|TABLE OF CONTENTS|
- - Zusammenfassung
2.1 Ultrasonic Inspection
2.2 High-Definition Radiographic (HDR) Inspection
2.3 Metallographic Analysis
3. Test Evaluation
3.1 Comparison between Ultrasonic and HDR Indications
3.2 Metallographic Verification
3.3. Round Bottom Holes
4. Calculating the POD
5. Conclusions and Prospects
When these components are welded, undesirable gas pores of sizes up to 0.5 mm dia. may be produced, especially when large sections are involved. Generally, these pores can be detected by ultrasonic inspection. In the assessment of ultrasonic inspection data, a major parameter apart from the incidence and spatial distribution of flaws is the detectability of pores, or POD (probability of detection), as a function of pore size.
This investigation was carried out to study the detectability of pores in titanium alloy welds by the ultrasonic immersion technique. High-definition radiography (HDR) was used as a reference method.
To determine the POD within confidence limits, the a-versus-a approach was selected. Principal problems embarrassing this approach proved to be:
|(i)||For real pores, the amplitude scatter is remarkably wider than for round bottom reference holes of known size, and|
|(ii)||the a-versus-a relation for the round bottom holes is not linear on a logarithmic scale, as would be expected, and the POD results vary widely with the decision threshold selected.|
Fig. 1 EB welded titanium compressor rotor
For characterizing the reliability of a non-destructive testing (NDT) system, a parameter called probability of detection (POD) is used. To calculate the POD, two different approaches have been described :
|(i)||Assess the test data in terms of the evidence it provides of the presence of a flaw (experimental POD), or|
|(ii)||correlate flaw size "a" with its associated signal amplitude "a".|
Fig. 2 C-scan of specimen (above) and scanning setup (below)
Fig. 3 20 by 20mm area selected for POD investigation (The large-area indications do not represent pores, but cracks generated by extreme weld parameters. These cracks were not involved in the investigation).
The specimen was inspected by ultrasonic immersion technique using a 40-mm spot-focused 25 MHz probe. This technique is maximally compatible with the test objective and especially with the required degree of defectivity. Used as a reference standard was one each round bottom hole 100, 200 and 400 um in diameter. The gain adjustment was selected for the 400-pm hole to produce an 80% full screen height (FSH) signal. At this setting the amplitude of the 100-um round bottom hole reached 28.24 FSH. This value was first taken as a lower limit, meaning that all indications below it would not be assessed. But when it became clear that for the POD calculation, also smaller indications should be included, a limit was set a 10% FSH relative to the basic noise of the unflawed structure.
The influence of flaw depth or depth position in the focused sonic field was considered by inspecting once with the focus in the weld plane and then again with the focus 1.5 mm behind it.
To keep the number of indications manageable for quantitative evaluation, a 20 by 20mm area (Fig. 3) was later selected for comparison with the radiograph.
The available specimen was perfectly suited for radiographic inspection, its pores all extending in a strictly confined plane, the fused volume. This permitted the radiated volume to be reduced by removing the remaining material to the thickness of the fusion zone plus a margin for a total of about 3 mm. In this manner, defectivity was maximized. The section was radiographed at 10X magnification.
2.3 Metallographic Analysis
To resolve disparities in pore size data derived from radiographic and ultrasonic inspection, respectively, various indications were verified metallographically. It was then seen that the radiographic size data were essentially correct.
Following machine evaluation, a "manual" evaluation was made by an inspector While this is a rather lengthy procedure, it beats automatic evaluation, especially in its ability to separate closely adjacent individual indications.
The 10X indications appearing on the high-definition radiograph were transferred to a transparent foil and registered with the accordingly scaled ultrasonic C-scan presentation.
When the radiograph was evaluated, however, it was seen that estimating the size of some of the indications was made extremely difficult by the low contrast.
For each indication, size "a" from the radiograph and signal amplitude "a" from ultrasonic inspection were tabulated for POD computation. For the purpose, the C-scan presentations were evaluated once automatically using the "ScanMaster" system and then again, for comparative purposes, manually by the inspector with an assist from image processing.
3.2 Metallographic Verification
Metallographic verification showed that substantial amplitude differences can occur even on apparently ideally round pores. This was impressively apparent on a 250 um dia. pore. Ultrasonic inspection should clearly have witnessed it, but the signal it produced did not rise above the basic signal threshold.
Fig. 4 Round bottom hole signal amplitude versus hole diameter
involve two further thresholds: (i) signals below a recording threshold ath < adec cannot be distinguished from background noise and (ii) adoption of a saturation threshold asat takes account of the fact that the applicability range of NDT systems is subject to an upper limitation imposed by saturation behaviour. Assuming a normal distribution for the signals and a linear relationship between the logarithms of the mean of the signal amplitude and the actual flaw size, the POD is given by the hatched area (Fig. 5).
Fig. 5 Schematic representation of POD model according to .
Fig. 6 POD results of round bottom holes for a signal threshold of 28.24% FSH and signal amplitudes in percent FSH
Fig. 7 POD results of round bottom holes for a signal threshold of 10% FSH and signal amplitudes in percent FSH
Fig. 8 Weld pore signal amplitude a versus pore diameter a
Fig. 9 POD results from the weld pores for a saturation threshold of 95% and a signal threshold of 10%.
|(i)||Owing to the relative moderate scatter of experimental data, the POD curves and the 95% confidence interval are spaced closely together.|
|(ii)||Since only few data reach or exceed 70% FSH, the result is little affected by the selection of saturation threshold asat|
|(iii)||Whereas the selection of noise threshold adec heavily impacts the position of the POD curve and hence the size 90/95 (the size at which the POD reaches 90% for a confidence level of 95%)|
|(1)||automatic identification of indications; sound field focus in the flaw plane|
|(2)||automatic identification of indications; sound field focus 1.5mm behind the flaw plane|
|(3)||as in (1), but indications evaluated by an inspector.|
In all three experiments the signal amplitudes exhibited notable variations for the respective pore size. In the POD analysis (Fig. 9), this caused wide confidence intervals. The 95% confidence curves reached 90% POD not until pore sizes were 0.4-0.8mm, depending on the experiment. Since pores of these sizes do not occur in the investigation, this finding invites critical analysis.
The investigation has produced essential insights on three counts:
The POD model used has recently come under critical discussion [2-4], with critical doubt focusing exactly on the linear relationship between system response and defect size. An attempt is therefore made to develop new POD concepts that take account of the actual relationship, starting with the physical method and its implementation . Playing a key role in the discussion is also the fact that flaws of like size can be expected to provide a wide spectrum of system response, as has been borne out by the present investigation. In  and , therefore, new recommendations are being developed that attempt to remedy the problem by making allowance for "effective reflectivity".