Applied Composite Materials (v.20, #4)

Wear Modalities and Mechanisms of the Mining Non-asbestos Composite Brake Material by Jiusheng Bao; Yan Yin; Zhencai Zhu; Minming Tong; Yuhao Lu; Yuxing Peng (331-339).
The mining brake material is generally made of composite materials and its wear has important influences on the braking performance of disc brakes. In order to improve the braking reliability of mine hoisters, this paper did some tribological investigations on the mining brake material to reveal its wear modalities and mechanisms. The mining non-asbestos brake shoe and 16Mn steel were selected as braking pairs and tested on a pad-on-disc friction tester. And a SEM was used to observe the worn surface of the brake shoe. It is shown that the non-asbestos brake material has mainly five wear modalities: adhesive wear, abrasive wear, cutting wear, fatigue wear and high heat wear. At the front period of a single braking the wear modality is mainly composed of some light mechanical wear such as abrasive, cutting and point adhesive. With the temperature rising at the back period it transforms to some heavy mechanical wear such as piece adhesive and fatigue. While in several repeated brakings once the surface temperature rises beyond the thermal-decomposition point of the bonding material, the strong destructive high heat wear takes leading roles on the surface. And a phenomenon called friction catastrophe (FC) occurs easily, which as a result causes a braking failure. It is considered that the friction heat has important influences on the wear modalities of the brake material. And the reduction of friction heat must be an effective technical method for decreasing wear and avoiding braking failures.
Keywords: Wear modality; Wear mechanism; Non-asbestos brake material; Friction heat; Emergency braking

Using of the corrugated skins and morphing technology is a good idea to provide the desired performance and improve aerodynamic efficiency. Corrugated structures and skins are flexible in the direction of corrugation and stiff in the transverse direction. In this paper a simple analytical model for the effective stiffness of the trapezoidal corrugated composites is developed in symmetrical and unsymmetrical lay-up. The elongation and effective stiffness in longitudinal and transverse directions of trapezoidal corrugated skins and flat composites are extracted using strain energy and Castiglione’s theorem. Various dimensions of trapezoidal element for unidirectional and plain woven fabrics of E-glass/Epoxy are investigated. Trapezoidal corrugated composites were modelled by commercial FEM software ABAQUS and compared to analytical model. Analytical model is validated by experimental results from bending and tensile tests. Finally, load-displacement curves in the tensile and bending tests are studied and their different stages of behavior are identified. Results of FEM, experimental and analytical simulation show that how the corrugated composite skins can afford obviously larger deformation than the flat one and they are good solution to use in the morphing applications.
Keywords: Composite structure; FEM; Corrugated skin; Mechanical behavior; Trapezoidal geometry

A new solid-like shell element was formulated which is suitable for analysis of laminated and sandwich composite structures. Then, a multiscale analysis technique was implemented to the shell element formulation so that micro-level stresses and strains (i.e. stresses and strains in reinforcing fibers and the binding matrix) in those structures can be computed. The shell element has three displacement degrees of freedom per node like a 3-D solid element. Therefore, the shell elements can be stacked easily on top of one another like 3-D solid elements in order to represent multiple layers through the thickness of laminated and sandwich structures. The effect of a thin resin or adhesive layer in laminated and sandwich composite structures was investigated on both static and the dynamic responses of the structures using the developed shell elements. The study showed an apparent effect of the resin/adhesive layer even though it is very thin. As a result, the present shell element can be used effectively to include those thin layers in finite element analysis models of laminated and sandwich composite structures.
Keywords: Solid-like shell element; Laminated and sandwich composite; Adhesive layer; Resin layer; Multiscale analysis

