Fracture Mechanism Of Composites Research Articles

Study of the microstructure and mechanical properties of composites fabricated by the reaction method in an Al–TiO 2–B 2O 3 system

This paper investigates the microstructure, mechanical properties and fracture mechanism of composites fabricated by the exothermic dispersion method in the Al–TiO 2–B 2O 3 reaction system. When the B 2O 3/TiO 2 mole ratio is zero, the reinforcements of the composites are composed of α-Al 2O 3 and Al 3Ti. The α-Al 2O 3 particulates are segregated at matrix boundaries and cannot enter the aluminum matrix grains due to poor wetting with the matrix and the Al 3Ti rods distribute uniformly throughout the matrix. The ultimate tensile strength and elongation of the composites at room temperature are 250.4 MPa and 4.0%, respectively. The fracture mechanism of the composite can be characterized by a crack nucleus initiating in the Al 3Ti rod and then propagating to the interface because of the poor properties of Al 3Ti. When the B 2O 3/TiO 2 mole ratio reaches 1, the Al 3Ti phase is almost eliminated, the ultimate tensile strength and elongation increase to 320.8 MPa and 10.6%, respectively, and the tensile fracture surface is composed of fine ductile dimples. When the test temperature is about 723 K and the B 2O 3/TiO 2 mole ratio is 1, the elongation increases to 20.5% and the ultimate tensile strength decreases to 85.6 MPa.

Microstructure and mechanical properties of Al–2Mg alloy base short steel fiber reinforced composites prepared by vortex method

Fabrication and characterization of cast Al–2Mg alloy matrix composites reinforced with short steel fibers are dealt with in the present study. Three types of steel fiber were used: uncoated, copper coated and nickel coated. All the composites were prepared by the liquid metal route using vortex methods. When tested in tension, all composites exhibited improvement in strength due to high relative strength of steel fibers. The ductility was lowered except for the composite with copper coated fibers. Copper coated fiber reinforced composites gave the highest strength. Higher strength accompanied with appreciable ductility demonstrated by composites with copper coated fibers is attributed to the solid solution and fiber strengthening as well as good bonding at the interface. Composites reinforced with uncoated and Ni coated steel fibers did not exhibit strengthening to the level exhibited with copper coated fibers because brittle intermetallic phases are formed at the interface. These phases promote initiation and facilitate propagation of cracks. The observed fracture mechanism of composites was dimple formation, fiber breakage and pullout of fibers. Fracture surface of uncoated and Ni coated composites showed extensive pull out of fibers as well as fiber breakage confirming the above inference. In case of the copper coated composites dimple formation and coalescence was more extensive. EDX analysis showed a build up Cu, Ni, and Fe at the interface.

Microstructure and tensile properties of in situ synthesized (TiC + TiB + Nd 2O 3)/Ti-alloy composites at elevated temperature

A novel (TiC + TiB + Nd 2O 3)/Ti-alloy composites were in situ synthesized utilizing the reaction between Ti, C, Nd and B 2O 3 through homogeneously melting in a non-consumable vacuum arc remelting furnace. Ti–6.6Al–4.6Sn–4.6Zr–0.9Nb–1.0Mo–0.32Si alloy was chosen as the matrix Ti-alloy. The phases were identified by X-ray diffraction (XRD). The microstructures were examined by means of optical microscopy (OM), scanning electron microscopy (SEM) and backscattered electron microscopy. There are three kinds of reinforcements: TiC with near-equiaxed shape, TiB with whisker shape and Nd 2O 3 with sphere shape. Tensile properties of the as-cast composites were tested with strain rate 0.001 s −1 at 600, 650 and 700 °C. The results showed the tensile strength of the composites increased significantly at elevated temperature while keeping good plasticity. With the temperature increases, the improvement in tensile strength decreases. The fracture surfaces were examined by scanning electron microscopy. The fracture mechanism of composites turns from reinforcements’ crack to reinforcements’ debonding between 650 and 700 °C.

Shear strength of epoxypolysulfone glass-reinforced plastics in a wide range of loading rates

The shear strength of unidirectional epoxypolysulfone glass-reinforced plastics in the range of six decimal orders of loading rates has been measured. ED-22 epoxydiane resin (analog of DGEBA) modified by PSK-1 aromatic polysulfone served as a matrix. It has been shown that the strength values linearly increase with increasing logarithm of the loading rate; the sensitivity of composites to the rate of application of a force increases with increasing quantity of PSK-1 introduced into the matrix, and the fracture mechanism of composites is the thermofluctuation one. The laws revealed do not differ from those established by us earlier for glass-reinforced plastics based on epoxy binders modified by active diluents.

