Characterization for elastic constants of fused deposition modelling-fabricated materials based on the virtual fields method and digital image correlation

Fused deposition modelling(FDM), a widely used rapid prototyping process, is a promising technique in manufacturing engineering. In this work, a method for characterizing elastic constants of FDM-fabricated materials is proposed. First of all, according to the manufacturing process of FDM, orthotropic constitutive model is used to describe the mechanical behavior. Then the virtual fields method(VFM) is applied to characterize all the mechanical parameters(Q, Q, Q, Q) using the full-field strain,which is measured by digital image correlation(DIC). Since the principal axis of the FDM-fabricated structure is sometimes unknown due to the complexity of the manufacturing process, a disk in diametrical compression is used as the load configuration so that the loading angle can be changed conveniently. To verify the feasibility of the proposed method, finite element method(FEM) simulation is conducted to obtain the strain field of the disk. The simulation results show that higher accuracy can be achieved when the loading angle is close to 30?. Finally, a disk fabricated by FDM was used for the experiment. By rotating the disk, several tests with different loading angles were conducted. To determine the position of the principal axis in each test, two groups of parameters(Q, Q, Q, Q) are calculated by two different groups of virtual fields. Then the corresponding loading angle can be determined by minimizing the deviation between two groups of the parameters. After that, the four constants(Q, Q, Q, Q) were determined from the test with an angle of 27?.

Tensile strength of sea ice using splitting tests based on the digital image correlation method

The splitting test is a competitive alternative method to study the tensile strength of sea ice owing to its suitability for sampling.However,the approach was questioned to the neglect of local plastic deformation during the tests.In this study,splitting tests were performed on sea ice,with 32 samples subjected to the regular procedure and 8 samples subjected to the digital image correlation method.The salinity,density,and temperature were measured to determine the total porosity.With the advantage of the digital image correlation method,the full-field deformation of the ice samples could be determined.In the loading direction,the samples mainly deformed at the ice-platen contact area.In the direction vertical to the loading,deformation appears along the central line where the splitting crack occurs.Based on the distribution of the sample deformation,a modified solution was derived to calculate the tensile strength with the maximum load.Based on the modified solution,the tensile strength was further calculated together with the splitting test results.The results show that the tensile strength has a negative correlation with the total porosity,which agrees with previous studies based on uniaxial tension tests.

Precise 3D shape measurement of three-dimensional digital image correlation for complex surfaces

Three dimensional-digital image correlation （3D-DIC） is a widely used optical metrology in the experimental mechanics community because of its reliability, practicality, and flexibility. Although the precision of digital image correlation （DIC） has been thoroughly studied theoretically and numerically, verification experiments have seldom been performed, especially fbr complex surfaces with a small field of view （FOV）. In this work, the shape of a 1-yuan coin was measured using 3D-DIC; the shape was complex due to the presence of many fine details, and the FOV was relatively small because the coin diameter was only 25 mm. During the experiment, a novel strategy for speckle production was developed： white paint was simply sprayed onto the surface. Black paint was not used; instead, taking advantage of the reflective nature of the coin surface, polarized light and a Polaroid filter were introduced, and the polarization direction was carefully adjusted, ensuring that the spray pattern was extremely thin and that high-quality speckle images with significant contrast were captured. The three-dimensional coin shape was also successfully determined for comparison using a stylus profiler. The results demonstrate that 3D-DIC provides high precision in shape measurement even for complex surfaces with small FOV. The precision of 3D-DIC can reach 1/7000 of the field of view, corresponding to about 6 ~tm in this experiment.

