基于非局部理论的超声振动磨削纳米复相陶瓷损伤机理研究

发布时间：2018-08-22 07:23

【摘要】：难加工材料(如陶瓷、钛合金等)的可加工性差,加工效率低,限制了它们在工业中的应用。大量的研究已经证明,超声振动磨削加工具有较高的材料去除率、较小的磨削力,较低的表面损伤和较高的加工精度,迄今为止,而对这种加工方法所产生这些优点的本质机理研究的还不够深入。本文基于非局部理论,着重探索了超声波与ZTA纳米复相陶瓷材料的微观作用机理,从材料本构特性、宏观力学特性和微观形貌等方面研究了超声激励对纳米陶瓷加工特性的影响,揭示超声加工时所产生的延性高效现象的理论本质,为进一步完善超声加工的理论奠定基础。本文主要研究内容如下:考虑ZTA陶瓷的内禀长度和外部超声波波长之间的相互作用关系,通过引入非局部理论,建立超声振动下的非局部本构模型。通过分析超声纵波和横波在陶瓷材料的波频散特性,得出波速和频率的关系,解释了波速的衰减现象,根据波速的计算公式,分别获得纵向波和横向波标准化非局部模量(非局部模量与经典模量的比值)随频率以及影响域的变化规律。为了研究纳米复相陶瓷超声作用下的非局部磨削应力特性,以ZTA纳米复相陶瓷为加工对象,提出一种新型的二维超声振动磨削技术,基于超声振动理论和压痕断裂力学,给出了新型二维磨削下材料去除率模型和磨削力模型。基于该磨削力模型,通过核函数建立宏-微观联系,建立陶瓷材料超声激励裂纹尖端的非局部弹性解析应力场,发现发生在裂纹尖端的最大应力是有限的,该结论从理论上消除了物理上不现实的裂纹尖端应力奇异性,显示了裂纹尖端应力场的本来面目。为超声振动加工延性去除机理的研究提供理论保证和指导。为研究超声主要参数(频率、振幅等)与陶瓷磨削时的非局部应力之间的关系,建立了超声振动拉伸和超声振动三点弯曲试验装置,基于波动理论,完成了具有复杂曲线的复合多段式变幅杆设计,对研究超声参数(频率和振幅)对陶瓷的作用机理提供理论很大的帮助,同时,基于波动理论和非局部理论完成了三点弯曲试样和拉伸试样的设计。通过超声激励三点弯曲和轴向拉伸试验,研究了超声振动下材料的断裂行为,分析了ZTA纳米复相陶瓷材料在超声振动条件下裂纹形成及扩展方式,得出不同超声参数下非局部弹性核函数与长程作用域的关系是断裂应力随着振幅和频率的增加而减小。通过对三点弯曲和拉伸断口的电镜观察表明,超声拉伸下的断口有明显的晶粒断面,在大量的晶粒上有裂纹出现,说明ZTA陶瓷发生了穿晶/沿晶混合断裂。随着超声激励振幅和频率的增大,断口的空隙逐渐增多,平整度也得到很大的改善,对试件拉伸断口物相分析,发现超声激励作用在很大程度上改善了陶瓷材料的力学性能,ZrO2在应力诱导下发生t→m相变从而吸收应变能,有较好的塑性力学性能,不仅从理论上而且在试验上均验证了非局部衰减率的理论变化规律。通过二维超声辅助磨削方式进行了磨削试验,基于非局部弹性理论和冲击断裂力学,从二维超声辅助磨削技术的加工特性进行解释,分析了二维超声振动参数及磨削加工参数对磨削力的影响规律。借助于扫描电镜,研究了ZTA纳米复相陶瓷二维超声辅助磨削表面/亚表面损伤特征,并用非局部理论做了对比验证,指出二维超声辅助磨削对获得具有较好微观特性的ZTA陶瓷磨削表面十分有益。研究表明非局部本构模型对于超声振动加工陶瓷硬脆材料力学特征的适用性,符合其显著增加延性域切削深度的现象。
[Abstract]:The poor machinability and low machining efficiency of difficult-to-machine materials (such as ceramics, titanium alloys, etc.) limit their application in industry. A large number of studies have proved that ultrasonic vibration grinding has higher material removal rate, lower grinding force, lower surface damage and higher machining accuracy, and so far, this processing method has been used. Based on the non-local theory, the micro-mechanism of ultrasonic and ZTA nano-composite ceramics is explored in this paper. The influence of ultrasonic excitation on the machining characteristics of nano-ceramic is studied from the aspects of constitutive properties, macro-mechanical properties and micro-morphology. The main contents of this paper are as follows: Considering the interaction between the intrinsic length of ZTA ceramics and the external ultrasonic wavelength, a non-local constitutive model under ultrasonic vibration is established by introducing the non-local theory. The dispersion characteristics of ultrasonic P-wave and S-wave in ceramic materials are analyzed. The relationship between wave velocity and frequency is obtained. The attenuation phenomenon of wave velocity is explained. According to the calculation formula of wave velocity, the variation of normalized non-local modulus (the ratio of non-local modulus to classical modulus) of longitudinal wave and transverse wave with frequency and influence region is obtained. In this paper, the non-local grinding stress characteristics of nano-composite ceramics under ultrasonic loading are studied. A new two-dimensional ultrasonic vibration grinding technology is proposed