Compressive response of Z-pinned woven glass fiber textile composite laminates: Experiments

In the first of this two part sequel, experimental results pertaining to the compressive response and failure of Z-pinned S-Glass fiber, plain-weave laminated composites are presented. These experiments are motivated by a need to understand the effect of Z-pinning on the strength and stiffness of these composites. A series of experiments are performed based upon density of the Z-pins and the diameter of the Z-pins. It is concluded that the damage zone around a Z-pin plays an important role in influencing the stiffness and strength of the Z-pin composite. In part 2 of this sequel, a 3D finite element (FE) based numerical model (based upon the composite microstructure acquired from scanning electron micrograph-SEM images) are used to capture details of the observed failure mechanisms and to provide predictions of the stiffness and strength of the composite.     

Predictive modelling for optimization of textile composite forming

Wrinkling often occurs during textile composite forming and is a major problem for manufacturers. The prediction of this defect is, therefore, of major importance for the design and optimization of textile composite structures. Numerical simulations of forming for textile composites over a hemisphere have been conducted using a rate/temperature-dependent hybrid FE model. The hybrid FE model incorporates a fully predictive multi-scale energy model which determines the shear resistance of the textile composite sheet. The effects of varying the normal force distribution across the edges of the blank and blank size, together with the effect of changes in forming temperature on the final fibre pattern and wrinkling behaviour, are investigated. Predictions are evaluated against press-formed components. The results from the simulation and the experiments have good correlation and show that wrinkling can be minimized by optimizing the force distribution around the edge of the manufacturing tool and by careful choice of forming temperature.     

Strength prediction of multi-layer plain weave textile composites using the direct micromechanics method

Previously developed micromechanical methods for stiffness and strength prediction are adapted for analysis of multi-layer plain weave textile composites. Utilizing the direct micromechanics method (DMM) via finite element modeling, three methods are presented: (a) direct simulation of a multi-layer plain weave textile composite; (b) micromechanical analysis of a single layer of interest from the force and moment resultants acting on that layer; and (c) application of the previously developed quadratic stress-gradient failure theory to the layer of interest. In comparison to direct modeling, the other two techniques show only 5% difference over a number of random test cases. Several practical design examples of strength prediction are included to illustrate the importance and accuracy of method implementation.     

Cryogenic delamination growth in woven glass/epoxy composite laminates under mixed-mode I/II fatigue loading

This paper investigates the fatigue delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates under mixed-mode I/II conditions at cryogenic temperatures. Fatigue delamination tests were performed with the mixed-mode bending (MMB) test apparatus at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K), in order to obtain the delamination growth rate as a function of the range of the energy release rate, and the dependence of the delamination growth behavior on the temperature and the mixed-mode ratio of mode I and mode II was examined. The energy release rate was evaluated using three-dimensional finite element analysis. The fractographic examinations by scanning electron microscopy (SEM) were also carried out to assess the mixed-mode fatigue delamination growth mechanisms in the woven GFRP laminates at cryogenic temperatures.     

Use of hypar-shell structures with textile reinforced cement matrix composites in lightweight constructions

The main goal of this paper is to define a design procedure for modular, lightweight and freeform structures by quantifying the relative importance of serviceability limit states and ultimate limit states. The modular building stones of the freeform structures under study are sandwich panels with a foamed polyurethane core and TRC (textile reinforced concrete) faces, shaped in the form of hyperbolic paraboloids (hypars). The shape of these modular building stones allows the production of structural elements on a reusable doubly-curved mould. For the dimensioning of the global modular structure, two states are important according to the Eurocodes: the ultimate limit states and the serviceability limit states. Due to the lightweight aspect of the modular structure, the serviceability limit states will gain in importance: stiffness and crack formation become important factors, as does the influence of repeated loading. These factors and their influence on the final design of the proposed structures will therefore be discussed in this paper.     

Cryogenic through-thickness tensile characterization of plain woven glass/epoxy composite laminates using cross specimens: Experimental test and finite element analysis

This paper investigates the through-thickness tensile behavior of woven glass fiber reinforced polymer (GFRP) composite laminates at cryogenic temperatures. Tensile tests were carried out with cross specimens at room temperature and liquid nitrogen temperature (77 K), and the through-thickness elastic and strength properties of the woven GFRP laminates were evaluated. The failure characteristics of the woven GFRP laminates were also studied by optical and laser scanning microscopy observations. A three-dimensional finite element analysis was performed to calculate the stress distributions in the cross specimens, and the failure conditions of the specimens were examined. It is found that the cross specimen is suitable for the cryogenic through-thickness tensile characterization of laminated composite materials. In addition, the through-thickness Young's modulus of the woven GFRP composite laminates is dominated by the properties of the matrix polymer in the given temperature, while the tensile strength is characterized by both, the fiber to matrix interface energy and the cohesion energy of the matrix polymer.     

