Damage modelling of leaded free cutting steel under hot forming conditions

Abstract

In this thesis the influence of stress-state on ductile damage in free cutting steel under hot forming conditions is examined. The industrial motivation for the project focuses on edge cracking in hot rolling. A brief outline of the hot rolling process conditions is necessary to define the important parameters affecting edge cracking. Triaxiality was, thus, identified as the key parameter relating to damage under hot rolling conditions. Based on a detailed literature review, the appropriate testing and modelling methodology were identified for this body of work. A high temperature, uniaxial tension test program was implemented to identify the effect of triaxiality on damage under hot forming conditions. Double notched bars with varying notch radii were utilised, thus inducing different stress triaxialities due to geometrical constraints. Based on the resultant stress-strain data the effect of triaxiality on ductility and the strain to failure was investigated. Subsequently, unbroken notches from tested double notched samples were sectioned and optically examined to reveal damage initiation sites. Interesting damage features were identified and correlated with sample geometry (i.e. triaxiality) and testing conditions. Finite element analysis of the double notched samples revealed the effect of triaxiality on the local stress-state. The accuracy of the mechanical analysis from such simulations was improved by incorporating the thermal gradients induced during high temperature Gleeble tests. Three stress parameters were examined in relation to their effect on the experimentally observed damage; maximum principal stress, effective stress and hydrostatic stress. The maximum principal stress and equivalent stress were most clearly correlated to damage development under multiaxial conditions for this particular free cutting steel. Based on the results of the stress-state investigation of the double notched samples, a multiaxial damage expression was developed that reproduced the experimentally observed damage characteristics. The new multiaxial damage model was calibrated using a combination of uniaxial and multiaxial stress-strain data and damage profiles. The model was shown to have good accuracy in predicting both the stress-strain data and the damage initiation sites as a function of geometry and damage conditions. Finally, an extensive range of temperature and strain rate conditions were simulated for all tested sample geometries, and an additional sample geometry, to fully understand how testing conditions affect damage characteristics and under what triaxialities this is prone to happen.