TECHNICAL REPORT 92-14 - Nagra

NAORA NTB 92-14 - 22-
The (true) stress-strain relations from the experiments for the two strain rates
are shown in Figure 2.2; the curves at 900 °C match those at 800 °C and
therefore are not included in the figures. The temperature dependency of
the modulus of elasticity E and the yield limit RpO.2 is given in Figure 2.3.
The two further parameters which are necessary for a time independent,
thermomechanical calculation, i.e. the Poisson ratio v and the coefficient of
thermal expansion CY, are represented in Figure 2.4. The values of these parameters
are taken from (RICHTER 1973) for a weldable fine-grained steel.
The coefficient of thermal expansion CY is reduced at 800 °C, thus indicating
some transformation plasticity at the ferrite-austenite transformation.
2.2.3 Creep tests
In this work, creep is understood as a flow which is time dependent and
which is possible for any stress; yielding is understood as a flow which
arises only when the stress is above a certain threshold level, namely the
yield limit.
The influence of creep on the material behaviour increases with increasing
temperature; at temperatures such that the creep response time, even under
low stresses, is smaller than the time constant of the temperature history,
only small stresses can build up. It is therefore necessary to estimate the
limiting temperature below which creep can be neglected for time horizons
of the order of the duration of the cooling period after completion of the
weld: significant residual stresses can build up only below that temperature.
In order to address this issue, to determine the creep behaviour of OS-40 cast
steel and to develop an adequate mathematical description of the primary
and secondary creep, a series of creep tests under constant loading were
carried out on samples (diameter 8 mm, gauge length 38 mm) of OS-40
cast steel at the three temperatures 400 °C, 550°C and 700 °C (ROSSELET
1990). Figure 2.5 shows the creep curves obtained in the tests. It can be
seen that already ,at 400 °C the primary creep rate is important. Also at
this temperature an important strain hardening is observed, i.e. the creep
rate decreases rapidly with time (Figure 2.5a). By contrast, at 700 °C (Figure
2.5c), the strain hardening is practically compensated by the recovery
and primary creep is not observed.
2.2.4 Implementation of the material properties into a finite element
calculation
The time independent stress-strain relations are represented in the finite
element calculations either by a multilinear, temperature- and strain ratedependent
or by a bilinear, temperature-dependent material law . The former
is implemented into the material model by a table, the latter is given by the
input. The bilinear material law is based on the experimental values with a