TECHNICAL REPORT 92-14 - Nagra

nagra

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

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