Reliability model of drilled composite materials

Recent studies by the authors have investigated the influence of the quality of the drilling on the fatigue behaviour of composites. From microscopical observations, a three-dimensional criterion using the Hashin’s theory has been established. It predicted well which component (fiber or matrix) has been damaged first. Firstly, a Monte Carlo technique considering the angles uncertainty of the different layers of a laminated composite was used to predict the statistical distributions of its mechanical properties. Secondly, a statistical distribution has been affected to each variable of the Hashin’s criterion in order to obtain a probability of failure rather than a deterministic value (less or equal to 1). Thus, it is possible to affect at any value of the criterion a probability.     

Simulation of the bone healing process of fractured long bones applied with a composite bone plate with consideration of the blood vessel growth

The healing process of long bones such as the tibia was simulated on the basis of a mechanoregulation theory by taking blood vessel growth into consideration. The tissue differentiation process of calluses by taking into consideration blood vessel growth was simulated by a user subroutine program based on the mechanoregulation model and a diffusion equation. Composite bone plates made of a plain weave carbon/epoxy composite (WSN3k) and a plain weave glass/polypropylene composite (Twintex) were applied to the fracture site to investigate the effect of plate modulus on the healing performance. The simulation results revealed that the flexible composite bone plate made of Twintex [0]₁₈, which had a slightly higher Young’s modulus than a cortical bone, provided the highest healing performance. Moreover, it was found that the effect of the plate modulus on the healing performance reduced when the blood vessel growth at the fracture site was considered, which reflected a more realistic bone healing process.     

Multiscale modeling of unsaturated flow in dual-scale fiber preforms of liquid composite molding III: reactive flows

The woven, stitched or braided fabrics used in liquid composite molding (LCM) display partial saturation behind moving flow-front in an LCM mold which is caused by delayed impregnation of fiber tows. In this part 3 of the present series of three papers, a novel multiscale approach proposed in parts 1 and 2 [1] and [2] is adapted for modeling the unsaturated flow observed in the dual-scale fabrics of LCM under non-isothermal, reactive conditions. The volume-averaged species or resin cure equation, in conjunction with volume-averaged mass, momentum and energy (temperature) equations, is employed to model the reactive resin flow in the inter-tow (gap) and intra-tow (tow) regions with coupling expressed through several sink and source terms in the governing equations. A coarse global-mesh is used to solve the global (gap) flow over the entire domain, and a fine local mesh in form of the unit-cell of periodic fabrics is employed to solve the local (tow) flows. The multiscale algorithm based on the hierarchical computational grids is then extended to solve the dual-scale flow under reactive conditions. The simulation is compared with a two-color experiment and a previously published two-layer model. Significant differences between the temperatures and cures of the gap and tow regions of the dual-scale porous medium are observed. The ratio of pore volumes in the tow and gap regions, the effective thermal conductivity in the tows, and the reaction rate are identified as the important parameters for temperature and cure distributions in the gap and tow regions.     

Importance of in-plane shear rigidity in finite element analyses of woven fabric composite preforming

A finite element made of woven unit cells under biaxial tension and in-plane shear is proposed for the simulation of fabric forming. The simulation is made within an explicit dynamic approach and is based on a simplified dynamic equation accounting for tension and in-plane shear strain energy. The biaxial tensile properties (given by two surfaces) and the in-plane shear properties (given by a curve) can be determined both by biaxial tensile tests and picture frame experiments or obtained by mesoscopic 3D finite element analyses of the woven unit cell. The interior load components of the proposed finite element are calculated explicitly and simply from the tensions and shear torque on four woven cells. The results obtained by the simulations of a hemispherical forming process on a very unbalanced fabric are compared to experiments. It is shown that the tension strain energy permits to describe the asymmetry of the response but that the computation of wrinkles and of the deformed states when the locking angle is exceeded needs to take the in-plane shear stiffness and its evolution with shear angle into account.     

