含孔洞岩石单轴压缩下变形破裂规律的实验研究

采用数字散斑相关量测方法(DSCM)和数字照相量测软件系统PhotoInfor,通过观测表面位移场、应变场及总体变形在整个加载过程中的变化特点,对大小为100mm×100mm×100mm的含有孔洞的岩石试件在单轴压缩下的变形破裂演变规律进行了实验研究。结果表明:1)DSCM方法能够对岩石变形破裂的特点进行全程有效捕捉,有助于加深对岩石变形局部化过程的全面认识;2)含孔洞岩石试件局部化变形起始于峰值点处轴向位移的80%处,应力峰值点附近变形局部化最为剧烈;3)岩石表面平均最大剪应变随轴向位移的变化曲线,总体上能够很好地反映岩石变形破裂的演变过程和规律,可作为分析岩石变形演变的一个参考特征参数。     

纤维增韧轻骨料混凝土单轴受压应力-应变全曲线试验研究

为探究纤维增韧后轻骨料混凝土应力-应变全曲线,完成了不同混凝土强度等级、纤维种类及掺量下的9组棱柱体单轴受压试验,分析了破坏过程和破坏特征,系统研究了各因素对峰值应力、峰值应变和弹性模量的影响,并结合已有研究给出了各曲线特征点计算模型,考虑纤维轻骨料混凝土自身特征,建立了分段式纤维轻骨料混凝土应力-应变全曲线模型。研究表明:轻骨料混凝土破坏特征与普通混凝土显著不同,掺入纤维有效抑制了其内部微裂缝的开展,起到阻裂、增韧的效果,且随混凝土强度等级、纤维种类及掺量变化差异明显;纤维增韧后试件的应力-应变曲线下降段坡度趋于平缓,脆性得到有效改善;建议的应力-应变全曲线模型与试验结果吻合良好,能够准确描述纤维增韧轻骨料混凝土在单轴受压作用下的受力变形特征。     

数字散斑相关法在陶瓷材料准静态破坏 行为研究中的应用

研究了引入高精度数字散斑相关法光学测量技术对A９５ 陶瓷材料在准静态压缩试验中的变形过 程,得到了该材料在压缩变形过程不同时刻的试件表面的全场位移分布,推导出应力应变曲线与材料的本构 关系。与传统方法相比较,数字散斑相关法所具有的非接触、高精度、全场测量等特点,避免了人为干扰,可 以对试件表面全场的变形进行研究。     

基于数字体相关方法的3D打印材料内部变形测量EI北大核心CSCD

采用工业CT设备获取3D打印材料试样受载变形前后的三维数字体图像,并使用数字体相关(digital volume correlation,DVC)方法计算出试样内部三维位移场和三维应变场,研究试样在载荷作用下的内部变形与损伤演化规律,通过设计制作具有1700 N拉压加载能力的原位加载装置,结合微纳CT设备建立拥有原位加载与观测能力的DVC方法实验装置,其最高成像分辨率可达10μm。结果表明:对采用激光选区熔化(selective laser melting,SLM)工艺方法制作的3D打印铝合金试样进行原位拉伸实验,并使用DVC方法获取试样内部的全场位移与应变数据,测得的弹性模量数据与常规实验方法结果完全吻合,验证了使用DVC方法对内部位移场和应变场进行测量的有效性。     

基于SPM与DIC技术的应变场测量改进方法

微纳米尺度下的应力/应变测量方法是实验力学研究的重要方向之一。现有的许多研究忽略了待测试件表面的散斑图质量、系统随机误差、计算流程等因素对最终的应变场测量精度的影响。围绕基于扫描探针显微镜的数据图像相关技术实现过程,提出科学的评估准则、优化位移场计算方法。实验结果表明,所提出的应变场计算方法在精度控制方面的性能得到显著提升。     

基于VIC-3D技术的沥青混合料界面力学特性实验研究

为研究沥青混合料开裂及裂缝发展规律,通过SBS改性沥青混合料半圆试件三点弯曲实验,基于数字图像相关技术,利用VIC-3D(video image correlate,3D)采集系统对试件从加载到破坏全过程进行图像采集,并利用VIC-3D软件计算得到试件在受弯拉过程中三维位移场、应变场的变化及变化速率,分析并得出裂缝出现及发展规律;利用扫描电子显微镜对开裂试件断口进行观察,结合位移场、应变场及其变化速率揭示了试件开裂及裂缝发展的机理。结果表明,利用水平方向应变随时间变化规律可探究裂缝的发展轨迹;沥青混合料的开裂和裂缝发展沿沥青砂浆与粗集料的界面区发生,且趋向于表面情况相对差的集料的界面区;裂缝发展速率与界面区的厚度有关,可根据界面区厚度的代表值预测裂缝发展速率。     

碳纤维布加固钢筋混凝土梁的DSCM实验研究

通过用数字散斑相关方法(DSCM)对碳纤维布加固混凝土梁的实验,研究了碳布加固混凝土梁的承载力及变形行为。确定了碳纤维布加固混凝土梁在不同加载条件下的强度特征。并且用数字散斑相关方法实时记录了加载过程中反映试件变形信息的光学散斑图,确定了一些局部的变形特征,分析了其断裂机理。     