The purpose of this work is to evaluate the influences of fatigue and environmental conditions (−55 °C, 23 °C, and 82 °C/Wet) on the ultimate compression strength of notched carbon-fiber-reinforced poly(phenylene sulfide) composites by performing open-hole compression (OHC) tests. Analysis of the fatigue effect showed that at temperatures of −55 and 23 °C, the ultimate OHC strengths were higher for fatigued than for not-fatigued specimens; this could be attributed to fiber splitting and delamination during fatigue cycling, which reduces the stress concentration at the hole edge, thus increasing the composite strength. This effect of increasing strength for fatigued specimens was not observed under the 82 °C/Wet conditions, since the test temperature near the matrix glass transition temperature (T g) together with moisture content resulted in matrix softening, suggesting a reduction in fiber splitting during cycling; similar OHC strengths were verified for fatigued and not-fatigued specimens tested at 82 °C/Wet. Analysis of the temperature effect showed that the ultimate OHC strengths decreased with increasing temperature. A high temperature together with moisture content (82 °C/Wet condition) reduced the composite compressive strengths, since a temperature close to the matrix T g resulted in matrix softening, which reduced the lateral support provided by the resin to the 0° fibers, leading to fiber instability failure at reduced applied loads. On the other hand, a low temperature (−55 °C) improved the compressive strength because of possible fiber-matrix interfacial strengthening, increasing the fiber contribution to compressive strength.
Keywords: Fatigue; Environmental conditioning; Residual strength; Thermoplastic composite; Notched laminate

The low-velocity impact characters of 3-D braided carbon/epoxy composites were investigated from experimental and finite element simulation approaches. The quasi-static tests were carried out at a constant velocity of 2 mm/min on MTS 810.23 material tester system to obtain the indentation load–displacement curves and indentation damages. The low-velocity tests were conducted at the velocities from 1 m/s to 6 m/s (corresponding to the impact energy from 3.22 J to 116 J) on Instron Dynatup 9250 impact tester. The peak force, energy for peak force, time to peak force, and total energy absorption were obtained to determine the impact responses of 3-D braided composites. A unit cell model was established according to the microstructure of 3-D braided composites to derive the constitutive equation. Based on the model, a user-defined material subroutine (VUMAT) has been compiled by FORTRAN and connected with commercial finite element code ABAQUS/Explicit to calculate the impact damage. The unit cell model successfully predicted the impact response of 3-D braided composites. Furthermore, the stress wave propagation and failure mechanisms have been revealed from the finite element simulation results and ultimate damage morphologies of specimens.
Keywords: 3-D braided composites; Low-velocity impact; Unit cell model; Finite element analysis; Impact response behaviors

Effect of Fibre Volume Fraction on Mixed-Mode Fracture of a Fabric Carbon/Epoxy Composite by Victor Feret; Hossein Ghiasi; Pascal Hubert (415-429).
Variation in fibre volume fraction is a common characteristic of composites made by an injection moulding process. The effect of this variation on fracture toughness is not yet fully investigated. This paper examines the fracture in fabric carbon/epoxy composite laminates under a wide range of combined mode-I and mode-II delamination. A total of 60 double cantilever beam and edge-notched flexure specimens are manufactured by resin transfer moulding with two different fibre volume fractions. It was observed that increasing the fibre volume fraction decreased the initiation fracture toughness in all mixed-mode ratios. This behaviour is believed to relate to the fact that the initiation fracture energy is dominantly absorbed by the resin-rich regions at the delamination tip. In contrast, an increase in fibre volume fraction was found to increase the propagation fracture toughness at high mode-I contribution where the fibre bridging is believed to be the major energy dissipating mechanism. Fractographic analysis also demonstrated that an increase in contribution of mode-II delamination is accompanied by a decrease in fibre bridging and an increase in shear hackles.
Keywords: Mixed-mode delamination; Fracture; Fabric composites; Fibre volume fraction; Fractography

The objective of this study is to investigate the impact of two geometric design parameters (injection slot width and final composite thickness) on achieving complete wetout of the fiber reinforcement as well as the corresponding maximum chamber wall resin pressure inside a resin injection chamber for both the attached-die and detached-die configurations for various injection chamber lengths and compression ratios. This work compares the performances of the detached-die with the attached-die resin injection chamber configurations and determines the feasible manufacturing solutions.
Keywords: Pultrusion; Slot width; Chamber length; Compression ratio

