陶瓷材料抗热震断裂性和抗热震损伤性统一的理论

从裂纹体出发，基于能量守恒原理，论述了ＨａｓｓｅＩｍａｎ关于抗热震断裂性和损伤性统一理论的建立、意义和应用。解释了材料在热震破坏过程中不同阶段显示的不同抗热震主导机制，明确裂纹在抗热震方面的重要作用，提出营造裂纹的原理及途径。     

Residual stress and fracture toughness of sintered body of ZrO2-GO composite ceramics material

Zirconia ceramics and carbon-based materials are widely adopted in medical and dental applications due to their excellent biocompatibility and aesthetics. However, fracture toughness of ceramic materials limits their application in clinical dentistry because of the existence of residual stress. In this study, zirconia/graphene oxide (ZrO₂-GO) composite ceramics were fabricated by hot-press sintering. Residual stresses developed on the surface of ZrO₂-GO composite ceramics were evaluated by X-ray residual stress analysis and indentation techniques. The variation of surface residual stress with GO content was evaluated, and found to be consistent with that of fracture toughness. The generation of residual stress was found to be directly related to fracture toughness. Residual stress calculated by theoretical formula of indentation method was consistent with that measured by X-ray diffraction in line with the content of GO. Based on above results, it is concluded that 0.1–0.15 wt% GO composite ceramics possessed better mechanical properties.     

13R22.5无内胎全钢载重子午线轮胎胎踵裂原因分析及改善措施

采用有限元软件分析13R22.5无内胎全钢载重子午线轮胎胎圈部位材料施工结构对胎踵裂的影响。结果表明:钢丝加强层外敷贴锦纶包布层可以分散轮辋轮缘对胎踵部位的挤压应力,减轻应力集中,改善胎踵裂问题。通过成品室内和道路试验验证了有限元分析的正确性。     

Mechanical behavior and failure mechanisms of boron carbide based three-layered laminates with weak interfaces

The effect of weak interfaces on failure mechanisms of a three-layered composite was studied. Three-layered B₄C/B₄C–C_nanofibers laminates have been produced using a hot pressing technique. The laminates were designed with thick (∼2.6 mm) outer layers of B₄C and a thin (∼90 μm) center layer of B₄C–70 wt.% C_nanofibers. Based on the difference in coefficients of thermal expansion and Young's moduli of the pure B₄C and B₄C–70 wt.% C_nanofibers layers, it was estimated that low tensile thermal residual stress with a magnitude of 11.3 ± 2.5 MPa was developed in the thick B₄C outer layers, and compressive residual stress with a magnitude of 455.7 ± 5 MPa was developed in the thin central B₄C–70 wt.% C_nanofibers layer. The apparent fracture toughness of laminates was measured and based on the estimated fracture toughness values, a threshold stress was calculated.     

Polymer Network Mobility and Environmental Stress Cracking Resistance of High Density Polyethylene

Zero shear viscosity and molecular weight between entanglements (M _e ) are determined from dynamic oscillatory shear experiments. Lower M _e value means higher number of entanglements in the system and is associated with increasing strain hardening stiffness. With the understanding that strain hardening is related to environmental stress cracking resistance (ESCR) of high density polyethylene (HDPE), M _e is then related to the ESCR of several resins in this study. The inversely proportional relationship between M _e and ESCR indicates that low network mobility due to an increasing number of chain entanglements increases the ESCR of HDPE.     

带腐蚀缺陷采油管线剩余强度和剩余寿命的评价

为了评价某采油厂采油管道实际工况,基于ANSYS有限元法对管道腐蚀缺陷进行模拟,研究了不同缺陷类型下,缺陷长度、宽度、深度因素对管道失效的影响,分析了不同腐蚀类型缺陷对管道最大工作压力的影响。利用检测数据,对管道最大腐蚀速率进行了计算,并对管道剩余寿命进行了预测。结果表明:缺陷长度和深度是管道失效的直接影响因素,等效应力和工作压力成正比关系,管线的剩余寿命为10.2 a。此结果对管道的安全运行及维护具有重要的参考价值。     

Research advances in residual thermal stress of ceramic/metal brazes

Brazing, as a common method of bonding ceramic and metal, has been applied in microelectronics, aerospace, machinery and other domains extensively. The residual thermal stress in the brazed joint has direct effects on the mechanical properties of the joint, so how to control the generation of residual thermal stress has become the vital point. In this paper, the methods of reducing residual thermal stress in the brazing process in recent years are reviewed. The generation and effects of residual thermal stress in the brazed joint are introduced. Besides, the methods of detecting residual thermal stress are discussed, and different methods of reducing residual thermal stress in brazed joints are also analyzed. Finally, the future development directions of reducing residual thermal stress in the brazed joint are proposed.     

