High velocity impact on NCF reinforced composites

In the current paper, a series of high velocity impact tests using ?50 and ?25 mm ice spheres and 0.32 g granite stones on non-crimp fabric (NCF) composite plates are reported. The impact tests were performed using an air gun and velocities between 100 m/s and 199 m/s. The impact events were monitored using a high-speed camera, with a 20 million frames per second capacity, as well as by a displacement transducer for out-of-plane displacement measurements of the impacted plates. NCF composite plates of two different thicknesses were impacted. The composites were manufactured from carbon fibre and epoxy resin by vacuum infusion.     

Meshfree modeling and homogenization of 3D orthogonal woven composites

In this paper, evaluation of 3D orthogonal woven fabric composite elastic moduli is achieved by applying meshfree methods on the micromechanical model of the woven composites. A new, realistic and smooth fabric unit cell model of 3D orthogonal woven composite is presented. As an alternative to finite element method, meshfree methods show a notable advantage, which is the simplicity in meshing while modeling the matrix and different yarns. Radial basis function and moving kriging interpolation are used for the shape function constructions. The Galerkin method is employed in formulating the discretized system equations. The numerical results are compared with the finite element and the experimental results.     

Analytical elastic stiffness model for 3D woven orthogonal interlock composites

This research presents the development of an analytical model to predict the elastic stiffness performance of orthogonal interlock bound 3D woven composites as a consequence of altering the weaving parameters and constituent material types.     

Split Hopkinson pressure bar testing of 3D woven composites

Results from a series of split Hopkinson pressure bar (SHPB) tests on 3D woven tetxile composites (3DWC) are presented. These tests were done to determine the rate dependent compression response of 3DWC. Three different configurations of the 3DWC, corresponding to compression response in the plane of the material and through-the-thickness direction (out-of-plane) were studied. The rate dependent responses were compared against quasi-static test results and it was found that 3DWC showed an increase in strength in all three directions studied, however, accompanied by a transition in the failure mechanism. The in-plane orientations showed the largest increase in (about 100%) strength at the elevated rates of loading. A follow-on paper provides finite element based results that correspond to the experimental results presented here.     

Development of woven fabric reinforced all-polypropylene composites with beta nucleated homo- and copolymer matrices

In the present work self-reinforced polypropylene composites (SRPPC) were developed and investigated. As reinforcement a fabric, woven from highly stretched split PP yarns, whereas as matrix materials α and β crystal forms of isotactic PP homopolymer and random copolymer (with ethylene) were selected and used. The composite sheets were produced by film-stacking method and compression moulded at different processing temperatures keeping the holding time and pressure constant. The quality of the composite sheets was assessed by optical microscopy, density and peel-strength measurements. The SRPPC specimens were subjected to static tensile and flexural, and dynamic falling weight impact tests and the related results were analyzed as a function of processing temperature and polymorphic composition. Based on the results the optimum processing temperature was determined and found by 20–25 °C above the related matrix melting temperature. It was established that the β-modified PP homopolymer based one-component SRPPCs possessed similar attractive mechanical properties as the intensively studied α-random PP copolymer based two-component ones.     

An experimental study of failure initiation and propagation in 2D woven composites under compression

Experimental research was conducted to investigate the compressive failure of orthogonal 2D woven composites. Damage initiation and the effect of weave architecture were studied using a Four Point Bending (FPB) test setup. Scanning Electron Microscope (SEM) and Digital Imaging (DI) observations show the individual failure of the load-aligned tows, which behave as structural elements at the internal reinforcement level. Weave architecture and internal geometry are seen to affect location and morphology of the damage initiation. Damage propagation and the effect of stacking were studied using a reduced Compact Compression (rCC) test setup. Optical micrographs show that stacking configuration can have a significant effect on the damage morphology. Additionally, results also suggest the effect of stacking is a function of the weave architecture and internal geometry.     

2D and 3D imaging of fatigue failure mechanisms of 3D woven composites

A detailed investigation of the failure mechanisms for angle-interlocked (AI) and modified layer-to-layer (MLL) three dimensional (3D) woven composites under tension–tension (T–T) fatigue loading has been conducted using surface optical microscopy, cross-sectional SEM imaging, and non-destructive X-ray computed tomography (CT). X-ray microCT has revealed how cracks including surface matrix cracks, transverse matrix cracks, fibre/matrix interfacial debonding or delamination develop, and has delineated the complex 3D morphology of these cracks in relation to fibre architecture. For both weaves examined, transverse cracks soon become uniformly distributed in the weft yarns. A higher crack density was found in the AI composite than the MLL composite. Transverse cracking initiates in the fibre rich regions of weft yarns rather than the resin rich regions. Delaminations in the failed MLL specimen were more extensive than the AI specimen. It is suggested that for the MLL composite that debonding between the binder yarns and surrounding material is the predominant damage mechanism.     

