Applied Composite Materials (v.20, #5)

In this paper, the effect of various aging environments on the painted surface finish of unidirectional carbon fibre composite laminates, manufactured by autoclave and a novel out-of-autoclave technique was investigated. Laminates were exposed to water immersion, 95 % relative humidity and cyclic environments for 552 h and the surface finish was evaluated using visual and wave-scan distinctness of image (DOI) techniques. It was found that the laminate surface finish was dependent on the amount of moisture in the aging test. Minor surface waviness occurred on the laminates exposed to the cyclic test, whereas, surface waviness, print through and DOI values were all significantly higher as the laminates absorbed larger quantities of moisture from the hygrothermal and hydrothermal tests. The water immersion test, which was the most detrimental to the surface finish of the painted laminates, produced dense blistering on the autoclave manufactured laminate surface whereas the out-of-autoclave laminate surface produced only a few. It was found that the out-of-autoclave laminate had high substrate surface roughness which resulted in improved paint adhesion and, therefore, prevented the formation of surface blistering with aging.
Keywords: Carbon fibre composites; Epoxy; Environmental degradation; Aging; Surface analysis; Automotive

A series of finite element analysis on the cracked composite cylindrical shells under combined loading is carried out to study the effect of loading condition, crack size and orientation on the buckling behavior of laminated composite cylindrical shells. The interaction buckling curves of cracked laminated composite cylinders subject to different combinations of axial compression, bending, internal pressure and external pressure are obtained, using the finite element method. Results show that the internal pressure increases the critical buckling load of the CFRP cylindrical shells and bending and external pressure decrease it. Numerical analysis show that axial crack has the most detrimental effect on the buckling load of a cylindrical shell and results show that for lower values of the axial compressive load and higher values of the external pressure, the buckling is usually in the global mode and for higher values of axial compressive load and lower levels of external pressure the buckling mode is mostly in the local mode.
Keywords: Crack; Laminated cylindrical shells; Buckling analysis; Internal and external pressure; Axial compression; Bending moment

Analysis of Crushing Response of Composite Crashworthy Structures by Matthew David; Alastair F. Johnson; H. Voggenreiter (773-787).
The paper describes quasi-static and dynamic tests to characterise the energy absorption properties of polymer composite crash energy absorbing segment elements under axial loads. Detailed computer tomography scans of failed specimens are used to identify local compression crush failure mechanisms at the crush front. The varied crushing morphology between the compression strain rates identified in this paper is observed to be due to the differences in the response modes and mechanical properties of the strain dependent epoxy matrix. The importance of understanding the role of strain rate effects in composite crash energy absorbing structures is highlighted in this paper.
Keywords: Composite structures; Crashworthiness; Energy absorption; Crush test method; Composites failure modes; Dynamic crushing response

In this study, the Fiber-Metal Laminates (FMLs) containing glass fiber reinforced polypropylene (GFPP) and aluminum (Al) sheet were consolidated with Al foam cores for preparing the sandwich panels. The aim of this article is the comparison of the flexural properties of FML/Al foam sandwich panels bonded with various surface modification approaches (silane treatment and combination of silane treatment with polypropylene (PP) based film addition). The FML/foam sandwich systems were fabricated by laminating the components in a mould at 200 °C under 1.5 MPa pressure. The energy absorbtion capacities and flexural mechanical properties of the prepared sandwich systems were evaluated by mechanical tests. Experiments were performed on samples of varying foam thicknesses (8, 20 and 30 mm). The bonding among the sandwich components were achieved by various surface modification techniques. The Al sheet/Al foam sandwiches were also consolidated by bonding the components with an epoxy adhesive to reveal the effect of GFPP on the flexural performance of the sandwich structures.
Keywords: Fiber/Metal Laminates (FML); Al foam; Sandwich composites; Interface; Flexural behavior

As new lightweight textile material, 3D five directional and full five directional braided composites (5DBC and F5DBC) have tremendous potential applications in the aerospace industry. Before they are used in primary loading-bearing structures, a rational characterization of their mechanical properties is essential. In this paper, three types of unit-cell models corresponding to the interior, surface and corner regions of 5DBC and F5DBC are proposed. By introducing the reasonable boundary conditions, the effective stiffness properties of these two materials are predicted and compared by the three unit-cells models. The detailed mechanical response characteristic of the three unit-cell models is presented and analyzed in various loading cases. Numerical results show good agreement with experiment data, thus validates the proposed simulation method. Moreover, a parametric study is carried out for analyzing the effects of braiding angle and fiber volume fraction on the elastic properties of 5DBC and F5DBC. The obtained results can help designers to optimize the braided composite structures.
Keywords: Textile composites; Braiding; Three unit-cells; Mechanical properties; Stress distribution; Finite element analysis

