TECHNICAL REPORT 92-14 - Nagra

NAGRA NTB 92-14 - IT -
The above-mentioned aim (2) is the subject of the second part of this work. After a
description of the material properties of GS-40 cast steel, the one-dimensional FIBRE
model is discussed, which should give an insight into the parameters involved in a thermomechanical
calculation of a welding process. Next the thermomechanical calculation
for the evaluation of residual stresses using the finite element method is described; it
includes the calculation procedure and the verification of the proposed user-supplied
material model for the simulation of the material behaviour at high temperature and
during multipass welding and stress relief.
The calculations were performed with the commercially available finite element code
ADINA (ADINA 1984). The output from ADINA was used as input to the postprocessor
ORVIRT (BASS & BRYSON 1983) when fracture calculations are considered.
During the simulation of the welding process, the need for a user-supplied material
model for the simulation of material behaviour at high temperature became obvious.
This was implemented in the finite element code SOLVIA (SOLVIA 1987) which was
derived from ADINA version 84.
The work presented allows to conclude the following:
• The commercially available finite element code ADINA is suitable for the stress
analysis of high level nuclear waste overpacks under repository loading and during
welding.
• It was demonstrated that the behaviour of an overpack can be modelled to a level
of accuracy within engineering limits with the help of the finite element code
,,-\'DINA. The verJJication and validation work performed in the COMPAS project
treated creep, fracture mechanics problems and buckling; the experiments were
simulated by two-dimensional or three-dimensional models considering elast(}plastic
material behaviour and large displacements. The material model used to
simulate the material behaviour at high temperature was verifyed with an experiment
where two annuli are welded together by means of electr(}-beam welding.
• The assumptions used in the thermomechanical calculations for the simulation of
the welding process are important for the residual shape of the workpiece, but not
for the residual stresses. The calculated residual stresses are high and, therefore,
they are limited by the yield limit. The calculated tensile stresses around the weld
may reach the yield limit.
• The stress relief obtained in a post-weld heat treatment may be maintained during
the subsequent cooling to ambient temperature when the cooling process is performed
appropriately. Rapid cooling may result in large residual stresses which
will not be reduced in all cases, even if viscous effects are considered.