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

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.