Precursory Failure Of Rock Samples Under Three Point Bending Based On Acoustic Emission And Infrared Radiant Temperature Field

After a long period of geological changes,the rock has a complex structure and various failure modes.Up to now,the mechanism of rock failure is not clear.Accurately predicting the occurrence of rock failure can provide a guarantee for safe production.In order to further study the failure characteristics and precursor law of the rock under tensile-shear conditions,a threepoint bending test of diabase samples containing a single crack was carried out using a research method combining physical testing and numerical simulation,and the stress state near the incision was changed by changing the position of the prefabricated crack.In the physical experiment,the acoustic emission instrument and the infrared thermal imager were used to jointly monitor the failure rule and precursor characteristics of the diabase under tensile-shear fracture conditions.Acoustic emission instrument obtains rock fracture precursor information and damage degree.Infrared radiation imager studies on the characteristics of radiation temperature and frequency domain of thermal image sequence.Numerical simulation can display the distribution of the stress field on the sample surface,which can make up for the shortcomings of the two monitoring methods.The main results are as follows:(1)Obvious brittle fractures occur in the diabase specimens under the condition of threepoint bending.The failure modes and energy consumption of specimens vary with the location of the pre-crack.The coefficient of offset influence has the first-order exponential decay fitting relationship with the offset ratio.The support distance d from the left support of the pre-crack increases,the bending resistance decreases and the dispersion of fracture energy consumption in the same working condition decreases.(2)Acoustic emission tensile signals are predominated,and tensile cracks penetrate the diabase before the final failure.The acoustic emission activity is intense,and the event rate,ring counts,energy and amplitude all increases suddenly,and the value of r drops sharply approaching to the rock failure.(3)The acoustic emission frequency spectrum is divided into three regions of low frequency,intermediate frequency and high frequency.The dominant frequency only has a large dominant frequency band in the intermediate frequency region.The secondary dominant frequency has one band in each of the three frequency domains,and the secondary dominant frequency band in the intermediate frequency area is the most obvious.The dominant frequency and secondary dominant frequency are dense in the high-frequency area approaching to the rock failure.Compared with the dominant frequency,the secondary dominant frequency tends to migrate from the intermediate frequency to the low frequency and high frequency.According to the distribution of acoustic emission RA & AF and the proportion of high frequency and low frequency of the dominant frequency and secondary dominant frequency,the tensile and shear fracture conditions during rock failure can be judged.(4)The damage evolution of rock samples is divided into three stages: micro-damage stage I,damage manifestation stage II and damage instability precursor stage III.The sequence of emergence of damage precursors characterized by acoustic emission parameters: event rate <ring counts <energy counts.(5)Before the sample is finally destroyed,the maximum temperature and average temperature of infrared radiation both appear the quiet period.Average infrared radiation temperature is positively correlated with stress and strain,and has a cubic polynomial relationship with the cumulative mechanical work curve.The vertical high-frequency band in the center of the two-dimensional spectrogram gradually widens and expands laterally,and the sample shows "?" after being destroyed.(6)The numerical simulation of pre-crack initiation,propagation and fracture morphology are in good agreement with physical tests.As d decreases,the stress increases,the hardness and brittleness increase.The starting point of continuous occurrence of acoustic emission can be used as a precursor of brittle fracture.The numerical simulation has the highest priority,followed by acoustic emission,and the lowest infrared radiation.There is a critical position in the offset notch,which determines the tensile and shear stress state and fracture path of the crack.