Mechanical characterization of hierarchical carbon fiber/nanofiber composite laminates

Susceptibility to matrix driven failure is one of the major weaknesses of continuous-fiber composites. In this study, helical-ribbon carbon nanofibers (CNF) were dispersed in the matrix phase of a continuous carbon fiber-reinforced composite. Along with an unreinforced control, the resulting hierarchical composites were tested to failure in several modes of quasi-static testing designed to assess matrix-dominated mechanical properties and fracture characteristics. Results indicated CNF addition offered simultaneous increases in tensile stiffness, strength and toughness while also enhancing both compressive and flexural strengths. Short-beam strength testing resulted in no apparent improvement while the fracture energy required for the onset of mode I interlaminar delamination was enhanced by 35%. Extrinsic toughening mechanisms, e.g., intralaminar fiber bridging and trans-ply cracking, significantly affected steady-state crack propagation values. Scanning electron microscopy of delaminated fracture surfaces revealed improved primary fiber–matrix adhesion and indications of CNF-induced matrix toughening.     

A survey of test methods for multiaxial and out-of-plane strength of composite laminates

This review paper gives an overview of test methods for multiaxial and out-of-plane strength of composite laminates, with special consideration of non-crimp fabrics (NCF) and other textile systems. Tubular and cruciform specimens can provide arbitrary in-plane loading, while off-axis and angle-ply specimens provide specific biaxial loadings. Tensile and compressive out-of-plane strength may be determined by axial loading of specimens with a waisted gauge section, while bending of curved specimens allow determination of the out-of-plane tensile strength. Tests suited for out-of-plane shear strength include the short beam shear test, the inclined double notch test and the inclined waisted specimen. Testing of arbitrary tri-axial stress states using tubular or cruciform specimens with superimposed through-the-thickness loading is highly complex and significant problems have been reported in achieving the intended stress states and failure modes. Specific tri-axial stress states can be obtained by uniaxial loading of specimens with constrained expansion, as in the die channel test.     

Dynamic micromechanical modeling of textile composite strength under impact and multi-axial loading

Micromechanical finite element modeling has been employed to define the failure behavior of S2 glass/BMI textile composite materials under impact loading. Dynamic explicit analysis of a representative volume element (RVE) has been performed to explore dynamic behavior and failure modes including strain rate effects, damage localization, and impedance mismatch effects. For accurate reflection of strain rate effects, differences between an applied nominal strain rate across a representative volume element (RVE) and the true realized local strain rates in regions of failure are investigated. To this end, contour plots of strain rate, as well as classical stress contours, are developed during progressive failure. Using a previously developed cohesive element failure model, interfacial failure between tow and matrix phases is considered, as well as classical failure modes such as fiber breakage and matrix microcracking. In-plane compressive and tensile loading have been investigated, including multi-axial loading cases. Highly refined meshes have been employed to ensure convergence and accuracy in such load cases which exhibit large stress gradients across the textile RVE. The effect of strain rate and phase interfacial strength have been included to develop macro-level material failure envelopes for a 2D plain weave and 3D orthogonal microgeometry.     

Mechanics of textile composites: Micro-geometry

Textile fabric geometry determines textile composite properties. Textile process mechanics determines fabric geometry. In previous papers, the authors proposed a digital element model to generate textile composite geometry by simulating the textile process. The greatest difficulty encountered with its employment in engineering practice is efficiency. A full scale fiber-based digital element analysis would consume huge computational resources. Two advances are developed in this paper to overcome the problem of efficiency. An improved contact-element formulation is developed first. The new formulation improves accuracy. As such, it permits a coarse digital element mesh. Then, a static relaxation algorithm to determine fabric micro-geometry is established to replace step-by-step textile process simulation. Employing the modified contact element formulation in the static relaxation approach, the required computer resource is only 1–2% of the resource required by the original process. Two critical issues with regards to the digital element mesh are also examined: yarn discretization and initial yarn cross-section shape. Fabric geometries derived from digital element analysis are compared to experimental results.     

