This paper deals with an experimental study for evaluation of TOFD technique for determination of size of the surface breaking cracks. The study was confined to simulated cracks. The steel test blocks used for the study contained 0.5mm wide vertical slits of various heights ranging from 0.91mm to 30mm. Another set of blocks contained inclined slits (10°, 15°) inclination of various heights ranging from 2.56mm to 19.82mm. Both the vertical and inclined slits were opened to the top surface. TOFD equipment Model MICROPLUS of M/S AEA Technology, UK with manual scanner along with longitudinal angle beam probes of 45° -4MHz were used for the study. The blocks were scanned along the slits / defects and across the slits. The scanned images were analysed for the sizing. The results of the study indicated an average error of ±0.13 for depth in vertical slits and ±0.05 for inclined slits whereas the average error in length measured was ±0.36mm for vertical slits and ±0.29mm for inclined slits. However difficulty was experienced using TOFD to size defects extending less than about 2mm depth. This is due to the presence of the lateral wave, which obscures the tip-diffracted signals from the defects close to the surface and also due to the inherent lack of time resolution near the surface.

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This paper deals with the experimental work carried out to demonstrate the feasibility of ultrasonic Synthetic Aperture Focusing Technique (SAFT) to obtain improved detection and sizing of vertical/inclined (10° and 15°) simulated cracks underneath different claddings. Crack heights ranging from 1.68mm to 19.04mm underneath stainless steel, Inconel and ferritic steel cladding could be sized with an accuracy of ±0.1 to ±0.3 mm. The problems encountered in TOFD with regard to sizing of near surface cracks was successfully overcome by SAFT. Mis-oriented (inclined) defects embedded below the cladding suffer added disadvantage due to loss of reflected energy due to mis-orientation. Using SAFT even these defects could be sized accurately. Introduction:Pressure vessels heat exchangers and many other components used in many applications such as nuclear reactors, petrochemical, power plants and paper mills are typically fabricated from thick carbon steel covered with austenitic stainless steel cladding. The purpose of the clad is to protect the carbon steel from the environment in the vessel; typically, the clad is not considered a structural part of the vessel. Clad is applied by welding using either an automated or manual process. These clads pose problem for the ultrasonic inspection during the routine inservice inspection. Inspection procedures based on NDE play an important role in structural integrity assessment and this involves the knowledge of the defects present in the structure in the areas particularly subjected to stresses induced by the pressurized thermal shock (PTS). These defects include sub-clad defects of the planar type generally perpendicular to the surface such as under clad cracks; weld cracks, lack of fusion and defects in the cladding. The most commonly used NDE method for these applications is ultrasonic testing. Evaluation of the acceptability of detected flaws according to ASME Section XI requires measurement of the depth of the flaw extending into the base metal. Accurate crack depth or size determination and cracks growth monitoring are essential for safe operation of critical components in industrial plants (1) . To obtain accurate defect sizes using conventional ultrasonic NDT equipment, considerable skill is required because; there are many factors, which affect the accuracy and sizing ability of the ultrasonic inspection. Two main factors that affect ultrasonic inspection of cladded components are: (a) the effect of clad, and (b) the beam spread of ultrasonic beam at longer range. The presence of a welded cladding layer particularly austenitic type, seriously affects the ultrasonic inspection due to its coarse grained anisotropic structure and the surface irregularities this leading to beam skewing, scattering and mode conversion of ultrasonic waves. The heavy scattering of ultrasonic waves in coarse grain structure of the clad results in poor signal to noise (S/N) ratio, in turn resulting in poor detection sensitivity. This problem is minimized to some extent by utilization of...