In this paper thermo-chemical simulation of the pultrusion process of a composite rod is first used as a validation case to ensure that the utilized numerical scheme is stable and converges to results given in literature. Following this validation case, a cylindrical die block with heaters is added to the pultrusion domain of a composite part and thermal contact resistance (TCR) regions at the die-part interface are defined. Two optimization case studies are performed on this new configuration. In the first one, optimal die radius and TCR values are found by using a hybrid genetic algorithm based on a sequential combination of a genetic algorithm (GA) and a local search technique to fit the centerline temperature of the composite with the one calculated in the validation case. In the second optimization study, the productivity of the process is improved by using a mixed integer genetic algorithm (MIGA) such that the total number of heaters is minimized while satisfying the constraints for the maximum composite temperature, the mean of the cure degree at the die exit and the pulling speed.
Keywords: Pultrusion; Finite difference; Optimization; Genetic algorithms; Thermal contact resistance

In this paper, a study on skin delamination growth in stiffened composite panels made of carbon fibres reinforced polymers and subjected to compressive load is presented. A robust (mesh and time step independent) numerical finite elements procedure, based on the Virtual Crack Closure Technique (VCCT) and on the fail release approach, is used here to investigate the influence of skin delamination size and position on the damage tolerance of stiffened composite panels. Four stiffened panels configurations with skin delaminations differently sized and positioned are introduced. Bay delaminations and delaminations under the stringer foot are considered. The novel numerical procedure has been used to simulate the delamination growth for all the investigated panel configurations and to evaluate the influence of the delaminations’ geometrical parameters on the growth development. As a confirmation of the applicability and effectiveness of the adopted numerical tool, the numerical results, obtained for all the analysed configurations, in terms of grown delaminated area, displacements and strains measured in various panel locations, have been compared with experimental data available in literature.
Keywords: Skin delaminations; Damage growth; Stiffened panels; FEM

Acoustic Emission as a Tool for Damage Identification and Characterization in Glass Reinforced Cross Ply Laminates by D. G. Aggelis; N.-M. Barkoula; T. E. Matikas; A. S. Paipetis (489-503).
Loading of cross-ply laminates leads to the activation of distinct damage mechanisms, such as matrix cracking, delaminations between successive plies and fibre rupture at the final stage of loading. This study deals with the investigation of the failure of cross ply composites by acoustic emission (AE). Broadband AE sensors monitor the elastic waves originating from different sources of failure in coupons of this material during a tensile loading-unloading test. The cumulative number of AE activity, and other qualitative indices based on the waveforms shape, were well correlated to the sustained load and mechanical degradation as expressed by the gradual decrease of elastic modulus. AE parameters indicate the succession of failure mechanisms within the composite as the load increases. The proposed methodology based on Acoustic Emission for the identification of the damage stage of glass reinforced cross ply laminates is an initial step which may provide insight for the study of more complex laminations.
Keywords: Acoustic emission; Composite materials; Cross ply laminates; Damage monitoring; Failure modes

Curvature saturation has been observed in bi-stable composite laminates when the side length exceeds a critical value. This is the curvature which the stable cylindrical shell converges to after cooling down. Conventional models of the displacement field fail to predict the correct shapes. This is especially true when the laminate forms a wound-up cylinder which can occur when the longer side is several times the critical length. This paper presents a saturated curvature model to predict the cured shape for bi-stable laminates. The finite element analysis is also carried out to capture the cured shape. Both the analytical model and finite element method give the accurate cured shape. The cured shapes are measured experimentally. The results from the proposed model and finite element analysis are compared with the experimental and show a good agreement.
Keywords: Bi-stable; Unsymmetric laminates; Saturated curvature; Cured shape; Rayleigh-Ritz

Glass fibre reinforced plastic (GFRP) composites are an economic alternative to engineering materials because of their superior properties. Some damages on the surface occur due to their complex cutting mechanics in cutting process. Minimisation of the damages is fairly important in terms of product quality. In this study, a GFRP composite material was milled to experimentally minimise the damages on the machined surfaces, using two, three and four flute end mills at different combinations of cutting parameters. Experimental results showed that the damage factor increased with increasing cutting speed and feed rate, on the other hand, it was found that the damage factor decreased with increasing depth of cut and number of the flutes. In addition, analysis of variance (ANOVA) results clearly revealed that the feed rate was the most influential parameter affecting the damage factor in end milling of GFRP composites. Also, in present study, Artificial Neural Network (ANN) models with five learning algorithms were used in predicting the damage factor to reduce number of expensive and time-consuming experiments. The highest performance was obtained by 4-10-1 network structure with LM learning algorithm. ANN was notably successful in predicting the damage factor due to higher R2 and lower RMSE and MEP.
Keywords: Glass fibre reinforced plastic composites; End milling; Damage factor; ANN

