Sasaasa

Ans 5.

In Fig., 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 specimens. The specimen contains microcracks (which are a statistical sample from the rock microcrack population): the larger the specimen, the greater the number of microcracks and hence the greater the likelihood of a more severe flaw. With respect to the tensile testing described earlier, it has been said (Pierce, 1926) that "It is a truism, of which the ramifications are of no little interest, that a chain is only as strong as its weakest link". The elastic modulus does not vary significantly with specimen size because the relation between overall stress and overall strain is an average response for many individual aspects of the microstructure. However, the compressive strength, being the peak stress that the specimen can sustain,

is more sensitive to exfremes in the distribution of microstructural flaws in the sample. A larger sample will have a different flaw distribution and, in general, a more 'extreme' flaw. Also, this statistical effect will influence the form of the post-peak curve.

There have been many attempts to characterize the variation in strength with specimen size using extreme value statistics and, in particular, Weibull's theory, but it should be remembered that this theory is based

on fracture initiation being synonymous with fracture propagation, which is not the case in compression. Thus, if extreme value statistics are to be applied to the analysis of compressive strength, then some form of parallel

break-down model is required, rather than the weakest-link Weibull.

Here, we discuss the complementary effect, the shape effect, when the size (i.e. volume) of the specimen is preserved but its shape changes. In Fig., we illustrate the effect of shape variation in uniaxial compression.

The...