Materials and Product Technologies

摘要: In this paper, several new main compositions are presented with Mn2+ substituted by Ti4+ and Co2+ in Manganese-zinc ferrites in consideration of the strong influence of Ti4+ and Co2+ on thermal stability by different mechanisms. Moreover, the effects of these substitutions on such parameters as permeability, electrical resistivity, density, and power loss are analyzed. The experimental results show that the proper substitution of Ti4+ or Co2+ in manganese-zinc ferrites can increase the thermal stability remarkably. Furthermore, the effects of composition of Ti4+ and Co2+substitutions have interactive process, especially when Manganese-zinc ferrites with high thermal stability in 20°C~140°C are achieved when the substitution of Ti4+ is 0.10mol%and Co2+ is 0.05mol%.

摘要: A viscoplastic constitutive model with void damage is developed to analyze the macroscopic mechanical response and damage mechanism of lead-free solder alloy in CBGA packaging under cyclic thermal loading. The constitutive model is implemented into ABAQUS through its user defined material subroutine. Two-dimensional nonlinear finite element analysis of a ceramic ball grid array(CBGA) package is conducted to simulate the viscoplastic deformation and damage failure process of the lead-free solder joint under cyclic thermal loading. The damage model is helpful for optimization and reliability of electronic package.

摘要: For injection mold with core, during the injection molding process, the pressure on the core is usually uneven and will cause the core to deform. In this paper, on the basis of some predigestions and assumptions of the model, formulas for forecasting the deformation of the circular cross-section and the rectangular cross-section cores under three different injection ways are analyzed. The theoretical analysis results of a core with special section are validated through finite element software. At the end, some suggestions are given to minish the core deformation when the calculation value is too large.

摘要: This paper proposes a Hamiltonian perturbation approach for analyzing the dynamic characteristics of the damaged structure. Firstly, structural vibration governing equation is transformed to the general expression of state variables composed of displacement and momentum. On the basis of the conjugate symplectic orthogonal relation, the first-, and second- order perturbation expression of the damaged structural eigensolution are obtained. Finally, the numerical simulation and cantilever experiment prove the effectiveness of this approach.

摘要: The temperature-dependent tensile strength is an important indicator used to evaluate combination property of short-fiber-reinforced elastomer matrix composite. Some short-fiber-reinforced elastomer matrix composites are manufactured in the molding preparation process, and the tensile tests of fiber, matrix and the composites are carried out at different temperatures. The fiber length and orientation distributions are statistically analyzed. The influence of temperature on the micromechanical stress distribution and transfer in the composite is investigated, and the thermal stresses in the fiber, matrix and fiber-matrix interface are obtained. Based on the theory of micromechanical stress distribution and transfer of the fibrous composite, the mixture law is modified, and a model for predicting the temperature-dependent tensile strength of this kind of composite is developed. Moreover, the mechanism of the tensile fracture of the composite at various temperatures is discussed. Research indicates that the tensile strength is largely related to the temperature, mechanical performances of the main components of the composite and some microstructural parameters, such as short fiber aspect ratio, volume fraction and orientation distribution. The tensile strength of SFRE decreases with increasing temperature. The tensile strength increases with the increase of fiber length when the fiber length is no larger than critical fiber length. There exists a critical fiber volume fraction where the tensile strength of SFRE reaches the maximum. The tensile fracture of the composite depends largely on the temperature, the bond strength of fiber-matrix interface and the average length of reinforcing short fibers. The temperature-dependent tensile strengths predicted by the presented model are in good agreement with experimental data.

摘要: Fatigue crack growth rate experiments of center-cracked tension (CCT) specimens of the 2024-T3 aluminum alloy under constant-amplitude load in corrosive environment are carried out with 3 kinds of loading frequency. The fatigue crack growth rates in 3.5% NaCl solution are obtained by using seven-point incremental polynomial method. A probabilistic approach is presented for fatigue crack growth rate in corrosive environment with log-normal random variable model. The reliability analysis of crack propagation is conducted based on the experimental data. The crack exceedance probability at given service time and the distribution of the service time at given crack size are obtained by using the reliability analysis approach. The effect of loading frequency on crack propagation is studied. It is shown that the fatigue crack growth rate is increasing with loading frequency decreasing in corrosive environment. The predicted results by the presented method match the experimental results very well.

