Split Hopkinson Pressure Bar System Research Articles

Design of a continuously repeated impact method with constant amplitude based on Hopkinson bar: Principle and impact fatigue life testing

The lack of reliable testing methods and standards hinders systematic research on the impact fatigue performance of materials. The aim of developing an impact fatigue test method is to generate repeated impact loads with a constant amplitude, which is difficult to achieve using various existing impact test devices. This study investigated the principles of cyclic loading-resetting and constant-amplitude loading based on the split Hopkinson pressure bar (SHPB) system, established a novel impact fatigue test device, and verified the developed method by testing the impact fatigue life of a Ti-6Al-4V alloy. Electromagnetic valves, laser sensors, and a vacuum pump were used in the developed device to launch and reset the striker bar; a servo motor and laser sensors were used to reset the loading bars and specimen, and the entire system was controlled by a programmable controller to achieve continuously repeated loading. To achieve constant-amplitude loading, the conditions required to ensure that the specimen moves away from the incident bar before the returning stress wave reloads the specimen was derived. It was found that a constant-amplitude repeated impact on the specimen could only be achieved through the precise design of the geometric configuration and material of the loading bars. This method is considerably simpler than various existing methods based on energy absorption and is more applicable to impact fatigue tests. The verification tests showed that the highest loading frequency of the device was 0.5 Hz, the loading rate exceeded 105 kN/s, and the amplitude error of the repeated impact loads did not exceed 2.45 %. Under loads with the same amplitude, the impact fatigue life of a Ti-6Al-4V alloy was considerably shorter than the non-impact (low strain rate) fatigue life, which indicates the necessity of investigating the strain rate effect on the fatigue performance of materials.

Study on the dynamic constitutive model of coal at different depths in Pingdingshan mining area

The environment of deep in-situ occurrence is complex, and secondary dynamic disasters occur frequently. Exploring the dynamic constitutive models of coal at different depths is the foundation for deep resource development, which can help promote the safe and efficient mining of coal resources at different depths and reveal the mechanism of sudden dynamic disasters. Dynamic mechanical testing of coal at different depths of the Pingdingshan mine (China) was carried out using a split Hopkinson pressure bar system with confining pressure. On the basis of statistical damage theory, dynamic constitutive models of coal at different depths were explored, and the theoretical models were compared and verified using the experimental results. An expression relating to the infinitesimal element strength of coal and loading rate was derived, and the parameters σ1dp, Ed, and ε1dp were determined. Inversion verification of the model showed that the experimental results agree well with the theoretical results prior to the peak stress. These research results are expected to be applied to the exploitation of deep resources, laying a foundation for the law of deep earth science and the construction of deep engineering.

Influence of GGBFS and alkali activators on macro-mechanical performance and micro-fracture mechanisms of geopolymer concrete in split Hopkinson pressure bar tests

With superior low-carbon emission and low-resource consumption, geopolymer concrete (GPC) is receiving increasing attention as an optimal alternative to conventional building materials for cleaner and sustainable construction. However, the practical application of GPC poses great challenges since its dynamic mechanical performance remains far from well understood. In this study, via a split Hopkinson pressure bar (SHPB) system, a series of dynamic compression tests at strain rates of 45 s-1 to 150 s-1are conducted on GPC specimens with different alkaline activators (AAS)-binder mass ratios (i.e., 10%, 15%, 20%, and 25%) and ground granulated blast furnace slag (GGBFS)-binder mass ratios (i.e., 0%, 30%, 50%, 70%, and 100%). Our testing results comprehensively explored the influence of mix proportions and loading rates on the dynamic mechanical performance of GPC, including the dynamic strength and deformation characteristics, energy evolution and utilization, and progressive failure behavior. The permanent strain, energy dissipation density, and dynamic strength of GPC specimens generally increase with increasing strain rate, while the average fragment size decreases. Under higher loading rates, GPC specimens are featured by denser microcracks, shorter propagation paths, and more prominent stepwise fractures. In addition, GGBFS has a greater impact on dynamic strength, strain rate sensitivity, and energy utilization than AAS. Micro-fracture effect of GPC with different mix proportions is investigated. An appropriate content of alkaline activator (i.e., 20%) and higher GGBFS content (i.e., 100%) promotes the binder depolymerization-aggregation reaction, reduces initial defects inside concrete specimens, and facilitates a more compact block structure.

