Investigation On Micro-mechanical Behaviors And Machining Properties Of KDP Crystal

KDP crystal is a high-performance nonlinear optical material which has been widely used in the field of national core strategy, such as energy, national defense, aerospace, basic nuclear physics research, etc. However, because of KDP crystal's special material properties (e.g., soft texture, high brittleness, easy deliquescence, sensitive to temperature change, easy-cracking), it has already become a typical difficult-to-process material. The profound explorations of the micro-mechanical behaviors and the generation mechanism of damages during the ultra-precision machining processes are the necessary theoretical guarantees of the realization of KDP crystal's processing methods which are both high efficiency and low damage. The smooth particle hydrodynamics (SPH) mesh-free simulation algorithm has been introduced in order to eliminate mesh distortion when using the finite element method to solve the regional deformation problems. The research of the micro elastic-plastic mechanics behaviors (e.g., extrusion, scratching) in the ultra-precision processes has been accomplished by using nano-indentation and nano-scratching techniques, In addition, the relevant testing experiments have been carried out so as to verify the correctness of simulation outcomes. The detailed researching contents are shown below:First of all, the material constitutive model of KDP crystal's (001) plane has been built based on the elastic-plastic theory. Then the nano-indentation SPH simulations which include various types of indenters have been carried out. The results show that the distribution of equivalent stress is radial circular arc under 8mN maximum load. Different degree of stress concentration has been observed at the contact position between KDP crystal and indenters. The equivalent stress which residual depth is 1.3-1.5?m is mainly located at the indentation edges at the end of the unloading process. No variation of equivalent stress has been found contrasting the maximum loading moment. The relationship between load and the influence depth of equivalent stress is approximate linear increasing when the load lies within the range of 0?8mN. For each kind of indenter, the plastic damage layer's depth is in accordance with the equivalent stress's influence depth under the same loading conditions. Moreover, both the plastic property and the yield limit on (001) plane have been obtained through nano-indentation tests. The rationality of simulations has been proved as well.Secondly, the anisotropic mechanical model of KDP crystal's tripler plane has been established by using "ABAQUS-EXPLICIT" dynamics module. The nano-indentation numerical calculations based on SPH method has been carried out. The correlation coefficient between the experiment's load-indentation depth curve and the simulation one is 0.996328 which has been tested by OriginPro 8. This result not only the shows a relatively high goodness of fit, but also the correctness of the mechanical model has been proved. The yield strength of the tripler plane is 240MPa. Because of the anisotropy properties of the material, the irregular arc shape distribution of stress inside the workpiece has been observed. The relationship between the magnitude of load and the influence depth of equivalent stress is approximate linear increasing. The reflection effect of the plastic damage which exists at the material surface has also been discovered. When the load is lower than 2mN, the differences of residual stress's depth among the indenters are less than 0.2?m. As the load increases gradually, the differences are widening.Last but far from least, the exploration of the micro-tribology and the mechanical responses of KDP crystal's (001) plane has been achieved through using nano-scratching simulation based on SPH method. The reliability of model has been proved by the nano-scratching experiments. The research outcomes indicate that the majority of material deformation lies in the elastic-plastic field, and no brittle fracture has been occurred. The positive correlation relationships between the friction coefficient which has the anisotropic characteristics and the normal load have been observed. As the scratching depth increases, the equivalent stress perturbation on the material becomes more severe which indicates that to obtain a high quality crystal plate during the machining processes, a relatively small cutting depth is necessary. In addition, the plastic damage of workpiece is mainly located at both the bottom and the two sides of the scratching groove.