材料断口分形维数分析

本文分析了应用分形维数描述材料断口轮廓形貌几何特征的局限性。结果指出，分形维数对于材料断口粗糙度的变化是不敏感的，该参数与材料断口轮廓几何形态和断裂韧性之间不存在确定性的关系。     

陶瓷刀具材料断口形貌及裂纹扩展的分形特征

通过热压烧结工艺,制备了一种高性能的Si3N4基陶瓷刀具材料。利用X射线衍射仪和扫描电子显微镜分别对材料物相组成、微观形貌及裂纹扩展路径进行了分析。借助图像处理技术和分形理论,计算了断口形貌及裂纹扩展路径的分形维数,并揭示材料断裂机制。研究表明,Si3N4基陶瓷刀具材料表现为穿晶/沿晶的混合断裂模式,其裂纹扩展方式主要是偏转和桥联,断口形貌及裂纹扩展均具有明显的分形特征。当材料断口形貌越粗糙,裂纹扩展路线越不规则,分形维数值增大,表明断口微观结构的粗糙程度、裂纹扩展路线的不规则程度可用分形维数来刻画。     

基于纤维解离水曲柳压缩解离特征与能量耗散机制研究

目的 以水曲柳为研究对象,研究高应变率压缩载荷作用下水曲柳试件的解离特征和能量耗散机制。方法 利用压缩加载试验分析应变率、加载方向对受载水曲柳的形态特征影响和动力学特性,并利用弹塑性基本原理分析其受压解离的能量耗散机制。结果 解离后径向加载试件主要呈火柴棍状,弦向加载试件主要呈片状,轴向加载试件主要呈不规则块状,试件的解离程度随应变率的增大而增大;当应变率在400~1 000 s⁻¹时,水曲柳试件的应力-应变曲线由弹性阶段和屈服后弱线性强化阶段两部分组成;水曲柳试件的屈服强度随应变率的增大而增大,当应变率从400 s⁻¹增加到1 000 s⁻¹时,径向、弦向和轴向加载试件的屈服强度分别增加了0.45倍、1.34倍和0.71倍;木材原料沿径、弦向解离时主要依靠木材细胞的压缩变形来耗散能量,沿轴向解离时主要依靠木材细胞纵向结构的弯曲来耗散能量。结论 弦向最易解离,轴向最难解离;水曲柳是一种应变率敏感材料;木材原料径、弦向解离主要依靠压缩变形来耗散能量,轴向解离主要依靠弯曲变形来耗散能量,木材原料解离能够耗散能量的多少主要受加载方向、木材细胞的结构尺寸和力学性能的影响。     

D6AC钢冲击断口形貌的分形研究

本文利用扫描电镜对含Mn$夹杂物和含ZrN夹杂物的D6AC钢的冲击断口形貌进行了观察与分析;利用数字图象法测定了试样的断口分形维数,揭示了冲击断口形貌和冲击韧性与夹杂物含量之间的内在联系,探讨了材料冲击断口形貌与分形维数的关系,拓宽了分形几何学在材料领域的应用范围.     

多相材料断裂的分形特征

测量了多相材料复合涂层的断裂挠度和断口表面的分形维数，分析了复合涂层的断裂特征。结果表明：随着硬质相增加，复合涂层的断裂挠度和断口表面的分形维数降低。     

D6AC钢冲击断口形貌的分形研究

本文利用扫描电镜对含MnS夹杂物和含ZrN夹杂物的D6AC钢的冲击断口形貌进行了观察与分析;利用数字图象法测定了试样的断口分形维数,揭示了冲击断口形貌和冲击韧性与夹杂物含量之间的内在联系,探讨了材料冲击断口形貌与分形维数的关系,拓宽了分形几何学在材料领域的应用范围。     

断口定量分析的分形几何方法

近几年来,分形(fractal)几何已广泛地应用于材料的断裂分析。本文简要介绍了分形几何的发展概况及其几个主要定义,分析了一些简单不规则图形的分形维数的计算。把断口形貌统计地看作分形表面,进行了材料的断口的分析分析。根据断口的分形维数,人们可能去定量地追溯材料断袭时的力学行为。     

材料断口分形研究现状及发展前景

讨论了分形几何应用于断裂研究的几个基本问题,主要包括断面的分形特征与分形测量,分维与断裂韧性的关系以及金属断裂的多种分形模型.进一步阐述了分形理论在金属断裂方面的应用前景.     