Effects of strain rate and bedding on shale fracture mechanisms

Anisotropic shale is commonly distributed in nature and undergoes both dynamic compressive and tensile load in geological engineering. In this article, the coupling effects of strain rate and bedding structures on the fracture mechanisms of shale are investigated using a split Hopkinson pressure bar (SHPB) system. The tension–compression comparative study is conducted. The Brazilian disc method is modified, and a unified strain rate measurement method is established to comprehensively analyze shale’s fracture-related properties, including moduli, strength and energy dissipation. The results indicate that the energy dissipated by shale fracture is strongly related to its strength. The strain rate and bedding affect the anisotropic strength and dissipated energy by changing the crack density and crack propagation mode. At low or medium strain rates, the bedding orientation of shale determines the direction of crack propagation. However, at high strain rates, microcracks in different directions are widely activated in the shale, which increases the fracture degree and decreases the difference in fracture toughness between the shale’s matrix and bedding planes. This also leads to a decrease in anisotropy and causes the bedding planes to lose control over the direction of crack propagation. Additionally, the dynamic tensile strength of shale increases faster than the compressive one with the strain rate, leading to a reduction in the proportion of tensile failure under impact load. Based on the experimental findings, a new anisotropic damage constitutive model is developed to characterize the dynamic properties of shale. This model reflects the anisotropic damage evolution behaviors and aligns well with experimental results, offering a theoretical foundation for predicting shale’s dynamic fracture behavior. In addition to shale, the developed experimental methods and theoretical models can also be applied to the fracture analysis of other brittle transversely isotropic materials.

Effects of strain rate on dynamic deformation behavior and microstructure evolution of Fe-Mn-Cr-N austenitic stainless steel

In this work, the effects of strain rate on dynamic hot deformation behavior and microstructure evolution of Fe-Mn-Cr-N steel were studied under quasi-static and dynamic loading conditions using a Gleeble-3800 thermal-mechanical simulator and Split-Hopkinson pressure bar (SHPB) system. The stress-strain curves show that the yield and flow stresses are both significantly dependent on strain rate, also the strain rate sensitivity increases from 0.050 at a low strain rate (0.001–0.1 s−1) to 1.507 at a high strain rate (4500–5500 s−1). At a low strain rate (0.001–0.1 s−1), the microstructure evolution is dominated by dislocation slip and dynamic recovery (DRV). When strain rates are increased to 2500–3500 s−1, dislocation slip and mechanical twining together coordinate plastic deformation. When the strain rate was 4500–5500 s−1, the dynamic recrystallization (DRX) based on twinning formed in the deformation bands, and its fraction increased with the increasing strain rate due to adiabatic temperature rise (ΔT). The high micro-hardness value was obtained mainly because of the strain rate hardening (2500–3500 s−1), but the decrease at the strain rate of 4500–5500 s−1 is primarily attributed to recrystallization softening after deformation. The modified Johnson-Cook (J-C) model considering adiabatic temperature rise agrees with experimental results. It can be used to predict the plastic deformation behavior of Fe-Mn-Cr-N steel under high strain rate loading.

