Developments in Ultrasonic Phased Array Inspection III Improved Phased Array Mode Conversion Inspections Using Variable Split Aperture Processing R. ong, P. Cawley, Imperial College, United Kingdom J. Russell, Rolls-Royce Marine, United Kingdom ABSRAC Work is being conducted to develop phased array inspection of stainless steel welded pipes. For inspections of defects lying on the weld fusion face often we would like to use focused waves reflected and mode converted at the inner surface of the pipe. For inspections where the transmit and return legs of the wave propagation differ there are difficulties to generate a consistent B-Scan image for single depth focusing. A solution is to collect data as Full Matrix Capture (FMC) and apply a processing technique that allows focusing to be ideal for every pixel within the Bscan image. Further improvements to the Bscan image can be made by having a split aperture that varies as a function of focal depth. his paper illustrates the problem associated with displaying mode converted results, introduces the FMC principle, and describes the variable split aperture signal processing. he inspections were also modeled using the CEA CIVA software which was compared to experimental results. INRODUCION We are developing phased array techniques for the inspection of welds in stainless steel pipe lines [1,2] which often proves challenging. he inability to access both sides of the weld might warrant the use of waves that are mode converted at the inner surface of the pipe to propagate a wave at the desired angle which would maximize a specular reflection from a defect with a postulated inclination. Even where access to both sides of a weld is available, inspections using mode converted waves might be preferred rather than using waves that propagate directly through the weld since the structural complexity of austenitic stainless steel welds can disturb the ultrasonic wave propagation. For mode conversion inspections we use one of two configurations as shown in Figure 1a and Figure 1b. he delay laws for the inspections shown in Figure 1a and Figure 1b are both computed considering the same transmit wave propagation path towards the defect where a shear wave () is mode converted at the inner surface of the pipe into a longitudinal wave (). he delay laws required for the two inspections differ in that the reflected path off the defect is indirectly reflected back to the transducer along the transmit path for Figure 1a giving the total path traveled as but directly reflected back towards the transducer for Figure 1b giving a path traveled as.
Defect 1 on lower fusion face Defect 2 on upper fusion face Figure 1 - Phased array inspection techniques of weld fusion face, mode converted wave for defect lying on lower fusion face and mode converted wave for defect lying on upper fusion face Aperture Upper surface Summed A-scan Focal point Summed A-scan Focal point ower surface d Aperture Aperture Upper surface Focal point r 1 Focal point r 2 ower surface t 1 d) t 2 d Figure 2 - Wave propagation paths using single depth focusing for mode converted inspection, inspection showing wave propagation path for a point a distance d from the focal point, mode converted inspection and d) mode converted inspection showing wave propagation path for a point a distance d from the focal point
Generating a B-Scan image for Single depth focusing mode converted inpsections With conventional phased array data acquisition a B-scan image is generated by the grouping together of M adjacent elements to form an aperture [3]. he received time domain signals (A-scans) from all the elements in the aperture are summed to form a single A-scan. Focusing is achieved by delaying the excitation and reception of individual elements in the aperture such that propagated waves arrive simultaneously at a desired focal point. he aperture is sequenced (electronically scanned) along the length of the phased array. For a phased array with N elements this allows (N-M+1) scan locations. A summed A-scan is obtained at each step with all A-scans combined finally to form the B-scan image. Figure 2a illustrates the positioning of a single summed A-scan that goes to make up the B-scan image for a mode conversion inspection where delay laws are computed for single depth focusing. he point along the A-scan that coincides with the delay law for the central element of the aperture is positioned at the focal point. he A-scan is orientated with the longitudinal () wave propagation path. he difference in total travel time (dt) for a point along the propagation path, a distance (d) from the focal point, as shown in Figure 2b, is obtained from the delay laws for the centre of the aperture being proportional to the distance travelled and is given by Equation 1 dt = 2d / (1) v where v is the velocity of the longitudinal wave. hus for a linear discretisation, each point along the summed A-scan can be correctly located in the B-scan image though optimal focusing will only occur at the focal point when using single depth focusing delay laws. For a inspection the wave propagation path from the centre of the aperture to the focus is shown in Figure 2c where t and r signify the transmission and reception longitudinal wave propagation paths to the focus. he difference in total travel time (dt) for a point along the propagation path, a distance (d) from the focal point, as shown in Figure 2d and disregarding propagation in the water path, is given by Equation 2 ( t + r ) ( t + r ) 1 1 2 2 dt = 2d / v + (2) v where (t 2 + r 2 ) is not proportional to d. hus in order to position the summed A-scan appropriately within the B-scan image all the wave propagation paths must be known for all points which is not ideal for conventional phased array data acquisition. FOCUSING A A PIXES WIHIN A B-SCAN IMAGE USING FMC DAA Full Matrix Capture (FMC) [4,5] is an alternative to conventional phased array data acquisition which involves the collection of the complete set of time-domain data (A-scans) for all combinations of transmit and receive elements. FMC produces a matrix of data that can subsequently be used to generate any ultrasonic beam type that the array is capable of producing in post-processing. Having the full matrix of data allows focusing at all points in the image using the otal Focusing Method (FM) [6]. For inspections of a weld fusion face we prefer to use waves that propagate at a given angle in the component for which we have a physical understanding of the wave/defect interaction. However, for an image generated using FM the mean wave propagation direction varies between different points in the grid. Instead, we have proposed a technique, that we have termed AFM (Almost otal Focusing Method) [7], which allows focusing at every point within a B-scan image and yet retains a consistent wave propagation direction. he collection of FMC data and the ability to focus at all pixels within the B-scan image allows a possible solution for the positioning A-scans with the B-scan image for the mode conversion inspection.
