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The main objective of this research is to introduce a new coupled analytical–numerical procedure to calculate stiffness reduction due to matrix cracks in composite laminates subjected to general loading conditions. A novel micromodel is developed for transverse cracking of general symmetric laminates based on a variational approach. The proposed method overcomes the restrictions of previous similar approaches regarding damage in laminates with general symmetric stacking sequences. A polynomial series is used for calculating the stress perturbation due to intralaminar cracks in conjunction with the principle of minimum complementary energy. The numerical part deals with well-known Newton–Raphson procedure to solve the governing equations for the unknown parameters for the stress functions. Furthermore, the analytical part can be solved just one time by a symbolic calculation for unknown geometry parameters, material properties, and loading conditions, which has the benefit to be ready for Finite Element software implementations. Comparison between the existing experimental and analytical results and the result of the present study are in good agreement. [ABSTRACT FROM AUTHOR]

Critical energy release rate for delamination initiation in composites as a material property, supposed to be independent from non-material variables. However, a thorough literature review presented in this study shows that in many cases it may vary with the variation of layup configuration or geometrical and dimensions. This study is aimed to investigate the effect of interface layers orientation on fracture toughness by eliminating the other influential parameters such as stacking sequence, by selecting the anti-symmetric layup configuration of Double Cantilever Beam, [±θ/010/±θ]as, in which θ will be 0°, 30°, 45° and 60°. The energy release rates data have been calculated using different criteria and techniques to obtain the load and displacement at initial crack growth and the results were compared with the standard methods. The damage zone near the crack tip is also illustrated before and after the crack propagation by microscopic images of delamination front, and discussed for all investigated interface fiber angles. Experimental results show that the effect of interface layers orientation on fracture toughness of the investigated layup configurations based on the nonlinear technique as a standard procedure is negligible while other techniques show a considerable changes in the calculated energy release rate with the increase of interface layers angle from zero to 60 degrees. [ABSTRACT FROM AUTHOR]

Abstract: One of the main disadvantages of using composite patches in repair of metallic panels is development of thermal residual stresses due to the curing cycles of the bonded repairs. Fiber Metal Laminates (FMLs) have the overall larger thermal expansion coefficients when compared with the fully composite patches. In this study, the thermal residual stresses that occur due to the thermal expansion coefficient mismatch between the FML patch and underlying cracked aluminum panel are investigated using both numerical and experimental studies. The fatigue crack growth behaviors of centrally cracked aluminum panels in mode-І condition with single-side repairs of FML patches that are made by alternating layers of aluminum and glass fiber-epoxy are carried out. Sensitivity of the curing temperature on fatigue crack-growth life extension and crack-front shape of the repaired panels is also investigated. It is shown that implementing of various curing temperatures do not significantly affect the thermal residual stresses, crack-growth lives and crack-front shapes of the repaired panels by FML patches. [Copyright &y& Elsevier]

This paper focuses on the fiber-kinking failure mode of unidirectional laminated composites under the compressive loading. An available stress based fiber-kinking model is explained and improved on the bases of strain concept. In the improved model, a new fracture surface is considered and the stresses are updated according to this new fracture surface. By taking the advantage of damage variables, the models are implemented into a finite element code and the results of numerical analysis such as prediction of kink band angles are discussed in details and compared with the available results in the literature. It is shown that the predicted kink band angles using the improved model are in good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a numerical investigation on the buckling of composite laminates containing delamination, under in-plane compressive loads, is presented. For this purpose, delamination propagation is modeled using the softening behavior of interface elements. The full layerwise plate theory is applied for approximating the displacement field of laminates and the interface elements are considered as a numerical layer between any two adjacent layers where the delamination is expected to propagate. A non-linear computer code was developed to handle the numerical procedure of delamination buckling growth in composite laminates using layerwise-interface elements. The load/displacement behavior and the contours of embedded and through-the-width delamination propagation for composite laminates are presented. It is shown that delamination growth can be well predicted using this layerwise-interface elements with decohesive law. [Copyright &y& Elsevier]

Abstract: Thermal residual stresses, due to the curing cycle of the bonded composite patch affects the stress/strain states and the crack patching efficiency of the single-side repaired aluminum panels. In this study the thermal residual stresses that occur as a result of the thermal expansion coefficient mismatch between the patch and the underlying cracked aluminum panel are investigated. The fatigue crack-growth life and the crack-front configuration of centrally cracked aluminum panels in mode-I condition with various single-side repairs of glass/epoxy, graphite/epoxy and boron/epoxy composite patches are curried out. Sensitivity of the obtained residual stresses to different curing temperatures of the repaired panels with various composite patches is also investigated. It is shown that low curing temperatures (e.g., 60°C) with long curing cycles have not a considerable effect on fatigue crack-growth life of the repaired panels with glass/epoxy patch and have minor effects for repaired panels with graphite/epoxy and boron/epoxy composite patches. It is also shown that considering the thermal residual stresses, the obtained FEM fatigue life and crack-front shapes of the repaired panels using glass/epoxy patch are in good agreement with those obtained from the experiments. [Copyright &y& Elsevier]

Abstract: Three-dimensional analyses are conducted to study the fatigue crack growth of panels containing mixed-mode cracks, reinforced with composite patches. A procedure is developed to obtain the crack trajectories using dynamic mesh generation for repaired panels in both mode-I and mixed-mode conditions. It is shown that the fatigue crack growth life obtained using mid-plane fracture parameters of the repaired panels in mode-I condition are almost compatible with the experimental results, however, they are non-conservative in some cases. The calculated fatigue crack growth life using un-patched surface fracture parameters are too conservative. It is shown that in both mode-I and mixed-mode crack loading conditions the J-integral values at mid-plane of the cracked plates are reduced with increasing the patch thickness; however, they depend on the patch thickness at un-patched surface at the crack-tip. The crack propagation path may be changed by the amount of about 10% due to the various fibers orientations of the patch. It is also shown that the patch thickness has not a considerable effect on the crack propagation path. Furthermore, there are considerable differences between the life obtained using un-patched surface and mid-plane fracture parameters for various patches thicknesses and layers angles of the repaired panels. The optimum patch layer angle considering maximum fatigue crack growth life of the repaired panels with an inclined crack angel of 45° is also obtained. Finally, a simple formulation is presented to justify the trend of finite element results obtained for single-sided repaired panels with various patch thicknesses. [Copyright &y& Elsevier]