Test and analysis of stitched composite structures to assess damage containment capability

Integrally stitched composite technology shows promise in enhancing structural integrity of next-generation aircraft structures. The most recent generation of integrally stitched out-of-autoclave manufacturing is the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept. While the PRSEUS concept has been shown to provide damage-containment capability for composite structures while reducing overall structural weight, the mechanisms responsible for damage containment are not well understood. The objective of this thesis is to develop and validate an analysis methodology for predicting damage initiation, progression, and containment in full-scale composite structures with stitched interfaces. The damage containment mechanisms were examined using a full-scale PRSEUS fuselage panel. Tests were performed at the FAA Full-Scale Aircraft Structural Test Evaluation and Research (FASTER) facility in a joint NASA, Boeing, Drexel, and FAA test program. The panel, with a two-bay notch severing the central stiffener, was subjected to simulated flight load conditions of combined axial tension and internal pressure. Test results showed that damage was arrested by the stitched stiffeners and was contained within the two-bay region to a load level above the anticipated flight loads. Detailed posttest examinations were conducted using non-destructive inspection techniques and destructive teardown evaluations on regions of the panel where stable damage growth occurred to identify the dominant failure mechanisms. The posttest examination results suggest that the damage containment behavior observed was a result of interaction between damage propagation in the skin and delamination of the stitched skin-stiffener interface. A global/local finite element analysis approach was developed to simulate damage progression so as to better understand the key mechanisms that enable damage containment. The two dominant damage mechanisms identified from the posttest examination were considered in the analysis: through-the-thickness crack propagation in the skin and delamination at the stiffener interface. In order to analyze the through-the-thickness crack propagation with the cohesive zone model, a refined cohesive law characterization approach was developed for multidirectional laminates using compact tension (CT) tests. Tests and analyses of geometrically scaled CT specimens demonstrated the scaling capability of the cohesive law characterization methodology. In addition, several details were addressed in order to scale progressive damage analysis techniques to the structural scale in a computationally tractable manner including global/local boundary conditions, cohesive element integration within a shell element mesh, and element size considerations. Excellent correlation between calculated and measured damage propagation and strain redistributions was achieved. Results from parametric studies suggest that modest increases in the toughness of the skin-to-stiffener interface yield significant improvements in the peak damage containment load level. This new model is the first analysis methodology capable of predicting damage containment behavior in full-scale composite structures without nonphysical manipulations. This approach represents an important step toward damage tolerance evaluation of composite structures by analysis.