Applied Composite Materials (v.20, #6)

An approach to estimate the fiber/matrix interface shear stress of woven ceramic matrix composites during fatigue loading has been developed in this paper. Based on the analysis of the microstructure, the woven ceramic matrix composites were divided into four elements of 0o warp yarns, 90o weft yarns, matrix outside of the yarns and the open porosity. When matrix cracking and fiber/matrix interface debonding occur upon first loading to the peak stress, it is assumed that fiber slipping relative to matrix in the interface debonded region of the 0o warp yarns is the mainly reason for the occurrence of the hysteresis loops of woven ceramic matrix composiets during unloading and subsequent reloading. The unloading interface reverse slip length and reloading interface new slip length are determined by the interface slip mechanisms. The hysteresis loops of three different cases have been derived. The hysteresis loss energy for the strain energy lost per volume during corresponding cycle is formulated in terms of the fiber/matrix interface shear stress. By comparing the experimental hysteresis loss energy with the computational values, the fiber/matrix interface shear stress of woven ceramic matrix composites corresponding to different cycles can then be derived. The theoretical results have been compared with experimental data of two different woven ceramic composites.
Keywords: Ceramic matrix composites; Woven; Fatigue; Hysteresis loops; Interface shear stress

This paper presents a 2D braiding design system for advanced textile structural composites was based on dynamic models. A software package to assist in the design of braided preform manufacturing has been developed. The package allows design parameters (machine speeds, fiber volume fraction, tightness factor, etc.) to be easily obtained and the relationships between said parameters to be demonstrated graphically. The fabirc geometry model (FGM) method was adopted to evaluate the mechanical properties of the composites. Experimental evidence demonstrates the success of the use of dynamic models in the design software for the manufacture of braided fabric preforms.
Keywords: Integrated manufacturing; Dynamic model; Software; Design parameter

This paper presents the results of a study into a novel application of the “stacked-shell” laminate modelling approach to dynamically loaded bolted composite joints using the explicit finite element code PAM-CRASH. The stacked-shell approach provides medium-high fidelity resolution of the key joint failure modes, but is computationally much more efficient than full 3D modelling. For this work, a countersunk bolt in a composite laminate under in-plane bearing loading was considered. The models were able to predict the onset of damage, failure modes and the ultimate load of the joint. It was determined that improved debris models are required in order to accurately capture the progressive bearing damage after the onset of joint failure.
Keywords: Explicit finite element analysis; Bolted joints; Carbon fibre composites; Polymer matrix composites; Dynamic loading

This paper was based on the explicit finite element codes to predict the impact behavior of through-thickness stitched foam core sandwich composites. It is proposed that the extent of the impact damage can be characterized by the token parameters of cracking width, penetration depth and damage angle; and observations made during the simulative analysis with such damage parameters. The results show that the same tendencies and characteristics are shown on the numerical and test results of impact force-displacement plots, and a good agreement is also obtained in damage parameters. In comparing the unstitched types, the through-thickness stitched sandwiches are optimal for both the peak loads shown on the numerical plots at 25.0 J; and demonstrate the fewer extent of impact damage with a 63.5 and 6.0 % decreasing to the cracking width and penetration depth respectively, and where a 52.0 % increasing to the damage angle.
Keywords: Foam core sandwich; Impact behavior; Finite element analysis (FEA); Through-thickness stitched; Damage parameters

An Analytic Definition of the Border Polymerization Line for Axisymmetric Composite Rods by S. N. Grigoriev; A. N. Krasnovskii; I. A. Kazakov; K. V. Kvachev (1055-1064).
The paper describes a theoretical study of the border polymerization line for rods of circular cross-section during the pultrusion process. The influence of the pull speed, the type of filler and fiber volume fraction on the position of the border polymerization line is investigated. It is shown that pre-heating of the fiber/resin system helps to speed up the polymerization process.
Keywords: Composite material; Pultrusion; Border line; Die; Specific heat; Degree of polymerization