Effective characterization for the dynamic indentation and plastic parameters acquisition of metals

As a fundamental test technique for the mechanical properties of materials, indentation shows broad application prospects. Compared to the well-studied quasi-static indentation, relatively few and incomplete studies on the dynamic indentation test and characterization have been performed. In this work, a dynamic indentation test technique based on the split Hopkinson pressure bar system is developed, and the corresponding “three-wave method” is proposed to effectively acquire the time-resolved dynamic indentation load and depth. Systematic indentation tests under different loading rates and indenter cone angles are conducted on typical metal materials such as Cu, α-Fe, and α-Ti. Based on the dynamic indentation contact stiffness analysis and indentation theory the indentation hardness under different loading conditions are determined. A procedure for plastic parameter inversion of metal materials, including strain hardening and strain rate strengthening terms based on dynamic indentation, is provided. The effectiveness is verified by the traditional compression test results, providing a new method for characterizing the dynamic mechanical properties of metal materials.

Physical and mechanical properties of deflection-hardening hybrid fibre-reinforced recycled aggregate concrete

Recycled aggregate concrete (RAC) has been gaining a large interest in recent years due to the depletion of natural aggregates and environmental protection requirements. To improve the ductility of RAC for structural applications and to avoid corrosion risks involved with utilising steel fibres, this study proposes a deflection-hardening hybrid fibre-reinforced recycled aggregate concrete (FRRAC) by combining newly developed macro-basalt fibre (BF, 0.67 % by volume) and recycled macro-polypropylene fibre (PF, 0.67 % by volume). Its fresh, static and dynamic mechanical properties were investigated and compared with the mix containing macro-BF only (1.34 % by volume). A Ø 100 mm split Hopkinson pressure bar (SHPB) system was used to determine the dynamic mechanical properties in compression and tension. The hybrid FRRAC retained up to 90 % of its average first peak load at 3.5 mm crack width and showed better cracking bridging performance under dynamic splitting tension than the specimens with the same volume of macro-BF. Empirical formulae for dynamic increase factors and energy absorption capacities against varying strain rates have also been proposed for the newly developed hybrid FRRAC.

Dynamic compressive properties of UHPC under one-dimensional stress wave: The influence of steel fiber morphology and curing regime

The low matrix strength and insufficient toughness of conventional concrete structures seriously threaten the service performance when subjected to intensive compressive waves. This study aims to comprehensively investigate the impact of steel fiber morphology and curing regime on the dynamic compressive properties of Ultra-high performance concrete (UHPC) under one-dimensional stress wave. Seeking to improve its impact resistance without changing the mix ratio. Three steel fiber types (straight, wave, hooked) and two curing regimes (steam at 90 °C, dry heat at 180 °C) were selected as experimental variables. Quasi-static mechanical properties of UHPC were initially calibrated, followed by dynamic compressive tests utilizing the Split Hopkinson Pressure Bar (SHPB) system. The compressive stress-strain relationships and corresponding dynamic mechanical parameters of six types of UHPC were calculated under five impact velocities (8.5 m/s, 10.8 m/s, 13.1 m/s, 15.2 m/s, 17.3 m/s). Furthermore, the dynamic failure mode and evolutionary process of UHPC under high-speed impact was meticulously captured by a high-speed camera. CT scanning was utilized to observe the crack propagation path and damage patterns under low strain rate. Finally, SEM and EDS testing was conducted to unravel the underlying mechanisms of the curing regime and fiber morphology on the dynamic compressive properties of UHPC. The recalibrated empirical DIFfc model suitable for UHPC under strain rates of 37 s-1 to 212 s-1 was proposed.

Hybrid Fabrication of Cold Metal Transfer Additive Manufacturing and Laser Metal Deposition for Ti6Al4V: The Microstructure and Dynamic/Static Mechanical Properties.

