Shearography checks helicopter blades

Paris-Due to safety considerations and a complex manufacturing process, the quality control of helicopter rotor blades demands 100% inspection. Helicopter manufacturer Eurocopter uses shearography, a laser technique based on speckle interferometry, to check its helicopter rotor blades. At Eurocopter's testing facility in Paris, the blades are put inside a vacuum chamber where they are subjected to a slight vacuum of 50 mbars. Shearography equipment and the Windows-based ISTRA program, both supplied by Dr. Ettemeyer GmbH (Neu Ulm, Germany), compare the shape of the blade under negative pressure conditions with blade shape under normal atmospheric pressure. The software analyzes changes between the two states to deliver a "good/bad" quality assessment. To do this, the interference pattern from the laser images is converted into strain measurements.

Necessary equipment includes two shearography cameras that measure blade surface deformation under the partial vacuum conditions. The cameras can observe an area of 600x800 mm on each side of the blade. Green laser light from a 5W NdYAG laser is coupled into two glass-fiber cables and guided to the shearography cameras. A diffuser system expands the laser beam to provide homogeneous laser illumination of the whole measuring field.

Blades to be inspected are loaded onto a slide that transports each blade into the vacuum chamber. The ISTRA program, running on a PC, automatically controls positioning of the shearography cameras, pressure inside the chamber, and evaluation of the results. Apart from the automatic inspection mode, there are blade definition and teaching modes for setting up. The system can detect defects as small as 2x2 mm, as well as shape changes over an area up to 10x10 mm.

Philippe Gaignard, Technical Quality Manager at Eurocopter France, says that the system has significantly reduced blade inspection times. "With the previous holographic technique, we needed about an hour for a medium sized blade. Now we can do it in just ten minutes."