Simulations of instability in dynamic fracture by the cracking particles method

Crack instabilities and the phenomenon of crack speed saturation in a brittle material (PMMA) are studied with a meshfree cracking particle method. We reproduce the experimental observation that the computed terminal crack speeds attained in PMMA specimens are substantially lower than the Rayleigh wave speed; the computed crack speeds agree quite well with the reported experimental results. We also replicate repetitive microcrack branching along with the increased rate of energy dissipation after attainment of a critical crack speed, even in the absence of microstructural defects. We show that the presence of microdefects changes the response only a little. The computations reproduce many of the salient features of experimental observations.     

聚丙烯腈基炭纤维中微孔的演变规律

利用二维小角X射线散射技术(SAXS),结合纤维孔结构解析理论及分形原理得到了炭纤维形成过程中纺丝、预氧化、低温和高温炭化等四个阶段样品的微孔结构信息。结果表明:原丝中孔隙沿纤维轴向择优取向,呈长梭状,其长轴与短轴的平均尺寸分别为24.3nm和19.2nm,长径比为1.27。遗留到预氧化阶段的原丝孔洞使得预氧化纤维出现高达1.85的长径比极大值,这可能与原丝线性结构向预氧丝耐热梯形结构转化有关。炭化阶段微孔尺寸迅速减小,长轴、短轴分别达到3.56nm和2.85nm左右,长径比也减小至1.24。分形状态研究表明:表面分形维数DS值介于2.42～2.88之间,且随工艺的进行逐渐增大,低温炭化阶段变化幅度较大,说明各级产品在微观结构上越来越复杂,亦证明低温炭化是促进炭纤维微观结构转变的重要的工艺段。     

WAXD/SAXS study and 2D fitting (SAXS) of the microstructural evolution of PAN-based carbon fibers during the pre-oxidation and carbonization process北大核心CSCD

Microstructutal evolution in polyacrylonitrile (PAN) fibers at different temperatures during pre-oxidation and carbonization under stretching was studied by. synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS). Microvoids were characterized by the classical SAXS method, and were compared with simulation results obtained by fitting 2D SAXS patterns to a model based on a dilute system of cylindrical microvoids randomly distributed and preferentially orientated along the fiber axis and having a log-normal size distribution. The WAXD results showed that the crystal size, d-spacing and preferred orientation decreased during pre-oxidation, and increased during carbonization. A diffraction peak for PAN fibers at 2 theta = 13. 6 degrees disappeared during the final stage of pre-oxidation, meanwhile a new peak at 2 theta = 23.6 degrees appeared, whose intensity increased during carbonization, indicating the formation of the graphite structure. The average length of the microvoids increased, and new microvoids were formed, which became oriented along the fiber axis as the fiber manufacturing process proceeded. The length of microvoids from simulation results is consistent with that from the classical method, indicating that the model is valid to describe the microvoid structure of fibers.     

Analysis of gas transport properties of PPO/PS blends by Xe NMR spectroscopy

Relationships among variations of microvoids and gas transport properties for miscible poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)/polystyrene (PS) blends in the glassy state have been investigated by Xe sorption, Xe permeation, and ¹²⁹Xe NMR measurements. Xe sorption isotherms of the blends can be interpreted successfully on the basis of the dual-mode sorption model. Decrease in the permeability of Xe is attributed to the decrease in the diffusivity of that in the Langmuir site. ¹²⁹Xe NMR spectra of ¹²⁹Xe in the blends show non-linear low-field shift with increasing sorption amount of Xe because of a fast exchange of Xe atoms between Henry and Langmuir sites. From the analysis of ¹²⁹Xe NMR chemical shifts, it is found that the mean volume of individual microvoids varies with a negative deviation against volume fraction of PPO in the blend. For PPO/PS blends, it has been clarified that the contraction of individual microvoids occurs by blending and highly affects gas transport properties.     

基于微观孔洞型损伤的疲劳蠕变寿命预测方法

孔洞型损伤是延性金属材料一种常见的破坏形式.对于材料疲劳蠕变破坏过程,文中提出等效孔洞体积概念,从微观唯象角度出发,综合考虑疲劳蠕变对孔洞体积长大的影响,得到一种微孔洞型疲劳蠕变寿命预测模型.以此模型对1.25Cr0.5Mo钢540℃应力控制下疲劳蠕变寿命进行预测,结果显示该模型具有很好的适用性.     

An Experimental Study on Electrical Discharge Machining of Manganese-Zinc Ferrite Magnetic Material

The objective of this research is to investigate the machining characteristics of manganese-zinc (Mn-Zn) ferrite magnetic material using electrical-discharge machining (EDM). The material removal rate, the surface topography, the surface roughness, the recast layer, and the chemical composition of the machined surface were studied in terms of EDM processing variables. Experimental results indicate that the morphology of debris revealed the mechanism of material removal. The surface microgeometry characteristics are not always uniform and homogenous and the EDM process produces much damage on the machined surface. The material removal rate, the surface roughness, and the recast layer are proportional to the applied discharge energy.