Research Of Subsurface Crack In Optical Glass Grinding Based On Dynamic Impact Of Single Abrasive

Subsurface cracks of optical glass and other hard brittle materials due to grinding have make great influence on the properties and applications of optical components in the existing environment of air and space.So the subsurface crack formation mechanism of hard brittle optical materials in grinding is one of the important basic scientific problems in the field of optical fabrication.Most of existing researches on optical glass grinding mechanism and subsurface crack formation are based on indentation fracture mechanics and static loading conditions,which didn’t consider the effect of the abrasive high-speed dynamic impact in grinding process and were not correspondent with the actual situation of the generation of cracks/subsurface crack.So the influence law of abrasive impact velocity on the generation of grinding crack/sub surface for optical glass has become an important issue.When dealing with the solid dynamics under the abrasive impact,the material inertia effect and strain rate effect must be taken into account.In this paper,the depth of subsurface crack for optical glass has been researched based on the solid stress wave theory,dynamic fracture mechanics theory and strain rate effect.In this paper,according to the characteristic of single abrasive cutting for optical glass,the solid dynamics stress wave mathematical model of single abrasive cutting dynamic impact effect for optical glass has been established based on the point impact load and elastic cylindrical stress wave theory.The mathematical models of dynamic crack initiation,dynamic crack propagation,dynamic crack extension and crack arrest based on stress intensity factors have been established according to the brittle fracture theory of dynamic fracture mechanics and dynamic crack propagation rule.The subsurface crack generation mechanism of optical glass single abrasive grinding under dynamic impact effect of abrasive has been analyzed with numerical simulation technology.According to the stress wave and the maximum tensile stress theory,the influence law of grinding velocity on internal stress wa ve of optical glass,the influence law of grinding velocity on interaction region and the influence law of action time on stress wave value have been analyzed.It is found that the maximum tensile stress of stress wave is proportional to the abrasive veloc ity,and the time for the maximum tensile stress achieves tensile limit decreases with the increase of the abrasive velocity,and the position of maximum tensile stress which achieves tensile limit and the contact area of the abrasive approach to each othe r.With the increase of the abrasive velocity,the stress wave intensity increases.The propagation process of radial cracks has been simulated by using the theory of stress wave and the dynamic stress intensity factor,and the depth of the crack has been obtained.The simulation results show that the radial crack propagation depth of the subsurface is reduced with the increase of abrasive impact velocity.According to the stress tensor theory,the principal direction of the stress tensor of the cylindrical stress wave has been analyzed,and the direction of the crack system has been defined as the maximum tensile stress direction.The direction of the crack system includes the radial and circumferential cracks.In order to study the formation mechanism of subsurface crack in the single abrasive cutting of optical glass,a single abrasive grinding experiment device has been developed.The method of cross section polishing has been used to detect the subsurface crack system.Through the single abrasive cuttin g experiment of optical glass,it is found that the micro cracks in subsurface includes the radial and circumferential crack.The existence of circumferential cracks is a typical characteristic of dynamic impact crack.According to the analysis of single a brasive grinding system of micro cracks and fracture surface morphology,it is found that brittle fracture granular disintegration occur on optical glass by dynamic impact under certain cutting depth.To research the influence law of abrasive impact veloci ty on the subsurface crack depth of optical glass in grinding process,the variation law of the moving contact arc in single abrasive grinding process for optical glass has been analyzed,and the coordination experimental scheme of grinding velocity and fe ed velocity is proposed.By this scheme,moving contact arc of single abrasive grinding is same under different grinding velocity and same cutting depth,and the experiment purpose that regarding the abrasive impact velocity as the single factor has been achieved.According to the coordination experiment of grinding velocity and the feed velocity,the subsurface crack depth detection of single abrasive grinding for optical glass in different grinding velocity has been carried out,which detected the value of subsurface crack depth in different grinding velocity and different cutting depth.Through the grinding parameter collaborative experiment,it is found that the surface crack depth decreased with the increase of the abrasive velocity under the same moving contact arc and cutting depth.The experiment results of surface grinding for optical glass show that in the case of other conditions unchanged,the increase of the grinding velocity results in the decrease of the subsurface crack layer depth,which is consistent with the experimental results of single abrasive grinding.The linear low-speed scratching experiment of optical glass has been compared with the linear dynamic impact test.The scratching gap profile of optical glass is not clear,and there’re long cracks around the scratching gap.While,there are obvious impact gap and circumferential cracks in the linear dynamic impact,and the long cracks and the internal expansion cracks are not obvious.The strain rate effect of optical glass K9 has been researched by using split Hopkinson pressure bar(SHPB),and the variation rule of strain rate under the grinding condition has been analyzed.The formula of subsurface crack depth based on strain rate effect of abrasive dynamic impact has been established,which revealed the influence of subsurface crack depth on single abrasive grinding for optical glass.Theoretical researches and experimental results show that the stress wave intensity in the abrasive interaction region,stress intensity factor in crack t ip,and the strain rate in the abrasive interaction region increase with the increase of velocity of abrasive,and then the dynamic elastic modulus of the material is increased,and the position of maximum tensile stress which achieves tensile limit and the contact area of the abrasive approach to each other.Therefore,the region of the crack is reduced,and the depth of the subsurface crack in the grinding process is reduced.