不同加载边界下混凝土巴西劈裂过程及强度的DIC实验分析

EXPERIMENTAL ANALYSIS OF PROCESS AND TENSILE STRENGTH FOR CONCRETE BRAZILIAN SPLITTING TEST WITH DIFFERENT LOADING BOUNDARIES BY DIC METHOD

平板、垫条、圆弧压模及平台试样等不同加载方式下,脆性材料巴西劈裂实验结果的差异一直是人们所讨论的问题.本文设计了混凝土平板直接加载与采用垫条加载实验,采用理论、高速相机与数字图像相关法(digital image correlation,DIC)相结合的方法,对试样表面应变场演化、起裂位置及裂纹扩展过程进行分析,探讨了不同的加载边界对试样应变集中演化及劈裂拉伸强度结果的影响.结果显示:(1)混凝土拉伸非线性特性导致的应变集中演化对不同加载条件非常敏感,平板加载时应变集中较早在加载端起始并向中心演化,即使满足巴西实验"中心起裂"条件,其强度仍低于垫条加载情况,两者差别达17.9%;(2)如果软垫条加载接触角合理设计,与平板直接加载相比,其应力场更稳定,有利于保证圆盘试样的应变集中及断裂均从中心起始,更好满足巴西劈裂实验条件;(3)仅校验巴西实验"中心起裂"有效性条件是不充分的,设计时必须谨慎考虑.研究结果对脆性材料巴西劈裂实验设计、测试分析具有重要参考意义.

Brazilian splitting test was widely used in the evaluation of tensile strength of brittle materials,but the results of strength are difference with different loading boundaries,such as flat plates,bearing strips,arc die,and flattened Brazilian disc,which has always been a research focus.In this paper,the splitting test was designed under different loading boundaries of direct flat plates and bearing strips,and analyzed theoretically,carried out experimentally as well.By the high-speed camera and digital image correlation(DIC)method,the process of the evolution of strain field,crack initiation and propagation on the surface of the specimen were studied in the splitting test.The influence of different loading boundaries on the tensile strength and the strain concentration evolution were further discussed.The results show that:(1)The evolution characteristics of strain concentration caused by the nonlinear tensile properties of concrete are sensitive to loading conditions,which lead to strain concentration earlier from the loading ends and evolves to the center of the specimen when loaded by flat plates.In this case,even the experimental condition of"center cracking"is satisfied,its strength still lower than that of bearing strips,the difference can reach 17.9%;(2)As long as the contact angles of bearing strips loading is designed properly,the stress field of the specimen can develop stably than that of direct flat plate loading,which is conductive to ensure strain concentration as well as fracture starting from the center.It is better to meet the requirements of the Brazilian splitting test;(3)It is not sufficient to only verify the validity condition of"center cracking",which must be carefully considered in the test design and analysis.The research results have important reference significance for the design and test analysis of the Brazilian splitting experiment of brittle materials.