Dynamic Uniaxial Compression Tests Research Articles

Analysis of fracture modes and acoustic emission characteristics of low‐frequency disturbed coal rock bodies with different cyclic amplitudes

AbstractFrequent mining operations significantly disturb the security of deep weakly cemented rock roadways in western mining areas, constituting one of the primary causes of deformation, instability, and failure within the coal‐rock body. In this paper, dynamic uniaxial compression tests of soft rock‐coal combinations under low‐frequency disturbance with different cyclic amplitudes were conducted based on acoustic emission to elucidate fracture modes. The findings are as follows: Different cycle amplitudes manifested significant degradation effects on the soft rock‐coal combination. With increasing cycle amplitude, the proportion of tensile cracks initially decreased before subsequently increasing, demonstrating a general upward trajectory. The sudden increase in the acoustic emission RA value, the large decrease in the b‐value, and AE counts reaching the peak mean that failure and destabilization of the specimen begin to occur. The results of this study will furnish theoretical direction for dynamic disaster monitoring and early warning in soft rock mines located in western mining regions.

Dynamic response and constitutive model of damaged sandstone after triaxial impact

In deep rock engineering, dynamic hazards such as blasting and earthquakes pose serious threats to the safety of underground rock structures, including mining and construction activities. To explore the dynamic mechanical properties and damage mechanisms of rocks under realistic ground stress conditions, this study utilized dynamic triaxial compression tests under hydrostatic pressure. These tests employed a modified split Hopkinson pressure bar (SHPB) technique at various strain rates. Wave velocities of the specimens were measured before and after the tests to calculate the damage factor based on the P-wave velocity. Additionally, dynamic uniaxial compression tests on the damaged sandstone specimens indicated a significant reduction in dynamic strength as the damage impact air pressure increased. There was also a notable correlation between the dissipated energy ratio and the level of rock damage, with more severe damage resulting in a smaller dissipated energy ratio. Macroscopic examination through sieve grain size results confirmed the formation of more cracks under high impact air pressure, while microscopic analysis using scanning electron microscopy (SEM) images uncovered the damage mechanisms within the rock post-impact. Ultimately, this study developed an improved ZWT model to effectively verify the dynamic response of the damaged sandstone.

Dynamic mechanical properties and failure mechanisms of rock-like materials containing main and auxiliary holes with various arrangement inclinations and spacing ratios

As a typical rock defect, circular holes significantly influence rock masses, diversifying their failure modes and ultimately weakening their bearing capacities. To study the effects of hole arrangement inclination and spacing ratio on the mechanical properties and failure mechanisms of rock-like specimens, dynamic uniaxial compressive tests were conducted by employing a modified split Hopkinson bar apparatus and digital image correlation. The results reveal a V-shaped trend in the peak stress, Young’s modulus, and absorbed energy proportion as the hole arrangement inclination increases from 0° to 90°. However, the peak strain is insensitive to changes in the hole arrangement inclination and spacing ratio. The hole spacing ratio has no significant effect on the Young’s modulus, but it causes a gradual reduction in the proportion of energy absorbed in the specimens. Moreover, shear and tensile cracks emanating from the main hole dominate the failure mode of specimens with main and auxiliary holes, as demonstrated by the resultant displacement and vertical strain contours. Rock bridges experience shear failure as the hole arrangement inclination increases from 15° to 75°, whereas the rock bridges of specimens with various hole spacing ratios tend to fail owing to tensile cracks. The interaction between main and auxiliary holes with an arrangement inclination of 0° is the weakest, suggesting that tunnel groups should be arranged perpendicularly to the direction of dynamic loading.

Into a rapid polymer characterization employing optical measurement systems and high-power ultrasonic excitation

This study presents a novel methodology for estimating the master curve of the complex modulus of viscoelastic materials using a combination of optical measurement systems and ultrasonic excitation. Traditional techniques for characterizing properties of viscoelastic materials are often time-consuming and encounter limitations that hinder their accuracy at high strain rates. To address this, a method was proposed that leverages two optical measurement systems to quickly assess material properties at multiple points on a sample. A high-power ultrasonic transducer was employed to excite the material at its first longitudinal natural frequency, creating non-uniform temperature variations and strain rates. A scanning laser Doppler vibrometer measured vibrations across the material, enabling computation of the complex modulus magnitude under varying conditions. These results were correlated with temperature readings obtained from an infrared camera. The constructed master curve using the proposed methodology closely resembled those established through quasi-static and dynamic uniaxial compression tests in the literature. Additionally, this method revealed a more substantial increase in complex modulus at high strain rates compared to traditional experiments, where this characteristic is less pronounced.

