Abstract
Background
The burst testing technique is used to determine the failure hoop stress of tubes that are subjected to internal pressure. In that technique, the tube is subjected to internal high fluid pressure until it bursts. This testing technique is costly and is not suitable if the available tube material is limited.
Objective
In this paper, a novel design of a ring expansion testing rig is presented to replace the burst testing technique for determining the failure hoop stress of circular uniform thin-walled ring specimens where t/d < 1/20; t is the wall thickness and d is the outer diameter of the ring. In addition, the ring expansion testing technique can be used to measure the mechanical properties of the tube material.
Methods
The testing rig consists of a multipiece mandrel assembled with two cones that are moving against the mandrel in opposite directions. The ring of the tested tube circumscribes the mandrel and deforms radially as the cones move axially against the mandrel. In addition to experiments, numerical analyses were carried out using the commercial finite element method (FEM) package ABAQUS. The FEM was used to optimize the mandrel design in terms of the optimum number of the mandrel pieces.
Results
The FEM revealed that at least six mandrel pieces should be employed in order to produce uniform hoop stress in the ring specimens. In addition, the FEM results were utilized in conjunction with the theoretical results to find the value of the shear factor (α = 0.06). The experimental results highlighted the effects of the ring cross-section dimensions on the obtained results.
Conclusions
It is concluded that the greater the number of mandrel pieces is, the more uniform the distribution of the hoop stress developed in the ring and, hence, the more accurate the determined hoop stress. In addition, the numerical and experimental results can be used to determine the coefficient of friction between the rig's contact surfaces of the test unit.