(State Key Laboratory of Geomechanics and Geotechnical Engineering，Institute of Rock and Soil Mechanics，Chinese Academy of Sciences，Wuhan，Hubei 430071，China)
Abstract：Brittleness is one of the most important properties for rock materials，to which many mechanical behaviors of rock are related. Existing indices of brittleness based on strength，stress-strain curve，loading and unloading tests，hardness and mineral composition were summarized，and merits and weakness of the indices were analyzed in detail. In order to evaluate the brittleness of rocks reasonably and accurately，a new index of brittleness reflecting the characteristic of plastic yielding and the effect of stress state was introduced on basis of the relative magnitude and the absolute velocity of post peak stress dropping. Axial and triaxial compression tests were conducted to examine the new index. The results of tests show that brittleness of cement mortar and marble both decrease with the increasing of confining pressure and the brittleness of marble is greater than the one of cement mortar under the same stress state，which agree with the actual brittleness of the two rock materials. Brittleness of limestone，marble，granite and red sandstone decrease in order and their axial strain at failure increase in order gradually，which conforms to that the low values of elongation means more brittle. The test results verified the reliability of the new index of brittleness nicely which may enrich and improve the existing evaluation methods of rock brittleness.
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2014, Vol. 33 
