Mechanical Properties and Failure Modes of CRCB Specimen Under Impact Loading

To explore the dynamic mechanical characteristics of coal-rock combined body (CRCB) load-bearing structures, impact tests were performed on CRCB specimens by using a separated Hopkinson pressure bar test device (SHPB) combined with an ultra-high-speed camera system. The propagation characteristics of stress wave , dynamic stress-strain relationship, energy evolution law, and distribution characteristics of CRCB crushed particles in the impact tests were analyzed. The obtained results showed that: with the increasing of impact velocity, the effect of the wave impedance difference between the CRCB specimens and incident bar on stress wave propagation is gradually weakened. The peak strength (sII) and peak strain of the CRCB had obvious strain-rate effects, the ratio of reflected energy decreases linearly. In addition, with increased impact velocity, the growth rate of the peak strength and ratio of absorbed energy gradually dropped, changing approximately as a power function. Macro-fractures of the CRCB mainly occurred at the coal or rock ends which is far away from the interface. When the stress at the crack tip is greater than the "weakened" coal or rock strength, the crack will continue to develop across the coal and rock interface. With the increasing of impact velocity and rock strength, the crushed coal particles gradually transform from massive to powdering, and the average size of crushed coal blocks decreases, which leads to a gradual increase in the fractal dimension of the CRCB specimens. Therefore, the monitoring and prevention of dynamic loads should be strengthened in the coal mines with thick and hard roofs.

Research on the Seam Formation Mechanism of Elliptical Bipolar Linear Directional Blasting Based on the SPH-FEM Coupling Method

Rock mass blasting is a complex process that involves the coupling of both discontinuous and continuous media. This paper aims to reveal the dynamic failure process between adjacent boreholes under an elliptical bipolar linear charge structure using the SPH-FEM (smooth particle hydrodynamics and finite-element method) coupling algorithm numerical simulation method. The numerical simulation results are compared with the existing experimental results, which proves the rationality of the algorithm. According to the numerical simulation results, the shaped jet will first shock the hole wall and form a stress concentration zone that will guide the formation of cracks during the stress wave propagation process. In the case of double-hole blast loading, there is a tendency for cracks coalescence to develop between adjacent boreholes due to the superposition of stresses between the double holes and the increase in damage and plastic strain. The best blasting results will be achieved with this structure when the distance between adjacent holes is 110 cm. Finally, the superiority of elliptical bipolar linear blasting in engineering blasting was verified through field experiments. The results of this study provide a reference for subsequent applications of elliptical bipolar structures in the field of rock blasting.

Stress Wave Propagation and Energy Absorption Properties of Heterogeneous Lattice Materials under Impact Load

A heterogeneous lattice material composed of different cells is proposed to improve the energy absorption capacity. The heterogeneous structure is formed by setting layers of body-centered XY rods (BCCxy) cells as the reinforcement in the body-centered cubic (GBCC) uniform lattice material. The heterogeneous lattice samples are designed and processed by additive manufacturing technology. The stress wave propagation and energy absorption properties of heterogeneous lattice materials under impact load are analyzed by finite element simulation (FES) and Hopkinson pressure bar (SHPB) experiments. The results show that, compared with the GBCC uniform lattice material, the spreading velocity of the stress of the (GBCC)3(BCCxy)2 heterogeneous lattice material is reduced by 18.1%, the impact time is prolonged 27.9%, the stress peak of the transmitted bar is reduced by 34.8%, and the strain energy peak is reduced by 29.7%. It indicates that the heterogeneous lattice materials are able to reduce the spreading velocity of stress and improve the energy absorption capacity. In addition, the number of layers of reinforcement is an important factor affecting the stress wave propagation and energy absorption properties.

Analysis of Dynamic Response Behavior of Crack under Impact Stress Wave

The structural parts of construction machinery mostly fail due to impact load, but current research on the failure behavior of the impact load has not established a complete theoretical system. Based on wave theory and fracture mechanics, this paper analyzed the wave behavior of shock stress waves and established a model of shock stress wave propagation. Given the dynamic response behavior of the stress and strain field at the crack tip, dynamic fracture mechanics theory was used to solve the dynamic fracture strength stress factor and evaluate the dynamic fracture performance of the structure with crack damage under shock waves. Through dynamic response analysis and numerical calculation of the typical SHPB (split Hopkinson pressure bar) test standard compact tension (CT) specimens under the short-term strong shock stress wave, the stress and strain evolution law of the material under the shock wave was analyzed, and the correlation of the shock stress wave was verified. This research work can meet the requirements of engineering design and has practical engineering significance, playing an important role in material safety design.

Towards an Understanding of the Effect of Adding a Foam Core on the Blast Performance of Glass Fibre Reinforced Epoxy Laminate Panels

This paper presents insights into the blast response of sandwich panels with lightweight foam cores and asymmetric (different thicknesses) glass fibre epoxy face sheets. Viscously damped elastic vibrations were observed in the laminates (no core), while the transient response of the sandwich panels was more complex, especially after the peak displacement was observed. The post-peak residual oscillations in the sandwich panels were larger and did not decay as significantly with time when compared to the equivalent mass laminate panel test. Delamination was the predominant mode of failure on the thinner facesheet side of the sandwich panel, whereas cracking and matrix failure were more prominent on the thicker side (which was exposed to the blast). The type of constituent materials used and testing conditions, including the clamping method, influenced the resulting failure modes observed. A probable sequence of damage in the sandwich panels was proposed, based on the transient displacement measurements, a post-test failure analysis, and consideration of the stress wave propagation through the multilayered, multimaterial structure. This work demonstrates the need for detailed understanding of the transient behaviour of multilayered structures with significant elastic energy capacity and a wide range of possible damage mechanisms. The work should prove valuable to structural engineers and designers considering the deployment of foam-core sandwich panels or fibre reinforced polymer laminates in applications when air-blast loading may pose a credible threat.

Experimental Study on Stress Wave Propagation Crossing the Jointed Specimen with Different JRCs

Most of the rock masses in the outer crust of the Earth are discontinuous. They are divided by joints, faults, fractures, etc. And those discontinuities, generally referred to as joints, greatly affect the property of the rock masses. The paper experimentally investigates the stress wave propagation crossing the jointed specimens. The tests were conducted on the split Hopkinson pressure bar (SHPB). The test specimens consist of two parts cast by cement mortar. Both parts have an irregular surface, and they were designed to match each other completely. The surfaces where two parts meet make an artificial joint. The surfaces of the joints were scanned by a three-dimensional scanner to obtain its actual topography and then to calculate the roughness of the surface, i.e., the joint roughness coefficient (JRC). A set of jointed specimens with JRC ranging from 0 to 20 were made and used in dynamic compression experiments. During the tests, signals were captured by strain gauges stuck on the incident and transmitted bars of the SHPB apparatus. The incident, reflected, and transmitted waves across the jointed specimens were obtained from the test records. We found out that more stress wave would transmit through the jointed specimen with larger JRC. Besides, collected data were processed to get the dynamic stress-strain relation of jointed specimens and the stress-closure curves of the joints. The results show that the joint increases the deformation of the specimen, and the stiffness of the jointed specimen would increase slightly when the joint is rougher.