Effect of initial pH and temperature of iron salt solutions on formation of magnetite nanoparticles

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.

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.

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

Time of flight diffraction (TOFD) is a well-developed ultrasonic non-destructive testing (NDT) technique which has been applied successfully for accurate sizing of defects in thick sections. Codes of practice such as ASME now permit TOFD for routine examination as an alternative to radiography for thick weldments. However, examination of thinner sections by TOFD has its limitations. The main limitation is that as the thickness of the specimen reduces, the lateral wave, diffracted wave and the back wall echo merge together and it is very difficult to identify and size the discontinuity. Also, the size of conventional transducers limits the required probe separation. Limited success has been obtained internationally through the application of miniature probes and software for extending TOFD to lower thicknesses. In these cases, the minimum thickness that has been examined is 7 mm. A new methodology based on a simple and novel combination of TOFD and the immersion technique has been proposed by the authors that successfully extends the application of TOFD to thinner sections down to 3 mm. Immersion coupling provides a delay line, the necessary angles and probe separation making it possible to examine the thin components successfully. This paper highlights the results of detailed experimental investigations on immersion TOFD and its successful application for the evaluation of welds in hexcan used for encapsulating nuclear fuel pins.

Immersion and TOFD (I-TOFD): A Novel Combination for Examination of Lower Thicknesses

Ultrasonic testing is a mandatory requirement during inservice inspection of the welds in the main and safety vessels of a prototype fast breeder reactor. Presently, conventional ultrasonic techniques have been proposed for use. Two of the main challenges likely to be encountered during ultrasonic testing are the high temperature of the vessels (around 423-473 K during shutdown) and the background radiation. Ultrasonic time of flight diffraction is now well established as a technique for NDT of thick (greater than 12.5 mm) weldments. The main advantage of using time of flight diffraction is its increased accuracy for discontinuity detection and fast scanning times. Conventionally, time of flight diffraction has been applied only for carbon steel weldments and at ambient temperatures. This paper highlights the successful application of time of flight diffraction for testing of stainless steel weldments at high temperature (423 K). Experimental studies reveal that the diffracted signal amplitude decreases and, hence, the time of flight diffraction image contrast decreases with increasing temperature. Additional gain was necessary to compensate for the decreased signal amplitude. Analysis of the experimental data indicated that the additional gain necessary to compensate for the lower signal amplitude varied nonlinearly (quadratically) with temperature. The errors in the discontinuity dimensional measurement were observed to be less than 5% compared to measurements at ambient temperature.

Time of Flight Diffraction Testing of Austenitic Stainless Steel Weldments at Elevated Temperatures

Temperature is one of the key parameters employed as an indicator of the proper functioning of an industrial component or plant. Thermocouples are widely used for temperature measurement. In the nuclear industry, stainless steel sheathed thermocouples are preferred due to their better mechanical properties. Performance of the thermocouple is related to a variety of parameters including fusion of the junction weld, wall thinning, and others. While general continuity checks help in assessing the thermocouple, a deeper insight into the physical construction through techniques like radiography help in evaluating the condition of the thermocouple and thus preventing premature failures. In this paper, the authors highlight the application of digital image processing techniques for the quantitative evaluation of features in radiographs of small diameter (less than 4 mm) sheathed thermocouples. Through the application of digital image processing techniques, element weld ball size, average spacing between sheath and weld ball, and other variables could be measured reliably. A flowchart has also been devised for the regular application of digital image processing techniques. Based on the experimental results, authors recommend that present standards can also incorporate the use of appropriate image processing techniques along with conventional radiography, especially for thermocouples with diameters under 4.0 mm, for enhanced reliability and sensitivity of testing.

R. Subbaratnam

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

Immersion and TOFD (I-TOFD): A Novel Combination for Examination of Lower Thicknesses

Time of Flight Diffraction Testing of Austenitic Stainless Steel Weldments at Elevated Temperatures

Image processing for enhanced quality assurance of sheathed thermocouples