Experimental Investigation of the Dynamic Tensile Properties of Naturally Saturated Rocks Using the Coupled Static–Dynamic Flattened Brazilian Disc Method

In a naturally saturated state, rocks are likely to be in a stress field simultaneously containing static and dynamic loads. Since rocks are more vulnerable to tensile loads, it is significant to characterize the tensile properties of naturally saturated rocks under coupled static–dynamic loads. In this study, dynamic flattened Brazilian disc (FBD) tensile tests were conducted on naturally saturated sandstone under static pre-tension using a modified split-Hopkinson pressure bar (SHPB) device. Combining high-speed photographs with digital image correlation (DIC) technology, we can observe the variation of strain applied to specimens’ surfaces, including the central crack initiation. The experimental results indicate that the dynamic tensile strength of naturally saturated specimens increases with an increase in loading rate, but with the pre-tension increases, the dynamic strength at a certain loading rate decreases accordingly. Moreover, the dynamic strength of naturally saturated sandstone is found to be lower than that of natural sandstone. The fracture behavior of naturally saturated and natural specimens is similar, and both exhibit obvious tensile cracks. The comprehensive micromechanism of water effects concerning the dynamic tensile behavior of rocks with static preload can be explained by the weakening effects of water on mechanical properties, the water wedging effect, and the Stefan effect.

Dynamic Splitting Behavior and the Constitutive Relationship of Frozen Sandstone Containing a Single Fissure

Fractured sandstone is widely distributed in mining areas throughout western China where the artificial freezing method is extensively adopted to construct vertical shafts. Blasting and excavation generate stress waves and break frozen fractured sandstone. Among the failure modes of frozen fractured rocks, tensile failure is very common. In this study, the dynamic tensile strength of fractured sandstone samples with four crack inclination angles (0°, 30°, 60°, and 90°) is tested by using a split Hopkinson pressure bar at four subzero temperatures (−5, −10, −15, and −20°C). Accordingly, a damage constitutive relationship that considers the effect of fissure angle and freezing temperature is established. The results show the following: (1) the fissure angle does not significantly affect the dynamic tensile strength of frozen fractured sandstone but mainly affects the failure mode of the sample. (2) The dynamic tensile strength of fractured sandstone has a negative linear correlation with the freezing temperature. (3) When the fissure angle is small, only tensile cracking occurs; when the fissure angle is large, tensile cracking occurs along both the loading direction and the fissure; and shear cracking occurs along the fissure as well. (4) Regardless of the fissure angle, tensile cracking is initiated at the stress-concentration zone and then propagates towards the loading end. Fissure ice provides both resistance to deformation and resistance to crack propagation which affects the crack propagation and coalescence mode. A dynamic constitutive relationship is established by considering the effects of fissure angle and freezing temperature on the dynamic properties of frozen fractured sandstone, which is proven to be highly reliable and provides a reference and basis to study the dynamic mechanical properties of similar rock types.

Numerical Study of Concrete Dynamic Splitting Based on 3D Realistic Aggregate Mesoscopic Model

In mesoscopic scale, concrete is regarded as a heterogeneous three-phase material composed of mortar, aggregate and interfacial transition zone (ITZ). The effect of mesoscopic structure on the mechanical behaviors of concrete should be paid more attention. The fractal characteristics of aggregate were calculated, then the geometric models of aggregate were reconstructed by using fractal Brownian motion. Based on the random distribution of aggregates, the concrete mesoscopic structure model was established. And the numerical model was generated by using grid mapping technology. The dynamic compression experiments of concrete under Split Hopkinson Pressure Bar (SHPB) loading verify the reliability and validity of the mesoscopic structural model and the parameters of the constitutive model. Based on these, a numerical study of concrete under dynamic splitting is carried out. By changing the parameters of the constitutive model, the effects of tensile strengths of aggregate, mortar and ITZ on the dynamic tensile strength of concrete are discussed. The results show that the dynamic failure of specimen usually occurs at the interfacial transition zone, then extends to the mortar, and the aggregates rarely fail. However, the increase of strain rate intensifies this process. When the strain rate increases from 72.93 s−1 to 186.51 s−1, a large number of aggregate elements are deleted due to reaching the failure threshold. The variation of tensile strengths of each phase component have the same effect on the dynamic tensile strength and energy of concrete. The dynamic tensile strength and energy of concrete are most affected by the tensile strength of mortar, following by the ITZ, but the tensile strength of aggregate has almost no effect.

Numerical Modeling of Concrete Spallation at Medium Strain-rate

Dynamic tensile strength of brittle materials such as concrete is usually obtained by performing laboratory investigations such as direct tensile, Brazilian splitting, and spall tests. This research presents a study aimed to investigate numerically the dynamic behavior of concrete exposed to tensile loading at medium strain-rate. The dynamic tensile behavior of concrete is investigated using the Modified Split Hopkinson Bar (MSHB) at strain-rate ranges from 33 to 80 s-1. The commercial finite element explicit code LS-DYNA is used to perform the numerical simulations of the MSHB tests. Karagozian & Case Concrete Model (K&C) is adopted to define the mechanical properties of the investigated specimens. The employed K&C material model is verified by using the experimental results obtained in [1]. The validation of the K&C material model is carried out with the comparison of the computed and experimental pull-back velocities of the specimens free end. The results of the analysis are used to enhance the understanding of strain-rate sensitivity of the concrete tensile strength.

Analysis of dynamic fracture properties of High-Performance

Concrete product research has experimented a deep change during the last decades in the search of novel products with improved mechanical properties. Conventional concrete (CC) is now being replaced by new high-performance-concrete (HPC) mixes, with compressive strengths ranging from to 80 to 200 MPa, offering great chemical resistance and improved durability. The use of these HPC mixes has been extended to a wide diversity of applications, from civil to military structures, allowing the development of new designs by increasing the compressive properties with a notorious self-weight decrease. As opposite to the improved quasistatic behaviour, HPC may be more susceptible to certain dynamic actions as compared to conventional mixes. Research in the dynamic behaviour of these new mixes is needed in order to ensure resilient structural designs. In the present paper, a compared analysis of the dynamic response of HPC mixes versus conventional concrete is carried. Using a Modified Hopkinson Bar, spalling tests are performed over cylindrical concrete specimens. Results on the dynamic tensile strength are compared for CC and HPC.

Dynamic Tensile Properties and Energy Dissipation of High-Strength Concrete after Exposure to Elevated Temperatures

In view of the devastating outcomes of fires and explosions, it is imperative to research the dynamic responses of concrete structures at high temperatures. For this purpose, the effects of the strain rate and high temperatures on the dynamic tension behavior and energy characteristics of high-strength concrete were investigated in this paper. Dynamic tests were conducted on high-strength concrete after exposure to the temperatures of 200, 400, and 600 °C by utilizing a 74 mm diameter split Hopkinson pressure bar (SHPB) apparatus. We found that the quasi-static and dynamic tensile strength of high-strength concrete gradually decreased and that the damage degree rose sharply with the rise of temperature. The dynamic tensile strength and specific energy absorption of high-strength concrete had a significant strain rate effect. The crack propagation law gradually changed from directly passing through the coarse aggregate to extending along the bonding surface between the coarse aggregate and the mortar matrix with the elevation of temperature. When designing the material ratio, materials with high-temperature resistance and high tensile strength should be added to strengthen the bond between the mortar and the aggregate and to change the failure mode of the structure to resist the softening effect of temperature.