基于声发射技术的非标准自密实混凝土三点弯曲梁动态断裂特性

对不同初始缝高比的自密实混凝土(Self-compacting concrete,SCC)非标准三点弯曲梁开展不同加载速率下的断裂试验,获得其断裂的荷载-裂缝嘴张开口位移曲线及峰值荷载、断裂韧度、临界缝高比增量、弹性模量和柔度系数等断裂参数,结合Pearson相关性检验公式及加载速率效应模型,定量分析初始缝高比、加载速...     

纳米粒子链/聚合物基复合体的界面形态及力学行为

介绍了纳米粒子链弹性力学属性，粒子链与高分子链缠结和吸附，以及界面形态和力学行为。同时研究原生粒子结合处的“脖颈”对增强高分子聚合物的作用。     

Fracture toughness of thermoset composites reinforced by perfectly bonded impenetrable short fibers

The fracture toughness of brittle thermoset resins could be improved significantly by perfectly bonded tough, short fibers through both crack trapping and bridging effects. In this paper, the crack trapping effect was studied through the analysis of the change of strain energy associated with the crack propagation across a regular array of fibers, and the bridging effect was discussed based on the Andersson–Bergkvist model. The fracture resistance increases with the fiber volume fraction, and is independent of the elastic properties of the matrix, the crack length, and the cross-sectional diameter of the fibers.     

Three-dimensional stress and displacement fields near an elliptical crack front

Local stress and deformation fields for an elliptical crack embedded in an infinite elastic body subjected to normal, shear and mixed loads are considered. Particular emphasis is placed on the direction of propagation of points along the crack border. A confocal curvilinear coordinate system related to a fundamental ellipsoid, and a local spherical coordinate system attached to the crack border are adopted. Using asymptotic analysis, this paper obtains the stress and displacement fields in a plane inclined to the 3D crack front. Results show that the present solutions are independent of the curvature of the ellipse, and different from those given by Sih (1991). Based on two different fracture criteria, crack growth analysis shows that a 3D crack would propagate in the direction of the normal plane along the crack front. As a result, the fracture initiation and propagation of a 3D flat crack can be analyzed in the plane normal to the crack front, and the local fields in the normal plane are the linear superposition of the plane strain mode-I, mode-II, and mode-III crack-tip fields.     

Spherical-impact damage and strength degradation in Si3N4-SiC composites

Four types of SiC-whisker/silicon nitride and SiC-particle/silicon nitride composites were produced, and their mechanical properties and impact damage behaviour examined. All of the composites exhibited elastic response behaviour at spherical impact with Hertz cone crack initiation. Impact resistance behaviour, however, was different for each composite. This was due to the different mechanical properties produced by their microstructures. A SiC-platelet/silicon nitride composite displayed the highest resistance to crack initiation and propagation, which resulted in high impact resistance to strength degradation. On the other hand, SiC-particle/silicon nitride, SiC-whisker/silicon nitride, and large SiC-whisker/silicon nitride composites showed less impact resistance, even though they have higher mechanical properties such as bending strength and fracture toughness.     

ELASTIC BRIDGING FOR MODELING FATIGUE CRACK PROPAGATION IN A FIBER-REINFORCED TITANIUM MATRIX COMPOSITE

Abstract— A model based upon linear elastic bridging and fiber crack tip shielding is proposed for predicting fatigue crack growth in a SCS-6/Ti-6–4 composite. The model is characterized by the fiber/matrix debond length rather than the fiber/matrix interfacial frictional shear strength used in most current fatigue models. Finite elements combined with fracture mechanics are applied for computing the local stress intensity. The local stress intensity in the matrix is then utilized to predict crack growth in the composite via comparison to monolithic fatigue crack propagation data for a similar Ti-6–4 matrix material.     

A theory for fatigue crack propagation

A new continuum mechanics model is developed for predicting fatigue crack propagation rates using a fracture mechanics approach. The model demonstrates the critical dependence of fatigue crack growth on the fatigue ductility exponent, the fatigue ductility coefficient, the elastic modulus and the fracture toughness; it is related to the stress intensity range, implying that fatigue crack growth is critically dependent upon the condition at the tip of the crack.Four materials are studied, namely a creep resistant stainless steels, FV535; a $\text{2}\text{1}\text{2}$ per cent nickel-chromium-molybdenum direct hardening steel, 2S96D; a nickel base heat resisting alloy INCO 901; and a ferrous alloy containing titanium carbide in a medium alloy tool steel matrix, known as Ferrotic C.The developed model provides a means of predicting crack propagation rates based on mechanical properties, and the simplified model provides a fundamental basis for a more general form of the Paris relationship.     

