A thermomechanical loading component facility has been built and developed for testing model copper slag tap components under ratchetting conditions due to combined damage resulting from creep and plasticity. Model copper slag tap components have been tested under dominant plasticity conditions at temperatures up to 300°C, and under balanced creep–cyclic plasticity conditions at temperatures up to 340°C. Overall ratchet deformations have been continuously measured to failure. A cycle jumping numerical technique has been used to analyse a multi-bar model of the slag tap using viscoplastic constitutive equations embodying softening due to combined cyclic plasticity and creep damage. Excellent predictions of lifetimes, ratchet strains and ratchet rates have been achieved, despite the details of the rupture processes not being faithfully modelled.