Stress relaxation modeling using creep data

Bolts in gas turbine units are often subjected to severe work conditions at high service temperatures, for which the relaxation of the initial stress, generated by the tightening couples, can become significant. Besides, since in such conditions re-tightening operations become necessary, the stress trend must be accurately calculated and predicted in order to avoid inappropriate value before every maintenance interval of the turbine. A constitutive equation based on the Continuum Damage Mechanics (CDM) formalism, describing the creep and stress relaxation behaviour of a martensitic steel, has been developed and evaluated. Creep tests were performed at 520°C with applied stresses producing a strain ? = 1 % in a time range of 1000-10000 h. The stress relaxation tests were performed at 520°C, with initial stress at 300 MPa corresponding to a strain in the range of 0.2 %. The specimens were reloaded several times in order to simulate the aforesaid service conditions of bolts. In this work it is proposed a set of equations, with parameters obtained by creep tests interpolation, able to accurately reproduce and well predict the experimental relaxation behaviour of the steel under investigation. The model, fitted on creep tests, includes a term accounting for the internal stress, entailing the capability to foresee the changes in stress relaxation during successive reloading of the same specimens (or, alternatively, during successive re-tightening of the same bolt). The advantage of such approach (i.e. calculating relaxation data from creep tests) consists mainly in the possibility to perform evaluations and assessments even without stress relaxation data, which are more expensive to collect experimentally and rarer to find in literature.