A closed-form approximate solution for the total strain energy release rate (G) of an elliptically shaped delamination has been developed from thin-film assumptions that treat the delamination as a clamped plate subject to enforced boundary displacements. Approximate equations are derived which couple the large deflection membrane stresses to the out-of-plane deflections. The model restricts the boundary to remain elliptical as the delamination grows. By applying the simple failure criterion that rapid growth of the delamination (which occurs when G reaches a critical value) precipitates the final failure of the laminate, the predicted failure stresses can be correlated with post-impact strength data available in the literature. The Mode I critical G is required for a good correlation, although the model calculates the total G averaged around the perimeter. For impact damage, data on delamination depth are rarely available. However, it was found that by assuming a delamination interface that results in the lowest failure strain, correlation with test data was obtained.
The behavior of plain woven fabric composites is studied using three-dimensional finite elements which allows detailed modeling of the geometric complexities and spatial material variations within the fabric. Damages in the composite constituents viz. yarn and pure matrix are modeled on a continuum basis and related to their material constitutive behavior. The 3D constitutive laws describing pure matrix and yarn behavior are developed using a damage mechanics based approach with the dissipated energy density as the damage parameter. The strain energy dissipation (SED) concept is employed to describe the damage state and current stiffnesses of the weave constituents. A progressive failure analysis of plain woven fabrics subjected to tension and in-plane shear is carried out considering both geometric and material nonlinearities. The initiation and progression of damage within the fabric is investigated and the significant damage mechanisms outlined.Keywords: Woven composites, finite element analysis, damage mechanics, constitutive laws, failure INTRODUCTIONMicromechanics of textile composites has been the focus of study by many investigators [1][2][3][4][5][6][7][8][9][10][11]. The proper description of the internal weave geometry and spatial variation in material properties within the fabric presents a formidable task in the analysis of such composites. Most of the analytical studies on such composites have focussed on predicting their elastic properties [1][2][3][4][5][6][7]. Studies relating to prediction of their strength and failure modes are relatively few [8][9][10][11].A detailed study of the significant stresses and strains developed within the fabric and possible damage mechanisms can be accomplished by the use of three-dimensional finite elements. Nonlinear material behavior of composites is due to damage accumulation, which causes changes in the stiffnesses of the material. It is well known that macroscopic failure is preceded by an accumulation of different types of microscopic damage and stiffness losses due to the accumulation of such damage, which cause significant load Existing FE modeling tools and preprocessors are not adequate for rapidly creating complex 3-dimensional models needed for the purpose. Therefore, mesh generation programs were developed which would easily interface with a general purpose FEA software, ABAQUS [12]. FE models can be generated for plain weaves and higher-order satin-weaves using a minimal definition of the textile architecture. In the present approach, damages in the composite constituents are modeled on a continuum basis and related to the material constitutive behavior. The 3D constitutive laws describing matrix and yarn behavior are developed using a damage mechanics based approach, with the dissipated energy density as the damage parameter. The strain energy dissipation (SED) concept is employed to describe the damage state and current stiffnesses of the weave constituents. The yarns are treated as transversely isotropic while the pure matrix pockets are assumed ...
A method is presented for computing strainenergy-release-rates (SERR) for delamination growth in a wide variety of composite structures. The method is based on a sublaminate analysis which treats portions of a laminate as higher-order plates. The plates may be stacked such that the displacements and tractions are identical at the shared interfaces. By assuming a constant cross-section in onedimension, the resulting systems of governing differential equations can be solved in closed form. A means of coupling plates end-to-end is also presented, allowing complex structures to be modeled in a manner similar to finite element analysis. The software (SUBLAM) that implements the analysis can be used to determine either interlaminar stresses, or SERR. The individual modes of the SERR (G I , G II , and G III ) can be computed. The present paper includes a series of examples which demonstrate the flexibility and accuracy of the SERR calculations.
A novel analysis method has been developed with the goal of providing ac curate assessments of the forces that can delaminate composite laminates and bonded structures. The method uses interconnecting high-order plates to represent the cross sec tion of a structural element. The plates can be both stacked and connected end-to-end. When stacked, the interfacial tractions generated between the plates can be calculated, providing a measure of the delaminating stresses. The plate equations are solved exactly, thus giving accurate numerical results even in cases where the stress gradients are steep. The plate theory used incorporates a linear distribution through the thickness for the u, v, and w displacements. This set of assumed displacements makes the plate shear deform able, and allows stretching in the thickness direction. Consequently, both shearing and normal tractions can be computed at the interfaces between stacked plates. The plate equations have been derived for completely general stacking sequences of composite plies; i.e. unsymmetric and unbalanced laminates are allowed. Both flat and cylindrically curved plates have been implemented. Beyond stress analysis, the method also provides strain-energy-release-rate (SERR) results for the growth of existing delaminations. The individual modes of the total SERR can be calculated for input to interactive crack growth laws.
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