1 Copyright © 2008 by ASME Proceedings of PVP2008 2008 ASME Pressure Vessels and Piping Division Conference July 27-31, 2008, Chicago, Illinois, USA PVP2008-61115 A NEW TIGHTENING METHODOLOGY FOR GASKETED JOINTS BASED ON NONLINEAR FINITE ELEMENT ANALYSIS Sayed A. Nassar, Zhijun Wu, Xianjie Yang Fastening and Joining Research Institute (FAJRI) Department of Mechanical Engineering Oakland University, Rochester, Michigan 48309 USA ABSTRACT A three dimensional nonlinear finite element model is developed for achieving a uniform clamp load in gasketed bolted joints. The model is used for both multiple and single pass tightening patterns. Geometric nonlinearity of the gasket is taken into account and plastic model parameters are experimentally determined. The effect of the tightening pattern, gasket loading and unloading history, and the preload level is investigated. The validity of the FEA methodology is experimentally verified. This study helps improve the reliability of gasketed bolted joints by minimizing the bolt-to- bolt clamp load variation caused by elastic interaction among the various bolts in the joint during initial joint bolt-up. Keywords: elastic interaction, multi-pass tightening, single- pass tightening, gasketed flange. 1. INTRODUCTION When a group of bolts are tightened in a gasketed joint, the elongation of each individual bolt causes it to structurally interact with other bolts in the same joint. As other bolts are subsequently tightened, the tension in previously tightened bolts is changed, most often reduced. After all bolts have been tightened, bolt tensions are no longer uniform. This phenomenon is referred to as elastic interaction [1]. Bibel and Ezell [2] and Bibel and Eric [3] reported that the elastic interaction caused some bolts in the flange to lose up to 98% of their initial preloads when adjacent bolts were subsequently tightened. It is well known that it is extremely difficult to achieve uniform bolt preloads in a gasketed joint with multiple bolts due to the combined effect of elastic interaction and gasket creep relaxation. Nonuniform and insufficient bolt loads in a flanged joint often increase the risk of leakage and/or fatigue failure under service loading. In practice, multi-pass bolt-up operation is usually conducted to achieve a more uniform bolt load. Kumakura and Saito [4] carried out two new multi-pass tightening patterns for pipe flanges. They reported that the proposed tightening method is effective to achieve uniform bolt tensions with multi-pass tightening strategy. Fukuoka and Takaki [5] presented a numerical approach to estimate the scatter in bolt preloads and to achieve uniform bolt preloads when tightening the bolts one by one using single-pass tightening of pipe flanges with an aluminum gasket. Takaki and Fukuoka [6] studied the effect of preload level and flange size on multi-pass tightening of bolted flanges with a compressed asbestos sheet gasket by conducting finite element bolt-up simulation. However, their FEA simulation results were not experimentally verified. Takaki and Fukuoka [7] determined the clamping forces in each pass using the elastic interaction coefficient method for a two-pass bolting up operation. They also concluded that three or four passes are required in order to achieve the desired uniform preloads in a gasketed flange. Although some researchers have proposed some models to predict the bolt tightening preloads of a gasketed joint [2, 7, 8], most of the models are based on an elastic deformation assumption of the gasket. However, many gasket materials often exhibit significant inelastic deformation even under relatively small loads. In this study, a more accurate finite element model for multi-pass and single-pass bolt tightening operations is introduced. A new tightening methodology of gasketed joint based on three-dimensional nonlinear finite element simulation is proposed, in order to explore the tightening strategies for achieving the uniformity of the desired bolt preload with