New Mandrel Design for Ring Hoop Tensile Testing

The determination of mechanical and fracture properties of anisotropic tubular materials along hoop direction needs the use of the ring hoop tensile test, for which, the obtained results are deceived by the effect of friction between the ring sample and the D-shaped block mandrel. Commonly, lubricants are applied to reduce the friction, which are inefficient in some specific cases; despite of that, it was noticed that scarce works have focused on the development of new mechanical mandrel designs or trying to improve the current ones to resolve the friction concern. The aim of this research is to correctly address the friction issue between the ring sample and the fixture mandrel to significantly reduce its effect on the ring hoop tensile test results without using any kind of lubricants. New mechanical design of D-shaped block mandrels are developed to carry out ring hoop tensile tests to simply characterize the mechanical behaviour of tubular materials. New mechanical D-shaped block mandrels were designed, manufactured and used to carry out experimental ring hoop tensile tests. An inverse identification method based on an artificial neural network trained by finite element simulation responses, was developed to efficiently segregate the flow stress curve from the influence of the friction, inherent in the global force-displacement curve for the classical ring hoop tensile test. The experimental force - displacement curves using five mandrel-types are established and quantitatively compared on the base of their ability to reduce the friction issue. The analysis of the finite element simulations, related to the investigation of the influence of the friction on the ring hoop tensile test results, shows that one of the new developed mandrels reduces the friction coefficient by about 10 times compared to that identified using the classical D-shaped block mandrel. It has been found that, the finite element simulation of ring hoop tensile test using the identified material parameters matches the experimental results. This investigation provides a useful fixture mandrel, which is able to drastically reduce the friction without resort to any lubricants to just determine the material flow stress curve using ring hoop tensile test, regardless the friction level between the sample and mandrel.