工程陶瓷引弧微爆炸加工冲击力建模及实验分析

介绍了引弧微爆炸加工过程中冲击力产生的原因,利用单因素实验确定影响冲击力的主要工艺参数,利用正交实验和回归分析的方法建立了冲击力指数型经验模型,得出工艺参数对冲击力的影响规律。实验结果表明:工作电流、工作气压和微爆炸发生器喷嘴直径是影响冲击力大小的主要工艺参数,冲击力随着工作电流的增大而减小,随着工作气压和喷嘴直径的增大而增大,而工作脉宽和工作距离对冲击力大小影响不显著;建立的冲击力模型与实验结果吻合良好。研究结果可以为引弧微爆炸加工过程的控制提供参考依据。     

Al2O3陶瓷引弧微爆炸加工温度场模拟

建立了Al2O3陶瓷引弧微爆炸加工(micro-detonationofstrikingarcmachining,MDSAM)过程的传热模型,基于有限元理论,利用ANSYS软件对加工过程中的温度场分布进行了模拟。结合材料性质,对模拟和实验得到的蚀坑尺寸进行了比较,并分析了加工参数对温度场的影响。模拟结果表明,Al2O3陶瓷引弧微爆炸加工时在给定的加工参数下的最高温度可达13435℃,且高温影响区范围很小,加工实验与模拟结果符合较好。随着脉冲宽度和工作电流的增加,加工区域的温度以及蚀坑的半径和深度增大;随着喷嘴半径的增大,加工区域的温度降低而蚀坑的径深比增大。模拟结果可为Al2O3 陶瓷引弧微爆炸加工过程中表面形貌的预测、材料去除机理的揭示以及加工参数的选择等提供参考。     

Estimating the tensile strength of super hard brittle materials using truncated spheroidal specimens

New approaches need to be introduced to measure the tensile capacity of super hard materials since the standard methods are not effective. To pursue this objective, a series of laboratory tests were constructed to replicate the fracture mechanism of diamond-based materials. Experiments indicate that under a certain compressive test condition, stresses normal to the axisymmetric line in truncated spheroidal specimens (bullet-shaped specimens) are in tension contributing to the tensile fracture of the material. From experimental and numerical studies, it is concluded that semi-prolate spheroidal specimens can be used to determine precisely the tensile strength of brittle stiff diamond-like composites.     

Deformation model for brittle materials and the structure of failure waves

Constitutive equations that describe the experimentally observed failure waves are proposed to model inelastic strains of brittle materials. The complete system of equations is hyperbolic, each equation of this system has divergent form. The model is based on the assumption that continual failure is the process of transition from an intact state to a “fully damaged” state described by the kinetics of the order parameter. The structure of stationary traveling compressive waves is analyzed using a simplified model. It is shown that in a certain range of amplitudes, the wave splits into an elastic precursor and a failure wave. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 3, pp. 164–172, May–June, 2007.     

Quasi-micromechanical constitutive theory for brittle damaged materials under tension

One of fundamental but difficult problems in damage mechanics is the formulation of the effective constitutive relation of microcrack-weakened brittle or quasi-brittle materials under complex loading, especially when microcrack interaction is taken into account. The combination of phenomenological and micromechanical damage mechanics is a promising approach to constructing an applicable damage model with a firm physical foundation. In this paper, a quasi-micromechanical model is presented for simulating the constitutive response of microcrack-weakened materials under complex loading. The microcracking damage is characterized in terms of the orientation domain of microcrack growth (DMG) as well as a scalar microcrack density parameter. The DMG describes the complex damage and its evolution associated with microcrack growth, while the scalar microcrack density factor defining the isotropic magnitude of damage yields an easy calculation of the effects of microcrack interaction on effective elastic moduli. Project supported by the National Natural Science Foundation of China (19891180).     

