| Testing and analysis of flush-head bolted lap joints determined the effect of various factors on load-elongation behavior. The study included three bolt sizes in aluminum and titanium alloy panels. The investigation also covered a wide range of panel thicknesses. Three-dimensional nonlinear finite element analyses utilizing solid elements were used to model the load-elongation behavior of the joints. The primary focus of this research was the bolt head geometry, but the influence of other factors was also studied. Eight hundred and seventy models were analyzed, and the results were summarized into charts and tables as an aid to airframe designers.; This research depended on the ability of the finite element method to reliably model the behavior of mechanically-fastened lap joints. Analytical models were built for three diverse fastener types for validation with experimental testing: solid rivets, blind bolts, and threaded bolts with nuts. Factors that influenced successful convergence of the analytical models were identified during this initial phase. These factors included the mesh density and distribution, contact constraints, material behavior, fastener installation strategy, and convergence tolerances. Fastener installation is a key factor that influences joint load-elongation behavior through induced cold work and clamping effects. As such, strategies were developed to model the squeezing of rivets, the bulbing of blind fasteners, and the tightening of nuts. A means for varying the convergence tolerance during an analysis was also developed and implemented.; Fifty-nine analytical models were built and one hundred and eighteen experimental tests were conducted during a validation phase. In all cases, the load-elongation predictions are in excellent agreement with the experimental test results.; Having established confidence in the analytical models, four studies of bolted joints were made. All four studies, in increasing levels of complexity, concerned the influence of various factors on the load-elongation behavior of bolted lap joints. The final study identified the most effective bolt head geometry for slip resistance over a range of panel thickness. Five head angles and four head heights were evaluated for three fastener sizes and three panel material types. Guides were then developed to aid designers in the selection of the most slip-resistant head geometry. |
