光弹贴片法研究混凝土Ⅰ-Ⅱ复合型裂缝扩展过程

EXPERIMENTAL INVESTIGATION ON DOUBLE-K FRACTURE PARAMETERS FOR SMALL SIZE SPECIMENS IN CONCRETE

该文采用光弹贴片方法对最大尺寸为2500mm×600mm×200mm的混凝土四点剪切梁,Ⅰ-Ⅱ复合型裂缝起裂、稳定扩展直至失稳破坏全过程进行了系统的研究,通过照相机拍摄光弹贴片所显示的裂缝扩展全过程,得到了混凝土裂缝稳定扩展阶段完整而直观的观测结果。根据光弹贴片所显示的彩色条纹序列,测得了四点剪切荷载作用下裂缝尖端附近主应变场分布,得到了荷载-裂缝扩展量曲线。根据光弹贴片所观察的裂缝扩展长度?a,借助有限元计算了裂缝扩展过程中尖端应力强度因子的变化,并模拟断裂过程。结果表明,对于混凝土Ⅰ-Ⅱ复合型断裂,裂缝在失稳扩展前存在着明显的亚临界扩展,采用光弹贴片方法可有效地记录这一过程,而以往不考虑裂缝扩展量得到的断裂准则是偏于保守的。试验结果还表明,在裂缝起裂后,虽然加载方式仍为Ⅰ-Ⅱ复合型,但断裂类型已由Ⅰ-Ⅱ复合型退化为纯Ⅰ型断裂。

For studying crack propagation process, four-point shearing specimens, with the maximum size of 2500mm×600mm×200mm, are utilized to systematically investigate the crack initiation, stable propagation and unstable failure of concrete by means of photoelastic coatings. Using camera, the complete crack propagation process of concrete that appeared as interference fringes on the photoelastic coatings was recorded and the whole process of stable propagation and unstable development of the main crack was recorded. Based on the colored strain sequence, the strain distribution around the crack tip was measured and the curve of P-a was also obtained. In the presence of the crack extended length of photoelastic coatings, the finite element method was applied to simulate the process of crack propagation and count the variation of stress intensity factor at the tip of crack during crack extension. The results show that there is an obviously crack stable propagation before an unstable propagation, and that it can be recorded using photoelastic coatings. Therefore, the failure criterion, taking no account of P-a, is conservative. It can be also concluded that fracture mode has been turned into I mode from Ⅰ-Ⅱ mixed mode after crack initiation, although the loading condition is Ⅰ-Ⅱ mixed mode.