Analytical Modeling of the Mechanics of Re-Torque

A secondary torque, i.e., re-torque, is generally applied in order to confer long term bolt tightness of certain gasketed-flange configurations that have undergone a primary torque with some relaxation. In some sense, the initial torque conditions the viscoelastic gasket material for long term performance under service loading. While prior research has been carried out to analytically model the mechanical response of gasket materials under either creep, stress relaxation, or creep relaxation, the mechanics of gasket re-torque has received much less attention. In the current study, a candidate fiber-glass reinforced gasket material is subjected to creep relaxation after a series of primary and secondary torques. Test variables considered here include values of either torque, dwell period, or gasket thickness. The over-arching goal addressed in this study is the identification of the conditions that confer the minimal initial dwell period without loss of long term load retention. In all cases, specimen-sized samples were used on a raised-face, serrated flange assembly. Based on the experimental test data and observations from scanning electron microscopy, an viscoelasticity model is developed to analytically predict the response of the time-dependent solid.