边界条件对混凝土四点剪切散斑试验结果影响的研究

STUDY ON INFLUENCE OF BOUNDARY CONDITIONS ON RESULTS OF FOUR POINT SHEAR CONCRETE SPEKLES EXPERIMENTS

用CCD摄像机对单边预制缺口和双边预制缺口混凝土梁分别在滑动支座和铰支座2种边界条件下,在四点剪切加载方式下的破坏过程进行全过程记录。从采集到的连续散斑图中可以清楚地看到混凝土试件不同的裂纹扩展过程和最终破坏形态。用数字散斑相关方法(DSCM)处理采集后的散斑图,得到混凝土梁表面在破坏过程中的水平方向位移场、竖直方向位移场、水平方向应变场、竖直方向应变场及剪应变场。从采集到的散斑图中,间隔一定时间挑选若干张图片与变形前的散斑图片进行运算处理,得到若干个位移场、应变场,将这些位移场和应变场按照时间顺序排列,得到试件在破坏过程中各场的演化过程。将各个场在试件开始破裂处的值进行对比,发现水平方向和竖直方向的位移值大小相近,应变值也处于同一数量级。结果表明预制缺口的附近并不是一个剪应力占主要地位的区域。试验表明,即使使用了滑动支座,支座处的摩擦力也没有被减小到一定的程度。因此,要使混凝土的四点剪切试验更接近于理想状态,一定要对支座处进行仔细处理。

Several tests are conducted in which the concrete specimens are divided into two groups： the single-edge notched（SEN） and the double-edge notched（DEN） concrete beams. Both of the two groups of specimens are loaded under four point shear force and two different types of boundary conditions. The whole failure process of the specimens is shot with charge coupled device（CCD）. From the speckle images, different failure phenomena are seen induding the propagation paths of cracks and final failure mode. The speckle images shot by the CCD are manipulated from which the displacement, strain and shear fields in strain horizontal and vertical directions in the surface of specimens are gained. Among the speckle images, some are picked out at certain intervals and compared with that before deformation in order to obtain diseplacement and strain fields. The evolution process of each field is gained by arrangind those fields to the time order. Comparing the displacement and strain fields gained at the place where the cracks start, it is shown that the magnitude of the horizontal and vertical displacements are close, and the values of the horizontal and vertical strains are in the same grade. The results demonstrate that the shear stress is not dominant in the zone near the notches. It also indicates that, though the sliding support is adopted, friction in the support is not reduced to a certain extend. Therefore the support should be treated carefully so as to get better results from four point shear test.