Mechanical Behavior Of Composite Materials Research Articles

Mechanical Properties of Composite Materials Made of 3D Stitched Woven-knitted Preforms

This article presents an experimental investigation carried out to understand the mechanical behavior of composite materials made of 3D stitched woven-knitted basalt fabrics. The innovative preforms used consist of two outer layers of a plain woven fabric combined with two inner layers of weft-knitted fabrics. The weft-knitted fabrics selected for the study were varied plain knit, 1 × 1 rib, Milano, and interlock. The fabric layers were stitched together with Kevlar yarns. These 3D stitched preforms were impregnated with polyester resin using resin transfer molding, and the corresponding composites obtained were tested under tensile, bending, and impact loads. The results obtained show that the type of knitted structure significantly influences the mechanical performance of the 3D stitched woven-knitted composites. The composite using interlock structure as the inner layers has the best results concerning energy absorption and tensile strength. The varied plain knit structure has proved to be the best suited to impart stiffness as it provides the highest Young’s modulus among the above four knitted structures.

Mechanical behaviour of composite materials made by resin film infusion

Innovative composite materials are frequently used in designing aerospace, naval and automotive components. In the typical structure of composites, multiple layers are stacked together with a particular sequence in order to give specific mechanical properties. Layers are organized with different angles, different sequences and different technological process to obtain a new and innovative material. From the standpoint of engineering designer it is useful to consider the single layer of composite as macroscopically homogeneous material. However, composites are non homogeneous bodies. Moreover, layers are not often perfectly bonded together and delamination often occurs. Other violations of lamination theory hypotheses, such as plane stress and thin material, are not unusual and in many cases the transverse shear flexibility and the thickness-normal stiffness should be considered. Therefore the real behaviour of composite materials is quite different from the predictions coming from the traditional lamination theory. Due to the increasing structural performance required to innovative composites, the knowledge of the mechanical properties for different loading cases is a fundamental source of concern. Experimental characterization of materials and structures in different environmental conditions is extremely important to understand the mechanical behaviour of these new materials. The purpose of the present work is to characterize a composite material developed for aerospace applications and produced by means of the resin film infusion process (RFI). Different tests have been carried out: tensile, open-hole and filled-hole tensile, compressive, openhole and filled-hole compressive. The experimental campaign has the aim to define mechanical characteristics of this RFI composite material in different conditions: environmental temperature, Hot/Wet and Cold.

Effect of Sericin on Mechanical Behavior of Composite Material Reinforced by Silk Woven Fabric

Effect of Sericin on Mechanical Behavior of Composite Material Reinforced by Silk Woven Fabric

Vinyl Ester—Glass Hollow Particle Composites: Dynamic Mechanical Properties at High Inclusion Volume Fraction

This study entails the analysis of dynamic mechanical properties of hollow particle filled composites, called syntactic foams. A theoretical model is developed to predict the dynamic mechanical behavior of composite materials containing particle volume fraction up to the packing limit. The modeling approach is based on a differential scheme that extrapolates the properties of infinitely dilute dispersions of hollow particles (microballoons) to high inclusion volume fraction. The model is capable of predicting storage modulus and loss tangent in a wide frequency band of mechanical vibrations. Theoretical predictions are validated with experimental results on 16 compositions of vinyl ester—glass microballoon syntactic foams. Vibration methods are used to assess the dependence of dynamic mechanical properties of syntactic foams on microballoon wall thickness and volume fraction. Theoretical predictions are in close agreement with experimental findings over a wide range of vibration frequencies. Results show that loss tangent generally decreases as the inclusion volume fraction increases and is almost unaffected by microballoon wall thickness and that storage modulus increases with increasing microballoon wall thickness. Therefore, the loss tangent and the storage modulus can be tailored by means of microballoon volume fraction and wall thickness.

