The rock fragmentation mechanism and plastic energy dissipation analysis of rock indentation

Zhu, Xiaohua,Liu, Weiji Techno-Press 2018 Geomechanics & engineering Vol.16 No.2

Based on theories of rock mechanics, rock fragmentation, mechanics of elasto-plasticity, and energy dissipation etc., a method is presented for evaluating the rock fragmentation efficiency by using plastic energy dissipation ratio as an index. Using the presented method, the fragmentation efficiency of rocks with different strengths (corresponding to soft, intermediately hard and hard ones) under indentation is analyzed and compared. The theoretical and numerical simulation analyses are then combined with experimental results to systematically reveal the fragmentation mechanism of rocks under indentation of indenter. The results indicate that the fragmentation efficiency of rocks is higher when the plastic energy dissipation ratio is lower, and hence the drilling efficiency is higher. For the rocks with higher hardness and brittleness, the plastic energy dissipation ratio of the rocks at crush is lower. For rocks with lower hardness and brittleness (such as sandstone), most of the work done by the indenter to the rocks is transferred to the elastic and plastic energy of the rocks. However, most of such work is transferred to the elastic energy when the hardness and the brittleness of the rocks are higher. The plastic deformation is small and little energy is dissipated for brittle crush, and the elastic energy is mainly transferred to the kinetic energy of the rock fragment. The plastic energy ratio is proved to produce more accurate assessment on the fragmentation efficiency of rocks, and the presented method can provide a theoretical basis for the optimization of drill bit and selection of well drilling as well as for the selection of the rock fragmentation ways.

A displacement solution for circular openings in an elastic-brittle-plastic rock

Huang, Houxu,Li, Jie,Rong, Xiaoli,Hao, Yiqing,Dong, Xin Techno-Press 2017 Geomechanics & engineering Vol.13 No.3

The localized shear and the slip lines are easily observed in elastic-brittle-plastic rock. After yielding, the strength of the brittle rock suddenly drops from the peak value to the residual value, and there are slip lines which divide the macro rock into numbers of elements. There are slippages of elements along the slip lines and the displacement field in the plastic region is discontinuous. With some restraints, the discontinuities can be described by the combination of two smooth functions, one is for the meaning of averaging the original function, and the other is for characterizing the breaks of the original function. The slip lines around the circular opening in the plastic region of an isotropic H-B rock which subjected to a hydrostatic in situ stress can be described by the logarithmic spirals. After failure, the deformation mechanism of the plastic region is mainly attributed to the slippage, and a slippage parameter is introduced. A new analytical solution is presented for the plane strain analysis of displacements around circular openings. The displacements obtained by using the new solution are found to be well coincide with the exact solutions from the published sources.

A New Approach of Rock Cutting Efficiency Evaluation by using Plastic Energy Dissipation Ratio

Weiji Liu,Xiaohua Zhu 대한토목학회 2019 KSCE JOURNAL OF CIVIL ENGINEERING Vol.23 No.2

Polycrystalline Diamond Compact (PDC) bit is extensively used in oil & gas drilling, the rock cutting efficiency of PDC cutter directly determines the drilling efficiency and costs. Hence, it is crucial to evaluate the rock cutting efficiency of PDC cutters. The Mechanical Specific Energy (MSE) is used as an index for long periods of time to evaluate the rock cutting efficiency, however, the energy dissipation in rock breaking cannot be further calculated in details, leading to inaccuracy. To address this problem, the new concept of Plastic Energy Dissipation Ratio (PEDR) and its model are presented, a new approach for rock cutting efficiency evaluation by using PEDR is also put forward. Compared with MSE, the PEDR can determine the Optimum Depth of Cut (DOC) under various conditions. The theoretical analysis shows that the critical DOC, governing the transition of ductile to brittle failure mode, is the optimal cutting depth, having the smallest PEDR and highest rock cutting efficiency. The test and simulation of rock cutting are carried out to verify the PEDR model, and the PEDR under different DOC, cutting velocities and rake angles are depicted and discussed. The results can provide a theoretical basis for the design of PDC cutter and optimization of drilling parameters.