This paper presents the integrated design of manufacturing of braided preforms by two types of novel 3D braiding technology: 3D Cartesian braiding and novel hexagonal braiding. The principles for design are first introduced and the ensuing software package development is subsequently discussed. The relationships between fiber volume fraction and braiding angle which are key parameters for fiber reinforcement composites were analyzed and compared. Meanwhile, several samples are carried out to verify the software. The result shows it is consistent between theoretical and experimental results. Combined with the Part I: 2D braiding section, many complex shape performs can be made, which will be usefully for design of advanced composites.
Keywords: 3D Cartesian braiding; Hexagonal braiding; Software package; Fiber volume fraction; Braiding angle

Thermoplastic Composites Reinforced with Textile Grids: Development of a Manufacturing Chain and Experimental Characterisation by R. Böhm; E. Hufnagl; R. Kupfer; T. Engler; J. Hausding; C. Cherif; W. Hufenbach (1077-1096).
A significant improvement in the properties of plastic components can be achieved by introducing flexible multiaxial textile grids as reinforcement. This reinforcing concept is based on the layerwise bonding of biaxially or multiaxially oriented, completely stretched filaments of high-performance fibers, e.g. glass or carbon, and thermoplastic components, using modified warp knitting techniques. Such pre-consolidated grid-like textiles are particularly suitable for use in injection moulding, since the grid geometry is very robust with respect to flow pressure and temperature on the one hand and possesses an adjustable spacing to enable a complete filling of the mould cavity on the other hand. The development of pre-consolidated textile grids and their further processing into composites form the basis for providing tailored parts with a large number of additional integrated functions like fibrous sensors or electroconductive fibres. Composites reinforced in that way allow new product groups for promising lightweight structures to be opened up in future. The article describes the manufacturing process of this new composite class and their variability regarding reinforcement and function integration. An experimentally based study of the mechanical properties is performed. For this purpose, quasi-static and highly dynamic tensile tests have been carried out as well as impact penetration experiments. The reinforcing potential of the multiaxial grids is demonstrated by means of evaluating drop tower experiments on automotive components. It has been shown that the load-adapted reinforcement enables a significant local or global improvement of the properties of plastic components depending on industrial requirements.
Keywords: Polymer matrix composites; Fabrics; Coupon testing

An experimental study was carried out to determine the notched tension characteristics of CCF300 fiber reinforced composite and T300 fiber reinforced composite subjected to normal and hygrothermal environment conditions. First, tests to failure with residual strength were carried out and the damage progressions were carefully investigated. Scanning electron microscope (SEM) was then employed for fractographic investigations. It was found that the CCF300/QY8911 composite specimens with poor interfacial adhesion between fiber and matrix exhibited higher residual strength, while the T300/QY8911 composite specimens with well adhesion had shorter ultimate strain. Damage progression mechanisms in the two material systems were very different. The major damage mechanism of CCF300/QY8911 composites was delamination generally occurring at lower stresses, which allowed higher levels of damage formation and resulted in higher ultimate strength. When changing to hygrothermal environment condition, greater damage was observed and off axis plies were extensively pulled out from the adjacent plies. On the other hand, all of the T300/QY8911 composite specimens failed due to fiber fracture with less damage, leading to a lower residual strength regardless of the environmental conditions. Further examinations showed that the failure mode of CCF300/QY8911 composite was much more sensitive to the hygrothermal environment, for the micro-failure mode had changed from matrix failure and fiber breakage to fiber/matrix splitting and fiber bundle pullout.
Keywords: Hygrothermal environment condition; Notched strength; Damage mechanism; Carbon fibers

The aim of this work is to study the influence of weave structure on the crack growth behavior of thick E-glass/polyester woven fabric composites laminates. Two different types of laminates were fabricated: (i) balanced: plain weave (taffetas T)/chopped strand mat weave (M) [T/M]6 and (ii) unbalanced: 4-hardness satin weave (S)/chopped strand mat weave [S/M]7. In order to accurately predict damage criticality in such structures, mixed mode fracture toughness data is required. So, the experiments were conducted using standards delamination tests under mixed mode loading and pure mode loading. These tests were carried out in mode II using End Load Split (ELS) tests and in mixed-mode I+II by Mixed Mode Flexure (MMF) tests under static conditions. The test methodology used for the experiments will be presented. The experimental results have been expressed in terms of total strain energy release rate and R-curves. The fracture toughness results show that the T/M interface is more resistant to delamination than the S/M interface.
Keywords: E-Glass/polyester; Woven fabric composite; Delamination; ELS; MMF; R-curve