for ZTA nano-composite ceramics. Based on ultrasonic vibration theory and indentation fracture mechanics, a new material removal rate model and grinding force model under two-dimensional grinding are presented. The non-local elastic analytical stress field at the crack tip excited by ultrasonic wave in ceramic materials is established by establishing the macro-micro relationship with the kernel function. It is found that the maximum stress at the crack tip is finite. The conclusion theoretically eliminates the physical unrealistic stress singularity at the crack tip and shows the true nature of the stress field at the crack tip. In order to study the relationship between the main ultrasonic parameters (frequency, amplitude, etc.) and the non-local stresses in ceramic grinding, a three-point bending test apparatus for ultrasonic vibration stretching and ultrasonic vibration was established. Based on the wave theory, the complex multi-section curves with complex curves were completed. The design of H-type horn is of great help to the study of the mechanism of ultrasonic parameters (frequency and amplitude) acting on ceramics. At the same time, the design of three-point bending specimen and tensile specimen is completed based on wave theory and non-local theory. In order to analyze the mode of crack formation and propagation of ZTA nanocomposite ceramics under ultrasonic vibration, it is concluded that the relationship between non-local elastic kernel function and long-range region under different ultrasonic parameters is that the fracture stress decreases with the increase of amplitude and frequency. The fracture surface of ZTA ceramics has obvious grain cross section and cracks appear in a large number of grains, indicating that transgranular/intergranular mixed fracture occurred in ZTA ceramics. ZrO2 can absorb strain energy by t_m phase transformation under stress induction, and has better plastic mechanical properties. The theoretical variation of non-local attenuation rate is verified not only theoretically but also experimentally. Grinding experiments are carried out by two-dimensional ultrasonic assisted grinding method based on non-local elasticity. The influence of two-dimensional ultrasonic vibration parameters and grinding parameters on grinding force is analyzed. The surface/subsurface damage characteristics of two-dimensional ultrasonic-assisted grinding of ZTA nanocomposite ceramics are studied by means of scanning electron microscopy and non-local theory. It is pointed out that two-dimensional ultrasonic-assisted grinding is very useful for obtaining ZTA ceramics grinding surfaces with good micro-characteristics. The results show that the non-local constitutive model is applicable to the mechanical characteristics of ultrasonic vibration machining of hard and brittle ceramics, which is in line with the phenomenon that the cutting depth in ductile domain is significantly increased.
【学位授予单位】：河南理工大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ174.6

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