Balanza comercial Ecuador – China

The encoded scans of the pipe sample can be used to show the presence of a defect, but further processing of the data is required in order to extract sizing data for the defect. From the colour maps of the encoded scans of the sample, the clear areas of the pipe can be identified by the constant wave velocity and amplitude of both the CSH0 and CSH1 modes. If these values are constant throughout the length of the sample then it can be said to be defect free in terms of the corrosion patch style defects that the system here is designed to locate.

As shown in the encoded scans, the most convenient way of locating whether a defect is present in the sample is to look for an area where the amplitude and arrival time of the CSH1 mode differs from its normal value. The mode that is affected by the defect can be extracted from the rest of the signal using time gating. This allows the affected signal to be considered separately for all of the axial distance steps, as shown in Figure 5.15.

Figure 5.15: Presentation of just the long path CSH1 mode section from the encoded scan, showing the mode at all distance points. Here the change in amplitude at the defect becomes more obvious.

The reduced time signal here for all distance steps can be manipulated to present the relative amplitude of the mode at each distance step by calculating the Root Mean Squared level (RMS) of the waveform. This is the value of the square root of the average of the squares of the waveform values:

XRM S= v u u t1 N N X n=1 |Xn|2 (5.2)

This yields an RMS value for every distance step in the scan, which effectively generates an amplitude profile of the CSH1 mode for each distance point in the axial scan.

Changes in the amplitude of the CSH1 mode will indicate that a defect is present that has redistributed the CSH1 mode energy, due to the mode interacting with the defect. This will manifest as a drop in amplitude of the CSH1 mode as the probe is moved past the defect area. The change in amplitude of the CSH1 mode along the length of the sample will be a valid method of detecting the defect whether the remaining thickness is above or below the cut-off thickness of the sample due to the difference that will be seen in the selected time gate. The raw amplitude profile from this data can be presented as in Figure 5.16, highlighting the axial size and position of the defect.

Figure 5.16: Amplitude profile of the long path CSH1 mode through the axial length of the pipe, showing a decrease in amplitude as the probes pass the axial position of the defect.

Taking into account the width of the transducer in the axial direction, the length of the defect in the axial direction can be calculated. If a single defect is assumed in the pipe then this process can be carried out with minimal user input. This is achieved by normalising the value of the RMS at each axial point to a baseline

value by taking an average of a section that is assumed to be defect free. The minima of the profile along the length of the pipe can then be found, which will correspond to a point where the path of the ultrasound is centred over the defect so that the guided wave energy is unable to travel around the pipe without interacting with the defect. The value of the amplitude at a point is then compared to the average value of the non defective area and when the value reaches this average value the full extent of the defect’s effect on the signal is acquired.

Subtracting the active area of the transducer from this defect extent then gives the axial length of the defect. This is an effective method for finding the axial extent of a single defect in the sample, as the minima of the profile will be easy to find, with the amplitude of the profile returning to the average value as the probe passes out of the defect area. The procedure would become more complex for multiple defects in the sample axially due to the presence of several minima, with the accuracy of the measurement of the axial extent likely to decrease if the defects are close together axially. If the defects are close to each other axially, then the amplitude profile would not be able to return to its maximum value after passing over the first defect due to the signal still being affected by the other defect, which would complicate the extraction of information from the profile, the effect of this is illustrated in more detail in Chapter 6.

The furthest extent of the defect effect is shown in Figure 5.17, with the full extent of the defect being this value minus the active area of the transducer, giving an approximate axial extent of (47± 1)mm. This is in reasonable agreement with the measured value of (48.6± 0.1)mm.

This technique can be applied for either the long path signals or the short path signals, depending on where the defects are expected to lie around the circum- ference. The mode that is affected can be found from the colour map, allowing the relevant area of the signal to be gated out of the full signal for further consideration. If this information is not known, both can be tested to check for the presence of defects in the signal. The use of the quantitative A-scan data rather than relying on the qualitative appearance of the encoded scan is important, as the appearance of the defect in the colour map is likely to be different than the actual data due to the limited number of colour levels in the map. However, when the defects are very shallow (below 10%) or have a small circumferential extent, a combination of the two methods may be necessary as the drop in amplitude for the CSH1 mode may be minimal as the mode will be only affected by a slowing of the wave in the case of the CSH1 mode so the energy will still be contained in the window. The minimum detectable defect will be considered in Chapter 6.

Figure 5.17: Amplitude profile of the long path CSH1 mode through the axial length of the pipe, normalised to an average value of the defect free section of pipe, with the outer extent of the defect’s effect shown by the red lines.