Fuzzy Reasoning to More Accurately Determine Void Areas on Optical Micrographs of Composite Structures by Jesus A. Dominguez; Lanetra C. Tate; M. Clara Wright; Anne Caraccio (1125-1134).
Accomplishing the best-performing composite matrix (resin) requires that not only the processing method but also the cure cycle generate low-void-content structures. If voids are present, the performance of the composite matrix will be significantly reduced. This is usually noticed by significant reductions in matrix-dominated properties, such as compression and shear strength. Voids in composite materials are areas that are absent of the composite components: matrix and fibers. The characteristics of the voids and their accurate estimation are critical to determine for high performance composite structures. One widely used method of performing void analysis on a composite structure sample is acquiring optical micrographs or Scanning Electron Microscope (SEM) images of lateral sides of the sample and retrieving the void areas within the micrographs/images using an image analysis technique. Segmentation for the retrieval and subsequent computation of void areas within the micrographs/images is challenging as the gray-scaled values of the void areas are close to the gray-scaled values of the matrix leading to the need of manually performing the segmentation based on the histogram of the micrographs/images to retrieve the void areas. The use of an algorithm developed by NASA and based on Fuzzy Reasoning (FR) proved to overcome the difficulty of suitably differentiate void and matrix image areas with similar gray-scaled values leading not only to a more accurate estimation of void areas on composite matrix micrographs but also to a faster void analysis process as the algorithm is fully autonomous.
Keywords: Composite void analysis; Optical micrographs analysis; Void content composites; Void content resin; Void content matrix; Fuzzy reasoning; Fuzzy logic; Image segmentation; Image binarization; Binary segmentation

Mode I Fracture Toughness Testing of Composite Pipes by G. Perillo; A. T. Echtermeyer (1135-1146).
A common industrial production process for axially symmetric composites is filament winding. For this type of material, interlayer properties cannot be evaluated using the available ISO and ASTM standards because the curved surfaces and partially interwoven fibers of cylindrical parts can affect the results. This paper presents a special test geometry for measuring the critical energy release rate GIc directly from actual filament wound products. Different specimen geometries were investigated numerically with respect to their stress state at the crack tip and tested experimentally. The results were compared against those of flat specimens tested according to ISO/ASTM standards and made from the same constituent materials as the original test pipe. Testing specimens taken from an actual filament wound pipe yields more realistic results of GIc = 1,220 J/m2 than testing flat specimens (GIc = 330 J/m2), especially made for fulfilling the test standard’s requirements.
Keywords: Critical energy release rate; Filament wound composite; DCB test; Fracture

Multiple delamination causes severe degradation of the stiffness and strength of composites. Interactions between multiple delamination, and buckling and postbuckling under compressive loads add the complexity of mechanical properties of composites. In this paper, the buckling, postbuckling and through-the-width multiple delamination of symmetric and unsymmetric composite laminates are studied using 3D FEA, and the virtual crack closure technique with two delamination failure criteria: B-K law and power law is used to predict the delamination growth and to calculate the mixed-mode energy release rate. The compressive load-strain curves, load-central deflection curves and multiple delamination process for eight composite specimens with different initial delamination sizes and their distributions as well as two angle-ply configurations 04//(±θ)6//04 (θ = 0° and 45°, and “//” denotes the delaminated interface) are comparatively studied. From numerical results, the unsymmetry decreases the local buckling load and initial delamination load, but does not affect the global buckling load compared with the symmetric laminates. Besides, the unsymmetry affects the unstable delamination and buckling behaviors of composite laminates largely when the initial multiple delamination sizes are relatively small.
Keywords: Composites; Buckling and postbuckling; Delamination; Finite element analysis (FEA)

