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In this paper, the influence of different fiber materials on the dynamic splitting mechanical properties of concrete was investigated. Brazil disc dynamic splitting tests were conducted on plain concrete, palm fiber-reinforced concrete, and steel fiber-reinforced concrete specimens using a split Hopkinson pressure bar (SHPB) test device with a 100 mm diameter and a V2512 high-speed digital camera. The Digital Image Correlation (DIC) technique was used to analyze the fracture process and crack propagation behavior of different fiber-reinforced concrete specimens and obtain their dynamic tensile properties and energy dissipation. The experimental results indicate that the addition of fibers can enhance the impact toughness of concrete, reduce the occurrence of failure at the loading end of specimens due to stress concentration, delay the time to failure of specimens, and effectively suppress the expansion of cracks. Steel fibers exhibit a better crack-inhibiting effect on concrete compared to palm fibers. The incident energy for the three types of concrete specimens is roughly the same under the same impact pressure. Compared with plain concrete, the energy absorption rate of palm fiber concrete is decreased, while that of steel fiber concrete is increased. Palm fiber-reinforced concrete and steel fiber-reinforced concrete have lower peak strains than plain concrete under the same loading duration. The addition of steel fibers significantly impedes the internal cracking process of concrete specimens, resulting in a relatively slow growth of damage variables.

This study explored the dynamic behaviors and fracturing mechanisms of flawed granite under split‐Hopkinson pressure bar testing, focusing on factors like grain size and flaw dimensions. By means of digital image processing and the discrete element method, Particle Flow Code 2D (PFC2D) models were constructed based on real granite samples, effectively overcoming the limitations of prior studies that mainly relied on randomized parameters. The results illustrate that the crack distribution of granite is significantly influenced by grain size and flaw dimensions. Tension cracks predominate and mineral boundaries, such as between feldspar and quartz, become primary crack sites. Both flaw length and width critically affect the crack density, distribution, and dynamic strength of granite. Specifically, dynamic strength tends to decrease with the enlargement of flaws and increase with an increase in flaw angles up to 90°. This research examines granite's mechanical properties under split‐Hopkinson pressure bar testing, focusing on factors such as grain size and flaw dimensions. The study reveals that tension cracks are prevalent across grain sizes, with most intergrain cracks occurring between feldspar and quartz. Larger flaws in granite seem to act as stress‐relief channels, leading to reduced crack density. Highlights This research uses digital image processing and the discrete element method to accurately create granite models, capturing their authentic mineral content and spatial distributions. This study explores the mechanical properties and fracture mechanisms of granite subjected to dynamic loadings, highlighting critical factors including grain size, and the orientation and size of flaws. Analysis of correlation coefficients between various factors offers in‐depth insights into their individual and combined impact on the behaviors of granite under dynamic conditions.

Granite, as a typical anisotropic hard and brittle rock, is generally in a uniaxial, biaxial, or triaxial state of stress in deep underground engineering and is frequently influenced by extreme dynamic loads including earthquakes, occasional burst, and excavation disturbance. A series of dynamic compression tests were conducted on granite using a true triaxial split Hopkinson pressure bar (SHPB) system on initially anisotropic granite. Using X-ray three-dimensional (3-d) computed tomography (CT) scanning technology and ultrasonic testing, the mechanical properties and fracture behaviors of anisotropic granite under combined dynamic and static loading conditions were studied. Results show that the failure mode of granite changes from damage, to splitting failure, and finally to crushing failure with the rising strain rate under dynamic uniaxial compression. In addition, the axial prestress increases the proportion of transmitted energy in the rock and significantly influences failure strain of the rock. Under biaxial static prestresses, the granite is split along the direction of its free faces and the failure mode is affected by the value of the anisotropy index in the direction of free faces. Under the triaxial static prestresses, the rock is very unlikely to be damaged and the damage effect occurs only after multiple impacts. In the same prestress state, the strain rate of granite increases with the increasing impact velocity and the prestress constraint reduces the dynamic strain rate in the granite. The initial anisotropy of the longitudinal wave velocity of granite is gradually enhanced as the cyclic impact continues. Increasing the strain rate will induce more significant evolution of initial anisotropy of granite, while the prestress constraint restricts the development of initial anisotropy. The results improve understanding of the evolution of impact damage and the dynamic failure characteristics of anisotropic rocks under high geostress.HighlightsConducted a series of dynamic fracture tests of granite under unprestressed, uniaxial, biaxial and triaxial prestressed conditions.Revealed the dynamic impact damage evolution mechanism of granite under multi-axial stress constraints from a microscopic perspective.Investigated the confining pressure effect and strain rate effect on dynamic damage anisotropy of granite under multi-axial stress constraint.