Deformation field and crack analyses of concrete using digital image correlation method

The study on the deformation distribution and crack propagation of concrete under axial compression was conducted by the digital image correlation (DIC) method. The main parameter in this test is the water-cement (fT/C) ratio. The novel analysis process and numerical program for DIC method were established. The displacements and strains of coarse aggregate, and cement mortar and interface transition zone (ITZ) were obtained and verified by experimental results. It was found that the axial displacement distributed non-uniformly during the loading stage, and the axial displacements of ITZs and cement mortar were larger than that of coarse aggregates before the occurrence of macrocracks. The effect of W/C on the horizontal displacement was not obvious. Test results also showed that the transverse and shear deformation concentration areas (DCAs) were formed when stress reached 30%-40% of the peak stress. The transverse and shear DCAs crossed the cement mortar, and ITZs and coarse aggregates. However, the axial DCA mainly surrounded the coarse aggregate. Generally, the higher W/C was, the more size and number of DCAs were. The crack propagations of specimens varied with the variation of W/C. The micro-crack of concrete mainly initiated in the ITZs, irrespective of the W/C. The number and distribution range of cracks in concrete with high W/C were larger than those of cracks in specimen adopting low W/C. However, the value and width of eraeks in high W/C specimen were relatively small. The W/C had an obvious effect on the characteristics of concrete deterioration. Finally, the characteristics of crack was also evaluated by comparing the calculated results.

The improved genetic algorithms for digital image correlation method

We present a global optimization method, called the genetic algorithms (GAs), for digital image/speckle correlation (DISC). The new algorithms do not involve reasonable initial guess of displacement and deformation gradient and the calculation of second-order spatial derivatives of the digital images, which are important challenges in practical implementation of DISC. The performance of a GA depends largely on the selection of the genetic operators. We test various operators and propose optimal operators. The algorithms are then verified using simulated images and experimental speckle images.

Residual stress determination in friction stir butt welded joints using a digital image correlation-aided slitting technique

This paper presents an incremental cutting method for evaluating the longitudinal residual stresses in a butt welded thin plate via combining the traditional residual stress measurement methods and the advanced optical technique.The proposed approach,which can be called digital image correlation（DIC）-aided slitting technique,introduces a successive extension slot to a specimen and employs the DIC technique to measure the released displacement profiles of the cutting sections after each cutting increment.Then the displacement profiles are used to directly calculate the residual stress distributions up to the slot tip and hence,a stress distribution can be obtained after a cutting increment.Finally,all of the stress distributions are averaged to ultimately determine the original residual stress field.This method does not include any complex experimental operations or tedious derivation,and the resolution of stress variation is greatly improved by the continuous measurement of the released displacements.The presented method has been preliminarily verified by a specimen with residual stress introduced by a four-point bending test.The results show that residual stresses determined by the DIC-aided slitting technique agree well with those from finite element（FE） prediction.The residual stress in a friction stir welded aluminum specimen obtained by the presented technique is also consistent with the evaluations given by X-ray diffraction.Furthermore,the residual stresses obtained by the DIC-aided slitting technique demonstrate higher accuracy and stability than the evaluations derived by the DIC-aided contour method.

Elastic constants characterization on graphite at 500℃ by the virtual fields method

In this paper the elastic constants of graphite at elevated temperature were experimentally investigated by using the virtual fields method （VFM）. A new method was presented for the characterization of mechanical properties at elevated temperature. The three-point bending tests were performed on graphite materials by an universal testing machine equipped with heating fumace. Based on the heterogeneous deformation fields measured by the digital image correlation （DIC） technique, the elastic constants were then extracted by using VFM. The measurement results of the elastic constants at 500℃ were obtained. The ef- fect on the experimental results was also analyzed. The successful results verify the feasibility of using the proposed method to measure the properties of graphite at high temperature, and the proposed method is believed to have a good potential for further applications.

Optimisation Method for Determination of Crack Tip Position Based on Gauss-Newton Iterative Technique

In the digital image correlation research of fatigue crack growth rate,the accuracy of the crack tip position determines the accuracy of the calculation of the stress intensity factor,thereby affecting the life prediction.This paper proposes a Gauss-Newton iteration method for solving the crack tip position.The conventional linear fitting method provides an iterative initial solution for this method,and the preconditioned conjugate gradient method is used to solve the ill-conditioned matrix.A noise-added artificial displacement field is used to verify the feasibility of the method,which shows that all parameters can be solved with satisfactory results.The actual stress intensity factor solution case shows that the stress intensity factor value obtained by the method in this paper is very close to the finite element result,and the relative error between the two is only−0.621%;The Williams coefficient obtained by this method can also better define the contour of the plastic zone at the crack tip,and the maximum relative error with the test plastic zone area is−11.29%.The relative error between the contour of the plastic zone defined by the conventional method and the area of the experimental plastic zone reached a maximum of 26.05%.The crack tip coordinates,stress intensity factors,and plastic zone contour changes in the loading and unloading phases are explored.The results show that the crack tip change during the loading process is faster than the change during the unloading process;the stress intensity factor during the unloading process under the same load condition is larger than that during the loading process;under the same load,the theoretical plastic zone during the unloading process is higher than that during the loading process.

