Residual Strength Analyses of Riveted Lap-Splice Joints

Aging aircraft may develop multiple-site damage (MSD) that can reduce the structural integrity of fuselage structures. The existence of small cracks emanating from adjacent rivet holes in a fuselage lap-splice joint is of major concern. Small collinear cracks greatly reduce the residual strength of a panel with a lead crack. Recent studies predicting the residual strength of flat and curvilinear panels with riveted lap-splice joints gave encouraging results but some difficulties arose in modeling small cracks at rivet-loaded holes. Thus, there was a need to conduct detailed fracture analyses of the crack-linkup phenomenon in lap-splice joints with rivet-loaded fasteners. The objective of this paper is to analyze the crack-linkup behavior in riveted-stiffened lap-splice joint panels with small MSD cracks at several adjacent rivet holes. Analyses are based on the STAGS (STructural Analysis of General Shells) code with the critical cracktip-opening angle (CTOA, Ψc ) fracture criterion. To account for high constraint around a crack front, the “plane strain core” option in STAGS is used. The importance of modeling rivet flexibility with fastener elements that accurately model load transfer across the joint is discussed. Fastener holes are not modeled but rivet connectivity is accounted for by attaching rivets to the sheet on one side of the cracks that simulated both the rivet diameter and MSD cracks. Residual strength analyses made on 2024-T3 alloy (1.6-mm thick) riveted-lap-splice joints with a lead crack and various size MSD cracks were compared with test data from Boeing Airplane Company. Analyses were conducted for both restrained and unrestrained Buckling conditions. Comparison of results from these analyses and results from lap-splice-joint test panels, which were partially restrained against buckling indicate that the test results were bounded by the failure loads predicted by the analyses with restrained and unrestrained conditions. ________________________________________________________________________ * National Research Council Resident Research Associate, NASA Langley Research Center, MS 188E, Hampton, VA 23681