QMT Features: September 2011
NDT for composites - a headache?
Getting rid of that NDT headache - an optical, non contact technique, laser shearography, can provide high speed 100% inspection of composites. By Rob Wood, Dantec Dynamics

Rising oil prices and the demand to lower CO2 emissions is continuing to push aerospace and other transportation industries into weight savings through the use of composite materials including sandwich constructions. These new often complex designs using relatively unknown materials and manufacturing processes cause headaches for Non-Destructive Testing (NDT) personnel.

Sandwich materials and adhesively bonded structures have always been difficult for traditional NDT technologies to inspect however over recent years Laser Shearography has come to the forefront for inspection of composite materials. It has been applied in many industries including, from aerospace to new energy and defence sectors.

Laser Shearography is an optical, non contact technique; this gives its main advantage, high inspection speed. NDT can often provide a bottleneck in the production process, Shearography can reduce this and allow for 100% area inspections with measurement speeds of 2m² per minute. The main principle of Shearography uses an interferometric camera to measure surface deformations, often of 10’s of nanometres, caused by subsurface discontinuities or weaknesses while the parts are under a small load. The load is traditionally heat or vacuum, typically the surface temperature changes by less than a °C with heat loading. The results are shown in real time to the user and systems can be configured to automatically detect and measure defects including: delaminations disbonds, impact damage, wrinkles, kissing bonds, separation of structural components, crushed core and porosity.

Additionally structural information will be shown including, ply drops, overlaps, bulkheads, splices and ribs. This is another advantage of Shearography as it performance tests parts showing defects or weaknesses directly related to the material and structural strength. Traditional NDT techniques show all discontinuities including ones which may not be related to the strength of the material.

Established technique
Shearography isn’t a new technique with initial research into the technique carried out in the early 80’s at institutes in the UK, US and Japan. Early systems were based on wet film rather than modern CCD cameras. Technology development in CCD cameras, lasers and computing hardware coupled with price reductions in these technologies and the continued use of composites has led to large advances in the technique over the last ten years. Shearography reached a fundamental milestone in Aerospace industry acceptance with inclusion in ASNT SNT-TC1A for Level III Shearography Certification in 2006. Since 2008 Shearography has also been incorporated in the Standard Document, NAS 410, 2008 Rev 3, which is used for the European EN4179 Certification.

The technique is based on another called Electronic Speckle Pattern Interferometry (ESPI) this is a full field technique for accurate measurement of deformation and strain. When a surface is illuminated with a coherent laser light source a stochastic interference pattern is created. This ’speckle’ pattern is projected on a camera’s CCD chip. In contrast to ESPI where the speckle is compared with a reference light path, Shearography uses a reference created by shearing the image of the test object to create a double image.

This makes the method much less sensitive to external vibrations and noise. The superposition of the double image, a shearogram, represents the surface of the test object in an unloaded state, (this image can be thought of as an interferometric foot print). By inducing a small strain in the material using thermal, pressure, vibration or mechanical loading, the material deforms. If the material has non-homogeneous properties, the deformation of the surface will not be uniform – more deformation at weak areas. A new shearogram is recorded in the now loaded state and is compared with the unloaded image. If a flaw is present it will be seen in this result as a small deformation. The basic Shearography principle is to subtract two images of a test object; before and after load. Thereafter the information (intensity) from those images are subtracted, and then the surface deformation can be displayed due to the speckle information. To increase the sensitivity of the technique, a real-time phase shift process is used in the sensor. This uses a stepping mirror that shifts the reference beam and enhances the results with directional information.  

Composite manufacturing is constantly developing automated production lines. In parity with this trend, robotic Shearography equipment is also available as a standard industrial product. With performance capable of inspecting up to 2m² per minute, this is absolutely cutting edge performance in worldwide NDT. The system operates in a production environment inside a vacuum chamber. With a sensor mounted on a robot, it excites the production parts with vacuum or up to 3 kW of heat load. Objects are illuminated with 8 laser diodes and the Shearography sensor reads out real time phase stepping results. The image shows an automated robotic system that Dantec Dynamics have recently installed at a leading business jet manufacturer in the USA. Dantec’s Q-810 vacuum hood system is also used in an aerospace production environment to check particular structures.

The Q-810 measures an area roughly the size of an A4 sheet of paper in 10 seconds. It uses the vacuum hood to attach directly to the structure then vary the pressure in the chamber to create the load, alternatively the chamber can be filled with heat. It is a simple 2 button operation with the results presented real time to the user on the integrated touchscreen. Shearography is not just limited to the production environment with the Q-810 able to measure in-field or in-service at routine inspection points during the life of the structure. The Royal Air Force in UK uses 5 Shearography systems for in-field inspection with one application to inspect the 50m² E3D Sentry's Rotodome during service.

Another industry which has seen recent growth in the use of composite materials and hence Laser Shearography is the wind energy industry, specifically turbine blades. Like the aerospace industry robotic or gantry mounted Shearography systems can be used at the production stage to look for defects, the main one being wrinkles which are folds or creases in the fibre plies. During its lifetime, a blade is exposed to many factors that can lead to damage, thus resulting in downtime and lost revenue. Pre-installation inspection for defects and condition monitoring of blades using Laser Shearography can save time and money. Low-impact inspections can be made up-tower via ropes with minimal access costs and rapid servicing time, requiring no surface preparation and yielding instant results. Defects and damage including wrinkles, delaminations, cracks, water ingress, voids, lightening damage and many more, can be detected and monitored using Dantec’s Q-810.

For current and future innovative industrial composite structures, Shearography offers contactless large area rapid inspection, with unique detection capabilities. Shearography is ideally suited to sandwich material applications and the technology is industrially mature and certified according to all main Aerospace NDT standards since 2008..l

Author – Rob Wood – application specialist for Dantec Dynamics Ltd.
email: rob.wood@dantecdynamics.com

To see a system running and discuss your applications visit the Materials Testing 2011 exhibition at the International Centre, Telford from the 13th-15th September, organised by the British Institute of Non-Destructive Testing (BINDT).Dantec Dynamics will be exhibiting on stand D11.

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