Universality and Specificity of Fractal Dimension of Fractured Surfaces in Materials

1. IntroductionSince Mandelbrot et al.11] showed .that fracturedsurfaCes are fractals in nature and that the fractaldimension of fractured surfaCes correlates well withthe toughness of ~erials. Many authors have doneexperimellts on this probleml2,3]. Now, it is generally accepted that the fractured surface is a fractalwith self-affine property if the applied stress is ModeI loading. Several years later, Bouchaud et al.14] reported thst the rouglmess index of the fractured surfaces would be …     

Perimeter-Area Relation and Fractal Dimension of Fracture Surfaces

We have theoretically analysed the perimeter-area relation and simulated its application to measuring the fractal dimension of fracture surfaces. It is proved that the fractal dimension Dobtained by slit island method (SIM) is related to the dependence of measured area A(δ) ofthe slit island on yardstick δ. So in some cases, the dimension D obtained by SIM is dependenton yardstick and in other cases independent on yardstick δ. But in all cases, when δ→0 thedimension D obtained by SIM approaches the real fractal dimension (similar dimension) of coastline' of the island. We analysed some experimental data and found some new and interestingcharacteristics of crack propagation in steels.     

粉末NBR-PVC共混型热塑性弹性体断裂面的分形维数

本文利用扫描电镜对粉末NBR-PVC共混型热塑性弹性体拉伸断面形貌进行了观察与分析;基于断面小岛周长-面积关系,测定了试样的断口分形维数,考察了NBR含量、拉伸断面分形维数、力学性能(拉伸强度、断裂伸长率、邵氏硬度)之间的关系。结果表明,NBR-PVC弹性体拉伸断面的分形维数随着NBR含量的增加而增加,当NBR含量超过29份后其分形维数呈下降趋势;分形维数与材料的拉伸强度、断裂伸长率和邵氏硬度变化关系一致。     

高强度海工钢断裂行为与板厚相关性分析

采用裂纹尖端张开位移(CTOD)试验对177.8mm厚的A517GrQ高强度齿条钢开展了沿厚度方向不同取样位置、不同试样厚度条件下的断裂韧性分析.结果表明:钢板沿厚度方向不同位置的断裂行为差异明显,同一温度下的CTOD特征值从表面至中心位置逐渐降低;随着试样厚度的增加,钢板的CTOD特征值先增加而后缓慢下降,其相关性满足关系式:δm=0.095t0.5e-0.009t1.17+0.65(1-e-0009t1.17).     

Multirange Fractal Analysis on the Negative Correlation between Fractal Dimension of Fractured Surface and Toughness of Materials

Applying the concept of multirange fractals, a new explanation to the Williford's multifractal curve on the relationship of fractal dimension with fracture properties in materials has been given. It 5hows the importance of factorizing out the effect of fractal structure from other physical causes and separating the appropriate range of scale from multirange fractals     

纳米高强钢铁材料增塑研究进展

钢铁是制造业以及结构应用的主要材料,这很大程度上是因为它们拥有良好的强度与塑性,且价格低廉.材料工程界一直在不停研究更优越的强度和塑性相结合的材料.具有纳米晶/超细晶结构的纳米钢铁材料显示出优异的力学性能,例如卓越的硬度和强度,作为高强钢应用非常有吸引力.然而,超强纳米钢铁材料通常在环境温度下具有低塑性,这极大地限制了它们的应用.由于晶粒细化方法提高强度受到塑性的限制,新的高强度水平下增强塑性的方法成为钢铁材料高性能化的研究热点.为了提高超细晶/纳米晶钢铁材料的塑性,考虑通过调整微观组织结构来提高其加工硬化能力.通过对已经报道的同时具有高强度和良好塑性的纳米结构钢铁材料的实验数据、组织结构的归纳,总结了优化纳米高强钢铁材料塑性的三种基本方法:纳米第二相、微纳复合结构和多相不均匀复合结构.这些增塑方法的主要机理是利用组织结构的改变提高超细晶金属的加工硬化能力以维持其良好的均匀塑性变形,以及利用组织相变提高金属的塑性.这些不均匀纳米结构类似于复合物,具有共同的材料设计和力学原理.本文归纳了钢铁材料常用的强化方法,综述了纳米/超细晶高强钢铁材料提高塑性的方法,尤其是通过突出介绍一些新颖的纳米结构设计来实现钢铁材料的高强高塑,总结了高强高塑纳米钢铁材料的变形机制,以期为纳米晶金属增强塑性研究提供参考.     

