Contrast enhancement in visualisation of woven composite tow architecture using a MicroCT Scanner. Part 1: Fabric coating and resin additives

MicroCT scanning is a non-destructive inspection method which was used to visualise tow architecture in woven composites with the ultimate goal of three-dimensional model generation. This has been achieved in the past for glass fabric composites, but is problematic when applied to carbon fabrics. Using X-rays, it is difficult to discriminate between elements of the composite, particularly the region between co-aligned neighbouring tows. This presents difficulty when viewing such composites using X-ray MicroCT scanning. Additives were used to enhance contrast during scanning. The most successful techniques were coating of fabrics with gold, copper, and an iodine contrast agent. Resin particle additive techniques were also trialled, with limited success. Good visualisations of glass fabrics were possible without contrast enhancement. Three-dimensional reconstructions of interior tow architectures were then made from the scans of contrast enhanced specimens. This research can be viewed as a starting point in developing methods for generating contrast between neighbouring tows within a three-dimensional woven preform using MicroCT scanning.     

Numerical simulation of injection/compression liquid composite molding. Part 2: preform compression

In the injection/compression liquid composite molding process (I/C-LCM), a liquid polymer resin is injected into a partially open mold, which contains a preform of reinforcing fibers. After some or all of the resin has been injected, the mold is closed, compressing the preform and causing additional resin flow. This paper addresses compression of the preform, with particular emphasis on modeling three-dimensional mold geometries and multi-layer preforms in which the layers have different mechanical responses. First, a new constitutive relation is developed to model the mechanical response of fiber mats during compression. We introduce a new form of nonlinear elasticity for transversely isotropic materials. A special case of this form is chosen that includes the compressive stress generated by changes in mat thickness, but suppresses all other responses. This avoids the need to model slip of the preform along the mold surface. Second, a finite element method, based on the principle of virtual displacement, is developed to solve for the deformation of the preform at any stage of mold closing. The formulation includes both geometric and material nonlinearities, and uses a full Newton–Raphson iteration in the solution. An open gap above the preform can be incorporated by treating the gap as a distinct material layer with a very small stiffness. Examples show that this approach successfully predicts compression in dry preforms for three-dimensional I/C-LCM molds.     

Analysis of the structure and deformation of a woven composite lamina using X-ray microdiffraction

X-ray diffraction (XRD) is an important tool for studying multiphase materials because it can resolve parameters from each phase independently. When coupled with a high-flux, microfocussed X-ray beam, scanning microdiffraction experiments are possible. This technique can investigate how reciprocal-space parameters vary as a function of real-space sample geometry for heterogeneous materials. Consequently, multiphase materials can be imaged in terms of those parameter variations. This study reports on the use of microfocussed X-ray diffraction (μXRD) to both image and follow the deformation of a multiphase material. In this case, this technique is applied to the study of a woven fibre-reinforced composite (FRC) lamina. Such systems are difficult to study with other experimental techniques because the fibres are inaccessible and the matrix is often opaque. However, using μXRD it is possible to assess both sample geometry and stress field information simultaneously.     

Development of damage in a 2D woven C/SiC composite under mechanical loading: I. Mechanical characterization

An investigation has been undertaken to determine the damage mechanisms and the associated mechanical response of a 2D reinforced composite of carbon fibers in an SiC CVI-processed matrix subjected to uniaxial tensile and compressive loadings at room temperature. Under tension loading, an extended non-linear stress/strain response was evidenced and related to a multi-stage development of damage involving transverse matrix microcracking, bundle/matrix and inter-bundle debonding as well as thermal residual stress release. This tensile behavior proved to be damageable-elastic with respect to a fictitious thermalstress-free origin of the stress/strain axis lying in the compression domain. In compression, after an initial stage involving closure of the thermal microcracks present from processing, the composite displayed a linear-elastic behavior until failure. The extent of damage over the material was characterized quantitatively at the microscale by the decrease of the average transverse microcrack spacing and at the macroscale by the decrease of both the longitudinal Young's modulus and the in-plane Poisson's ratio.     

Development of damage in a 2D woven C/SiC composite under mechanical loading: II. Ultrasonic characterization

The non-linear mechanical behavior of a CVI-processed 2D woven C/SiC composite has been investigated by means of an ultrasonic method. This method provided the complete variation of the stiffness tensor of the material required to fully identify anisotropic damage, which was otherwise inaccessible by classical strain measurements. The various damage mechanisms induced by mechanical loading and their influence on the tensile behavior were determined and analyzed by comparing the variations of the components of the stiffness tensor obtained from the ultrasonic measurements with the prediction of microcracks by a system of slit cracks derived from a micromechanical model. Two damage modes were thus emphasized: transverse microcracking characterized by a deterministic accumulation and a random development of longitudinal microcracking, i.e. fiber/matrix and bundle/matrix debonding. Comparisons with the results obtained in Part I from both classical strain measurements and microstructural observations are also made and discussed, whenever possible.     