High Temperature Residual Properties of Carbon Fiber Composite Sandwich Panel with Pyramidal Truss Cores by Jiayi Liu; Zhengong Zhou; Linzhi Wu; Li Ma; Shidong Pan (537-552).
A study on the mechanical property degradation of carbon fiber composite sandwich panel with pyramidal truss cores by high temperature exposure is performed. Analytical formulae for the residual bending strength of composite sandwich panel after thermal exposure are presented for possible competing failure modes. The composite sandwich panels were fabricated from unidirectional carbon/epoxy prepreg, and were exposed to different temperatures for different time. The bending properties of the exposed specimens were measured by three-point bending tests. Then the effect of high temperature exposure on the bending properties and damage mechanism were analyzed. The results have shown that the residual bending strength of composite sandwich panels decreased with increasing exposure temperature and time, which was caused by the degradation of the matrix property and fiber-matrix interface property at high temperature. The effect of thermal exposure on failure mode of composite sandwich panel was observed as well. The measured failure loads showed good agreement with the analytical predictions. It is expected that this study can provide useful information on the design and application of carbon fiber composite sandwich panel at high temperature.
Keywords: Sandwich panel; High temperature exposure; Bending properties; Composite

This paper presents a method of joining carbon-fibre plies and rigid cellular foam core with stitching for producing light-weight composite structures. After resin infusion and consolidation, the stitched sandwich panel exhibits superior damage tolerance as well as improved transverse properties due to the presence of through-thickness fibre reinforcement. First part of the paper deals with the conceptual development of a multi-needle stitching machine for rigid foams. A needle penetration model for computing the penetration forces has been reported—there is a good agreement between the experimental and theoretical penetration force-displacement curves. A number of sandwich panels with orthogonal and bias stitch orientations have been developed and examined for stitch quality with the aid of X-ray tomography. The paper also presents results from quasi-static indentation, three-point bending and transverse compression tests, on both the stitched and unstitched sandwich panels.
Keywords: Multi-needle stitching; Rigid foam; Sandwich structures; Through-thickness; Damage tolerance

Flexible woven composites have been widely used in geotextiles and light weight building structures. The stab resistance behavior of the flexible woven composite is an important factor for the application design. This paper reports an analytical model for predicting stab resistance of flexible woven composites under perpendicular stab with a blunt steel penetrator. The analytical model was established based on the microstructure and the deformation shape of the flexible woven composite under normal penetration. During the quasi-static stab penetration, the strain energies of warp and weft yarns and resins have been calculated. The stab resistance was calculated from the strain energies of the flexible woven composite. Furthermore, the contributions of the warp and weft yarns, resins to the stab resistance have been analyzed. It was found the three constituents have near the same contribution to the stab resistance. The higher value of weaving density, strength of yarns and especially the higher strength coating resins will lead the higher stab resistance. With the analytical model, the stab resistance would be expected to be designed in an efficient way with an acceptable precision.
Keywords: Flexible woven composite; Fabric architecture; Stab resistance; Strain energy

We characterize the combined Mode I and Mode III delamination fracture behavior of woven glass fiber reinforced polymer (GFRP) composite laminates at cryogenic temperatures. The eight-point bending plate (8PBP) tests were conducted at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) using a new test fixture. A three-dimensional finite element analysis was also performed to calculate the energy release rate distribution along the delamination front, and the delamination fracture toughnesses were evaluated for various mixed-mode I/III ratios. Furthermore, the microscopic examinations of the fracture surfaces were carried out with scanning electron microscopy (SEM), and the mixed-mode I/III delamination fracture mechanisms in the woven GFRP laminates at cryogenic temperatures were assessed. The fracture properties were then correlated with the observed characteristics.
Keywords: Cryomechanics; Toughness testing; Finite Element Analysis (FEA); Polymer-Matrix Composites (PMCs); Delamination; Mixed mode fracture