The titanium alloy components utilized in the aviation field are typically large in size and possess complex structures. By utilizing multiple additive manufacturing processes, the precision and efficiency requirements of production can be met. We investigated the hybrid additive manufacturing of Ti-6Al-4V using a combination of cold metal transfer additive manufacturing (CMTAM) and laser metal deposition (LMD), as well as the feasibility of using the CMT-LMD hybrid additive manufacturing process for fabricating Ti-6Al-4V components. Microstructural examinations, tensile testing coupled with digital image correlation and dynamic compressive experiments (by the split Hopkinson pressure bar (SHPB) system) were employed to assess the parts. The results indicate that the interface of the LMD and CMTAM zone formed a compact metallurgical bonding. In the CMTAM and LMD zone, the prior-β grains exhibit epitaxial growth, forming columnar prior-β grains. Due to laser remelting, the CMT-LMD hybrid additive zone experiences grain refinement, resulting in equiaxed prior-β grains at the interface with an average grain size smaller than that of the CMTAM and LMD regions. The microstructures reveal significant differences in grain orientation and morphology among the zones, with distinct textures forming in each zone. In the CMT-LMD hybrid zone, due to interfacial strengthening, strain concentration occurs in the arc additive zone during tensile testing, leading to fracture on the CMTAM zone. Under high-strain-rate dynamic impact conditions, the LMD region exhibits ductile fracture, while the CMTAM zone demonstrates brittle fracture. The hybrid zone combines ductile and brittle fracture modes, and the CMT-LMD hybrid material exhibits superior dynamic impact performance compared to the single deposition zone.

Incorporation of Disposed Face Mask to Cement Mortar Material: An Insight into the Dynamic Mechanical Properties

Incorporating masks into building materials offers a potential solution to the environmental threat of disposable masks with promising material performance. However, research on their dynamic properties is lacking to further determine the application range of the new composite. This study addresses this gap by shredding face masks into strips and incorporating them into mortars at varying volume ratios. The integrity and compactness of the mortar was measured and characterized by P-wave velocity, while dynamic compression properties were explored using a split Hopkinson pressure bar (SHPB) system. Subsequently, sieve analysis was conducted on the fractured specimens. The results indicate that incorporating masks generally improves the mortar integrity and the fragmentation after impacting. The dynamic uniaxial compression strength (DUCS) decreased for all mixing designs compared to plain ones under a constant loading rate. Meanwhile, the dissipated energy density showed a similar trend to the P-wave velocity, exhibiting less pronounced enhancement at higher loading rates. According to the dynamic characteristics, a dynamic constitutive model based on the Lemaitre principle and Weibull distribution of damage is developed and validated. The test results are further understood through the perspective of the mechanism of mask inclusion.

Lightweight ambient-cured geopolymer composite with expanded clay: quasi-static and dynamic properties

The increased awareness of sustainability and concerns about global warming have prompted the construction industry to explore green sustainable materials such as geopolymer composites as an eco-friendly alternative to ordinary Portland cement (OPC). This study developed ambient-cured lightweight geopolymer composites (LWGPCs) by incorporating different volume fractions of expanded clay (EC). The physical and quasi-static characteristics of LWGPCs with various EC contents were investigated. Empirical formulae for the quasi-static compressive and splitting tensile strength, as well as the elastic modulus of LWGPCs were proposed. A Ø 100-mm split Hopkinson pressure bar (SHPB) system was employed to investigate the dynamic compressive and splitting tensile properties of LWGPCs. The failure processes and failure patterns of LWGPCs, as well as the stress–strain curves and the energy absorption capacities under different strain rates were compared. Based on the testing results, empirical formulae of the DIF (dynamic increase factor) for LWGPCs compressive and split tensile strength as well as the energy absorption capacities were proposed. This study offers valuable insights into the material performance of ambient-cured LWGPCs with EC as sustainable materials under quasi-static and dynamic loading, paving the way for further developing and application of lightweight and sustainable engineering materials in construction.