Search for conditions of compressive fracture of hard brittle ceramics at impact loading

In this paper we discuss three different experimental configurations to diagnosing the modes of inelastic deformation and to evaluating the failure thresholds at shock compression of hard brittle solids. One of the manifestations of brittle material response is the failure wave phenomenon, which has been previously observed in shock-compressed glasses. However, based on the measurements from our “theory critical” experiments, both alumina and boron carbide did not exhibit this phenomenon. In experiments with free and pre-stressed ceramics, while the Hugoniot elastic limit (HEL) in high-density B₄C ceramic was found to be very sensitive to the transverse stress, it was found relatively less sensitive in Al₂O₃, implying brittle response of the boron carbide and ductile behavior of alumina. To further investigate the effects of stress states on the shock response of brittle materials, a “divergent flow or spherical shock wave” based plate impact experimental technique was employed to vary the ratio of longitudinal and transversal stresses and to probe conditions for compressive fracture thresholds. Two different experimental approaches were considered to generate both longitudinal and shear waves in the target through the impact of convex flyer plates. In the ceramic target plates, the shear wave separates a region of highly divergent flow behind the decaying spherical longitudinal shock wave and a region of low-divergent flow. Experiments with divergent shock loading of alumina and boron carbide ceramic plates coupled with computer simulations demonstrated the validity of these experimental approaches to develop a better understanding of fracture phenomena.     

Pulse shaping techniques for testing brittle materials with a split hopkinson pressure bar

We present pulse shaping techniques to obtain compressive stress-strain data for brittle materials with the split Hopkinson pressure bar apparatus. The conventional split Hopkinson pressure bar apparatus is modified by shaping the incident pulse such that the samples are in dynamic stress equilibrium and have nearly constant strain rate over most of the test duration. A thin disk of annealed or hard C11000 copper is placed on the impact surface of the incident bar in order to shape the incident pulse. After impact by the striker bar, the copper disk deforms plastically and spreads the pulse in the incident bar. We present an analytical model and data that show a wide variety of incident strain pulses can be produced by varying the geometry of the copper disks and the length and striking velocity of the striker bar. Model predictions are in good agreement with measurements. In addition, we present data for a machineable glass ceramic material, Macor, that shows pulse shaping is required to obtain dynamic stress equilibrium and a nearly constant strain rate over most of the test duration.     

Deformation of brittle materials with scattered cracking

A structural approach combined with the energy method is used to construct a continuum model of the joint process of deformation and scattered cracking in elastic-brittle isotropic materials. The proposed approach allows one to match the form of the symmetry of the effective deformation properties and damageability of the material depending on the stress state character.     

基于COMSOL的脆性材料相场断裂模型

相场法通过一系列微分方程描述材料断裂过程,避免了繁琐的裂纹面追踪,在模拟裂纹的萌生、扩展和分叉等方面具有优势。介绍了基于相场法的脆性材料断裂模型,给出了脆性材料断裂问题相场法控制方程的推导过程,提出了基于分步迭代法在COMSOL中实现脆性材料相场断裂模型的方法。再现了脆性材料单元模型和单边缺口平板受拉及受剪作用下的开裂过程,模拟的裂纹扩展路径与已有文献的结果相近,验证了程序的合理性。针对脆性材料相场断裂模型包含的诸多参数,采用Morris法对影响荷载-位移关系的脆性材料断裂模型参数进行了全局敏感性分析,结果表明,杨氏模量(E)、临界能量释放率(G_c)和位移增量(Δu_x)是影响模型荷载-位移关系输出结果的主要参数。基于COMSOL实现的相场断裂模型能够有效模拟脆性材料的裂纹萌生和扩展断裂过程,模型参数E,G_c和Δu_x对材料断裂性能的提升或模型参数反演效率的提高具有重要影响。     

Engineering mechanics and materials research in the information technology age

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Calculation of strains for brittle materials taking into account limiting failure

Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 3, pp. 137–143, May–June, 1989.