Structural behavior of large-scale triangular and trapezoidal threaded steel tie rods in assembly using finite element analysis
Abstract Pre-stressed steel tie rods are used extensively as load carrying components in structures like suspension bridges, steering systems in automotive vehicles, wings and fuselage of aircrafts, spokes of Ferris wheels, etc. The governing variables for the bearing strength of a large-scale steel tie rod (of diameter usually larger than 60 mm) include the mechanical strength of the tie rod body, geometric configuration of the thread teeth and the number of turns of thread engagement. The optimal number of turns of threaded connections of large scale steel tie rods has been predicted by full scale rupture experiments and reported previously by the authors. This paper builds up further on that work and investigates the structural behavior of the two types of threaded connections of steel tie rods, viz. (i) the triangle threaded connection, and (ii) the trapezoidal threaded connection, using finite element analysis in ANSYS. The axial displacement, equivalent stress distribution, interfacial contact pressure and sliding distance on the engaged thread teeth under different axial loads are presented and analyzed. The numerical simulations verify and expound the results of full-scale tensile rupture experiments. The concordant comparison between experimental measurements and finite element analysis provides working guidelines and support for the design and fabrication of steel tie rods in practical operations.
Determination of load distribution in a threaded connector with yielding threads
A modified spring model was developed to predict the load distribution in the threaded portion of a connector having yielded threads. Distinction was made between a compression case (nut and bolt) and a tension case (turnbuckle). The load distribution was described using simple second order difference equations and linear first order constraints. The load distributions for elastic and yielded threaded connectors are compared, and a discussion of how the yielded threads affect the load distribution is given.
Fatigue life improvement of threaded connections by cold rolling
Failures in drillstring threaded connections continue to afflict the oil industry at a cost of millions of dollars per year. In an attempt to reduce these failures the industry has generally adopted the fatigue-life-enhancing technique of cold rolling the threaded connections which act as stress concentrations. However, this beneficial process is only specified as optional in the industry standards, and where it is performed there is very little detail provided about this important process. This paper describes full-scale fatigue tests that investigate the cold-rolling process. From the development of cold-rolling equipment and the gathering of fatigue experimental data, several key factors have been identified that influence the operational lifetime of drillstring threaded connections.