A three-dimensionally integrated microstrip antenna (3DIMA) is a microstrip antenna woven into the three-dimensional woven composite for load bearing while functioning as an antenna. In this study, the effect of weaving direction of conductive yarns on electromagnetic performance of 3DIMAs are investigated by designing, simulating and experimental testing of two microstrip antennas with different weaving directions of conductive yarns: one has the conductive yarns along the antenna feeding direction (3DIMA-Exp1) and the other has the conductive yarns perpendicular the antenna feeding direction (3DIMA-Exp2). The measured voltage standing wave ratio (VSWR) of 3DIMA-Exp1 was 1.4 at the resonant frequencies of 1.39 GHz; while that of 3DIMA-Exp2 was 1.2 at the resonant frequencies of 1.35 GHz. In addition, the measured radiation pattern of the 3DIMA-Exp1 has smaller back lobe and higher gain value than those of the 3DIMA-Exp2. This result indicates that the waving direction of conductive yarns may have a significant impact on electromagnetic performance of textile structural antennas.
Keywords: 3-Dimensional reinforcement; Electromagnetic performance; Computational modeling; Microstrip antenna

In this work, a new family of sandwich structures with fiber metal laminate (FML) faces is investigated. FMLs have benefits over both metal and fiber reinforced composites. To investigate the bending properties of sandwich beams with FML faces and compare with similar sandwich beams with fibrous composite faces, 6 groups of specimen with different layer arrangements were made and tested. Results show that FML faces have good resistance against transverse local loads and minimize stress concentration and local deformations of skin and core under the loading tip. In addition, FML faces have a good integrity even after plateau region of foam cores and prevent from catastrophic failures, which cannot be seen in fibrous composite faces. Also, FML faces are lighter than metal faces and have better connection with foam cores. Sandwich beams with FML faces have a larger elastic region because of simultaneous deformation of top and bottom faces and larger failure strain thanks to good durability of FMLs. A geometrical nonlinear classical theory is used to predict force-deflection behavior. In this model an explicit formula between symmetrical sandwich beams deflections and applied force which can be useful for designers, is derived. Good agreement is obtained between the analytical predictions and experimental results. Also, analytical results are compared with small deformation solution in a parametric study, and the effects of geometric parameters on difference between linear and nonlinear results are discussed.
Keywords: Sandwich structures; Fiber metal laminates; Flexural properties; Large deformation; Parametric study

CCF300/BMI composites are relevant materials for supersonic aircraft due to their high specific properties. However in aeronautical applications, the composites are exposed to severe environmental conditions, and it is known that hot and humid environments can degrade some aspects of the material performance especially the compressive strength. In this paper, the effect of moisture and temperature on the compressive failure of unidirectional CCF300 carbon fiber reinforced bismaleimide(BMI) matrix composites were studied. Also scanning electron microscope (SEM) was employed for fractographic investigations. It is observed that the plastic deformations at the fiber/matrix and interlaminar interface as well as residual stresses lower the compressive strength of the material. The failure of specimens tested in hot and wet conditions always occurs as a result of out-of-plane microbuckling that is attributed to the reduction of matrix strength. In addition, the fiber microbuckling model, fiber kinking model and combined model were employed for the compressive strength prediction of the UD CCF300/QY8911 composites subjected to different environment conditions. The comparison was done between these models. Results show that the combined model is more suitable for the compressive strength prediction of CCF300/QY8911 composite systems when suffering severe environment conditions.
Keywords: Moisture and temperature; Unidirectional; Prediction; Compressive strength; Composites

