Investigation of the effects of thermal gradients present in Gleeble high-temperature tensile tests on the strain state for free cutting steel

A common feature of uniaxial high-temperature tension tests, and to a certain extent compression tests, performed using a Gleeble thermomechanical testing system that employs direct resistance heating for the characterisation of the rheological behaviour of materials is longitudinal and radial thermal gradients. The aim of this article is to experimentally quantify the axial thermal gradients for a given tensile specimen geometry of free cutting steel during heating and deformation, and design a modelling methodology to simulate their influence on the strain distribution as compared to the assumption of isothermal heating and deformation. For this purpose, a feedback algorithm was developed to control the electric current input in a similar manner to that applied by the Gleeble testing system, which implemented via the UAMP user subroutine in ABAQUS for use in electro-thermal simulations of the direct resistance Joule heating used by the Gleeble testing system. The predicted temperature fields were compared with the temperature distributions recorded experimentally along the gauge section of the tensile specimens. Finite element simulations of Gleeble tensile tests were carried out under isothermal conditions and using the temperature distributions calculated by the feedback algorithm for a range of strain rates and temperatures in order to evaluate the difference in predicted stress state. The results show that an isothermal assumption should only be used conservatively in finite element simulation of the Gleeble thermomechanical test employing direct resistance heating to avoid significant errors.