Numerical Analysis of Thermodynamic Behaviour of Through-Thickness Stitched Sandwich Laminate by Ai Shigang; Mao Yiqi; Pei Yongmao; Fang Daining; Tang Liqun (1161-1171).
Effects of stitching angle on mechanical properties, thermal protection capability and induced thermal stress of stitched sandwich laminate (SSL) are numerically analyzed by ABAQUS codes. Interest centers on the potential for microcracking in the vicinity of the through-thickness stitches and the skins/foam interfaces. Two numerical models, in-depth heat transfer and thermoelastic deformation, are coupled to yield the transient response of the SSL. Six different stitching angles are considered and the simulation results showed that: the heat conductivity ability of the SSL is improved as the stitching angle increasing, which alters the mechanical behaviour and the thermal stress state of the SSL.
Keywords: Stitching angle; Stitched sandwich laminate; Thermal stress; Heat transfer; Periodic boundary condition

In resin injection pultrusion, the liquid resin is injected through the injection slots into the fiber reinforcement; the liquid resin penetrates through the fibers as well as pushes the fibers towards the centerplane causing fiber compaction. The compacted fibers are more difficult to penetrate, thus higher resin injection pressure becomes necessary to achieve complete reinforcement wetout. Lower injection pressures below a certain range (depending upon the fiber volume fraction and resin viscosity) cannot effectively penetrate through the fiber bed and thus cannot achieve complete wetout. Also, if the degree of compaction is very high the fibers might become essentially impenetrable. The more viscous the resin is, the harder it is to penetrate through the fibers and vice versa. The effect of resin viscosity on complete wetout achievement with reference to fiber-reinforcement compaction is presented in this study.
Keywords: Pultrusion; Resin injection; Resin viscosity; Fiber reinforcement compaction; Operational envelope

In this study, an optimal finite element model of Kevlar woven fabric that is more computational efficient compared with existing models was developed to simulate ballistic impact onto fabric. Kevlar woven fabric was modeled to yarn level architecture by using the hybrid elements analysis (HEA), which uses solid elements in modeling the yarns at the impact region and uses shell elements in modeling the yarns away from the impact region. Three HEA configurations were constructed, in which the solid element region was set as about one, two, and three times that of the projectile’s diameter with impact velocities of 30 m/s (non-perforation case) and 200 m/s (perforation case) to determine the optimal ratio between the solid element region and the shell element region. To further reduce computational time and to maintain the necessary accuracy, three multiscale models were presented also. These multiscale models combine the local region with the yarn level architecture by using the HEA approach and the global region with homogenous level architecture. The effect of the varying ratios of the local and global area on the ballistic performance of fabric was discussed. The deformation and damage mechanisms of fabric were analyzed and compared among numerical models. Simulation results indicate that the multiscale model based on HEA accurately reproduces the baseline results and obviously decreases computational time.
Keywords: Woven fabric; Ballistic impact; Finite element; Hybrid element analysis; Multiscale model; Fabric deformation

Contrast Enhancement of MicroCT Scans to Aid 3D Modelling of Carbon Fibre Fabric Composites by Luke P. Djukic; Garth M. Pearce; Israel Herszberg; Michael K. Bannister; David H. Mollenhauer (1215-1230).
This paper presents a methodology for volume capture and rendering of plain weave and multi-layer fabric meso-architectures within a consolidated, cured laminate. Micro X-ray Computed Tomography (MicroCT) is an excellent tool for the non-destructive visualisation of material microstructures however the contrast between tows and resin is poor for carbon fibre composites. Firstly, this paper demonstrates techniques to improve the contrast of the microCT images by introducing higher density materials such as gold, iodine and glass into the fabric. Two approaches were demonstrated to be effective for enhancing the differentiation between the tows in the reconstructed microCT visualisations. Secondly, a method of generating three-dimensional volume models of woven composites using microCT scan data is discussed. The process of generating a model is explained from initial manufacture with the aid of an example plain weave fabric. These methods are to be used in the finite element modelling of three-dimensional fabric preforms in future work.
Keywords: MicroCT; Polymer matrix composites; Volume rendering; Microstructures

