By the end of 2010, Airbus will have the world's most advanced Laser UT system for the inspection of composites. The equipment will be installed at Technocampus EMC2 (Nantes, France), the technology centre for the development of industrial applications of composite materials created jointly by Airbus, EADS-IW and CETIM.
Laser-ultrasonics (LUS) is generally defined as a technology in which one laser generates ultrasonic waves and another laser coupled to a detection system detects the associated ultrasonic displacements. This new testing technology offers important technical advantages over conventional techniques, such as the angle of incidence of the laser beam on complex surfaces of aeronautical components may be varied by a value 45 times higher than that of conventional probes without the need for ultrasonic coupling, thus allowing the header to be located at a much greater distance from the part surface.
In 1998, Lockheed Martin built the first LUS facility for the Joint Strike Fighter proposal. Two additional facilities were subsequently built for the F-22 and F-35 programs, resulting in the first big scale implementation of LUS for the inspection of composites for the aeronautic industry (1).
However, despite a long development period and some industrial successes, LUS has not yet been implemented in the aeronautic industry on a large scale. Two main reasons have limited the spread of LUS for the inspection of composites: the lack of reliability of various prototypes used to validate the technology for line production and, secondly, the acquisition cost of the LUS equipment. Now, a Spanish company, Tecnatom in alliance with its North American technological partner iPhoton Solutions LLC, have introduced a new approach that addresses these limitations. The strategy consists of adopting technologies from high-volume industries to address
the cost and reliability issues simultaneously.
LUS uses one laser to generate ultrasonic waves and another laser coupled to an interferometer to detect the corresponding ultrasonic displacements. The CO2 laser is the recognized choice for generation in composite materials because its wavelength (10.6 ìm) is strongly absorbed in most organic materials. The CO2 laser is absorbed in a very thin layer (10 to 200 ìm deep) at the surface of the composite, typically in the polymer matrix material or the peel ply. This absorption produces a local temperature elevation which in turn creates a local thermal expansion. This sudden thermal expansion generates the ultrasonic waves. This generation mechanism launches longitudinal ultrasonic waves in a direction normal to the surface, independent from the angle of incidence of the CO2 laser beam. This characteristic constitutes the main benefit of LUS for the inspection of composites.
Parts can be inspected with a very high tolerance relative to the angle of incidence.
The detection of the ultrasonic waves is accomplished by a long-pulse single-frequency laser combined with an interferometer. The detection laser beam illuminates the area where the generation laser hits the part. The detection laser light is collected and sent to a confocal Fabry-Perot interferometer. The mechanical displacements created by the ultrasonic waves induce small changes in the optical frequency of the detection laser. Those small changes in optical frequency are demodulated by the Fabry-Perot interferometer, resulting in signals very similar to the signals obtained by conventional piezoelectric transducers.
The generation and detection laser beams are aligned on top of each other and sent to an optical scanner such as a two-dimensional galvanometer system. The two laser beams are indexed over the sample surface while firing the lasers. Each laser shot results in an ultrasonic signal. Figure 1 shows a typical laser-ultrasonic signal at a single spot in a composite material. Figure 1 also shows images generated from one scan line, and from a two-dimensional scan area.
Typical optical scanners and laser powers limit the scanning area to approximately 1 m2 and to incidence angles below 45°. For the inspection of composite parts that have complex shapes or large sizes, or both, the optical scanner or the part needs to be repositioned to completely inspect the part. This repositioning can be obtained in an automated manner by putting the optical scanner on a robot.
Robotic systems for LUS
The first LUS systems mounted on robots (1,2) used gantry-type robots. Optical alignment of the CO2 laser beam in the optical scanner must be precisely maintained to obtain valid ultrasonic results. Optical fibres cannot be used in an industrial environment to transmit the CO2 laser beam. Therefore, the most obvious solution is to move the CO2 laser along with the optical scanner. This approach requires gantry robots because only this type of robot can move equipment as large and heavy as an industrial CO2 laser. Gantry robots present several disadvantages, the most important being the cost.
