Li Kezhi

Elasto-plastic FEM analysis and experimental study of diametral growth in tube spinning

Abstract An elasto-plastic incremental finite-element computer program has been developed to simulate the forming process in order to be able to gain understanding of the characteristics of tube spinning theoretically. The distributions of stress, strain, residual stress and residual strain in a workpiece and the expansion of the plastic deformation regions in the process of loading during tube spinning can be obtained through the calculations. The analytical results show that the forming of a bell mouthing and diametral growth in tube spinning are due mainly to the growth of circumferential stress and strain. A large number of experimental studies employing different process parameters in tube spinning has been carried out and the influences of the process parameters on diametral growth analyzed. The quantitative relationships between the process parameters and diametral growth have been obtained in this paper.

Research on the distribution of the displacement in backward tube spinning

Abstract The distribution of the displacements in different sections of the blank for backward tube spinning has been obtained by elasto-plastic finite-element simulation. The results reveal the characteristics of metal flow on backward spinning and provide a theoretical basis for controlling the dimensional precision and improving the quality of spun parts in practical production.

Numerical design of the die land for shape extrusion

Abstract In the extrusion of shapes with a flat-faced die, the proper design of the die land is of critical importance in avoiding the generations of geometrical defects. A methodology for the design of the die land, which consists of a simulation–adjustment iteration, is proposed in this paper. The metal flow in extrusion is simulated by the three-dimensional finite element method and the die land is adjusted according to the simulation result. A criterion for the judgment of the proper die land is suggested by the author. The extrusion of an L-section shape is chosen as the computational example and the result is compared with industrial design.

The effect of applied stress on damage mode of 3D C/C composites under bend-bend fatigue loading

The bend-bend fatigue behavior of 3D integral braided carbon/carbon composites (3D C/C) was examined. Fatigue test was conducted under load control at a sinusoidal frequency of 10 Hz to obtain stress-fracture cycles (S-N) relationship. The fatigue limit of the C/C was found to be 203 MPa (92% of the static flexural strength), the lag loops of fatigue load-displacement were transformed from elasticity to anelasticity and the flexibility of specimens were enhanced with increase in applied stress. It is revealed that the interfacial sliding abrasion played an important role in the fatigue failure process, and the extent and speed of sliding abrasion were controlled by the level of applied stress.

Modeling of CVI process in fabrication of carbon/carbon composites by an artificial neural network

The chemical vapor infiltration (CVI) process in fabrication of carbon-carbon composites is very complex and highly inefficient, which adds considerably to the cost of fabrication and limits the application of the material. This paper tries to use a supervised artificial neural network(ANN) to model the nonlinear relationship between parameters of isothermal CVI(ICVI) processes and physical properties of C/C composites. A model for preprocessing dataset and selecting its topology is developed using the Levenberg-Marquardt training algorithm and trained with comprehensive dataset of tubal C/C components collected from experimental data and abundant simulated data obtained by the finite element method. A basic repository on the domain knowledge of CVI processes is established via sufficient data mining by the network. With the help of the repository stored in the trained network, not only the time-dependent effects of parameters in CVI processes but also their coupling effects can be analyzed and predicted. The results show that the ANN system is effective and successful for optimizing CVI processes in fabrication of C/C composites.

Joining of C/C Composites and GH3128 Ni-based Superalloy with Ni-Ti Mixed Powder as an Interlayer

Abstract A modified SiC coating was prepared on the surface of C/C composites by a pack cementation process. Ni-Ti mixed powder was used as an interlayer to join SiC coating modified C/C composites and GH3128 Ni-based high-temperature alloy (GH3128) by a vacuum hot-pressing diffusion process. The chemical element distribution, microstructure and mechanical properties of the joint between C/C composites and GH3128 were investigated by X-ray diffractometry, scanning electron microscopy, energy-dispersive spectrometry and material universal testing machine. Results show that the SiC modified coating on the surface of C/C composites not only improves fully the wettability of C/C composites to the Ni-Ti intermediate layer, but also alleviates effectively the thermal stress in the joint due to mismatch thermal expansion. Under the same joining condition, the shear strength of the joint with SiC modified coating at room temperature is up to 22.50 MPa, whereas that of the joint without SiC coating modification is almost 0.

