Study On Damage Evolvement Rules Of Rock Under Cyclic Load

As a kind of non-homogeneity & brittle materials, the damage failure of rock is a very important problem relating to many engineering fields under cyclic load. In the paper, studies on damage evolvement characteristics of rock have been presented with advanced rock mechanics test equipment named on MTS815 and 12CHs SAMOS^TM system acoustic emission (AE) instrument under cyclic load.In this paper, the acoustic emission rules have been studied using the methods of experiment research and theory analysis with different upper limited stress level, different lower limited stress level, different load rates and different saturation under cyclic load. Based on the theory that AE is the macroscopical behavior of damage, and is consistent with the damage, the damage evolvement rules have been analyzed; At the same time, the damage evolvement equations have been established; At last, the AE correlation dimension have been calculated using fractal theory, and analyzed the changing rules of correlation dimension. The results could be summarized as follows:(1)AE characteristics of rock have been studied under different load conditions under uniaxial stress state, and the damage evolvement equation based on AE parameter has been improved; The cyclic stress threshold has been studied through the AE Kaiser effect & Felicity effect experiment, and the upper stress of Kaiser effect is the 70% of peak stress.(2) The upper stress is the most important factor to AE under cyclic load, which determined the changing tendency of AE; The lower stress advanced the time of AE in every cycle at the end of cyclic load, that is to say, the Felicity ratio becomes smaller; Load rate affects the AE rate mostly, especially in the phase of main crack expanding, which improves the rate of main crack forming and expanding during the cyclic load, and reduces the time of failure; The AE results of different saturation experiment shows that the bigger saturation, the lower AE rate, and the total counts become few, at the same time the lower energy releases at the failure.(3) There are different load rates and stress ranges in cyclic load, there are different AE changing modes; Different mode predicts the different failure rate of rock, that is to say, the type of stabilization needs more time of failure, and the type of mutationist needs little time of failure; The cyclic damage evolvement mode based on AE parameter has been established; The space-time evolvement characteristics have been analyzed through AE location, and damage evolvement rule has been studied from time and space; At the same time several factors influencing the precision of AE location have been studied.(4) The correlation dimension shows the downward tendency with the cyclic ratio increasing under cyclic load, but there are some discrepancies under different experiment condition; The AE correlation dimension is according with the AE total counts which show the cumulate damage, and the correlation dimension which has the correlativity with the damage will change because of the cumulate damage under cyclic load. The downward tendency of correlation dimension shows that the crack of forming and expanding have the tendency from chaos to regularity; The AE correlation dimension changing is consistent with the crack changing, that is to say, the obvious dimension changing shows that the crack changing quickly ,and the steady dimension shows that the crack state has little change.