The deep fissured rock mass is affected by coupled effects of initial ground stress and external dynamic disturbance. In order to study the effect of internal flaw on pre-stressed rock mechanical responses and failure behavior under impact loading, intact granite specimens and specimens with different flaw inclinations are tested by a modified split Hopkinson pressure bar (SHPB) and digital image correlation (DIC) method. The results show that peak strain and dynamic strength of intact specimens and specimens with different flaw angles (α) decrease with the increase of axial static pressure. The 90° flaw has weak reduction effect on peak strain, dynamic strength and combined strength, while 45° and 0° flaws have remarkable reduction effect. Specimens with 90° flaw are suffered combined shear and tensile failure under middle and low axial static pre-stresses, and suffered shear failure under high axial static pre-stresses. Specimens with 45° and 0° flaws are suffered oblique shear failure caused by pre-existing flaw under different axial static pre-stresses. Besides, based on digital image correlation method, it is found that micro-cracks before formation of macro fractures (include shear and tensile fractures) belong to tensile cracks. Tensile and shear strain localizations at pre-existing flaw tip for specimen with 45° and 0° flaws are produced much earlier than that at other positions.

In protective applications, polyurethane (PU) is a key material, yet the microstructural mechanisms governing its dynamic mechanical properties are not well understood. This study investigates the influence of hard segment content on the low strain rate compression and high strain rate impact properties of waterborne polyurethane (WPU) by modulating the NCO/OH ratio. Mechanical responses are characterized using a universal testing machine and a split Hopkinson pressure bar (SHPB) system. Additionally, the hydrogen bonding and microphase separation structure are analyzed using FTIR, DSC, DMA, and SAXS. These findings reveal that the glass transition temperatures (TgDSC and TgDMA) shift toward higher temperatures with increasing hard segment content, which is attributed to the intensified hydrogen bonding cross‐linked network, as corroborated by FTIR and SAXS analyses. The WPU demonstrates a pronounced strain rate sensitivity across a broad range of strain rates (10−4–104 s−1). Notably, the 45 wt.% hard segment WPU523 sample shows heightened sensitivity, attributed to complex hydrogen bonding heterogeneity and a higher Herman's orientation factor during loading, the key to WPU's dynamic mechanical response. This study explores how hard segment content (HS) modulates waterborne polyureathane's (WPU's) dynamic mechanical properties under low/high strain rates. Increasing HS enhances hydrogen bonding and microphase separation, with abundant hard domains acting as physical crosslinks. However, optimal HS content improves strain‐rate sensitivity, driven by HS‐regulated physical crosslinks balancing structural reinforcement and chain mobility, thereby promoting molecular chain alignment.

In cold climate regions, rock engineering structures are subjected to repeated processes of freeze–thaw weathering and consequently the integrity of these structures will gradually deteriorate. The resulted reduction in rock strength makes the structures become increasingly more vulnerable to external loads, particularly to dynamic loads such as blasting or earthquakes, even when these loads are below the original designed capacity. In this work, the reductions in static and dynamic strengths of sandstones after they are treated with different number of freeze–thaw cycles were studied using conventional UCS experiments and impact tests with split Hopkinson pressure bar apparatus. Based on the experimental results, a decay model was used to describe the reduction of rock strength with the increasing number of freeze–thaw weathering cycles. For the prediction of the degradation of dynamic rock strength corresponding to freeze–thaw weathering, a model describing the dynamic increase factor for the dynamic rock strength corresponding to different strain rates and specimen sizes was proposed and its parameters are obtained by regression analysis of published experimental data. These two models were then combined into a unified model which can be used to describe the reduction in the dynamic strength of rocks when they are subjected to repeated freeze–thaw weathering processes. Though only tested on sandstones, the proposed unified model, with different parameters, is expected to be applicable to other types of rocks as long as the rocks undergo the same or similar damage mechanism when they are subjected to freeze–thaw weathering processes.