Coatings for glass fibers in a cementitious matrix

Despite the glass fiber being considered as a reinforcement of cementitious materials for several decades, the structural applications are limited by its poor corrosion resistance in a highly alkaline environment. Here, we describe an inexpensive and applicable process by which a nanometer-scale polymer surface coating layer based on styrene–butadiene with varied concentrations, as environmental barrier interphase, was applied to the alkali-resistant glass fibers. Our data indicates that the polymer coating designs the glass fiber with significantly improved both corrosion resistance to alkali and fracture energy absorption capability of its composites. Moreover, an assessment of changes in the fiber surface chemical and nanomechanical properties is provided and a strong correlation between the tensile strength, roughness and Griffith fracture predictions is noted. We examine the fracture mechanisms of composites at different scales based on a mechanical–statistical and an energy-geometry analysis.

Mechanical properties and fracture mechanism of ZA-27/TiO2 particulate metal matrix composites

The mechanical properties and the fracture mechanism of composites consisting of ZA-27 alloy reinforced with titanium dioxide particles were investigated with the primary objective of understanding the influence of the particulate reinforcement on the mechanical behaviour of the ZA-27 alloy. The titanium dioxide particle content in the composites ranged from 0 to 6 per cent, in steps of 2 wt %. The composites were fabricated by the stir casting technique in which the reinforcement particles were dispersed in the vortex created in the molten matrix alloy. The study revealed improvements in Young's modulus, ultimate tensile strength (UTS), compressive strength, yield strength and hardness of the composites as the titanium dioxide content was increased, but at the expense of ductility and impact strength. The fracture behaviour of the composite was also significantly influenced by the presence of titanium dioxide particles. Eventual fracture was a result of crack propagation through the matrix as well as through the reinforcing particles. Scanning electron microscopy and fractography analyses were carried out to provide suitable explanations for the observed phenomena.

Fracture mechanics of composites with residual stresses, imperfect interfaces, and traction-loaded cracks

By partitioning the total stresses in a damaged composite into either mechanical and residual stresses or into initial and perturbation stresses, it was possible to derive two exact results for the energy release rate due to crack growth. These general results automatically include the effects of residual stresses, traction-loaded cracks, and imperfect interfaces. These effects are normally not needed in the fracture mechanics of homogeneous materials, but they are commonly needed for the fracture mechanics of composites. The general results were used to consider mode I fracture in composites, fracture and thermal cracking for two-phase, isotropic composites, and interfacial fracture in the microbond and single-fiber, pull-out tests. The analysis of interfacial fracture illustrates the importance of including friction effects in the energy release rate and not as part of the toughness of the composite. Many composite damage modes consist of a series of events instead of stable crack propagation. A new analysis method, referred to as finite fracture mechanics, is proposed which predicts that the next event occurs when the total energy released by that event exceeds some critical value or toughness for that type of event. A finite fracture mechanics model for microcracking that can correlate the results from many laminates is described.

Detection of particle damage event by acoustic emission during tensile straining of SiC particle reinforced aluminium alloy composites

The fracture mechanisms of composites which contain coarse second phase (30 µmSiC) have been studied by acoustic emissions measurement. The fracture mechanism for these composites is different from that for composites containing small particles. In the latter case, it is well known that the void nucleation during fracture occurs at interface between matrix and reinforcement. The experimental data and the current model for fracture were compared, and fracture strain has been estimated using a modified particle fracture model based on the Evensen void growth model. Acoustic emissions were detected from fractured particle, although there are some differences in AE responses caused by difference in heat treatment. Void nucleation occurs by particle fracture in all of the materials. A modified equation for the fracture strain has been proposed and this equation provides a good quantitative agreement with experimental data.

Exact and variational theorems for fracture mechanics of composites with residual stresses, traction-loaded cracks, and imperfect interfaces

By partitioning the total stresses in a damaged composite into either mechanical and residual stresses or into initial and perturbation stresses, it was possible to derive several exact results for the energy release rate due to crack growth. These general results automatically include the effects of residual stresses, traction-loaded cracks, and imperfect interfaces. The exact energy release rate results were expressed in terms of exact solutions to reduced composite stress analysis problems. By considering the common situation where the initial stresses are known exactly, but the perturbation stresses are only known approximately, it was possible to derive rigorous upper and lower bounds to the energy release rate for crack growth. Some of the new fracture mechanics equations were applied to crack closure calculations, to fiber fracture and interfacial debonding in the fragmentation test, and to microcracking in composite laminates.