Effect of stitch density and stitch thread thickness on compression after impact strength and response of stitched composites

In this paper, the damage failure and behaviour of stitched composites under compression after impact (CAI) loading are experimentally investigated. This study focuses on the effect of stitch density and stitch thread thickness on the CAI strength and response of laminated composites reinforced by through-thickness stitching. Experimental findings show that stitched composites have higher CAI failure load and displacement, which corresponds to higher energy absorption during CAI damage, mainly attributed to greater energy consumption by stitch fibre rupture. The coupling relationships between CAI strength, impact energy, stitch density and stitch thread thickness are also revealed. It is understood that the effectiveness of stitching has high dependency on the applied impact energy. At low impact energy range, CAI strength is found to be solely dependent on stitch density, showing no influence of stitch thread thickness. It is however observed that stitch fibre bridging is rendered ineffective in moderately stitched laminates during compressive failure, as local buckling occurs between stitch threads, resulting in unstitched and moderately stitched laminates have similar CAI strength. The CAI strength of densely stitched laminates is much higher due to effective stitch fibre bridging and numerous stitch thread breakages. At high impact energy level, CAI strength is discovered to be intimately related to both stitch density and stitch thread thickness. Since CAI failure initiates from impact-induced delamination area, stitch fibre bridging is considerable for all specimens due to the relatively large delamination area present. Stitch threads effectively bridge the delaminated area, inhibit local buckling and suppress delamination propagation, thus leading to increased CAI strength for laminates stitched with higher stitch density and larger stitch thread thickness. Fracture mechanisms and crack bridging phenomenon, elucidated by X-ray radiography are also presented and discussed. This study reveals novel understanding on the effectiveness of stitch parameters for improving impact tolerance of stitched composites.     

Monotonic and cyclic behaviour of isolated FRP anchors loaded in shear

Using anchors made of fibre reinforced polymers (FRP) is an increasingly accepted method to delay the delamination of FRP sheets from the concrete surface and to enhance the capacity of FRP strengthened concrete structures. For many applications, FRP anchors are primarily loaded in shear. When used for seismic retrofitting schemes, the anchors are subjected to cyclic loads which may lead to premature fatigue failure. To date, however, shear strength of FRP anchors has experienced much less attention than their tension resistance. This paper documents tests on isolated FRP anchors which were conducted to determine the seismic shear capacity of FRP anchors and to propose design rules. To this end, a test setup was developed which allows direct and reverse loading of FRP anchors.     

In-plane and through-thickness properties,failure modes,damage and delamination in 3D woven carbon fibre composites subjected to impact loading

Two noncrimp 3D woven carbon fibre composites (through thickness angle interlock) of binder volume fractions 3% and 6% were characterised for their response to applied deformation. Experiments were performed at quasi static, medium and high strain rates under a large variety of load cases (tension in warp/weft direction, interlaminar/intralaminar shear, through thickness tension/compression, 3-point bending and plate bending). During the study, novel experimental methods were developed in order to address several challenges specific to 3D composite materials. The results show that, while the different binder volume fractions of 3% and 6% have only a small effect on the in-plane stiffness (warp and weft direction), its effect on the delamination resistance in plate bending experiments is considerable. This is a very important result for the use of these materials in the future. The availability, in previous publications, of complementary data for the matrix and the interface between matrix pockets and fibre bundles makes the comprehensive data set a generically useful reference for hierarchical numerical modelling strategies.     

Characterisation of mechanical behaviour and damage analysis of 2D woven composites under bending

In this paper, flexural loading of woven carbon fabric-reinforced polymer laminates is studied using a combination of experimental material characterisation, microscopic damage analysis and numerical simulations. Mechanical behaviour of these materials was quantified by carrying out tensile and large-deflection bending tests. A substantial difference was found between the materials' tensile and flexural properties due to a size effect and stress stiffening of thin laminates. A digital image-correlation technique capable of full-field strain-measurement was used to determine in-plane shear properties of the studied materials. Optical microscopy and micro-computed tomography were employed to investigate deformation and damage mechanisms in the specimens fractured in bending. Various damage modes such as matrix cracking, delaminations, tow debonding and fibre fracture were observed in these microstructural studies. A two-dimensional finite-element (FE) model was developed to analyse the onset and propagation of inter-ply delamination and intra-ply fabric fracture as well as their coupling in the fractured specimen. The developed FE model provided a correct prediction of the material's flexural response and successfully simulated the sequence and interaction of damage modes observed experimentally.     