The mechanical properties of rocks are significantly affected by chemical corrosion. To explore the influence of chemical corrosion on the weakening laws of sandstone mechanical properties, the porosity and pore size distribution (PSD) of sandstone samples immersed in different chemical solutions was measured by the nuclear magnetic resonance (NMR) technique. The damage variable based on the change of porosity was proposed to analyse the chemical damage to the sandstone samples. Moreover, both compressive and Brazilian tensile tests under static and dynamic conditions were carried out using a conventional servo-controlled testing machine and a split Hopkinson pressure bar (SHPB) system. The results showed that the porosity and proportion of macropores of the sandstone increase after chemical corrosion. The weakening laws of compressive and tensile strength of the sandstone under static and dynamic states are similar, and the relations among them and the damage variable are exponential. The dynamic tensile strength is most sensitive to the effects of chemical corrosion. The order of the degree of damage of chemical solutions on mechanical properties of sandstone is: DH2SO4 > DNaOH > DDistilledwater. Based on the experimental data, the relationships between the mechanical properties and chemical damage variable can be described as exponential equations. Additionally, the variations of dynamic increase factors versus chemical damage variable, the relationship between PSD and the strength of the chemically corroded sandstone, and the corrosion mechanism are also investigated.

Deep coals are mostly in a three-dimensional (3D) unequal stress state. In the hope of revealing dynamic characteristics of deep coals under impact load, dynamic failure experiments were performed on coal samples under dynamic load through the triaxial Split–Hopkinson pressure bar test system. In addition, the dynamic characteristics of coal in the multi-axial pre-stress state were analyzed by sampling the signals of incident, reflected and transmitted waves. Moreover, the relationships of dynamic peak strength, macroscopic fracture morphology of coal with axial pressure and confining pressure were explored. The following conclusions were drawn: The 3D pre-stress state exerts an obvious constraint on the dynamic failure of coal. With increase in strain, the dynamic initial stress of coal increases linearly first, and then grows at a reduced rate until the peak strength. After that, the mechanical curve rebounds notably. With increases in vertical force σ 2 and horizontal force σ 3 , the dynamic strength of coal increases gradually. Under uniaxial impact, coal is broken into particles or powder, while in 3D pre-stress state, coal presents macroscopic fracturing. The dynamic strength factors of coal vary obviously with the increase of confining pressure. The research results can provide reference for the study on dynamic characteristics of coal under multi-axial constraints and for the prevention and control of dynamic disasters induced by dynamic loads in deep coals.

This paper presents the results of an experimental study on the dynamic fracture behaviour of 3D printed rock-like disc specimens with various pre-existing flaw configurations under high strain rate loading. The 3D printing technology is utilized to prepare disc specimens containing a single or a pair of unfilled or filled flaws. A split Hopkinson pressure bar is employed to generate high rate loading on the specimens, while the digital image correlation (DIC) technique is adopted to determine the type of new cracks, and their initiation, propagation paths and coalescence types. The results show that the dynamic strengths of the 3D printed specimens are higher than the quasi-static ones. When under high strain rate loading, not only can the specimens with filled flaws carry more load than the corresponding specimens with an unfilled flaw, but also their cracking pattern is different as compared to the unfilled flaw counterpart. It is interesting to note that the dynamic peak loads are not dependent on the flaw inclination angle, while the quasi-static peak loads show obvious flaw inclination angle dependence. Moreover, DIC results reveal that under some specific flaw configurations, the filling material undergoes shear strain concentration and a shear band develops inside the filled flaws. Overall this study confirms the strong effects of the flaw configurations and filling material on the deformation and crack patterns of the 3D printed rock-like materials under impact loading.