Page 172 - Handbook of Materials Failure Analysis

168 CHAPTER 7 Investigation of failure behavior of tubular components

test. The engineering stress-strain data is then converted to the true stress-strain data
using standard equations of uniaxial tensile tests. The problem with this method is
the determination of the gauge-length and the condition of stress-state, which is not
purely uniaxial tension in this specimen (due to the curvature of the specimen). Hence,
the inverse FE analysis procedure has inherent advantages for determining the mechan-
ical properties from these nonconventional specimens as presented in Figure 7.3.
The true stress-strain curve as obtained from FE analysis has been presented in
Figure 7.5 along with the stress-strain data as obtained from the conventional tech-
nique. It can be observed that the true stress-strain curve as obtained from inverse FE
analysis shows a strain-hardening behavior whereas the data obtained from the con-
ventional technique shows a flat curve after a strain of 8%. This material is known
to exhibit substantial strain-hardening behavior at room temperature and hence, it
can be observed that the data as obtained from FE analysis represents the material
behavior satisfactorily.

4 FRACTURE EXPERIMENT ON TUBULAR SPECIMENS
USING CONICAL MANDRELS
For evaluation of fracture resistance behavior of fuel-clad tubes, one needs to sim-
ulate the loading condition as seen by the fuel-clad inside a rector. The fuel-pellets
exert radial load on the fuel-clad due to expansion and cracking of fuel-pellets. Inter-
nal pressure also rises in the fuel-clad tubes due to accumulation of fission gas after
operation of the reactor for a long time. All these loading conditions need to be sim-
ulated in the fracture tests so that the fracture behavior of the component in the actual
condition can be evaluated. This data is needed for integrity analysis of fuel-clad
tubes for longer operation of reactor and for achieving higher fuel burn-up. Loading
with an internal conical mandrel as well as split semi-cylindrical mandrels are some
of the options which have been used in this work. These tests simulate the loading
condition of a fuel-clad tube in a nuclear reactor during its extended operation.
In the first fracture test setup, the conical mandrel with a half conical angle of 4° has
beenused.Themandrelhasanincreasingdiameterandasitisgraduallyinsertedintothe
axially cracked fuel-clad tube, the two crack-tips (on diametrically opposite sides) are
loaded in mode-I. The state of stress is predominantly tensile with combination of con-
tactpressureduetomandrelandthe shear stressduetofrictionalinteractionbetweenthe
mandrel and the interior wall of the clad. Zircaloy-4 fuel-clad tubes with internal diam-
eter of 14.42 mm, thickness of 0.4 mm, and length of 50 mm were used in the tests. The
axially cracked tubular specimens have different a/W ratios ranging from 0.1 to 0.5,
whereaisdenotestheinitialcracklengthandWdenotesthetotallengthofthespecimen.
The geometry and dimensions of the conical loading mandrel are shown in Figure 7.6a,
whereas the details of the specimen are shown in Figure 7.6b. The mandrel has a lower
straight cylindrical portion which goes inside the tubular specimen and it is used to pre-
vent bending of the specimen during the test.
Theconicalportionofthemandrelopensthecracksurfaceofthespecimenwhenitis
pushed into the fuel-clad specimen. Figure 7.7 shows the picture of the test setup with