Brittle Matrix Composites Research Articles

Seeding ductile nanophase in ceramic grains.

Transgranular brittle fracture is the dominant failure mode of brittle materials, including ceramics and ceramic matrix composites. However, strengthening these materials without sacrificing their toughness has been a big challenge. In this study, an innovative approach is proposed to achieve coordinated strengthening and toughening of ceramics-based composites by introducing specific ductile coherent nanoparticles into ceramic grains. As an example, the WC-Co cemented tungsten carbides were used to demonstrate how this brittle material can achieve ultrahigh strength without losing toughness by seeding metallic nanoparticles inside WC grains. The mechanisms for inducing the formation and modulating the amount, size, and distribution of such nanophase within the ceramic grains were disclosed. The fraction of transgranular ruptures of the brittle ceramic phase was reduced significantly due to the presence of the ductile coherent in-grain nanoparticles. Both the strength and strain limit of the cemented carbides were remarkably increased compared to their counterparts reported in the literature. The coordinated strengthening and toughening strategy proposed in this work is applicable to a broad range of ceramics and ceramic matrix composites to obtain superior comprehensive mechanical properties.

Abnormal mechanical hysteresis behavior of Tyranno-ZMI SiC/SiC containing Ti3Si(Al)C2

This work studied the effect of tough phase Ti3Si(Al)C2 on the mechanical hysteresis behavior of SiC/SiC. Different from continuous fibers reinforced brittle ceramic matrix composites, the mechanical hysteresis behavior of SiC/SiC containing Ti3Si(Al)C2 shows some abnormal phenomena: as peak applied stress increases during cyclic loading-unloading-reloading tests, the thermal residual stress values exhibit highly dispersion and the thermal misfit relief strain shows abnormally slow growth. These abnormal phenomena are caused by the reduction of transvers cracks (perpendicular to loading fibers) and the generation of hoop cracks (parallel to loading fibers). The plastic deformations of Ti3Si(Al)C2 prevents the transverse cracking of modified matrix, while promoting the hoop cracking of SiC matrix prepared by chemical vapor infiltration (CVI-SiC). Hoop cracking occurs within the transition zone containing amorphous SiO2 layer and carbon layer in CVI-SiC matrix. The combination of weak transition zone and strong modified matrix finally leads to the occurrence of hoop cracking.

New Self-Repairing System for Brittle Matrix Composites Using Corrosion-Induced Intelligent Fiber.

Brittle matrix composites such as concrete are susceptible to damage in the form of cracks. Most of the current self-repair and self-healing techniques have repair limits on crack widths or high costs of an external stimulator, or have an unfavorable effect on the composite’s strength. This paper proposes a new concept of corrosion-induced intelligent fiber (CIF) and a new self-repairing system that uses the CIFs to close cracks in brittle matrix composites within a corrosive environment without external help, and without compromising the strength. The CIF comprises an inner core fiber and an outer corrodible coating that are in equilibrium, with the core fiber in tension and the corrodible coating in compression. The preparation steps and shape recovery mechanism of the CIF and the self-repair mechanism of the CIF composites are explained. Based on these concepts, this paper also describes several mechanical models built to predict the magnitude of pre-stress stored in the core fiber, and the maximum pre-stress released to the matrix composites, and the minimum length of the reliable anchor ends of CIF. The sample calculation results show that the recovery strain was 0.5% for the CIF with the steel core fiber and 12.7% for the CIF with the nylon core fiber; the maximum crack closing force provided by the CIF to concrete can be increased by increasing the amount of the CIFs in concrete and the initial tensile stress of the core fiber. This paper provides some suggestions for enhancing the self-repair capability of brittle composites in complex working environments.

