NAGRA NTB 92-14 - 27 - STRESS STRAIN Fig. 2.7: Iteration procedure for visco elasto-plastic material behaviour. be guessed from an extrapolation to the strain c tot as shown in Figure 2.7. This procedure is unsafe when small stresses are assumed for a large c tot • Interpolation is also not in place due to a bad convergence in some cases. What remains is the simple trial and error method according to the 'regula falsi' as shown in the flow chart of the FIBRE programme (Fig. 2.8), where the above equations are written in an incremental form for a time increment dt. Since the FIBRE programme has to explain the effect of the different parameters involved in a thermomechanical calculation, no effort was put into an accurate time integration or into an evaluation of its iteration algorithms. 2.3.2 Material properties and loading The thermo elasto-plastic material behaviour was taken as defined in section 2.2 (material properties). Three different material property sets were investigated; they differed in terms of the viscosity parameters. Material property set 1 is thermo elasto-plastic, the material property sets 2 and 3 are visco thermo elasto-plastic, whereby material property set 2 is more viscous at low temperature and material property set 3 more viscous at high temperature (Fig. 2.9). Between 400 °C and 700 °C material property set 2 uses in the creep parameters determined for GS-40 cast steel at a time t of 1 s. The loading of the fibre is a function of the temperature and three types of temperature evolutions are considered. The first one consists of 3 cycles, one of which is shown in Figure 2.10. This form is a rough approximation of the temperature evolution as it was determined for the welding of the overpacks (MEYER & ATTINGER 1987). The second and the third type of loading have the same form and temperature as the first one, but the time axis is delayed by a factor of 10 and 100, respectively.