| The problem of crack propagation of rock materials has been an important issue in geotechnical engineering.It has important theoretical guiding significance for the design,construction and stability maintenance of geotechnical engineering.Based on the theoretical analysis,laboratory experiment and numerical simulation,the problem of crack propagation in brittle rock materials is analyzed in this paper.The study focused on the uniaxial compressive strain rate effect of two-dimensional pre-existing cracked rock materials and the study of uniaxial compression of three-dimensional built-in coin-shaped cracked rock materials.The 3D printing technology is introduced to make the rock specimen containing built-in coin shaped cracks.The self contained intellectual property based micro focus CT wireless automatic loading and unloading device is applied for the first time to achieve the observation of the internal crack propagation of rock materials during loading.The main results are as follows:?1?In order to understand the theoretical criterion of the composite crack initiation,the theoretical criterion of the composite crack initiation is summarized and classified.In this paper,the theoretical criterion for the crack initiation of a type I-II composite crack in brittle materials is summarized.According to the hypothesis difference of the plastic zone at the crack tip,the fracture theory criterion of brittle materials is divided into four types.According to the four types of criteria,a theoretical criterion is derived,and the theoretical calculation of the crack angle of the prefabricated cracks under the given boundary conditions is theoretically calculated.The initial derivation of the equivalent tensile stress criterion for three-dimensional brittle fracture is preliminarily derived.The application of fracture criteria should be differentiated according to the different stages of different materials?metal or rock materials?and crack initiation.For example,the maximum circumferential stress criterion is applied to the joint fracture in the main rock mass with opening displacement,and the fault in the rock mass dominated by shear displacement is explained by the maximum shear stress criterion.The cause of cracks in concrete,ceramics,rocks,or other brittle composites is the fracture propagation that occurs after the leading edge of the crack tip is first damaged.It is appropriate to use the strain energy criterion to describe the crack propagation of brittle materials.?2?The effect of strain rate on the crack propagation of a two-dimensional rock material under uniaxial compression of prefabricated cracksBased on the MTSC 45.105 universal testing machine,the quasi-static uniaxial compression strain rate effect test of precast single crack gypsum specimen was carried out.According to the quasi-static strain rate lying the range of 10-5-10-3/s,six strain rates were selected.At the same time,the high-speed camera was used to capture the crack initiation and propagation under different strain rates.The stress-strain curves and crack propagation under different strain rates were analyzed.The change law of some relevant mechanical parameters,including crack initiation stress,peak stress and elastic modulus under different strain rates,was summarized.The crack initiation angle was analyzed and compared with the theoretical calculation result.The strain rate effect of the law of energy evolution was carried out.The results show that when the strain rate is less than or equal to 2×10-4/s,the stress and strain curve fluctuates obviously,and the stepwise rise or fall occurs.The crack is well developed.Those mechanical parameters such as cracking stress,peak stress and elastic modulus fluctuate slightly.The crack initiation angle is about 82°,which is close to 82.3°calculated by the theory of strain energy density factor.When the strain rate is more than 2×10-4/s,the stress-strain curve before its peak is smooth,the stress after its peak decreases rapidly,which shows a more pure brittle characteristic.Those mechanical parameters such as crack initiation stress,peak stress and elastic modulus obviously increase,compared with their counterparts obtained under the low strain rate.The crack initiation angle increases to 105°,which is larger than the theoretical calculation result.The strain rate effect of the energy evolution during crack initiation was analyzed.The airfoil crack initiation process was analyzed based on energy release and dissipation theory?3?The discrete element method is difficult to simulate the strain rate effect of laboratory experiments.The discrete element method can simulate the sensitivity of the material parameters,so as to better guide the laboratory experiments.The results of sensitivity analysis of prefabricated crack parameters show that:The crack initiation position has nothing to do with the crack length and the crack opening degree,but only related to the crack inclination angle.When the pre-cast crack angle is 15°,the crack initiation position is near the middle of the pre-crack.When the prefabricated crack angle is more than 15°,the crack initiation position is at both ends of the prefabricated crack.After a certain stage of the crack expansion of the main wing,it always extends along the direction of the maximum principal stress until the perforation test results in damage.The crack inclination angle is highly significant for the cracking stress,crack initiation ratio,peak stress,and strength weakening ratio.The crack length presents a significant increase in the