Assessment of Mixed Uniform Boundary Conditions for Predicting the Mechanical Behavior of Elastic and Inelastic Discontinuously Reinforced Composites

The combination of heterogeneous volume elements and numerical analysis schemes such as the Finite Element method provides a powerful and well proven tool for studying the mechanical behavior of composite materials. Periodicity boundary conditions (PBC), homogeneous displacement boundary conditions (KUBC) and homogeneous traction boundary conditions (SUBC) have been widely used in such studies. Recently Pahr and Zysset (2008) proposed a special set of mixed uniform boundary conditions for evaluating the macroscopic elasticity tensor of human trabecular bone. These boundary conditions are not restricted to periodic phase geometries, but were found to give the same predictions as PBC for the effective elastic properties of periodic open cell microstructures of orthotropic symmetry. Accordingly, they have been referred to as “periodicity compatible MUBC” (PMUBC). The present study uses periodic volume elements that contain randomly positioned spherical particles or randomly oriented short fibers at moderate volume fractions for assessing the applicability of PMUBC to modeling composite materials via volume elements that deviate from orthotropic symmetry. Macroscopic elasticity tensors are evaluated with PBC, PMUBC and KUBC for elastic contrasts in the range 2 ≤ sr ≤ 30. For one configuration the isotropic contributions to the macroscopic elastic tensors obtained with PBC and PMUBC are extracted and compared. In addition, macroscopic elasticplastic responses for different hardening behaviors are studied with PBC and PMUBC. Only small differences between the predictions obtained with PBC and PMUBC are found, validating the PMUBC for studying volume elements the overall behavior of which shows minor contributions of lower than orthotropic symmetry. keyword: Mixed Uniform Boundary Conditions, Periodicity Boundary Conditions, Finite Element Simulation, Composites 1 Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, A-1040 Vienna, Austria

Small-scale mechanical behavior of intermetallics and their composites

To model and predict the mechanical behavior of composite materials, knowledge of the properties of their constituent phases as well as how they interact with each other is required. Although these mechanical properties can sometimes be deduced from measurements on bulk specimens, it is not always possible due to difficulties in producing bulk materials with compositions and structures similar to those of the individual phases in the composite. In such cases, in situ measurements of mechanical properties are needed. We review here our recent work on the application of nanoindentation and neutron diffraction to investigate the in situ mechanical responses of Cr-Cr 3Si and NiAl-Mo, two model eutectic composites that were chosen because they can be processed by directional solidification to yield well-aligned lamellar and fibrous microstructures. Phase-specific mechanical properties were measured and correlated with bulk behavior. We will discuss recently developed techniques that improve the accuracy of mechanical property measurements at small microstructural length scales.

Flexural Properties of Fiber‐Reinforced Composites

AbstractTo describe the macroscopic mechanical behavior of composite materials with biaxial weft‐knitted fabric reinforcement, a difference between the flexural and tensile stiffness must be taken into account. Due to the periodic structure of the composite, the material behavior can be investigated on a unit cell. A homogenization technique based on the Hill‐Mandel energy balance is used to replace a heterogenous Boltzmann medium by a homogenous COSSERAT continuum. To simulate the material behavior, this homogenization method is applied to the Binary Model. Tensile, shear and bending tests were performed to verify the simulation results. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Directional damage and failure of composite materials under cyclic loading

Fibre orientation and shape of structure create directional effects that must be taken into account in mechanical behaviour of composite materials. Some orientations are more sensitive to strain and failure. This sensitivity is analysed by considering the feature of loading (direction and type that depend on the sign of stress components). We suggest a model able to identify mechanical properties in various directions by using an iso-strain quadratic surface in the space of stress. And we define a directional damage parameter, which depends on the plastic residual strain and the loading cycles. For failure, we use a quadratic surface similar to Hill criterion, the coefficients of which depend on loading cycles, for each type and direction of loading. Computations are validated on unidirectional glass.

Surface modification of boron fibres for improved strength in composite materials

When boron fibres are combined with an organic matrix, such as an epoxy resin, a high-performance composite structure is created. This study investigates the surface chemistry of plasma- and organosilane-treated boron fibres with the key aim to improving the adhesion properties between the boron fibre and the epoxy matrix. Optimisation of this interfacial region plays a critical role in influencing the mechanical behaviour of composite materials and has considerable industrial applications in the aerospace and manufacturing industries. The surface chemistry of a model boron surface and boron fibres was monitored using a combination of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). Initial investigation of the as-received fibres showed the presence of silicone contamination on the fibre surface, which would affect adhesion. Removal of this contaminant through solvent cleaning and plasma oxidation provided an ideal surface for attachment of the organosilane adhesion promoter. A model for the interaction of the organosilane with a boron surface is proposed. The pull-out strength of boron fibres, with different surface treatments, embedded in the epoxy resin was measured using a custom designed adhesiometer. Compared with as-received boron fibres, a 6-fold improvement in the apparent interfacial shear strength was achieved for the organosilane treated fibres. Optical microscopy was used to determine the failure mechanisms between the fibre and epoxy resin. Typically, as the surface treatment improved adhesion, the locus of failure changed from the boron–epoxy interface to failure within the epoxy and ultimately fibre breakage.