Dimensional Changes in CFRP/PMI Foam Core Sandwich Structures by M. John; T. Skala; T. Wagner; R. Schlimper; M. Rinker; R. Schäuble (601-614).
A sandwich structure was observed from the beginning of life—starting at the vacuum assisted manufacturing process up to several environmental in service conditions. Strain variations due to curing process at the manufacturing, well as thermal fluctuations and humidity environments in service were detected and reviewed by their influence on the overall dimensional stability. The CFRP/PMI foam core sandwich structure was checked for an application as a primary structure in commercial aviation with its specific environmental requirements.
Keywords: Foam core sandwich; Cure residual strain; Thermal strain; Hygrothermal strain

Elevating work platforms are hoists equipment that are increasingly used in many applications, like in the construction industry and in the maintenance field. The maintenance of the hub of the wind turbines, for example, can be done through the use of a working platform; these structures have to reach great heights and obviously they have to satisfy the constraints induced by the highway standards, like the maximum axle load and the maximum overall dimensions. To satisfy these requests the material of the structures changed from the classic structural steel (S235 JR, S275 JR or S355JR) to high strength steel (S700 to S1100 or more), characterized by a significantly higher specific resistance. The idea of this paper is to use a composite material for the construction of the arms of an elevating platform in order to reduce the global weight of the machine. The analyses on the new kind of platform show the technical possibility to change the material of the arms with composite materials and this produces a significant reduction of the weight of the machine components, about 50 %. Being a feasibility study, still remain open some problems such as the mechanical behavior of the used composite materials (fatigue, environment effects, etc.).
Keywords: Composite structures design; Carbon fiber; Lifting platform

The mechanical properties of advanced composites are essential for their structural performance, but the surface finish on exterior composite panels is of critical importance for customer satisfaction. This paper describes the application of wavelet texture analysis (WTA) to the task of automatically classifying the surface finish properties of two fiber reinforced polymer (FRP) composite construction types (clear resin and gel-coat) into three quality grades. Samples were imaged and wavelet multi-scale decomposition was used to create a visual texture representation of the sample, capturing image features at different scales and orientations. Principal components analysis was used to reduce the dimensionality of the texture feature vector, permitting successful classification of the samples using only the first principal component. This work extends and further validates the feasibility of this approach as the basis for automated non-contact classification of composite surface finish using image analysis.
Keywords: Carbon fiber; Surface analysis; Image analysis; Wavelet texture analysis

In the present study the reliability estimation of the pultrusion process of a flat plate is analyzed by using the first order reliability method (FORM). The implementation of the numerical process model is validated by comparing the deterministic temperature and cure degree profiles with corresponding analyses in the literature. The centerline degree of cure at the exit (CDOCE) being less than a critical value and the maximum composite temperature (T max) during the process being greater than a critical temperature are selected as the limit state functions (LSFs) for the FORM. The cumulative distribution functions of the CDOCE and T max as well as the correlation coefficients are obtained by using the FORM and the results are compared with corresponding Monte-Carlo simulations (MCS). According to the results obtained from the FORM, an increase in the pulling speed yields an increase in the probability of T max being greater than the resin degradation temperature. A similar trend is also seen for the probability of the CDOCE being less than 0.8.
Keywords: First order reliability method (FORM); Pultrusion process; Reliability analysis; Curing; Computational modeling

The loading/unloading tensile behavior of unidirectional C/SiC ceramic matrix composites at room temperature has been investigated. The loading/unloading stress–strain curve exhibits obvious hysteresis behavior. An approach to model the hysteresis loops of ceramic matrix composites including the effect of fiber failure during tensile loading has been developed. By adopting a shear-lag model which includes the matrix shear deformation in the bonded region and friction in the debonded region, the matrix cracking space and interface debonded length are obtained by matrix statistical cracking model and fracture mechanics interface debonded criterion. The two-parameter Weibull model is used to describe the fiber strength distribution. The stress carried by the intact and fracture fibers on the matrix crack plane during unloading and subsequent reloading is determined by the Global Load Sharing criterion. Based on the damage mechanisms of fiber sliding relative to matrix during unloading and subsequent reloading, the unloading interface reverse slip length and reloading interface new slip length are obtained by the fracture mechanics approach. The hysteresis loops of unidirectional C/SiC ceramic matrix composites corresponding to different stress have been predicted.
Keywords: Ceramic matrix composites; C/SiC; Hysteresis loops; Matrix cracking; Interface debonding; Fiber failure

