Finite Element Analysis of Elastoplastic Elements in the Iwan Model of Bolted Joints

Abstract

The Iwan model is composed of elastoplastic elements and is widely used to represent the stiffness degradation of bolted joints under mixed-mode loading (normal and tangential loading). The latest static methods of parameter identification established the relationship between the elastoplastic elements and the contact pressure under normal loading. Under mixed-mode loading, the parameters of the Iwan model are dynamic for the evolution of contact conditions. Therefore, static parameter identification methods are not suitable for the dynamic Iwan model. A new technique was proposed to identify the parameters of the elastoplastic elements in this paper. Firstly, several different finite element models were established. The influence of the contact method and the thread structure were analyzed, and a reliable and efficient bolted-joint modeling method was proposed. Secondly, the evolution of contact conditions was studied. The dynamic elliptical contact model and the ellipticity discrete method were proposed. Finally, the residual stiffness of the Iwan model was analyzed to establish the mapping between the residual stiffness and the bending of the screw. The results can provide a technique for identifying the parameters of the dynamic Iwan model.

Contact Pressure Distribution and Pressure Correction Methods of Bolted Joints under Mixed-Mode Loading

“…The classic Iwan model is composed of n Jenkins elements, as shown in Figure 2a [25]. The Jenkins element is composed of a spring with stiffness k t /n and a friction resistor with critical sliding force f * i /n in series.…”

“…In the dynamic DF modelling technique, the yield force of each Jenkins element is related to the contact pressure by Coulomb friction law, as shown in Figure 2b. The function of contact boundary in microslip is [25]:…”

“…However, the basic assumption of this method is that the pressure distribution remains constant under mixed-mode loading. Wang et al [25] found that the assumption was not tenable and proposed a new approach to solve the DF by discretizing the dynamic contact region. This method provides a technique to solve the DF under mixed-mode loading without the assumption of constant pressure.…”

Finite-Element Analysis on Energy Dissipation and Sealability of Premium Connections under Dynamic Loads

“…In the early stage, Iwan [23] built an energy dissipation model of bolted connections based on spring-damped vibration theories. Wang et al [24] optimized the Iwan model to apply Affected by loads such as the weight of tubing string, internal and external fluid pressure, viscous damping when fluid flows in tubing string and Coulomb friction force, premium connections are subjected to loads such as axial force, internal and external pressure, bending moment, etc. During high flow rate fracturing and high production gas operation, the dynamic load change of tubing caused by the change of fluid pressure and flow rate in tubing causes vibration of tubing [14][15][16].…”

“…In the early stage, Iwan [23] built an energy dissipation model of bolted connections based on spring-damped vibration theories. Wang et al [24] optimized the Iwan model to apply it to the study of energy dissipation in more complex situations. Zhang et al [25,26] studied the normal stiffness, normal damping, and energy dissipation of the bolt bonding surfaces under different external loads.…”