Study On The Key Physical Mechanism Of Fracture And Deformation In KDP Crystal Materials

KDP crystal is widely used in controllable nuclear fusion and laser weapon system.In the ultra-precision machining of KDP crystal,the single point diamond flying-cutting technology is adopted.And due to a series of complex characteristics of KDP crystal,such as soft,easy to tide and brittle,it is very easy to produce cracks in the process of ultra-precision machining.And the energy accumulation and laser damage will occur at the crack in the process of optical frequency conversion.Such situation will seriously affect the lifetime of KDP crystals,so it is of great significance to study the mechanism of crack propagation in ultra-precision machining of KDP crystal.When studying the mechanism of crack formation during the single point diamond flying cutting process of KDP crystal,the research method of process test can’t capture the whole process of crack propagation.For conventional finite element method,the crack propagation path can only be carried out along the boundary of the element,so the detailed mechanism of crack propagation in KDP crystals can’t be accurately revealed.The extended finite element method effectively compensates for the shortcomings of the experimental method and the conventional finite element method,and the crack tip can passes through the interior of the element through the step function,so it can simulate crack propagation process of KDP crystal more accurately,and the crack propagation mechanism of KDP crystal during single point diamond flying-cutting process can also revealed.Combined with extended finite element theory,the numerical model of KDP crystal is established by ABAQUS in this paper.And the parameter combination involved in the process of flying-cutting process is planned based on the basic principle of orthogonal test.A total of 18 sets of parameter combinations are planned,and the single point diamond flying-cutting process of KDP crystal is numerically calculated under 18 sets of parameter combinations,and the variation of the cutting force and the crack tip propagation are obtained.And the main factors affecting the change of the cutting force and the crack propagation are found by the corresponding analysis methods.The single factor method is used to reveal the specific influence mechanism of the main factors.The BP neural network model is established by using MATLAB software,and the model is used to predict the maximum principal stress at the crack tip of KDP crystal in single point diamond flying-cutting process,and the correctness of the model is verified.The results showed that: the cutting depth and feed are the main factors affecting the cutting force of KDP crystal single point diamond flying-cutting process,and the cutting force is gradually increasing with the increase of the cutting depth and the increase of the feed;the spindle speed and the cutting depth are the main factors affecting the crack tip extension of KDP crystal single point diamond flying-cutting process;the maximum principal stress and the extension trend of the crack tip increase with the spindle speed increasing,and the growth rate is the fastest in the range of 600r/min to 800r/min;the maximum principal stress and the extension trend of the crack tip increase with the cutting depth increasing,and the growth rate when the cutting depth is less than 25μm is higher than the growth rate when the cutting depth is greater than 25μm;the prediction mechanism established by the BP neural network model can accurately predict the maximum principal stress of the crack tip under different parameters combination conditions,it provides a useful reference for the control of the single point diamond flying-cutting process in KDP crystal.