Propagation of SV waves in a periodically layered media in nonlocal elasticity

Effect of nonlocality on the dynamic behavior of laminated composites is investigated by means of dispersion of vertically polarized harmonic shear waves propagating in the direction parallel to layering. Nonlocal elasticity is briefly summarized and the dispersion relation for symmetric and anti-symmetric waves are obtained within the framework of both classical and elasticity. The numerical structure of the problem is investigated as to reduce some possible incorrect results which may take place in the solution. Results are displayed in a serious of figures. Advantages of nonlocal elasticity in representing the mechanical behavior of composite materials is discussed.

On the interaction between a dislocation and a circular inhomogeneity with imperfect interface in antiplane shear

The solution of appropriate elasticity problems involving the interaction between inclusions and dislocations plays a fundamental role in many practical and theoretical applications, namely, it increases the understanding of material defects thereby providing valuable insight into the mechanical behavior of composite materials. Although the problem of a three-phase circular inclusion interacting with a dislocation in antiplane shear has been presented [Xiao and Chen, Mech. Mater. 32 (2000) 485], the analysis is limited to the classical perfect bonding condition. The current paper considers the solution for a homogeneous circular inclusion interacting with a dislocation under thermal loadings in antiplane shear. The bonding along the inhomogeneity–matrix interface is considered to be imperfect with the assumption that the interface imperfections are constant. It is found that when the inhomogeneity is soft, regardless of the level of interface imperfection, the inhomogeneity will always attract the dislocation. As a result, no equilibrium positions are available. Alternatively, when the inhomogeneity is hard, an unstable equilibrium position is found which depends on the imperfect interface condition and the shear moduli ratio μ 2/ μ 1.

Numerical simulation of fiber reinforced composite materials––two procedures

Abstract In this work, two methodologies for the analysis of unidirectional fiber reinforced composite materials are presented. The first methodology used is a generalized anisotropic large strains elasto-plastic constitutive model for the analysis of multiphase materials. It is based on the mixing theory of basic substance. It is the manager of the several constitutive laws of the different compounds and it allows to consider the interaction between the compounds of the composite materials. In fiber reinforced composite materials, the constitutive behavior of the matrix is isotropic, whereas the fiber is considered orthotropic. So, one of the constitutive model used in the mixing theory needs to consider this characteristic. The non-linear anisotropic theory showed in this work is a generalization of the classic isotropic plasticity theory (A Continuum Constitutive Model to Simulate the Mechanical Behavior of Composite Materials, PhD Thesis, Universidad Politecnica de Cataluna, 2000). It is based in a one-to-one transformation of the stress and strain spaces by means of a four rank tensor. The second methodology used is based on the homogenization theory. This theory divided the composite material problem into two scales: macroscopic and microscopic scale. In macroscopic level the composite material is assuming as a homogeneous material, whereas in microscopic level a unit volume called cell represents the composite (Tratamiento Numerico de Materiales Compuestos Mediante la teori de Homogeneizacion, PhD Thesis, Universidad Politecnica, de Cataluna 2001). This formulation presents a new viewpoint of the homogenization theory in which can be found the equations that relate both scales. The solution is obtained using a coupled parallel code based on the finite elements method for each scale problem.

The formulation of homogenization method applied to large deformation problem for composite materials

In order to analyze the mechanical behaviors of composite materials under large deformation, the formulation of the homogenization method is described. In this formulation, assuming that the microstructures in a local region of the global structure are deformed uniformly and that consequently the microscopic periodicity remains in the local region under large deformation, the microscopic deformation is precisely defined by the perturbed displacement and product of macroscopic displacement gradient and microscopic coordinates. Finally, microscopic and macroscopic equations are obtained. The above mentioned assumption of the periodicity of microstructures is experimentally validated. The computer program is also developed according to this formulation, and the large deformation is analyzed for the unidirectional fiber reinforced composite material and the knitted fabric composite material.

Microscopic and Mesoscopic Evaluations of Materials. Hierarchical Modelling and Local Stress Analysis of Composite Materials and Structures.

