| ABSTRACT: | TIME OF FLIGHT DIFFRACTION (TOFD) TECHNIQUE FOR ACCURATE SIZING OF
SUBCLADDING EMBEDDED CRACKS
S. Baby1, T. Balasubramanian1, R.J.Pardikar2, M.Palniappan3, and R.Subbartanam3
1Department of Physics, National Institute of Technology, India; 2NDTL, Bharat Heavy Electricals Limited,
India; 3 Indira Gandhi Centre for Atomic Research, India
This paper deals with an experimental study for evaluation of Time-of-flight diffraction (TOFD) for
determination of size of the embedded subcladding cracks. TOFD equipment Model MICROPLUS of M/s
AEA Technology, UK with manual scanner along with longitudinal angle beam probes of 45¡-4MHz was
used for the study. The steel test blocks used for the study contained 0.5mm wide vertical/inclined simulated
cracks (machined slits)inclination (10¡ and 15¡) of various heights ranging from 1.68mm to 19.04mm
underneath the Stainless steel, Inconel and Ferritic cladding. The experimental results have shown excellent
correlation between the actual and estimated through thickness height and length of the simulated cracks
with a mean error of ±0.18mm (vertical slits)and ±0.14mm (inclined slits). The proven level of accuracy
attainable is found to be within ±0.2mm in terms of critical through wall extent and ±0.9mm in terms of
horizontal dimension. It has been observed that TOFD suffers from a near surface effect caused by its
inherent lateral wave. Difficulty was experienced using TOFD to detect and size cracks starting from the
interface between inconel /Stainless Steel cladding and the ferritic base i.e. (5mm/6mm below the scanning
surface). This is due to lateral wave, which obscures the tip-diffracted signals from the defects close to the
surface. However, these cracks could be well detected and sized by scanning through the cladding from the
opposite surface of the blocks. Even the small cracks of the order of 1.9mm could be detected as long as
they are not influenced by the lateral wave.
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