EH36钢焊接接头焊趾处应力集中对高低周复合疲劳的影响

为研究船舶结构用钢EH36钢焊接接头焊趾处应力集中对高低周复合疲劳的影响,对不同焊缝形貌20 mm厚EH36钢双面全熔透对接接头进行高低周复合疲劳试验.研究发现,高低周复合疲劳对接头寿命的降低幅度随着高周应力比的增大而增大;结合数字图像相关法测量结果分析,发现焊趾处应力集中系数越大,接头的高低周复合疲劳寿命越低,其降低幅度随着高周应力比的增大而增大.基于试验数据对模型进行验证,结果表明,应力集中系数高的接头疲劳寿命低且寿命预测结果分散性增大;Palmgren-Miner模型和Zhu模型由于未充分考虑高低周耦合损伤作用,导致寿命预测结果相对危险.修正后Zhu模型在充分考虑低周疲劳对高低周复合疲劳寿命的影响后,预测寿命大于实际寿命的预测点较少,预测结果相对安全,且分散性较小,误差也较小.

高温后混凝土单轴拉伸和压缩破坏规律及力学性能研究

本文对常温及高温作用后的混凝土进行单轴拉伸和压缩试验,同时使用数字图像相关方法(DIC)观测混凝土试件表面的变形演化过程,探究高温后混凝土拉伸和压缩过程中裂缝的变形演化及力学性能衰减规律。结果表明,随着温度升高,混凝土表面裂缝的萌生和扩展时间逐渐提前,破坏时表面应变集中区的宽度及相应的应变数值逐渐增大,裂缝扩展程度更加显著,当温度达到800℃时,甚至呈多裂缝扩展形态。裂缝的显著扩展增加了混凝土的耗能,使高温损伤后混凝土的延性增加。比较高温后混凝土拉伸和压缩应变云图可知,裂缝开裂引起的应变集中区宽度相近,但压缩试件的应变数值略高于拉伸试件。裂缝的萌生和扩展影响混凝土的力学性能,随着温度升高,混凝土抗拉强度、抗压强度及弹性模量不断下降,峰值应力处的应变逐渐增加;拉伸应力应变曲线由双阶段逐渐转变为三阶段,破坏特征由脆性转变为延性;而压缩应力应变曲线始终为三阶段,与单轴拉伸相比,压缩破坏的延性特征更加明显。

钢带嵌缝加固砌体结构抗剪性能试验研究

为提高砌体结构抗剪强度,同时保留原有风貌,采用“弓”字型钢带或钢筋对砌体试件进行嵌齿缝加固。利用数字图像相关方法对加固后的砌体结构进行沿通缝截面抗剪强度试验研究,并建立砌体抗剪试件有限元模型,分析钢带宽度对砌体抗剪强度的影响。结果表明:嵌入钢筋或钢带,均能够有效限制层间分离,提高砌体完整性,破坏模式由单剪破坏转化为双剪破坏。承载能力大幅提高,单面钢带、双面钢带、双面钢筋加固的试件极限荷载分别提高了23.63%,61.18%,35.02%,极限荷载对应位移提高了16.38%,23.41%,16.95%。钢带或钢筋的拉结作用可以提高砌体结构的延性,改善砌体结构脆性破坏的特征。