A hybrid artificial neural network-based identification systemfor fine-grained composites

Recent interest in the development of innovative building materials has brought about the need for a detailed assessment of their mechanical fracture properties. The parameters for these need to be acquired, and one of the possible ways of doing so is to obtain them indirectly - based on a combination of fracture testing and inverse analysis. The paper describes a method for the identification of selected parameters of mortars and other fine-grained brittle matrix composites. The cornerstone of the method is the use of an artificial neural network, which is utilized as a surrogate model of the inverse relation between the measured specimen response parameters and the sought material parameters. Due to the potentially wide range of composite mixtures and hence the wide range of experimental responses likely to be gained from individual specimens, an ensemble of artificial neural networks was created. It allows the entire range of variants to be covered and provides resulting parameter values with sufficient precision. Such a system is also easy to expand if a composite with properties outside the current range is tested. The capabilities of the proposed identification system are demonstrated on two selected types of fine-grained composites with different specimen responses. The first group of specimens was made of composite based on alkali-activated slag with standardized and natural sand investigated within the time interval of 3 to 330 days of aging. The second tested composite contained alkali-activated fly ash matrix, and the effect of the addition of natural fibers on fracture response was investigated.

Tensile behavior of carbon textile concrete composite captured using a probabilistic multiscale multiple cracking model

The fast growing amount of high-performance textile fabrics made of carbon, glass and basalt yarns open up enormous design scope for cementitious composites, that needs to be sorted and graded using novel characterization procedures. A combined experimental and numerical characterization method of the composite tensile behavior is presented here for concrete reinforced with non-impregnated carbon textile fabrics. Because of the heterogeneous structure of the bond layer, which consists of a non-impregnated multifilament yarn partially penetrated by the cement paste, this material combination poses particularly high demands on a realistic modeling approach. It is well known, that the tensile response of strain-hardening brittle-matrix composites is primarily shaped by the multiple cracking process of the matrix. To account also for the local damage developing in the heterogeneous bond layer ahead of a crack bridge during the multiple cracking process, we combine a probabilistic crack bridge model with a generalized version of a crack tracing algorithm reflecting the random matrix strength. The resulting probabilistic multiscale multiple cracking model has been calibrated and validated using a consistent series of crack bridge and composite tensile tests by comparing the predicted and measured stress–strain curves and crack spacing histories. The efficient, flexible and realistic modeling concept can serve as a basis for the development of rapid characterization methods and more economic and reliable design rules.

Effect of anisotropy on the ductile fracture in metal reinforcements of brittle matrix composites

Mechanisms of ductile failure in brittle matrix composites with metal reinforcements are investigated using a mechanistic approach to incorporate material anisotropy. Rapid void expansion with increasing remote strain and at nearly constant remote strain (cavitation) is modeled. Fracture energy, total energy absorbed, and cavitation limit is obtained as a function of material anisotropy. It is observed that for initial void volume fraction less than or equal to 10-4 cavitation phenomenon is seen. The critical stress at cavitation is found to increase with decreasing void size until it reaches the cavitation limit. Further, it is observed that the ductile fracture of metal reinforcements in the brittle matrix composite is sensitive to the material anisotropy.

Probabilistic multiple cracking model of brittle-matrix composite based on a one-by-one crack tracing algorithm

The paper describes a probabilistic model capturing the multiple-cracking behavior of unidirectional brittle-matrix composites loaded in tension. The approach to modeling of composite fragmentation introduces two salient features that enhance both efficiency and flexibility compared to existing simulation methods: First, the algorithm identifies the emerging cracks one by one within a minimum number of load increments. Second, the crack-tracing algorithm is based on an abstract description of a crack bridge behavior. As a result, it is possible to combine the crack-tracing algorithm with a wide variety of crack bridge models, i.e. non-linear, deterministic or probabilistic. In this way, specific phenomena of bond behavior in different types of composites can be accounted for. The random nature of matrix cracking is reproduced using a random field simulation of the matrix strength. The model is verified by reproducing analytical results available for constant bond-slip law. The feasibility and robustness of the algorithm is demonstrated using an interactive web application that is directly executable from within a public github.com repository.