crack initiation ratio,and is highly significant for the other three parameters.The crack opening is only highly significant to the crack initiation stress,and is not significant to the cracking ratio,the peak stress and the strength weakening ratio.?4?Study on the mechanical properties of 3D printed rock specimen and the law of three-dimensional built-in coin shaped crack propagation.Two types of hard and brittle material specimens were fabricated based on 3D printing technology:True rock type gypsum and translucent resin specimens.The printing methods of two types of test pieces,the physical properties of the complete test pieces,and the mechanical properties under uniaxial compression are introduced.Observation on crack propagation of prefabricated built-in coin shaped plaster specimens by using CT synchronous loading instrument with independent intellectual property rights.The visual observation of the crack propagation state of the real rock material under different stress states is realized in the whole stress and strain process.Based on the fractal theory,the complexity of built-in crack propagation in different stress states is analyzed.A uniaxial compression test was performed on the translucent resin specimens of different post-treatment methods.The crack propagation patterns of the translucent resin specimens with low temperature freezing built-in coin cracks were visually observed.3D printing technology can quickly,accurately and flexibly produce rock material specimens.This provides a new method for the fabrication of mechanical properties of rock materials and provides more possibilities for laboratory rock mechanics experiments.Compared with traditional casting gypsum specimens,there are some differences in physical and mechanical properties of 3D-printed gypsum specimens.3D printed gypsum test pieces have lower density,less strength and greater plastic deformation.The reason is that a lot of transparent glue is added to the3D printing process.Printed specimens are bonded by plaster and glue,resulting in insufficient compactness of 3D-printed gypsum specimens.The presence of glue also affects the strength and large plastic deformation of the specimen.The experimental results of crack propagation of a built-in coin shaped crack plaster specimen based on CT synchronous loading instrument are not ideal.3D printing technology still has some technical difficulties in making pre-existing crack rock specimens.Prefabricated cracks need prefabricated structure fixing cracks,which leads to abnormalities in the strength and destruction of the specimen.Resin material made by laser-curing printing technology is more suitable for simulating brittle hard rock.Extended secondary curing time,low temperature freezing,increased internal macro-cracks and other methods can improve the brittleness of three-dimensional printing resin.In this paper,the three-dimensional printing translucent photosensitive resin specimen is only a preliminary attempt in simulating brittle hard rock.The mechanical properties of the printed specimen are far from the real rock material.Solving this problem depends to a great extent on the more in-depth study of printing processes,mechanical properties of photosensitive materials,etc.?5?Discrete element analysis of crack propagation in three-dimensional built-in coin shaped cracked rock specimen.When the ratio of the crack radius to the specimen radius is between 0.2-0.6 and the inclination angle is 45°,the elastic modulus,secant modulus,deformation modulus,modulus of reduction,peak strength and peak strain of the numerical specimen decrease with the increase of the crack radius.When the ratio of the coin crack radius to the numerical specimen radius is 0.6 and the inclination angle is between 0°and 75°.The elastic modulus,secant modulus,deformation modulus,drop modulus,peak strength,peak strain,etc.of numerical specimens increase with the increase of coin crack angle.The stress-strain curve under uniaxial compression of three-dimensional built-in coin type specimen is lack of effective point of mutation,and the whole is smooth.It is not possible to determine the internal crack propagation process from the point of change of the stress strain curve.From the crack initiation and propagation mode,when the ratio between the crack radius and the numerical specimen radius is between 0.2-0.6 and the inclination angle is 45°,the smaller the crack radius of the numerical specimen is,the more the crack initiation stress is,the more difficult it is to crack.The actual rock specimens are different from the low temperature brittle resin specimens.It is difficult to observe the specific winged cracks or petal like cracks.The location of micro-crack growth is random,which is due to the heterogeneity of the rock material.In general,the initiation of micro cracks in the precast crack region has the advantage of time priority and quantity,showing the upward trend of the crack trend of the wrapped airfoil.Micro cracks in other locations begin to sprout at this time,leading to difficulties in the appearance of macroscopic cracks in the pre-crack area.When the coin crack angle is 45°,the stress-strain curve and micro crack growth curve of the coin crack test piece with different radiuses are divided into three phases according to their constitutive relations.Compaction stage,linear damage and macroscopic crack appearing stage,post peak stress drop and strain softening stage.The three phase constitutive equations are fitted. |
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