Damage Behaviors of Foam Sandwiched Composite Materials Under Quasi-Static Three-point Bending by Fa Zhang; Ramadan Mohmmed; Baozhong Sun; Bohong Gu (1231-1246).
This paper reports the quasi-static three-point bending damage behaviors of foam sandwiched composites in finite element analyses (FEA) and experimental. Finite element calculations were performed to characterize the static response of foam sandwich composites with different ply angle face sheets. Quasi-static three-point bending tests were conducted with a MTS materials testing system to obtain the load–displacement curves and energy absorption under quasi-static bending. A crushable foam model was used in order to explore the mechanical behaviors of core materials, while the Hashin criterion was employed to predict the failure of the face sheets. The load–displacement curves show a satisfactory agreement between the experimental and numerical results. The finite element calculations can also be used to obtain the failure mode included the core damage, face sheet damage and face-core interface damage. It can be observed that the damage at the core material can be classified as either core cracking or core crushing. The damage of the face sheet was through matrix cracking and delamination, with fiber breakage. The significant indentation occurs as a result of the fiber breakage. The face-core interface crack was typically induced by the cracks initiated from the tensile side and propagated to the compressive side.
Keywords: Foam sandwiched composites; Stacking sequence; Quasi-static three-point bending; Finite element analyses (FEA); Failure modes

Thermo-Chemical Modelling Strategies for the Pultrusion Process by Ismet Baran; Jesper H. Hattel; Cem C. Tutum (1247-1263).
In the present study, three dimensional (3D) numerical modeling strategies of a thermosetting pultrusion process are investigated considering both transient and steady state approaches. For the transient solution, an unconditionally stable alternating direction implicit Douglas-Gunn (ADI-DG) scheme is implemented as a first contribution of its kind in this specific field of application. The corresponding results are compared with the results obtained from the transient fully implicit scheme, the straightforward extension of the 2D ADI and the steady state approach. The implementation of the proposed approach is described in detail. The calculated temperature and cure degree profiles at steady state are found to agree well with results obtained from similar analyses in the literature. Detailed case studies are carried out investigating the computational accuracy and the efficiency of the 3D ADI-DG solver. It is found that the steady state approach is much faster than the transient approach in terms of the computational time and the number of iteration loops to obtain converged results for reaching the steady state. Hence, it is highly suitable for automatic process optimization which often involves many design evaluations. On the other hand sometimes the transient regime may be of interest and here the proposed ADI-DG method shows to be considerably faster than the transient fully implicit method which is generally used by the general purpose commercial finite element solvers. Finally, using the proposed steady-state approach, a design of experiments is carried out for the curing characteristic of the product based on pulling speed and part thickness.
Keywords: Numerical modeling; ADI; Finite difference method; Pultrusion; Curing

Processing Optimization of Deformed Plain Woven Thermoplastic Composites by John R. Smith; Uday K. Vaidya (1265-1272).
This research addresses the processing optimization of post-manufactured, plain weave architecture composite panels consisted of four glass layers and thermoplastic polyurethane (TPU) when formed with only localized heating. Often times, during the production of deep drawn composite parts, a fabric preform experiences various defects, including non-isothermal heating and thickness variations. Minimizing these defects is of utmost importance for mass produceability in a practical manufacturing process. The broad objective of this research was to implement a design of experiments approach to minimize through-thickness composite panel variation during manufacturing by varying the heating time, the temperature of heated components and the clamping pressure. It was concluded that the heated tooling with least area contact was most influential, followed by the length of heating time and the amount of clamping pressure.
Keywords: Thermoforming; Plain woven composite; Design of experiment; Processing optimization

