Using UT to characterize defects in composites detected with Digital Shearography

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1 Using UT to characterize defects in composites detected with Digital Shearography Jasson Gryzagoridis 1, Dirk Findeis 2 Mechanical Engineering Department 1 Cape Peninsula University of Technology 2 University of Cape Town Cape Town South Africa E- Mail: gryzagoridisj@cput.ac.za Abstract This paper reports on a nondestructive application where the use of traditional ultrasonic testing may aid in characterizing the defects that are detected using the laser based optical interference technique known as Digital Shearography. The composite materials tested are both of the laminate and sandwich type construction. The portable Digital Shearography system employed with regard to evaluating the integrity of composite materials has proven to be particularly user friendly, revealing the whole field under test in almost real time. Barely visible or invisible impact damage, delaminations and cracks in composite specimens were localized using mild surface heating between the two captured images required by the Digital Shearography technique. Subsequent testing, using a portable ultrasonic tester, aimed at characterizing the defects as they were detected in terms of their extent and depth position relative to the near or test surface. Keywords: Digital Shearography, Nondestructive testing, Composite materials, Laminates, Sandwich Composites, Voids, De-laminations, Ultrasonic Testing, A,B,C-Scans, Thickness determination. 1.0 Introduction Digital Shearography as has evolved to this date since its introduction by Hung [1] is slowly becoming a useful nondestructive testing/evaluation technique particularly in detecting defects or flaws in structures made of composite materials. The technique is a laser based optical method that determines the gradients of surface displacements on a test piece. It is an interferometric technique that relies on combining two speckled images of the surface of the component under inspection; one before and the other one after the specimen has been perturbed mildly for example by applying heat, pressure or vacuum, or a mechanical force. The speckle that one observes on a diffuse surface that is illuminated by a monochromatic (single wavelength) light beam, as from a laser, is the result of light interference within the peaks and valleys on a real surface. When the surface is distorted due to any disturbance as mentioned above the speckle pattern of the surface changes. Comparing the two speckle pattern images or making them interfere with each other by adding or subtracting them, gives rise to an image of the surface of the component under test with what a lot of people refer to as zebra like stripes superimposed on it. This pattern is known as the interference fringe pattern and the individual lines that make up the pattern are known as fringes. When observing the fringe pattern, obtained through the Digital Shearographic technique performed using a typical laboratory setup as depicted in figure 1, any abrupt 1

2 changes in the direction or the density of the fringes signifies a subsurface disturbance that affects the surface displacement and hence its displacement gradients. The disturbance may be the result of a defect or flaw such as a crack, delamination, void, porosity etc. The anomalies that may be observed in the fringe pattern basically display the two dimensional location and approximate size of the defect. Laser Object Shearing device Beam expander Mirror CCD Figure 1: Typical Laboratory Shearography set-up Typical laboratory Shearographic NDT systems, similar to the one depicted in fig.1 above, include personal computers housing software to process the images of the object under test. The images are obtained through a digital camera or a closed couple device viewing the object through some shearing optics and are stored in the image digitizer housed in a personal computer. A single wavelength (a laser) light source is used to illuminate the object and produce the required speckled image. There is a vast amount of examples in the relevant literature on the use of Digital Shearography in identifying various kinds of defects, however the majority deal with the process of identifying and locating the presence of defects, but are unable to further characterize these defects; specifically how deep below the near surface (the surface under inspection) these defects are located. In other words it is becoming evident that unless one embarks with a turnkey solution to a particular problem with parametric studies on the particular type and composition say of a given composite material, characterization of the defect/flaw solely by evaluating the fringe pattern is not possible. A turnkey solution in this context is a procedure that attempts to provide further information about the defects that have been detected by evaluating the fringe pattern that has emerged under controlled perturbation conditions that were established though parametric studies of the particular situation. Such turnkey solutions are sound approaches that yield acceptable results as evidenced by examples that appear in the literature [2, 3]; however it is an avenue one wishes to avoid if possible, bearing in mind the copious time and expense this particular method requires. So what is left? How do we get around the inherent weakness associated with Digital 2

