Impact tests on post-fire concrete confined by Carbon Fiber-Reinforced Polymer/Plastic (CFRP) sheets were carried out by using Split Hopkinson Pressure Bar (SHPB) experimental setup in this paper, with emphasis on the effect of exposed temperatures, CFRP layers and impact velocities. Firstly, according to the measured stress-strain curves, the effects of experiment parameters on concrete dynamic mechanical performance such as compressive strength, ultimate strain and energy absorption are discussed in details. Additionally, temperature caused a softening effect on the compressive strength of concrete specimens, while CFRP confinement and strain rate play a hardening effect, which can lead to the increase in dynamic compressive strength by 1.8 to 3.6 times compared to static conditions. However, their hardening mechanisms and action stages are extremely different. Finally, nine widely accepted Dynamic Increase Factor (DIF) models considering strain rate effect were summarized, and a simplified model evaluating dynamic compressive strength of post-fire concrete confined by CFRP sheets was proposed, which can provide evidence for engineering emergency repair after fire accidents.

This paper presents the investigation of the dynamic mechanical properties of coal rock under complex stress conditions at depth, based on the improved Separate Hopkinson Pressure Bar Test System. A total of 15 groups of coal samples were used to perform dynamic impact tests under different conditions. The changing rules of dynamic strength, crushing, fractal dimension and damage modes of coal under different stress conditions were analyzed. A total of nine groups of coal samples were selected for numerical simulation using ANSYS/LS-DYNA. The results show that: (1) The stress-strain curves of coal specimens under different strain rates, different confining pressures and axial pressures have basically the same trend and the curves show a certain jump forward. (2) The peak dynamic stress of the coal specimens increased linearly with the increase of strain rate and confining pressure, and the ambient pressure limited the expansion of internal cracks of the coal specimens under impact loading. Based on the experimental and simulated data, the maximum relative errors between the experimental and simulated data were determined to be 2.9578% for Group A, 6.177% for Group B, and 6.382% for Group C, respectively. (3) The damage modes of the coal samples under the three-dimensional dynamic-static combined loading were mainly \"X\" type and \"conical\" shear damage. The fractal dimension increases with the increase of strain rate, decreases with the increase of confining pressure, and first decreases and then increases with the increase of axial pressure. This research achievement can provide theoretical support for the prevention of dynamic disasters in deep coal mine engineering.

Characterized by light weight and high strength, composites are widely used as protective materials in dynamic impact loading under extreme conditions, such as high strain rates. Therefore, based on the excellent tensile properties of continuous fiber and the good flexibility and toughness of the bionic spiral structure, this study uses a multi-material 3D printer to incorporate continuous fiber, and then modifies the G-CODE file to control the printing path to achieve the production of a continuous fiber-reinforced Polylactic Acid composite helicoidal (spiral angle 60°) structure (COF-HP). Dynamic behavior under high-strain-rate impact experiments have been conducted using the Split Hopkinson Pressure Bar (SHPB). Stress-strain curves, impact energy curves and high-speed camera photographs with different strain rates at 680 s-1 and 890 s-1 have been analyzed to explore the dynamic process and illustrate the damage evolution. In addition, some detailed simulation models considering the incorporation of continuous optical fiber (COF) and different strain rates have been established and verified for deeper investigations. The results show that the COF does enhance the impact resistance of the laminates. When the porosity is reduced, the maximum stress of the continuous fiber-reinforced composite material is 4~7% higher than that of the pure PLA material. Our findings here expand the application of COF and provide a new method for designing protective materials, which have broad application prospects in the aerospace and automotive industries.