Optimization of Composite Material System and Lay-up to Achieve Minimum Weight Pressure Vessel by Haris Hameed Mian; Gang Wang; Uzair Ahmed Dar; Weihong Zhang (873-889).
The use of composite pressure vessels particularly in the aerospace industry is escalating rapidly because of their superiority in directional strength and colossal weight advantage. The present work elucidates the procedure to optimize the lay-up for composite pressure vessel using finite element analysis and calculate the relative weight saving compared with the reference metallic pressure vessel. The determination of proper fiber orientation and laminate thickness is very important to decrease manufacturing difficulties and increase structural efficiency. In the present work different lay-up sequences for laminates including, cross-ply [0 m /90 n ] s , angle-ply [±θ] ns , [90/±θ] ns and [0/±θ] ns , are analyzed. The lay-up sequence, orientation and laminate thickness (number of layers) are optimized for three candidate composite materials S-glass/epoxy, Kevlar/epoxy and Carbon/epoxy. Finite element analysis of composite pressure vessel is performed by using commercial finite element code ANSYS and utilizing the capabilities of ANSYS Parametric Design Language and Design Optimization module to automate the process of optimization. For verification, a code is developed in MATLAB based on classical lamination theory; incorporating Tsai–Wu failure criterion for first-ply failure (FPF). The results of the MATLAB code shows its effectiveness in theoretical prediction of first-ply failure strengths of laminated composite pressure vessels and close agreement with the FEA results. The optimization results shows that for all the composite material systems considered, the angle-ply [±θ] ns is the optimum lay-up. For given fixed ply thickness the total thickness of laminate is obtained resulting in factor of safety slightly higher than two. Both Carbon/epoxy and Kevlar/Epoxy resulted in approximately same laminate thickness and considerable percentage of weight saving, but S-glass/epoxy resulted in weight increment.
Keywords: Composite pressure vessel; FEA; Classical lamination theory; Optimization; ANSYS

The effect of holes on the fatigue life of a non-oxide ceramic composite processed via chemical vapor infiltration (CVI) was examined at 1,200 °C in laboratory air and in steam. The effect of holes on tensile strength at 1,200 °C was also evaluated. The composite comprised laminated woven Hi-Nicalon™ fibers in an oxidation inhibited matrix, which consisted of alternating layers of silicon carbide and boron carbide. Fiber preforms had pyrolytic carbon fiber coating with boron carbon overlay applied. Unnotched specimens and specimens with a center hole having a radius to width ratio of 0.24 were tested in tension-tension fatigue at 0.1 Hz and at 1.0 Hz. The fatigue stresses ranged from 100 to 140 MPa in air and in steam. Fatigue run-out was defined as 105 cycles at 0.1 Hz and as 2 × 105 cycles at 1.0 Hz. The net-section strength was less than the unnotched ultimate tensile strength. Comparison of notched and unnotched data also revealed that the fatigue performance was notch insensitive in both air and steam environments. Composite microstructure, as well as damage and failure mechanisms were investigated.
Keywords: Ceramic-matrix composites (CMCs); Fatigue; High-temperature properties; Mechanical properties; Fractography; Notch sensitivity

Effect of Compaction and Preforming Parameters on the Compaction Behavior of Bindered Textile Preforms for Automated Composite Manufacturing by Wangqing Wu; Binyan Jiang; Lei Xie; Florian Klunker; Santiago Aranda; Gerhard Ziegmann (907-926).
The effect of compaction and preforming parameters on the Fiber Volume Fraction (FVF) and the Residual Preform Thickness (RPT) of bindered textile preforms during a compaction experiment was investigated by using Taguchi method. Four compaction and preforming parameters of compaction temperature (A), binder activation temperature (B), binder content (C) and binder activation time (D) were selected and optimized with respect to the FVF at specified compaction pressure (0.2 MPa) and the RPT after compaction. The results reveal that the compaction behavior of bindered textile preforms has been significantly influenced due to the presence of preforming binder. From all the selected experiment parameters the compaction temperature is the most influential factors on the FVF and RPT. The significant sequence of the parameters for the resulting FVF can be concluded as ABDC, which represents compaction temperature, binder activation temperature, binder activation time and binder content respectively, while this sequence is changed as ADCB as far as the RPT is concerned. The FVF during compaction and RPT during release were correlated with the compaction and preforming parameters using a modified four-parameter-compaction-model which has been proposed for describing the compaction behavior of bindered textile preforms.
Keywords: A. Compaction behavior; B. Taguchi method; C. Bindered textile preforms; E. Composites manufacturing