Tensile fatigue behavior of plain-weave reinforced Cf/C–SiC composites

The fatigue behavior of plain-weave C_f/C–SiC composites prepared by liquid silicon infiltration (LSI) was studied under cyclic tensile stress at room temperature. The specimens were loaded with stress levels of 83% and 90% of the mean static tensile strength for 10⁵ cycles. The cross-sections and fracture surfaces of the fatigued specimens were examined by optical microscopy (OM) and scanning electron microscopy (SEM), respectively. The results show that the specimens can withstand 10⁵ fatigue cycles with a stress level of 90% of the static tensile strength. The retained strengths after fatigue for 10⁵ cycles with stress levels of 83% and 90% are about 19% and 11% higher than the static tensile strength. Due to the observation of the microstructures a relief of the thermal residual stress (TRS) caused by stress-induced cracking is probably responsible for the enhancement. Furthermore, the fracture surfaces indicate that the fatigue stress results in interfacial debonding between the carbon fiber and matrix. Additionally, more single-fiber pull out was observed within the bundle segments of fatigued specimens.     

The effect of local residual stress on the fractal dimension of silicate glasses

Local residual stress caused by impacts, machining and indentation results in a decrease in strength in most materials that fail in a brittle manner. The ratio of the critical crack size, c, and the fracture mirror size, r, also is affected by the existence of local residual stress. The global fracture toughness of non-R curve materials is not affected by the local residual stress. The fractal dimension of the fracture surface as characterized by the fractal dimensional increment, D*, is directly related to the square of the fracture toughness. This paper addresses the question of the effect of the local residual stress on the fractal dimension of the fracture surface. We derive a relationship between the fractal dimensional increment and the c/r ratio for materials fractured with and without local residual stress. We then compare the prediction with two cases of experimental results. We show the fractal dimension remains constant with the change in the c/r ratio for local residual stress conditions.     

A phenomenological energy-based model to characterize stress-softening effect in elastomers

A phenomenological energy-based model for stress-softening of isotropic, incompressible hyperelastic rubberlike materials is derived here. In this model, the microstructural damage is characterized by an exponential softening function that depends on the current magnitude of the strain-energy function and its maximum previous value in a deformation of the virgin material. Theoretical models are presented for uniaxial, equibiaxial and pure shear deformations by using Gaussian and non-Gaussian material molecular network models. The accuracy of the resulting constitutive equations is demonstrated on uniaxial, equibiaxial and pure shear experimental data provided in the literature. Comparisons between the energy-based model and the strain intensity based phenomenological model described in [Elías-Zúñiga A, Beatty MF. ZAMP 2002;53:794-814. [1]] show that the model developed here is slightly superior in following experimental data.     

吸热型碳氢燃料的裂解及结焦研究进展

首先比较了燃料热裂解和催化裂解的优缺点,由于二者均具有一定局限性,使引发热裂解成为燃料目前最具优势的裂解方式,重点介绍了引发热裂解的研究进展。燃料裂解形成的碳沉积主要有丝状和无定形状2种形态,分别对它们的形成机理、影响因素及抑制措施进行了介绍。     

Microstructure and mechanical properties of plasma-sprayed LaMgA111O19 thick ceramic coatings

In the research, the effect of different critical plasma spray parameters (CPSP) on the microstructure and mechanical properties of plasma-sprayed LMA coatings with thickness of 797 μm were investigated. As a result, the porosity of coatings was increased from 12.14% to 24.88% with the decrease of CPSP from 1.20 to 0.86, while bonding strength of coatings was obviously reduced from 15.98 ± 0.36 MPa to 4.87 ± 0.7 MPa. Relatively, Young's modulus and hardness of the coatings exhibited a decreasing tendency with the decrease of CPSP. When the CPSP was decreased from 1.20 to 0.97, the residual compressive stress of coating surface varied from ?162.10 ± 12.13 MPa to ?93.49 ± 3.28 MPa, and that obtained from cross-section was decreased from ?116.02 ± 5.92 MPa to ?70.68 ± 3.99 MPa. Meanwhile, the fracture toughness of coating was improved from 0.62 ± 0.05MPa?m^1/2 to 1.34 ± 0.05 MPa?m^1/2, which was higher than that of cross-section of coating. The microstructure and mechanical properties of LMA thick coatings were strongly dependent on the CPSP.