基于数字图像技术的致密砂岩纯剪裂缝断裂过程实验

纯剪应力状态下致密砂岩断裂过程是理解致密砂岩破坏失效机理的关键,然而在纯剪应力状态下,初始裂缝是发育扩展为I型裂缝或II型裂缝却一直有所争议。本文基于新型光学测量技术数字图像相关法和双切边压缩实验理论方法,开展了双切边压缩实验并通过数字图像相关法得到了位移和应变场。实验结果表明:双边开槽压缩实验能在槽尖(预制裂缝尖端)产生纯剪应力状态;致密砂岩试件起裂过程包含2个阶段的I型裂缝,第一阶段裂缝从槽尖起裂,第二阶段裂缝从试件中心两槽尖相连区域起裂。本文研究成果为数字图像技术应用在岩石断裂力学领域提供了新思路,对分析水力压裂中致密砂岩纯剪裂缝的演化规律有一定参考价值。

基于数字图像相关法的轮轨力测量研究

针对应变计式轮轨力测量方法的不足,研究一种基于数字图像相关法的非接触式轮轨力测量方法。通过受力分析以及有限元计算,确定钢轨表面非接触测点区域位置。根据轨旁检测的特点设计测量系统的整体方案并分析了其测量原理,对相机进行了分析选型,以实现轮轨力作用下钢轨表面测点区域变形散斑图像的高质量采集。基于数字图像相关法对所采集的变形散斑图像进行分析,采用1阶形函数描述钢轨的变形,并使用零均值归一化差值平方和函数作为相似度的度量指标;为了提高系统的测量分辨精度,在整像素匹配算法的基础上,通过对位移初值进行迭代计算进一步得到亚像素级位移,并使用最小二乘法拟合位移场结果得到相应的应变场。在实验室设计并搭建了具有移动加载功能的轮轨力测量试验系统;通过静态标定试验准确建立了反映钢轨应变和轮轨垂向力大小数学关系的线性标定曲线,并分析讨论了相机工作距离和光照强度等因素对轮轨力测量的影响;在移动载荷作用下进行了轮轨力动态测量试验,与接触式应变计法测量结果进行比较,非接触式数字图像相关法在跨中位置的最大轮轨力测量误差不超过0.3%。动静态测量试验结果表明,数字图像相关法能够实现轮轨力的精确测量,是一种有效的非接触式轮轨力测量方法。

基于应变的应变硬化指数演化行为研究

为准确描述变形过程中双相高强钢板的应力应变关系,针对3种双相高强钢板应变硬化指数的演化行为进行了试验研究与理论模型预测。借助单向拉伸试验机和自主研发的DIC应变测量系统,获得了3种材料的真实应力-真实应变曲线,基于经典的Hollomon流动应力模型,得到了其应变硬化指数的演化历程,发现3种材料应变硬化指数与真实应变呈非线性变化关系。为了更好地预测应变硬化指数的演化行为,提出了可以描述其值变化的非线性数学模型,并结合经典的Hollomon流动应力模型,提出了基于n值变化的修正的Hollomon-n流动应力模型,最后通过3种材料的真实应力-真实应变曲线验证了所提出模型的有效性。

碳酸钙晶须对水泥砂浆单轴压缩特性的影响

为了解碳酸钙晶须对水泥砂浆单轴压缩特性的影响,分别对不同碳酸钙晶须掺量的水泥砂浆进行了单轴压缩试验,研究了单轴压缩全过程荷载-位移曲线的特征及其影响因素,基于数字图像相关法研究了单轴压缩下砂浆的变形特征,并借助扫描电镜对试样进行了断面微观分析。结果表明:砂浆的抗压强度在碳酸钙晶须掺量0~3%时逐渐提高,继续增大碳酸钙晶须掺量到5%时,抗压强度降低;砂浆的应变场显示,掺加适量的碳酸钙晶须可延缓局部裂缝扩展,砂浆内部结构受晶须掺量的增加会从“桥连强化”转为“团聚弱化”负向作用。研究结果可为碳酸钙晶须在水泥基材料中应用提供一定参考。

拉伸载荷下含缺陷管道宽板力学响应与断裂失效试验

为探究缺陷与焊缝的相对距离和夹角对X65螺旋埋弧焊钢管失效的影响,开展宽板拉伸试验,采用三维数字图像相关法分析轴向拉伸载荷下缺陷到焊缝的距离、缺陷与焊缝的夹角对管道宽板的力学响应和断裂失效行为的影响特征.结果表明:焊缝和缺陷的存在,以及缺陷到焊缝的相对距离和夹角对管道宽板的应变变化和断裂失效行为有影响,且不同的相对距离和夹角对其抗拉强度、应变分布和断裂失效行为的影响程度不同;当缺陷与焊缝的夹角为30°且当缺陷到焊缝的距离为39 mm时,管道宽板的抗拉强度最低、裂纹萌生及断裂失效所需位移载荷最小,此时其承载能力最差,最容易断裂失效.