Study on the Numerical Relationship between Dynamic Compressive Strength and Crushing Energy Consumption in Dynamic Point Load Test under Different Corrosion Conditions

Compared with most dynamic tests, point load test is widely used in engineering practice because of its wide requirements for test conditions. In offshore mining engineering, the surrounding rock is affected by dynamic load impact and corrosion of various aqueous solutions for a long time. Therefore, it is of great significance to study the influence of hydrochemical corrosion on dynamic mechanical properties of rock to ensure the stability of surrounding rock. In this paper, a large number of dynamic point load and dynamic uniaxial compression tests were carried out on the granite mined from a large mine by using the Split Hopkinson pressure bar test device after the corrosion of pH values of 2, 5, and 7 solutions, respectively. Based on the energy theory, the crushing energy consumption in the test process is analyzed, and finally the fitting formula of the dynamic compressive strength and the crushing energy consumption in the dynamic point load test is obtained through fitting analysis. The results show that when the crushing energy consumption in the dynamic point load test is used to estimate the dynamic compressive strength, a certain crushing energy consumption (the final value of this test is 70 J in this paper) is more accurate, and it can estimate the dynamic compressive strength under various corrosion conditions under this crushing energy consumption. However, with the change of crushing energy consumption in the dynamic point load test, this transformation relationship is no longer accurate, and it is necessary to modify this transformation relationship with corrosion condition as the influencing factor. This conclusion can provide reference for obtaining the dynamic compressive strength of rocks under different corrosion conditions in engineering field in the future.

Biaxial tensile behavior of Ti-6Al-4V under proportional loadings at high strain rates

To study the mechanical performance of materials under dynamic and proportional biaxial tensile loadings, the electromagnetic Hopkinson bar system was employed. Dynamic biaxial tensile experiments were conducted by this novel system. Dynamic uniaxial tensile and compressive tests were also performed. The digital image correlation (DIC) technique combined with the high-speed camera was utilized to analyze the deformation in the gauge section of the improved cruciform specimens. The cruciform specimen design was improved by finite element method (FEM) method to adapt to dynamic biaxial loadings. An inverse analysis (IA) method was adapted to deduce the true stresses in cruciform specimens. Based on the Yoon criterion, a strain rate and stress state dependent yield criterion and a related constitutive model were developed to account for the asymmetric yielding and strengthening behaviors under high strain rates. The new constitutive model was implemented in the FEM analysis and the numerical results agreed well with the experiments.

The Influence of the Strain Rate and Prestatic Stress on the Dynamic Mechanical Properties of Sandstone-A Case Study from China.

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).

Evaluation of Rock Brittleness Index under Dynamic Load

Rock is a typical brittle material, and the evaluation of its brittleness index has important guiding significance for hard rock resource exploitation, unconventional oil and gas resource exploitation, mechanical driving efficiency, rock burst prediction, and dynamic disaster prevention and control. At present, brittleness index often measures the brittleness of rock under static load; thus, whether it is applicable to dynamic load is worth exploring. In this study, static and dynamic uniaxial compression tests and Brazilian splitting tests were carried out on five kinds of rocks, including fine granite, coarse granite, shale, marble, and sandstone, using the INSTRON−1346 test system and split−Hopkinson pressure bar (SHPB), respectively. The brittleness index values of different rocks under static and dynamic load were determined, and the changes in the brittleness of rocks under different loading methods and different strain rates were studied. The definition of brittleness and the applicability of existing brittleness indices were also discussed. It was found that the loading rate amplified the variation of the brittleness characteristics of rock. When static load changes to dynamic load, the brittleness of rocks increases, and the brittleness relationship between different rocks remains unchanged. The more brittle the rock is under static load, the greater the range of brittleness enhancement is under dynamic load. It was also found that the brittleness of sandstone had an obvious effect on the strain rate. The brittleness of rock increases with the increase in strain rate, and the greater the strain rate, the greater the brittleness enhancement degree. These research results can provide reference values for dynamic disaster prevention and safe construction of deep rock projects such as mines and tunnels.