17-4PH末级长叶片叶根的疲劳强度与缺口效应

采用成组法和升降法开展了室温某17-4PH末级长叶片叶根纵、横方向光滑与缺口试样的疲劳试验,获得了两个方向上材料的S-N曲线,并对典型疲劳断裂试样进行了断口宏微观形貌观察。结果表明:叶片叶根纵、横方向的疲劳强度基本一致,在长寿命区,纵向疲劳强度略高于横向;光滑试样的疲劳断裂主要呈现单裂纹源特征,而缺口试样呈现多裂纹源汇聚的断裂形貌。结合试样缺口根部的弹塑性应变分析,进一步讨论了叶片叶根纵、横方向的疲劳缺口效应。     

水泥断口表面形貌的分形维数定量表征研究

利用分形几何学理论定量表征了几种水泥断口表面的微观形貌特征,并且考察了断口表面分形维数与其抗压强度的关系.结果表明,水泥断口的表面分形维数介于2～3之间,且线性回归分析的相关系数均大于0.98,强的相关性表明实验所选用的水泥断口具有明显的分形特征;水泥的抗压强度与其断口表面分形维数值呈正相关关系.     

A discrete cohesive model for fractal cracks

The fractal crack model described here incorporates the essential features of the fractal view of fracture, the basic concepts of the LEFM model, the concepts contained within the Barenblatt-Dugdale cohesive crack model and the quantized (discrete or finite) fracture mechanics assumptions proposed by Pugno and Ruoff [Pugno N, Ruoff RS. Quantized fracture mechanics. Philos Mag 2004;84(27):2829-45] and extended by Wnuk and Yavari [Wnuk MP, Yavari A. Discrete fractal fracture mechanics. Engng Fract Mech 2008;75(5):1127-42]. The well-known entities such as the stress intensity factor and the Barenblatt cohesion modulus, which is a measure of material toughness, have been re-defined to accommodate the fractal view of fracture.For very small cracks or as the degree of fractality increases, the characteristic length constant, related to the size of the cohesive zone is shown to substantially increase compared to the conventional solutions obtained from the cohesive crack model. In order to understand fracture occurring in real materials, whether brittle or ductile, it seems necessary to account for the enhancement of fracture energy, and therefore of material toughness, due to fractal and discrete nature of crack growth. These two features of any real material appear to be inherent defense mechanisms provided by Nature.     

Fracture of case-hardened steel in bending

An analytical model is presented for the fracture of case-hardened steel specimens in bending. The case-hardened steel is characterized as a material having a gradient of fracture toughness in the case and a constant fracture toughness in the core. Linear elastic fracture mechanics is used to predict load/crack length/displacement relationships and absorbed energy. The model is used to explain the influence of case depth, core fracture toughness and residual stress on the maximum fracture load and the influence of core thickness on absorbed energy.     

Fracture toughness of transverse cracks in graphite/epoxy laminates at cryogenic conditions

Stress singularity of a transverse crack normal to ply-interface in a composite laminate is investigated using analytical and finite element methods. Four-point bending tests were performed on single-notch bend specimens of graphite/epoxy laminates containing a transverse crack perpendicular to the ply-interface. The experimentally determined fracture loads were applied to the finite element model to estimate the fracture toughness. The procedures were repeated for specimens under cryogenic conditions. Although the fracture loads varied with specimen thickness, the critical stress intensity factor was constant for all the specimens indicating that the measured fracture toughness can be used to predict delamination initiation from transverse cracks. For a given crack length and laminate configuration, the fracture load at cryogenic temperature was significantly lower. The results indicate that fracture toughness does not change significantly at cryogenic temperatures, but the thermal stresses play a major role in fracture and initiation of delaminations from transverse cracks.     