Textile composite double dome stamping simulation using a non-orthogonal constitutive model

Stamping is one of the most effective ways to form textile composites in industry for providing high-strength, low-weight and cost-effective products. This paper presents a fully continuum mechanics-based approach for stamping simulation of textile fiber reinforced composites by using finite element (FE) method. A previously developed non-orthogonal constitutive model is used to represent the anisotropic mechanical behavior of textile composites under large deformation during stamping. Simulation are performed on a balanced plain weave composite with 0°/90° and ±45° as initial yarn orientation over a benchmark double dome device. Simulation results show good agreement with experimental output in terms of a number of parameters selected for comparison. The effects of meshing and shear moduli obtained from bias extension test and picture frame test on forming simulation results are also investigated.     

The manufacture and characterisation of a novel,low modulus,negative Poisson’s ratio composite

Relatively few negative Poisson’s ratio (auxetic) composites have been manufactured and characterised and none with inherently auxetic phases [Milton G. J. Mech. Phys. Solids 1992;40:1105–37]. This paper presents the use of a novel double-helix yarn that is shown to be auxetic, and an auxetic composite made from this yarn in a woven textile structure. This is the first reported composite to exhibit auxetic behaviour using inherently auxetic yarns. Importantly, both the yarn and the composite are produced using standard manufacturing techniques and are therefore potentially useful in a wide range of engineering applications.     

Rate-dependent phenomenological model for self-reinforced polymers

The visco-elastoplastic nature of self-reinforced polymers (SRPs) implies that their mechanical behaviour depends on strain rate. Such dependence, when significant, must be taken into account in order to predict the impact response of these materials. In this paper, the strain rate dependence of the mechanical behaviour of a self-reinforced polypropylene (SRPP) and a self-reinforced poly(ethylene terephthalate) (SRPET) is determined and constitutively modelled. To do this, stress–strain curves corresponding to constant strain rates are deduced for each material by using a characterization method presented and validated in previous works. The strain rate dependence of the stress–strain response is quantified based on the ‘strain rate sensitivity coefficient’, defined by G’Sell and Jonas for their material model for semi-crystalline polymers. Such dependence is found to be higher in the SRPET than in the SRPP and, moreover, in both materials it depends on strain. Finally, a modified phenomenological constitutive model based on the G’Sell–Jonas one is proposed. The results show that the modified model improves the prediction of the original model reproducing accurately the rate-dependent behaviour of both SRPs.     

Development of fibrous preforms for FRP pipe connections

This paper describes the experimental work carried on at the University of Minho concerning the design of weft-knitted fleecy fabrics for application in pipe connections produced with composite materials reinforced by 3D weft-knitted fabric preforms. The specifications of a T tube composite connexion have been established according to the information of the composites producer company involved in the project. The weft-knitted fleecy fabric has been optimized to perform a better mechanical performance, i.e., to increase the stiffness due to the use of straight fleece yarns. Special knitting techniques, developed by the authors, have been applied to produce 3D shaped performs to be impregnated by using RTM techniques. A special mould has been produced according to the required geometry. The results of the mechanical tests made on the final produced samples are presented, discussed and compared with those imposed in the initial specifications.     

Simulating tape resin infiltration during thermoset pultrusion process

The thermoset tape pultrusion is a widely adopted manufacturing process to produce long, constant cross-section composite structural parts. For high volume production, low cost can be achieved by maximizing the production rate which is a function of the material and process parameters, more specifically the rate of resin infiltration and resin cure. During resin infiltration, the resin saturates the dry reinforcement either under positive pressure in the pressure chamber, or, by the action of capillary and surface forces, within the resin bath. In either case, the saturation must be completed as the tape is squeezed into the final cross-sectional form at the entrance of the heated mold where the resin will be cured to form the composite part.This paper models the resin infiltration process during pultrusion, by modifying the pre-existing simulation tool for liquid molding processes. The formulated capability can be used not only to optimize the impregnation dynamics within the pressure chamber, but can also be used to predict the required forces for the selected pulling rate. The proposed model does allow one to handle a variety of tape cross-sections, not just rectangular prisms.     