Effect of sludge ceramsite particle grade on static and dynamic mechanical properties of alkali-activated slag lightweight concrete at early age

Sewage sludge (SS) produced in water supply and drainage plants is a kind of solid waste with pollution and high water content. SS mixed with fly ash and other materials to make sludge ceramide (SC) can be used as light environmental protection building materials. Alkali-activated lightweight concrete (ASLC) is prepared by mixing SC with alkali-activated slag mortar with low carbon emission, which has lightweight and early strength characteristics. Using the split Hopkinson pressure bar test system, the dynamic and static uniaxial compressive tests of ASLC at an early age were carried out. The influence of aggregate particle gradation on the mechanical properties of ASLC was comprehensively evaluated from static compressive strength and dynamic peak stress. Experiments show that small-size SC has better mechanical properties and lower air content. SEM test shows that the paste has closely meshed with SC, which proves that SC has good compatibility with alkali-activated slag material. The air content of ASLC was quantified by mesopore characterization. The results show that the increase in SC size will increase the air content of the specimen and weaken the compressive performance of ASLC.

Constrained shape-memory behaviors of multiple network elastomers

Most existing shape-memory polymers (SMPs) only generate low recovery stress, which restricts the applications of SMPs. To address this issue, here we investigate the shape-memory behaviors of multiple network elastomers (MNEs), a type of novel tough soft materials. The dynamic mechanical tests and uniaxial compression tests are used to characterize the mechanical behaviors of MNEs across the glass transition region. The constrained recovery tests show that the recovery stress depends on the deformation temperature, applied strain, and the number of networks. For double network elastomers with an 85 % programmed strain, the peak recovery stress and the final recovery stress are both larger than 20 MPa, which is significantly larger than the values reported in the literature.

Dynamic fracture analysis of sandstone specimens containing different inclusions

Infilling materials are commonly distributed in natural rocks, which play a key role in the stability of rock engineering. To reveal the enhancing mechanism of inclusions, dynamic uniaxial compressive tests were carried out on sandstone specimens containing double elliptical flaws using a split Hopkinson pressure bar (SHPB) system and the digital image correlation (DIC) technique, and three inclusion conditions (R1, R2 and R3) and three flaw inclination angles (0°, 45° and 90°) were taken into consideration. The results indicate that the inclusions (R2 and R3) with a similar wave impedance to sandstone matrix can maximally enhance the dynamic strength of flawed specimens by 46.82% and 42.83%, respectively. The crack initiation and propagation behavior were significantly changed after the flaws were filled with different types of inclusions. The dominant crack in the bridge area can be identified by strain histories and stress intensity factors (SIFs) at the inner inclusion tips. The findings are projected to deepen the understanding of interaction mechanism of rock matrix and inclusions.

Dynamic mechanical behavior of rocks containing double elliptical inclusions at various inclination angles

Inclusions embedded in rock materials highly affect their mechanical response and fracture behavior under external loadings. In this study, dynamic uniaxial compressive tests were performed on sandstone specimens containing double elliptical inclusions with various inclination angles to investigate the effects of inclusion conditions on the dynamic mechanical properties and failure mechanism of rock specimens using a split Hopkinson pressure bar (SHPB) system and the digital image correlation (DIC) technique. The results reveal that the inclusions effectively enhance the dynamic strength of flawed specimens, but its variation trend versus the flaw inclination angle is similar to that of unfilled specimens. Remarkable transformations were observed in the initiation position and propagation path of cracks after filling the flaw, which can be represented by the vertical displacement distribution along the inclusion periphery. The interaction mechanism between the double inclusions was dominated by a shear slip crack at the flaw inclination angle of 45° and tensile cracks at the flaw inclination angles of 0° and 90°. The effect of inclusions on the final failure pattern becomes slight with the increasing flaw inclination angle.

