Adhesively bonded composite repairs exhibit significant advantages in terms of mechanical efficiency compared to those effected using mechanical fasteners. However, they are at the same time extremely sensitive to process parameter variations. Small deviations against the repair specifications and subsequent flaws, could lead to disproportionally larger consequences to the final mechanical performance of the repair and to the integrity of the structure. Moreover, the inevitable differences between laboratory and repair shop conditions could induce additional “problematic” areas, which need to be reliably traced before the certification of methods and the release of individual aircraft to flight. Consequently, the existence of reliable and easy to apply NDT techniques is of capital significance to repair reliability and to flight safety. For this reason, existing NDT principles need to be adapted to the specificities of bonded composite repairs, in order to guarantee quality and durability in order to achieve flightworthiness and certification.
This proposal seeked to address this need by comparative evaluation of three different NDT techniques (namely piezoelectric ultrasound, shearography and laser ultrasound). The work assessed flaw detectability (e.g. delamination, debonding, porosity, foreign object inclusion etc.), functional reliability, repeatability of results, operational constraints, overall performance and applicability to the typical bonded composite repair cases of the aeronautical industry.
The latest developments in the NDT technology were used, in terms of methodology and equipment, which are already available among the consortium members. Moreover, as most aircraft structural repairs were not performed under ideal conditions in a laboratory, the comparative evaluation was undertaken in real-life maintenance environment, through detection and verification of flaws in actual repaired aircraft structural components.
All methods showed good indication of feature locations and similar sizes, for a sample without a repair patch, but LU and ACU showed smallest feature dimensions, i.e. 10x10mm generated feature. Additionally, ACU and MCU were able to indicate depth of features since they were in pulse-echo mode. Inspection sensitivity is expected to be superior with LU method due to its small footprint, although in one dimension only since the other dimension was limited by distance between generation and detection.
None of the methods required surface preparation, but only LU and LS where non-contact and non-invasive, other methods did require a gel or water which create contamination issues in particular for repair patch samples. As far as safety is concerned, LU has the highest level, requiring eye protection and if performed on a real sample it would require a special shielding around inspection zone. LS although safe to use, it was recommended to avoid direct eye contact with beam, all other methods do not require particular health and safety issues. Both LU and LS required a level of expertise to generate and interpret data, but CU methods required a minimum of ultrasound testing (UT) level 1. Inspection speeds were much higher with LS, but its level of detail was the lowest. If both speed and detail were required, it was recommended to have a two-level approach. Use LS as a first rough estimate and if an indication is detected, then use LU or CU to extract detailed information.
LU and CU have both shown similar capabilities in identifying artificial delaminations and they both require some level of expertise or training. However, LU had the obvious advantage, over CU, of being non-contact and non-invasive. LS as LU had the advantage of being non-contact and non-invasive, in addition possessed fastest inspection speeds, but had poorest sensitivity. As far as safety is concerned LU has the highest level requiring eye protection and if it were performed on an in-service sample, it would require a special shielding around inspection zone. LS although safe to use, it was recommended to avoid direct line of sight with beam. Inspection speeds were highest using LS, but its level of detail was the lowest. If both speed and high resolution were required, it was recommended to have a two-level approach. Use LS as a first rough detection system and if an indication was found, then use LU or ACU to acquire additional detail. Feature depth information may be extracted from LU data using additional signal processing and interpretation. Finally, LU inspection speeds can significantly increase by using a higher repetition rate laser, but at a higher cost.
All methods showed good indication of a general area where defects were inserted, for a sample with repair patch, but only MCU and LU were able to identify individual flaws, where smaller indications were 10mm and 6mm, respectively. These flaw dimensions fall within the aerospace industry requirements of reporting for repair a delamination size which is greater or equal to 8mm diameter or 8x8mm. In this comparison, although ACU and MCU were expected to achieve such resolution due to sensor diameters of 5mm and 6.25mm, LU method showed higher resolution capability and therefore better compliance. Additionally, only LU was capable of detecting 15mm intended flaw. However, ACU and MCU were able to indicate depth of features and only ACU and LU were capable of detecting patch details.