This paper shows an integrated structural design optimization of a composite rotor-hydrofoil of a water current turbine by means the finite elements method (FEM), using a Serial/Parallel mixing theory (Rastellini et al. Comput. Struct. 86:879–896, 2008, Martinez et al., 2007, Martinez and Oller Arch. Comput. Methods. 16(4):357–397, 2009, Martinez et al. Compos. Part B Eng. 42(2011):134–144, 2010) coupled with a fluid-dynamic formulation and multi-objective optimization algorithm (Gen and Cheng 1997, Lee et al. Compos. Struct. 99:181–192, 2013, Lee et al. Compos. Struct. 94(3):1087–1096, 2012). The composite hydrofoil of the turbine rotor has been design using a reinforced laminate composites, taking into account the optimization of the carbon fiber orientation to obtain the maximum strength and lower rotational-inertia. Also, these results have been compared with a steel hydrofoil remarking the different performance on both structures. The mechanical and geometrical parameters involved in the design of this fiber-reinforced composite material are the fiber orientation, number of layers, stacking sequence and laminate thickness. Water pressure in the rotor of the turbine is obtained from a coupled fluid-dynamic simulation (CFD), whose detail can be found in the reference Oller et al. (2012). The main purpose of this paper is to achieve a very low inertia rotor minimizing the start-stop effect, because it is applied in axial water flow turbine currently in design by the authors, in which is important to take the maximum advantage of the kinetic energy. The FEM simulation codes are engineered by CIMNE (International Center for Numerical Method in Engineering, Barcelona, Spain), COMPack for the solids problem application, KRATOS for fluid dynamic application and RMOP for the structural optimization. To validate the procedure here presented, many turbine rotors made of composite materials are analyzed and three of them are compared with the steel one.
Keywords: Water current turbines (WCT); Composite materials; Rotor turbine design and analysis; Finite element method (FEM); Multi-objective Optimization

In this study, it was aimed to compare mechanical behavior of double-strap joints with aluminum (AA2024-T3) or 16-ply laminate of carbon/epoxy composite (T300/934) patches of different orientation angles at their overlap area subjected to bending moment. For this purpose, AA2024-T3 aluminum was used as adherend, while the adhesive was a two-part paste (DP 460). Six different types of joint samples were subjected to bending moment. The effect of patch material on failure load and stress distribution was examined experimentally and numerically. In the numerical analysis, the composite patches were assumed to behave linearly elastic, while adherend and adhesive layers were assumed to be nonlinear. It was found that the data obtained from 3-D finite element analysis were coherent with experimental results. Meanwhile, experiments showed that fiber orientation angles of the patches markedly affected the failure load of joints, failure mode and stress distributions appeared in adhesive and composite.
Keywords: Adhesive; Joint design; Ply stacking sequence; Non-linear finite element; Three-dimensional effects

Vacuum assisted resin transfer molding (VARTM) is one of the important processes to fabricate high performance composites. In this process, resin is drawn into the mold to impregnate the fiber reinforcement to a form composite. A resin distribution layer with high permeability was often introduced on top of the fiber reinforcement to accelerate the filling speed. Due to the difference of the flow resistance in the resin distribution layer and the reinforcement as well as the resulting through thickness transverse flow, the filling flow field is intrinsically three-dimensional. This study developed a two-layer model with two-dimensional formulation to simulate the filling flow of the VARTM process with a resin distribution layer. Two-dimensional flow was considered in each layer and a transverse flow in the thickness direction was estimated between the two layers. Thermal analysis including the transverse convection was also performed to better simulate the temperature distribution.
Keywords: Resin transfer moulding (RTM); Resin flow; Numerical analysis

Analytical Model for Prediction of Reduced Strain Energy Release Rate of Single-Side-Patched Plates by Y. W. Kwon; W. Y. Lee; A. S. McGee; D. C. Hart; D. C. Loup; E. A. Rasmussen (1321-1339).
A study was undertaken to develop an analytical model that can predict how much reduction in Strain Energy Release Rate (SERR) can be achieved by repairing a cracked plate using a single-side bonded patch. The plate may be subjected to inplane or out-of-plane bending loading. Furthermore, the plate may be flat or curved in a cylindrical shape. The model helps to select patch material (i.e., elastic modulus of the material) and the appropriate patch size in order to reduce the SERR at the crack tip of the patched base plate. In other words, the analytical model can be utilized to select the patch material and patch dimensions required to achieve the desired SERR for a cracked base plate with known modulus, thickness, and crack size. The model is based on axial and bending stresses of the single-side strap joint configuration, which are related to the SERR at the crack tip of a plate with a single-side patch repair. In order to verify the analytical model, finite element analyses were conducted to determine stresses as well as SERR in many different patched plates. The numerical study confirmed the validity of the analytical model in predicting the reduction ratio of SERR resulting from the single-side patch repair.
Keywords: Composite patch; Bonded repair; Single-side patch; Strain energy release rate