Research On Damage Mechanism Of Ultrasonic Vibration Grinding Nano-composite Ceramics Based On Nonlocal Theory

Poor machinability of hard-to-machine materials(such as advanced ceramics,titanium)limits their applications in industries.Ultrasonic vibration grinding(UVG)is a cost-effective these materials.It is generally known that UVG causes the micro-fracture of the workpiece,which are capable of achieving high material removal rate while maintaining the lower surface/subsurface damage,reduced lower grinding force and improved accuracy.However,the mechanism of UVAG so far has not been further researched.In this paper,the interaction mechanism between the ultrasonic wave and microstrcture was researched based on nonlocal theory,and the effect of ultrasonic vibration on grinding characteristics of nano-composite ceramics were studied from material constitutive properties,macroscopic mechanical properties and material surface microtopography,etc..The essential theory of the high efficiency and ductility phenomenon under ultrasonic machining is revealded,and It has laid a foundation for the further perfection of ultrasonic machining theory.The main work and the achievements of this dissertation are list as follows:The mechanics behavior of the ceramics under the action of ultrasonic vibration is analyzed according to the relationship between the internal character length and the external character length(i.e.ultrasonic wave length)for ZTA nano-composite ceramics.To interpret this phenomenon,the nonlocal theory is introduced to establish the constitutive model under the ultrasonic condition.The wave speed attenuation and the relationship between wave speed and frequency are obtained through analyzing the frequency dispersion characteristic of longitudinal wave and transverse wave propagation in ceramics.According to expressions of phase velocity,the variation of normalized nonlocal moduli(longitudinal and transverse propagation)with different frequencies are calculated(the ratio of nonlocal moduli and classical moduli).The nonlocal stress attenuation rule can be obtained when the normalized nonlocal moduli are integrated.In order to study thenonlocal stress characteristicof nano-composite ceramics under ultrasonic vibration grinding,a new kind method of two-dimensional ultrasonic vibration grinding(TUVG)was put forward.Based on the theory of ultrasonic vibrationand indentation fracture mechanics,the material removal rate and grinding force model under TUVG were established.Based this grinding force model,Nonlocal elastic stress field which is near the crack tip of ceramics materials under ultrasonic excitation is established.It is found that the maximum stress that occurs near the crack tip is limited.The stress singularity of crack tip which is unrealistic in physics is eliminated theoretically and the nature of stress field near the crack tip are revealed.It provides a theoretical guarantee and guidance for ultrasonic vibration machining ductile removal mechanism of ceramics.For the study of the relationship between ultrasonic main parameters(frequency,amplitude,etc.)and ceramic grinding nonlocal stress,the ultrasonic vibration tensile and three-point bending experiment device were built.The composite sections horns having complex curve were designed based on the wave dynatics theory,which provided great help for studing action mechanism of the ultrasonic parameters(frequency and amplitude)on the ceramic,at the same time,three-point bend specimens and the tensile samples were designed based on the nonlocal theory and Ultrasonic wave theory.Through ultrasonic irradiation three-point bending and axial tensile test,the fracture behaviors of materials under the ultrasonic vibration were studied,cracks formation and propagation of the ZTA material nanocomposite ceramics under the condition of ultrasonic vibration were analyzed.The relationships between nonlocal elasticity nuclear function and long-range scope under different ultrasonic parameters were obtained from these tests,which are fracture stress decreases with the increase of amplitude and frequency.The SEM photographs of three point bending and tensile fracture of ZTA ceramics show that there are obvious grain sections and large amounts of micro-crack on the fracture morphology under ultrasonic irradiation,which happened to transgranular/the intergranular mixed fracture for the ZTA ceramic under ultrasonic action.the gap roughness have been greatly improved with the increase of ultrasonic excitation amplitude and frequency of fracture increase gradually.Through phase analysis of tensile fracture specimen,it finds that mechanical properties of ceramic materials are improved significantly under ultrasonic excitation,the ceramics materials present phase transition t?m which is induced by stress to absorb strain energy and itshows better plasticity mechanics performance,which the theory of nonlocal attenuation change rule is verified not only in the experiment but also in the theoretically.Through two-dimensional ultrasonic assisted grinding way grinding experiment was carried out,based on the nonlocal elastic theory and impact fracture mechanics,the influence of the ultrasonic vibration parameters and grinding parameters on grinding force are analyzes studied.Was studied by means of scanning electron microscopy(sem),ZTA two-dimensional ultrasonic assisted grinding nanocomposite ceramics surface/subsurface damage characteristic,and contrasted the nonlocal theory is verified,points out that the two-dimensional ultrasonic assisted grinding to obtain good microscopic characteristics of ZTA ceramic grinding surface is very good.Based on the nonlocal elastic theory and impact fracture mechanics,two-dimensional ultrasonic assisted grinding experiment was carried out,the influence rule of ultrasonic parameter(such as frequency and amplitude)and grinding parameters on the grinding force was analyzed from characteristics of processing.Was studied,ZTA the surface/subsurface damage characteristicof nanocomposite ceramics under two-dimensional ultrasonic assisted grinding was studied by means of scanning electron microscopy(SEM),and the nonlocal theory is verified.It is found that the ultrasonic parameter have the effect on the microcosmic characteristic and material removing style,which is the reason to illustrates the improvement of surface quality essentially with ultrasonic grinding.The applicability of the nonlocal constitutive model which has been established to mechanics characteristic of ceramics materials under ultrasonic vibration machining correspond with the phenomenon that the cutting depths are increased significantly in ductile domain.