Composite Structural Integrity NDT with Automatic Shearography Measurements

This month's "NDT Solution" reports on successful industrial implementation of a technology developed after extensive laboratory studies. The authors describe the integration of automatic shearography testing systems into the production lines of aerospace components. The feature describes a detailed testing procedure developed by a French helicopter producer in testing rotor blades for structural discontinuities. G.P. Singh Associate Technical Editor

 

Introduction
Shearography has been validated as a fast and reliable testing technique for composite materials in aerospace components. Following several years of evaluation of the technique to show the principal applicability and prove the required sensitivity, the first production lines for aerospace components are now being equipped with automatic shearography testing systems.

Shearography, although very similar to electronic speckle pattern interferometry (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 operating the equipment typically requires less technical understanding. It is generally used qualitatively because additional information and processing is required to determine the absolute value of the deformation.

The consequent development of small and effective sensors facilitates an easily made application in complex systems. In this way it becomes possible to achieve an economic integration in automatic production testing systems.

 

Automatic Shearography Testing System for Helicopter Rotor Blades
Helicopter rotor blades are highly sophisticated products composed of different materials and components. They are safety relevant components and therefore complete quality control has to be assured.

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shearography offers the advantages of a noncontact, full field test and an overall significantly increased testing speed.

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Each rotor blade is manufactured as composite, with foam or honeycomb materials as the core of the blade and covered on the outside with one or more layers of fiber reinforced plastics. As reinforcement, carbon, aramid or glass fibers are used. In certain highly loaded areas (for example, 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. A thorough and complete testing of the blades is therefore required after production. After repairing any discontinuities on the rotor blades, a test of the repaired area is also required. The French helicopter producer Eurocopter S.A., Paris, installed such a system for automatic and complete testing of rotor blades for structural discontinuities.

The rotor blades are guided into a special mounting in the system. The precise repositioning of the rotor blades is required for fully automatic testing. The rotor blades are mounted in a 10 m (33 ft) long vacuum chamber (Figure 1) and loaded with a relative pressure difference of up to 5 kPa (50 mbar). At this load, debondings and structural discontinuities show up as tiny deformations of the surface of the rotor blades with amplitudes in the range of few micrometers. Two miniaturized shearing cameras are positioned on a separate guiding system on each side of the rotor blade and observe both sides of it (Figure 2). This allows the simultaneous testing of both sides of the rotor blade during one loading cycle. The test areas are illuminated by a 5 W Nd:YVO₄ 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 testing of areas up to 600 by 800 mm (24 by 31 in.) 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 testing of the largest rotor blades.

Figure 1 - Helicopter rotor blade testing system.

Figure 2 - View inside the vacuum chamber with rotor blade in testing position.

 

The complete system is operated by user friendly software which provides three different levels of operation (Figure 3). The supervisor (expert level) has complete access to all data structures and free configuration of the complete system. After the definition of a test cycle and teaching of "good" rotor blades (specialist level), an operator is able to run a fully automatic test cycle (operator level). The operator level of the software is designed to be used with a touchscreen monitor instead of a mouse and keyboard. Figure 4 shows the control panel of the rotor blade testing system.

Figure 3 - Software of rotor blade testing system.

Figure 4 - Control panel of rotor blade testing system.

 

The test results are fully and automatically analyzed by comparing the measured data with a set of earlier taught master data. This allows the operator to distinguish between structural information and anomalies. Figure 5 shows some typical test results with anomalies. The automatic anomaly localization is carried out during the test cycle and indicates the anomaly position on the screen. Sizes and positions of the anomalies are printed in a test report, which is automatically prepared after every test cycle.

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Figure 5 - Test results on both sides of an 800 by 600 mm (31 by 24 in.) field on the rotor blade with discontinuities. The typical double indications show local delaminations.

 

Automatic Shearography Testing System for Thermal Protection Parts
In the aerospace industries lightweight sandwich constructions are very common. The German Man Technologies GmbH in Augsburg produces the thermal protection parts of the new European Ariane 5 launcher, which are made of carbon reinforced composite materials using honeycomb structure cores and monolithic structures in one part. These thermal protection parts are in quite complex shapes, being cylindrical or conical and containing flanges and edges. In an extensive validation process, shearography has been chosen for its performance and testing speed as the technique for complete testing of all components. Two different stressing methods are used for the shearography testing of these components. The honeycomb composite parts are tested with thermal load because their structure is, compared 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 industrial robot (Figure 6). The robot is mounted on a vibration isolated base plate in the center of a 20 m (66 ft) vacuum chamber. Additionally, the robot head carries three halogen heaters for thermal loading of the components (Figure 7).

Figure 6 - Six-axis industrial robot in automatic testing system of thermal protection parts.

Figure 7 - Sensor, illumination optic, thermal loading and discontinuity marking system on the tip of the robot.

 

The complete testing process, including the positioning of the heaters and the sensor during the test and setting all of the test parameters, can be taught to the system in the expert mode (Figure 8). The operator level (Figure 9) allows only the choice between different automatic testing cycles. The automatic test, therefore, does not require extensive knowledge about the testing system.

Figure 8 - Software's expert level of thermal protection part testing system.

Figure 9 - Software's operator level of thermal protection part testing system.

 

The robotīs repositioning accuracy of less than 0.1 mm (0.004 in.) enables reproducible measurements of any complex part. The automatic anomaly recognition function of the system compares the results of good parts with the actual measurement results. Besides delaminations and debondings, the automatic anomaly detection function also shows missing or badly positioned honeycomb fillers.

The operator is supported in the localization of the detected anomalies by a laser scanner mounted near the shearography sensor on the robot's head. Using the measurement information, the laser scanner points directly to the position of the discontinuity at the surface of the test part. This makes it easy for the operator to manually mark the position of the anomaly on the part.

 

Conclusion
The recent improvements of shearography sensors and the development of automatic testing techniques enabled the successful integration of automatic shearography testing systems into the production lines of aerospace industries. In comparison with conventional testing techniques, shearography offers the advantages of a noncontact, full field test and an overall significantly increased testing speed.

 

Acknowledgments
The authors would like to acknowledge the cooperation of Eurocopter S.A., Paris, and Man Technologies GmbH, Augsburg, for data and photographs.

 

References
Ettemeyer, Andreas and Thomas Walz, "Automatic Shearography Inspection for Helicopter Rotor Blades," Application Report No. 03-97, Neu-Ulm, Germany, 1997.

Honlet, Michel, Andreas Ettemeyer, Thomas Walz and Christophe Bouju, "Automated Inspection of Helicopter Rotor Blades Using Shearography," ECNDT, Copenhagen, Denmark, 1998.

Tyson, John, "Portable Interferometry Systems for Rapid NDI," USAF Aircraft Structural Integrity Program, San Antonio, Texas, 1998.

 

* Ettemeyer GmbH and Company, Heinz-Ruehmann-Str. 207, D-89231 Neu-Ulm, Germany; 49 731 985830; e-mail <sales @ettemeyer.de>.

 

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