Study On Subsurface Damage Of Optical Quartz Glass Based On SPH Method

Optical quartz glass is widely used in high-end manufacturing such as precision instruments due to its excellent properties.However,its physical properties of hard and brittle make it easy to cause brittle fracture in the process of processing.At present,the research on hard-brittle materials is mainly focused on reducing or even eliminating the subsurface damage and improving the ductile machining capability.The experimental method cannot observe the changes of materials at the same time,and the experimental results are uncertain due to the influence of uncontrollable factors.However,the SPH Simulation method has been widely used in the mechanism research of various hard and brittle materials in recent years with its unique advantages.In this paper,the subsurface damage mechanism and material removal mode of optical quartz glass are studied in detail by establishing the SPH simulation model of optical quartz glass and the varying cutting-depth scratching experiment.The specific work has done as follows:The single grain scratching SPH simulation model was established to study the subsurface damage of optical quartz glass combined with the scratch elastic stress field model.The cracks generate firstly at the elastic-plastic deformation boundary(?=b)in front of the grain(?=28°)due to the influence of the maximum principal tensile stress.The generation and propagation of cracks in the scratching SPH simulation are consistent with those described in indentation fracture mechanics.The increase of scratching speed leads to the increase of dynamic fracture toughness,then the maximum depth of subsurface crack and the number of subsurface cracks decreases significantly.The subsurface residual stress is concentrated at the bottom of the scratch,and the residual stress on both sides of the scratch surface would cause the generation and propagation of the Hertz crack.With the increase of scratching speed or the decrease of scratch depth,the scratching force is relatively stable,and the residual stress dispersion is relatively small,which can suppress the generation of subsurface cracks.The research results could help to restrain the subsurface damage in grinding process.Through the orthogonal cutting model of optical quartz glass,the influence of tool rake angle and edge radius on material removal mode and subsurface damage was studied.The tool negative rake angle and the large tool edge radius promote the material in the chip forming area to be downward suppressed and form a high stress influence area,and at the same time produce a large range of compressive stress.The increase of compressive stress would reduce the stress intensity factor K_I,thus realizing the ductile machining of optical quartz glass.When the tool edge radius or tool negative rake angle is relatively large,the more stable the cutting force,and the smaller the dispersion of residual tensile stress.However,when tool edge radius is too large,the normal force is very large,would cause serious subsurface damage to the optical quartz glass.When the tool rake angle ranges from-15°to-35°or tool edge radius is 0.3?m~0.5?m,the optical quartz glass is not only machined in a stable ductile region,but also has less subsurface damage.The varying cutting-depth scratching experiment of optical quartz glass was carried out.Based on the analysis of the surface morphology of the scratches,it can be seen that when the scratch depth is small,the degree of brittle fracture of the optical quartz glass during the scratching is relatively small.For the critical value of brittle-ductile transition,the experimental results,the theoretical calculation results at micro-nano scale and the SPH simulation results are all in the range of 0.16?m~0.36?m.Besides,the generation and propagation of subsurface cracks and scratching force are both consistent with the results obtained by the SPH simulation analysis,which further verifies the effectiveness of simulation.