Global behaviour of a composite stiffened panel in buckling. Part 2: Experimental investigation

The present study analyses an aircraft composite fuselage structure manufactured by the Liquid Resin Infusion (LRI) process and subjected to a compressive load. LRI is based on the moulding of high performance composite parts by infusing liquid resin on dry fibres instead of prepreg fabrics or Resin Transfer Moulding (RTM). Actual industrial projects face composite integrated structure issues as a number of structures (stiffeners, …) are more and more integrated onto the skins of aircraft fuselage.A post-buckling test of a composite fuselage representative panel is set up, from numerical results available in previous works. Two stereo Digital Image Correlation (DIC) systems are positioned on each side of the panel, that are aimed at correlating numerical and experimental out-of-plane displacements (corresponding to the skin local buckling displacements of the panel). First, the experimental approach and the test facility are presented. A post-mortem failure analysis is then performed with the help of Non-Destructive Techniques (NDT). X-ray Computed Tomography (CT) measurements and ultrasonic testing (US) techniques are able to explain the failure mechanisms that occured during this post-buckling test. Numerical results are validated by the experimental results.     

An energy based damage mechanics approach to modelling impact onto woven composite materials: Part II. Experimental and numerical results

Laminated composites have found an increasing use in many industries, particularly in transport, e.g. in the design of aircraft, helicopters, boats, cars, etc. Many of these composite components are fabricated from woven carbon based materials, which are more drapeable than conventional uni-directional (UD) composites, but generally have less stiffness and strength. To accurately design such components or structures to withstand severe external loadings, such as high velocity impact or a crash event, is conceptually a difficult task for the composite designer. Unlike metallic components, which can yield and dissipate energy via plasticity, composites can only dissipate energy by different damage or fracturing processes, which usually degrade the stiffness of the structural component.This paper presents the application of the energy based damage model, previously described [Iannucci L, Willows M. An energy based damage mechanics approach to modelling impact onto woven composite materials: Part I Numerical model, Composites A, in press, doi:10.1016/j.compositesa.2005.12.013], suitable for modelling the progressive failure of woven carbon composites under high strain loading. The approach is based on a damage mechanics methodology, and has been implemented into the explicit dynamic DYNA3D code for plane stress shell elements. An interface modelling strategy is also presented to determine the corresponding maximum delamination envelope during a dynamic simulation.The form of the stress–strain–damage curve for woven carbon and the relevance of experimentally measured material damage constants are discussed. The simulation results are compared to three CRAG impact experimental tests at three distinct energy levels. A detailed parameter study is performed on the magnitude of the intralaminar energy release rate used to propagate the damage in woven carbon composites. Conclusions are drawn on the assumed form of the damage evolution curve, and its applicability to stochastic modelling techniques.     

Oxidation of gas phase trichloroethylene and toluene using composite sol-gel TiO2 photocatalytic coatings

The previously developed composite sol-gel (CSG) process is proposed for the deposition of thick (10-50 microm) porous films of photocatalytic TiO2. The CSG titania was developed by binding pre-calcined TiO2 particles with TiO2 sol. It had relatively high surface area (15-35 m2/g) and good resistance against mechanical stress and abrasion. Photocatalytic activity tests were carried out on trichloroethylene (TCE) and toluene, and compared with those of standard Degussa P-25 titania. The CSG photocatalyst provided good photo-efficiency in removing both pollutants from contaminated air streams. When compared with P-25 titania, the CSG photocatalyst showed a similar photo-efficiency with first-order kinetic rate constants not significantly different from that of P-25. For both photocatalysts the rate of photocatalytic oxidation of TCE was significantly greater than that obtained for toluene. Overall, the combination of better mechanical integrity, resistance against abrasion, and comparable photocatalytic efficiency of the CSG titania versus that of P-25 titania, make the composite sol-gel (CSG) photocatalyst a viable alternative for industrial applications where long term stability, superior mechanical properties, and good photo-efficiency are of critical value.     

High performance nano-titania photocatalytic paper composite. Part I: Experimental design study for TiO2 composite sheet using a natural zeolite microparticle system and its photocatalytic property

Nanosized titanium dioxide (TiO₂) photocatalytic papers were successfully prepared using a natural zeolite microparticle retention aid in papermaking. Applying a factorial experimental design, natural zeolite has been demonstrated to be the most significant and interactive factor for increasing the TiO₂ retention rate in paper stock. The photocatalysis of as-prepared sheets was evaluated with toluene as one of the representative volatile organic compounds (VOCs). Under UV irradiation, it was shown to be effective in removing gaseous toluene by photodecomposition, assisted by adsorption. It was revealed that natural zeolite plays an important role in both increasing retention rate of TiO₂ nanoparticles and enhancing photocatalytic efficiency of the paper. The photocatalytic paper can be potentially applied for environmental purification.     

