Sizing of cracks embedded in sub-cladding using the ultrasonic synthetic aperture focusing technique (SAFT)

Abstract

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...

A time-domain synthetic aperture ultrasound imaging method for material flaw quantification with validations on small-scale artificial and natural flaws

“…Li et al [10] proposed an adaptive weighting technique based on a focusing-quality index to suppress sidelobes and improve the performance of SAFT. Baby et al [19] presented the experimental work demonstrating the feasibility of ultrasonic SAFT to obtain improved detection and sizing of vertical/inclined simulated cracks underneath different claddings. Brekow et al [20] developed the defect sizing on power plant components using PA-based SAFT.…”

Simulation of ultrasonic phased array technique for imaging and sizing of defects using longitudinal waves

“…The reflected signal is picked up and the amplitude of the reflected signal gives an indication of the size of the defect. Advanced ultrasonic methods like the time of flight diffraction (TOFD) technique [7], synthetic aperture focusing technique (SAFT) [8] and phased array techniques [9,10] also been extensively used to accurately detect and size cracks in critical engineering components.…”