Mechanical properties and fractography of zircon-particle-reinforced ZA-27 alloy composite materials

The mechanical properties and the fracture mechanism of composites consisting of ZA-27 alloy reinforced with Zircon particles were investigated with the primary objective of understanding the influence of the particulate reinforcement on the behaviour of the ZA-27 alloy. The zircon particle content in the composites ranged from 1 to 5% in steps of 2% by weight. The composites were fabricated by the ‘compocasting’ technique in which the molten mass was squeeze-cast under pressure. The study revealed improvements in ultimate tensile strength, yield strength, hardness, and Young's modulus of the composites, but at the cost of ductility and impact strength, which decreased with increase in zircon content. The fracture behaviour of the composites was altered significantly by the presence of the zircon particles. Crack propagation through the matrix and the reinforcing particles resulted in the final fracture. Scanning electron microscopy and fractography analyses have been carried out to furnish suitable explanations for the observed phenomena.

Analytical and experimental methods for a fracture mechanics interpretation of the microbond test including the effects of friction and thermal stresses

An energy release rate model based on a generalized fracture mechanics of composites was developed for analyzing the microbond test. This model, which extended a previous model, includes both friction at the fiber/matrix interface and residual thermal stresses. A series of microbond tests on macroscopic specimens were carried out for evaluating the model. In some specimens we could observe debond crack growth. These results could be interpreted with a fracture mechanics R-curve which led to a measured interfacial fracture toughness. In many specimens, debond crack growth could not be observed. We developed an approximate method for determining interfacial fracture toughness even without knowledge of debond crack size. The macroscopic specimens were designed for studying the optimal approach to analysis of microbond specimens. The geometry of the macroscopic specimens, however, could also be used to measure the mode II toughness of adhesive bonds.

Fracture Mechanics of Composites With Residual Thermal Stresses

The problem of calculating the energy release rate for crack growth in an arbitrary composite in the presence of residual stresses is considered. First, a general expression is given for arbitrary, mixed traction, and displacement boundary conditions. This general result is then applied to a series of specific problems including statistically homogeneous composites under traction or displacement boundary conditions, delamination of double cantilever beam specimens, and microcracking in the transverse plies of laminates. In many examples, the energy release rate in the presence of residual stresses can be reduced to finding the effect of damage on the effective mechanical properties of the composite. Because these effective properties can be evaluated by isothermal stress analysis, the effect of residual stresses on the energy release rate can be evaluated without recourse to any thermal elasticity stress analyses.

On the fracture of the unidirectional composites in compression

The experimental studies of the fracture of unidirectional composite materials under loading along the reinforcing elements showed that one of the major mechanism of this fracture is the stability loss in material structure. Up to the present time, numerous theoretical and experimental investigations of the mentioned problems have been made. Note that in these investigations (carried out from a theoretical viewpoint), the problems considered have been studied in the framework of the Euler approach and those have been reduced to investigations of the corresponding eigenvalue problems. In the present paper, a method for investigating the problem of fracture (stability loss) of a unidirectional composite in compression has been given, which uses as a criterion the value of the compressive force for which the initial infinitesimal curvature given to the fibre or layer starts to increase and grows indefinitely. In these cases the investigations are carried out in the framework of the piecewise-homogeneous body model with the use of the exact three-dimensional non-linear equations of the deformable solid body mechanics.Note that the results obtained with the application of the proposed approach to the investigation of the mentioned fracture problems coincide with those which have been obtained in the framework of the Euler approach. Moreover, the approach is also employed successfully for the theoretical investigations of the problems of the fracture mechanics of composites in compression, where the study in the framework of the Euler approach are impossible. In the present paper the fracture (stability) problems of composites with viscoelastic components in compression are also studied by the use of the proposed approach.

Effect of thermomechanical processing on the microstructure and mechanical properties of Al [sbnd]Mg (5083)/SiCp and Al [sbnd]Mg (5083)/Al 2O 3p composites. Part 3: Fracture mechanisms of the composites

In this paper, the last of three articles, the fracture mechanisms of as-cast composites, extruded composites and extruded monolithic 5083 aluminum alloy deformed in uniaxial tension are discussed. Dimple fracture is observed in both the monolithic 5083 aluminum alloy and the extruded composites, whereas the fracture of the as-cast composites is mainly caused by decohesion at matrix/particle interfaces and crack propagation along dendrite boundaries. For extruded composites the fracture is mainly nucleated by particle cracking and final failure occurs by shear coalescence of the small void sheets of matrix between the large reinforcing particles (or between clusters of particles). The large voids caused by fracture/decohesion of reinforcing particles induce localization of deformation in the matrix which accelerates the nucleation, growth and coalescence of the small voids in the matrix. Localization of deformation is one of the important factors which decrease the ductility of the composites. Three types of composites reinforced by SiC particles, artificially oxidized SiC particles prior to their incorporation and Al 2O 3 particles are examined. The effects of the interfacial reactions on the fracture of the composites are also presented.