Structure and oxidation behavior of a plasma sprayed yttrium silicates coated SiC-C/C with a glass outer sealant from 1 573K to 1 873K

Different compositions of yttrium silicates coatings were deposited on SiC-C/C by plasma spraying and an outer borosilicate glass was applied on the yttrium silicates coatings surfaces. The structure of the multi-layer coatings was characterized by XRD and SEM analyses. High temperature oxidation behavior of the multi-layer coatings coated C/C composites was investigated. Results show that SiC/2SiO2·Y2O3/1.5SiO2·Y2O3/ SiO2·Y2O3/glass multi-layer coating has better high temperature oxidation resistance, protecting carbon/ carbon composites from oxidation at 1 773 K in air for 164 h with the weight loss of 1.65%. The oxidation weight loss of the coated C/C with time accorded with parabolic rule in the temperature range 1 573 K–1 873 K; and the corresponding oxidation activation energy of the coated carbon/carbon composites is 132.2 kJ/mol.

Microstructure and Mechanical Properties of C/C-ZrC Composites

采用ZrOCl 2 溶液浸渍法把锆化合物引入碳纤维预制体, 经热处理、热梯度化学气相渗透致密化和高温石墨化工艺制备了C/C-ZrC复合材料。性能测试结果表明, C/C复合材料的弯曲强度和模量随ZrC含量的增加而增大, ZrC含量为12.08wt%时, 其强度和模量分别为42.5 MPa 和9.6 GPa, 比未改性试样分别提高了70.0%和43.3%。基体中结合较弱的微米级ZrC颗粒的存在不利于碳基体强度的提高, 但其对材料最终性能的影响是次要的, 碳基体中亚微米/纳米级ZrC颗粒的存在和良好的ZrC-C界面结合, 提高了碳基体的强度和模量, 进而提高了复合材料的最终性能。

Influences of molding processes and different dispersants on the dispersion of chopped carbon fibers in cement matrix

Chopped carbon fiber-reinforced cement composite (CFRC) has become one of the potential smart materials in recent years. The key process of preparing this kind of composite at the early stage is how to disperse carbon fibers evenly into the cement matrix to achieve CFRC composite with good properties. In this study, a three-step mixing process and a six-step mixing process for the preparation of CFRC were suggested, respectively. The fracture morphology was observed by scanning electron microscopy. The influence of mass fractions of three common dispersants methyl cellulose (MC), carboxymethyl cellulose sodium (CMC) and hydroxyethyl cellulose (HEC) on the dispersion of short carbon fibers in water under certain temperature was investigated prior to the manufacture of CFRC. The correlation between mass fraction and viscosity was discussed, and the dispersion effect was assessed from the structure of dispersants and molding process. A hypothetical capsule theory was proposed to reasonably explain the dispersion effect of MC, CMC, and HEC. The experiments showed that pre-dispersion by ultrasonic vibration improved the dispersion of carbon fibers greatly. The dispersivity of carbon fibers was closely related to the category and mass fractions of dispersants. With the same mass fraction of dispersants at a certain temperature, the dispersion effect was in order of HEC > CMC > MC. Meanwhile, when the mass fraction of HEC was between 0.6 and 0.8 wt% by weight of cement and the mass fraction in the aqueous solution was between 1.65 and 1.80 wt%, carbon fibers dispersed most ideally and distributed further uniformly in the cement matrix.

C/C-ZrC复合材料的微观结构和力学性能研究

采用ZrOCl 2 溶液浸渍法把锆化合物引入碳纤维预制体, 经热处理、热梯度化学气相渗透致密化和高温石墨化工艺制备了C/C-ZrC复合材料。性能测试结果表明, C/C复合材料的弯曲强度和模量随ZrC含量的增加而增大, ZrC含量为12.08wt%时, 其强度和模量分别为42.5 MPa 和9.6 GPa, 比未改性试样分别提高了70.0%和43.3%。基体中结合较弱的微米级ZrC颗粒的存在不利于碳基体强度的提高, 但其对材料最终性能的影响是次要的, 碳基体中亚微米/纳米级ZrC颗粒的存在和良好的ZrC-C界面结合, 提高了碳基体的强度和模量, 进而提高了复合材料的最终性能。