Transition from softening to hardening related with strain rate in Zr-based metallic glasses

The dynamic deformation and fracture behavior of Zr-based metallic glasses related with strain rate were systematically investigated by dynamic uniaxial compressive tests over a wide range of strain rate. The dynamic fracture strength with the increase of the strain rate shows two stages, i.e., the transition from strain-rate softening to strain-rate hardening. In the first stage, the temperature rise within the shear bands gradually increases due to increasing strain rate. The viscosity in the shear bands greatly reduces and the free volume keeps increasing, which results in the occurrence of strain-rate softening and significantly promotes the propagation of shear bands of Zr-based MGs. However, in the second stage, the increasing temperature in the shear band results in the occurrence of localized crystallization of Zr-based MGs. The shear bands are difficult to extend due the strong interaction between multiple shear bands and dendritic crystal, leading the occurrence of strain-rate hardening.

Research on the Conversion Relationship between Dynamic Point Load Strength and Dynamic Compressive Strength Based on Energy System

Point load test is a simple and fast method to obtain the compressive strength of rock. According to the maximum load at failure and the distance between two cone ends, the compressive strength of specimens can be calculated by the empirical formula. Because the test process does not need to process the sample into laboratory size type, so it is widely used. At the same time, with the rapid development of rock dynamics, the dynamic strength index of rock becomes more and more important under dynamic load. But dynamic point load is seldom mentioned by scholars. In order to explore the correlation between dynamic point load test and dynamic compressive strength, a large number of dynamic point load and uniaxial dynamic compressive strength tests were carried out using a split Hopkinson pressure bar for granite. Based on the energy theory, the crushing energy consumption during dynamic point load and uniaxial dynamic compressive strength tests were statistically analyzed, and the transformational relation between uniaxial dynamic compressive strength and crushing energy consumption during the dynamic point load test was determined. At the same time, the influence of the cone angle on crushing energy consumption during the dynamic point load test was investigated by changing the conical angle. The results show that with the increase in conical angle, the energy consumption of rock crushing decreases gradually during the dynamic point load test. And the decreasing trend tends to be gentle. The conclusion can provide a reference for obtaining the numerical value of the dynamic compressive strength of the rock in the future.

The effect of temperature on the high-strain-rate response of Co-Al-W- base alloys: Experiments and modeling

Two novels Powder Metallurgy (PM) cobalt-based superalloys with a γ/γ’ dual-phase microstructure have been subjected to dynamic uniaxial compression tests at temperatures from 25 °C to 850 °C, and a high strain rate of 2500 s−1, to investigate the effect of temperature on their high-strain-rate response. Compression tests have been performed using a Split Hopkinson Pressure Bar (SHPB), focusing on the temperature-dependent anomalies of the flow stress at high temperatures for both alloys. The analysis of the experimental results indicates an important strain-rate sensitivity and thermal softening effect with a noticeable positive stress peak at high temperatures. Finally, a Johnson-Cook-type constitutive model is developed to describe the flow stress as a function of the temperature, including the anomalous positive peak temperature. The modified JC model presents a good correlation to predict the behavior of both Co-based superalloys over wide ranges of temperatures through simulating the experimental camping with Abaqus. This model offers a potential instrument to simulate and optimize high impact events applications.

Mechanical behavior of a typical polymer bonded explosive under compressive loads

ABSTRACT The compressive mechanical behavior and damage mode of polymer bonded explosives (PBXs) based on hexahydro-1, 3, 5-trinitro-1, 3, 5-s-triazine (RDX) were systematically investigated via experiments and simulations. The strength sensitivity on strain rate of PBX–JH14C was studied using quasi-static and dynamic uniaxial compressive tests. The axial and radial stress history curves were measured using a passive confining experiment, and characterizations of the behavior of PBX–JH14C were studied under dynamic multi-axial loads. The results showed that the dynamic stress-strain curves of PBX–JH14C exhibited an evident strain rate effect. Combining theoretical and meso analyses, the damage characteristics of PBX–JH14C under low speed impacts were studied. A constitutive theory was developed for modeling the dynamic mechanical response of PBX–JH14C by using the Boltzmann superposition principle with a Prony series representation. Mathematical presentation of the relaxation modulus can be obtained by fitting the modulus master curve. The model was verified by comparing the finite element simulations with experimental results. Furthermore, it was observed that the theoretical prediction coincided with experimental results. The damage mode and mechanisms of PBX–JH14C were discussed based on the experimental and theoretical studies. In the absence of confinement, interface debonding and matrix cavitation were the predominant failure mechanisms under low speed impact.

