Experimental and Numerical Analysis of Mode I Fracture Process of Rock by Semi-Circular Bend Specimen

The semi-circular bend (SCB) specimen is widely used to measure fracture toughness of brittle materials such as rock. In this work, the stress field, fracture process zone (FPZ), and crack propagation velocity of SCB specimen are analyzed during the fracture process of rock specimens. The FPZ of specimen is obtained by experimental and numerical methods under a three-point bend test. The stress concentration zones of specimen present a heart shape at peak load points. FPZ forms before macro fracture occurs. The macro fracture form inside FPZ in a post-peak region of a load–displacement curve. The crack propagation process of specimen include two stages, namely the rapid crack initial development stage, and the final crack splitting stage. The maximum crack propagation velocity of specimen is about 267 m/s, and the average crack propagation velocity is about 111 m/s.

Study on Crack Propagation of the Tunnel Model under Dynamic Loading

Crack defects make it difficult to predict the dynamic fracture of tunnel specimens under an impact load. To study the impact of the velocity and crack location on a roadway under dynamic load, specimens with tunnel-type voids were made using polymethyl methacrylate. The split-Hopkinson bar was used as the loading method, and a digital laser dynamic caustics system was used to observe the fracture process of the specimens. The dynamic fracture process was evaluated by the crack propagation velocity, displacement, and dynamic stress intensity factor. To predict and verify the test results, ABAQUS was used to simulate the test process. It was found that the results of the simulated combinations of the crack propagation path and initial fracture toughness change law are consistent with the test results. The initial fracture toughness and the peak value of the crack propagation velocity increased with the increase of the impact velocity. The crack propagation law and trajectory were affected by the location of the prefabricated cracks.

Development of a Model of Crack Propagation in Multilayer Hard Coatings under Conditions of Stochastic Force Impact

The article deals with the problems of cracking in the structure of multilayered coatings under the conditions of stochastic loading process. A mathematical model has been proposed in order to predict the crack propagation velocity in the coating while taking the influence of interlayer interfaces into account. A technique for calculating the probability density distribution of the coating fracture (failure rate) has been developed. The probability of a change in the crack growth direction is compared with the experimental data that were obtained as a result of the studies focused on the pattern of cracking in the Zr,Nb-(Zr,Nb)N-(Zr,Nb,Al)N and Ti-TiN-(Ti,Cr,Al)N coatings under the conditions of the real stochastic loading of cutting tools during the turning. The influence of the crystalline structure of the coating on the cracking pattern has been studied. The investigation has found the significant effect of the crystalline structure of the coating layers on the cracking pattern.

Formation of excess atomic volume and its role in the processes of fracture of nickel single crystal

Molecular dynamics simulation of crack propagation peculiarities in a nickel single crystal under uniaxial tension along the cubic direction was carried out. It was found that at room temperature regions with excess atomic volume are formed near the tips of the opening crack. Subsequently nanopores are formed in these areas which then merge with the crack stimulating high-speed opening. It is shown that if dislocations begin to form at the crack tip in a region with an increased atomic volume the crack propagation velocity in this direction significantly decreases.

Dynamic Tensile Test of Granite and Its Tensile Sensitivity

Through the dynamic splitting tensile test under various loading rates, different mechanical parameters have been analyzed; not only the dynamic peak stress but also the dynamic peak strain has a good linear relationship with the strain rate. The tensile sensitivity obtained from the dynamic tensile test increases with strain rate gradually, and it shows a nearly linear relation, which fully indicates that the granite specimen is a strain rate sensitive material. Moreover, with the numerical simulation, the damage area of the specimen is consistent with the actual failure mode of the specimen. Furthermore, the influenced factor on the dynamic tensile strength is discussed, which illuminates that the most fundamental reason of the rate effect is that the stress wave velocity is faster than the crack propagation velocity in the specimen during the impact process.

Harmfulness Defect in Double-Notched Specimen of S533 Steel

The analytical solving of fracture mechanics equations remains insufficient for complex mechanisms, hence the use of finite element methods (FEM). The paper considered a test specimen with double S355 steel notches, stressed in tension, causing crack opening with two planes of symmetry. The refinement of the mesh is carried out at the bottom of the notch using the elements of Barsoum. The stress concentration coefficient and the crack propagation velocity are evaluated.

Rock Dynamic Crack Propagation under Different Loading Rates Using Improved Single Cleavage Semi-Circle Specimen

The objective of this paper is to investigate the complete process of dynamic crack propagation in brittle materials under different loading rates. By using Improved Single Cleavage Semi-Circle (ISCSC) specimens and Split Hopkinson Pressure Bar equipment, experiments were conducted, with the fracture phenomenon and crack propagation of tight sandstone investigated. Meanwhile, the process of crack propagation behaviour was simulated. Moreover, with the experimental–numerical method, the crack propagation dynamic stress intensity factor (DSIF) was also calculated. Then, the crack propagation toughness of tight sandstone under different loading rates was investigated and illustrated elaborately. Investigation results demonstrate that ISCSC specimens can achieve the crack arrest position unchanged, and the numerical simulation could effectively deduce the actual crack propagation, as their results were well matched. During crack propagation, the crack propagation DSIF in the whole process increases with the rising loading rate, and so does the crack propagation velocity. Several significant dynamic material parameters of tight sandstone are also given, for engineering reference.