Fracture mechanics of composites with nonhomogeneous interphases and nondilute fiber volume fractions

A linear elastic fracture mechanics model is developed for a periodic composite geometry with a single cracked layer under uniform longitudinal strain. The interface between the fiber and the matrix can be nonhomogeneous, homogeneous or perfect. Four fracture mechanics models are considered: (1)dilute fiber volume fraction composite with nonhomogeneous interphases.(2)periodic composite with nondilute fiber volume fraction.(3)periodic composite with irregular fiber spacing near a matrix layer.(4)hybrid composites with more than one type of fiber. The stress intensity factors at the crack tips and the interface stress fields are studied to understand the fracture mechanics of composites as a function of the relative elastic moduli of the fiber, the matrix and the interphase, and the global and local fiber volume fractions.

Elastoplastic state of axisymmetrically loaded laminated bodies of revolution made of isotropic and orthotropic materials

i. A. N. Guz', "Construction of a theory of the stability of unidirectional fiber composites," Prikl. Mekh., 5, No. 2, 62-70 (1969). 2. A. N. Guz', Stability of Three-Dimensional Deformable Solids [in Russian], Naukova Dumka, Kiev (1971). 3. A. N. Guz', Stability of Elastic Bodies with Finite Strains [in Russian], Naukova Dumka, Kiev (1973). 4. A. N. Guz', Principles of a Three-Dimensional Theory of the Stability of Deformable Bodies [in Russian], Vishch. Shkola, Kiev (1986). 5. A. N. Guz', Fracture Mechanics of Composites in Compression [in Russian], Naukova Dumka, Kiev (1989). 6. A. N. Guz' and I. A. Guz', "Local instability of composite laminates," Dokl. Akad. Nauk SSSR, 312, No. 3, 210-212 (1990).

Catastrophes in the fracture mechanics of composites with cracks

Catastrophes in the fracture mechanics of composites with cracks

The boundary-layer method in the fracture mechanics of composites of periodic structure

The problem of a rectilinear crack in a composite material of doubly — periodic structure is considered. It is assumed that the dimensions of the crack are considerably greater than the cell of material periodicity. A boundary-layer method based on the use of the asymptotic method of averaging periodic structures, taking additional solutions of boundary-layer type /1/ into account to allow the edge effect that occurs near the boundary of the crack outline to be considered, is proposed for analysing the stress field in the neighbourhood of a macrocrack. Analysis of the stress field in highly inhomogeneous (composite) materials with an idealized smooth macrocrack is usually performed by replacing the inhomogeneous composite medium by a certain homogeneous anisotropic medium that is equivalent to the composite material with respect to the average reaction. Such an approach enables the computation of the average stress field in the composite with a macrocrack to be reduced to solving elasticity theory problems for an anisotropic homogeneous material with a mathematical slit. If the material has a periodic structure (as is true of many composites), the average (effective) characteristics of the equivalent should be determined by the method of averaging periodic structures /1–3/ which yields an asymptotically correct approximation to the exact solution of the problem for the initial inhomogeneous medium. The averaging method here allows the local structure of the fields being investigated to be determined with a high degree of accuracy. This approach was used in /4/ to analyse the stress field near a macrocrack in laminar composites of periodic structure. In a number of cases formulas were obtained for the stress intensity factors that express them in terms of the characeristics of the individual composite components and parameters which fix the crack location in the laminar material. A more rigorous approach to the estimation of the state of stress of a composite material with a crack is proposed.

Fracture behavior in stitched multidirectional composites

Abstract Various two-dimensional (2D) and three-dimensional (3D) composite materials in different geometries and compositions were fabricated and examined. The 3D composites were produced by the techniques of stitching. Damage resistance and fracture mechanisms of composites were studied as a function of loading directions at both low and high loading rates. The effects of the type, concentration and distribution of the third-direction fibers on the mechanical behavior of an otherwise 2D composite were investigated. Failure analyses were conducted to determine the failure causes and crack propagation behavior by means of in situ optical microscopy and scanning electron microscopy. Multidirectional composites were found to possess a greater damage resistance than did the conventional 2D composites. The third-direction fibers were found to reduce effectively the degree of delamination and to increase the flexural strength of composites by increasing the interlaminar fracture surface energy. Problems associated with the fabrication of 3D composites were identified and discussed.

Diffraction of acoustic waves by two nonparallel circular cylinders

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