Modeling and experimental verification of dielectric constants for three-dimensional woven composites

In order to determine the dielectric constants of 3D orthogonal woven single fiber type (SFT) and hybrid composites from their component dielectric properties, a theoretical model is proposed based on the rule of binary mixtures. The model shows that with the same fiber volume fraction, a component with a larger cross-sectional area perpendicular to the electric field has a greater contribution to the composite dielectric constant. For experimental verification, SFT basalt/epoxy and aramid (Kevlar 129)/epoxy as well as interply and intraply basalt/aramid/epoxy 3D orthogonal woven hybrid composites were fabricated and their dielectric properties were measured using the waveguide method at a frequency range of 8–12 GHz. At 10 GHz, the experimental results agreed well with the calculated results from the model for the SFT composites, while a positive hybrid effect on the dielectric constant was observed for the two hybrid composites.     

Modeling and predicting the compression strength limiting mechanisms in Z-pinned textile composites

Motivated by experimental results on Z-pinned plain weave glass fiber textile composites that show kink banding of fiber tows to be a strength limiting mechanism of failure in compression, computational results are presented for the effects of Z-pin diameter and Z-pin density on compression strength. Distortion to the textile fiber tows introduced by the insertion of Z-pins is found to be the dominant cause for initiating kink bands while the type of bond between the Z-pin and the surrounding matrix is found to influence the post-kinking response. When the Z-pin diameter remains unchanged, the composite strength decreases as the Z-pin density increases, while, when the Z-pin density is fixed, the composite strength decreases as the Z-pin diameter decreases in agreement with experimental observations.     

Finite element simulation and experimental study on mechanical behavior of 3D woven glass fiber composite sandwich panels

The results of finite element simulation followed by an experimental study are presented in order to investigate the mechanical behavior of three-dimensional woven glass-fiber sandwich composites using FE method. Experimental load–displacement curves were obtained for flatwise compressive, edgewise compressive, shear, three-point bending and four-point bending loads on the specimens with three different core thicknesses in two principal directions of the sandwich panels, called warp and weft. A 3D finite element model is employed consisting of glass fabric and surrounding epoxy resin matrix in order to predict the mechanical behavior of such complex structures. Comparison between the finite element predictions and experimental data showed good agreement which implies that the FE simulation can be used instead of time-consuming experimental procedures to study the effect of different parameters on mechanical properties of the 3D woven sandwich composites.     

Modelling and simulation of earthquake resistant 3D woven textile structural concrete composites

Concrete is a composite material composed of water, sand, coarse granular material called aggregate and cement that fills the space among the aggregate particles and glues them together. Conventional building structures are made up of steel skeleton with concrete impregnation. These are very heavy weight structures with steel vulnerable to corrosion. The conventional concrete structures tend to undergo large deformations in the event of a strong earthquake. Mechanical simulation of various textile structural concretes is carried out successfully for their ductility behaviour. 3D woven reinforced concretes display superior ductile character showing ray of hope to develop seismic resistant building. Simulation of three 3D woven fabrics and their composites was carried to predict ductility and strengths of fabric reinforced concrete structures. Maximum deformation was observed for beam reinforced with orthogonal interlock fabric under the same load and minimum deformation was observed for plain concrete. Maximum equivalent stress was observed to be highest for plain concrete followed by beam reinforced with angle interlock fabric followed by orthogonal fabric and warp interlock fabric under similar loading conditions. From the results it was clear that 3D fabric reinforced structures are more ductile than the traditional steel reinforced structures. Hence 3D fabric reinforced concrete structures are much better in strength and ductility as compared to conventional construction materials. Among the three 3D fabric, orthogonal fabric reinforced composites are most ductile and are also less stiff. They can deform more than the other two fabric composites. Hence, orthogonal fabric reinforced composites can undergo higher deformations without collapsing. These composites can be more elastic under earthquake shaking.     

Stress distributions in bluntly-notched ceramic composite laminates

We present a methodology for determining stress distributions ahead of blunt notches in plates of fiber-reinforced ceramic–matrix composites subject to uniaxial tensile loading, accounting for the effects of inelastic straining due to matrix cracking. The methodology is based on linear transformations of the corresponding elastic distributions. The transformations are derived from adaptations of Neuber’s law for stress concentrations in inelastic materials. Comparisons are made with results computed by finite element analysis using an idealized (bilinear) form of the Genin–Hutchinson constitutive law for ceramic composite laminates. Effects of notch size and shape as well as the post-cracking tangent modulus are examined. The comparisons show that, for realistic composite properties, the analytical solutions are remarkably accurate in their prediction of stress concentrations and stress distributions, even in cases of large-scale and net-section inelasticity. Preliminary assessments also demonstrate the utility of the solution method in predicting the fields under multiaxial stressing conditions.     

