Determination of the mechanical properties of textile-reinforced composites taking into account textile forming parameters

Textile manufacturing and textile processing parameters have a significant influence on the local fibre architecture of a textile-reinforced composite part. Consequently, local quantities like mechanical or permeability properties can vary significantly. Numerical approaches to predict the influence of the manufacturing processes on the local orientation exist, but are often only used to prevent problems during the manufacturing process itself and not for evaluation of the mechanical performance of the part. This paper presents a possible solution for this problem. A multi-level simulation approach is used to predict local mechanical properties of orthogonal and sheared textile-reinforced composites. The results of draping simulations are then combined with the predicted mechanical properties using an interpolation technique. The proposed method is illustrated with a numerical case study.     

Experimental validation of forming simulations of fabric reinforced polymers using an unsymmetrical mould configuration

Forming simulations are performed on woven textile composites using (a) an explicit finite element method and (b) a kinematic mapping scheme. Both methods are compared with experimental results that are obtained by determining the fiber orientations of glass/PP woven composites thermoformed on an extended hemispherical shaped mould with different orientations of the blank. It was found that the kinematic mapping approach severely fails in predicting the fiber reorientation that occurs during stamp forming for non-symmetrical forming configurations. The FEM-simulation gives a reasonably good prediction of the fiber reorientation and seems the most promising technique in having good draping simulations. The reported experimental results can be used as a benchmarking case in the assessment of the quality of forming simulation methods.     

Compressibility and relaxation of a new sandwich textile preform for liquid composite molding

In liquid composite molding, such as RTM or SRIM, the dry reinforcement is first compressed in the mold, and then the resin is injected into the mold cavity and cured. Knowledge of the compressibility of the reinforcement is important in order to estimate the mold closing force and the attainable range of fiber volume fraction. Moreover, in a lay-up composed of several types of fabrics, the compressibility of each fabric should be known to predict the thickness of each fabric layer, which is a prerequisite for the mold filling simulation. In this work, the relaxation and compressibility of a new sandwich fabric, Multimat®, and its components (a weft knit and a random mat) were studied and compared with a woven fabric. The different fabric relaxation behavior is explained in terms of fabric stiffness and volumetric dissipation energy. Fabric compression tests were performed by taking the fabric relaxation behavior into account. Fabric compression curves are fitted with the power law equation and agree well with the clamping pressure experimentally measured with a pressure transducer in an RTM mold. The influence of the number of layers was also investigated. Finally the compressibility of the Multimat is correlated to its components with a simple model.     

Finite element modelling of SMA textiles: superelastic behaviour

The paper describes guidelines for building a finite element model of a unit cell of a textile made of superelastic NiTi wires and illustrates the application of the model for woven and knitted textiles. The goal of the analysis is prediction of the tensile diagram of the textile based on the fabric structure and the superelastic tensile diagram of the wires. The differences between the superelastic behaviour of the fabrics and that of individual wires are discussed. Predictions for a knitted NiTi fabric are compared with experimental data.     

Elastic compliance of a partially debonded circular inhomogeneity

In the present work, we predict contribution of a partially debonded circular inhomogeneity into the material overall elastic compliance. Debonding at the matrix/inclusion boundary is modeled as interfacial arc cracks. The change in the elastic compliance caused by interface cracking is estimated through the accompanying energy change that is related to the mode I and mode II stress intensity factors at the crack tips. The sum of the crack compliance and the inhomogeneity compliance (with perfect bonding) gives the total compliance of the debonded inhomogeneity. The latter is obtained in terms of the material properties and crack length. Additional analysis shows that the replacement of an interface crack with a crack in a homogenized medium is an inadequate approach when seeking approximate solutions. The paper also provides guidelines how to determine properties of a fictitious perfectly bonded orthotropic inhomogeneity that has the same contribution into the material compliance as the debonded isotropic one. This problem is of practical importance when modeling damage accumulation in composite materials by means of homogenization schemes.     

The effect of adding carbon nanotubes to glass/epoxy composites in the fibre sizing and/or the matrix

Carbon nanotubes (CNTs) were integrated in glass fibres epoxy composites by either including CNTs in the fibre sizing formulation, in the matrix, or both. The effects of such controlled placement of CNTs on the thermophysical properties (glass transition temperature and coefficient of thermal expansion) and the Mode I interlaminar fracture toughness of the composites were studied. The present method of CNT-sizing of the glass fibres produces an increase of almost +10% in the glass transition temperature and a significant reduction of ?31% in the coefficient of thermal expansion of the composites. Additionally, the presence of CNTs in the sizing resulted in an increased resistance of crack initiation fracture toughness by +10%, but a lowered crack propagation toughness of ?53%. Similar trends were observed for both instances when CNTs were introduced only in the matrix and in combination of both matrix and sizing.     

Assessment of the tensile properties of coir,bamboo and jute fibre

Natural fibres are studied as alternatives for man-made fibres to reinforce composites while keeping the weight lower. The assessment of the value of some commonly available tropical fibres for the composite industry starts with the determination of the strength, E-modulus and strain to failure through single fibre tensile tests. The mean strength and standard deviation is calculated following the normal and Weibull distribution resulting in the questionable benefit of applying the Weibull distribution. Furthermore, a correction method assesses the real fibre elongation from the measured clamp displacement. This procedure seems to be useful for strong, brittle fibres to produce more reliable results for the E-modulus and strain to failure.     

Fatigue tensile behavior of carbon/epoxy composite reinforced with non-crimp 3D orthogonal woven fabric

An experimental study of the in-plane tension-tension fatigue behavior of the carbon fiber/epoxy matrix composite reinforced with non-crimp 3D orthogonal woven fabric is presented. The results include pre-fatigue quasi-static test data, fatigue life diagrams, fatigue damage progression, and post-fatigue quasi-static test data for the warp- and fill-directional loading cases. It is revealed that the maximum cycle stress corresponding to at least 3 million cycles of fatigue life without failure, is in the range of 412-450 MPa for both loading directions. This stress range is well above the static damage initiation threshold and significantly above the first static damage threshold (determined by the onset of low energy acoustic emission). The second static damage threshold, determined by the onset of high energy acoustic emission and related to the appearance of local debonds and intensive transverse matrix cracking falls within this range. The established correlation between a 3000,000 cycle fatigue stress limit on one side and the second static damage threshold stress on the other is of a high practical importance, because it will significantly reduce the amount of future fatigue tests required for this class of composites. Surprisingly, for equal maximum cycle stress level, the fatigue life under fill-directional loading appears about three times shorter than that under warp-directional loading. The 100,000 cycle, 500,000 cycle and 1000,000 cycle fatigue loading with 450 MPa maximum cycle stress has resulted in so high variations of post-fatigue static modulus, strength and ultimate strain, that no consistent and statistically meaningful trends could have been established; further extensive experimental studies are required to reliably quantify this effect.     

Analysis of knitted fabric reinforced composites: Part I. Fibre orientation distribution

The fibre orientation distributions in different types of warp knitted fabric are studied. The fibre orientations are represented by orientation tensors. This allows for the production of second- and fourth-order approximations of the orientation distribution function, which contain the relevant part of the orientation distribution for second- and fourth-order tensorial properties, respectively. Also, the symmetry that is present in the knitted fabrics can be analysed with the help of orientation tensors. It is shown that all the knitted fabrics are ‘almost' monoclinic. Determination of the ‘nearly' on-axis coordinate system is of interest for the data reduction of tensile test data.