|NDT.net Apr 2005 Vol. 10 No.4|
Mobile shearographyMichael Kalms, Werner Jueptner
BIAS - Bremen Institute of Applied Beam Technology
Klagenfurter Str.2, 28359 Bremen, Germany
Corresponding Author Contact:
Michael Kalms, Email: email@example.com
© SPIE - The International Society of Optical Engineers.
ABSTRACTBy reason of their sensitivity , accuracy and non-contact as well as non-destructive characteristics, modern optical methods such as digital speckle shearography have found an increasing interest for NDT applications on the factory floor. With new carbon fiber technologies and other lightweight constructions in aircraft and automotive manufacturing, adapted examination designs and especially developed testing methods are necessary. Shearography as a coherent optical method has been widely accepted as an useful NDT tool. It is a robust interferometric method to determine locations with maximum stress on various material structures. However, limitations of this technique can be found in the bulky equipment components, the interpretation of the complex shearographic result images and at the work with non-cooperative surfaces (dark absorber, bright shining reflectors). We report a mobile shearography system that was especially designed for investigations at aircraft and automotive constructions.
1. INTRODUCTIONDigital speckle shearography is an appropriate method for the non-destructive inspection of technical and cultural objects. The method is based on the digital correlation of two speckled wavefronts representing two states of the object under test, loaded and unloaded condition. Fig.1 gives an impression of a typical speckle pattern caused by laser illumination on an optical rough object. To every technical surface belongs an own individual speckle field and this fact is applied in all speckle metrology technologies. An optical interferometric technique needs a reference wave. A special quality of shearography is that the technique generates the reference wave automatically. By shearing the wavefront scattered from the illuminated object surface, one of both wavefronts serves as the reference for the other . This makes the technique very robust against environmental disturbances.
The working principle of shearography is shown in Fig.2. The object is illuminated by a laser and observed by a sensor (e.g. CCD target). A shearing element such as a wedge is placed in front of the lens. Consequently the camera sees the usual and a sheared speckle image of the object. These two images interfere on the sensor area and result in another speckle field, which carries the interferometricly sensitive object information. After loading the object e.g. thermally or mechanically, the new speckle distribution which appears now on the detector will be compared with the first one. This leads to a macroscopic fringe pattern shown in Fig.3a. After applying a phase-shifting technology  the grey values of the shearogram will be transformed into a phase value distribution (Fig.3b). With known algorithms the generated wrapped phase may be transformed into the demodulated or so-called unwrapped version. With all three images non-destructive investigations are possible and will be executed. An interesting point is that these result images represent the mechanical extension of a loaded object.
With adequate loading and knowledge about the internal object structure it is possible to detect damages like pores, delaminations, cracks or even faults in bonded composites. Some examples will be shown in the chapters below.
2. THE MOBILE SYSTEMCarbon fiber technology and other lightweight constructions are used more and more for the production of automobiles and airplane parts. Modern airliner are already equipped with such components as e.g. in the vertical and horizontal stabilizer, rudder, airbrakes and spoiler. The application of these new materials is accompanied by new requirements for optimal dimensioning. In this case, the investigation with respect to material and construction imperfections is of high interest. In order to receive a high safety of operation, possible damages must be recognized prematurely within control examinations to prevent the total breakdown of the device.
An appropriate testing approach must meet non-destructive evaluation, inspection in an industrial environment and must also be mobile, flexible and easy to handle. The developed shearographic system fulfils these demands. It is a comprehensive testing device for the inspection of aircraft structures. But it is also possible to apply it at other structures and materials as well as in automobile or in other manufacturing works. Moreover, the technique is practicable even by working with sensitive cultural objects. The great advantage of the system is the adjusted balance of all single elements to a complete measurement procedure. It means that only with the alignment of all involved parameters it is feasible to get optimal measurement results. Fig.4 gives a first impression. Important consideration is that the only external resource is a conventional AC 230V (16A) power supply. The necessary pneumatic pressure will be generated and controlled internal with a pneumatic pump driver. A portable computer contains both control and evaluation unit.
The lightweight construction integrates all for the technique necessary components. An easy handling of the hardware tools means integration and a miniaturized housing of the equipment. Examples are visualized with Fig.5 and Fig.6. An especially developed diode laser delivers an optical power output of nearly 2 Watt (Fig.5). The.dimension is only 20x20x40mm. The shearographic sensor head is also an integrated lightweight construction. It contains the optical elements, shearing and phase-shifting units and additionally the camera. The measurement concept based on a Twyman-Green interferometer.
