Abstract: To investigate the extension characteristics of coal samples semi-submitted by liquid nitrogen with different water saturation, the anthracitic coal samples taken from Shanxi province were semi-submitted in liquid nitrogen with the self-made experimental setup. The ultrasonic velocity change and the cracks’ width extension of coal samples subjected to liquid nitrogen were tested. The results show (1) the saturation of coal samples has great effects on the cracks’ extension subjected to liquid nitrogen. The ultrasonic velocity change and the cracks’ width extension of the saturated coal samples are maximum, Those of the semi-saturation coal samples become worse. Those of dry coal samples are worst. (2) The nearer to the surface of the liquid nitrogen, the greater the temperature change of coal samples is. And there is much more significant effect of liquid nitrogen. (3) The main reason of the cracks’ extension of the saturating and semi-saturation coal samples is the frost heave pressure caused by the phase from water to ice. Thermal stress due to temperature gradient induces the cracks’ extension of dry coal samples.

Key words: cracks&rsquo, width extension rate