TABLE OF CONTENTS Having described how the complete stress-strain curve can be obtained experimentally, let us now consider the effects of the specimen size and shape, loading conditions, and time and temperature effects.
It is well known that many materials exhibit a size effect in terms of strength, with smaller specimens indicating a higher strength than larger specimens. This was probably first recognized by Leonardo da Vinci, who found that longer wires were not as strong as shorter wires of the same diameter. In more recent times, Griffith (1921) showed that thin filaments of glass displayed much higher tensile strengths than thick filaments. Similarly, there are ductility effects as the temperature of a material is increased.
Thus, it is prudent to consider the effects of specimen geometry, loading conditions and environment on the complete stress-strain curve. This is because we need to understand the effects of these variables in order to be able to predict the mechanical behaviour of rock under conditions which may differ from those under which a specimen of the same rock was tested in the laboratory. The discussion below on these effects describes the general trends that have been observed in laboratory tests over the years.
In Fig. 6.11, we illustrate how the complete stress-strain curve varies with specimen size, as the ratio of length to diameter is kept constant. The main effects are that both the compressive strength and the brittleness are reduced for larger...
