Fracture Initiation Under Pure Shear In Quasi-brittle Materials Via Imaging And Acoustic Emission

As a kind of sliding crack,mode ? crack actually contains two kinds of cracks: one is mode ? crack whose boundary is not affected by any normal stress;the other is compression-shear crack whose boundary is not only sheared but also compressed.compression-shear cracks exist in all kinds of engineering structures and are easy to be realized in laboratory.But as for pure mode ? cracks,it is extremely rare in the laboratory.Many so-called pure mode ? cracks are compression-shear cracks or even other modes of cracks.This is because:(1)the realization of pure mode ? cracks requires a pure shear stress state at the crack tip,which is a challenge for many fracture experiments;(2)more importantly,whether fracture initiation under pure shear stress state can develop along the direction of pure mode ? cracks is a question in itself.Many researches have shown that fracture initiation in quasi-brittle materials follows the maximum tangential stress criterion,i.e.the maximum tensile stress criterion.It is in conflict with shear failure.The purpose of this study is to observe the fracture initiation and propagation under pure shear stress in quasi-brittle materials.In order to understand the fracture failure mechanism of quasi-brittle materials,a mode ? fracture test method,named Double Edge Notched Compression test(DENC),is introduced in this study.The testing procedure is pretty simple,and a pure shear stress state can be created theoretically at the notch tip.During the loading stage,digital image correlation and acoustic emission are adopted to monitor the fracture initiation and propagation.The existence of pure shear stress state at the crack tip are verified by the displacement and strain field obtained tough digital image correlation.Besides,the results from DIC and AE show that sandstone and granite specimens have two stages of cracks.The first stage of cracks initate at the notch tip,and they are mode I cracks.The second stage of crack initate at the central area of the specimen and is also mode I cracks.Graphite specimens only have the first stage of cracks,and they are mode I cracks.According to the theoretical analysis,the initiation of mode I cracks under pure shear stress is highly possible.Therefore,to achieve pure ? crack,pure shear stress state is not enough.For rocks and other quasi-brittle materials,this conclusion not only explains the difficulty of realizing pure mode ? cracks in the laboratory,but also indicates various testing methods for mode ? cracks proposed by researchers in the past 30 years.If they only realize the pure shear stress state at the crack tip,they are likely to form mode I cracks,which results in the so-called mode ? cracks.It's not credible.For rocks and other quasi-brittle materials,cracks will not produce mode ? cracks under pure shear stress,and the cracks will extend perpendicular to the maximum tensile stress.In order to realize mode ? fracture of quasibrittle materials,normal stress perpendicular to the crack surface must be applied on the crack surface to limit the effect of tensile stress on the crack tip.