冲刷作用下砂岩宏观力学特性及微观结构

水流环境下库岸边坡岩石劣化会严重危害坡体稳定性,如何精细化揭示动水冲刷作用下岩石宏观力学特性及微观结构对库区灾害防治具有重要意义。为模拟库岸边坡所处的水流环境,设计了一套岩石抗冲刷试验装置。以砂岩为研究对象,考虑自然、静水浸泡和动水冲刷3种工况。结合室内试验、数字图像相关方法、扫描电镜和分形理论,从宏观力学参数、应变场分布特征、微观结构等角度系统探讨了不同水环境对岩石力学特性的影响。研究结果表明:(1)动水冲刷后的岩石抗压强度和弹性模量软化系数比饱水试件分别降低了0.07和0.06;(2)应变场分异速率-轴向应变曲线在峰值应力前出现拐点P,与断裂过程区的集聚相关,可作为岩石破裂的前兆信号;相较于饱水作用,冲刷作用使得破裂前兆点出现变早,这是因为冲刷作用加剧了岩石内部缺陷的发育,更易形成裂纹;(3)相较于饱水试件,动水冲刷后砂岩内部矿物颗粒和胶结物流失程度加大,微观结构劣化更为显著,这种变化在扫描电镜图像的分形维数计算结果中得到了印证,冲刷试件的分形维数最大。

沥青混凝土起裂韧度试验研究

开展了沥青混凝土半圆弯曲断裂试验,获得了裂缝扩展全过程荷载-裂缝口张开位移曲线,结合线性相关陡降法研究了温度、加载速率及预制裂缝长度对沥青混凝土起裂韧度的影响。应用数字图像相关方法对沥青混凝土起裂及裂缝扩展全过程损伤变形特性进行了研究。结果表明:线性相关系数陡降法所得起裂荷载处于损伤产生时刻对应荷载范围内,验证了该方法应用于沥青混凝土起裂韧度研究的可行性。随温度和预制裂缝长度增加,起裂韧度逐渐降低;随加载速率增大,起裂韧度呈现出上升趋势。

弹载天线模块热变形仿真预测与试验研究

以典型的弹载天线模块为研究对象,采用ABAQUS软件建立其热变形仿真预测模型。同时,在相同条件下,采用所搭建的基于数字图像相关法(DIC)的热变形测量系统提取并计算变温前、后弹载天线模块的热变形,以验证热变形仿真模型的准确性。结果表明,ABAQUS的仿真结果与DIC方法测量的弹载天线模块热变形结果吻合度较好,验证了所建立的有限元仿真模型能准确且有效地评估弹载天线模块的热变形情况。

Physical model test and application of 3D printing rock-like specimens to laminated rock tunnels

Weak structural plane deformation is responsible for the non-uniform large deformation disasters in layered rock tunnels,resulting in steel arch distortion and secondary lining cracking.In this study,a servo biaxial testing system was employed to conduct physical modeling tests on layered rock tunnels with bedding planes of varying dip angles.The influence of structural anisotropy in layered rocks on the micro displacement and strain field of surrounding rocks was analyzed using digital image correlation(DIC)technology.The spatiotemporal evolution of non-uniform deformation of surrounding rocks was investigated,and numerical simulation was performed to verify the experimental results.The findings indicate that the displacement and strain field of the surrounding layered rocks are all maximized at the horizontal bedding planes and decrease linearly with the increasing dip angle.The failure of the layered surrounding rock with different dip angles occurs and extends along the bedding planes.Compressive strain failure occurs after excavation under high horizontal stress.This study provides significant theoretical support for the analysis,prediction,and control of non-uniform deformation of tunnel surrounding rocks.