Paper presented at the 8th ECNDT, Barcelona, June 2002

Abstract

Shearography has been validated as fast and reliable inspection technique for aerospace components. Following several years phase of evaluation of the technique, meanwhile, shearography has entered the industrial production inspection.

The applications basically range from serial inspection in the production line to field inspection in assembly and to applications in the maintenance and repair area. In all applications, the main advantages of shearography, as very fast and full field inspection and high sensitivity even on very complex on composite materials have led to the decision for laser shearography as inspection tool.

In this paper, we present some highlights of industrial shearography inspection. One of the first industrial installations of laser shearography in Europe was a fully automatic inspection system for helicopter rotorblades. Complete rotor blades are inspected within 10 minutes on delaminations and debondings in the composite structure.

In case of more complex components, robotic manipulation of the shearography camera has proven to be the optimal solution. An industry 6-axis robot gives utmost flexibility to position the camera in any angle and distance. Automatic defect marking systems have also been introduced to indicate the exact position of the defect directly on the inspected component. Other applications are shearography inspection systems for abradable seals in jet engines and portable shearography inspection systems for maintenance and repair inspection in the field. In this paper, recent installations of automatic inspection systems in aerospace industries are presented.

1. Introduction

Following a several years phase of evaluation of the technique to show the principal applicability and prove the required sensitivity, now several production lines for aerospace components have been equipped with automatic shearography inspection systems..Shearography, although very similar to ESPI, is typically used for nondestructive testing rather than for material analysis and strain measurement. The shearography method is less susceptible to environmental noise and typically requires less of a technical understanding in order to operate the equipment. It is typically used qualitatively, because additional information and processing is required to determine the absolute value of the deformation. The consequent development to realize small and well working sensors facilitate nowadays an easy made application in complex systems. In this way it becomes possible to achieve an economic integration in automatic production inspection systems.

2. Automatic shearography inspection system for helicopter rotor blades

Helicopter rotor blades as highly sophisticated products are composed from different materials and components. They are safety relevant components and, therefore, 100 % quality control has to be assured.

Each rotor blade is manufactured as composite, with foam or honeycomb materials as core of the blade, covered on the outside with one or more layers of fiber reinforced plastics. As reinforcement carbon fibers, kevlar fibers or glass fibers are used. In special, highly loaded areas, as, e.g. at the front edge of the blade, metallic layers serve as additional reinforcement. Consequently, the production of these rotor blades follows a rather complicated and complex procedure. Therefore a 100 % inspection of the blades is required after production. After repairing of defective rotor blades, an inspection of the repaired area is also required. The rotorblades are mounted in a 10 m long vacuum chamber, fig. 1 and loaded with a relative pressure difference of up to 50 mbar .At this load, debondings and structural defects show up as tiny deformations of the surface of the rotor blades with amplitudes in the range of few micrometers. Two miniaturized shearing cameras, fig. 2 are positioned on a separate guiding system on each side of the rotor blade and observe both sides of the rotor blade. This allows the simultaneous inspection of both sides of the rotor blade during one loading cycle..Fig. 1: Helicopter rotorblade inspection system

Fig. 1: Helicopter rotorblade inspection system

Fig. 2: View inside the vacuum chamber with rotorblade in testing position

The inspection areas are illuminated by a 5 Watt Nd:YAG laser, coupled into two fiber coupling systems. The laser beam expanders are positioned together with the shearography cameras on the guide and provide a homogeneous illumination on the whole measurement field. This allows an inspection of areas up to of 600 x 800 mm² on each side of the rotor blade. After each loading and measurement cycle, the shearography cameras are moved on the linear guide to the next measuring area. Up to 15 measurement steps are required for complete inspection of the largest rotorblades.

The inspection results are fully automatically analyzed, by comparing the measured data with a set of earlier taught master data. This allows to distinguish between structural information and defects. The automatic defect localization is carried out during the test cycle and indicates the defect position on the screen. Sizes and positions of the defects are printed in a test report, which is automatically prepared after every test cycle.

3. Automatic shearography inspection system for thermal protection parts

Fig. 3: 6-axis industry robot in automatic inspection system of thermal protection parts.

In the aerospace industries lightweight sandwich constructions are very common. As example, the thermal protection parts of the European ARIANE 5 launcher are made of carbon reinforced composite materials using honeycomb structure cores and monolithic structures in one part. These thermal protection parts show quite complex shapes, as cylindrical or conical and contain flanges and edges. In an extensive validation process shearography has been chosen as technique for 100 % inspection of all components for its performance and inspection speed. Two different stressing methods are used for the shearography inspection of these components. The honeycomb composite parts are inspected with thermal load, because their structure is, comparing to the monolithic areas, porous and not completely sealed. The sealed monolithic carbon structures are stressed by vacuum. Due to the great variations of shapes of.the components the shearography camera is positioned by a very flexible 6-axis industry robot, fig.3.

The robot is mounted on a vibration isolated base plate in the center of a 20 m³ vacuum chamber. Additionally, the robot head carries 3 halogen heaters for thermal loading of the components, fig.4.

The robot's repositioning accuracy of less than 0.1 mm enables reproducible measurements of any complex part. The automatic defect recognition function of the system compares the results of "good" parts with the actual measurement results. Besides delaminations and debondings, the automatic defect detection function also shows missing or badly positioned honeycomb fillers.

The operator is supported with the localization of the detected defects by a defect marking system, mounted near to the shearography sensor on the robot head, fig.4. Using the measurement information, fig. 5, the software directs a laser beam onto the position of the defect at the surface of the test part. This makes it easy for the operator to manually mark the position of the defect on the part.

Fig. 4: Sensor, illumination optic, thermal loading defect marking system on the tip of the robot.

Fig. 5: Software interface with defect and marking menue.

4. Inspection of abradable seals

In order to optimize efficiency of jet engines, the gap between the rotating blades and the casing should be as small as possible. Therefore, designers introduce a socalled abradable seal between the blade and the casing. This seal is made from felt metal and has to be renewed after certain operation intervals.

The replacement seal is soldered manually to the casing. The manufacturer demands a 100% inspection of each replaced seal to avoid any damage caused by bad bonding between casing and seal. Fig. 6 shows a laser shearography inspection system for abradable seals. The jet engine section to be inspected is positioned inside the system. A shearography camera looks on a 30° section of the circumference while the section is excited by a electrodynamic shaker with white noise. The white noise excitation causes the unbonded areas of the seal to vibrate and produce an amplitude indication in the shearography system. The complete inspection time for one 300 segment is less than 30 seconds !

Fig. 6: Automatic shearography system for abradable seals inspection.

Fig. 7: Portable shearography system for field inspection of composite materials

5. Portable Inspection Systems

In many cases, as, e.g. maintenance or assembly of aeroplanes, it is not possible, to transport the part to be inspected into a stationary test system. For such purposes, a portable shearography inspection system has successfully been developed and used in industry. In Fig. 6 such a system is presented. It consists of a portable vacuum hood, which sucks directly to the surface to be inspected. Inside the hood, a shearography camera records all deformation of the component's surface, when a vacuum is pulled. Debondings or other defects will show up in typical deformation patterns, fig. 7. The operator can view the inspection result with ahead up display and therefore is flexible to operate on scaffolds or inside openings. Typical inspection time is less than 30 seconds per image. One image covers approx. 220 x 180 mm².

6. Conclusion

Recent improvements of shearography sensors and the development of automatic evaluation techniques enabled the successful integration of automatic shearography inspection systems into the production lines of aerospace industries. In comparison to conventional inspection techniques, shearography offers the advantages of non-contact, full field inspection and an overall significantly increased inspection speed.

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