增龄对人牙本质失水收缩的重要性

牙本质内的矿物质随着年龄的增长呈现增长的趋势．由于矿物质与蛋白胶原比例的改变,可能导致牙本质脱水时氢键强度和收缩程度的变化．因此,该研究的目的是确定牙本质收缩随年龄增长的量变．将磨牙牙本质的冠部制成试件,青年组:23≤age≤34,老年组:52≤age≤62,贮存在水或Hanks平衡盐溶液（HBSS）中．利用数字图像相关法(DIC)观察牙本质试件在自由空气状态下的尺寸变化过程．结果表明青年人的牙本质不管在哪个方向或存储时间上,其收缩都明显大于老年人(p<0.01)．平行于牙本质小管方向上的应变从牙髓腔侧到釉牙本质界(DEJ)侧依次递增,而垂直于牙本质小管方向的应变则依次递减．即从牙髓腔侧到DEJ牙本质收缩的各向异性增强,该现象在青年人牙本质中尤其明显．     

Whole-field thickness strain measurement using multiple camera digital image correlation system

Three Dimensional digital image correlation(3D-DIC) has been widely used by industry, especially for strain measurement. The traditional 3D-DIC system can accurately obtain the whole-field 3D deformation. However, the conventional 3D-DIC system can only acquire the displacement field on a single surface, thus lacking information in the depth direction. Therefore, the strain in the thickness direction cannot be measured. In recent years, multiple camera DIC (multi-camera DIC) systems have become a new research topic, which provides much more measurement possibility compared to the conventional 3D-DIC system. In this paper, a multi-camera DIC system used to measure the whole-field thickness strain is introduced in detail. Four cameras are used in the system. two of them are placed at the front side of the object, and the other two cameras are placed at the back side. Each pair of cameras constitutes a sub stereo-vision system and measures the whole-field 3D deformation on one side of the object. A special calibration plate is used to calibrate the system, and the information from these two subsystems is linked by the calibration result. Whole-field thickness strain can be measured using the information obtained from both sides of the object. Additionally, the major and minor strain on the object surface are obtained simultaneously, and a whole-field quasi 3D strain history is acquired. The theory derivation for the system, experimental process, and application of determining the thinning strain limit based on the obtained whole-field thickness strain history are introduced in detail.     

基于三维数字图像相关方法的面部表情变形测量研究

目前关于人脸面部表情的相关研究正在逐渐应用到各领域,而其研究大多为基于数据库的定性分析。本文将三维数字图像相关方法用于人脸面部表情研究。首先,针对人脸面部图像的具体特点,在深入研究方法原理的基础上,提出以参数优化和消除刚体位移来提高实验测量精度。在此基础上,对普通表情和微表情状态下面部肌肉变形进行精确的计算和定量的研究,分析特定表情的形成原因。实验采用自传式回忆的方法唤起被测试者的基本情绪,随之进行模仿来诱发面部普通表情;采用指导性表达抑制的方法诱发被测试者的微表情。通过对特定表情状态下的肌肉运动进行全场和局部的计算,获得精确的三维位移场和位移矢量场。结果表明,面部微表情状态与普通表情状态的运动规律基本一致,只是位移幅度存在显著差异。实验测量结果与实际情况基本吻合,符合常规认知。不同的是本文将特定表情状态下面部肌肉的运动规律从以往的感性认知上升到精确计算和定量分析的水平,为面部表情的自动识别及形成机理的深入研究提供了良好的基础。     

Characterization of mechanical properties of aluminum cast alloy at elevated temperature

The tensile response, the low cycle fatigue(LCF) resistance, and the creep behavior of an aluminum(Al) cast alloy are studied at ambient and elevated temperatures.A non-contact real-time optical extensometer based on the digital image correlation(DIC)is developed to achieve strain measurements without damage to the specimen. The optical extensometer is validated and used to monitor dynamic strains during the mechanical experiments. Results show that Young's modulus of the cast alloy decreases with the increasing temperature, and the percentage elongation to fracture at 100℃ is the lowest over the temperature range evaluated from 25℃ to 300℃. In the LCF test, the fatigue strength coefficient decreases, whereas the fatigue strength exponent increases with the rising temperature. The fatigue ductility coefficient and exponent reach maximum values at 100℃. As expected, the resistance to creep decreases with the increasing temperature and changes from 200℃ to 300℃.     

基于数字散斑相关实验测量的材料构型力的计算方法

材料构型力学主要研究材料中的缺陷(夹杂、空穴、位错、裂纹、塑性区等)的构型(形状、尺寸和位置)改变时,所引起的系统自由能的变化。本研究将基于数字散斑相关技术,实验测量材料试件的位移场分布,随后通过材料构型力的定义式,计算求得弹塑性材料中缺陷构型力的分布。其方法概括如下:位移场通过数字图像相关技术测得;应变及位移梯度场利用三次样条拟合获得;线弹性材料应力通过简单线弹性本构方程获取,而塑性材料的表面应力场通过Ramberg-Osgood本构方程计算求得;弹塑性应变能密度分布则由应力-应变曲线数值积分获得。该方法对普通弹性材料或者弹塑性材料均适用,可以用于各种不同的缺陷及缺陷群的材料构型力测量。     