基于CT切片的三维混凝土细观层次力学建模

基于CT 切片研究混凝土数值模型的三维重构,可以为揭示混凝土破坏机理研究提供更好的研究对象。基于数字图像技术和MATLAB 图像处理工具,针对混凝土截面CT 切片研究混凝土三维模型的重构技术。应用CT技术获取混凝土试件截面切片,应用底帽变换等方法将CT切片处理为三值图片,考虑孔隙影响,并基于体数据法重构混凝土三维模型。基于网格映射方法,重点研究了单元属性控制因素,提出更精确的单元属性识别判断准则,得到了与试件真实骨料配比相近的三维有限元网格模型。对重构的混凝土试件的四点弯拉断裂过程的非线性有限元分析表明,数值分析的弯拉曲线与破坏形态都与实验结果有较好的相似性,数值模型能较好的模拟实际混凝土试件的破坏。     

考虑应变梯度效应的三点弯梁模型解析研究--第一部分:局部化带的传播

为分析应变软化和由此带来的应变局部化问题,将梯度塑性理论引入裂纹带模型。以拉应变局部化区域代替裂纹带,在三点弯梁裂纹带(具有一定尺寸的带宽由特征长度确定)内部存在着不均匀分布的拉应变,这与实验结果相符。对拉应变进行积分,得到了拉应变局部化区域的张拉位移的理论表达式,结果表明:该位移与拉应力成线性规律,拉应变局部化区域的宽度越大,弹性模量越小或降模量越小,则该位移越大。此外,采用应力平衡条件、应变软化的本构关系及平截面假定,还得到了拉应变局部化区域的扩展规律,结果表明:下降模量越大、三点弯梁高度越小及弹性模量越小,则在相同的拉应力的情况下,拉应变局部化区扩展的长度越小;抗拉强度对拉应变局部化区扩展长度的最大值没有影响。此外,还研究了梁中部横截面内中性轴到具有最大承载能力的点的距离的变化规律。     

碳纤维编织复合材料弯曲损伤破坏声发射监测

为研究碳纤维编织复合材料的弯曲损伤与破坏行为,结合声发射(AE)与数字图像相关(DIC)方法互补实验技术,对复合材料在四点弯曲载荷作用下的失效过程进行监测,动态获取AE特征信号和对应的散斑图像,并根据DIC算法得到复合材料在加载过程中的全场变形和应变。结果表明:AE信号在碳纤维编织复合材料中近似呈指数衰减,与谐振式AE传感器相比,宽频带式AE传感器表现出更好的测量性能;复合材料弯曲加载前期,无明显损伤出现,AE信号较少,对应的位移场和应变场变化平稳;复合材料弯曲破坏对应较高持续时间、幅度和相对能量的AE信号。复合材料损伤演化过程的AE响应行为及对应位移场、应变场的变化反映复合材料的变形与损伤破坏过程,为碳纤维编织复合材料的无损评价、健康监测与失效分析提供借鉴。     

Optimized butterfly specimen for the fracture testing of sheet materials under combined normal and shear loading

The present paper is concerned with multi-axial ductile fracture experiments on sheet metals. Different stress-states are achieved within a flat specimen by applying different combinations of normal and transverse loads to the specimen boundaries. The specimen geometry is optimized such that fracture initiates remote from the free specimen boundaries. Fracture experiments are carried out on TRIP780 steel for four different loading conditions, varying from pure shear to transverse plane strain tension. Hybrid experimental–numerical analyses are performed to determine the stress and strain fields within the specimen gage section. The results show that strain localization cannot be avoided prior to the onset of fracture. Through-thickness necking prevails under tension-dominated loading while the deformation localizes along a band crossing the entire gage section under shear-dominated loading. Both experimental and simulation results demonstrate that the proposed fracture testing method is very sensitive to imperfections in the specimen machining. The loading paths to fracture are determined in terms of stress triaxiality, Lode angle parameter and equivalent plastic strain. The experimental data indicates that the relationship between the stress triaxiality and the equivalent plastic strain at the onset of ductile fracture is not unique.     

Experimental study on damage evolution of rock under uniform and concentrated loading conditions using digital image correlation

Damage evolution and crack propagation in sandstone specimens have been observed by digital image correlation method. To investigate deformation and failure process of rock under different loading conditions, uniaxial compression and indentation tests were performed. Through the experiment, displacement and strain fields are simultaneously obtained that can visually display the distribution, mode and evolution of deformation and cracking in rock. Experimental results show that the damage distributes diffusely in rock at early loading stage, and the measured apparent strain increasingly concentrates with loading because of the nucleation of crack; propagation of the crack leads to the eventual failure of the specimen. Damage factor is calculated on the basis of deviation of apparent strain, and localization factor is presented to describe the level of deformation localization. The combined use of two factors can well represent the damage evolution of rock under compression.     

