|NDT.net - February 2003, Vol. 8 No.2|
Paper presented at the 8th ECNDT, Barcelona, June 2002
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.
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.
Fig. 3: 6-axis industry robot in automatic inspection system of thermal protection parts.
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.
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
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