真三轴卸载煤爆实验破坏特征演化分析

Damage Evolution in the Fracture Process of Coal Burst during True Traxial Unloading Test

对煤岩体进行真三轴卸载煤爆实验,获得了其临界破坏应力、破坏碎屑分形维数及声发射参数特征,引入处理声发射波形的短时傅里叶变换方法获得时频演化特征,利用离散元颗粒流数值方法对该过程进行模拟揭示其细观损伤机制。研究结果表明:煤爆临界破坏应力状态为28.6 MPa/17.8 MPa/8 MPa,煤爆时刻声发射能量快速释放,能率达到峰值;量测粒径尺寸大于10 mm的喷出碎屑尺寸,按照粒度-数量方法算得的分形维数值为2.04;声发射时频特性在煤岩体煤爆实验过程中经历了由单峰低幅低频向双峰高频演化再过渡到单峰高幅高频,最后变为多峰高幅低频的过程,预示着破裂源由单一小尺度不断向复杂大尺度演化的损伤机制;通过颗粒流软件对煤爆过程进行模拟,发现煤岩模型试件内部主要存在张拉损伤,而随着模型加卸载不断进行,剪切破裂逐级增多,与声发射时频特性相吻合。

True traxial unloading test was performed on coal sample and then the critical failure strength for coal burst and fragment fractal dimension as well as acoustic emission(AE)parameters evolution were determined.Using short-time Fourier transform(STFT)method to process AE wave,we can obtain its time frequency feature.Finally,coal burst process was mimicked by particle flow code(PFC)to reveal its mesoscopic damage mechanism.The main research results are as follows:critical burst failure stress state is 28.6 MPa/17.8 MPa/8 MPa,AE energy release quickly at burst time and energy ratio reaches its peak value,the fractal dimension of ejected fragments with dimension larger than 10 mm is about 2.04 using particle size-quantity method.AE time frequency evolution has a transition from unimodal wave with lower amplitude and frequency to bimodal wave with higher frequency and then back to unimodal wave with higher amplitude and frequency and at last multimodal wave with higher amplitude and lower frequency,indicating the damage evolution from single damage source in small scale to complex damage source in large scale.PFC 2 D can better simulate coal burst process and reflect the whole meso damage process of coal sample.It can be seen that tensile fracture mode mainly exists in coal sample during burst whereas shear fracture increases as the experiment proceeding,which agrees with the AE time frequency feature.