Deformation and failure mechanism in AISI 4340 steel under ballistic impact

Deformation and failure mechanism in quench-hardened AISI 4340 steel under ballistic impact is investigated. The influence of microstructure on damage evolution is also evaluated. Strain localization and shear failure along adiabatic shear bands are the dominant deformation and failure mechanisms. The time and critical strain for the commencement of strain localization is influenced by strain rate and microstructure. The microstructure of the steel sample also influenced the type of adiabatic shear bands formed during impact. Failure mechanism involves nucleation of micro-voids, which clusters to form bigger pores. Extremely fine micro-cracks are initiated adjacent to the pores and in shear flow direction along the shear bands. These micro-cracks become interconnected and grow to macro-cracks, which cause fracture of some of the investigated cylindrical steel samples under impact. The susceptibility of the adiabatic shear bands to cracking increases with decreasing tempering temperature of the steel.     

岩石双孔爆破过程的流形元法模拟

在流形元基本理论基础上,引入断裂力学的裂纹产生及扩展判据,应用二阶流形元对岩石双孔爆破过程进行了研究,模拟了双孔同时起爆和毫秒延时起爆两种条件下裂纹的产生和扩展、块体的形成以及漏斗形成过程,分析了爆炸载荷作用下岩石的破坏规律以及起爆条件对该过程的影响,验证了流形元法在研究冲击载荷作用下岩石从连续体到不连续体破坏过程的准确性和有效性,为研究类似问题提供一个新的思路和方法。     

On the relationship between the work of adhesion and the critical shear stress for the onset of flow instabilities

The relationship between the critical shear stress for the onset of flow instabilities and the work of adhesion at the interface has been evaluated experimentally for a series of polyolefins on several steels, brasses, and coated steels. The critical shear stress was considerably affected by the chemical nature of the die. Low-surface-energy materials were found to produce smooth extrudates presumably by inducing slippage at the wall. Steel, tungsten carbide, and brass surfaces promoted sharkskin defect at shear stresses above the first critical shear stress. A linear relationship between the critical shear stress and the work of adhesion was found to be valid for values of work of adhesion smaller than approximately 30 mN/m. This indicates that slippage occurs due to a breakdown of the adhesion at the interface. For values of work of adhesion greater than approximately 30 mN/m a stronger interaction at the interface may induce a cohesive failure at the interface and subsequently, extrudate distortions.     

论“岩体结构控制论”

经过长期实践和研究,作者于1984年提出岩体结构控制论是岩体力学的基础理论,并全面、系统地以岩体结构控制论为指导研究了岩体变形、岩体破坏及岩体力学性质的基本规律;提出岩体变形系山岩体材料变彤和岩体结构变形共同贡献的,岩体破坏系受岩体材料破坏和岩体结构破坏控制的;岩体力学性质不仅决定于岩体材料力学的性质,而且受控于岩体结构力学效应及环境因素力学效应。在此基础上,作者提出了岩体可以划分为连续介质、碎裂介质,块裂介质及板裂介质四种岩体力学介质,从而建立了完整的岩体结构力学理论体系。     

Ply-by-Ply Failure Analysis of Laminates Under Dynamic Loading

Ply-by-ply failure analysis of symmetric and anti-symmetric laminates under uniform sinusoidal transverse dynamic loading is performed for a specified duration. The study investigates the first ply failure load, followed by the detection of successive ply failures along with their failure modes using various failure theories. Some of the well-established failure theories, mostly used by the researchers, are considered for the failure prediction in laminates. The finite element computational model based on higher order shear deformation displacement field is used for the failure analysis and the complete methodology is computer coded using FORTRAN. The ply-discount stiffness reduction scheme is employed to modify the material properties of the failed lamina. The failure theories used in the analysis are compared according to their ability to predict failure load, failed ply, failure mode and progression of failure. The failure analysis is performed for both the cross-ply and angle-ply laminates with all edges simply supported and clamped. The significance of fibre orientation and stacking sequence in terms of the strength of a laminate and failure progression is also highlighted.     

化学实验失败体验:教育现象学的视角

近10年来，教育现象学在我国迅猛发展，得到愈来愈多的关注，教育现象学已逐渐形成一套较为成熟与可操作的研究方法与步骤。运用教育现象学研究方法对学生实验失败体验进行研究，得出根本主题：化学实验失败（包括实验意外）体验丰富，其增强或消减受多种因素（老师指导、同伴合作与竞争、自身、时间长短）的单一与相互影响。并从空间性、实体性、时间性、相关性等维度对根本主题的意义进行了论述。并与国际相关研究进行了比较，最后指出了该方法的适用范围和应用前景。     

连续损伤力学基临界奇异指数与破坏时间预测

响应量在临近破坏时呈现出临界幂律奇异性加速特征,是一种被广泛证实的灾变破坏前兆,并被火山、滑坡和岩石破坏实验等后验预测结果证实为一种对破坏时间进行短临期预测的可行方法.但是,奇异性指数测量值的较大分散性导致了对其具体取值的争议和预测效果的不确定性.因此,理解奇异性指数取值特征及其内在物理控制因素,成为了一个核心问题.本文基于连续介质损伤力学和材料时间相关失效特征,构建了刻画损伤加速发展通向破坏过程的力学模型.导出了恒名义应力蠕变加载和控制名义应力随时间线性增大两种典型加载方式下,损伤和应变率加速发展通向破坏的临界幂律奇异性前兆特征.阐明了临界幂律奇异性指数取值依赖于材料损伤与承受真应力之间的非线性关系这一内在物理根源,表明了实际测量中奇异性指数的分散性不完全归结于测量数据误差,而是有着内在物理控制因素.针对破坏前奇异性指数的不确定性,建议了在未知奇异性指数条件下预测破坏时间的方法,并基于花岗岩脆性蠕变破坏实验进行了验证和说明.      

Covalent Organic Frameworks toward Diverse Photocatalytic Aerobic Oxidations

Photoactive two-dimensional covalent organic frameworks (2D-COFs) have become promising heterogenous photocatalysts in visible-light-driven organic transformations. Herein, a visible-light-driven selective aerobic oxidation of various small organic molecules by using 2D-COFs as the photocatalyst was developed. In this protocol, due to the remarkable photocatalytic capability of hydrazone-based 2D-COF-1 on molecular oxygen activation, a wide range of amides, quinolones, heterocyclic compounds, and sulfoxides were obtained with high efficiency and excellent functional group tolerance under very mild reaction conditions. Furthermore, benefiting from the inherent advantage of heterogenous photocatalysis, prominent sustainability and easy photocatalyst recyclability, a drug molecule (modafinil) and an oxidized mustard gas simulant (2-chloroethyl ethyl sulfoxide) were selectively and easily obtained in scale-up reactions. Mechanistic investigations were conducted using radical quenching experiments and in situ ESR spectroscopy, all corroborating the proposed role of 2D-COF-1 in photocatalytic cycle.     

Hodge numbers of nodal double octics

Let B be a surface of even degree d in P³ with nodes as the only singular points. In [1] Clemens proved that the topology of the small resolution of the double covering of P³ branched along B depends not only on the number of nodes but also on the so-called defect, a non-negative integer describing their configuration.

The aim of this note is to give an elementary proof of the Clemens. result and to present some geometric interpretation of defect.