TECHNICAL REPORT 92-14 - Nagra

NAGRA NTB 92-14 - 13 -
Table 9: Observed failure for the intermediate testwork on simplified
overpacks.
I Overpack I II 114 I IS
Description Standard Standard Corroded surface
Welding type TIG Electro-beam TIG
Wall thickness [mm] IS.0 IS.0 IS.0 -7- 7.S
Experimental data
Failure pressure [MPa] 87 83 7S
Buckling mode [lobes] 3 3 3
Further calculations were carried out by PSI/NAGRA to evaluate the relative
effects of different material laws. The variation of the Poisson ratio was
found to have only a small effect; a larger Poisson ratio tended to reduce
strains at intermediate pressures and increased them at higher pressures. It
is concluded from the studies with a bilinear material law that, where a
multi-linear material model is not available, a 'tangent modulus' model, i.e.
a straight line between the yield limit and the ultimate strength, is most
accurate when the high strains are localized An increase in the strength of
the weld material was found to reduce the deformation in the base; this is
consistent with test observations.
The last two overpacks were not analysed by finite element calculations.
Overpack 14 was geometrically identical to the standard overpack II, but
with electro-beam welds used throughout instead of TIG welds. Overpack
14 failed in a manner very similar to that of II but buckling occurred at a
lower pressure (Table 9). As the welding method was the only parameter
which differed from overpack II to 14, this implies that TIG welding leads
to a stronger overpack than electro-beam welding. This can be explained
by noting the larger and hence stronger heat affected zone of the former.
Overpack IS represents a heavily corroded overpack. Craters were formed
by electrolysis in the surface of the overpack; they had an average surface
area of approximately SOO mm 2 and a local reduction in the wall thickness
of up to SO %, although the average thickness reduction was only S %.
Comparison of the experimental results with those for overpack II indicates
that the reduction in failure pressure due to the simulated corrosion is nearer
to the average reduction in wall thickness than to the maximum reduction.