Experimental study on dynamic fracture of UHPC-NC specimens after high temperature burning treatment

For the ultra-high performance concrete (UHPC) overlaying normal concrete (NC) structures subjected to high-temperature burning, the reliable interface performance between UHPC and NC is the foundation for achieving their good co-work behavior. This paper focused on experimental and theoretical research on crack propagation at the interface of these UHPC-NC configurations under stress waves. A bi-material notched semi-circle bend (BNSCB) configuration was proposed, and the impact experiments were conducted on the BNSCB specimens using a split Hopkinson pressure bar system (SHPB). Three temperatures (20℃, 100℃, and 300℃) were designed in this test, examining the interfacial impact performance of the BNSCB specimen subjected to the high-temperature burning. Based on the finite element method and crack propagation gauge, the dynamic fracture toughness at the crack tip of the interface of the BNSCB specimen was calculated. The results show that: (1) the K1 values for the untreated BNSCB specimens increased by 23.3% and 22.6%, respectively, compared to the heat-treated BNSCB specimens. (2) the parameter K2 at the crack tip of the BNSCB specimen is much smaller than K1, and the parameter K1 plays a dominant role in crack initiation; (3) with the increase in loading rate from 580 GPa/s to 740 GPa/s, both untreated and heat-treated BNSCB specimens show significant improvement in the parameters G and K, with a more pronounced improvement observed in untreated compared to heat-treated BNSCB specimens.

Determination of critical energy for coal impact fracture under coupled static-dynamic loading

Deep coal is constantly subjected to high static loading, and the accompanying mining disturbances will exacerbate the energy changes in the coal specimen. This study utilized the Split Hopkinson Pressure Bar (SHPB) system to examine the critical energy required for coal impact fracture under one-dimensional (1D) and three-dimensional (3D) coupled static-dynamic loading. The results revealed that a sufficiently large impact disturbance was required to cause deformation of the coal specimen when the initial static energy storage was low. Additionally, the coal with higher initial static energy storage was more likely to fracture under minor impact. Compared to the 1D loading, the 3D loading resulted in a higher critical impact energy and velocity for intact coal to fracture. Additionally, a model was developed to demonstrate the inherent energy evolution of coal specimen breakage under coupled static-dynamic loading, based on the relationship between energy and coal specimen deformation. Reducing the initial energy storage, weakening the dynamic absorbed energy, and increasing the critical instability energy of the coal matrix were the primary recommendations for disaster prevention. And corresponding prevention and control measures are suggested. This research can provide a scientific foundation and guidance for predicting dynamic disasters in order to ensure safe and efficient mining engineering.

Hybrid fibre reinforced recycled aggregate concrete: dynamic mechanical properties and durability

Research into recycled aggregate concrete (RAC) has been on the rise as there is a need to preserve natural resources and promote sustainable infrastructure practices. In our previous study, a new hybrid-fibre reinforced recycled aggregate concrete (FRRAC) with micro basalt fibre (BF) and recycled macro polypropylene fibre (PF) was developed and its physical and mechanical properties were investigated. As a continuous work, this study investigated the dynamic mechanical properties and durability of the developed FRRAC. Dynamic compressive and splitting tensile strength tests were undertaken using a Ø 100 mm split Hopkinson pressure bar (SHPB) system. The failure processes and failure modes of RAC and FRRAC specimens under different strain rates were recorded using a high-speed camera. For dynamic compressive strength, the stress-strain curves, dynamic increase factor for compressive strength (DIFf’c) and energy absorption capacities of RAC and FRRAC were evaluated and compared in the strain rate range from around 27.9 s-1 to 138.0 s-1. The stress-time histories, crack-opening displacement values and dynamic increase factor for splitting tensile strength (DIFfst) at the strain rates of around 3.03 s-1 to 8.17 s-1 of the two materials were obtained and compared. The test results indicated that RAC specimens were slightly more strain rate sensitive than natural aggregate concrete (NAC) under dynamic compression and splitting tension. On the other hand, FRRAC specimens showed similar strain rate sensitivity as their counterparts with no fibres under dynamic compression but showed lower DIFfst values with increasing strain rate under splitting tension. Empirical formulae for DIFf’c, DIFfst and energy absorption capacities against strain rates were proposed. In addition, the durability performance was assessed by examining the water absorption, volume of permeable voids (VPV), water sorptivity and chloride ion penetration resistance. Durability test results indicated that RAC and FRRAC in this study are suitable for structural applications.

