Fractal Experiment Research Of Damaged Concrete Under Impact Loading

Concrete is a project material of extensively application. There are very important meanings to study the microstructural characteristic of damaged concrete under impact load and explore the inherent laws between the fractal characters of concrete surface crack graph and its impact load. A series of impact load tests and theory analysis were carried on concrete samples, including four groups of C30 plain concrete samples and polypropylene fiber concrete samples, using SHPB device. Base on the tests, researchs were carried on concrete dynamic uniaxial compression stress and crack fractal dimension and analysis were carried on the laws between impact loading and fractal dimension of concrete surface crack. The primary research achievements are as following:1,According to the characteristic of the experiment of brittle materials such as rock and concrete, the SHPB was improved and the constitutive-curve of C30 plain and polypropylene fiber concrete were obtained.2,Experimental results indicate that concrete material have significant strain-rate effect. The strengthening mechanism of appropriate amount of incorporated polypropylene fibers is to restrict the crack from developing, and it can remarkably improve the toughness of concrete material.3,Summary and analysis was carried on the calculation method of commonly used fractal dimensions, and finally determined to use the box-counting dimension. According to the calculation method of box-counting dimension and digital image memory principles, a direct computational method of 2D digital image box-counting dimension was designed basing on the computation function of Matlab. This method establishes the foundation of exploring the inherent laws between the fractal characters of concrete surface crack graph and the impact load they endure.4,According to the experimental results, it is shown that the surface crack distribution of damaged concrete exhibits fractal properties, and it can be used as a description parameter of concrete damage degree. Concrete surface crackle fractal dimension is correlate to the pressure state, its values keep correlated with their impact loads. With the loads increase, fractal dimension value, strain-rate and dynamic uniaxial compression stress are crescent.5,According to the experiment, a quantitative description of dynamic uniaxial compression stress and fractal dimension was try, and quantitative relationship was fitted.