Working Bolts Near to Yield: Theory, Experience and Recommended Practice

For many years 50% yield was a favored generic level for bolted flanged joints, however in pursuit of leak free performance this is no longer the case. In line with the guidance given in modern documents such as Appendix O of ASME PCC-1-2010 Guidelines for Pressure Boundary Bolted Flange Assembly [1] and with the increasing number of joints requiring corrosion resistant alloys often it is necessary and desirable to take bolts to values closer to their yield point or more accurately their 0.2% proof stress. The paper will examine the issues associated with tightening bolts to values well in excess of 50% of their 0.2% proof stress and analyze the effects on the during tightening and subsequent re-tightening. The analysis will include results of actual testing of bolts tightened to high stresses and give recommendations on a sound engineering approach to this practice which considers the risks and benefits. Analysis will also include a statistical evaluation of the effects of torque and tension scatter at high levels of target bolt stress.

Theoretical and Numerical Predictions of Burst Pressure of Pipelines

To accurately characterize plastic yield behavior of metals in multiaxial stress states, a new yield theory, i.e., the average shear stress yield (ASSY) theory, is proposed in reference to the classical Tresca and von Mises yield theories for isotropic hardening materials. Based on the ASSY theory, a theoretical solution for predicting the burst pressure of pipelines is obtained as a function of pipe diameter, wall thickness, material hardening exponent, and ultimate tensile strength. This solution is then validated by experimental data for various pipeline steels. According to the ASSY yield theory, four failure criteria are developed for predicting the burst pressure of pipes by the use of commercial finite element softwares such as ABAQUS and ANSYS, where the von Mises yield theory and the associated flow rule are adopted as the classical metal plasticity model for isotropic hardening materials. These failure criteria include the von Mises equivalent stress criterion, the maximum principal stress criterion, the von Mises equivalent strain criterion, and the maximum tensile strain criterion. Applications demonstrate that the proposed failure criteria in conjunction with the ABAQUS or ANSYS numerical analysis can effectively predict the burst pressure of end-capped line pipes.