ESPI of Aerospace Composites Using Digital Shearography

Transcription

1 ESPI of Aerospace Composites Using Digital Shearography Jasson GRYZAGORIDIS and Dirk FINDEIS, Mechanical Engineering Department University of Cape Town Cape Town, South Africa Abstract. It is demonstrated that a portable digital shearography system is capable of obtaining measurements, in addition to the gradients field of normal surface displacements, the actual field of normal to the surface displacements, commonly identified as displacement interferograms obtained usually through digital holographic set-ups, labeled ESPI (Electronic Speckle Pattern Interferometry). This has been accomplished as reported here, by increasing excessively the shear or the distance separating the two images in a shearography system, which employs the modified Michelson interferometer as the shearing device, and allowing a portion of a reference surface to reflect on the beam splitter of the digital shearography s head. The technique is demonstrated by the results obtained when testing manufactured defects on composite specimens from an Oryx helicopter main rotor blade. 1.0 Introduction In the quest of assessing and maintaining structural integrity, emphasis is being placed in detecting and quantifying defects, albeit resulting during the manufacturing processes or during in-service conditions, on components of structures containing new materials and in particular those of composite nature. Typically a composite material is a hybrid made of at least two individual materials or components which will retain their individuality with regard to their properties even after the forming process. In choosing the individual components that make up a composite, a very simple rule is adhered to, one material will be the core or matrix and the other will act as the reinforcement. The final product is a material that has been chosen on an optimum combination of its constituents, capable of being shaped to the designer s choice, exhibiting totally new mechanical and chemical properties normally of the orthotropic type. That is, the composite material will have different stiffness and strength depending on the orientation of the force that may be applied to it. There is a great variety of composite products which are produced by an equally great variety of manufacturing processes. It is therefore expected that in the realm of such a large range of products, derived from a large range of manufacturing processes, flaws and defects will be created even at microscopic level. These defects eventually, more likely during service, are liable to grow and lead to catastrophic failures. The type of defects that have been identified that potentially may lead to catastrophic events are associated to a large extend with the method of the forming of composites. For

2 example the defect known as delamination is the most common defect or damage that occurs between adjacent layers of a laminated composite as a result of inadequate adhesion or insufficient adhesive agent applied between the two layers, or as a result of impact. Optical laser based non-destructive testing techniques are becoming more trendy lately as being particularly suitable for the detection of defects in structures and components made from composite materials, and they are classed as young technologies slowly increasing in volume and breadth their track record of successful applications[1]. These techniques are becoming more popular as opposed to the traditional methods because they are non contacting, real time, present a whole field view and are operator or user friendly in identifying and locating a flaw. The technique of Digital Shearography, as it is evidenced in the literature, has been continuously providing examples of success in detecting defects among the wide range of types of flaws/defects in composites. Just to mention a few of these defects; delaminations, cracks, inclusions, voids, impact damage, broken filaments/mats etc. Shearography has its origin as a strain measurement technique (1974) and later as a nondestructive testing tool (1982) reported by Hung [2, 3]. Technological advances in lasers, digital cameras, frame grabbers etc. enabled researchers to refine the technique and in some instances produce portable equipment. Examples are Digital Shearographic portable equipment developed at the Non Destructive Testing Laboratory of the University of Cape Town (UCT) [4], or the Applied Research Lab. of the Pennsylvania State University [5], as well in industry, Dantec-Ettemeyer [6]. 1.1 Digital Shearography Digital Shearographic NDT systems, similar in schematic form to the one as depicted in figure 1, include personal computers housing software to process the images of the object under test. The images are obtained via a digital camera viewing the object through some shearing optics and are stored in the image digitizer. A single wavelength light source preferably a laser is used to illuminate the object and produce the required speckled image. Laser Object Shearing device Beam expander Mirror Figure 1. Typical Shearographic system for out-of-plane surface displacement gradients

