Progressive failure analysis of laminated composite structures

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Virginia Tech


A methodology for progressive failure analysis is presented that is applicable to thin composite structures undergoing large deformations. The method is based on a nonlinear shell theory that accounts for geometric and material nonlinearities. An isoparametric, displacement-based finite element formulation is used to discretize the nonlinear shell equations and an iterative solution is obtained via the Newton-Raphson method. Phenomenological failure criteria are used to predict the onset of damage in the material. Damage is treated using nonlinear constitutive relationships where the material properties are degraded to simulate loss of load-carrying capability. Unlike traditional ply discount models where properties are degraded in an entire ply, the damage models are evaluated at several material points within each ply during the element integration. This allows damage to be localized within an element or ply, and reduces finite element mesh sensitivity since localized damage can develop even in a relatively coarse mesh.

An experimental program was conducted to validate the progressive failure analysis. Six foot diameter graphite/epoxy frames—representative of ring stiffeners for rotorcraft fuselage—were tested in quasi-static crush tests that simulated crash loading of the rotorcraft. Excellent agreement was obtained between the analysis and experiments: the analysis successfully predicted the failure load, the magnitude of unloading at failure, and the residual stiffness of the frames after failure for several different frame configurations. Several parameters affecting the progressive failure analysis are investigated including nonlinear solution procedures, material failure data, damage models, and finite element mesh effects.