Hyperelastic model for large deformation analyses of 3D interlock composite preforms

A hyperelastic constitutive law is proposed to describe the mechanical behaviour of 3D layer to layer angle interlock composite reinforcements. The objective of this model is to simulate shaping of thick textile preforms for RTM processes. After the identification of the independent deformation modes of initially orthotropic reinforcements, a strain energy potential is built up based on strain invariants representative to those modes assuming an additive composition of them. The parameters of the proposed constitutive model are identified using standard and specific mechanical tests performed on a 3D interlock material. Then, the model is validated on forming simulations on a single curve and double curve shapes. Three point bending tests on thick interlock reinforcements have been analysed experimentally and numerically. The specific transformation of cross sections is depicted by the proposed hyperelastic model.     

Review of z-pinned composite laminates

This paper reviews published research into polymer composite laminates reinforced in the through-thickness direction with z-pins. Research into the manufacture, microstructure, delamination resistance, damage tolerance, joint strength and mechanical properties of z-pinned composites is described. Benefits of reinforcing composites with z-pins are assessed, including improvements to the delamination toughness, impact damage resistance, post-impact damage tolerance and through-thickness properties. Improvements to the failure strength of bonded and bearing joints due to z-pinning are also examined. The paper also reviews research into the adverse effects of z-pins on the in-plane mechanical properties, which includes reduced elastic modulus, strength and fatigue performance. Mechanisms responsible for the reduction to the in-plane properties are discussed, and techniques to minimise the adverse effect of z-pins are described. The benefits and drawbacks of z-pinning on the interlaminar toughness, damage tolerance and in-plane mechanical properties are compared against other common types of through-thickness reinforcement for composites, such as 3D weaving and stitching. Gaps in our understanding and unresolved research problems with z-pinned composites are identified to provide a road map for future research into these materials.     

Analytical method using gamma functions for determining areas of power elliptical shapes for use in geometrical textile models

Textile models are often assumed to have homogenous and well defined cross-sections. For these models, the use of a power elliptical cross-sectional shape has been found to be beneficial as different shapes can be created, e.g. lenticular, elliptical or rectangular, with a single function. The cross-sectional area of a power ellipse is usually determined numerically as the analytical determination of the cross-sectional area is not straightforward. This short communication presents an analytical solution for this shape.     

Evaluation of the mechanical behavior of epoxy composite reinforced with Kevlar plain fabric and glass/Kevlar hybrid fabric

In this study, composite plates were manufactured by hand lay-up process with epoxy matrix (DGEBA) reinforced with Kevlar fiber plain fabric and Kevlar/glass hybrid fabric, using to an innovative architecture. Results of the mechanical properties of composites were obtained by tensile, bending and impact tests. These tests were performed in the parallel direction or fill directions of the warp and in a 90° direction. FTIR was used in order to verify the minimum curing time of the resin to perform the mechanical tests, and scanning electron microscopy was used to observe reinforcement and matrix fractures. Composites with Kevlar/glass hybrid structure in the reinforcing fabric showed the better results with respect to specific mechanical strength, as well as bending and impact energy.     

An elastic–plastic constitutive model for ceramic composite laminates

Existing phenomenological constitutive models are unable to capture the full range of behaviors of ceramic composite laminates. To ameliorate this deficiency, we propose a new model based on the deformation theory of plasticity. The predictive capabilities of the model are assessed through comparisons of computed and measured strain and displacement fields in open-hole tension tests. The agreements in the magnitude of strains and in the size and shape of shear bands that develop around a hole are very good over most of the loading history. Some discrepancies are obtained at high stresses. These are tentatively attributed to non-proportional stressing of some material elements: a feature not captured by the present model.     

Numerical prediction of in-plane permeability for multilayer woven fabrics with manufacture-induced deformation

A unit cell based Computational Fluid Dynamics model is presented for predicting permeability of multilayer fabric structures. In Liquid Composites Moulding processes, fabric lay-ups undergo significant manufacture-induced deformation, combining compression, shear, and inter-layer nesting. Starting from the configuration of un-deformed fabric, the deformation is simulated geometrically by accounting for self-imposed kinematic constraints of interweaving yarns. The geometrical modelling approach is implemented in the open-source software TexGen. The permeability tensor is retrieved from flow analysis in ANSYS/CFX, based on TexGen voxel models. Using only measured geometrical data for un-deformed fabrics, deformed plain weave fabric and twill weave fabric lay-ups were modelled and their permeability tensors predicted. Comparison with experimental data demonstrates the generally good accuracy of predictions derived from the proposed numerical method.     