Fracture toughness measurement of thin-film silicon

The fracture toughness of single‐crystal silicon thin films oriented to (100) and (110) was investigated by tensile testing under both 〈100〉 and 〈110〉 loading conditions. The specimen was fabricated from a p‐type Czochralski (CZ)‐grown wafer and passed through a thermal process during the fabrication of the test device. The measured fracture toughness is dependent on the loading direction in the tensile test and independent of the specimen surface orientation. The test results were 1.94 MPa√m in the 〈100〉 direction and 1.17 MPa√m in the 〈110〉. In these tests, no longitudinal size effect on the fracture stress or fracture toughness was observed. The SEM photographs obtained from the fracture specimens after the tensile test show that the crack initiated from the notch tip and propagated straight in the across‐the‐width direction on the (110) or (111) cleavage plane.     

混凝土断裂及亚临界扩展的细观机制

通过模型和三点弯曲断裂SEM试验,详细研究了混凝土断裂全过程及亚临界扩展的细观机理。结果表明:混凝土断裂是一个复杂的不规则过程,存在明显的亚临界扩展现象。混凝土亚临界扩展路径是曲折的,并非经典断裂力学假定的平直路径,混凝土亚临界扩展和临界失稳扩展呈现分形特征。用起裂断裂韧性iICK和分形等效断裂韧性feICK,来描述混凝土抵抗初裂和临界失稳扩展的能力。给出了考虑亚临界扩展弯折效应的混凝土亚临界扩展长度、混凝土起裂断裂韧性iICK和分形等效断裂韧性feICK,的计算表达式。计算表明:混凝土失稳断裂时的分形等效断裂韧性feICK ,与混凝土亚临界扩展的分维数D成正比。     

Al－Fe系梯度功能材料断口特征及其多尺度分形研究

研究了Ａｌ－Ｆｅ系梯度功能材料的试样不同部位的断口特征；基于离散分形布朗随机场模型，对试样断口进行多尺度分形研究，结果表明断口的分形参数Ｈ值也呈梯度分布，它反映了该材料的韧性呈梯度分布。     

The mechanics of self-similar and self-affine fractal cracks

In this paper we study the mechanical attributes of the fractal nature of fracture surfaces. The structure of stress and strain singularity at the tip of a fractal crack, which can be self-similar or self-affine, is studied. The three classical modes of fracture and the fourth mode of fracture are discussed for fractal cracks in two-dimensional and three- dimensional solid bodies. It is discovered that there are six modes of fracture in fractal fracture mechanics. The J-integral is shown to be path-dependent. It is explained that the proposed modified J-integrals in the literature that are argued to be path-independent are only locally path-independent and have no physical meaning. It is conjectured that a fractal J-integral should be the rate of potential energy release per unit of a fractal measure of crack growth. The powers of stress and strain singularities at the tip of a fractal crack in a strain-hardening material are calculated. It is shown that stresses and strains have weaker singularities at the tip of a fractal crack than they do at the tip of a smooth crack.     

取样方向对含单边裂纹牛卡纸拉伸断裂行为的影响

目的 探究取样方向对含单边裂纹牛卡纸拉伸断裂行为的影响,以期提高牛卡纸为原料制作的包装盒等成品的强度。方法 首先在不同取样方向上制备含单边裂纹的牛卡纸试样,然后基于数字图像相关(Digital Image Correlation, DIC)方法,结合显微张力测试台测得不同取样方向与拉伸断裂载荷的关系图,分析不同取样方向下裂纹尖端微观应变场,最后结合电镜扫描测试进行对比分析。结果 结果表明,通常情况下,含单边裂纹的牛卡纸依然符合纸张纵向拉伸载荷大于横向拉伸载荷的规律,取样方向越靠近横向,裂尖区域应变值越大,发生断裂时的拉伸载荷越小。结论 牛卡纸取样方向和纤维与纤维的结合强度是影响牛卡纸拉伸断裂行为的重要因素。     

包装箱用竹木复合层合板断裂特性的研究

采用二次回归正交组合设计法分析了含水率、裂纹长度和加载速率三个因素对竹木复合层合板断裂韧性(K1c)的影响.结果表明含水率对竹木复合层合板断裂韧性的影响最大;建立了含水率、裂纹长度和加载速率三个因素和断裂韧性(K1c)之间的回归方程."Z"字型裂纹为纤维本身破坏形式,"T"字型和"V"字型为纤维间破坏形式.