Coal seam drilling is a simple, economical, and effective measure commonly used to prevent and control rock burst. Following rock burst, coal exhibits significant dynamic characteristics under high strain-rate loading. Our purpose was to determine the physical processes associated with impact damage to drilled coal rock, and its mitigation mechanism. An impact test was carried out on prefabricated borehole coal specimens, and the impulse signals of the incident and transmission rods were monitored. The crack initiation, expansion, and penetration of coal specimens were video-recorded to determine the mechanical properties, crack expansion, damage modes, fragmentation, and energy dissipation characteristics of coal specimens containing different boreholes. The dynamic compressive strength of the coal specimens was significantly weakened by boreholes under high strain-rate loading; the dynamic compressive strength and the dynamic modulus of elasticity of coal rock showed a decreasing trend, with increasing numbers of boreholes and a rising and decreasing trend with increasing borehole spacing; the number and spacing of boreholes appeared to be design parameters that could weaken coal-rock material under high strain-rate loading; during the loading of coal and rock, initial cracks appeared and expanded in the tensile stress zone of the borehole side, while secondary cracks, which appeared perpendicular to the main crack, expanded and connected, destroying the specimen. As the number of boreholes increased, the fractal dimension (D) and transmission energy decreased, while the reflection energy increased. As the borehole spacing was increased, D decreased while the reflective energy ratio decreased and increased, and the transmissive energy ratio increased and decreased. Drilling under high strain modifies the mechanical properties of impact damaged coal rock.

To study the dynamic mechanical properties and damage evolution mechanism of Beishan deep granite under medium and high strain rates, dynamic mechanical tests for the deep granite specimens with different strain rates were conducted using the split Hopkinson pressure bar (SHPB) device. The improved Zhu-Wang-ang (ZWT) dynamic constitutive model was established, and the relationship between strain rate and strain energy was investigated. The test results show that the strain rate in the dynamic load test is closer to the strain rate in the rock blasting state when the uniaxial SHPB test is applied to the granite specimens in a low ground stress state. Peak stress has a linear correlation with strain rate, and the dynamic deformation modulus of the Beishan granite is 152.58 GPa. The dissipation energy per unit volume and the energy ratio increase along with the strain rate, whereas the dissipation energy per unit volume increases exponentially along with the strain rate. There is a consistent relationship between the damage degree of granite specimens and the dissipation energy per unit volume, which correspond to one another, but there is no one-to-one correspondence between the damage degree of granite specimens and the strain rate. To consider the damage and obtain the damage discount factor for the principal structure model, the principal structure of the element combination model was improved and simplified using the ZWT dynamic constitutive model. The change of damage parameters with strain rate and strain was obtained, and the dynamic damage evolution equation of Beishan granite was established by considering the damage threshold.

The mechanical properties and impact resistance of conventional self-compacting concrete (SCC) need to be further improved. In order to explore the dynamic and static mechanical properties of copper-plated steel-fiber-reinforced self-compacting concrete (CPSFRSCC), the static mechanical properties and dynamic mechanical properties of CPSFRSCC with a different volume fraction of copper-plated steel fiber (CPSF) are tested, and a numerical experiment is carried out to analyze the experimental results. The results show that the mechanical properties of self-compacting concrete (SCC) can be effectively improved by adding CPSF, especially for the tensile mechanical properties. The static tensile strength of CPSFRSCC shows a trend that increases with the increase in the volume fraction of CPSF and then reaches the maximum when the volume fraction of CPSF is 3%. The dynamic tensile strength of CPSFRSCC shows a trend that increases first and then decrease with the increase in the volume fraction of CPSF, and then reaches the maximum when the volume fraction of CPSF is 2%. The results of the numerical simulation show that the failure morphology of CPSFRSCC is closely related to the content of CPSF; with the increase in the volume fraction of CPSF, the fracture morphology of the specimen gradually evolves from complete fracture to incomplete fracture.

