TECHNICAL REPORT 92-14 - Nagra

NAGRA NTB 92-14 - 7 -
Table 6: Critical pressure for growth of a 5 mm long crack in the uniformly
corroded overpack.
I Partner
OAP PAFEC
CEA CASTEM - INCA
STEAG COSMOS/M
ENSA ANSYS
PSI/NAGRA ADINA
I Calculation 1)
two-dimensional
three-dimensional
standard
no crack calculation
three-dimensional
elastic
elasto-plastic
Pressure I
I [MPa]
21.2
21.2
20.3
-
21.6
20.0
18.3
1): A two-dimensional elastic calculation in a small displacement formulation
was used as a standard.
The critical pressure was defined as that which caused the stress intensity
factor !{I to reach the critical value !{IC.
This test was chosen to compare the crack/fracture calculation capabilities
of the codes. The predicted critical pressures are listed in Table 6. The
PSI/NAGRA result shows that plasticity near the crack tip region reduces
the pressure required for unstable crack growth. It is interesting to note that
their effective stress and effective plastic strain countours for the elastoplastic
calculation confirm that plastic deformation is confined locally to the
crack tip region. This suggests that plasticity is a factor that needs careful
consideration in future fracture calculations.
1.3 Preliminary ring tests
The preliminary ring tests were intended as a series of tests which could be
easily defined and would not require expensive test facilities (OAP 1989c).
As a result, four variations on a simple ring under transverse loading were
tested. The first (Rl) is a simple uncracked ring, the second (R2) and third
(R3) are rings with external and internal cracks respectively, and the fourth
(R4) is a ring which has weld material deposited on the outside, leaving
a ring consisting of two materials and a heat affected zone. All the rings
were machined from the same piece of a mild steel bar. The geometry
of the rings (outer diameter 160 mm, inner diameter 128 mm, crack depth
8 mm) was chosen to be representative of a section through a thick-walled
overpack. However, the loading is somewhat artificial in that the ring is
squeezed between two fiat plattens of the loading device.
This problem involved large deformations, large strains and cracking, as
well as interfaces with moving contact points. The results for the cross-head