Singular stress analyses of V-notched anisotropic plates based on a novel finite element method

A super singular wedge tip element whose stiffness matrix is based on numerical eigensolutions is incorporated into standard hybrid-stress finite elements to study singular stress fields around the vertex of anisotropic multi-material wedge. The numerical eigensolutions are obtained by an ad hoc finite element eigenanalysis method. To demonstrate the validity of the method, singular stresses for some typical anisotropic single-material/bimaterial wedges are investigated. All numerical results show present finite element method converges rapidly to available solutions with few elements. The present method is applicable to dealing with the problems with more complex geometries.     

尖锐V形切口脆断准则研究的进展

介绍在线弹性断裂力学基础上单奇异性V形切口脆性断裂准则研究的进展.把复合型V形切口脆性断裂准则分为应力参数型和能量参数型,并重点综述这两类准则的研究现状、适用性和局限性,讨论了特征距离的确定,展望了V形切口断裂准则将来可能的研究方向,指出Ⅱ型成分占主导的复合型V形问题,断裂机制和准则,在理论与实验上还需进一步研究.     

V-notched rail shear test applied to geopolymer composites

This work presents a systematic study of the shear properties of a potassium-based geopolymer reinforced with distinct types of fibers. Chopped basalt reinforcements in lengths from 3 mm up to 50 mm and 13 μm in diameter were compared with thicker 20-mm length, basalt mini bars, sand-coated basalt mini rods, and steel fibers. The samples were tested under a V-notched rail shear tests (ASTM D7078), coupled with optical measurements, namely, digital image correlation, allowing a novel study of their crack patterns and failure modes under shear loading. In general, the use of chopped fibers resulted in shear strengths of up to 9 MPa and shear moduli of 4.3 GPa, with no significant variation with fiber length increments, neither in shear stress nor strain at peak load (0.1%). Mini bars and steel fiber reinforcements resulted in slightly lower shear stresses of 7.1 and 8.4 MPa, respectively. They exhibited greater strain values at peak loads, up to 2.1% which were attributed to fiber-matrix enhanced adhesions, thereby allowing gradual debonding and increased ductility. This effect was also recorded for mini rods, but at much lower strength levels, which did not contribute to their multiple cracking capacities. The alignment of the mini rods in 45° directions resulted in a 50% increase in shear stress, showing the feasibility of tailoring the manufacturing process to attend to distinct demands.     

Temperature effect on elastic and fracture behaviour of lead-free piezoceramic BaTiO3

Ferroelectric ceramics, especially piezoceramics, are widely used in the field of sensors, micromanipulators, and capacitors. Barium titanate and its derivates are the first choices between lead-free materials. The main aim of the paper is to clarify fundamental processes taking place in the vicinity of Curie temperature with a focus on the fracture behaviour of pure polycrystalline barium titanate. The explanation of observed changes in the mechanical behaviour of this material is based on the experimental approach supported by numerical simulations utilising features of the real microstructure on the grain level. Several model materials with various grain microstructures were manufactured from the submicron barium titanate powder sintered at various temperatures. Two resulting materials with a suitable distribution of grains were selected for further investigation. The grain size influenced not only the exact position of the temperature of the Curie point but also the kinetics of the lattice transformation, elastic, and fracture properties. The significant drop observed in the fracture resistance was attributed to the development of localised internal thermal stresses, which was supported by the results of the performed numerical simulations. The coincidence of the volume change of neighbouring grains due to lattice transformation together with a significant variation in elastic properties can lead up to a 20% decrease in the measured fracture toughness. Understanding this behaviour is essential for the processing and correct application of lead-free barium titanate materials.     

双频激振下带V形缺口轴的疲劳寿命研究

针对金属轴类零件在实际复杂工况下易产生应力集中而发生疲劳破坏的问题，利用双频激振系统，研究带V形缺口轴的疲劳寿命随缺口几何参数的变化规律。首先，提出了促进轴疲劳裂纹萌生的激振频率控制曲线，同时采用响应曲面法中的Box-Behnken设计法对V形缺口的夹角、圆角半径和深度进行三因素三水平的实验设计；其次，建立了疲劳寿命多元回归预测模型，并采用方差分析法对模型进行可靠性评价；最后，利用响应曲面和等高线图分析了缺口的夹角、圆角半径和深度对轴疲劳寿命的影响规律，并进行了预测模型的应用。研究结果表明：疲劳寿命预测值与实测值之间的误差在4.2%以内，预测精度较高，预测模型可靠；缺口几何参数对疲劳寿命从大到小的影响次序是缺口深度、缺口圆角半径、缺口夹角，以圆角半径和深度的交互作用对轴疲劳寿命的影响最为显著。研究结果可为金属轴类零件的抗疲劳设计提供重要参考。