for each radius and the flat sections (as shown in the schematic diagram). Accurate positioning of the curved arrays is essential for obtaining accurate results, and this can be particularly difficult on long composite parts. When accurate positioning is maintained, very good results can be obtained. The time-of-flight and amplitude C-scans shown on the right are from a hat-section test specimen with known defects, all of which were successfully detected and imaged using the probes illustrated in the schematic drawing. The middle row of Figure 3 shows B-scans for the top convex radius obtained for an actual composite stringer using a linear probe (as illustrated in the left-hand photograph). B-scans were obtained with the linear array without SAUL (middle image) and with SAUL (righthand image). Applying the surfaceadaptive SAUL algorithm (described later in the paper) allows a backwall signal to be measured, but the lateral extent over which there is a strong backwall signal is relatively short. The uniformity of measured signals and the lateral extent of the backwall signal are greatly improved using a 4x16 matrix array optimized for use on aerospace composites (a schematic drawing of the matrix probe is shown in the bottom-left corner of the figure). geometry parameters [3]. The increase in processing time is partially offset by a reduction in measurement time (compared to electronic scanning) achieved by firing all elements at once rather than firing by groups. SAUL has been automated in the M2M instrumentation so that the user only needs to input a few parameters as part of the ultrasonic setup process. SAUL’s ability to compensate for probe misalignment As discussed above and reported by Hopkins et al. [1], probe positioning is critical for obtaining satisfactory results, particularly for curved arrays. The complex shapes of composite parts and part-to-part variability increase the positioning challenge especially for long parts. The ability to fully automate inspection processes therefore depends on being able to address positioning errors and part variability. One automation approach is to install extremely precise positioning systems. Such systems work well, but Fig. 4: SAUL correction for a misaligned linear array Surface-Adaptive ULtrasound (SAUL) The innovative SAUL technique developed by the CEA and implemented in M2M instrumentation is used as a solution to the problems posed by complex geometries including probe positioning and part variability [2]. The objective of the adaptive technique is to generate a wave front that is normal to the front surface of the test specimen. The specimen shape is estimated in real time from the front-surface echoes. An iterative algorithm then optimizes delay laws based on minimizing the error function determined from calculating the travel times to the surface, which is also done on the fly. The delay laws are continually updated to adapt to the changing Fig. 5: Experimental setup and defects for scans No83 August - September 2013 / jec composites magazine 71