NOTCHED MEMBER FATIGUE LIFE PREDICTIONS COMBINING CRACK INITIATION AND PROPAGATION

Abstract Fatigue lives of notched members are considered to be divided into crack initiation and propagation phases. Apparent size effects caused by crack propagation through the strain gradient of the notch are accounted for if initiation is defined as a crack size within the local notch field. The extent of this field may be estimated from fracture mechanics analysis, with its size being of the order of one tenth of the notch radius. Plasticity effects must be properly handled in predicting crack initiation, but linear elastic analysis is generally satisfactory for handling the propagation phase.     

The interaction between a penny-shaped crack and a spherical inhomogeneity in an infinite solid under uniaxial tension

Summary This paper presents an approximate three-dimensional analytical solution to the elastic stress field of a penny-shaped crack and a spherical inhomogeneity embedded in an infinite and isotropic matrix. The body is subjected to an uniaxial tension applied at infinity. The inhomogeneity is also isotropic but has different elastic moduli from the matrix. The interaction between the crack and the inhomogeneity is treated by the superposition principle of elasticity theory and Eshelby's equivalent inclusion method. The stress intensity factor at the boundary of the penny-shaped crack and the stress field inside the inhomogeneity are evaluated in the form of a series which involves the ratio of the radii of the spherical inhomogeneity and the crack to the distance between the centers of inhomogeneity and crack. Numerical calculations are carried out and show the variation of the stress intensity factor with the configuration and the elastic properties of the matrix and the inhomogeneity.     

Modelling fatigue crack propagation of a cracked metallic member reinforced by composite patches

The concept of fracture for material elements at front of a crack for fatigue crack propagation was extended to the fatigue crack propagation of a cracked metallic member reinforced with a composite patch in this paper. From static mechanics and linear elastic fracture mechanics, force transfer on a cracked member through a composite patch was analyzed and a formula connecting the stress intensity factor with crack length was obtained. Thereafter, a fracture model for fatigue crack propagation of a repaired cracked metallic member was proposed. A new expression for the fatigue crack propagation rate has thus been derived. The expression was verified objectively by the test data. It is in good agreement with the test results.     

Cohesive zone modelling of fracture in polybutene

Fracture properties of isotactic polybutene-1 have been investigated. Fracture tests have been conducted and relevant properties at initiation have been determined according to linear elastic fracture mechanics. Two distinct fracture mechanisms have been identified, one of them causing partial instability during crack propagation. Numerical modelling has been performed using a cohesive zone approach. In particular, the identification of suitable cohesive laws has been tried using parametric identification and two different experimental methods. Results suggest that two different cohesive laws may be needed in order to describe crack initiation and crack propagation.     

On a thermo-mechanical effect and criterion of crack propagation

A thermo-mechanical effect from partial conversion of fracture work into heat energy during crack propagation is considered with a simple mathematical model. It is assumed that the heat production zone in the vicinity of the crack tip is very small. Thus, the crack propagation process can be viewed as propagation of the crack in elastic material with a point thermal heat source fixed at the tip of the crack. This thermal heat source generates its own temperature and stress fields around the crack tip. As shown in this paper it also generates a negative stress intensity factor that specifies fracture mode I and has to be accounted for in the energetic fracture criterion. The model developed may help to explain many experimental observations such as the increase in the specific surface energy that accompanies an increase in the crack speed and why fracture mode I has a special role in crack propagation phenomena.     

高温等静压烧结碳化硅基复相陶瓷的强化与增韧

本文通过Si₃N₄、TiC及SiC晶须补强SiC基复相陶瓷的高温等静压烧结,研究了复相陶瓷的显微结构与力学性能,探讨了晶须及第二相颗粒对复相陶瓷的强化与增韧机理．结果表明,不同的补强颗粒及晶须在基体中的作用也不同,Si₃N₄的引入将在基体与第二相颗粒之间产生径向压应力,阻碍裂纹的扩展,TiC的引入将在基体与第二相颗粒之间产生径向张应力,诱导裂纹的偏转;SiC晶须的引入也将产生阻碍裂纹扩展的机制,从而达到SiC基复相陶瓷强化与增韧,改善其力学性能．     

Modelling on crack propagation behaviours at concrete matrix‐aggregate interface

A numerical model is proposed to simulate crack propagation at concrete matrix‐aggregate interface. One single aggregate surrounded by concrete matrix is taken to demonstrate the behaviours of crack penetration into concrete matrix and crack growth along the interface. Influences of side‐edge constraint, aggregate direction, and interface fracture energy on the crack propagation behaviours are respectively investigated. The results show that, tensile constraint on the side edge, a smaller angle between tensile axis and aggregate, and higher fracture energy lead to a higher rupture strength of the interface. Once the interface crack starts to grow, it propagates to the two ends of aggregate major axis drastically and further penetrates into the matrix. Nevertheless, these factors have no appreciable influence on crack propagation path. By mapping interface crack into major axis, ordinary crack is generated. Using the above simplification, modelling of multiple crack propagation in concrete is efficiently achieved.     