A series of conventional dynamic uniaxial compressive (CDUC) tests and coupled static dynamic loading (CSDL) tests were performed using a split Hopkinson compression bar (SHPB) system to explore the variable dynamic mechanical behavior and fracture characteristics of medium siltstone at a microscopic scale in the laboratory. In the CDUC tests, the dynamic uniaxial strength of the medium sandstone is rate-dependent in the range of 17.5 to 96.8 s-1, while the dynamic elastic modulus is not dependent on the strain rate. Then, this paper proposes a generalized model to characterize the rate-dependent strength from 17.5 to 96.8 s-1. In the CSDL tests, with increasing initial prestatic stress, the dynamic elastic modulus and dynamic strength increase nonlinearly at first and then decrease. The results show that two classical morphological types (i.e., Type I and Type II) are observed in the dynamic stress-strain response from the CDUC and CSDL tests. By scanning electron microscopy (SEM), microscopic differences in the post-loading microcrack characteristics in the behavior of Type I and Type II are identified. In Class I behavior, intergranular fracture (IF) usually initiates at or near the grains, with most cracks deflected along the grain boundaries, resulting in a sharp angular edge, and then coalesces to the main fracture surface that splits the specimen along the direction of stress wave propagation. In contrast, Class II behavior results from the combined IF and transgranular fracture (TF).

The development of island construction concrete can serve as a basis for the development and utilization of island resources. Complying with the principle of using local materials to configure seawater coral aggregate concrete (SCAC) that is able to meet the requirements of island and reef engineering construction could effectively shorten the construction period and cost of island and reef engineering construction. In this paper, quasi-static mechanical experiments and dynamic mechanical experiments were carried out on SCAC with different limestone powder contents. High-speed photography technology and Digital Image Correlation (DIC) were used to monitor the dynamic failure process and strain field of SCAC, and the influence of limestone powder content on the dynamic and static mechanical properties of SCAC was investigated. The results showed that, when the limestone powder content was 20% and 16%, the quasi-static compressive strength and quasi-static tensile strength exhibited the best improvement. Additionally, with increasing limestone powder content, the dynamic tensile strength of SCAC first showed and increasing trend and then a decreasing trend, reaching its maximum value when the limestone powder content was 16%. Moreover, the maximum strain value of SCAC with the same limestone powder content increased with increasing strain rate grade, showing an obvious effect on strain rate.

Studying the mechanical behavior of rocks under real-time high-temperature conditions is of great significance for the development of energy caverns, nuclear waste disposal projects, and tunneling engineering. In this study, a real-time high-temperature impact compression test was conducted on Sejila Mountain granite to explore the effects of temperature and external load on its mechanical properties. Based on the concepts of damage mechanics and statistics, a coupled thermal-mechanical (T-M) damage constitutive model was established, which considers the temperature effect and uses the double-shear unified strength as the yield criterion. The parameter expressions were clarified, and the accuracy and applicability of the model were verified by experimental data. The research results indicated that high temperatures had an obvious damaging and deteriorating effect on the strength of the granite, while an increase in impact velocity had an enhancing effect on the strength of the granite. The established constitutive model theoretical curve and test curve showed a high degree of agreement, indicating that the coupled T-M model can objectively represent the evolution process of damage in rocks and the physical meaning of its parameters is clear.

During the excavation of underground projects, the rock masses left as the bearing support system are also subjected to dynamic loads from the excavation activities ahead. These rock masses have been damaged and fractured during the initial exposure (dynamic loads) and are subjected to static loads in the subsequent process as the support system. In this study, granite rock samples and specimens with different angles were produced, preloaded with different confining pressure, and under a combination of dynamic and static loading tests using a modified dynamic and static loading system: split Hopkinson pressure bar (SHPB). The peak strain and dynamic modulus of elasticity are weakened by the inclination angle in a similar way to the strength, with the specimens showing an evolutionary pattern from tensile strain to shear damage. The change in the inclination angle of flaws would weaken the dynamic and combined strengths, and a larger inclination flaw results in a significant decrease in its strength. Fractal analysis revealed that the fractural dimension was closely related to the fissure angle and showed a good linear correlation with the strain rate. This study will provide an important security assurance for deep mining.