Sizing of flaws using ultrasonic bulk wave testing: A review

In this paper, the half-skip configuration of the Total Focusing Method (TFM) is used to image and size surface-breaking cracks. The TFM is an ultrasonic array post-processing technique which is used to synthetically focus at every image point in a target region. This paper considers the case of inspecting for cracks which have initiated from the far surface of a parallel-sided sample using an array on the near surface. Typically, only direct ray paths between the array and image points are included in the TFM algorithm and therefore the image obtained for this case consists only of root and tip indications; no specular reflection from the crack faces is captured. The tip indication often has such a poor signal-to-noise ratio that reliable crack depth measurement is challenging. With the Half-Skip TFM, instead of using directly-scattered signals, the image is formed using ultrasonic ray paths corresponding to the ultrasound that has reflected off the back surface and has then undergone specular reflection from the crack face back to the array. The technique is applied to experimental and simulated array data and is shown to measure the depth of small cracks (depth

Accurate depth measurement of small surface-breaking cracks using an ultrasonic array post-processing technique

Ultrasonic time of flight diffraction (TOFD) for sizing defects is based on the time of flight of the diffracted echo that is generated when a longitudinal wave is incident on a crack tip. This technique has the limitation during near-surface inspection due to signal superposition. Here, this limitation is overcome by using the shear wave-diffracted signal (instead of longitudinal wave) and hence called S-TOFD. Experiments were conducted on samples with defect tip closer to the surface of a flat plate sample to illustrate the utility of the S-TOFD technique. An increase in the flaw sizing accuracy, by using the shear wave-diffracted echoes from the tip and through the application of a signal processing technique (ESIT), was demonstrated.

Shear-wave time of flight diffraction (S-TOFD) technique

Ultrasonic sizing of short surface cracks.

Signal quality enhancement using higher order wavelets for ultrasonic TOFD signals from austenitic stainless steel welds

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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Time of flight diffraction (tofd) technique for accurate sizing of surface breaking cracks

This paper deals with an experimental study for evaluation of the TOFD technique for determination of size of cracks embedded in sub-cladding. It extends the results presented in the previous papers ( 1 , 2 ) . The studywas confined to simulated cracks (machined slits) underneath the cladding. It was found that crack heights ranging from 1.68 mm to 19.04 mm underneath stainless steel, Inconel and ferritic cladding could be sized with an accuracy of ′0.2 mm. Difficulty was experienced using TOFD to size cracks starting from the interface between Inconel/stainless steel cladding and the ferritic base ie (5 mm/6 mm below the scanning surface). However, these cracks could be detected and sized by scanning through the cladding from the opposite surface of the blocks. Even the small cracks of the order of 1.9 mm could be detected as long as they are not influenced by the lateral wave. Experimental errors are presented.

M. Palaniappan

Papers

Time of flight diffraction (tofd) technique for accurate sizing of surface breaking cracks

Time-of-flight diffraction (TOFD) technique for accurate sizing of cracks embedded in sub-cladding