Flexural and impact response of woven glass fiber fabric/polypropylene composites

Impact tests with a falling dart and flexural measurements were carried out on polypropylene based laminates reinforced with glass fibers fabrics. Research has shown that the strong fiber/matrix interface obtained through the use of a compatibilizer increased the mechanical performance of such composite systems. The improved adhesion between fibers and matrix weakly affects the flexural modulus but strongly influences the ultimate properties of the investigated woven fabric composites. In fact, bending tests have shown a clear improvement in the flexural strength for the compatibilized systems, in particular when a high viscosity/high crystallinity polypropylene was used. On the contrary, the low velocity impact tests indicated an opposite dependence on the interface strength, and higher energy absorption in not compatibilized composites was detected. This result has been explained in terms of failure mechanisms at the fiber/matrix interface, which are able to dissipate large amounts of energy through friction phenomena. Pull-out of fibers from the polypropylene matrices have been evidenced by the morphological analysis of fracture surfaces after failure and takes place before the fibers breakage, as confirmed by the evaluation of the ductility index.     

Ductile deformation mechanisms and designing instructions for integrated woven textile sandwich composites

To suggest designing instructions for integrated woven textile sandwich composites (IWTSCs), anti-crush properties of IWTSC and the corresponding ductile deformation mechanism were investigated. Quasi-static out-of-plane crushing and dynamic impact tests were carried out. Typical deformation curves with a relative stable deformation plateau were obtained from tests. Failure of IWTSC is ductile through coupled compression–shear deformation. An analytical plastic model was proposed to explain ductile mechanism of IWTSC qualitatively, including densification caused by interactions among inclined piles. Combining with qualitative analysis, comparisons between two kinds of IWTSC panels with piles of different density and thickness reveal the key to design a ductile IWTSC.     

Compressive failure of laminates containing an embedded wrinkle; experimental and numerical study

An experimental and numerical study has been carried out to understand and predict the compressive failure performance of quasi-isotropic carbon–epoxy laminates with out-of-plane wrinkle defects. Test coupons with artificially induced fibre-wrinkling of varied severity were manufactured and tested. The wrinkles were seen to significantly reduce the pristine compressive strength of the laminates. High-speed video of the gauge section was taken during the test, which showed extensive damage localisation in the wrinkle region. 3D finite element (FE) simulations were carried out in Abaqus/Explicit with continuum damage and cohesive zone models incorporated to predict failure. The FE analyses captured the locations of damage and failure stress levels very well for a range of different wrinkle configurations. At lower wrinkle severities, the analyses predicted a failure mode of compressive fibre-failure, which changed to delamination at higher wrinkle angles. This was confirmed by the tests.     

Assessment of fatigue damage evolution in woven composite materials using infra-red techniques

Thermoelastic stress analysis (TSA) is used to study the growth of fatigue damage in single and two ply, 2 × 2 twill woven composite materials. Test specimens were subjected to a uniaxial tensile cyclic loading with maximum stresses of 10%, 15% and 20% of the ultimate failure stress. The development of fatigue damage locally within the weft yarns is monitored using high resolution TSA. The specimens were subsequently inspected using optical microscopy to evaluate the location and extent of cracks. Cracks were found in the weft fibres, running transverse to the loading direction. It is demonstrated that the lighter weight fabric is more resilient to damage progression. A signature pattern is identified in the TSA phase data that indicates the onset and presence of fatigue damage in the composite material.     

Unit cell modelling of textile laminates with arbitrary inter-ply shifts

Deformation and failure mechanisms of textile laminates are strongly affected by mutual shift of the plies. To model arbitrarily stacked laminate within a traditional framework of multi-scale modelling, one must construct a representative volume element (RVE), which includes all the plies. This is a time consuming and computationally expensive work. As an alternative, the paper suggests a technique that allows setting problems on one unit cell of a single ply, i.e. a volume smaller than RVE of the laminate. The technique approximates the stress field in a ply by combination of stress fields obtained in two additional problems. Boundary conditions (BC) in these problems imitate the interaction of the unit cell with surrounding media. Once these problems are solved, the solution for arbitrary number of plies is composed analytically. The proposed technique respects inter-ply configurations, accounts for the number of plies, distinguishes the ply position, and reproduces the meso stress state with a good accuracy. The technique is validated against reference solutions obtained for the entire laminate.