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Durability and Intelligent Nondestructive Evaluation of Adhesive Composite Joints

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Final rept. 1 Sep 1996-31 Aug 2000

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This project addressed durability and damage and fracture development and monitoring in adhesive composite joints for aerospace structural applications. Experimental and theoretical studies of static and fatigue behavior of single lap adhesive joints were performed on joints with unidirectional UD and cross-ply CP adherends made from a graphite-epoxy composite and an adhesive used in the Air Force. Joints with and without deliberately introduced cracks and delaminations were analyzed. Instrumented testing was performed by a digitally controlled servohydraulic testing machine. Damage and fracture evolution and mechanisms were analyzed by acoustic emission AE, video microscopy, and off-line optical, scanning electron, and atomic force microscopies. A new method of acoustic emission analysis of histories of damage and fracture mechanisms was developed patent pending. The method was shown to be especially effective for fatigue damage and fracture evolution studies. Nonlinear finite element analysis was used to compute mixed mode energy release rates for the bond cracks and delaminations as functions of crack lengths. The results showed that both UD and CP joints subjected to fatigue exhibited gradual crack propagation over a substantial portion of fatigue life. Static fracture mechanics characterization of joints under pure Mode 1,11, and mixed mode loadings was also performed and exploratory results on fatigue fracture under pure mode loads are also discussed. Overall, the project provides better understanding of static and fatigue behavior of adhesive composite joints. The results can be used in the development of intelligent nondestructive evaluation approaches with predictive capabilities.

Subject Categories:

  • Aircraft
  • Adhesives, Seals and Binders

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