Fractal dimension analysis of concrete specimens under different strain rates

To analyze the relationship between the dynamic mechanical performance and fractality of concrete, the specimens of a separate Hopkinson pressure bar (SHPB) from a concrete component using the coring method were obtained. This study designed three common concrete strength grades, C40, C50, and C60 and conducted dynamic splitting tensile tests, together with scanning electron microscopy (SEM) tests on these specimens. Based on the experimental results, the variations in the dynamic splitting strength (DSTS), dynamic increase factor (DIF), and energy absorption (EA) efficiency were analyzed. A semi-empirical formula for a unified DIF value was proposed. A digital image algorithm based on fractal theory was designed to evaluate the fractal dimension (FD) of the impact damage area crack pattern, and regression models were established between strain rate, DIF, and FD. Based on SEM tests, the relationship between dynamic splitting of the core sample and microstructure was analyzed, and a model was established between FD of SEM image and strain rate. The test results demonstrated that the dynamic mechanical performance of concrete has a significant strain-rate effect. The damage characteristics of concrete under impact can be quantitatively evaluated by the FD characteristics under impact loads, and there was a significant correlation between the strain rate, DIF, and crack FD. The internal and interaggregate cracking of the core samples increased and expanded continuously. The change in microstructure was consistent with the changes in the macroscopic failure modes, and FD of the specimen pore area increased with an increase in the strain rate. The research results organically linked FD and dynamic performance, failure mode, SEM images, and other mechanical attributes, providing a reference for using image information to study the dynamic mechanical performance of concrete materials.