形变渗氮08F钢冲击断裂表面不同方向的分形分析

本文利用垂直截面法测量了０８Ｆ钢经不同形变渗氮复合处理后冲击断裂表面的分形维数。试验结果表明，材料的冲击能ＡＫ和断裂表面的分形维数ＤＦ均随形变量的增加而增大，且变化规律也十分相似，冲击断口Ｔ、Ｓ两方向上的分形维数ＤＴ和ＤＳ均与冲击能的对数ｌｎＡＫ之间呈正变化的线性关系，且Ｄｓ〉Ｄｒ。     

PP/纳米碳酸钙复合材料冲击强度与断口表面分维关系的研究

应用脂肪酸、钛酸酯偶联剂分别对纳米级碳酸钙(Nano-CaCO3)进行表面处理,并用熔融共混法制备PP/Nano-CaCO3复合材料.在室温条件下,测量PP/Nano-CaCO3复合材料的V型缺口冲击强度.采用投影覆盖法测算复合材料的冲击断口表面分维,考察断口表面分维与复合材料冲击强度的关系.结果表明,断口表面分维在2.3673～2.4120之间,相关系数均大于0.95,强的相关性说明断口表面分形特性显著;复合体系冲击强度与断口表面分维之间近似呈指数函数关系.     

ANN Optimization Method for High Strength High Fracture Toughness Steels

The two-target optimization problem for high strength high fracture toughness steels has been investigated. An effective method for two-target optimization of multi-variable non-linear complicated system is developed by combining simulated annealing algorithm with artificial neural network     

高强度钢超高周疲劳的研究进展

近年来,高速列车、汽车、航天器中的核心工程构件承受的疲劳循环已达108～1010周次甚至更高.目前的研究结果表明,高强钢材料在10 7周次以上的超高周疲劳阶段内仍会发生疲劳断裂,不存在传统的疲劳极限.因此,研究高强钢的超高周疲劳特性不仅有助于理解疲劳机理,而且有利于研究材料超高周疲劳设计及寿命评估方法.论述了高强钢超高周疲劳研究的背景和意义,介绍了近几年超高周疲劳的研究成果,包括S-N曲线的特征、裂纹萌生特征和扩展机理、断面上鱼眼形貌等,并给出了未来超高周疲劳的研究方向.     

高强度低磁钢研究及发展概况

介绍了低磁钢的主要种类,综述了氮强化、沉淀硬化及形变强化高强度低磁钢的发展概况。指出进一步开发高强度、高韧性低磁钢以及加强工业生产用加压冶金技术研究是我国高强度低磁钢发展的重要方向。     

高强度结构钢的大气应力腐蚀试验

本文报道了两种高强度结构钢GC-4和30XICHA试样在北京和海南岛进行大气应力腐蚀试验的有关情况及长达20年的试验结果。     

中碳高强度钢腐蚀疲劳研究

文中研究了35CrMo 及40CrNiMo 钢淬火不同温度回火时,在3.5%NaCl 盐雾介质中的腐蚀疲劳行为以及35CrMo 钢在3.5%NaCl 水溶液,0.1NHCl+3.5%NaCl 水溶液和空气中的腐蚀疲劳行为。结果表明,40CrNiMo 钢随回火温度升高,盐雾介质中腐蚀疲劳抗力增加,裂纹止裂倾向也增大。5.5Hz 时随介质的 pH 值降低,裂纹扩展加速。电镜断口观察表明,盐水介质腐蚀疲劳是以氢脆机制为主,盐雾介质疲劳是以阳极溶解机制为主。     

