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Micro-Stress Bound Estimate Enabled Optimization of Structural Composite Repair for the Next Generation Aircraft

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Final rept. 1 Mar 2004-28 Feb 2007

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The completed effort integrated the rigorous microlevel fiber, matrix, sizing stress bound recently obtained by Lipton 1,2 under AFOSR sponsorship with the AFRL-developed ply level multibasis spline approximation stress analysis tools 3-5. A robust multiscale analysis framework was developed and applied within and beyond the scope of the present grant. The developments included extension of the stress bound estimates in heterogeneous materials with periodic microstructure to include residual stress effects and, notably, regions of nonperiodic microstructure. Such regions included ply interfaces in fiber reinforced composite materials, which are present in all laminated composite structures. The improved accuracy of the multiscale analysis was verified by comparison with three-dimensional analysis of model cross ply composite laminate, where fibers were modeled explicitly. Our analysis capability for the first time addressed the composition of the repair patch of a composite scarf repair in addition to more conventional geometric parameters, such as scarf angle and repair depth. We found that altering the conventional ply by ply replacement schema of the repair patch does not lead to premature failure, but may delay the failure of the adhesive and increase the strength of the repaired composite. This work is well coordinated with the Materials Integrity Branch of the Air Force Research Laboratorys Materials and Manufacturing Directorate, which is presently executing an experimental program to verify our findings. We have transitioned the periodic RVE based micromechanical failure criterion evaluation algorithm to Sikorsky Aircraft, where it was applied to failure analysis of helicopter flex beam. We are also using our methodology to investigate the applicability of various micromechanical failure criteria to failure prediction of composites under complex states of stress.

Subject Categories:

  • Helicopters
  • Laminates and Composite Materials

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