IGCAR : Annual Report - Indira Gandhi Centre for Atomic Research

IGC
Annual Report 2007
characterization is not possible.
This problem has been
successfully overcome at the
Quality Assurance Division
using a novel combination of
TOFD and immersion. The
principle is illustrated as a
schematic sketch in Fig.1.
For initial studies, three
stainless steel plates with
artificial reference defects were
chosen. Plate 1 was 10 mm
thick and had five side drilled
holes of 2 mm diameter at
different locations. Plate 2 was
5 mm thick with 5% and 10%
notches and two side drilled
holes (2 mm diameter) and
Plate 3 was 3 mm thick with a
5% and 10% notch. Since the
plates were quite large and the
region of interest was small, to
maintain the required water
path, a dam was constructed.
Fig. 2(a) shows the
experimental setup.
Fig.1 Beam path and angle of propagation through water and
material in the immersion TOFD experiment
The parameters to be
considered were thickness of
the water column over the
material and probe tilt (Fig.1).
Based on the time of flight
equations, the time difference
between lateral wave and back
wall echo was computed as a
function of probe tilt for
specimens of different
thickness. It was observed that
as the probe separation and the
propagation angle inside the
material decreases, this time
difference increases which
leads to better spatial
resolution (Table1).
Fig.2 (a) Experimental setup (b) 3.15 mm thick welded hexagonal sheath
(c) TOFD image of a welded plate having no defect (slop is due to
curvature of the plate) and (d) TOFD images indicating lack of
penetration and porosity.
The required lower probe
separations were not possible
in conventional TOFD because
of the size of the probes. Based
on the experiments on the
reference specimens, further
examinations were carried out
on welded specimens of
thickness 3.15mm. These
specimens pertain to the
hexagonal sheath of the dummy
fuel sub-assembly. The weld
specimens had defects such as
lack of penetration and porosity
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