用“周长－面积关系”测量断裂表面分形维数的不合理性分析

给出了分形图形的周长和面积之间的理论关系式，以此阐明了用“周长－面积关系”测量断裂表面分形维数的不合理性。     

花岗石抛光表面的粗糙度、分形维数及其关系研究

对两种典型的不同晶粒度的花岗石抛光表面的分形特性和粗糙度特性进行了研究，分析了花岗石抛光表面的分形维数、粗糙度与金刚石磨盘目数的关系，同时也对组成花岗石的重要成分的分形特性及粗糙度做了分析。研究了花岗石抛光表面粗糙度与分形维数的相互关系，并对两者作为花岗石表面质量评价指标的适用性进行了讨论。     

Reliability of Measured Fractal Dimension and Roughness Exponent as Materials Dimensionless Parameters for Fracture Surfaces

The effects of accuracy of measured fractal dimension D and roughness exponent H are investigated in this paper with a view to examine the reliability of D and H as materials dimensionless parameters of fracture surfaces, D and H are different from general physical quantity, because they are dimensionless quantities and often appear as exponents in a theoretical function or formula. In many cases, the error of the physical quantity related to D or H may far exceed 10%, if D or H has error around 10%. The required accuracy of fractal dimension and roughness exponent should be higher, but it depends on the specific material, the associated physical quantity and the scale of measurement.     

取样长度与间隔对花岗石磨削表面分形维数计算的影响

利用高斯滤波对花岗石磨削表面轮廓数据进行处理,再用结构函数分析法,对不同取样长度及不同取样间距时花岗石磨削表面轮廓的分形特性进行了研究,分析了取样长度及取样间距对花岗石磨削表面轮廓分形参数计算结果稳定性的影响,确定了花岗石磨削表面分形参数计算的合适尺度。     

Relationship between Fracture Toughness and Fractal Dimension of Fracture Surface of Steel

The relationship between fracture toughness and fractal dimension of fracture surface was analysed based on a united fractal model of microscopic fracture modes. It was indicated that fracture toughness is a multivariant function of yield strength, microstructural length, fractal dimension of fracture surface and volume fraction of inclusions, other than a monovariant function of fractal dimension of fracture surface. This new fractal model of fracture toughness was verified to fit well with a set of experimental data of a ultrahigh strength steel with equal microstructure and grain size but different volume fraction of inclusion.     

高钢级管线钢的特征参量及其与强韧性的关系

利用力学实验及EBSD等微观分析手段,研究表征高钢级管线钢的特征参数,并探讨其对强韧性的影响.结果表明:在X70,X80高钢级管线钢中,C,Cu,Mn对材料的强韧性有重要的影响;其组织特征为针状铁素体、粒状贝氏体和少量下贝氏体组成;有效晶粒尺寸、组织均匀性、贝氏体含量是表征高钢级管线钢组织特征的重要参数;随着有效晶粒尺寸的降低、贝氏体含量的增加及组织的均匀性的提高,高钢级管线钢表现出更加优异的强韧性匹配特征.     

Modeling of Advanced High Strength Steels with the realistic microstructure–strength relationships

The objective of the work is to consider the first-order effects of the realistic microstructure morphology in the macroscale modeling of the multiphase Advanced High Strength Steels (AHSS). Instead of using constitutive equations at macroscale, the strength–microstructure relationship is studied in the forms of micromechanical and multiscale models that do not make considerable simplifications with regard to the microscale geometry and topology. The trade-off between the higher computational time and the higher accuracy has been offset with a stochastic approach in the construction of the microscale models. The multiphase composite effects of AHSS microstructure is considered in realistic microstructural models that are stochastically built from AHSS micrographs. Computational homogenization routines are used to couple micro and macroscale and resultant stress–strain relations are compared for models built with the simplified and idealized geometries of the microstructure. The results from this study show that using a realistic representation of the microstructure, either for DP or TRIP steel, could improve the accuracy of the predicted stress and strain distribution. The resultant globally averaged effective stress and strain fields from realistic microstructure model were able to accurately capture the onset of the plastic instability in the DP steel. It is shown that the macroscale mechanical behavior is directly affected by the level of complexities in the microscale models. Therefore, greater accuracy could be achieved if these stochastic realistic microstructures are used at the microscale models.