Fabrication, Structure, and Mechanical Properties of Composite Oxide Fibers for Brittle-Matrix Composites

Abstract—Composite fibers of starting mixtures of aluminum, yttrium, and calcium oxides and fibrous composite materials consisting of these fibers and a molybdenum matrix are fabricated by internal crystallization. The structure, the strength, and the cracking resistance of the fibers are studied in the temperature range 20–1400°C. Inductile components are shown to be used to form composite materials with quasi-plastic behavior, and their cracking resistance can be 20–25 MPa m1/2.

A homogenized formulation to account for sliding of non-meshed reinforcements during the cracking of brittle matrix composites: Application to reinforced concrete

Non-linear finite element modelling of complex structures made of composites, such as reinforced concrete, remains a challenge because, until now, the only way to consider the important phenomenon of sliding between the reinforcements and the brittle matrix of the composite has been to mesh the reinforcements and their interfaces explicitly. This method is accurate but so expensive in terms of computational resources that only critical small elements of composites structures are modelled using it. To get around this limit, a method avoiding the meshing of composite reinforcements is proposed. It consists in treating the sliding between reinforcements and matrix with a differential formula of reinforcements directly as a continuous field superimposed to the displacement field of the matrix. The method needs a minor modification of the finite element code, which can take advantage of its analogy with the anisotropic thermal formulation. After the analytical presentation of the method, two theoretical cases of study are given to confront the results obtained with this method without meshing of reinforcements, with reference results obtained using a complete mesh of the matrix, reinforcements and interfaces.

Composite oxide fibers and brittle matrix composites based on them

Composite oxide fibers were obtained on the basis of various initial mixtures of oxides of aluminium, calcium, barium and yttrium. The fibers contain components that provide the necessary strength and fracture toughness. High strength is provided by alumina. Fracture toughness is mainly determined by the formation of layered components of calcium hexaaluminate or barium hexaaluminate in the fiber structure. Relatively weak planes in the structure of the hexaalumiates play the role of brakes of cracks in composites consisting of non-plastic components. Composites with brittle matrices were made on the basis of these fibers, tests of which showed their “quasiplastic” behavior under load. The strength values in the temperature range of 20–1400 °C and fracture toughness correspond to the requirements for structural materials.

Effect of interface strength on the mechanical behaviour of bio-inspired composites: A molecular dynamics study

Nanoscale composites inspired from biological materials such as nacre and bone are excellent candidates for improving the mechanical properties of ceramic matrix composites (CMC) or in general any brittle-matrix composites. These composites are based on the regularly staggered model (RSM) and on the stair-wise staggered model (SSM), and consist of stiffer platelets embedded in a compliant matrix. The interface between the platelets and the matrix plays a key role as an enormous amount of interfacial area exists in the nanocomposite materials. We investigate the influence of interface strength on the overall mechanical properties and deformation mechanisms of bio-inspired brittle-matrix nanocomposites. Atomistic simulations are used for this purpose and the interface strength is controlled by varying the binding energy between the platelet atoms and the matrix atoms. We found that the mechanical properties of both models initially increase with the interface strength. However, above a critical value, the trend changes drastically between the two models. The RSM model continues to maintain the flow stress and toughness at higher interface strengths, due to the persistence of deformation mechanisms such as bulk matrix plasticity and platelet pullout. In contrast, in the SSM model the flow stress and toughness are deteriorated as softening of stress and fracturing of platelets occur at higher interface strengths. The knowledge gained from this study paves new way for the development of bio-inspired ceramic-matrix composites with improved mechanical properties.

Characterization model of thermal shock resistance for fiber reinforced brittle matrix composites over a wide range of cooling environment temperatures

In this study, a thermal shock resistance model over a wide range of cooling environment temperatures for fiber reinforced brittle matrix composites is developed. The model takes into account the combined effects of cooling environment temperature and temperature-dependent material parameters. The critical temperature difference causing matrix cracking of SiC fiber reinforced reaction bonded Si3N4 composite and Nicalon fiber reinforced borosilicate composite are predicted over a wide range of cooling environment temperatures. The results show that the thermal shock resistance of composites is strongly dependent on the cooling environment temperature. Moreover, the quantitative influences of matrix Young's modulus, thermal expansion coefficient of matrix, and interfacial shear stress on thermal shock resistance with increase of cooling environment temperature are studied in detail. And some insights into improving the thermal shock resistance of fiber reinforced brittle matrix composites are obtained.

