Page 167 - Handbook of Materials Failure Analysis

2 Material 163

evaluate the load-displacement behavior of the test. By comparing the load-
displacement response of FE analysis with that of experiment, the material stress-
strain curve was evaluated.
Investigation of fracture behavior of these fuel-clad tubes is of utmost importance
to the designers and plant operators. It is important to ensure the structural integrity
of these fuel-clad tubes under various types of operational transients as well as acci-
dent scenarios [4–9]. One of the postulated design-basis accidents for the fuel-clad
tubes is the reactivity-initiated accident [9]. In such a situation, the control rod drops
or ejects very rapidly from the reactor. This in turn deposits a large amount of energy
in the fuel-pellets and leads to adiabatic heating and large fission gas release in the
fuel pins. The fuel-pellets expand thermally and may cause fast straining of the sur-
rounding Zircaloy clad tube through pellet-clad mechanical interaction.
The fracture behavior of these clad tubes is further complicated due to several
conditions, such as hydrogen pickup, high temperature, irradiation environment,
prior cold work, and heat treatment conditions, etc. In order to ensure that the clad
remains intact for a longer period in the reactor, fracture-mechanics principles can be
applied for the fitness-for-purpose of service assessment of these fuel pins. However,
it may not be possible to evaluate the fracture behavior of these thin-walled tubular
components using standard ASTM test techniques (ASTM Standard E 1820-13) [20].
This is because of the stringent requirements of the ASTM test methods [20] for sat-
isfaction of plane strain condition at the crack-tip. For this purpose, several types of
nonstandard specimens have been studied in literature [14,17–27] in order to eval-
uate the fracture resistance and transverse mechanical properties of thin-walled tubu-
lar components.
As standard fracture-mechanics specimens cannot be machined from these thin-
walled tubes, nonstandard specimens with axial cracks (which can be directly
machined from the fuel-clad tubes) have been used in this work for the fracture tests.
Two types of loading mandrels have been used, viz., (a) a conical mandrel, driven
through the axially cracked tubes simulating expansion of fuel-pellets inside the
tubes and (b) two semi-cylindrical mandrels, inserted in the axially cracked tube
and aligned in such a way as to open the diametrically opposite cracks. The effect
of various values of initial crack length on the fracture resistance behavior of the
tubes have been studied using these two different types of test setups simulating dif-
ferent types of loading conditions at the crack-tip of the specimens.

2 MATERIAL

In this work, the fracture behavior of thin-walled Zircaloy-4 fuel-clad tubes of
220 MWe Indian PHWR is studied. The chemical composition of the alloy is shown
in Table 7.1. The alloy is used in the stress-relieved-annealed condition in the fuel-
clad tubes. Due to the heavy deformation ( 80%) during the prior cold-work
(pilgering) process, the grains are not of uniform shape. The average grain size is
of the order of 5-6 μm in the elongated direction and 2-3 μm in the other directions.
From the micro-texture analysis, it was observed that the basal poles of the