Initiation of fracture at the interface corner of bi-material joints

Within the context of linear elasticity, a stress singularity of the form Hr^λ−1 may exist at the interface corner of a bi-material joint, where r is the radial distance from the corner, H is the stress intensity factor and λ−1 is the order of the singularity. Recent experimental results in the literature support the use of a critical value of the intensity factor H=H_c as a fracture initiation criterion at the interface corner. In this paper, we examine the validity and limitations of this criterion for predicting the onset of fracture in a butt joint consisting of a thin layer of an elastic-plastic adhesive layer sandwiched between two elastic adherends. The evolution of plastic deformation at the corner is determined theoretically and by the finite element method, and the solution is compared with the extent of the elastic singular field. It is shown that H_c is a valid fracture parameter if h>B(H_c/σ_Y)^1/(1−λ) where the non-dimensional constant B=100 for β=0 and B=13 for β=α/4. Here, h is the thickness of the adhesive layer, σ_Y is the uniaxial yield stress of the bulk adhesive and (α,β) are Dundurs’ parameters (Dundurs, J., J. Appl. Mech. 36 (1969) 650). Experimental results for aluminium/epoxy/aluminium and brass/solder/brass sandwiched joints are used to assess the role of plastic deformation on the validity of the failure criterion.     

Two-Parameter Model of a Mode I Crack in an Elastoplastic Body under Plane-Strain conditions

The Dugdale crack model is generalized to the case of plane strain. The governing equations are set up to determine the stresses in the plastic zone. Numerical results from specific problems are analyzed and compared with those for plane stress state and other cases. A relationship between the crack model and K _I-T theory is established in the case of small-scale yielding at the crack tip __________ Translated from Prikladnaya Mekhanika, Vol. 41, No. 6, pp. 44–55, June 2005.     

COD参量在压力容器安全评定中的应用

本文通过爆破试验研究了带裂纹容器屈服发生发展机理和断裂参量在加压过程中的变化,按照COD设计曲线对各类缺陷进行了评定。指出:由于对亚临界缺陷COD值存在三个阶段(增加—冻结—增加),所以用这一设计曲线进行安全评定有一定局限性。     

杉木种子园无性系结实稳定性的遗传变异

在安徽西田林场杉木种子园对从福建洋口林场引进的25个无性系,每系固定6株样株,分系分株连续进行6a的结实量观测与分析。结果表明,杉木无性系结实相对产量在年度、重复间稳定性较强,但结实量的年度方差分量较大,占总方差的36．72％,而无性系仅占10．12％,表明种子产量稳定性受气候、生物、环境、人为措施及无性系基因型等多种因素综合影响。同年度中,影响产量的主要因素则是无性系的基因型。以结实量的变异系数(cv)或生态价系数两种方法评价无性系结实稳定性的结果基本一致,但应以多年的结实量与cv或生态价系数综合判断,对无性系再选择均有较高的可靠性和实用性。按该法可把25个杉木无性系划分为高产稳定、中产稳定、低产稳定等9种结实类型。     

Effects of the strain-hardening exponent on two-parameter characterizations of surface-cracks under large-scale yielding

The influence of strain hardening exponent on two-parameter J-Q near tip opening stress field characterization with modified boundary layer formulation and the corresponding validity limits are explored in detail. Finite element simulations of surface cracked plates under uniaxial tension are implemented for loads exceeding net-section yield. The results from this study provide numerical methodology for limit analysis and demonstrate the strong material dependencies of fracture parameterization under large scale yielding. Sufficient strain hardening is shown to be necessary to maintain J-Q predicted fields when plastic flow progresses through the remaining ligament. Lower strain hardening amplifies constraint loss due to stress redistribution in the plastic zone and increases the ratio of tip deformation to J. The onset of plastic collapse is marked by shape change and/or rapid relaxation of tip fields compared to those predicted by MBL solutions and thus defining the limits of J-Q dominance. A radially independent Q-parameter cannot be evaluated for the low strain hardening material at larger deformations within a range where both cleavage and ductile fracture mechanisms are present. The geometric deformation limit of near tip stress field characterization is shown to be directly proportional to the level of stress the material is capable of carrying within the plastic zone. Accounting for the strain hardening of a material provides a more adjusted and less conservative limit methodology compared to those generalized by the yield strength alone. Results from this study are of relevance to establishing testing standards for surface cracked tensile geometries.     

An expeditious synthetic approach towards the synthesis of Bis-Schiff bases (aldazines) using ultrasound

Aldazines (Bis-Schiff bases) 1–24 were synthesized using aromatic aldehydes (heterocyclic and benzaldehydes) and hydrazine hydrate under reflux using conventional heating and/or via ultrasound irradiation using BiCl₃ as catalyst. Ultrasonication conditions with cat. BiCl₃ proved to be an effective, environmentally friendly synthetic procedure. This methodology is robust in the presence of electron donating and electron withdrawing groups affording desired products with high yields (>95%) in just a couple of minutes vs. hours using conventional heating.     

Tensile yielding and microstructures of blends of isotactic polypropylene and linear low-density polyethylene

Thin sheets of isotactic polypropylene (iPP) and linear low-density polyethylene (LLDPE) blends were studied by tensile testing, optical microscopy (OM), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). Eyring's two-process yielding theory was used to analyze the data of yield stress as a function of strain rate and temperature, and satisfactory curve-fitting results were obtained. Furthermore, stress whitening was found to have occurred in the necked regions of tensile specimens under a certain range of yielding conditions. These conditions corresponded to the activation of Process II yielding of Eyring's theory. The whitening was found to be a result of formation of microvoids that initiated at the interface between iPP and LLDPE. © 1995 John Wiley & Sons, Inc.     

紬丝纺加工过程中单纤维机械性能的变化

本文对(纟由)丝纺原料在加工过程中单纤维的机械性能变化进行了探讨,对单纤维断裂强度、断裂伸长率、初始模量、屈服应力和断裂功等一系列机械性能指标及其内在联系进行了测试分析,对今后改进工艺条件和提高(纟由)丝纱线质量提供了有益的参考和依据。