A Study Of The Failure Process In Brittle Flawed Rocks And Geophysical Failure Forecast By Integrated Acousto-Optical Data

The deformation and failure characteristics and the underlying failure mechanisms of brittle flaw-contained rock are the basic scientific problems for the effective prevention of rock mass engineering instability.Using the acoustic emission(AE)technique,digital imaging technique and the peridynamics,this thesis deals with the stress-induced damage progressions and catastrophe in brittle rocks.Meanwhile,this thesis forecasts the catastrophic rupture time based on the pseudo-prospective modelling of precursory AE time series.Exploring the damage progressions and catastrophe in brittle rocks is conducive to deeply understanding the deformation and cracking characteristics,along with the fracture mechanisms.Accordingly,this study provides the theoretical basis for effectively controlling rock engineering instability,and this study also has a positive significance in the establishment of excellent early warning systems for rock engineering disasters.This thesis highlights the following four aspects:(1)Damage progressions and crackling noises in brittle failure of granite vs.sandstone;(2)Quantitative correlations among multiple physical field characteristics at the unstable cracking phase in flawed granite;(3)Progressive failure of brittle rocks with non-isometric flaws and the underlying mechanism;and(4)Forecasts of catastrophic rupture in brittle rocks using precursory AE time series.The main conclusions are summarized as follows:(1)Three types of grain-scale microcracks,i.e.,grain boundary cracks,intragranular cracks,and transgranular cracks,are generated in the granite due to its well-fused interlocking texture.The progressive clustering of these three types of grain-scale microcracks enables a visible FPZ,i.e.,white patch.Process zone nucleation mechanism is revealed as the microcrack clustering effect.The step-rise characteristics of the AE event rate before the first macrofracturing event and the apparent white patching phenomenon are the most significant acousto-optical evidence for process zone nucleation in the granite.In this thesis,the combinative acousto-optic-mechanical methodology is established,and the complete cracking process of the granite is classified into six levels:(I)crack closure,(II)linear elastic deformation,(III)process zone nucleation,(IV)crack initiation and stable crack growth,(V)critical energy release and unstable crack growth,and(VI)failure and post-peak softening.These six cracking processes are distinguishable by five characteristic stresses:(1)crack closure stress?_cc,(2)micro-crack nucleation stress?_cn,(3)crack initiation stress?_ci,(4)crack damage stress?_cd,and(5)peak stress?_UCS.In this thesis,the process zone nucleation level,of which the stress threshold is the micro-crack nucleation stress,is considered as one cracking level of the granite for the first time.However,the clustering of microcracks is insignificant in the brittle failure of the sandstone such that the fracture process zone is difficultly detected acoustically and optically.(2)Process zone nucleation,intensification,progression into macrocracks,and reactivation in the granite share the mechanisms of the initiation,growth and coalescence of grain-scale microcracks in the forms of grain boundary cracks,intragranular cracks and transgranular cracks.It is the first time that the robustly high AE event rate level registered in the stress build-up phase during the unstable crack growth stage in the granite has been mechanically correlated with the process zone nucleation,intensification and reactivation.The relatively low AE event rate level registered in the stress drop phase has been mechanically correlated with the progression of the FPZs into macrocracks.For the granite,the nucleation and reactivation of the FPZs by the stress build-up and the subsequent progression of the FPZs into macrocracks are the main mechanisms for observations of the reversed U-shaped curve relation between the AE event rate and the loading time during the unstable crack growth stage.(3)The interaction of stress fields around the tips of pre-existing flaws exhibits the stress buildup effect when the tips of pre-existing flaws are vertically aligned with the uniaxial loading;whereas,the interaction of stress fields around the tips of pre-existing flaws displays the stress shielding effect when the vertical distance of the adjacent tips of pre-existing flaws are relatively large.For the non-isometric flaw configurations,the ultimate failure is commonly triggered by the shear fracturing from the relatively long pre-existing flaw,while the short flaw is conditionally involved in ultimate failure especially when the interaction between the long and short flaws exhibits the stress buildup effect.It is noted that the short flaw is involved into the early fracturing process including the crack initiation,propagation and coalescence.The stress buildup effect and the stress shielding effect are closely dependent on the stress path,the flaw configuration feature and the evolving path of newly-generated cracks.This thesis also deals with the effects of flaw length ratios on the mechanical properties,the cracking characteristics and the ultimate failure patterns.(4)The accelerating damage behaviour in the vicinity of catastrophic rupture can be approximately described by the power law relation.The precursor with a power-law singularity is the physical base on which the catastrophic rupture in brittle rocks may be forecasted.Based on such experimental observation,pseudo-prospective failure forecast model in the framework of time-reversed Omori law is proposed.Firstly,the precursory AE time series can be quantitatively identified by the AE rate characteristics using the best linear correlation in a retrospective way.Importantly,the acoustically-determined onset-of-acceleration of damage is faithfully supported by the quantitative analysis of photographic data using the developed dual-factor characterization method.The catastrophic rupture of brittle rocks is successfully forecasted by using pseudo-prospective failure forecast model.The higher-quality forecast of catastrophic rupture,being both more precise and less biased,is achieved as the precursory time series of RT rate proceeds towards rupture.Pseudo-prospective modelling of precursory AE time series obtains a higher-quality forecast than retrospective modelling.Moreover,the use of amplitude as a precursor is conducive to the improvement of forecast quality and the increase of warning time.Strong disturbances due to the flaw-to-flaw interaction are introduced into the forecast results.The analysis shows that the choice of the appropriate precursor and the achievement of AE location are the two important directions to lengthen the warning time.