Investigation on Damage Characteristic and Constitutive Model of Deep Sandstone under Coupled High Temperature and Impact Loads

To investigate the coupling damage characteristics of rock after high-temperature treatment under impact load, dynamic uniaxial compression tests for deep sandstone specimen under laboratory conditions varying with high temperature (i.e., 25°C, 100°C, 300°C, 500°C, 700°C, and 900°C) and strain rate (i.e., 170 s-1, 205 s-1, and 240 s-1) were performed using splitting Hopkinson pressure bar (SHPB) system. Coupling damage variable of deep sandstone was deduced based on the Lemaitre equivalent strain theory. Moreover, the damage parameters of deep sandstone were systematically determined according to the test data, and the effects of high temperature and strain rate on damage growth curves were investigated. Finally, a dynamic compound damage constitutive model, which could consider the coupling damage, was established and verified to describe the dynamic mechanical characteristic of deep sandstone. Theoretical and experimental results indicated that the simulated stress-strain curves matched the test data well and the proposed coupling damage constitutive model could reflect the high temperature-induced weakening and strain rate strengthening effect.

Dynamic mechanical characteristic and fracture evolution mechanism of deep roadway sandstone containing weakly filled joints with various angles

To investigate the influence of weakly filled joint angles on the dynamic mechanical properties and fracture evolution mechanism of sandstone, dynamic uniaxial compression tests were performed on sandstone specimens with joint angles of 0°–90° under strain rates of 35–125 s−1 and an interval of 15° using a modified split Hopkinson pressure bar (SHPB) apparatus. The dynamic stress–strain relationship, dynamic strength, dynamic peak strain, energy characteristic, failure mode, typical failure progress, crack coalescence category, fracture micro-mechanism, and theoretical analysis were examined in this research. The test results indicated that both the dynamic compressive strength and peak strain of jointed sandstone first decreased and then increased with the increase in the joint angle. To maximise the energy reflection coefficient and minimise the absorbed energy density, a critical joint angle α(approximately 45°), was employed. The specimens with joint angles of 0°, 15°, and 30° exhibited considerable plastic deformation capacity, whereas the brittle failure feature of specimens with joint angles of 60°, 75°, and 90° was more typical than that of intact specimens. The stress bimodality phenomena of the specimen with a joint angle of 45° were more distinct than those of the specimen with a joint angle of 60°. Four types of crack coalescence behaviours were identified during the dynamic fracturing processes of the jointed specimens using a high-speed camera; subsequently, these were compared with the crack coalescence categories of jointed specimens under static loading. Moreover, the micro-fracture morphology well-revealed the macroscopic fracture behaviours of jointed sandstone specimens. It was further confirmed that the variable tendency of the dynamic compressive strength of jointed specimens based on experimental results agreed with the theoretical analysis.

Dynamic Uniaxial Compressive Tests on Limestone

Dynamic Uniaxial Compressive Tests on Limestone

An Experimental Study of Dynamic Compression Performance of Self-Compacting Concrete.

To investigate the dynamic performance of self-compacting concrete (SCC), the dynamic uniaxial compression tests at eight different loading strain rates were performed on the ordinary concrete and SCC cubic specimens. Based on the tests, the compression failure patterns and stress–strain curves of both kinds of concrete were obtained. The results show that SCC performs more brittle than ordinary concrete by showing the diagonal crack failure pattern of SCC at a high strain rate. Besides, with the increase of loading strain rate, the peak compressive stress of SCC is slightly lower than that of ordinary concrete, but the increase of elastic modulus is slightly higher than that of ordinary concrete. The peak compressive strains of the two kinds of concrete are discrete under the influence of loading strain rate, thus putting forward the relation equation for the loading strain rate and peak compressive stress increase coefficient of the two kinds of concrete. Besides, based on the theory of elastic–plastic damage and considering the dynamic extension of damage, the dynamic constitutive relation with good applicability between ordinary concrete and SCC was established.