models used in the conceptual design phase. The approach showed promising results on different styling products test cases.     

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.     

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.     

利用DSP的实时图像识别系统的设计与应用

以TMS320C6205为处理器,设计一种实时图像识别系统。用SAA7111和FPGA完成无 DSP干预的图像采集与存储,通过DMA实现图像传输与DSP识别的并行执行。系统将触发、采集、存储、识别等功能集成在一块PCI卡上,提高了系统的可靠性和安全性。与基于采集卡和PC机的图像识别系统相比,该系统在保持相同的图像识别准确率的前提下,可以将识别速度提高约28.7%。     

数字时代常州文化旅游品牌形象塑造与创意实践研究

文章对常州文化旅游品牌发展面临的优势和不足进行归纳分析,以数字时代信息手段为支撑,探讨新时代常州文化旅游品牌形象的塑造方法,将本地传统文化旅游资源以现代审美的艺术视角进行视觉形象、品牌故事和理念、影响力的再造是数字时代大势所趋。通过创意实践研发新颖独特的文化旅游品牌、产品或服务,可以为地区经济和文化旅游产业的进一步发展带来新的增长契机,做出更大贡献。     

Measurement of Fracture Parameters for a Mixed-Mode Crack Driven by Stress Waves using Image Correlation Technique and High-Speed Digital Photography

Abstract:  Measurement of fracture parameters for a rapidly growing crack in syntactic foam sheets using image correlation technique and high-speed photography is presented. The performance of a rotating mirror-type multi-channel high-speed digital camera to measure transient deformations is assessed by conducting benchmark tests on image intensity variability, rigid translation and rigid rotation. Edge-cracked foam samples are subjected to eccentric impact loading relative to the initial crack plane to produce mixed-mode loading conditions in a three-point bend configuration. High-speed photography is used to record decorated random speckles in the vicinity of the crack tip at a rate of 200 000 frames per second. Two sets of images are recorded, the first set before impact and the second after impact. Using image correlation methodology, crack-tip displacement field histories and dominant strains from the time of impact up to complete fracture are mapped. Over-deterministic least-squares analyses of crack-tip radial and tangential displacements are used to obtain mixed-mode fracture parameters. The measurements are compared with complementary finite element results. The fracture parameters determined from radial displacements seem more robust even when fewer number of higher order terms in the crack-tip asymptotic expansion are used.     

Determination of Young’s modulus in polyester-Al2O3 and epoxy-Al2O3 nanocomposites using the Digital Image Correlation method

Young’s modulus of nano-composite systems composed of unsaturated polyester and epoxy resins with alumina nanoparticles of different sizes has been experimentally estimated. The nanoparticles used were spherical alpha-Al₂O₃ having 30-40 and 200 nm in diameter. Young’s modulus was estimated using an inverse problem that is solved by means of the classical Levenberg-Marquardt technique. A cantilever beam under bending was used in the experiments and the experimental procedure was performed using the Digital Image Correlation method, which is a well-established optical-numerical method for estimating full-field displacement. Experimental results indicate that Young’s modulus increases with increasing nanoparticle volume fraction. Finally, the estimated Young’s moduli were compared with classical theoretical models, showing that the experimental results are in agreement with literature data.