The influence of the laser illumination is fundamental in the shearographic technique. The evaluation of the measurement result depends direct proportional to the quality of the illumination. The investigation of large scaled components leads to a careful preparation of both the used light system and the elements of the beam expansion system. To the objectives a constant illumination over the whole test area and avoiding bright spots on the object surface should be given great attention. The object illumination in the mobile set-up will be realized with two mutual incoherent laser sources. Each laser source illuminate more than a half of the object area. Both mutual incoherent waves produce their own reference beam by shearing. Consequently, the sensor sees two independent interferometric sensitive speckle fields . As the result two independent shearograms are generated. In Fig.6 we notice the shearographic sensor with two installed diode laser.
The software of the shearographic system is based on the shell of the BIAS Fringe Processor TM . It is a Microsoft Windows XP TM based software system. The user interface of the software is built up quiet effectively and easy to use with push-button operations. It is a fast single-monitor system (video real time in the shearographic mode) which works with conventional hardware components like usual frame-grabbers, standard PC graphic cards and CCD cameras. The systems is also flexible to work with sensors newer generation like metal-oxide semiconductors (CMOS). A feature of the shearographic software is the handling with several different windows. In this way it is possible to grab in the shearographic process and to observe the results while the live-mode under the same condition is still running. The Fringe Processor Shell calculates automatically the phase-shifting image and its unwrapped-frame (demodulation) and shows them on the screen (Fig.7). While the shearographic live-mode is running it is also possible for the operator to communicate with external hardware, such as the loading system, with sliders and push-buttons on the Shell main window.
One way to transport thermal load into a material is the application of infrared radiation sources. The absorption peak of most plastics, such as CFRP, is in the infrared region. In our test equipment we use high power infrared radiator bars just as they are used in automobile manufacturing to dry the varnish (drawn red in Fig.8). The power load can be adjusted by a portable computer system or can be set externally to a fixed level for a safe daily operation. To load both, light honeycomb sandwich combinations and also high stiffed monolithic structures we introduced three infrared radiation sources. For lightweight components the two vertically oriented bars are sufficient to bring the necessary load into the object under test. To work with more stiffed structures a third high power radiator can be hooked up if required.
To be more mobile to meet industrial demands, the thermal loading system is mechanically combined with the shearographic sensor and the light source in one single cabinet. Figure 8 shows the construction. It is a lightweight creation with a high safety level in handling and operation. Inside the head the position of laser source and shearografic sensor can be noticed. Every element can be simply adjusted over three angles. The complete head can be connected self-aspirated with the object under test. Therefore, the pneumatic suction pipes (Fig.8 blue) will be driven within two controlled cycles. If one cycle fails an alert will be activated and the other cycle guarantees the function until the operator finishes the measurement. The pneumatic pipes are additionally variable in all directions for a comfortable work with spherical surface areas.
3. TESTING OF TECHNICAL OBJECTSThe following examples should visualize some results of our work by testing airplanes or airplane components. The shearographic investigations were made in cooperation with Airbus Germany (NDT-Group Bremen) and EADS Military Aircrafts (Material Testing Facility Manching, Germany) [4,5,6].
With Fig.9 and Fig.10 the application of the mobile shearographic equipment in a real maintenance environment is shown. In Fig.9 an aircraft wing will be investigated under thermal loading and in Fig.10 the aluminium body of the aircraft. The red cabinet in the blue cage contains the compact driver hardware. Inside the cabinet the power and the pneumatic supply for all components is situated. Beside the operators head the shearographic tubus is in working condition.
The first here presented test sample is an aluminium honeycomb structure from the aircraft body (Fig.10). It is a metal-metal bonded aluminium type with pocket damages through the honeycomb structure. The size of the smallest fault is 10x10mm. The successful shearographic measurement result is shown in Fig.12. All damages are clearly visible. Besides, an unknown damage is located in the result pattern, also to be seen in the radiographic investigation as the white marked area (Fig.11).
The inspected material in all next presented results is CFRP. With Fig.13 we see a honeycomb sandwich structure which is introduced e.g. in the rudder. The inspected damage is water in the honeycomb core. In Fig.14 we notice a repaired monolithic stringer structure. The skin over the stringer was damaged by an impact. This record was made after the repair. The single CFRP-layers are clearly visible. The next results show monolithic stringer structures with different sorts of damages. In Fig.15 we notice delaminations of the skin with different extensions. The sizes range from 10mm to 50mm. In Fig.16 we have stringer debondings. The detected faults in the frame range from 50mm to 200mm.
With Fig.17 we see the inspection of a complete aircraft part. It is an example for a large scaled object. The sample is an airbrake which is located at the wings of the airplanes (Fig.17a). The task was to find all damages of the airbrake while the shearographic sensor had to view the whole sample. The results are shown in Fig.17c and Fig.17d. An interesting point is, that the airbrake was so mounted, that the located damages were on its backside during the measurement procedure.
4. SUMMARYIn conclusion, a comprehensive shearographic testing device for the inspection of aircraft structures is achieved. The presented measurement concept is tested successfully with different aircraft materials and structures as soon as lightweight monolithic construction components, thin laminates and honeycomb-structures.