Study On The Machining Damage And Crack Propagation Of K9 Glass Based On SPH Method

In the field of spaceflight and nuclear fusion,optical glass is widely used because of its good physical and optical properties.K9 glass as a kind of widely used in optical glass,its surface quality is increasingly required.At present,the ultra-precision machining technology and processing method of K9 glass have become an important research direction in the field of advanced manufacturing.Limited by the physical properties of the material,K9 glass is a kind of hard-brittle material that is difficult to be machined and easy to cause subsurface damage which will decline the surface quality during the processing.The appearance and propagation of crack almost have no researchers conducted on due to the difficulty in observing them in experiments.Simulation is a very effective method to study the change of material in the process of machining.Smooth particle hydrodynamics?SPH?method can be used to simulate the ultra-precision machining process.Therefore,in this paper,based on the shortcomings of current research and the difficulty of phenomenon observation in experiment process,smooth particle hydrodynamics method is used to simualte a series of research:material removal forms of K9 glass in different grinding depth,the influence of the grain shapes on the grinding quality and the process of crack propagation.The varied cutting-detph scratch experiment is carried out and the validity of the simulation results is verified by the experiments,the work has done as follows:The models of K9 glass SPH single-grain grinding and the most commonly used four grain shapes grinding were established by using JH-2 constitutive model.By analyzing the grinding force and the surface quality after grinding,it is found that the K9 glass can be completely removed in plastic region when the grinding depth is 0.1?m.Brittle fracture appears in the grinding depth of 0.2?m and amount of brittle fractures appear in the grinding depth of 0.4?m.At the grinding depth of 0.5?m,K9 glass is completely removed in brittle region.The average value of grinding force during stable grinding is calculated and the relation between grinding force and grinding depth is established asF_R=0.55078a_p^1.15356.By analysing,the negative rake of sharp grain as the cutting edge is the most stable in the grinding process.The scratch strain depth and residual stress depth were both very small and the surface quality of the scratch was the highest.The material removal rate is the highest by spherical grain and the generation of cracks can be surpressed,but the subsurface damage is also the largest,which provides a basis for the selection of abrasive shapes in actual machining.The K9 glass suimulation model of varied cutting-depth scratch with single grain was established and the process of crack propagation was studied.According to the basic principle of brittle solid fracture,crack propagation types and fracture toughness formula,a method for calculating the initial length of lateral crack by using critical fracture stress and fracture toughness is proposed.The reversibility of crack growth was observed by studying the relationship between the scratch depth and crack depth.It was found that K9 glass material had delayed crack propagation when it was fractured.Based on the energy balance theory,the energy change in the material is explained.The variation of the scratch force and the scratch morphology were recorded in the varied cutting-depth scratch experiment for K9 glass.The surface morphology was measured and compared with the simulation,the results indicate that the measured results were consistent with the scratch variation and crack appearance in the simulation process.Due to the scale difference,the error of the force ratio is existance.The experimental scratching forces and the simulation forces have a high consistency,which verified the validity of the simulation results.