Mechanical and electrical response of carbon nanotube-based fabric composites to Hopkinson bar loading

The numerous structural applications of composites, coupled with their complex, rate-dependent mechanical behavior necessitate research into their mechanical response under dynamic loading scenarios. While the damage mechanisms of composites under dynamic compression loading are well-understood, measuring the occurrence of damage in a non-invasive manner is challenging. Toward this end, we investigate the electrical response of an embedded percolating carbon nanotube network in woven fabric/epoxy composites to dynamic compression loading. The percolating network is established through the use of a non-uniform dispersion of carbon nanotubes, achieved using a fiber sizing agent. The resulting conductive network is sensitive to delamination and damage occurring near the fiber surfaces. The dynamic mechanical response of the composite specimens is explored using Hopkinson bar methodology. Definite increases in baseline resistance of the conductive composite specimens are seen after repeated impacts demonstrating the ability of the carbon nanotube network of these conductively modified composites to respond electrically to damage induced during dynamic loading.     

Effect of manufacturing defects on mechanical properties and failure features of 3D orthogonal woven C/C composites

For high performance 3D orthogonal textile Carbon/Carbon (C/C) composites, a key issue is the manufacturing defects, such as micro-cracks and voids. Defects can be substantial perturbations of the ideal architecture of the materials which trigger the failure mechanisms and compromise strength. This study presents comprehensive investigations, including experimental mechanical tests, micron-resolution computed tomography (μCT) detection and finite element modeling of the defects in the C/C composite. Virtual C/C specimens with void defects were constructed based on μCT data and a new progressive damage model for the composite was proposed. According to the numerical approach, effects of voids on mechanical performance of the C/C composite were investigated. Failure predictions of the C/C virtual specimens under different void fraction and location were presented. Numerical simulation results showed that voids in fiber yarns had the greatest influences on performance of the C/C composite, especially on tensile strength.     

Damage evolution of novel 3D textile-reinforced composites under fatigue loading conditions

For the simulation of the material degradation process during multiaxial fatigue loading of 3D textile-reinforced composites a new physically based damage model is developed based on the fracture mode concept (FMC) of CUNTZE and the continuum damage mechanics. For the damage analysis and the model parameter identification cyclic tests under superposed tension/compression–torque loading are performed.     

Shock loading response of sandwich panels with 3-D woven E-glass composite skins and stitched foam core

Sandwich composite are used in numerous structural applications, with demonstrated weight savings over conventional metals and solid composite materials. The increasing use of sandwich composites in defense structures, particularly those which may be exposed to shock loading, demands for a thorough understanding of their response to suc highly transient loadings. In order to fully utilize their potential in such extreme conditions, design optimization of the skin and core materials are desirable. The present study is performed for a novel type of sandwich material, TRANSONITE^® made by pultrusion of 3-D woven 3WEAVE^® E-glass fiber composites skin preforms integrally stitched to polyisocyanurate TRYMER^TM 200L foam core. The effect of core stitching density on the transient response of three simply supported sandwich panels loaded in a shock tube is experimentally studied in this work. The experimental program is focused on recording dynamic transient response by high-speed camera and post-mortem evaluation of imparted damage. The obtained experimental results reveal new important features of the transient deformation, damage initiation and progression and final failure of sandwich composites with unstitched and stitched foam cores. The theoretical study includes full 3-D dynamic transient analysis of displacement, strain and stress fields under experimentally recorded surface shock pressure, performed with the use of 3-D MOSAIC analysis approach. The obtained theoretical and experimental results for the transient central deflections in unstitched and two stitched foam core sandwiches are mutually compared. The comparison results reveal large discrepancies in the case of unstitched sandwich, much smaller discrepancies in the case of intermediate stitching density, and excellent agreement between theoretical and experimental results for the sandwich with the highest stitching density. The general conclusion is that further comprehensive experimental and theoretical studies are required in order to get a thorough understanding of a very complex behavior of composite sandwiches under shock wave loading.     

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 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.     

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.     

Experimental determination of the mode I delamination fracture and fatigue properties of thin 3D woven composites

The mode I delamination fracture toughness and fatigue strength of thin-section three-dimensional (3D) woven composite materials is experimentally determined. The non-crimp 3D orthogonally woven carbon–epoxy composites were thin (2 mm) and consequently their through-thickness z-binder yarns were inclined at a very steep angle (about 70°) from the orthogonal direction. The steep z-binder angle has a marked effect on the delamination toughening and fatigue strengthening mechanisms. Experimental testing revealed that the fracture toughness and fatigue resistance increased progressively with the volume content of z-binders. However, the steep angle caused the z-binder yarns bridging the delamination crack to deform and fail in shear and through-thickness tension, rather than in-plane tension which usually occurs in thick 3D woven composites. Mode I pull-off tests on a single woven z-binder yarn embedded within the composite revealed that the crack bridging traction load, strain energy absorption and failure mechanism were strongly affected by the steep angle.