In this study, the effects of processing temperature and vacuum applying rate on the forming quality of C-shaped carbon fiber reinforced epoxy resin matrix composite laminates during hot diaphragm forming process were investigated. C-shaped prepreg preforms were produced using a home-made hot diaphragm forming equipment. The thickness variations of the preforms and the manufacturing defects after diaphragm forming process, including fiber wrinkling and voids, were evaluated to understand the forming mechanism. Furthermore, both interlaminar slipping friction and compaction behavior of the prepreg stacks were experimentally analyzed for showing the importance of the processing parameters. In addition, autoclave processing was used to cure the C-shaped preforms to investigate the changes of the defects before and after cure process. The results show that the C-shaped prepreg preforms with good forming quality can be achieved through increasing processing temperature and reducing vacuum applying rate, which obviously promote prepreg interlaminar slipping process. The process temperature and forming rate in hot diaphragm forming process strongly influence prepreg interply frictional force, and the maximum interlaminar frictional force can be taken as a key parameter for processing parameter optimization. Autoclave process is effective in eliminating voids in the preforms and can alleviate fiber wrinkles to a certain extent.
Keywords: Thermosetting composite; Hot diaphragm forming process; C-shaped; Prepreg; Defect

R-Curve Behavior of Si3N4/BNNT Composites by Hang-Hai Yu; Shou-Ren Wang; Li-Ying Yang (947-960).
The crack propagation rsistance behavior of Si3N4 ceramics reinforced by boron nitride nanotubes (BNNT) has been discussed in the work. And, bending strength and fracture toughness of Si3N4/BNNT composites were tested by three point bending method. It is shown that crack propagation resistance of BNNT/Si3N4 composites is increased distinctly owing to addition of BNNT. It is attributed to the pinning and bridging roles of BNNT. One kind of mathematical model was constructed for calculating crack propagation resistance of Si3N4 ceramics and BNNT/Si3N4 composites. Crack resistance curve (R-Curve) of Si3N4 ceramics and BNNT/Si3N4 composites was also calculated. Crack propagation of them was simulated using finite element methods. The results show that strong shielding area is formed in crack tip owing to existence of BNNT and crack propagation is prevented by strong stress shielding roles.
Keywords: Si3N4 ceramics; Boron nitride nanotubes; Composites; Finite element; Crack propagation resistance behavior

Infrared thermography is a powerful non-destructive testing technique which can be used for the detection of damage in advanced materials such as ceramic matrix composites. The purpose of this study is to apply a non-destructive methodology for analyzing, in real-time, the thermal effects in ceramic matrix composites caused by cyclic loading. Mechanical stresses induced by cyclic loading cause heat release in the composite due to failure of the interface, which results in increasing the material’s temperature. The heat waves, generated by the thermo-mechanical coupling, and the intrinsic energy dissipated during mechanical cyclic loading of the specimen, were detected by an infrared camera. The results were correlated with acoustic emission events that occurred during the damage accumulation process of the material.
Keywords: Glass Matrix Composites; Infrared Thermography; Acoustic Emission; Damage evolution

Damage Model and Progressive Failure Analyses for Filament Wound Composite Laminates by Marcelo Leite Ribeiro; Dirk Vandepitte; Volnei Tita (975-992).
Recent improvements in manufacturing processes and materials properties associated with excellent mechanical characteristics and low weight have made composite materials very attractive for application on civil aircraft structures. However, even new designs are still very conservative, because the composite failure phenomenon is very complex. Several failure criteria and theories have been developed to describe the damage process and how it evolves, but the solution of the problem is still open. Moreover, modern filament winding techniques have been used to produce a wide variety of structural shapes not only cylindrical parts, but also “flat” laminates. Therefore, this work presents the development of a damage model and its application to simulate the progressive failure of flat composite laminates made using a filament winding process. The damage model was implemented as a UMAT (User Material Subroutine), in ABAQUSTM Finite Element (FE) framework. Progressive failure analyses were carried out using FE simulation in order to simulate the failure of flat filament wound composite structures under different loading conditions. In addition, experimental tests were performed in order to identify parameters related to the material model, as well as to evaluate both the potential and the limitations of the model. The difference between numerical and the average experimental results in a four point bending set-up is only 1.6 % at maximum load amplitude. Another important issue is that the model parameters are not so complicated to be identified. This characteristic makes this model very attractive to be applied in an industrial environment.
Keywords: Damage model; Composite laminates; Progressive failure analysis; Filament winding