A general-purpose micromechanics model was developed so that the model could be applied to various composite materials such as reinforced by particles, long fibers and short fibers as well as those containing micro voids. Additionally, the model can be used with hierarchical composite materials. The micromechanics model can be used to compute effective material properties like elastic moduli, shear moduli, Poisson’s ratios, and coefficients of thermal expansion for the various composite materials. The model can also calculate the strains and stresses at the constituent material level such as fibers, particles, and whiskers from the composite level stresses and strains. The model was implemented into ABAQUS using the UMAT option for multiscale analysis. An extensive set of examples are presented to demonstrate the reliability and accuracy of the developed micromechanics model for different kinds of composite materials. Another set of examples is provided to study the multiscale analysis of composite structures.
Keywords: Micromechanics; Composite; Multiscale analysis; Fibrous composite; Particulate composite; Hierarchical composite; Micro void

The tensile-tensile fatigue behavior of unidirectional C/SiC ceramic matrix composites at room and elevated temperature has been investigated. An approach to estimate the interface shear stress of ceramic matrix composites under fatigue loading has been developed. Based on the damage mechanisms of fiber sliding relative to matrix in the interface debonded region upon unloading and subsequent reloading, the unloading interface reverse slip length and reloading interface new slip length are determined by the fracture mechanics approach. The hysteresis loss energy for the strain energy lost per volume during corresponding cycle is formulatd in terms of interface shear stress. By comparing the experimental hysteresis loss energy with the computational values, the interface shear stress of unidirectional C/SiC ceramic composites corresponding to different cycles at room and elevated temperatures has been predicted.
Keywords: Ceramic matrix composites; C/SiC; Fatigue; Interface shear stress; Hysteresis loops; Matrix cracking; Interface debonding

Study on the Effect of Cure Cycle on the Process Induced Deformation of Cap Shaped Stiffened Composite Panels by Yong Miao; Jian-chuan Li; Zhi-hong Gong; Juan Xu; Kai He; Jian Peng; Yi-hua Cui (709-718).
Cap-shaped stiffened composite panels offer many excellent properties such as low density, high strength, high stiffness to weight ratio, and design flexibility. During their manufacturing processes, however, thermo-curing inherently produces the undesired residual stresses and cure deformations, which limits the applications of composite structures in a certain degree. In order to reduce the cure deformation, in this paper, the effect of cure cycle (curing temperature, curing pressure, cooling rate) on the process-induced deformation of cap-shaped stiffened composite panels was presented. A simple mathematical model based on the curing dynamics was established to predict the deformation of the cap-shaped stiffened composite panels. The deformation calculated by the mathematical model and experimental studies were compared, and an Error Correction Model was established. The Error Correction Model showed a good agreement with the experimental results.
Keywords: Cure cycle; Process-induced deformation; Mathematical model

Wavelet coefficients based on spatial wavelets are used as damage indicators to identify the damage location as well as the size of the damage in a laminated composite beam with localized matrix cracks. A finite element model of the composite beam is used in conjunction with a matrix crack based damage model to simulate the damaged composite beam structure. The modes of vibration of the beam are analyzed using the wavelet transform in order to identify the location and the extent of the damage by sensing the local perturbations at the damage locations. The location of the damage is identified by a sudden change in spatial distribution of wavelet coefficients. Monte Carlo Simulations (MCS) are used to investigate the effect of ply level uncertainty in composite material properties such as ply longitudinal stiffness, transverse stiffness, shear modulus and Poisson’s ratio on damage detection parameter, wavelet coefficient. In this study, numerical simulations are done for single and multiple damage cases. It is observed that spatial wavelets can be used as a reliable damage detection tool for composite beams with localized matrix cracks which can result from low velocity impact damage.
Keywords: Spatial wavelet; Matrix crack; Gabor wavelet; Damage detection; Composite beam; Monte Carlo Simulation