Experimental Study On The Mechanism Of Stick-slip Shear Failure Of PMMA

The study on the mechanism of stick-slip shear failure is of great significance to understanding shallow earthquakes failure processes and many other mechanical problems related to stick-slip in engineering practice and scientific research.This paper focuses on studying the viscous and sliding phase in the process of stick-slip.By using the PMMA samples with friction faults and the specific experiment system,high-speed records of shear stress and slip displacement along the fault can be captured during the stick-slip shear failure processes.High-speed records of shear stress and slip displacement clearly show both the propagation of the stick-slip failure and details of the local slip-weakening behavior of the fault.The results support an idea that stick-slip instability can be viewed as a dynamically propagating slip failure growth under mode II conditions in terms of fracture mechanics,a slip failure nucleus is formed at a point and then extend through the entire fault.There is an obvious acceleration process of rupture velocity,which can accelerate to subshear or supershear velocity after crack-tip expands from nucleation position.There is a critical crack length Lc beyond which crack grows dynamically.The whole fault is divided into nucleation zone and the dynamic rupture propagation zone according to the critical crack length.The local breakdown near a propagating crack-tip is referred to as slip-weakening of the fault.The interrelationships among shear stress,slip displacement,slip velocity and slip acceleration show the details of dynamic behavior during the local breakdown process near a crack-tip.And based on the experimental results,two different constitutive models are presented to describe the breakdown process at a position close to the beginning of the nucleation and the breakdown process in the dynamic rupture propagation zone.In addition,experiments are conducted at different normal stress of 2MPa-4MPa,and three prefabricated groups of specimens with different roughness of fault surface are used: group?with a maximum static friction coefficient of 0.50;group?with a maximum static friction coefficient of 0.38;group ? with a maximum static friction coefficient of 0.31.The effects of normal stress and fault roughness on the shear failure characteristics are studied.Based on experimental results,conclusions are drawn as follows:(1)In most cases,the critical crack length is smaller and the acceleration process of crack tip velocity is shorter with an increase in the normal stress,and the terminal velocity is also larger.While for a given normal stress,the smoother the fault is,the smaller the critical crack length is,the faster the rupture velocity accelerates and the larger the terminal velocity is.(2)The local stress drop in the slip-weakening process can be divided into two types: local dynamic stress drop(0(35)(28)-d r? ? ?)and local breakdown stress drop((35)(28)-b p r? ? ?).With the increasing normal stress,the local dynamic stress drop and the local breakdown stress drop gradually increase.Besides,the local dynamic stress drop and the local breakdown stress drop increase as the fault become smoother.(3)The critical slip-weakening displacement is insensitive to normal stress.However,the critical slip-weakening displacement decreases as the fault gets to be smoother.(4)The local fracture energy increases with the increase of the normal stress.In addition,the smoother the fault is,the smaller the fracture energy is.This article main innovation is mainly:(1)For the small deformation and relatively small displacement at each position,the relative displacements is measured by metal strain gauges which are across the two blocks.(2)On the larger-scale of simulated faults,the recordings of stick-slip processes are subject to unknown attenuation and propagation effects because of long measurement distance,so that the resolution of the stick-slip history is low.This paper utilizes the small-scale PMMA samples and high-speed recording systems,the slip failure nucleation and its transition process to unstable rupture have both been observed.The details of the local deformations and motions of the local breakdown process near a crack-tip have been studied.Four distinctive phases of the local breakdown processes are found in both the nucleation zone and the dynamic rupture propagation zone and two constitutive models are established.(3)Experiments are conducted at different normal stresses with different prepared roughnesses of faults.The effects of normal stress and fault roughness on the stick slip processes are systematically studied.In addition,the effects of normal stress and fault roughness on crack-tip propagation,local breakdown stress drop and critical crack length are studied for the first time.The dependence of the stick-slip parameters on friction coefficient,which characterize the failure processes,is also studied for the first time.(4)This paper has assumed that the point at which the crack-tip velocity begins to accelerate rapidly to a value close to Rayleigh or super-shear velocities(1000m/s~1500m/s)is the onset of unstable rupture.According to this,the nucleation zone and the dynamic rupture propagation zone are divided.The calculation results of critical crack length show that it is reasonable to devide the nucleation zone and the dynamic rupture propagation zone according to the rupture velocity.