In the numerical simulation of mechanical behaviors of composite materials and structures, a novel hierarchical modelling technique is proposed. Not only the global deformation but also the mesoscopic stresses are analyzed precisely with less human efforts than conventional methods. Four level hierarchy is defined for the textile composites, and the homogenization method and finite element mesh superposition technique are adopted. The latter technique makes it possible to overlay arbitrary local fine mesh on the global rough mesh. A panel with a rib made of knitted fabric composite material is analyzed by the proposed method. Three-dimensional modelling of the mesostructure of knitted fabric composite material is also shown. The validity of the proposed method was examined, and the mesoscopic stresses are calculated easily.

Bridging at the Nanometric Scale in 2.5D Cf–SiC Composites

This short paper presents the matrix microcrack bridging at a nanometric scale which was evidenced during creep tests of 2.5D Cf–SiC composites. It also shows the importance of the investigations of mechanical behavior of composite materials at all the different scales: from the macroscopic down to the nanoscopic one.

Testing of composite materials at high rates of strain: advances and challenges

This review paper is devoted to discussing the development and the present state of some key aspects of the mechanical behaviour of composite materials under impact loading. The experimental techniques employed for determining the behaviour of composite materials at high strain rate in tensile, compressive and shear loading are presented and discussed. The discussion includes their fundamental advantages and limitations. Directions for future research are also indicated.

Insertion of an interphase synthesised from a functionalised silicone into glass-fibre/epoxy composites

The fibre/matrix interface in unidirectional composite materials is modified by introducing a controlled interphase having a low glass transition temperature. The synthesis of this interlayer is based on polyurethanes and sol-gel chemistries. The interphase is introduced by a solvent process on-line during filament-winding. The mechanical behaviour of composite materials studied by means of off-axis tensile tests and three-point bending measurements (by varying the span-to-depth ratio) is the same than that for composite materials based on commercially sized glass fibres. Thus, the elastic behaviour is not modified by the introduction of a low-modulus and low-thickness interphase. On the other hand, the impact toughness, studied by using the Charpy test, is enhanced by the soft interphase. These experimental studies confirm the theoretical models developed previously in the literature and the conclusions obtained on microcomposites.

Digital image-based modeling applied to the homogenization analysis of composite materials

The systematic methodologies to derive accurate microstructural models are developed for studying the mechanical behaviors of composite materials. Since the geometric information of a microstructure is often given by an image or a set of images, the direct interpretation of the geometry is possibly by digitizing it. By identifying each pixel or voxel with a finite element (FE) and accompanying appropriate image processing, an FE model can be automatically generated. It is also emphasized that the digitized models can be suitable for solving the FE equations by utilizing the uniformity of the FE mesh. The finite element analysis (FEA) with the homogenization method enables the prediction the thermo-mechanical behavior of the periodic microstructure (unit cell) as well as the global mechanical response of a structural component, while we are taking into account the specific effect of the geometric structural configuration of the microstructure through digitization. Several kinds of the digitizing techniques are presented to illustrate the potential of digital image-based (DIB) FE modeling of the unit cell. Keeping the microstructural design in mind, the modification of the plane image is introduced and the virtual realization of the unit cell geometry is presented so that a microstructural analysis utilizing the homogenization method would be realistic.

The Mechanical Behaviour of Composite Materials under Impact Loading

The Mechanical Behaviour of Composite Materials under Impact Loading

The role of material nonlinearities in composite structures

In today's global market the optimal use of composites, to fulfil their potential for weight savings, is now being increasingly pursued as a means of obtaining a competitive advantage. However, the optimum design of composite structures and bonded composite joints requires a detailed understanding of the fundamental mechanical behaviour of composite materials and the adhesive systems. Whereas the time-dependent behaviour of the adhesive systems is reasonably well documented and has, to some degree, been accounted for in the design of bonded joints this has not yet occurred for advanced graphite-epoxy composites. Indeed, although it is recognised that the matrix-dominated response of many composite materials and structures is highly nonlinear, even at room temperature, this is often neglected in the design process. This restriction means that the full potential of the fibre matrix system may not be achieved. To this end the present paper, which focuses on the (matrix dominated) shear stress-shear strain behaviour of the AS4/3501-6 graphite-epoxy resin system, presents the results of an experimental investigation into the nonlinear behaviour of graphite-epoxy.