Research On Nonlocal Constitutive Relation Of Nano-composite Ceramics And Ductile Regime Grinding Mechanism Under Ultrasonic Excitation

The machining process of ceramics materials is followed with various behavioraleffects under ultrasonic excitation such as the mechanical effects, chemical effects,electromagnetic effects and thermal effects, etc. And it turns the study of the problemfrom a single area to multi-areas which are multi-disciplinarians, multi-fields, andintertwined areas between the macro and micro structures. At this point, in order toexplain the ameliorative phenomenon of processing performance, the research which isbased on nonlocal theory pays more attention to the microcosmic acting mechanismbetween ultrasonic and ZTA ceramics, and illustrates the affects of ultrasonic excitationon grinding characteristics of ZTA nano-composite ceramics from aspects of materialconstitutive properties, microscopic mechanical properties and SEM of grinding surfaceetc. The theory essence of the high efficiency and ductility phenomenon is disclosedunder ultrasonic machining, and it lays the foundation for improving the theory ofultrasonic machining. The main work and the achievements of this paper are list asfollows:Based on nonlocal mechanics theory of brittle material fracture and throughconsidering the influence factors of ultrasonic excitation parameters, nonlocal elastickernel parameters and the formula of elastic kernel function are established andnonlocal constitutive model which contains the influence parameters of ultrasonicexcitation is deduced. The influence of ultrasonic vibration frequencies and amplitudesto kernel function is discussed which mainly presents parabolic shape with open sidedown. The kernel function shows increasing trend before the critical amplitude andfrequency. It is the most obvious near the critical amplitude and frequency and thenonlocal effects are also the strongest. Since then the influences of frequencies andamplitudes to the kernel function present decay phenomenon. The connection betweenmacro and micro is built through the kernel function, and the nonlocal elastic analysisstress field which is near the crack tip of ceramics materials under ultrasonic excitationis established. The maximum stress that occurs near the crack tip is limited. The stresssingularity of crack tip which is unrealistic in physics is eliminated theoretically and thetrue features of stress field near the crack tip are revealed. It provides theoretical guarantee and guidance to the ductile removal mechanism under the ultrasonic vibrationmachining.Through axial tensile experiments of ultrasonic excitation and considering therelationship between nonlocal elastic kernel function and long-range scope, it can beobtained that fracture stress decreases at first and then increases which presentsparabolic shape with open side up when amplitudes and frequencies increase. When theamplitudes are near the critical value (30kHz,15.3218μm) and the frequencies are nearthe critical value (75W,35kHz), the decay rate is the minimum and the nonlocal effectsare strongest. The influence parameter of nonlocal kernel parameters is fitted throughthe results of tensile test. The influence coefficient of frequency is given to equal to1.2,and the nonlocal elastic kernel function and the nonlocal constitutive model areobtained under ultrasonic excitation. Through the phase analysis of tensile fracture, itsays that the mechanical properties of ceramic materials are improved significantlyunder ultrasonic excitation. The ceramic materials present phase transition tâ†'m whichis induced by stress and it shows better plastic mechanical properties. From the SEMobservation of tensile fracture, it can be seen that there are obvious grain cross-sectionin tensile fracture under ultrasonic excitation and cracks are emerged in a lot of grains.And it says that ZTA ceramics present transgranula and intergranular mixed fracture.Along with the increase of frequencies and amplitudes under ultrasonic excitation, thegap of fracture increases gradually, and the roughness also gets a lot of improvement.Near the amplitude (30kHz,15.3218μm) and frequency (75W,35kHz), the section is thetrimmest. The theoretical change rule of nonlocal decay rate is verified in theexperiment and ultrasonic vibration parameters are optimized.Based on the nonlocal elastic theory and the maximum tensile stress criterion, thetheoretical formula of the strain fracture toughness in crack plane is deduced, and thecleavage mechanism of fracture in ceramics materials is reflected from the micromechanical mechanism. Through testing experiments of hardness and fracturetoughness under ultrasonic excitation, it can be found that the hardness value underultrasonic excitation is as low as70.4%~58.7%of that in ordinary situation. And thefracture toughness value in ordinary situation is68.7%~85.1%of that under ultrasonicexcitation. Through the test results, the change rule of critical depths in different ultrasonic frequencies is analyzed and the influences of ultrasonic vibration on materialremoval performance are studied. Through the tests of grinding force and theobservation of the surface morphology, it can be found that, the ductile grinding depthof ZTA is about2μm in ordinary grinding, it is4μm in the frequency of20kHz, it is8μm in30kHz, it is12μm in35kHz and it is10μm in40kHz. The results are consistentwith the theory.According to cohesion theory and nonlocal energy balance theory, the fracturesurface energy formula and the velocity formula of dynamic crack propagation arederived whose physical significance is more definite. Through the photos of TEM andSEM at the crack tip under ultrasonic excitation, the essence of ultrasonic machining inductile domain is discussed from the angle of energy and microscopical testing. Themicroscopic mechanism of crack propagation under ultrasonic excitation is analyzed. Itcan be found that ultrasonic excitation causes many phenomena around the main cracksuch as dislocation clouds, micro cracks clouds and crack bifurcation or deflection, andthe mechanism that can absorb energy is formed. Crack propagation rate decreases andthe fracture surface energy of materials increases. This mechanism makes significanteffects on the diving force of crack propagation by forming shield effect on outerultrasonic load at crack tip, thereby it plays a role of expanding the ductile domain. Andthe nonlocal constitutive model is connected with the shield effect of cracks underultrasonic excitation through the stress intensity factor, mechanical energy release rateand fracture surface energy.The applicability of the nonlocal constitutive model which has been established tomechanics characteristic of ceramics materials under ultrasonic vibration machiningcorrespond with the phenomenon that the cutting depths are increased significantly inductile domain. Ultrasonic grinding is an effective processing method to hard and brittlematerials which can expand ductile removal domain and improve the toughness of thematerials.