Memory effects in rock salt under triaxial stress state and their use for stress measurement in a rock mass

Regularities of memory effects in rock salt specimens under triaxial stress state Were investigated. Each specimen was subjected to two loading cycles . The first cycle was axisymmetric triaxial compression (sigma (1) > sigma (2) = sigma (3)). The second cycle was uniaxial compression in the directio n of a, of the first cycle. Distinct acoustic emission (AE) and deformation memory effects were observed in the second cycle at the stress value equal to a linear combination of the first cycle principal stresses given by sig ma (1) - (k + 1)sigma (3), where k is about 0.5-0.6 for rock salt. Anomalie s in deformation curves were found to be more reliable than the AE methods in distinguishing memory symptoms. The necessary pre-requisite for memory f ormation in the first cycle was that sigma (1) exceeded the elastic limit, corresponding to the given confining stress sigma (3). Inflections in uniax ial stress versus axial strain and lateral strain curves, in the second cyc le, were observed at equal stress values if in the first cycle sigma (1) ex ceeded the elastic limit and memory-forming damage Was induced. If there wa s no memory-forming damage, those inflections were seen at different stress values. This characteristic was used to distinguish between true memory ef fects and natural characteristic points in deformation curves derived from rock salt testing. A new memory symptom was established, namely a turn poin t in curve "uniaxial stress versus differential coefficient of lateral stra ins". The results form a basis for application of the memory effects for st ress measurement in rock salt masses.