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With the increase in PC processor capabilities, shearography technology has become a very easy and robust measurement tool for the inspection of composite parts. Nowadays, the technology provides fast, reliable and easy-to-comprehend results during inspection of a variety of composite materials and rubber parts throughout the industrial world.
(Published on June-July 2005 – JEC Magazine #18)
BY JÖRG COLLREP,SALES MANAGER INTERFEROMETRY & TIRE TESTING,STEINBICHLER OPTOTECHNIK GMBH
The technology was invented 25 years ago and for many years, it was used in just a few, very specific applications. Steinbichler Optotechnik is a leading supplier of interferometry systems for the automotive, electronic and aerospace industries and for research institutes, including high-performance systems for vibration and stress analysis. The company develops the full product line from software to hardware and thus provides customised solutions for every industry sector. Shearography is one specific method among other interferometry technologies where solid-state lasers are used to create typical images. One advantage of shearography is that test results are provided within seconds, and that the operator can actually see what happens on the object surface under load. The technology uses coherent light emitted by a laser source to create the typical speckle pattern on a surface. During the test, the surface is excited with a very low load (typically vacuum or heat), and during both object states, an image is acquired. The software then subtracts these two images automatically and, directly afterwards, provides the result. Today, solid-state lasers are being replaced by commonly used diode lasers. These provide sufficient illumination power and accuracy, combined with dramatically reduced costs.
Increased detecting capabilities
With the optimisation of laser wavelengths, heat sources, processing algorithms and other data acquisition tools, the detecting capabilities of the shearography system have increased. Comparisons between different technologies have shown that the POD (Probability Of Detection) factor of shearography is 100% - especially for close-to-surface defects, where other technologies have obvious limitations. Today, defect sizes of 5mm in diameter can be detected in most materials. The depth of the defect inside the object can only be assumed, due to the nature of the measurement technology. Therefore, shearography does not substitute any other NDT technique, but it is an excellent tool for quickly scanning large areas. In case anomalies are found, further inspection techniques like ultrasound can be added.
for close-to-surface defects, where other technologies have obvious limitations. Today, defect sizes of 5mm in diameter can be detected in most materials. The depth of the defect inside the object can only be assumed, due to the nature of the measurement technology. Therefore, shearography does not substitute any other NDT technique, but it is an excellent tool for quickly scanning large areas. In case anomalies are found, further inspection techniques like ultrasound can be added.
Based on 20 years’ experience in this field of technology, Steinbichler Optotechnik has adapted the shearography technology into one of the harshest environments for optical measurement technologies, the tyre industry. In the past, no one imagined that optical technologies could survive these conditions and still deliver accurate results for the tyre retreading or manufacturing industry. Tyre rubber is a very cooperative material for shearography testing, due to its homogeneous response to vacuum loading. Today, nearly all retreaders worldwide rely on this technology to improve both incoming and outgoing inspection. Even aircraft tyre retreaders use shearography test systems to increase their production output by effective quality-assurance measures. Due to the integration of these test systems, the quality and even the number of retreading processes per tyre can be improved with a direct pay-off to safety, cost, and the lifetime of a retreaded tyre. The experience gained in this field gave the background for the development of the current shearography systems ISISmobile 3000 and ISIS 5000, as well as the latest system configuration ISISrobot.
Effective quality assurance for composites
The use of composite materials is no longer restricted to aerospace applications. They are also used in many other sectors to reduce weight, optimise structures and improve the performance of components. One of the most significant features of composite materials is their sensitivity to any impact damage that not visible on the surface. Due to the nature of the material’s structure, this may result in a reduction of both lifetime and/or mechanical stability and, in the worst case, even in the damage or loss of a complete unit, vehicle or aircraft. To increase the safety and quality of such components, NDT testing with shearography systems is an effective and highly accurate quality-assurance method.
The currently available image quality provides such clear images that even untrained operators can distinguish defects from faultfree areas or object structures. With the help of the user-friendly image processing software ISISplus, the systems can be operated in a supervisor or operator mode as well as in a fully automatic mode for stationary or robot systems to inspect large objects. The software tool was developed jointly with Airbus Germany to achieve a common user interface that would allow very fast introduction into the software capabilities, as well as special image processing for advanced and fully integrated reporting.
The ISISplus software is split into two main operation modes, the supervisor and the operator mode. The supervisor mode is intended for the use by NDT inspector level III with access to all software tools, image processing and report generation. Furthermore, the supervisor can define programmes that can later be selected by the operator as an automatic routine. The supervisor is responsible for setting the correct parameters for the specific object under test with the help of state-of-the-art software routines. Thus, user inputs can be drastically reduced. The software now provides tools like the automatic adjustment of camera settings in accordance with object reflection. The type of surface preparation required in the past is now obsolete with this unique tool, representing the integration of a very important customer requirement. Another feature of the ISISplus software is the option to filter global deformations out of the result image.
The operator mode is typically used to run automatic routines set by the supervisor, which increases the part throughput during the inspection process. Even the final judgement of accepted or rejected defects can be done at a later stage by the supervisor or the people in charge.
A mobile system for aerospace applications
ISISmobile 3000 was the first mobile shearography system developed by Steinbichler Optotechnik for composite testing in the aerospace industry. The system was also jointly developed with Airbus Germany (Bremen) to define the most suitable sensor size and capabilities for the in-field service of aircraft structures. It is fully movable and contains a lightweight sensor head, a controller, an image processing PC and a tablet PC for remote control. Thus, the system can be moved to any area and direction of the aircraft surface. Even an upside-down placement of the sensor head is possible. The sensor head has an automatic self-attaching mechanism working with pneumatic suction cups, giving the operator maximum flexibility to inspect the requested area. With one single measurement, the sensor acquires an image sized 350mm x 270mm. For standard excitation, the system contains two thermal units; an optional vacuum chamber can be easily attached onto the frame. Other integrated features include remote control of the measurements and automatic report generation. The system is classified in Laser Class 1.
ISIS 5000 provides the same features as the mobile unit while using a vacuum chamber as test environment. Typically, the system is used in a production area for quality assurance of parts with sizes up to 2.5m x 2m. The system allows a 100% inspection by turning the part only once, and uses a 5-axis system to control and move the measuring head. For advanced applications, even the mobile measuring head can be installed in the chamber so synergy effects are provided as far as possible. The ISISplus software controls the measurement process. Thus, the time effort for an operator training is reduced to a minimum.
In this particular unit, one of the biggest advantages offered by Steinbichler Optotechnik is integrated. 3D digital data either from CAD or digitised data can be combined with result images from other measurement technologies. Thus, the result image can be virtually placed on the object surface data. The operator now has the capability to detect and locate defects on the object with detailed position information to allow precise repair actions.
A mobile sensor attached to a robot is the latest development for shearography applications with ISIS. This combination gives the customer an even higher flexibility for inspection. Optionally, the robot can be installed either inside a larger vacuum chamber or outside, and provides the same result traceability as the ISIS 5000 stationary system. By a simple mouse click, the operator can proceed from the measurement to the report generation within a fully automatic process while only needing minimum operator input. This system is another example of how optical measurement technologies can be transferred into an industrial environment while providing an easy-to-operate measurement unit.
With the growing use of composite materials, the application area for shearography test systems is also increasing. Today, many materials like honeycomb or aluminium sandwich structures, sandwich structures combined with foam materials, PCBs, rubber-coated parts, plastics and other materials can be inspected with this technology.
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