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Applied Composite Materials: An International Journal for the Science and Application of Composite Materials (v.15, #4-6)
Tensile Performance and Damage Evolution of a 2.5-D C/SiC Composite Characterized by Acoustic Emission by Yiqiang Wang; Litong Zhang; Laifei Cheng; Junqiang Ma; Weihua Zhang (pp. 183-188).
The tensile behavior of a unique 2.5 D C/SiC composite and associated damage evolution was characterized by means of acoustic emission (AE) technique. The results show that the stress–strain curves exhibit mostly nonlinear behavior, which corresponds well with the AE activities. All the evolutions of damage characteristics indicate the phenomenon of matrix cracking saturation which is in agreement with AE results. Microstructural observations reveal extensive matrix cracks and the fracture of yarns always occurred in yarn crossover areas due to the unique weave architecture.
Keywords: Mechanical properties; Damage; Acoustic emission; Fiber architecture
Delamination Damage Analyses of FRP Composite Spar Wingskin Joints with Modified Elliptical Adhesive Load Coupler Profile by S. K. Panigrahi; B. Pradhan (pp. 189-205).
Three-dimensional non-linear finite element analyses (FEA) for delamination damage onset and its growth in Graphite Fiber Reinforced Plastic (GFRP) composite Spar Wingskin Joints (SWJ) with modified elliptical adhesive load coupler profile for varied ratios of base width to height of the spar have been presented in this paper. Both in-plane and out-of-plane normal and shear stress variations on the interfacial surface of the wingskin between the spar and the wingskin have been evaluated. Coupled stress failure criterion has been used to predict the locations of initiation of failures due to delamination induced damages. Based on the stress and delamination damage analyses, suitable geometry of the modified elliptical adhesive load coupler profile of the SWJ has been recommended. The delamination damage has been observed to be initiated from the toe-end of the interfacial surface of the spar and the wingskin of the SWJ. Subsequently, the delamination propagations have also been studied by calculating the individual and the total Mode of Strain Energy Release Rate (SERR) along the delamination front using Modified Crack Closure Integral (MCCI) technique based on Linear Elastic Fracture Mechanics (LEFM) approach. It is seen that SERR variations along the delamination front i.e. across the width of the SWJ are not uniform. Therefore, a straight delamination front may grow into a curved delamination front as the delamination propagates. Also, it is found that Mode I SERR (G I) governs the delamination propagation predominantly for the SWJ. Accordingly, suitable delamination arresting mechanism has been suggested.
Keywords: GFRP; Delamination; FEA; Spar wingskin joints
Curing Deformation Analysis for the Composite T-shaped Integrated Structures by Jun Li; XueFeng Yao; YingHua Liu; ShenShen Chen; ZheJun Kou; Di Dai (pp. 207-225).
Curing deformation of the T-shaped integrated structures is discussed in this paper. The mechanism of the deformation is analyzed for the T-shaped integrated structures, and a simple mathematical model for the deformation of the T-shaped integrated structures is established. Compare the mathematical model with the finite element analysis, the results show a good agreement. From the simple mathematical model, it can be seen that both cure shrinkage and thermal expansion are the major factors to produce the deformation of the typical T-shaped integrated structures and the tool-part contraction is the secondary factor. Therefore, it is important for the T-shaped integrated structures to select suitable fabrication process and the appropriate tool. The different geometry and material parameters of the T-shaped integrated structures are studied, and then a regression model is established.
Keywords: Cure; Warpage deformation; Spring-in; Regression analysis
Comparison of Tensile Strength of Composite Material Elements with Drilled and Molded-in Holes by A. Langella; M. Durante (pp. 227-239).
