A new set of six phenomenological failure criteria for fiber-reinforced polymer laminates denoted LaRC03 is described. These criteria can predict matrix and fiber failure accurately, without the curve-fitting parameters. For matrix failure under transverse compression, the angle of the fracture plane is solved by maximizing the Mohr-Coulomb effective stresses. A criterion for fiber kinking is obtained by calculating the fiber misalignment under load and applying the matrix failure criterion in the coordinate frame of the misalignment. Fracture mechanics models of matrix cracks are used to develop a criterion for matrix failure in tension and to calculate the associated in situ strengths. The LaRC03 criteria are applied to a few examples to predict failure load envelopes and to predict the failure mode for each region of the envelope. The analysis results are compared to the predictions using other available failure criteria and with experimental results.
SUMMARYThe initiation and evolution of transverse matrix cracks and delaminations are predicted within a meshindependent cracking (MIC) framework. MIC is a regularized extended finite element method (x-FEM) that allows the insertion of cracks in directions that are independent of the mesh orientation. The Heaviside step function that is typically used to introduce a displacement discontinuity across a crack surface is replaced by a continuous function approximated by using the original displacement shape functions. Such regularization allows the preservation of the Gaussian integration schema regardless of the enrichment required to model cracking in an arbitrary direction. The interaction between plies is anchored on the integration point distribution, which remains constant through the entire simulation. Initiation and propagation of delaminations between plies as well as intra-ply MIC opening is implemented by using a mixed-mode cohesive formulation in a fully three-dimensional model that includes residual thermal stresses. The validity of the proposed methodology was tested against a variety of problems ranging from simple evolution of delamination from existing transverse cracks to strength predictions of complex laminates without a priori knowledge of damage location or initiation. Good agreement with conventional
The relationships between a resistance curve (R-curve), the corresponding fracture process zone length, the shape of the traction-displacement softening law, and the propagation of fracture are examined in the context of the through-the-thickness fracture of composite laminates. A procedure for superposing linear cohesive laws to approximate an experimentallydetermined R-curve is proposed. Simple equations are developed for determining the separation of the critical energy release rates and the strengths that define the independent contributions of each linear softening law in the superposition. The proposed procedure is demonstrated for the longitudinal fracture of a fiberreinforced polymer-matrix composite. It is shown that the R-curve measured with a Compact Tension Specimen test cannot be predicted using a linear softening law, but can be reproduced by superposing two linear softening laws.
This paper describes a design study for structural optimization of an upper cover panel of a typical passenger bay of a blended wing body transport airplane. For these type of airplane the structure that obtained a common consent among designers is a sandwich structure. However unresolved issues of sandwich structures, like core crushing, might prevent their employment and suggest the study of an alternative all composite structure. A hat-stiffened laminated composite panel concept is considered as an alternative to the sandwich configuration. The initial geometry of the hat stiffener configuration is determined with the PANDA2 program by restricting the design to uniform axial properties. Because of pressure loading, a more efficient design has variable properties. Such designs are obtained by combining the STAGS finite element program with the optimization program in the Microsoft EXCEL spreadsheet program using response surfaces. Buckling and stress response surfaces are constructed from multiple STAGS analyses and are used as optimization constraints. The optimization conducted with the response surfaces results in considerable weight savings compared to the uniform cross-section design, albeit at the 2 expense of a more complex design. Moreover, after the initial optimization cycles are performed a subdomain of the design space is identified where simple approximate analyses, such as Euler-Bernulli beam theory and Kirchhoff plate theory applied to laminated composites, can be used to predict the behavior of the structure.
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