A stiffness volume averaging based approach to model non-crimp fabric reinforced composites

A representative volume element (RVE) based model to evaluate the mechanical performance of non-crimp fabric (NCF) composites has been developed and presented hereafter. By means of the stiffness averaging method, the modelling procedure is able to simulate the NCF’s elastic properties taking into account the effects of process-induced defects and final geometrical configuration. Microscopy analysis has been used to quantitatively evaluate the effects of tow waviness, stitching geometrical parameters and matrix porosity; these features have been included as sub-models into the final model. Numerical predictions have been compared to the results of tensile tests performed on composite coupons. A geometrical parameter characterising the undulation of the tows has been introduced and a sensitivity analysis has been performed on the model with different undulations.     

A novel methodology for the rapid identification of the water diffusion coefficients of composite materials

The paper presents a novel methodology for the rapid identification of the water diffusion coefficients of composite materials. The methodology consists in employing a numerical parametric Proper Generalized Decomposition (PGD) method allowing incorporating the diffusion coefficients among the number of degrees of freedom. Compared to classical identification schemes, often based on Finite Element Method (FEM) iterations, the proposed method allows achieving consistent CPU time gain. The method is general and can be applied when diffusion anomalies take place or when diffusion–reaction coupling must be taken into account, moreover can deal with anisotropic materials. However, for the scope of illustration, in the present case, it is applied to the simple case of “classical diffusion” (Fick’s model with constant boundary conditions) and concerns isotropic materials.     

Characterization of a novel natural cellulose fabric from Manicaria saccifera palm as possible reinforcement of composite materials

Identifying novel natural fibers/fabrics with proper properties as reinforcement material is a new challenge in the field of bio-composites. Hence, the aim of this paper is to study the possibility of using a natural fabric extracted from Manicaria saccifera palm as a novel reinforcement in composites. This fabric was extensively characterized by chemical composition analysis, infrared spectroscopy (FTIR) analysis, morphological studies (SEM), thermo-gravimetric analysis (TGA) and physical /mechanical properties studies. From SEM analysis it was identified globular protrusions spread uniformly over the fiber which could help the mechanical interlock with the resin. As well, Manicaria fabric showed good thermal stability, low density, low moisture content and good tensile properties. Further, their properties are comparable to most natural cellulose fabrics and some synthetic fabrics, such as fiber glass fabrics. Manciaria saccifera fabric showed to be a suitable candidate as natural reinforcement material for the development of bio- composite.     

A new analytical model on predicting the elastic properties of 3D full five-directional braided composites based on a multiunit cell model

A new analytical model based on a multiunit cell model is proposed to predict the elastic properties of 3D full five-directional braided composites (F5DBC). The stiffness-volume averaging method is applied to predict the elastic properties of unit cell models in meso-scale and specimens in global-scale by using the multi-scale modeling procedures. The contribution of all unit cells to the elastic properties of specimen is considered in the analytical model. The predicted elastic properties are in good agreement with the available experimental data, demonstrating the applicability of the model. Also, the effects of the braiding angle and the fiber volume fraction on the elastic properties are discussed in detail. The elastic constants of each unit cell are analyzed and the effect of the number of yarn carriers on the mechanical properties is also investigated. Results indicate that it is convenient to apply the present analytical model to predict the elastic properties of 3D F5DBC due to high computational efficiency.     

Mechanical hinge system for delamination tests in beam-type composite specimens

During the experimental study of composite delaminations external loads are usually applied by means of steel or aluminium parts bonded to the surface of beam-type specimens. The bonded joints between the metallic parts and the composite specimen might fail, especially when the tests are carried out under extreme temperatures or fatigue conditions. In addition, the point of application of the external load does not coincide with the neutral axis of the specimen beam, inducing non-linear effects that can lead, for example, to incorrect estimations of fracture toughness. In this paper, the relative importance of the non-linear effects in delamination tests is evaluated and the corresponding correction factors discussed. Next, the design of an improved mechanical hinge that avoids non-linear effects, eliminates bonded joints and can be adapted to different specimen thicknesses is introduced.