Development and characterization of self-sensing CNF HPFRCC

Cement based brittle matrix composites that show deflection hardening called high performance fiber reinforced cementitious composites (HPFRCC) have the potential of offering high resiliency and environmentally sustainable benefits in numerous applications. HPFRCCs have been used in numerous applications; however, more information is needed to fully understand, predict the behavior, and add functionality to HPFRCCs. This experimental research program aims to develop and characterize a new type of HPFRCC, composed of polyvinyl alcohol microfibers, carbon nanofibers (CNF), and a high volume of fly ash to form a self-sensing HPFRCC. Processing of the CNF and the HPFRCC composite matrix are studied to ensure adequate fresh mix properties and fiber dispersion. The multi-functionality of the CNFs allow for increased mechanical properties and enhanced strain and damage sensing capabilities. Digital image correlation was used extensively to capture the tensile strain and provide a visualization of the behavior of the composite under increasing displacements. CNF contents of 0.167, 0.333, 0.5, and 1.0 % were studied. Through laser diffraction analysis, sonication of CNF in a CNF/water/superplasticizer solution was found to improve particle dispersion. Mechanical testing showed that dosages of CNFs less than or equal to 0.333 % increased compressive strength, tensile strength, and stiffness. CNF had little effect on tensile ductility and toughness. Piezoresistive self-sensing strain measurement was achieved in all specimens. The addition of CNF improved the strain-sensing and allowed for stronger correlations between measured strains and correlated gauge factors. Ultimately, a CNF content of 0.333 % showed optimal results in terms of enhanced mechanical properties and self-sensing behavior for strain.

Fracture Surface Morphology of Delamination Failure of Polymer Fiber Composites Under Different Failure Modes

The delamination of fiber reinforced polymer composites is one of the most common failures encountered in industrial applications. The most unique macroscopic and microscopic fracture surface features of the delaminations under different failure modes are of interests not only for practical failure analysis investigations but also it helps to reveal the physics behind the delamination phenomenon. In this work, fracture surface morphology of the delaminated carbon fiber polymer composites under mode I, mode II, and mixed-mode I/II loading conditions is investigated mainly with scanning electron microscopy. The unique fractographic features are identified and discussed. The results on ductile and brittle matrix composites have shown their own features, and most important of all the alignment angle of fibrils in the resin-rich ductile matrix could be correlated with the delamination mode.

A New Approach to Simulate Interface Damage in Brittle Matrix Composites

Under mechanical loading, brittle matrix composites develop various damage processes which include cracking of matrix and fiber/matrix interface. Cohesive zone models (CZM) are widely used to model interfacial damage. The aim of this paper is to propose an alternative approach based on a damage model associated to a diffuse meshing method. In this study, the deflection of a matrix crack at the fiber/matrix interface in a single fiber composite is simulated using CZM and the results are compared with those obtained with the new approach. The simulation of a tensile test (in the fiber direction) on a brittle single fiber composite is performed by using the damage model and the result show a good agreement with the reference one obtained with CZM. On this basis, it is shown that the damage model is able to reproduce complex phenomenon as periodic matrix crack deflections at the fiber/matrix interface.

