混凝土材料拉伸强度的应变率强化效应实验研究

混凝土材料的拉伸强度具有明显的应变率强化效应。采用同批制作的混凝土圆盘和圆杆试样,利用霍普金森杆加载,进行动态劈裂实验和一维杆应力波层裂实验。借助超高速摄像机、数字图像相关(DIC)等测试方法观察实验中试件破坏过程,以准确测量混凝土材料在不同应变率下的拉伸强度。在劈裂实验中注重加载方式,以保证试件满足求解弹性解析解的中心起裂条件;在层裂实验中精确确定层裂发生位置和时刻,获得了可靠的材料拉伸强度,并分析劈裂拉伸与层裂拉伸强度的应变率强化规律。研究结果表明:对于动态劈裂实验,DIC方法显示圆盘中心起裂与否的临界应变率约为10 s^-1;而对于层裂实验,采用DIC方法可以得到多次层裂对应的拉伸强度及相应的应变率(10～100 s^-1);通过线性拟合拉伸强度应变率强化规律,层裂拉伸强度随应变率增长的斜率较劈裂实验结果偏高,混凝土的层裂拉伸强度动态增强因子可达5以上。所得研究结果对脆性材料动态拉伸强度及应变率强化效应的准确测量,具有一定的参考意义。

An Experimental Investigation on the Strain Rate-dependent Tensile Strength of Plain Concretes

Concrete-like materials have a clear strain-rate strengthening effect on tension strength. All specimens, including Brazilian discs and bars, were prepared and cured under the same laboratory conditions. The dynamic Brazilian split tests and the spalling tests were conducted by employing the Hopkinsion pressure bar as a loading platform. The failure processes of specimens in the tests were observed by using ultra-high speed camera and digital image correlation (DIC) method. In quasistatic or dynamic Brazilian split test,the disk specimen is assured to be splitted at the center of the disk. In spalling test, DIC analysis is applied to determine the exact timing and location of spall fracture, specifically in the case of multiple spallations. The dynamic tensile strengths of plain concrete discs and bars were obtained through these tests. The strain rate strengthening rules of split and spall tensile strengths are analyzed. The research results show that, for the dynamic split test, the central breaking only happens at relative low strain rate, and the critical strain rate is about 10 s^-1; the tensile strength and strain rates of multi-spalling can be achieved accurately by using a DIC-based analytical method; dynamic strain rate streng-thening rule of tensile strength are got by linearly fitting the experimental data, the increasing slope of tensile strength in spalling test is larger than that in dynamic Brazilian split test, and the dynamic increase factor of concrete can reach to above 5. The test results, based on the accurate measurements of tensile strength, can be used as a reference for the design and reconsideration of civil and military engineering.Key