Holes are generally obtained through drilling operations; this causes a property decrease for polymer composites reinforced by fibers, brought about by damage due to fiber continuity interruption, and to delamination between the laminate layers. In this study, specimens with circular holes, both drilled and molded-in, obtained in different ways, are tested in order to investigate on whether it is possible to avoid the decrease in mechanical properties of components with holes. In particular, a number of laminates were manufactured by RIFT (Resin Infusion under Flexible Tool), a closed mold process capable of obtaining large and complex forms, impregnating, under vacuum, a dry preform placed on the rigid mold. At specific points of these laminates, molded-in holes are generated during the resin infusion phase, operating in two different ways: displacing or cutting the fibers in the dry preform. Tensile tests were carried out in order to compare the mechanical properties of elements in composite materials which have molded-in holes generated during the impregnation process, with the properties of those with holes produced after the resin cure by drill operations.
Keywords: Drilled hole; Molded-in hole; Infusion process
Analysis of Bird Impact on a Composite Tailplane Leading Edge by M. Guida; F. Marulo; M. Meo; M. Riccio (pp. 241-257).
One of the main structural requirements of a leading edge of a tailplane is to ensure that any significant damage caused by foreign object (i.e. birdstrike, etc...) would still allow the aircraft to land safely. In particular, leading edge must be certified for a proven level of bird impact resistance. Since the experimental tests are expensive and difficult to perform, numerical simulations can provide significant help in designing high-efficiency bird-proof structures. The aim of this research paper was to evaluate two different leading edge designs by reducing the testing costs by employing state-of-the-art numerical simulations. The material considered was a sandwich structure made up of aluminium skins and flexcore as core. Before each test was carried out, pre-test numerical analyses of birdstrike were performed adopting a lagrangian approach on a tailplane leading edge of a large scale aircraft using the MSC/Dytran solver code. The numerical and experimental correlation have shown good results both in terms of global behaviour of the test article and local evolution of some measurable parameters confirming the validity of the approach and possible guidelines for structural design including the bird impact requirements.
Keywords: Birdstrike; Finite element analysis; Aircraft tailplane leading edge
Thermoelastic Characterization and Evaluation of Residual Stresses in Bi-Directional Fibrous Composites by G. Karami; N. Grundman; N. Abolfathi; A. Naik (pp. 259-272).
The thermoelastic behavior of bi-directional fibrous composites will be studied through the use of a finite element-based micromechanical model. The model is used to study the effect of the crossing angle of the fibers on the composite’s coefficient of thermal expansion (CTE) and the residual thermal stresses that develop after a temperature change. The effect of the fiber volume fraction (V f ) on the same results is also studied. For anisotropic materials, one can see that in addition to normal strains, shear strains will also be developed due to temperature change. This method will lend itself to evaluate the coefficients of thermal expansions not only due to normal expansion, but also due to shear expansion for composites with no principal directions. In this micromechanical model, parallelepiped unit cells incorporating the fibers at different cross angles are created to represent the periodic microstructure of the angular bi-directional composite. The volume averaged stresses per unit temperature of the individual constituents are used to study the residual thermal stresses that develop. Two different sets of materials are used to test this model. Results show that when the fiber’s cross angle is not 0o or 90o, shear strains are created. Also, residual stresses in the unit cell are functions of the cross angle between the fibers.
Keywords: Micromechanics; Fibrous composites; Bidirectional; Coefficient of thermal expansion (CTE); Residual stresses
Predicting the Crushing Behavior of Axially Loaded Elliptical Composite Tubes Using Artificial Neural Networks by Hany El Kadi (pp. 273-285).
In this research work, the artificial neural networks (ANN) technique is used in predicting the crushing behavior and energy absorption characteristics of axially-loaded glass fiber/epoxy composite elliptical tubes. Predictions are compared to actual experimental results obtained from the literature and are shown to be in good agreement. Effects of parameters such as network architecture, number of hidden layers and number of neurons per hidden layer are also considered. The study shows that ANN techniques can effectively be used to predict the crushing response and the energy absorption characteristics of elliptical composite tubes with various ellipticity ratios subjected to axial loading.
Keywords: Composite tubes; Artificial neural networks; Crushing behavior; Ellipticity ratio