混凝土一维应力层裂实验的全场DIC分析

基于74mm直径分离式Hopkinson杆(SHPB)实验平台进行了混凝土杆的一维应力层裂实验.采用超高速相机(采样频率:2 $\mu$s/frame)结合数字图像相关法(DIC),记录混凝土试件中的动态位移场实时变化情况,探讨了混凝土在拉伸断裂过程中的表面位移场及速度场演化规律.针对实验中出现的多重层裂现象,基于一维应力波传播理论,指出各个位置在发生层裂时,其最大拉应力均由透射压缩波与反射拉伸波叠加而成,各处层裂发生时均处于一维应力状态.并提出了根据层裂位置左右两点速度趋势变化判断层裂发生时刻的判据.该判据可以给出所有层裂的起裂时间,结合DIC分析直接给出了混凝土多重层裂应变.结果显示混凝土的拉伸强度具有明显的应变率效应,在30 s$^{-1}$的应变率下,其拉伸强度的动态增强因子(DIF)可以达到5.与传统的波叠加法和自由面速度回跳法相比,DIC全场分析法不受加载波形限制,可以精确给出每个层裂的位置和起裂时间,从而得到试件在高应变率加载下不同位置处的断裂应变、拉伸强度及相应应变率,提高了测量效率. 

FULL FIELD DIC ANALYSIS OF ONE-DIMENSIONAL SPALL STRENGTH FOR CONCRETE 1)

The one-dimensional stress spalling experiment of concrete bar was carried out based on a $\varPhi $74 mm SHPB experimental platform. The displacement and velocity field on the surface of concrete bar were measured by using digital image correlation method (DIC), which can digitalize and calculate the photos of movement of specimen recorded by an ultra-high speed video camera with the high-resolution sampling rate of 2 $\mu $s/frame. The strain field also can be achieved by DIC method. The analyzed results of displacement and strain fields show that multiple spalling occurs in a sequence of time near the far end of the concrete bar. It has been confirmed that the concrete bar stays in the one-dimensional stress state when the fracture occurs for that the tensile stress of each position is superimposed by the transmission compression wave and the reflection tensile wave, so that the one-dimensional stress wave propagation analysis can be applied. We put forward a criterion for judging the occurring moment of the spall according to the change of the velocity trend of the two point across the crack position. The criterion can give the starting time of all the spall cracks, and for each spalling crack the tensile failure strain, failure strength, and the strain rate is determined directly. The results show that the tensile strength of the concrete bar exhibits a strong strain rate dependency, with the dynamic increase factor (DIF) reaching 5 as the strain rate is 30 s$^{-1}$. Compared with the traditional methods (wave superposition method, pull-back method), the DIC full-field analysis method, which is not limited by the loading waveform, can give the exact starting time of each crack position. Therefore, it is possible to analyze the fracture strain and strain rate of the specimen at different positions, where have different strain rates, under higher strain rate loading.