Table of Contents ECNDT '98 Session: Aerospace

Automated And Non-destructive Inspection of Composite Helicopter Rotor Blades Using Advanced Shearography MICHEL HONLET* ANDREAS ETTEMEYER AND THOMAS WALZ Dr. Ettemeyer GmbH & Co., D-89231 Neu-Ulm, Germany CHRISTOPHE BOUJU EUROCOPTER S.A., F-93126 La Courneuve, France *Corresponding Author Contact: HONLET Optical Systems GmbH, D-89284 Pfaffenhofen, Germany ; honlet@t-online.de

TABLE OF CONTENTS

Summary Introduction Helicopter Rotor Blade Inspection System Automatic Defect Analysis Conclusion

Summary

Helicopter rotor blades are highly sophisticated products composed of a variety of materials and components. They are safety relevant components and therefore, 100% quality control has to be assured. The French helicopter producer, Eurocopter S.A., Paris, has installed a system for automatic inspection of rotor blades on structural defects. In this system, laser shearography is used as an inspection technique. The rotor blades are positioned in a vacuum chamber and loaded with a relative pressure difference. Bonding and structural defects show up as tiny deformations of the surface and are recorded with two shearography cameras, positioned on both sides of the rotor blade.

As this helicopter blade inspection system is the first fully automatic production control system in european aerospace industry based on laser shearography, this application is an important step in bringing shearography techniques into production control. In this paper, the complete inspection system is presented.

Introduction

Helicopter rotor blades are highly sophisticated products, composed of a variety of materials and composites. They are safety relevant components and, therefore, 100% quality control has to be assured. Each rotor blade is manufactured as a composite, with foam or honeycomb materials forming the core of the blade, covered with one or more layers of fibre reinforced plastics on the outside. For further reinforcement, carbon, kevlar or glass fibers are used. In highly stressed areas, such as the front edge of the blade, metallic layers provide additional reinforcement.

The production of these rotor blades follows a rather complicated and complex procedure and, consequently, a 100% inspection of the blades is required after production. Now, for the first time, a fully automatic inspection system for helicopter rotor blades has been installed in the aerospace industries. The system is integrated into the manufacturing line and allows 100% inspection of each manufactured rotor blade.

Fig 1: Automatic shearography inspection system for helicopter rotorblades.

Fig 2: Shearography camera included in the rotor blade inspection system. Its small dimensions (only a few inches) and light weight allowing easy positioning.

Fig 3: View inside of the vacuum chamber and rotor blade waiting to be inspected.

Fig 4: Control panel of the helicopter system: two monitors show the measuring results of both sides of the rotor blade.

Helicopter Rotor Blade Inspection System

The new, automatic inspection system uses laser shearography and a vacuum chamber for inspection. The main visible part of the helicopter rotor blade inspection system is the huge vacuum chamber with the dimension of approx. 12 x 5 x 2,5 m³, fig. 1. The rotor blade is positioned inside the vacuum chamber and loaded with slight pressure changes of less than 50 mbar. The pressure change in the vacuum chamber will produce slight deformations on the surface of the rotor blade. A laser shearography system can observe these deformations and automatically indicate defects, such as delaminations, debondings, etc., which show up by typical deformation patterns.

. A ventilation system facilitates fast pressure sequences during operation. The typical pressure difference is a few mbar. Despite the large dimensions of the vacuum chamber, the test pressure difference is realized within 5 seconds. Safety valves stop the evacuation system at the maximum allowable pressure difference of 50mbar.

The rotor blade is positioned on a sledge and fixed by automatic clamps in the center and the sledge is driven into the chamber. Fig. 2 shows a view of the rotor blade from inside the vacuum chamber at the test position. The internal chamber dimensions are designed for inspection of large main rotor blades. Two shearing cameras are positioned on a separate guiding system on each side of the rotor blade. They allow simultaneous inspection of both sides of the rotor blade during one pressure cycle.

Two laser shearography systems measure the deformation of the rotor blade surface at slight pressure changes of 50 mbar. Fig. 3 shows a shearography camera with laser illumination, inspecting a rotor blade.Each camera can observe an area of 600 x 800 mm² (24" x 31") on each side of the rotor blade.

The inspection areas are illuminated by the green light of a 5 Watt, frequency doubled NdYAG laser. The complete laser system is integrated in the control rack of the system. No external cooling is required for laser operation. The laser light is coupled into two monomode glass fibre cables and guided to the shearography cameras. There, a diffuser system expands the laser beam, to provide homogenous laser illumination on the whole measuring field.

Automatic Defect Analysis

The complete system is operated by the user friendly Windows 95 software ISTRA. The software controls the exact positioning of the shearing cameras, the automatic pressurization of the vacuum chamber and automatic evaluation of the measuring results. Two monitors show the inspection results of both cameras on the control panel, fig. 4. Three different operation levels allow automatic inspection (operator level), definition of new rotor blades and teaching of "good" rotor blades (specialist level) and complete access to all data structures and free configuration of the complete system (expert level). Touch screen monitors allow operation of the system without keyboard or mouse.

For each rotorblade inspection sequences are defined and stored in a database. These data contain all parameters for definition of the optical and mechanical properties during the inspection cycle. Using the rotor blade code, all parameters are automatically loaded and the inspection is started. If similar rotor blades have been inspected earlier, the measuring results are automatically compared with the stored master data of earlier inspections. Deviations are automatically indicated on the monitor.

The operator can decide to use these data as "good" master data, or to classify as "defect". In this way the system is being taught, learning the properties of each rotor blade by the experience of earlier measurements. Fig. 5 shows the menu, as it appears to the operator. Only limited numbers of operations are allowed. They can be started by just touching the monitor. Keyboard and mouse are not required. The expert can make use of full functionality of the system and, therefore, has a more complex menu, fig. 6.

The inspection result is printed out, indicating the defect position and defect size. The accuracy of the defect position is within 5 mm, the minimum defect size is 10x10mm². Fig. 7 shows some delaminations , as they show up at the inspection.

Fig 5: Screen shot of the operator test screen: only limited numbers of functions are visible. Large buttons allow start and stop of tests just by touching the monitor. Fig 6: Screen shot of the expert test screen: Inspected areas are marked, the live image and inspection results are shown in the lower right window. All necessary informations (pressure, position, etc.) are indicated on the screen.

Fig 7: Inspection results on both sides of a 800x600mm2 (31x24") field on the rotor blade with defects. The typical double indictations show local delaminations.

Conclusion

The helicopter rotor blade inspection system has been succesfully installed in the production line. In comparison to the earlier manual inspection, the elapsed time for a complete inspection cycle of large, main rotor blades has been significantly reduced.