7mm Immersion Phased Array 128 el - 2MHz- angle 7 o Upper Fusion face Defect Approx 60mm 169mm ower Fusion face Figure 3 - Experimental arrangement using a conformable membrane device for mode conversion inspection of defect located on the upper weld fusion face EXPERIMENA DAA Experimental data for a inspection was obtained to assess the effectiveness of generating an AFM B-scan image that is a consistent solution to the problem associated with inspection. Experimental testing was completed on a flat plate welded test piece with the weld cap left undressed to replicate the target application [7]. he test piece was manufactured by Sonaspection UK [8] from two flat stainless steel (304) plates of greater than 50mm thickness, with lower and upper weld fusion faces of 25 and 10 degrees respectively. he ack of Side Wall Fusion Defect, which lay at the bottom of the upper weld fusion face angled at 10º, was embedded in the test piece using an in-house Sonaspection technique. he 3 rd generation membrane device [1, 2] being developed by Rolls Royce Marine [9], shown as a schematic in Figure 3, was used for the inspections and incorporated a standard linear 128 element, 2 MHz, 0.75mm pitch phased array probe from Imasonic, France [10]. he phased array was angled in the device housing at 7º and the height of the first element above a plane surface is 7mm. hese parameters were chosen to reduce the likelihood of a surface reflection appearing in the B-scan image [9]. A Peak ND phased array controller [11] was used to obtain experimental FMC data which was then processed using the Imperial College software [2]. Experimental results for the inspection of the defect are shown in Figure 4. Results were processed as for conventional phased array controller display software where the B-scan image is generated in the same manner as for inspections using a linear discretisation for each point along a summed A-scan. Single depth focusing was used when processing the results with a depth of focus of 15mm, 25mm, 35mm shown in the B-scan images of Figure 4a, 4b and 4c respectively using transmission and reception apertures of 32 elements. A line represents the weld fusion face which is approximately located in the B-scan image. he distance from the top of the defect to the top surface of the test block is 21.5mm. he reflection off the defect is most appropriately positioned within the B-scan image when using the 25mm focal depth. For all other focal depths the reflection off the defect is misplaced within the B-scan image and the signal to noise ratio (SNR) is reduced. he SNR drops 7.7dB for the 35mm focal depth and 4.5dB for the 15mm focal depth. Figure 4e shows the B-scan image when processing the results using the AFM technique. he results show that the reflection off the defect is now almost correctly located in the B-scan image though the orientation of the reflection does not coincide with the angle of the fusion face. he reason for this is that the same aperture has been used for both transmission and reflection which was not appropriate since the return path does not reach the centre of the aperture.
