Papers by Author: R.H.C. Wong

Abstract: This paper presents the crack growth mechanism from a 3-D surface flaw on gabbro specimens using strain measurement and acoustic emission (AE) technique. Based on the results of strain and AE measurement, microcracks initiated inside the rock and extend to the surface of the specimen. With the observation from the measurements, four types of crack patterns initiate wing crack, anti-wing crack (opposite direction of wing crack), petal crack and compressive crack. The strain values of anti-wing cracks are larger 1 to 2 times than that of wing crack. The AE energy release from anti-wing crack is higher 2.5 times than that of wing crack, while the energy release form wing crack is the least but the compressive crack is the highest. Thus, the appearance of initiation and propagation of the anti-wing crack and compressive crack are very actively than that of the wing crack. The strain and AE measurement is not only to provide a clear concept on the mechanisms of crack growth form a 3-D surface flaw but also to provide useful knowledge on the AE property of the crack patterns.

Abstract: A number of instability problems in rock engineering projects are caused by crack propagation. However, crack growth mechanisms from 3-dimentional flaw are not fully understood, in particular for 3-D flaw case with varied dipping angle. This study focuses on 3-D surface flaw using real rock specimens containing a flaw with varied inclination angle α from axial loading and dipping angle γ from specimen surface under uniaxial compression. Acoustic emission technique was used for tracing the initiation and growth of micro-cracks inside of specimen. It was found that crack growth process is affected by the dipping angle γ of the 3-D flaw. When dipping angle γ ≠ 90º, the thickness of rock above the flaw plane is thinner than that of below the flaw plane. As a result, compressive crack and wing crack initiated easily from the thinner flaw tips. And, the normalized stress for crack initiation σi /σc, AE events and the AE energy for crack growth decreases with the dipping angle γ. However, for γ = 90º, the thickness of rock above and below of the flaw tips is the same, it was observed that anti-wing crack (crack growth direction opposite to wing crack) initiated first at a certain place away from the flaw tips, then wing crack and compressive crack emerged at the late stage. For this case, the stress σi /σc, AE events and the AE energy for crack initiation and propagation are at a high value. Thus, for rock mass contains flaws geometry with small dipping angle, some problems of crack propagation may be induced easily during excavation.

Abstract: Under extra compressive stress, some phenomena of rock spallings and fractures often exist on rock mass located in sidewalls of underground house and tunnels. It is the reason that the crack growth and coalescence initiation from original flaws (or faults) in rock mass. In the previous studies, many researchers took a flaw as a through flaw (2-dimentional model), but the flaws are not always through the whole rock mass in fact, most of them are only near the surface of rock mass, These are so named as surface flaws. They belong to three dimensional (2-D) flaws. Now, the reports on initiation and growth of 3-D surface flaw are few. So, for the investigation on growth patterns of 3-D surface flaw, a series of samples containing a surface flaw were carried out using frozen casting resin material at about -30°C temperatures. The surface flaw was made of a polyester film was used to model a single closed flaw on rock mass. The experimental results show that the wrapping wing crack (Mode I) initiated at the ends (or tips) of surface flaw first, and then formed a kinking zone (mixed crack zone) at a certain place at the middle of surface flaw region. Some petal cracks (Mode III) and shell-shaped cracks (Mode III) would grow at the middle place of flaw. A big fin crack (Mixed Mode) also emerged in middle of flaw and grown along loading direction. Finally, a team of large cracking curved faces deformed inside the resin specimen; the whole specimen would be splitted off by the initiation and growth of the cracks. The reasons lead to the fracture patterns of 3-D closed surface flaw were provided with brittle fracture mechanics theory in the article, preliminarily.

Abstract: Under the action of compressive load, the growth and coalescence containing flaws in brittle materials (rock and rocklike materials e.g.) will result in the local buckling and global fracture of rockmass. But, the mechanisms on propagation and coalescence of 3-dimensional internal flaws are not clear till now. We examine brittle fractures of manmade specimens using frozen casting resin and rocklike material to observe 3D internal flaws growth process at about -30° C. A team of specimens containing three internal flaws is measured; flaws are made of three parallel oblong aluminum films. The propagation and coalescence pattern of three internal flaws is observed under compressive stress. An interesting phenomenon is that the crack initiated from the second flaw quickly turns to the one induced from the third flaw and forms a bigger fracture plane, then splits the specimen. It shows that the flaw distribution pattern will greatly affect the flaws growth and coalescence process. The mechanisms that lead to the wing and anti-wing crack initiation and coalescence are described.

Abstract: It has long been recognized that the strength of brittle rocks decreases with the grain size. However, very few systematic investigation of this phenomenon has been made using numerical method. This paper presents the results of a numerical simulation using the Rock Failure Process Analysis code (RFPA2D) to investigate the effects of grain size on the uniaxial compressive strength and the failure behavior of Yuen Long marble. The Weibull distribution with two parameters (m that characterizes the strength heterogeneity, and σ0 that corresponds to the mean strength of an element) selected based on micromechanical basis is used in the RFPA2D code for simulation. The simulated stress-strain curves of Yuen Long marbles with different grain sizes under uniaxial compressive condition agrees well with the experimental study. The progressive failure process was captured in the numerical simulations. Our simulations also reproduced the influence of grain size, with strength scaling approximately with the inverse square root of grain size, which is in agreement with the previous experimental study.

Abstract: This study is to evaluate the effect of the heterogeneity on the failure processes and strength characterization of brittle rock containing the single pre-existing crack (or flaw) under uniaxial compression loadings. The numerical simulation reproduces the evolution of the stress and strain fields in flaw propagation process, the mode of acoustic emission related to the heterogeneity of rock and the phenomenon related to discontinuous. It is shown that the lower the value of the homogeneous index, the more influence of local variation on the propagation process of the pre-existing flaw, and there occurs more randomly distributed microfractures throughout the specimen. Studying the details of macrofracture formation in relatively homogeneous specimens, it is interesting to find that there exists a 'constant jump' propagation pattern of the wing crack, which is responsible for the formation of the pre-existing flaw. The numerical results also demonstrate that the stress-strain relation and strength characterization depends strongly on the heterogeneity of the specimen. The heterogeneous rock has a gentler post-peak behavior and lower strength, while the more homogeneous specimen has a higher strength, accordingly, the curve becomes more linear and the strength loss is also rapidly.

Abstract: A major challenge in rock mechanics has been the realistic modeling that can reveal the progressive accumulation of damage and shear localization in a brittle rock under compression. The Rock Failure Process Analysis code (RFPA^2D) is an efficient tool and realistic model to simulate such complexities. A key assumption of the code is that the heterogeneity of elastic moduli and failure strength are characterized by the Weibull distribution with two parameters (m and σ₀ ). However, these two parameters do automatically not relate to the microstructural parameters, such as grain size and microcrack statistics. Therefore, the purpose of this paper is to elucidate the micromechanical basis of these Weibull parameters, specifically how they depend on microstructural attributes such as grain size and crack statistics. Secondly, a methodology was developed to quantitatively determine the relevant micromechanical parameters for input into the RFPA^2D code. Finally, the methodology was implemented by quantifying the microcrack geometry and statistics of real rock and simulating its uniaxial compression and progressive failure behavior. The simulated result agrees well with the experimental study.