Determination of Fracture Toughness of Anisotropic Rocks Under Water Vapour Pressure by Semi-Circular Bend Test

Failure of rock materials is a process of crack propagation. Crack initiation takes place when the crack tip stress intensity K reaches a critical value called fracture toughness, K1C. The rock fracture toughness is known to be affected by the surrounding environment such as temperature, confining pressure and humidity. In order to examine the effect of humidity a series of semi-circular bend tests were performed under various water vapour pressures in a rock material that is known to be anisotropic. Water vapour promotes stress corrosion of rock and therefore the fracture toughness was found to have a decreasing trend with increasing water vapour pressure. The rate of decreasing the fracture toughness depends on the microcrack density that promotes the migration of water vapour into the rock. Also in an anisotropic rock the fracture toughness depends on the direction of crack in relation to the anisotropy of the rock material

Numerical modelling of monorail support requirements in decline development

This paper discusses support requirements for the proposed monorail system to be used in decline development. The monorail drilling and loading systems are systems that move on the rail (monorail) installed in the roof of the decline and supported by roof bolts, suspension chains and steel supports. However, due to the weight of the components of the two systems, it is imperative that the force in each roof bolt, suspension chain and steel support capable of suspending the weight of the heaviest component is determined. Numerical models that relate the weight of the monorail drilling and loading components to the required strength in the support system have been developed. Using these developed models, numerical values of the forces in each roof bolt, suspension chain and steel support, required to suspend the weight of the heaviest component of the monorail drilling and loading systems are determined

Automation design for monorail-based system processes

Currently, conventional methods of decline development put enormous cost pressure on the profitability of mining operations. This is the case with narrow vein ore bodies where current methods and mine design of decline development may be too expensive to support economic extraction of the ore. According to studies, the time it takes to drill, clean and blast an end in conventional decline development can be up to 224 minutes. This is because once an end is blasted, cleaning should first be completed before drilling can commence, resulting in low advance rates per shift. Improvements in advance rates during decline development can be achieved by application of the Electric Monorail Transport System (EMTS) based drilling system. The system consists of the drilling and loading components that use monorail technology to drill and clean the face during decline development. The two systems work simultaneously at the face in such a way that as the top part of the face is being drilled the pneumatic loading system cleans the face. However, to improve the efficiency of the two systems, critical processes performed by the two systems during mining operations must be automated. Automation increases safety and productivity, reduces operator fatigue and also reduces the labour costs of the system. The aim of this paper is, therefore, to describe automation designs of the two processes performed by the monorail drilling and loading systems during operations. During automation design, critical processes performed by the two systems and control requirements necessary to allow the two systems execute such processes automatically have also been identified

Evaluation of Mode I Fracture Toughness Assisted by the Numerical Determination of K-Resistance

The fracture toughness of a rock often varies depending on the specimen shape and the loading type used to measure it. To investigate the mode I fracture toughness using semi-circular bend (SCB) specimens, we experimentally studied the fracture toughness using SCB and chevron bend (CB) specimens, the latter being one of the specimens used extensively as an International Society for Rock Mechanics (ISRM) suggested method, for comparison. The mode I fracture toughness measured using SCB specimens is lower than both the level I and level II fracture toughness values measured using CB specimens. A numerical study based on discontinuum mechanics was conducted using a two-dimensional distinct element method (DEM) for evaluating crack propagation in the SCB specimen during loading. The numerical results indicate subcritical crack growth as well as sudden crack propagation when the load reaches the maximum. A K-resistance curve is drawn using the crack extension and the load at the point of evaluation. The fracture toughness evaluated by the K-resistance curve is in agreement with the level II fracture toughness measured using CB specimens. Therefore, the SCB specimen yields an improved value for fracture toughness when the increase of K-resistance with stable crack propagation is considered

Fracture toughness testing of geomaterials using semicircular specimen

© 2018 ISRM International Symposium 2000, IS 2000. All rights reserved. Fracture mechanics is primarily used to predict the failure of structures made of engineering materials. Principles of fracture mechanics are applicable for improving the strength and stability of mine structures. However, fracturing is considered to be favourable in certain other applications such as blasting and comminution in mining and mineral processing. Fracture properties such as fracture toughness are useful for the fracture characterisation of a material irrespective of the type of application. Mode I fracture toughness is a material property that is related to the critical stress intensity factor in the crack opening mode. In this investigation, semi-circular bend test specimens having a chevron notch are used to determine the fracture toughness. The relationship between the stress intensity factor and crack length is determined using the finite element method. The fracture toughness is shown to correspond to the minimum level of the stress intensity factor associated with crack extension. Fracture toughness is evaluated in a number of rock types using the experimentally determined fracture load and the normalised stress intensity factor

A study of crack initiation and crack growth in elastic and elastic-plastic materials using J-integral method

published_or_final_versionMechanical EngineeringDoctoralDoctor of Philosoph

Influence of specimen size on fracture toughness of sandstone when using SCB specimen

In order to investigate the effect of specimen size on the mode I fracture toughness using Semi-Circular Bend (SCB) specimen, we experimentally studied the fracture toughness using SCB and Short Rod (SR) specimens, both of which are International Society for Rock Mechanics (ISRM) suggested methods, for comparison. Fracture toughness of a strong sandstone having an unconfined compressive strength of 145 MPa was investigated. The test results revealed that the fracture toughness increased with specimen diameter and reached a limiting value. These experimental results further showed that the value of fracture toughness determined using SCB specimen is almost exactly the same as that using SR specimen, when the SCB specimen satisfies the minimum size requirement. However, SCB specimen being one without a chevron notch unlike SR specimen gives lower fracture toughness if the minimum size requirement is not satisfied

Development of a standard method for determining the plane strain fracture toughness of rock

A number of standard methods have been proposed to determine the Mode I plane strain fracture toughness of rock. The semi-circular bend (SCB) specimen has been widely used for fracture toughness determination of geomaterials owing to inherent favourable properties such as its simplicity, minimal requirement of machining and the convenience of testing. However, measurements of Mode I fracture toughness using SCB specimen have shown variability with specimen size. The minimum size requirements suggested by many theories are usually too large for practical applications. This research suggests an approximate, but relatively easily applied method, to satisfy the minimum size requirement. Fracture toughness of Kimachi sandstone was determined using the SCB specimen. By testing specimens of varying size, it was confirmed that the apparent fracture toughness increases with the specimen size. The trend line of the apparent fracture toughness displayed eventual stabilisation giving a value that can be considered as specimen size-corrected fracture toughness. This paper describes the method of determining the SCB specimen size-corrected fracture toughness and also highlights the advantages of using the SCB method over the other suggested standard methods