The gantry robot is typically the single most expensive element of a LUS system that includes such a robot The automobile industry has been using articulated robots in very large numbers for several years. Consequently, those robots are very reliable, readily available from several manufacturers, and reasonably priced.
LUS technology benefits from leveraging the robotic experience from the automobile industry by adopting articulated robots instead of gantry configurations.
In addition to the acquisition cost advantages, articulated robots have short delivery lead times, are easy and inexpensive to install, do not require as large a footprint, and benefit from a very large base of resources in terms of accessories, vendors, software and qualified labour.
The problem of optical alignment of the CO2 laser beam with the optical scanner is solved by exploring the large number of resources available for articulated robots. The laser processing industry has been using CO2 lasers and articulated robots for several years. A beam delivery system was developed to use CO2 lasers with articulated robots for material processing. The concept of beam delivery system consists in rigid tubes coupled together by rotation joints in which two mirrors are mounted. The CO2 laser propagates along the centre of the tubes. With near perfect alignment at the entrance of the beam delivery system, the CO2 laser beam transmits from the entrance to the exit of the beam delivery system through reflection via the mirrors located in the joints.(This concept is illustrated, in Figure 2.)
TECN IPLUS systems
A LUS concept, called TECN iPLUS, has been developed by Tecnatom, supported by its North American technology partner iPhoton Solutions, which uses a beam delivery system mounted on an articulated robot. To increase the working envelope, the robot, the beam delivery system and the CO2 laser are mounted on a linear rail. The beam delivery system requires a support and a tool balancer. Such a LUS system is called TECN iPLUS II and is shown in Figure 3.
The linear rail provides an almost unlimited working envelope to TECN iPLUS II in one direction. The total working envelope is approximately defined by the travel range of the linear rail, 3m. in the direction perpendicular to the linear rail and 5m. in height.
Articulated robots provide a flexibility which is not possible with gantry-based approaches. Some applications require the inspection of composite substructures inside larger structures, such as stringers inside a fuselage. These restricted access applications are not compatible with the beam delivery system of the TECN iPLUS II configuration. To respond to these restricted access applications, TECN iPLUS III was developed. In TECN iPLUS III systems, the beam delivery system is made of two standard beam delivery systems joined together on axis 3 of the robot. This approach, combined with a cantilevered linear rail, provides over 6m. of penetration inside a structure (a fuselage for example) in addition to the working envelope of TECN iPLUS II. (The TECN iPLUS III concept is shown in Figure 4 and an application is shown in Figure 5.)
To maximize the benefits of the TECN iPLUS systems, sophisticated ultrasonic data processing and system control are necessary. With its extensive experience in non-destructive testing, TECNATOM has developed an advanced data acquisition system (DAS), based on state-of-the-art electronics and proven software for mechanical controlling and US signal recording and evaluation. These two elements, electronics and software, assure the high productivity and the reliability that industrial manufacturing processes require. DAS is able to exchange ultrasonic data with other current and future formats. Additional productivity tools can be implemented in the data acquisition and evaluation software, such as an automated trajectories generator and an automatic defect analysis and evaluation module. (Figure 6, on page 11, shows an example of the processing results of a full fuselage inspection)
The integration of commercial articulated robots represents an important advancement toward the wide-spread adoption of LUS by aircraft manufacturers. The significant reduction in cost, compatibility with current robotic systems, ease of installation, and small plant footprint are among the advantages that can make the difference. Currently, a TECN iPLUS III system is in construction for a 2010 delivery. This system will establish a new standard for industrial LUS systems for aeronautic manufacturing.l
(1) M. Dubois, T. Drake, and M. Osterkamp, ‘Low-Cost Ultrasonic Inspection of Composites for Aerospace Applications with LaserUT™ Technologies’, Journal of JSNDI, Volume 57 Number 1, pp 11-18, 2008. (2008)
(2) C. J. Fiedler C. J., T. Ducharme, J. Kwan J., “The laser-ultrasonic inspection system (LUIS) at The Sacramento Air Logistics Center”, Review of Progress in QNDe, edited by Thompson D. O. and Chimenti D.e., Plenum Press, New-York, vol. 16, pp. 515-522, (1997).