Effect of magnetic properties of a composite superconductor on losses in variable magnetic field. Part 2: Experiment

The theory developed in Part 1 of this paper is compared with the results of experimental measurements of losses in multi-filamentary superconductors in a transverse field. The losses have been measured for different magnetic field variation patterns (sinusoidal, triangular, trapezoidal, single pulses). The numerical results obtained with the aid of the theory are in fairly good agreement with the experimental data. In the majority of the actual cases, analytical expressions for losses can be used.     

Losses in composite superconductors at a high level of magnetic field excitation: Part 2

Losses in a round composite superconductor are discussed. The calculated results are in good agreement with experimental data obtained. An analysis is provided of the effect of the collective interaction between turns in the winding at a high level of excitation, ie, under conditions where the saturated zone occupies a considerable part of the composite volume.     

Plasma spraying of zirconia-reinforced hydroxyapatite composite coatings on titanium: Part I Phase, microstructure and bonding strength

Plasma-sprayed hydroxyapatite (HA) coatings applied to metal substrates can induce a direct chemical bond with bone and hence achieve biological fixation of the implant. However, the poor bonding strength between HA and substrate has been of concern to orthopaedists. In this study, two submicrometre ZrO2 powders stabilized with both 3 and 8 mol% Y2O3 (TZ3Y and TZ8Y, respectively) were incorporated in a plasma-sprayed HA coating on Ti-6Al-4V substrate to investigate the change in phase, microstructure and bonding strength. The results show that ZrO2 composite coatings contain more unmelted particles and greater porosity. During plasma spraying, ZrO2 reacts with the CaO in HA to form CaZrO3 and accelerates HA decomposition to -TCP and Ca4P2O9. Nevertheless, bonding strength increases with increase of ZrO2 content in the range 0 to 10 wt% studied. The higher Y2O3-containing TZ8Y apparently exerts a greater strengthening effect than the lower Y2O3-containing TZ3Y.     

A study of damage development in a weft knitted fabric reinforced composite. Part 1: Experiments using model sandwich laminates

Coupons consisting of single layers of Milano weft knitted glass fabric reinforced epoxy resin have been tested with the knitted fabric oriented at various angles to the loading direction, alone and sandwiched between outer layers of unidirectional glass/epoxy reinforcement. The sandwich coupons enable the initiation of damage to be observed directly. Tensile tests have shown that the first damage occurs as microdebonding between the loop cross-over points in the knitted fabric structure. Matrix cracking damage develops from these initiation sites and the pattern of cracking is intimately related to the fabric architecture and the fabric orientation with respect to the loading direction. Cyclic tests of sandwich specimens, and a representation of the results in terms of cumulative strain, indicate that some of the acoustic emission activity during loading and unloading of the specimens is likely to be associated with the debonding and pulling-out of knitted fabric tows across the fracture surfaces of the matrix cracks.     

Late-stage fatigue damage in a 3D orthogonal non-crimp woven composite: An experimental and numerical study

Late-stage fatigue damage of an E-glass/epoxy 3D orthogonal non-crimp textile composite loaded in the warp direction has been investigated using a combination of mechanical testing, X-ray micro computed tomography (μCT), optical microscopy and finite element modelling. Stiffness reduction and energy dissipated per cycle were found to be complementary measurements of damage accumulation, occurring in three stages: a first stage characterised by rapid changes, a more quiescent second stage, followed by a third stage where the (decreasing) stiffness and (increasing) energy dissipation change irregularly and then rapidly, to failure. Microscopy of specimens cycled into the transition between the second and third stages showed macroscopic accumulations of fibre fractures in sections of warp tows which lying adjacent to the surface weft tows which are crowned-over by the Z-tows. At these locations, the warp tow fibres are subjected to stress concentrations both from transverse weft tow matrix cracks and resin pocket cracks.     

A study of fastener pull-through failure of composite laminates. Part 2: Failure prediction

The experimental study of fastener pull-through failure in composite laminates reported in Part 1 of this paper found pull-through failure to be characterised by substantial internal damage similar to that observed for low-velocity impacted composite panels. Damage is manifested in the form of a conically distributed network of matrix cracking and delaminations extending through-the-thickness from the fastener head outer edge, directed away from the fastener hole. Analysis is conducted in this paper to identify the mechanisms responsible for failure. Finite element analysis indicated high shear stresses at the fastener head outer edge to be responsible for the matrix cracking in this region. Tensile in-plane stresses are the cause of flexural failures found elsewhere in laminates of reduced bending stiffness. Fastener pull-through failure results from the tensile strength of the resin being exceeded. Matrix cracking was found to be the initial mode of failure with cracks aligning themselves perpendicular to the direction of principal stresses. Interply delamination is a secondary mode of failure and represents a propagation of cracking along the path of least resistance. Delaminations are induced due to excessive interlaminar shear and peel strains in the laminate due to through-thickness deformation and matrix cracking respectively. A numerical procedure for the prediction of failure was developed based upon a progressive damage model and a maximum principal strain criterion. Very good correlation between experimental and predicted pull-through failure loads, failure location and failure sequence were achieved. This research constitutes work performed as part of the Cooperative Research Centre for Advanced Composite Structures (CRC-ACS) task on highly loaded joints.     

