Extended AbstarctIntroductionIn recent years, China's tunnel engineering has developed rapidly, and concrete lining support technology has been widely used in tunnels, underground engineering projects and deep foundation pits. However, with increasing buried depth and ground stress, ordinary concrete lining support technology often has certain limitations. It is easy to crack during service and shows obvious brittleness when destroyed, which poses a threat to the safety of tunnel engineering. Therefore, in order to prevent cracking, steel fibers with random distribution are usually mixed into concrete to achieve, and in the engineering practice of tunnel lining detection, engineers urgently seek to use high-precision non-destructive testing methods to detect the health status of concrete lining. Therefore, two non-destructive testing methods, Digital image correlation technology (DIC) and Ultrasonic technology (UT), are used to characterize the cracking damage process of concrete, and the strong correlation and complementarity of DIC and UT are verified based on the law of energy evolution.MethodsThe specimens were designed with steel fiber and silica fume content as the control variables, and a total of 9 groups of orthogonal mix specimens were designed for splitting mechanical tests, with a water-glue ratio of 0.43, a mass steel fiber content of 40, 45 kg/m³, and 50 kg/m³, and the mineral admixture silica fume replaced cement with the same amount according to different volume parameters (3%, 6%, 9%), and the dimensions of the 10 groups of specimens in this experiment were: 100 mm × 100 mm × 100 mm.In accordance with CECS13-2009 Fiber Concrete TestMethodStandard, the electro-hydraulic servo pressure testing machine (WAW-1000) is used in the splitting test, the maximum test load is 1000 kN for the control parameters, the stress control loading speed is 0.04 MPa/s, and the failure standard is set to 30% of the peak load. Place the steel gasket up and down at the centerline position of the specimen, and the size of the gasket is 200 mm × 5 mm × 5 mm.Results and discussionIn this paper, by using DIC and UT, based on the energy evolution law, the crack propagation process of SFRC lining under different mix ratios is verified, and the strong correlation and complementarity between DIC and UT test results are verified. The influence of steel fiber and silica fume on fracture mechanical properties is revealed. The main conclusions are as follows:The peak stress at 40, 45 kg/m³ and 50 kg/m³ was increased by 52.73%,81.82% and 73.03% respectively compared with DZ, respectively, that is, the cleavage strength increased significantly when the steel fiber content was 45 kg/m³, and StF45-SF6% showed good safety reserve and ductility, and the increase of silica fume volume content at 50 kg/m³ led to a significant increase in the slope behind the peak of the stress-strain curve, which was negatively correlated with the cleavage strength.According to the cloud map analysis of DIC, it is found that the cracking resistance of steel fiber on the main crack begins to increase after the stress reaches the 90%P_Max, and the crack mainly extends longitudinally. When the stress drops below 70%P_Max, the main fracture begins to expand laterally, and the inhibition effect of steel fiber across the fracture path gradually decreases to form macroscopic cracks. Under the same steel fiber content, the silica fume volume content was inversely proportional to the CTOD_c, that is, silica fume could provide additional viscosity and had a good hindrance effect on the expansion of CTOD.According to the acoustic characteristics such as waveform, first wave amplitude and wave velocity, it can be divided into: no damage period, microcrack propagation period, macroscopic crack propagation period, and damage stability period. Based on the principle of energy evolution, it is verified that DIC and UT have strong correlation and complementarity, and the energy storage level of SFRC K_d increased by 12.5%~52.5% compared with DZ, and steel fiber significantly reduced the release rate of elastic strain energy.ConclusionsThe main summary of this paper is as follows: SFRC has a strong post-cracking bearing capacity, with steel fiber content of 45 kg/m³ and silica fume content of 6%, the highest energy storage level and the splitting strength can be increased by 81.82%; Based on the cloud map analysis of DIC, the bridging effect of silica fume is mainly manifested before the 90% peak stress. After 90% peak stress, the crack resistance of steel fibers is gradually enhanced, and the cracks are mainly propagated by longitudinal microcracks. After the stress drops to 70% of the peak stress, the steel fiber crack resistance is gradually weakened, and the fracture begins to expand horizontally rapidly. In addition, the ultrasonic velocity of SFRC showed a trapezoidal hierarchical attenuation trend during splitting damage. Analysis based on the energy evolution law, UT and DIC have strong correlation and complementarity in characterizing the crack propagation process.