Simple solutions of an opening in elastic-brittle plastic rock mass by total strain and incremental approaches

Park, Kyungho Techno-Press 2017 Geomechanics & engineering Vol.13 No.4

This study deals with simple solutions for a spherical or circular opening excavated in elastic-brittle plastic rock mass compatible with a linear Mohr-Coulomb (M-C) or a nonlinear Hoek-Brown (H-B) yield criterion. Based on total strain approach, the closed-form solutions of stresses and displacement are derived simultaneously for circular and spherical openings using original H-B and M-C yield criteria. Two simple numerical procedures are proposed for the solution of generalized H-B and M-C yield criteria. Based on incremental approach, the similarity solution is derived for circular and spherical openings using generalized H-B and M-C yield criteria. The classical Runge-Kutta method is used to integrate the first-order ordinary differential equations. Using three data sets for M-C and H-B models, the results of the radial displacements, the spreading of the plastic radius with decreasing pressure, and the radial and circumferential stresses in the plastic region are compared. Excellent agreement among the solutions is obtained for all cases of spherical and circular openings. The importance of the use of proper initial values in the similarity solution is discussed.

과지압으로 인한 암반의 점진적 취성파괴 과정의 수치해석적 연구

최영태(Young-Tae Choi),이대혁(Dae-Hyuck Lee),이희석(Hee-Suk Lee),김진아(Jin-A Kim),이두화(Du-Hwa Lee),유광호(Kwang-Ho You),박연준(Yeon-Jun Park) 한국암반공학회 2006 터널과지하공간 Vol.16 No.3

큰 초기응력을 받는 암반에서의 파괴 과정은 굴착경계에 평행하게 발생하는 응력 유도 균열에 의해지배된다. 특히 지압의 절대크기가 암반 강도의 일정 비율 이상이 되면 응력 집중에 의한 암반의 취성 파괴를 유발하고, 이러한 현상은 터널 굴착 시 발생하는 파괴음과, 굴착면에 평행한 형태로 암편이 탈락하는 취성파괴현상을 동반한다. Mohr-Coulomb과 같은 기존의 구성 모델은 일반적으로 마찰각과 점착력을 일정한 값으로 가정 하므로, 점진적인 암반의 취성파괴 현상을 모사하기 어렵다. 본 논문에서는 일반적인 수치해석 코드에서 취성파괴를 잘 모의할 수 있는 것으로 알려진 CW-FS 모델을 사용하여 유류 저장공동 주변 암반에 대한 수치해석을 실시하고, 그 결과를 선형 Mohr-Coulomb 모델의 결과와 비교하였다. 또한 마찰각과 점착력 성분의 전단 소성변형률 한계를 변화시키면서 해석을 실시하여, 유류 저장공동에서 관찰된 취성파괴와 비슷한 양상을 보이는 해석 결과를 찾아보았다. 결과적으로 모델은 견고한 암반에서의 취성파괴를 모의하는데 있어 적절한 해석방 . CW-FS 법이라는 것을 알 수 있었다. In rock mass subject to high in-situ stresses, the failure process of rock is dominated by the stress-induced fractures growing parallel to the excavation boundary. When the ratio of in situ stresses compared to rock strength is greater than a certain value, progressive brittle failure which is characterized by popping and spalling of rock debris occurs due to stress concentration. Traditional constitutive model like Mohr-Coulomb usually assume that the normal stress dependent frictional strength component and the cohesion strength component are constant, therefore modelling progressive brittle failure will be very difficult. In this study, a series of numerical analyses were conducted for surrounding rock mass near crude oil storage cavern using CW-FS model which was known to be efficient for modelling brittle failure and the results were compared with those of linear Mohr-Coulomb model. Further analyses were performed by varying plastic shear strain limits on cohesion and internal friction angle to find the proper values which yield the matching result with the observed failure in the oil storage caverns. The obtained results showed that CW-FS model could be a proper method to characterize essential behavior of progressive brittle failure in competent rock mass.