High-speed perforation of high-entropy alloy CrMnFeCoNi plates: Experiments and modeling

The perforation behavior of 2-mm thick CrMnFeCoNi high-entropy alloy plates is investigated via ballistic impact experiments using spherical stainless steel 304 projectiles with high-speed photography at impact velocities ranging from 500 ms−1 to 2213 ms−1. Postmortem target samples are characterized with macro photography, three-dimensional laser scanning, optical metallography, scanning electron microscopy, electron backscatter diffraction and microhardness tests. The absorbed energy and aperture area show a nearly linear increase with increasing the initial kinetic and absorbed energy, respectively. Dislocations, twins and kink bands are the main microstructural deformation modes. As the distance away from the bullet hole sidewall, both the deformation degree and hardness decrease. Besides, shear-dominated and tensile/shear-dominated fracture are found in the middle part and near the rear side of the bullet hole sidewall, respectively. Parameters of the Johnson–Cook(JC) constitutive model are obtained via the split Hopkinson pressure bar system. A finite element method (FEM) model comprised of the JC constitutive and JC progressive damage model is established and reproduces experimental observations.

Investigation on mechanical properties of Ti-6Al-4 V multilayer micro-lattice biomaterials under dynamic compression loading

Novel skull repair Ti-6Al-4 V multilayer micro-lattice biomaterials (MMBs) proposed recently have promising application in clinical practice. Herein, the dynamic compression performances of MMBs are concerned and investigated by Split Hopkinson Pressure Bar (SHPB) system and finite element (FE) methods. Typical shear band, strut yielding and damage deformation modes of MMBs are characterized by FE and SHPB experiment. Through density gradient regulation, deformation mode of MMBs is transformed from shear band to localized collapse pattern. It is interesting that MMBs with different unit cell sizes exhibit different strain rate sensitivities. For strain rate-insensitive MMBs (BCC(3), BCC(5) and BCCZ(3)), the influence of relative density on strength shows a linear increasing trend under both low and high strain rates. For strain rate-sensitive ones (BCC(7), BCCZ(5) and BCCZ(7)), the influence of relative density on strength under low strain rate is stronger than that under high strain rate. The strength range of MMBs matches skull/bone materials well under same loading strain rate. This work not only reveals dynamic mechanical properties of MMBs but also has enlightenment for investigating dynamic properties of porous metamaterials.

Dynamic compression responses of heterogeneous-structured CrMnFeCoNi high-entropy alloy at cryogenic temperatures

Dynamic compression experiments are conducted on a heterogeneous-structured CrMnFeCoNi high-entropy alloy (HEA) at different strain rates (1000–5000 s−1) and temperatures (123–293 K) with a split Hopkinson pressure bar system. Yield strength and strain hardening rate increase considerably with decreasing temperature at 5000 s−1 (low-temperature strengthening), but are insensitive to strain rate at 173 K, which can be well described by the Zerilli–Armstrong model. Compared to typical aluminum alloys and homogeneous-structured CrMnFeCoNi HEA, the heterogeneous-structured HEA exhibits higher thermal sensitivity and strain hardening rate. Microstructural characterizations show that dislocation density in the fine-grained domains increases more drastically than in the coarse-grained domains, inducing high deformation gradient between the coarse- and fine-grained domains. At 123 K, nano-scale primary and secondary deformation twins form in grain interior, along with a great number of stacking faults on twin boundaries; the twin width and spacing in coarse grains are considerably higher than in fine grains. In addition, the twin density at 123 K is significantly higher than that at 293 K. Overall, the increased deformation twinning and deformation gradient contribute to pronounced strengthening and strain hardening with decreasing temperatures in the heterogeneous-structured HEA under dynamic compression.