非连续数字图像相关方法在裂纹重构中的应用

数字图像相关方法作为一种新的非接触式位移测量方法,在力学工程中有广泛的应用前景,然而受限于标准方法对图像变形的连续性要求,这种高效的测量方法在断裂力学领域的推广受到了限制. 为解决这一问题,提出采用引入子区分离数学模型,代替标准方法的连续模型,来对非连续区域进行精确识别和匹配的非连续数字图像相关方法. 研究子区被裂纹等非连续分割后原始像素点的位移情况,并引入裂纹张开向量用以表征被分割子区的主区和副区的位移关系;从而建立子区分离模型的数学表达式,并且为所提出的模型设计相应的图像相关算法;然后将所提出的非连续数字图像相关方法应用于重构平板拉伸试验开裂过程中图像的位移. 研究结果表明,相比于标准的数字图像相关方法,所提出的非连续数字图像相关方法解决了图像相关法在非连续区域失效的问题,提高了数字图像相关方法对位移测量的正确率,特别是能够准确重构裂纹面及附近的位移场,其测量精度能够达到亚像素级别.      

On the Occurrence of Portevin–Le Châtelier Instabilities in Ultrafine-Grained 5083 Aluminum Alloys

The Portevin–Le Châtelier (PLC) instability is commonly observed in Al–Mg alloys and is manifested in serrated flow within the stress–strain response. We investigate the persistence of this instability with reduction in grain size by studying an ultrafine-grained (ufg) aluminum alloy (Al5083) and a conventional grain size Al5083. Micro-scale tensile tests combined with digital image correlation (DIC) reveal strength anisotropy and heterogeneity of the deformation in the three material directions (extrusion, rolled, and transverse). For the same applied displacement rate, the PLC effect in ufg-Al5083 is observed only over a small strain range immediately following the yield, while the coarse-grained Al5083 exhibits serrated flow over nearly the entire plastic strain range. These observations are explained using the stability analysis of Hähner (Acta Mater 45:3695–3707, 1997), and implications for nanocrystalline (nc) alloys are discussed.     

Evaluating the mechanical behavior of arterial tissue using digital image correlation

In this study, digital image correlation (DIC) was adopted to examine the mechanical behavior of arterial tissue from bovine aorta. Rectangular sections comprised of the intimal and medial layers were excised from the descending aorta and loaded in displacement control uniaxial tension up to 40 percent elongation. Specimens of silicon rubber sheet were also prepared and served as a benchmark material in the application of DIC for the evaluation of large strains; the elastomer was loaded to 50 percent elongation. The arterial specimens exhibited a non-linear hyperelastic stress-strain response and the stiffness increased with percent elongation. Using a bilinear model to describe the uniaxial behavior, the average minor and major elastic modulii were 192±8 KPa and 912±40 KPa, respectively. Poisson's ratio of the arterial sections increased with the magnitude of axial strain; the average Poisson's ratio was 0.17±0.02. Although the correlation coefficient obtained from image correlation decreased with the percent elongation, a correlation coefficient greater than 0.8 was achieved for the tissue experiments and exceeded that obtained in the evaluation of the elastomer. Based on results from this study, DIC may serve as a valuable method for the determination of mechanical properties of arteries and other soft tissues.     

Extraction of essential oils from Algerian myrtle leaves using instant controlled pressure drop technology

In the present work, the new extraction process of Détente Instantanée Contrôlée DIC (French, for instant controlled pressure drop) was studied, developed, quantitatively and qualitatively compared to the conventional hydrodistillation method for the extraction of essential oils from Algerian myrtle leaves. DIC was used as a thermomechanical treatment, DIC subjecting the product to a high-pressure saturated steam. The DIC cycle ends with an abrupt pressure drop towards vacuum, and this instantly leads to an autovaporization of myrtle volatile compounds. An immediate condensation in the vacuum tank produced a micro-emulsion of water and essential oils. Thus, an ultra-rapid cooling of residual leaves occurred, precluding any thermal degradation. An experimental protocol was designed with 3 independent variables: saturated steam pressure between 0.1 and 0.6 MPa, resulting in a temperature between 100 and 160 °C, a total thermal processing time between 19 and 221 s, and between 2 and 6 DIC cycles. The essential oils yield was defined as the main dependent variable. This direct extraction gave high yields and high quality essential oil, as revealed by composition and antioxidant activity (results not shown). After this treatment, the myrtle leaves were recovered and hydrodistilled in order to quantify the essential oil content in residual DIC-treated samples. Scanning electron microscope (SEM) showed some modification of the structure with a slight destruction of cell walls after DIC treatment.     

Residual stress evolution regularity in thermal barrier coatings under thermal shock loading

Residual stress evolution regularity in thermal barrier ceramic coatings (TBCs) under different cycles of thermal shock loading of 1100°C was investigated by the microscopic digital image correlation (DIC) and micro-Raman spectroscopy, respectively. The obtained results showed that, as the cycle number of the thermal shock loading increases, the evolution of the residual stress undergoes three distinct stages: a sharp increase, a gradual change, and a reduction. The extension stress near the TBC surface is fast transformed to compressive one through just one thermal cycle. After different thermal shock cycles with peak temperature of 1100°C, phase transformation in TBC does not happen, whereas the generation, development, evolution of the thermally grown oxide (TGO) layer and micro-cracks are the main reasons causing the evolution regularity of the residual stress.