A state-of-the-art review of mechanical characteristics and cracking processes of pre-cracked rocks under quasi-static compression

The mechanical characteristics and failure behavior of rocks containing flaws or discontinuities have received wide attention in the field of rock mechanics.When external loads are applied to rock materials,stress-induced cracks would initiate and propagate from the flaws,ultimately leading to the irreversible failure of rocks.To investigate the cracking behavior and the effect of flaw geometries on the mechanical properties of rock materials,a series of samples containing one,two and multiple flaws have been widely investigated in the laboratory.In this paper,the experimental results for pre-cracked rocks under quasistatic compression were systematically reviewed.The progressive failure process of intact rocks is briefly described to reveal the background for experiments on samples with flaws.Then,the nondestructive measurement techniques utilized in experiments,such as acoustic emission(AE),X-ray computed tomography(CT),and digital image correlation(DIC),are summarized.The mechanical characteristics of rocks with different flaw geometries and under different loading conditions,including the geometry of pre-existing flaws,flaw filling condition and confining pressure,are discussed.Furthermore,the cracking process is evaluated from the perspective of crack initiation,coalescence,and failure patterns.

Experimental study of pre-cracked concrete subjected to cryogenic freeze-thaw cycles based on an LNG concrete tank

To investigate the mechanical properties of concrete under the leakage condition for a liquefied natural gas storage tank,cryogenic freeze-thaw cycle tests were performed under liquid nitrogen refrigeration and water immersion melting.The effects of the cryogenic temperature,freeze-thaw cycle,pre-crack,and addition of steel fiber on the compressive strength,flexural strength,and splitting tensile concrete strength were analyzed.The experimental results show that the width of pre-cracks tends to expand after freeze-thaw cycles.When the freezing temperature is -80℃,the relative width of the pre-cracks expands by 1 to 2 times.However,when the freezing temperature is -120℃,the relative width of the pre-cracks expands by 2 to 5 times.Compared with the specimens without steel fibers,the specimens with steel fibers can still maintain a relatively complete appearance structure after the mechanical property tests.The compressive strength,flexural strength,and splitting tensile concrete strength decrease with the drop in the freezing temperature.After adding steel fibers,all of the three strengths increased.

Crack growth analysis for rock-like materials with ordered multiple pre-cracks under biaxial compression

The pre-burying iron sheets approach was used to prepare rock-like materials with ordered multiple pre-cracks. 60 specimens in total were prepared in these experiments. Through biaxial compression experiments, the influence of both the number of pre-cracks and pre-cracks angles to crack growth was analyzed. Meanwhile, species of rock bridge failure were summarized, namely, wing crack, secondary shear crack between horizontal pre-cracks and secondary shear crack between vertical pre-cracks. The wing crack plays a significant role in crack growth. Furthermore, fractal dimension was adopted to describe quantitatively the crack growth during the failure process. The results indicate that with the failure of specimens, corresponding fractal dimension for specimen monotonically increases, which indicates that the fractal dimension can be considered to the failure of the specimens quantitatively.

Mechanism on simulation and experiment of pre-crack seam formation in stope roof

The pre-crack blast technology has been used to control the induction caving area in the roof. The key is to form the pre-crack seam and predict the effect of the seam. The H-J-C blast model was built in the roof. Based on the theories of dynamic strength and failure criterion of dynamic rock, the rock dynamic damage and the evolution of pre-crack seam were simulated by the tensile damage and shear failure of the model. According to the actual situation of No. 92 ore body test stope at Tongkeng Mine, the formation process of the pre-crack blast seam was simulated by Ansys/Ls-dyna software, the pre-crack seam was inspected by a system of digital panoramic borehole camera. The pre-crack seam was inspected by the system of digital panoramic borehole in the roof. The results of the numerical simulation and inspection show that in the line of centers of pre-hole, the minimum of the tensile stress reaches 20 MPa, which is much larger than 13.7 MPa of the dynamic tensile strength of rock. The minimum particle vibration velocity reaches 50 cm/s, which is greater than 30-40 cm/s of the allowable vibration velocity. It is demonstrated that the rock is destroyed near the center line and the pre-crack is successfully formed by the large diameters and large distances pre-crack holes in the roof.

Characterization of stress corrosion crack growth of 304 stainless steel by electrochemical noise and scanning Kelvin probe

The fatigue pre-cracking 304 stainless steel （SS） specimens with lengths of 1.002 mm （L-crack） and 0.575 mm （S-crack） were prepared. Their corrosion behavior was studied by electrochemical noise （EN） in 4 mol/L NaC1 ＋ 0.01 mol/L Na2S203 solution under slow-strain-rate-testing （SSRT） conditions. Moreover, the characteristics of L-crack＇s surface morphology and potential distribution with scanning Kelvin probe （SKP） before and after SSRT were also discussed. Compared with S-crack, L-crack is propagated and the features of crack propagation can be obtained. After propagation, the noise amplitudes increase with increasing stress and accelerating corrosion, the white noises at low and high frequencies （WE and WH） of the later stage are one order of magnitude larger than that at early stage in the current power spectral densities （PSDs）. The potential PSDs also increase, but WH disappears. In addition, the crack propagation can be demonstrated according to variation of probability distribution, surface morphology and potential distribution.

Crushing Characteristics of Filament Wound Carbon Fiber/Epoxy Tube under Quasi-static Compression Condition

We investigated the effect of structural factor and amide grafted multi-walled carbon nanotubes（MWNTs-NH2） on crushing characteristics of filament wound CFRP tube under quasi-static compression conditon. It was found that CFRP tubes sequentially showed the brittle fracturing mode, the local buckling fracturing mode and transverse shearing fracturing mode with increasing winding angle, respectively, with the characterizations by mechanical testing, SEM and optical microscopy. Moreover, crack propagation initiated by pre-crack and subsequent failure in the tube were strongly dependent on pre-crack angle due to defl ection and penetration competition of crack evolution. The simulated compression failure behavior correlated well with the experimental results, revealing that the Chang-Chang failure criterion was effective in representing the quasistatic crushing characteristics of the tube. In addtion, the MWNTs-NH2 were sucessfully obtained by multistep functionization. The compressvie properties of the tubes were signifi cantly improved by the addition of the MWNTs-NH2 due to their uniform dispersion and high interfacial chemical reactivity, whereas the as-received MWNTs and other functionalized MWNTs were not as effective.

A Peridynamic Model for Analyzing Fracture Behavior of Functionally Graded Materials Used as an Interlayer

Functionally graded materials(FGMs)can be used els an interlayer.The interlayer connects two homogeneous materials and slows mutations in material properties,owing to the variability of parameters and microstructure.In this numerical simulation,a bond-based peridynamic(PD)model was used to perform dynamic fracture analysis of a continuous beam and a sandwiched structure with an FGM interlayer.Some simulation results of two convergence studies,which include grid refinement(m-convergence)and the reducing radius of PD horizon(^-convergence),were discussed.Under a four-point bending load,the propagation behaviors of single pre-crack in the continuous beam with the FGM interlayer at different locations are simulated to compare with the experimental results.For the sandwiched structure,the problems of a single crack and double cracks were considered.The numerical results show that the crack can deflect toward areas where the energy release rate is low.Additionally,the crack on the weak side for symmetric cracks only has fracture behavior,and if the double cracks are asymmetrical,all these cracks have fracture behaviors and propagation of the initial crack in the stiff region has a higher sensitivity than in the compliant region.