Papers by Keyword: Brittle Material

Abstract: The paper used computer simulation techniques to model the failure process of brittle materials. The simulation method is the finite element method. We have prepared a quad tree method program, through which the unit discrete planes structure, and the quick automatic generation of quadrilateral elements of good quality, and the discrete units of information and node information. We use object-oriented method of preparation of the finite element program, and use it for displacement in the destruction of the simulation, stress analysis. We simulate the failure process of brittle materials. Simulation, the choice of failure criteria and damage model are two key, they choose the proper relationship to the simulation results, good or bad. According to the characteristics of brittle materials, select the first strength theory as a failure criterion and a damage model suitable for brittle materials. The model assumes that a crack in the destruction of the cell boundary, and then through the crack surfaces and the distribution of the load to compensate for the energy released when the virtual cracks. Finally, the destruction of the material is given by the computer graphics form.

Abstract: A theoretical analysis for the effect mechanism of elliptic vibration assistance on the cutting of brittle materials is presented in this paper. The crack propagation in the chip formation zone in cutting of brittle materials is examined based on an analysis of the geometry and forces in the cutting region. The cutting model shows that the actual undeformed chip thickness is much decreased with elliptic vibration assistance, and the instantaneous rake angle of the cutting tool edge is also in a larger negative value than those in conventional cutting. These two conditions can ensure the compressive stress is much larger than the shear stress during the cutting process. Then the stress intensify factor is suppressed effectively and the shielding effect on the growth of pre-existing flaws is strengthened in the chip formation zone. The removal of brittle material tends to be in ductile mode without fracture. The characteristic examination of cutting force shows that the ratio of thrust force to cutting force with elliptic vibration is increased compared to that of conventional cutting. This can validate that the larger compressive stress can be generated in the chip formation zone with elliptic vibration assistance. The transition depth to brittle machining of silicon carbide with/without elliptic vibration assistance further supports the presented theory.

Abstract: Crack growth path was investigated experimentally, numerically and theoretically using two test specimens subjected to pure mode II loading. The specimens were (a) the center cracked circular disc (CCCD) specimen subjected to diametral compression often called the Brazilian disc and (b) the diagonally loaded square plate (DLSP) specimen containing inclined center crack and subjected to pin loading. A few CCCD and DLSP specimens made of two brittle materials (i.e. marble rock and PMMA) were tested under pure mode II conditions. It was observed that the fracture initiation directions and the fracture paths for the tested specimens differed significantly and grew in two different trajectories. However, it was shown that the experimentally observed fracture paths for both specimens can be predicted theoretically very well by using the incremental crack growth method. Several finite element analyses were performed to simulate the whole fracture trajectories of the tested CCCD and DLSP specimens. At each increment, the direction of fracture initiation for the tip of growing crack was determined using the fracture parameters (i.e. stress intensity factors and T-stress) based on the modified maximum tangential stress (MMTS) criterion. The main difference in the fracture trajectory was found to be related to the magnitude and sign of the fracture parameters (which depend strongly on the specimen geometry and loading configuration) and also the type of tensile or compressive loading in the CCCD and DLSP samples.

Abstract: In order to deeply study the influence of ultrasonic assistance on material removal characteristics of hard and brittle materials, a series of ultrasonic assisted single-point scratch experiments have been carried out in this paper. Experimental results show that the assisted ultrasonic vibration is benefit to increase the critical cutting depth and enlarge the ductile regime of material removal. The main reason can be explained as the influences of blank cutting phenomenon, the decrease of the normal cutting force under the same depth of cut, the decrease of the material hardness under ultrasonic excitation and the shielding effect of lateral crack.

Abstract: A theoretical discussion has been presented for the ductile chip formation in grinding of brittle materials. The single abrasive grit was dealt with a top-rounded cutter removing material of varying undeformed chip thickness. The force model in the chip formation zone was established. The stress analysis showed that larger compressive stress and shear stress can be generated in the chip formation zone, which shields the growth of pre-existing flaws in the material by suppressing the stress intensity factor. When the stress intensify factor is smaller than fracture toughness and the resolved shear stress exceeds the critical flow stress of the material, the ductile chip is formed. Experiments of monocrystal silicon grinding were conducted. The results show that the thrust force is much larger than the cutting force, which ensures the larger compressive stress in the chip formation zone and the formation of ductile chip.

Abstract: Based on fundamental ideas in tribology and basic concept of stress state in solid mechanics, the existence of frictional force on shear plane is discussed under uniaxial compression of brittle materials. On account of macroscopic fracture forms and mesoscopic fracture mechanisms, the key factors influencing shear fracture angle are analyzed. The results show that, when brittle materials are compressed and shear fracture occurs, shear fracture surface at the crack initiation point is consistent with the maximum shear stress. But the reason of shear fracture angle examined in experiment greater than 45º lies in that, the existence of frictional force between endface of specimen and pressure head of testing machine, and additional tensile stress produced in the materials when harder crystalline grain wedge in softer medium have changed original uniaxial compression stress state and the direction of maximum shear stress on next fracture path.

Abstract: Erosion tests on high strengh refractory castables were carried out using SiC grains at impact angles of 30°,45°,60°,and 90° with the velocity of 5m/s.In order to study the variation in stress and system energy with impact angles during solid particle erosion process,a single particle erosion model was designed by means of three-dimensional explicit dynamic software ANSYS/LS-DYNA according to experiment parameters. The Johnson-Holmquist brittle ceramic model was employed to model the failure of target material. The impact angles varied from 15° to 90° in increments of 15°.The simulation results were compared with erosion rate values from experiments. The results show that the variation trends of both the maximum stress of targets and system total energy loss are in a good agreement with experiment data,which increaes with increasing impact angle. The variation of erosion rate as a function of impact angle can be explained by the variation of the maximum stress of target material. The rule “the maximum erosion of typical brittle material occurs at 90°” is confirmed by the view of energy analysis.

Abstract: For machining of hard and brittle materials, iron electrodeposit is a kind of matrix material with potential advantages for manufacture of diamond tools. Aiming at the problem of difficult codeposition of diamond in iron deposit, this paper adopts orthogonal design of experiment to study the effects of solution pH value, cathodic current density, alkylphenol polyoxyethylene (10) ether (OP-10) concentration and ammonium chloride concentration on codeposition of diamond, and then Fe-based diamond bits were fabricated and drilling tests in granite were carried out. The results show that pH value, cathodic current density, OP-10 concentration and ammonium chloride concentration all have statistically significant effect on codeposition of diamond in iron deposit, whose contributions to the variance of the weight of codeposited diamond are 37.45%, 32.05%, 13.13% and 12.38%, respectively. The result of drilling test indicates that Fe-based diamond bit can achieve much higher penetration rate than common Ni-based diamond bit.

Abstract: Realize the basic process of fracture analysis in ANSYS. The fore treatment program of crack propagation simulation is compiled by parameterization method of apdl. The calculation and analysis is automatic. ANSYS is well for simulating the structures which contain cracks and bugs. When the parameters are suitable, the propagation of cracks can be simulated well in ANSYS. The different phases of crack propagation are simulated. To compare with the results by theory, perfect the formation process and mechanism of splitting cracks.

Abstract: The new concepts“crack nuclearation toughness”and “crack criterion number”given in this text were used for describing the different crack development steps and crack nuclearation critic condition and the fast developing critic condition of crack were suggested with them by means of Cottrellˊpiling up of dislocations model.The analysis pointed out that crack nuclearation toughness of brittle material is 1/2 of its fracture toughness and the criterion number of crack was a good parameter reflecting anti-crack toughness of material in crack developing process