Multi-level experimental and numerical analysis of composite stiffener debonding. Part 2: Element and panel level

In the framework of test analysis pyramid, large specimens were studied. To represent the bending behaviour during postbuckling, specimens composed of a plate with a stiffener were supported on five points and loaded transversely by two points, thus being subjected to “seven point” bending. By modifying the positions of the two loading points, symmetrical and antisymmetrical buckles that led to interface failure between the flange and the skin could be simulated. First a global numerical model of the test was made in order to assess the efficiency of the approach developed in the first part of this study. Predictions were in accordance with experiments despite strong scatter. Then, a global/local test method was considered. In this method, the global model considered shell elements although the local model used volume elements. The onset of delamination was correctly predicted at local level. Finally, the method was applied to two large stiffened panels subjected to compression and shear.     

Global behaviour of a composite stiffened panel in buckling. Part 1: Numerical modelling

The present study analyses an aircraft composite fuselage structure manufactured by the Liquid Resin Infusion (LRI) process and subjected to a compressive load. LRI is based on the moulding of high performance composite parts by infusing liquid resin on dry fibres instead of prepreg fabrics or Resin Transfer Moulding (RTM). Actual industrial projects face composite integrated structure issues as a number of structures (stiffeners, …) are more and more integrated onto the skins of aircraft fuselage. A representative panel of a composite fuselage to be tested in buckling is studied numerically.     

Instantaneous and time-dependent analysis of composite wood-concrete cross-sections using Dischinger’s equations of state: Part 2-time-dependent analysis

In the second and final part of this paper, the time-dependent analysis of a monosymmetric composite wood-concrete cross-section is presented. The relaxation procedure is used to assemble the stress and strain vectors due to creep and shrinkage effects. These are solved using the finite difference method and finally an illustrative example is used to demonstrate the method.

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Resume Dans la seconde et dernière partie de cet article, on présente l’analyse en fonction du temps d’une section transversale composite bois-béton monosymétrique. La relation fournit une méthode pour relier les vecteurs des contraintes et des déformations dues aux effets du fluage et du retrait. On donne ensuite une application de la méthode des différences finies, et enfin on illustre la démonstration par un example.

     

Plasma spraying of zirconia-reinforced hydroxyapatite composite coatings on titanium: Part II Dissolution behaviour in simulated body fluid and bonding degradation

The change of phase, morphology and bond strength of plasma sprayed hydroxyapatite (HA) coating and ZrO2/HA composite coatings immersed in simulated body fluid (SBF) for various periods of time was studied. X-ray diffractometry (XRD) and scanning electron microscopy (SEM) were used to identify the phase and observe the morphology of the coating surface before and after immersion. In addition, inductively coupled plasma emission spectroscopy (ICP) was used to measure the ion release rate of coatings in SBF for various periods of time. Observation of the morphology by SEM shows that the composite coating with the addition of ZrO2 in HA significantly reduced the dissolution rate of impurity phases in simulated body fluid. The argument was supported by measurement of Ca2+ ion concentration in SBF. During plasma spraying, less OH- ions were lost in a ZrO2-containing composite coating. This factor, together with the reduced effective surface of the ZrO2-containing HA coating, were attributed to the reduced dissolution rate of the composite coatings. All the plasma sprayed coatings degraded after immersion in SBF owing to dissolution of constituents in the coating, however, the addition of ZrO2 in HA improved the bonding strength of HA coating after immersion in SBF.     

Thermo-hygro-mechanical behavior of early-age concrete deck in composite bridge under environmental loadings. Part 2: strain and stress

Restraint of volume change in concrete accompanies the development of stress in concrete structures. When concrete is placed in the field, the stress development becomes very complicated at early ages due to the hydration and environmental interaction. This study investigates quantitatively the effects of various factors on the stress variation of early-age concrete decks in composite bridges under environmental loading. The test members were made to exhibit the early-age behavior of a composite bridge. The effects of parameters related to thermal and drying shrinkage stress were analyzed through a numerical model and compared with measured data. The risk of transverse cracking in early-age concrete decks was evaluated in the numerical parametric study. The present study provides better understanding of the behavior of early-age composite bridge under environmental loading, which can be efficiently utilized to reduce the risk of cracking at early ages.