Nonlinear mode-I fracture propagation characteristics and crack length quantitative measurement of quartzite under post-peak multi-cyclic loading
WANG Xiaoran
ZHOU Xin
LIU Xiaofei
WANG Enyuan
XIE Hui
SHAN Tiancheng
LIU Shuxin
Abstract：Rock engineering disasters result from the evolution result of crack initiation,propa-gation and fracture catastrophes in the surrounding rock.The severity of these disasters prima-rily depends on the dynamic propagation characteristics of fractures in the rock mass during the post-peak stage.To achieve precise characterization of the nonlinear fracture propagation in rocks and to quantitatively determine of dynamic lengths of cracks in the post-peak stage,this study focuses on quartzite beams with a prefabricated notch subjected to a three-point bending configuration.By utilizing a clip extensometer,the quasi-static propagation of mode-I fractures during the post-peak stage is achieved.The detailed crack propagation process in quartzite beam,under post-peak multi-cyclic loading,is visually tracked using acoustic emission(AE)and digital image correlation(DIC)techniques.Subsequently,a quantitative determination method for the nonlinear crack propagation length is proposed based on energy distribution of AE,displacement gradient from DIC,and mechanical compliance.The measured results are then compared with the theoretical model of fracture mechanics for validation.The results show that:with the increase of the post-peak cyclic number,the unloading slope of COD-P(i.e.,crack opening displacement-load)and LPD-P(i.e.,load point displacement-load)curves gradually becomes less steep,which indicates the crack gradually moves forward,and the bearing capacity of the quantize beam gradually decreases.During each cyclic process,the AE events become active starting from the pre-peak damage stress and continue until the post-peak unloading point,while the spatial distributions of these AE events tend to concentrate in the middle and upper regions of the crack.Additionally,the tips of AE events coincide with the in-tersection points of DIC horizontal displacement contour.During the post-peak cyclic loading of quartzite beam,the equivalent crack lengths determined using the AE energy distribution method closely align with the results obtained from the DIC displacement gradient method.Furthermore,the deviations from the traction-free crack length calculated using the unloading compliance method from the COD-P and LPD-P curves remain relatively stable at 5.1 mm.This specified value of 5.1 mm corresponds to the fracture process zone(FPZ)length of the quartzite.According to the Dugdale-Barenblatt model,the calculated FPZ length of quartzite is about 7.6 mm,which shows excellent agreement with the asymptotic critical value for an infi-nite size FPZ as derived from the extension of abovementioned results considering size effects.This agreement demonstrates the reliability of the proposed measurement approach for determi-ning the nonlinear crack propagation length,as well as emphasizes the significance of obtaining me-chanical parameters of rock at an engineering scale by extending the size effect criterion.
Keywords：mode-I fracture testpost-peak cyclic loadingacoustic-image-mechanical measure-mentunloading compliance approachfracture process zonesize effect
Publication Date：2024-07-30
Online Publishing Date：2025-08-15(First online date of this platform, not the publication date of the document)
Pages：16( 680-695 )