Compressive Strength and Griffith Criterion

3602 words 15 pages
The University of Hong Kong
Department of Civil Engineering

CIVL2002 M – Geology & Rock

Laboratory Report

Brazilian Test

A. Introduction As shown by the Griffith criterion, tensile strength of brittle materials is theoretical 1/8 of the compressive strength. Typically, tensile strength of rock materials is about 1/10 to 1/8 of the compressive strength. Hence, rock fails easily under tension. In design, rock should be subjected to minimum tensile stress. Several methods are commonly used to test the tensile strength of rocks: 1. Direct tensile test: Metal caps are cemented to the end-surfaces of the samples so that tensile load can be applied to the samples until failure. 2. Brazilian test: Compressive
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The number of specimens per sample tested should be determined from practical considerations. Normally, 10 specimens are recommended. Yet, in our experiment, only 8 specimens will be tested to obtain the tensile strength of the rock sample.

D. Apparatus 1. Loading Machine
It is a loading machine which applies compressive load to the rock specimens. It has a control panel showing the instantaneous applied load. At the moment when the rock specimen fails, one of the pointers will stop showing the failure compressive load

2. Steel Loading Jaws It fixed the rock specimen. Radius of jaws must be 1.5 times of specimen radius; 25mm penetration of guide pin has a clearance of 0.1mm; width of jaws must be 1.1 times of specimen thickness. 3. Electronic Calipers Measures the dimension of rock specimen (thickness and diameter) 4. Masking Tape
Wraps up the periphery of the rock specimens to reduce contact irregularities. It also prevents the specimens from breaking into pieces after failure, so that the failure plane can be observed.

Figure 5:
Schematic Diagram of the Setting Up of the Test
Figure 4:
Loading Machine