3 The technique of digital shearography requires, as already mentioned, the use of an image shearing device which is placed in front of the camera. Two laterally displaced images are focused by the camera to the convenience of the operator, in a horizontal, vertical or in any other direction in the x-y plane, by simply rotating the shearing device in front of the camera lens. A modified Michelson interferometer, as the one shown in figure 2, or a glass wedge or a birefringent prism are commonly utilized to shear the image. The Michelson interferometer type is preferred by the authors in that it allows the flexibility to vary the magnitude of the shear by manipulating one of the mirrors accordingly. This is an important feature in digital shearography because the magnitude of the shear is largely responsible for the sensitivity of the system as well as it will be shown shortly, a paramount feature for the purpose of this paper. Mirror Small angle Object Wave-front Mirror Separation distance, S y Sheared image x Figure 2. Image shearing device based on the Michelson interferometer Digital Shearographic non-destructive testing reveals the presence of flaws or defects as a localized disturbance in the fringe pattern depicting the gradient of surface displacements on the test piece. The fringe pattern is generated in response to any stress being applied on the object such as mechanical, thermal, pressure or vacuum, and emerges superimposed on the object s image, after the subtraction of two images of the object s surface, one before and the other after the stress was imposed. The anomalies in the fringe pattern basically display the position and approximate size of the defect, however not its depth relative to the surface quantitative interpretation of the fringe anomalies, with regard to the defect size and depth from the surface, requires a more involved process and has been the subject of considerable work by many researchers in the field. Even the most cursory literature survey indicates that it is unlikely that we can obtain a closed form solution for the problem but rather rely on turn key solutions to individual situations. Gryzagoridis at al. [7] Very often investigators require a graphical representation of the surface displacement above the defect and since digital shearography set-ups, like the one depicted in figure 1, reveal the gradient of the displacements normal to the surface, the investigators and researchers resort into employing software which transforms mathematically the gradients into surface displacements.

4 1.2 Electronic Speckle Pattern Interferometry Electronic Speckle Pattern Interferometry (ESPI) is a laser based, optical, non-contact, whole field, non-destructive testing technique capable of measuring surface displacements on an object under test. The displacement measurements can be effected quite independently of each other, either in the in-plane or out-of-plane (normal to the surface) directions relative to the observer, very much like digital shearography measurements, depending on the experimental set-up (see figure 3). The surface displacement field is represented in the form of interference fringes (zebra like stripes) superimposed on the image of the object (Gryzagoridis at al [8]). Any disturbance on this pattern, like the abrupt change of direction, or the density change of these fringes, represents an impediment in the surface displacement and in the absence of structural effects, obviously the presence of a flaw or defect. Laser Object Camera Beam expander Beam splitter Fiber optic (reference beam) Figure 3. Typical ESPI set-up for out-of-plane surface displacement 1.3 Dual-function, (Shearography/ESPI) When one compares the set-ups of Digital Shearography and ESPI (figures 1 and 3 respectively) the similarity is apparent, with the major difference being the reference beam. For example the reference beam in the Shearography set-up is generated by the actual surface of the test piece and therefore it carries the signals of the body motion. This of course is highly desirable in Shearography because it reduces the effect of environmental disturbances that de-correlate the speckle images. In the ESPI set-up the reference beam is produced from the original illumination beam and without being disturbed is combined with the beam reflected from the object by a beam splitter located after the camera s lens. Attempts to produce a dual purpose (ESPI and Shearography) camera have appeared in the literature. Fomitchov [9] has shown a device that utilizes a reference beam created by a beam splitter at the laser output. The reference beam is transported from the beam splitter into the camera via an optical fibre, and by removing one of the mirrors of the modified

5 Michelson interferometer, the optics result into an ESPI device. It is a workable solution however phase stepping is sacrificed by the removal of the mirror and the object beam or the distance of the object from the camera is fixed because of the use of the optical fibre. Bhaduri at al [10] developed a dual function ESPI system for the measurement of out-ofplane displacement and its slope change. They utilized the expanded beam of the laser to illuminate the object as well as a reference mirror. The light reflected from the reference mirror was scattered through a ground glass and was directed onto the cube beam splitter where it was combined with the reflected object image before entering the camera. This is a better solution then the one presented by Fomitchov [9] because it allows for phase stepping and the distance of the camera to object is variable. Perhaps here one notes a little difficulty in positioning the reference mirror next to the object and directing the reflection to the ground glass screen before entering the modified Michelson interferometer. It appears therefore that if a reference beam can be introduced in the shearography set-up and combined with the image of the object under test we would obtain an ESPI result. This is relatively easily accomplished by introducing in say one third of the illumination field of the object an optically diffuse flat surface. The surface will reflect its image onto the beam splitter where it would combine with the reflected image of the surface under test and create the fringe pattern that exhibits displacements normal to the surface (see figure 4). Of course introducing a neutral surface in front of the surface of the test piece (in about a third of the illumination field), reduces the overall area that can be tested, however the advantage of obtaining almost simultaneously Shearographic and ESPI data in such a simple manner by far outweighs the sacrifice of reducing the test area. What has been described above is depicted in figure 4 where we note that the shear distance is substantially increased? (by the tilting of one of the mirrors) to and beyond the point that the two images share no common portion. Under these conditions with the reference beam generated by the neutral surface, one image (the lower one in fig. 4 or the one transmitted through the beam splitter) of the portion of the surface under test is combined with the reference beam creating ESPI conditions. Reference surface for ESPI Test piece with internal defect Beam splitter Image plane Piezo stepped mirror Tilting or shearing mirror Large angle ESPI image Figure 4. Transformation of Shearography to ESPI