混凝土单轴动力压缩CT试验过程定量化分区研究

为了能定量地研究混凝土动力压缩CT试验过程,将基于模糊集合论的破损演化理论引入到混凝土动力CT试验研究中,基于λ1-λ2截集将混凝土CT扫描图分为孔洞裂纹（P0-λ1）、硬化水泥石(Pλ1-λ2)及骨料(Pλ2-1),并定义了混凝土的孔隙率、硬化水泥石率和骨料率。研究了动力单轴压缩CT试验过程中混凝土试样各扫描断面P0-λ1、Pλ1-λ2及Pλ2-1随加载的变化规律。研究结果表明：利用本方法实现了对混凝土各组分的定量化分区;可以定量地研究混凝土中P0-λ1、Pλ1-λ2及Pλ2-1随动力加载的具体变化规律;能较好地描述混凝土的受力破坏过程;并能定量地体现出试样破坏过程中应变的局部化。     

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.     

An elasticity microscope. Part I: Methods

An elasticity microscope images tissue stiffness at fine resolution. Possible applications include dermatology, ophthalmology, pathology, and tissue engineering. In addition, if the resolution approaches cellular dimensions, then this system may be very useful in understanding tissue micromorphology. Elasticity images can be reconstructed from displacement and strain fields measured throughout the specimen during controlled external loading. High frequency ultrasound is used to obtain these images by tracking coherent speckle motion during deformation. In this paper, methods are presented to track speckle in two dimensions with near unity correlation coefficients using a high frequency, single element focused transducer. These techniques include improved means for speckle tracking. Procedures to control boundary conditions for consistent specimen deformation and scanning techniques required to obtain a plane-strain state in the imaging plane are also discussed. To test these methods, a 50 MHz elasticity microscope was constructed     

冲击剪切载荷下SiCP/6151Al复合材料变形局部化及增强颗粒尺寸效应

利用特殊设计的"hat shape"试样,在分离式Hopkinson压杆和MTS通用材料试验机上实验研究了颗粒尺寸和应变率对颗粒增强金属基复合材料(SiC_P/6151Al)变形局部化行为的影响。结果表明:颗粒尺寸对复合材料的变形强化与变形局部化行为有显著影响。具体表现为:颗粒越小,复合材料流动应力越高,即强化效果越好;另一方面,对受载试样的微观检测发现,颗粒越小,复合材料剪切变形局部化越明显。同时发现,冲击载荷(高应变率)下复合材料更容易发生变形局部化。     

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.     

双面叠合混凝土剪力墙平面内和平面外抗震性能研究

双面叠合混凝土剪力墙由两侧预制混凝土板和中间后浇混凝土叠合层组成,具有整体性较好、大量节省模板和支撑、便于工业化生产、经济性好等优点,应用前景广阔。该文开展了剪力墙足尺模型的高轴压比下平面内和低轴压比下平面外低周反复荷载试验,对双面叠合剪力墙的抗震性能进行了研究与探讨。研究表明:双面叠合剪力墙在平面内和平面外低周反复荷载下的破坏形态与现浇剪力墙相同,均发生受弯破坏; 4片剪力墙滞回曲线特征相似,双面叠合剪力墙的耗能性能好于现浇剪力墙;双面叠合剪力墙的平面内承载力比现浇剪力墙高约5%,平面外承载力低约12%;平面内和平面外加载时双面叠合剪力墙均具有良好的延性,其延性系数分别为2.15和3.83,比现浇剪力墙分别高约25%和20%。双面叠合剪力墙基于规范计算值的抗弯及抗剪安全系数均达到1.1以上,双面叠合剪力墙破坏时其水平接缝处的剪力远小于其接缝抗剪承载力设计值。总体而言,双面叠合剪力墙具有良好的抗震性能,可在抗震设防地区推广应用。     

Application of DSCM in prediction of potential fatigue crack path on concrete surface

Prediction of potential fatigue crack paths on the surfaces of concrete structures was a difficult task in the past. To complete this task a Finite Element Method simulation by computers is usually employed, but bearing in mind that the numerical results are not always consistent with the experimental ones because of the effects of various environmental factors and construction quality on concrete structures. In order to inspect the damage process during the service life of structures and to confirm their detailed plan of repair and reinforcement, a new non-destructive technology is used for on-line health inspection of many important concrete structures in the world. Fortunately, along with the development of Digital Speckle Correlation Method (DSCM) in the last two decades, this new non-destructive technique has presented many advantages. To testify the feasibility and accuracy of this system, two series of concrete specimens were tested under flexural fatigue loading and a software UU^© matched to DSCM was employed in this work. DSCM can get full-field and local displacements by comparing two speckle images on target surfaces before and after deformation. Then the full-field and local strain fields on these regions can be deduced from the longitudinal and transversal displacements. By analysis of these strain fields, the starts of potential fatigue crack paths can be predicted precisely. It is testified that DSCM system is accurate and effective in on-line prediction of potential fatigue crack path of specimens made of heterogeneous materials under flexural cyclic loading. The fractural mechanisms of the two concrete series under flexural cyclic loading were discussed. Note that the fatigue testing conditions should be kept still and clean in order to produce precise results of DSCM system.