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