6 2.0 Experimental verification In order to validate the assumption that ESPI data could be obtained by suitably modifying the shearography set-up, along the suggestions contained in section 1.2 and as depicted in figure 4, a section of a main rotor blade from an Oryx helicopter was used. The test piece had a man made circular de-lamination on the far side and was heated with a heat lamp controlled by a variable resistor and a timer. In this manner the amount of heat into the test piece could be regulated and reproduced accurately. Using the digital shearography technique the defect was suitably identified through the characteristic double lobe and the resulting surface gradient plot and surface displacements plot are shown in figure 5. Figure 5. Images of a de-lamination on an Orix main rotor blade obtained using shearography. From left to right, the characteristic double lobe filtered phased stepped image depicting the flaw, followed by the surface gradient and surface displacement plots. With the knowledge of the location of the defect, as revealed by the shearographic testing we introduced next to the test piece a plain flat surface, partially into the object illumination beam, ensuring that the surface was visible by the camera (hence it was captured by the camera through the optics) and of course it did not cover the location of the defect. The heating lamp was activated keeping all the parameters identical to those during the shearographic test. The results demonstrated the characteristic circular fringes expected from an ESPI test which yielded the surface displacement plot depicted in figure 6. Figure 6. Images obtained after the insertion of the flat surface in front of the test piece. The first circular fringes image (on the left) is the ESPI output of the modified optics, the second image is a pseudo ESPI/Shearography image, followed by their respective surface displacement plots For completeness of the experimental verification, the last test involved obtaining the surface plots of the two lobes in a shearography set-up with a large shear i.e. allowing for complete separation of the lobes without a common portion of the image (magnitude of

7 shear identical to the one used for the transformation of the system from Shearography to ESPI). The results are depicted in figure Conclusions The transformation of a Shearography system to an ESPI is accomplished with ease by introducing, in part of the field of view of the component under test, a neutral surface. Typically if during a shearographic procedure a defect is identified or located, additional information about it can be obtained by transforming the system to an ESPI set-up. Admittedly the field of view is reduced; however in the context, it is of no consequence. A surface displacement plot obtained through an ESPI set-up should be more accurate, from the one deduced by intergrading the data obtained through Shearography, since the shearing distance is a factor which affects the shape of the surface deformation. Finally transforming Shearography into an ESPI system brings in the inconvenient environmental stability requirement, where full body motion, vibration, etc. affect or radically change the image of the fringe pattern around a defect, and depending on the severity of the disturbance, to the point of total extinction. References Figure 7. Images obtained from a very large magnitude shear. On the left are the filtered phased stepped image of the two lobes, followed by the left and right displacement maps respectively. [1] G W Carriveau Benchmarking of the state-of-the-art in Nondestructive Testing/Evaluation. Report no of the NDT Information Center (NTIAC) Austin Texas, U.S. Army Tank-Automotive Command [2] Y. Y. Hung, A speckle-shearing interferometer: a tool for measuring derivatives of surface displacement Opt. Commun. 11 (2) (1974) [3] Y. Y. Hung, Shearography: a new optical method for strain measurement and nondestructive testing Opt.Eng.21 (3) (1982) [4] Nondestructive Testing Laboratory, Mechanical Engineering, University of Cape Town, South Africa [5] Applied Research Laboratory at the Pennsylvania State University, State College, Pennsylvania, United States of America. [6] Automatic Shearography inspection system. Dantec-Ettemeyer, Gmb. Germany [7] J Gryzagoridis and D Findeis Benchmarking shearographic NDT for composites Insight Vol 50 No 5 May 2008 pp [8] J Gryzagoridis, D Findeis and W Bopape. Impact Damage Detection on Composites using Electronic Speckle Pattern Interferometry Comadem 2005 Aug 30th Sept 2, 2005, Cranfield, UK. [9] P A Fomitchov, S Krishnaswamy A Compact dual-purpose camera for shearography and electronic speckle-pattern interferometry Meas. Sci. Technol. 8 pgs (1997). [10] B Bhaduri, N K Mohan, M P Kothiyal, A dual-function ESPI system for the measurement of out-ofplane displacement and slope Optics and Lasers in Engineering, Volume 44, 6, June 2006, pgs