A comparison of high strain rate response and adiabatic shear band formation in additively and traditionally manufactured Inconel 718

3D printing of Inconel 718 has been increasingly commercialized, as such research comparing the mechanical properties of additively manufactured (AM) and traditionally manufactured (TM) components in both quasi-static and high rate regimes should be conducted. Herein, the high strain rate material response, as well as the formation of adiabatic shear bands (ASBs) in AM and TM Inconel 718 are studied using a split Hopkinson pressure bar (SHPB) system. A top-hat cylindrical specimen geometry was used to convert the compressive load to shear and conduct high rate shear tests in the SHPB. The quasi-static and high rate stress-strain data reported herein indicates that TM components have at least a 70 MPa higher compressive yield strength. Interestingly, the AM specimens were more susceptible to ASB formation than their TM counterparts. The critical shear strain at which an ASB forms was higher for the TM specimens compared to the AM specimens. This was attributed to the TM material having a higher critical stress for dynamic recrystallization initiation, which is required to ASB formation. The microstructures of the shear zone in the top-hat specimens were examined using an optical microscope (OM), scanning electron microscope (SEM), and transmission electron microscope (TEM). Dynamic recrystallization was observed at the center of the shear affected zone (SAZ) using TEM from material lifted out from the ASB. Due to the plastic deformation in the shear region, the material strain hardens and therefore has a higher hardness than the bulk material. At the center of the shear region, a peak in nano-hardness of 585 HV was recorded on the ASB. The outcome of this research will lead to the prevention, containment, and control of shear instability in AM structures.

Static compression behavior and strength weakening mechanism of dynamically damaged granite after water soaking

In backfill mining, the surrounding rock is dynamically damaged by intermittent mining and blasting. After backfilling, bleed water from the backfill seeps into the surrounding rock, leaving the surrounding rock in a water-rock coupling environment. To study the mechanical properties of surrounding rock in such a scenario, two types of granite specimens were prepared, namely, impact-damaged (ID), impact-damaged and water-soaked (IDWS) specimens. A modified split Hopkinson pressure bar (SHPB) system was first applied to execute the dynamic tests with different impact numbers and confining stresses to prepare the ID specimens. Subsequently, the IDWS specimens were prepared by water soaking tests on half numbers of the ID specimens. Finally, static uniaxial compression tests were performed on ID and IDWS specimens. The test results indicate that the peak strength of ID and IDWS specimens increases with the increase of confining stress, and first increases and then decreases with the increase of number of impacts. The total strength weakening factor of the IDWS specimens is composed of the impact strength weakening factor and the water-induced strength weakening factor, as well as the coupling effect of the two factors. Impact loading increases the number of microcracks and pores inside the IDWS specimens, resulting in a larger contact area between water and microcracks and pores. Compared with ID specimens, water induces more tensile cracks in the IDWS specimens according to the results of electron microscope scanning. This is manifested as the transfer from macro shear failure mode to macro tensile failure, and further reduces the peak strength of the IDWS specimens.

Insights into dynamic cracking and failure mechanisms of weakly cemented siltstone under impact loading

Soft strata composed of weakly cemented rock normally suffer from the dynamic load in underground energy mining engineering, yet there is a notable lack of research into the dynamic mechanical characteristics of such materials. In this study, the split Hopkinson pressure bar system was adopted to conduct compression tests under four different impact pressures to reveal the dynamic mechanical performances of the weakly cemented siltstone (WCS). Results showed that under different strain rates, dynamic stress-strain curves of the specimen displayed similar trends. The dynamic compressive strength, critical strain and dynamic toughness of WCS all increased as the strain rate increased, while the dynamic elastic modulus had almost no strain rate effect. Besides, combined with the high-speed camera, the dynamic failure process of WCS was analyzed, and the crack area ratio was defined to quantitatively analyze the damage evolution law of WCS under impact loading. Moreover, the dynamic failure mechanism of WCS was clarified from the microstructure level. Finally, the scaling law model for calculating the dynamic compressive strength was proposed for WCS, and the modeling results exhibited a close agreement with the test results. The research results can provide mechanical parameters and a theoretical basis for the safety assessment of weakly cemented rock strata during energy mining.