Mechanisms of interfacial bond in steel and polypropylene fiber reinforced geopolymer composites

The fiber-matrix interface properties play an important role in determining the overall mechanical performance of the fiber reinforced brittle matrix composites. This study is to evaluate the effects of wettability, chemical characterization and nanometric roughness of two different fibers, polypropylene (PPF) and micro steel fibers (MSF), superposed by shrinkage of fly ash based geopolymer binder on fiber-matrix interaction and the consequent mechanical properties of the corresponding composites. The fiber incorporation in geopolymer matrix was performed by 0.5, 1, 2, 3 and 4% of the total volume of each type of fibers. The results show that MSF has strong contact with the geopolymer paste since it tends to behave as a hydrophilic material that resulted in significant improvement in energy absorption and flexural strength of the composite; while PPF led to fiber-matrix debonding due to hydrophobic characteristics of the fiber compounds by geopolymer shrinkage that weakened the mechanical performances of the composites.

Stress analysis of a crack in a fiber-reinforced layered composite

Brittle matrix composites such as fiber-reinforced cements and ceramics are often bonded to dissimilar materials such as metals or other ceramics. The mismatch in elastic properties can affect the growth of matrix cracks located near the bonded interface. In this paper, a bonded system comprising of an elastic layer sandwiched between two identical elastic half-spaces is considered. A mathematical model is developed based on the distributed dislocation technique, for the analysis of a bridged straight crack embedded in the elastic layer. The bridging tractions are considered to be dependent on the crack opening displacement. The stress field is assumed to be plane stress or plane strain and all three-dimensional effects are disregarded.The numerical results include the critical stress required to initiate failure of the matrix and the failure of fibers bridging the crack. The dependencies of these critical values upon the thickness of the composite layer as well as the mismatch in elastic properties are presented in dimensionless form. The results can be used to estimate the residual strength of bonded composites with a flaw near the material interface.

Theoretical model of fiber debonding and pull-out in unidirectional hybrid-fiber-reinforced brittle-matrix composites

A theoretical model is developed to study the fiber debonding and pull-out in hybrid-fiber-reinforced brittle-matrix composites. By adopting the shear-lag model which includes the matrix shear deformation in the bonding region and friction in the debonding region, the relationship between the pull-out length and the debonding length of fiber is obtained by treating the interface debonding as a particular crack propagation problem along the interface. The interface debonding criterion for the hybrid-fiber-reinforced composites is obtained by applying the energy release rate relation in an interface debonding process. The analysis is applied to hybrid carbon/glass fiber-reinforced epoxy composites, and the theoretical results have a reasonable agreement with the experimental data. The hybrid effect is studied and the effect of material parameters is discussed.

Anisotropic unilateral damage with initial orthotropy: A micromechanics-based approach

A micromechanics-based damage model able to describe the brittle response of initially anisotropic materials is presented. A special emphasis is put on the account of damage-induced anisotropy and unilateral behaviour related to microcracks closure effects. These both features clearly influence the inelastic deformation of microcracked materials and lead to even more complex consequences in the context of initial anisotropy. The aim of this work is then to derive a new strain-based formulation which allows representing the related interactions between all these phenomena. This is achieved through a recent two-dimensional energy-based micromechanical analysis that accounts for the fully anisotropic multilinear response of orthotropic materials weakened by arbitrarily oriented microcracks. On the other hand, the thermodynamics framework gives a standard procedure for the development of the damage evolution law. Throughout the paper, attention is put on the mathematical and thermodynamical consistency of the model to avoid difficulties usually associated to the simultaneous description of damage-induced anisotropy and unilateral effects. In addition to elastic constants, the model requires the identification of only two parameters related to damage evolution. The model has been implemented within the commercial finite-element code ABAQUS, and various numerical simulations are presented to illustrate its capabilities. Especially, evolution of the material symmetry and influence of opening-closure states of microcracks on the damage process are illustrated in the case of brittle matrix composites subjected to different loading cases (axis and off-axis loads, tension and compression, tension followed by compression).

Corrigendum to ‘Brittle matrix composites with heterogeneous reinforcement: Multi-scale model of a crack bridge with rigid matrix’ [Composi. Sci. Technol. 89 (2013) 98–109

Corrigendum to ‘Brittle matrix composites with heterogeneous reinforcement: Multi-scale model of a crack bridge with rigid matrix’ [Composi. Sci. Technol. 89 (2013) 98–109