Outer pipe dia Reflection off defect B-scan image Inner pipe dia upper fusion face lower fusion face d) Figure 4 - Experimental results for inspection using single depth focusing. B-scan images shown for 15, 25, 35 mm single depth focusing and d) AFM focusing at all pixels 4 26 15 45 o 30 45 o 56 45 o 45 Figure 5 - Calculated split apertures required for the inspection of a defect at a distance of 15mm, 30mm, and 45mm from the lower surface of the component under test. Split aperture dimensions shown in mm
Outer pipe dia Reflection off defect B-scan image Inner pipe dia upper fusion face lower fusion face d) Figure 6 - Experimental results for inspection using split aperture AFM processing technique. B-scan images shown for mean split apertures of zero, 12, 24 and d) 36 elements SPI APERURE For a inspection the return leg of wave propagation may not reach the centre of the transmission aperture therefore it would be appropriate to have separate apertures for transmission and reception to achieve this. Calculations of appropriate split apertures dimensions for the inspection of a defect at various depths in a component are shown in Figure 5. For a feature located 15mm from the lower surface of the block a 27mm aperture offset of the receiver to the right of the transmitter would ideally be required as shown in Figure 5a. For a feature located 25mm from the lower surface of the block the 15mm offset now appears to the left, as shown in Figure 5b. Further improvements to the Bscan image for a inspection can be made by having a split aperture that varies as a function of focal depth. he calculations of appropriate split apertures dimensions, shown in Figure 5, were used when processing data for the inspection of the mid point of a defect located at 25mm from the lower surface of the test piece. Experimental results for inspection using a split aperture AFM processing technique are shown is Figure 6 where B-scan images are shown for various mean split apertures of zero, 12, 24 and 36 elements in Figures 6a, 6b, 6c and 6d respectively using transmission and reception apertures of 32 elements. he mean transmit and reception aperture separation occurs in the mid depth of the B-scan image with the required split aperture varying with depth from zero at the bottom of the image to twice the mean value at the top. For zero split aperture the reflection off the defect is correctly located though the reflection appears greater in size and orientated somewhat vertically. With increasing split apertures until the most appropriate shown in Figure 6d, the reflection off the defect reduces in size and becomes more orientated to its actual position on the fusion face. However, the SNR decreases with increasing degree of split aperture used by 4.8dB. his is due to the increased beam spreading for the greater propagation distance related to larger separations between the transmit and reception aperture as seen in Figure 5.
SIMUAED DAA he CIVA software [12] was used to obtain simulations of phased array inspection of a weld fusion face. he CIVA Defect Response module simulates beam-defect interactions and predicts the amplitude and time of flight of various echoes. For this paper we will be concerned only with the inspection of a defect shown in Figure 3 that lies on the lower part of the upper 10º weld fusion face. he phased array was modelled as a 128 element immersion array with a 0.75mm pitch angled at 7º and with a 7mm standoff to the surface under test. Data was collected in FMC mode which has become available in version 9 of the CIVA software. he simulated FMC data set was processed using the Imperial software. he simulated results for the inspection of defect are shown in Figure 7 using transmission and reception apertures of 32 elements. Simulated results for the inspection using the split aperture AFM processing technique are shown in Figure 7 where B-scan images are shown for split apertures of zero, 12, 24 and 36 elements in Figures 7a, 7b, 7c and 7d respectively. For the zero split aperture, shown in Figure 7a where the transmit and receive apertures are the same, the reflection off the defect is elongated. With increasing split apertures until the ideal split aperture shown in Figure 7d the elongation of the reflection off the defect is reduced and the reflection is ideally located on the upper weld fusion face. In addition, for increasing split apertures from zero to 30 elements the signal amplitude of the reflection off the defect reduces by 4dB. he simulated results in Figure 7 compare well to the experimental results shown in Figure 6 though the drop in signal amplitude for increasing split aperture is marginally less for the simulated results. Outer pipe dia Reflection off defect B-scan image Inner pipe dia upper fusion face lower fusion face d) Figure 7 - CIVA simulated results for inspection using split aperture AFM processing technique. B-scan images shown for mean split apertures of zero, 12, 24 and d) 36 elements
CONCUSION We have obtained simulated and experimental FMC data for inspections using mode converted ultrasonic waves. For inspections where the transmit and return legs of the wave propagation differ there are difficulties to generate a consistent B-Scan image for single depth focusing. We have shown that a solution is to collect data as Full Matrix Capture (FMC) and apply a processing technique that allows focusing to be ideal for every pixel within the B-scan image. Further improvements to the B- scan image where shown to be made by having a split aperture that varied as a function of focal depth. REFERENCES 1) ong, R., Russell, J., Cawley, P., Habgood, N., Non-Destructive Inspection of Components with Irregular Surfaces using a Conformable Ultrasonic Phased Array, Proc. 6 th Intl. Conf,