Experiment and analysis on cumulative damage and fragmentation of granite under cyclic impact

Abstract

The dynamic characteristics of rock under cyclic impact loading exhibited nonlinear behavior and high strain rate effects. To investigate the response of granite under medium strain rate impact, cyclic impact tests were conducted for 100 × 100 × 100 mm cubic specimens using a drop hammer impact machine at varying heights. The stress–strain, macroscopic fragmentation, and energy dissipation characteristics were analyzed under cyclic impact. High-speed cameras and Digital Image Correlation (DIC) technology were employed to record and analyze the strain evolution and crack propagation on the specimen surfaces. Ultrasonic testing was used to measure the wave velocity of the specimens. The cumulative damage characteristics of the granite under cyclic impact were assessed. The fragmented rock pieces were sieved, and the fractal dimensions of the fragmentation were analyzed to characterize the failure patterns. The mesoscopic cracks of the fragments after impact were analyzed by SEM. The stress-time curve of the specimen comprised four stages: rising stage, plateau stage, secondary rising stage, and unloading stage. The stress–strain curve was divided into compaction stage, linear elastic stage, elastoplastic stage and failure stage. With the increasing cyclic impact number, the peak stress decreased, energy efficiency increased, and damage accumulated. Concurrently, the strain concentration area gradually increased and showed an obvious localization phenomenon. As the impact height increased, the crack propagation rate accelerated, its resistance to deformation diminished, and the number of internal fracture planes increased, leading to a high fractal dimension of the rock fragmentation. With the increase of impact height, the specimens sequentially exhibited splitting failure, block failure, and pulverization. Meanwhile, the mesoscopic fracture of granite changed from being dominated by intergranular fracture to being dominated by transgranular fracture. The research results provide theoretical support and reference for stability assessments of rock engineering under impact loading.