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This work used short carbon fibers (SCFs) of different lengths to manufacture epoxy bulk mold compound (BMC) composites. Drop-weight-impact tests were performed on the intact carbon-fiber-reinforced-polymers (CFRPs) laminates to generate artificial damage. The damaged CFRP laminates were then repaired using the SCF-BMC composites. Compression after impact (CAI) tests were conducted to measure the compressive strength of the repaired laminates. Tensile properties and compressive properties of the SCF-BMC composites were also measured. The results show that 48-mm carbon fibers result in the largest tensile modulus, tensile strength, and compressive strength, while 12-mm carbon fibers result in the largest compressive modulus. CAI test results show that the compressive strength of the repaired laminate can be restored to 80.3% of that of the undamaged laminate. Therefore, SCF-BMC composites significantly contribute to repairing damaged composite laminates.

This paper considers a vibration-based damage assessment approach which is based on the general idea for signal cross-correlation. Here the method is demonstrated and validated on a composite laminate beam and it is applied for the purposes of delamination assessment in composite structures. The method uses two measures of cross-correlation between two vibration signals measured in different points on the structure in order to diagnose the delamination. The linear cross-correlation as well as a new measure for nonlinear cross-correlation, the mutual information, are introduced and applied for the purposes of delamination assessment. The delamination assessment is based on the comparison of the measures for the healthy and the damaged state of the structure. In this study, the method is applied using the free decay responses of the beam. Two delamination indices are introduced and they are used for the purposes of delamination detection and localization.

Buckling loads of laminated panels calculated by analytical approaches are usually based on the assumptions that the panels are subjected to uniform in-plane edge loads without cutouts, despite of the fact that real structural components are subjected to various kinds of non-uniform in-plane edge loads along with different sized cutouts. The main objective of this paper is to study the effects of reinforced/unreinforced circular cutouts and non-uniform in-plane edge loads on the buckling behavior of composite panels with different ply-orientations by the finite element technique. Furthermore, it addresses the effects of different boundary conditions and thickness of panels. To carry out the analyses, a nine-noded heterosis plate element and a compatible three-noded beam element are developed, including the effect of shear deformation and rotary inertia for both the plate and the stiffeners. In structural modeling, the plate and the stiffener elements are treated separately, with their displacement compatibility maintained using transformation matrices. It has been illustrated in this study that presence of larger-sized reinforced cutouts predominantly increases the buckling strength of the panel as compared to those with smaller sized cutouts.

The stability characteristics of web panels in two built-up members, i.e. box-sections and I-sections, are investigated here using a higher order plate bending element. At the beginning, the critical buckling loads of rectangular flat panels under various theoretical states-of-stress (pure shear, in-plane bending, shear and in-plane bending and combined stresses) and boundary conditions (simply supported and restrained by flange panels) are investigated. Then, the buckling behavior of an isolated flat panel is correlated with the stability characteristics of the web panels of built-up members under various loading conditions. The results on the stability behavior of the web panels are presented in nondimensional form for easy reference to designers and researchers.

The extended finite element method (X-FEM) is applied to the stress analysis of composite laminates having interlaminar planar delamination. In X-FEM analysis, the geometry of such delaminations can be modeled independent of the finite elements. The domain form of the contour integral can be used to compute the energy release rate in conjunction with X-FEM. As numerical examples, three-dimensional analyses for DCB and ENF test specimens were performed by X-FEM with various enrichment nodes, and the obtained results were examined. In addition, a model of the no-friction-contact condition by X-FEM was proposed and applied to ENF test analysis. Moreover, eigenvalue buckling analyses of a CFRP plate with delamination were performed by X-FEM as a practical example related to Compression After Impact (CAI) problems of composite materials. The numerical results show that X-FEM is an effective method for analyzing stress in composite laminates with delamination.

In this study, 3D thermal stresses in composite laminates under steady-state through thickness thermal conduction are investigated by means of a stress function-based approach. One-dimensional thermal conduction is solved for composite laminate and the layerwise temperature distribution is calculated first. The principle of complementary virtual work is employed to develop the governing equations. Their solutions are obtained by using the stress function-based approach, where the stress functions are taken from the Lekhnitskii stress functions in terms of in-plane stress functions and out-of-plane stress functions. With the Rayleigh–Ritz method, the stress fields can be solved by first solving a standard eigenvalue problem. The proposed method is not merely computationally efficient and accurate. The stress fields also strictly satisfy the prescribed boundary conditions validated by the results of finite element method (FEM) results. Finally, some of the results will be given for discussion considering different layup stacking sequences, thermal conductivities and overall temperature differences. From the results, we find that the thermal conductivity greatly affects the stress distributions and peak values of stresses increase linearly for the present model. The proposed method can be used for predicting 3D thermal stresses in composite laminates when subjected to thermal loading.

In this paper, two-dimensional (2D) orthotropic augmented finite element method (A-FEM) is applied to account for progressive failure of composite laminates under transverse loading, which considers all major cracking modes (delamination, fiber kinking/rupture matrix cracking). High-fidelity simulations of different stacking composite laminates under transverse loading are implemented. Both predicted load−deflection curves and damage evolution are in good agreement with that of experimental results, which demonstrates the numerical capability of A-FEM. In addition, the influence of stacking sequence on the failure mechanism is also studied by predicted damage evolution of laminates with different stacking sequence ${[90_2^{\circ }/0_2^{\circ }]}_{4S}$, ${[90_4^{\circ }/0_4^{\circ }]}_{2S}$ and ${[90_8^{\circ }/0_8^{\circ }]}_S$. Results show that the tensile matrix crack in the bottom laminar is always the first damage mode for the composite laminate, and the subsequent crack propagation is related to ply orientation of adjacent plies which have a blocking effect on crack propagation.

Carbon fiber-reinforced plastic (CFRP) composite materials are extensively used in engineering applications. These materials are used in the form of laminates for aerospace structures. Structures constructed of composite laminates may acquire impact damage, for example in the form of delamination. Therefore, clarification of the damage mechanism of composite laminates for structural design using composite materials is very important. In analytical studies, finite element models that consider delaminations in composite laminates have been used. Considerable effort and time are usually required in order to prepare the finite element meshes for creating models. In particular, for structures containing discontinuities such as delaminations the meshes cannot be constructed easily, even if the automatic mesh generation technique is used. Recently, Belytschko et al. proposed the extended finite element method (X-FEM) based on the concept of partition of unity. They applied this method to the evaluation of stress intensity factors and performed crack extension simulation. X-FEM can be used to simplify the modeling of continua containing several cracks and hence can be used to perform effective stress analyses related to fracture mechanics. In the present study, X-FEM is applied to buckling analyses of composite laminates with holes and delaminations. The interpolation functions of solid finite elements used in three-dimensional analysis are extended to perform eigenvalue analyses for buckling loads in composite laminates. The numerical results show that X-FEM is an effective method in performing buckling analyses in composite laminates with a hole and with a delamination, respectively.