Early age fracture properties of microstructurally-designed mortars

This paper compares the fracture properties as well as crack initiation and propagation of real and equivalent mortars. The development of the elastic modulus, tensile strength, and fracture energy at different hydration stages were determined by inverse analysis of load-displacement curves obtained by the compact tension test (CTT). Further, the impact of the moisture content on the aforementioned material properties was also tested on oven-dried equivalent mortars. Digital image correlation (DIC) was used to follow the crack initiation and propagation.The elastic modulus, tensile strength, and fracture energy support the validity of the equivalent mortars approach. The load-displacement curves obtained by the CTT were also compared to those simulated by finite element method showing excellent correlations. DIC revealed the formation of similar crack patterns at comparable load levels between the two mortars. At early age, the moisture content has a considerable influence on the tensile strength and the fracture energy.     

A model of brittle fracture propagation part I—continuum aspects

The micromechanism of crack propagation in steel is described and analyzed in continuum terms and related to the macroscopic fracture behavior. It is proposed that propagation of cleavage microcracks through favorably oriented grains ahead of the main crack tip is the principal weakening mode in brittle fracture. This easy cleavage process proceeds in the Griffith manner and follows a continuous, multiply connected, nearly planar path with a very irregular front which spreads both forward and laterally and leaves behind disconnected links which span the prospective fracture surface. A discrete crack zone which extends over many grains thus exists at the tip of a running brittle crack. Final separation of the links is preceeded by plastic straining within the crack zone and occurs gradually with the increasing crack opening displacement. It is suggested that in low stress fracture, straining of the links is the only deformation mode. However, it is recognized that under certain conditions plastic enclaves may adjoin the crack zone. This deformation mode is associated with high stress fracture, energy transition and eventually with crack arrest.

Energy dissipation resulting from the two deformation mechanisms is related to crack velocity, applied load and temperature and the crack velocity in a given material is expressed as a function of the external conditions. Fracture initiation and crack arrest are then discussed in terms of the conditions which are necessary to maintain the propagation process. Finally, the dimensions of a small scale crack tip zone for a steady state, plane strain crack are evaluated as functions of material properties and the elastic stress intensity factor.

The microstructural aspects of brittle fracture will be discussed in a separate Part 2 [1].     

Developing a stochastic model to predict the strength and crack path of random composites

We characterize fracture and effective stress–strain graphs in 2D random composites subjected to a uniaxial in-plane uniform strain. The fibers are arranged randomly in the matrix. Both fibers and matrix are isotropic and elastic–brittle. We conduct this analysis numerically using a very fine two-dimensional triangular spring network and simulate the crack initiation and propagation by sequentially removing bonds which exceed a local fracture criterion. In particular, we focus on effect of geometric randomness on crack path of random composites. Based on that two stochastic micro-mechanic models are presented that can predict with confidence the failure probability of random matrix–inclusion composites.     

Correlation of microstructure with mechanical properties and fracture toughness of A356 aluminum alloys fabricated by low-pressure-casting, rheo-casting, and casting-forging processes

Correlation of microstructure with mechanical properties and fracture toughness of three cast A356 aluminum alloys fabricated by low-pressure-casting, rheo-casting, and casting-forging was investigated in this study. Microfracture observation results showed that eutectic Si particles were cracked first, but that the aluminum matrix played a role in blocking crack propagation. Tensile properties and fracture toughness of the cast-forged alloy were superior to those of the low-pressure-cast or rheo-cast alloy. These results were interpreted by a simple fracture initiation model based on the basic assumption that crack extension initiated at a certain critical strain developed over some microstructurally significant distance.     

Crack propagation in a glass particle-filled epoxy resin

Crack propagation in an epoxy resin reinforced with spherical glass particles has been followed using a double-torsion test. In particular the effect of strain rate, volume fraction and particle size upon the stability of propagation, the Young's modulus, the critical stress intensity factor,K _Ic and the fracture energy,G _Ic has been studied. It has been shown that the crack propagation behaviour can be explained principally in terms of crack pinning, although it has been found that propagation is also affected by blunting the breakdown of the particle—matrix interface. It has been demonstrated that crack-front pinning is consistent with a critical crack opening displacement criterion.