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Abstract The uniaxial compression with stepped relaxation was conducted on coal samples with the testing system of rock mechanics RMT–150B. The aging characteristics of deformation，strength and failure of coal samples under uniaxial compression and uniaxial compression with stepped relaxation were analyzed. The results show that the stress-strain curves ahead of the peak values of coal samples at two loading conditions have four conventional stages，i.e. densification，elastic，yielding and failure. The stress-strain curves of uniaxial compression with stepped relaxations have a local drop of stress during the relaxation. The values of mechanical parameters of uniaxial compression are higher than ones of that with stepped relaxation，showing that the mechanical parameters of coal samples have aging characteristics. When the ratio of stress relaxation is lower than 70%，the characteristics of stress relaxation is not obvious. When the ratio of stress relaxation is higher than 70%，materials of lower strengths were gradually damaged by the energy stored in the materials of higher strength inside coal samples and the materials of higher strength carried higher stresses and were also be damaged gradually. The higher the ratio of the stress relaxation was，the shorter the length of time of failure was. There were noticeable surfaces of tension-shear fracturing for failure samples under uniaxial compression；but the failure cracks of coal samples in the stepped relaxation tests were in loading directions and many chippings and fragments were formed，showing clearly a characteristic of lateral expansion.
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Received: 17 September 2013
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2014, Vol. 33 
