Study on failure precursors in red sandstone under cyclic loading and unloading based on k-means clustering

Sun Guanghua; Qi Yuzhu; Liu Zhiyi; Han Kaiming; Guo Jiang; Wu Bingwei

doi:10.16186/j.cnki.1673-9787.2024070082

Abstract：Objectives In engineering practice,rocks are often subjected to cyclic loading and unloading conditions,making it essential to investigate the failure precursors in red sandstone under such conditions.Methods Mechanical monitoring tests with acoustic emission(AE)measurement were conducted on red sandstone under cyclic loading and unloading.The k-means clustering algorithm was employed to perform cluster analysis on AE characteristic parameters,aiming to investigate the rock damage and fracture pat-terns.Results The results indicate that acoustic emission(AE)events occur predominantly during the cy-clic loading phase,while very few events are recorded during the unloading phase,marking an"intermit-tent period".As the number of cycles increases,the peak stress gradually rises,and the number of AE events surges abruptly,entering an"active period".At the initial stage of cyclic loading and unloading,the AE b-value is the highest.As the number of cycles increases,the AE b-value gradually decreases and stabilizes within the range of 0.75～1.25.Before the rock enters the fracture stage,the AE b-value shows a sharp drop.However,a similar sharp decline in the AE b-value is also observed at the beginning of each loading cycle,even though the rock does not fracture.This suggests that using a sharp drop in the AE b-value as a precursor indicator for rock failure has limitations,and it is necessary to combine other phenom-ena to study rock fracture precursors.Based on k-means clustering,the AE signals during the rock damage and fracture process are classified into three categories:cluster 1,cluster 2,and cluster 3.Each cluster corresponds to different damage stages.In the late stage of cyclic loading and unloading,the energy of AE events in cluster 3 increases sharply,reflecting a significant enlargement of fracture scale in the sandstone during the final loading stage until failure.This can serve as a characteristic precursor to rock failure.Con-clusions Cluster 3 acoustic emission signals sporadically appear during the elastic deformation stage of the rock.As cycling accumulates,these signals become continuously and densely concentrated prior to the com-plete failure stage of the rock.Integrating the analysis of the AE b-value with the cluster-based signals can enhance the accuracy of predicting rock damage and fracture.