摘要: Based on two different cyclic plasticity models, fatigue crack growth for 16MnR steel specimens is simulated by using the same multi-axial fatigue damage criterion. The first plasticity model is the Jiang and Sehitoglu model and the second plasticity model is the simple nonlinear kinematic hardening model. The elastic-plastic stress-strain field near the crack tip is obtained respectively by using the two plasticity models. According to the same fatigue criterion, different fatigue damage near the crack tip is determined on the basis of stress-strain responses. The first plasticity model can accurately capture cyclic plasticity deformation behavior and predictions of fatigue crack growth rate are in agreement with the experimental results. However, lots of material constants in the model need to be fitted and more experimental tests should be conducted. The second plasticity model is very simple. The parameters of the model can be acquired easily by uniaxial fatigue tests. Compared with experimental data, the prediction results of fatigue crack growth rate lead to some errors by adopting the second plasticity model.

摘要: In order to investigate the fatigue behaviors in very high cycle fatigue regime of bearing steel GCr15 (Chinese standard), cantilever-type rotary bending fatigue test was carried out, by using an hourglass-shaped specimen. As a result, the S-N data were classified into surface grinding flaw or inclusion induced fracture mode, subsurface inclusion without ‘granular-bright-facet’ (GBF) area induced fracture mode and subsurface inclusion with GBF area induced fracture mode. Formation of a GBF area was clearly observed in the vicinity of subsurface inclusion on the fracture surface ruptured at high- cycle fatigue regime. The cause of the S-N curves of GCr15 obtained by this experiment continuously decreased, which is different from the common ‘duplex S-N’ curve. Experiments on fracture surface was carried out based on the ‘dispersive decohesion of spherical carbide’ model aimed to clarify the mechanism of the formation of GBF area.

摘要: In a turbofan engine, the high pressure rotor and the radial driveshaft, which transmit the power from the internal gear-box to the external gear-box, are geared by a spiral bevel gear pair. In this paper, a reasonably simplified dynamic model of the coupled rotors system is established, and then, the coupled stiffness matrix and coupled damping matrix of the spiral bevel gear pair are deduced. A shaft element method is proposed to investigate the lateral-torsional coupled vibration equations of the gear-rotor system. Furthermore, the mode shapes and unbalance responses of this two rotors coupled system are simulated. The results indicate that the system derives many new modes and the exciting forces on a rotor of the system would be passed to the other rotor for the gears meshing. When the rotor dynamics of a turbofan engine is being analyzed, the high pressure rotor and the radial drive shaft must be viewed as a whole. The dynamic balance precision of the rotors should be qualified properly, in order to improve the dynamic quality of the turbofan engine.

摘要: This paper describes an investigation about the grinding fluid optimization supply based on lubrication theory. The models for three-dimensional hydrodynamic flow pressure in contact zone between wheel and work are presented based on Navier-Stokes equation and continuous formulae. It is well known that hydrodynamic fluid pressure generates due to this fluid flux, and that it affects overall grinding resistance and machining accuracy. Moreover, conventional methods of delivering grinding fluid, i.e. flood delivery via a shoe or jet delivery tangential to the wheel via a nozzle, have been proved that they can not fully penetrate this boundary layer and thus, the majority of the cutting fluid is deflected away from the grinding zone. Therefore, in this paper, a new delivery method of grinding fluid, the minimum quantity lubricant (MQL)-near-dry green grinding is presented and analyzed for it not only reduces hydrodynamic lift force but also reduces grinding fluid cost to achieve green manufacturing. Experiments have been carried out to validate the performance of the MQL supply compared with conventional flood cooling. The experimental results have shown that the theoretical model is in agreement with experimental results and the model can well forecast hydrodynamic pressure distribution at contact zone between and workpiece and the MQL supply in grinding is feasible. Experiments have also been carried out to evaluate the performance of the MQL technology compared with conventional flood cooling. Experimental data indicate that the proposed method does not negatively affect to the surface integrity and the process validity has been verified.