3 Shearography of being unable to characterize completely a defect that it identified or located on a component under test? Ultrasonic Testing (UT) is an established NDT technique and one of the most frequently used method in detecting and quantifying defects in a large range of materials. The technique is based on the reflection of sound waves when they encounter a surface along their path of travel in the solid material of a component being tested. The reflected ultrasonic wave (echo) would therefore be either from the far surface of the component or from a surface or surfaces located in between the two boundaries as a result of discontinuities such as a cracks, voids, de-laminations etc. i.e. defects or flaws (see schematic figure 2). The time of travel associated with the speed of sound within the component s material and its thickness are data sufficient to characterise where the reflection emanates from and hence the depth that it occurs within the component from the near or test surface. Back wall echo Flaw echo Figure 2: Schematic example of the display (known as A scan) of a UT test on a material10 units thick with the defect located at 6 units depth. Modern ultrasonic testing equipment comprise of a portable unit that produces the ultrasonic waves and analyses their reflections from the solid under investigation through the interface of a dual function probe i.e. the transmitter and receiver of ultrasonic waves. These probes are of the order of magnitude of a few square millimetres of transmitting/receiving surface which must be in good contact with the surface of the solid under investigation. There lie the main weaknesses of ultrasonic testing. The need to have proper contact between probe and surface under test (normally overcome by using a fluid such as oil or water as a coupling between them) and the tedious operation and attention required to scan a surface without missing any of the required travel of the probe s footprint over the test surface. The suggestion here is that, Digital Shearography being a swift NDT technique capable of whole surface testing, (i.e. it eliminates the foremost weakness of ultrasonic testing) but unable to fully characterize the defects it locates, it can be complimented/aided by UT. Thus a defect or flaw is located swiftly and is characterized equally swiftly (now in terms of its depth) from the testing surface. This is perhaps an obvious solution to the main weaknesses of the two NDT techniques, nothing novel about it, however it is interesting or surprising that the literature does not abound with examples of such an approach having been 3

4 attempted. It is the purpose of this paper to report on the authors attempt of the marriage of these techniques when the component under test is a composite material of either the laminate or sandwich type. 2.0 Experimental set-up and results 2.1 Shearography Testing Samples of composites, both of the laminate and sandwich type, containing induced defects were evaluated using the authors [4]proprietary Digital Shearography prototype # 3 which is depicted in figure 3. The tests performed aimed at establishing firstly how close to real terms does Digital Shearography indicate the size of the defect by virtue of observing/evaluating the fringe pattern obtained and secondly can UT determine the position of the detected defect within the thickness of the specimen, hence its depth from the viewing and probe contact surface. Figure 3: UCT s Portable Digital Shearography system All the composite material specimens were tested by warming up their surface, with hot air from a hair drier or the use of an infrared lamp equipped with an electronic timer, for a few seconds and while observing the dynamic fringe pattern (as the specimen cooled to ambient conditions) a decision was made when to freeze/store the pattern in the computer. The criteria used here was basically to obtain relatively free of noise and as clear as possible shearographic images depicting the presence of the flaw beneath the test surface Test number one A specimen with manufactured defects was prepared from a section of a main rotor helicopter blade as shown in figure 4. Nine flat bottom holes were created allowing a range of thickness of core material from 2 to 64 mm. The positions of the defects were exposed to 1, 2 or 3 seconds of heating with the infrared lamp and typical results are shown in figure 5. Figure 4: Test specimen prepared from a helicopter main rotor blade 4

5 e f g Figure 5: Core thickness below test surface (e) 43, (f) 57, (g) 64 mm Top row heated for 2 seconds, bottom row for 3 seconds In figure 5 above it is noted that the number of fringes forming shearography s double bull s eye (typical for a delamination/void) increased as expected with longer period of heating the surface of the specimen. Of interest of course is that the size of the fringe pattern remains practically the same and approximates fairly well the size of the flaw (44 mm dia.) that was created with a hole-saw when manufacturing the defects. This obviously is where a UT test should be performed to ascertain the depth at which the simulated voidis located. Figure 6: Turnkey solution for main rotor blade 5

6 The tests were carried on to their obvious conclusion of establishing a relationship between number of fringes and depth of core material andthe position (depth wise)of the void. The whole test procedure is in fact a parametric study of a turnkey solution involving a main rotor helicopter blade. Provided the same protocol of testing, as was briefly reported here, is followed for identical components, by counting the fringes of any defect located and using similar figures obtained to that of figure 6, one would characterize the depth of the defect Test number 2 The second test we report here was performed for the purpose of further verification of the assumption that the size of the defect is fairly closely related to the size of the double bull s eye indicating the presence of a void or delaminations within a solid as we have seen in figure 5 above. For the purpose, an11.0 mm thick plywood sample 100 mm in width and 235 mm in length was prepared with four flat bottom holes of equal depth (5.0 mm) and varying diameters 44, 26, 21 and 18, mm. Figure 7 (left) depicts the specimen and an intensity shearogram (right) of the developed fringe pattern of the near surface (blind to the holes) after the specimen was heated for 2-3 seconds with hot air from a hair drier. The shearogram (obtained with minute shearing of the images) depicts the size of the defects with sufficient (acceptable?) accuracy to guide the positioning of a UT probe for depth determination. Figure 7: The plywood specimen on the left and on the right depicting its near surface captured intensity shearogram Test number 3 Test number 3 is fairly similar to the second test as detailed above, in that it aims to support the claim that the size of the flaw can be inferred from the shearogram, but in addition it serves to cover another parameter; that is the type of material as we are now dealing with being a sandwich type (skin, core, skin) composite specimen. Figure 8 depicts the test specimen constructed from a 1.0 mm thick E glass fibre skin, with a 5.5 mm thick Nomex (hexagonally un-symmetrical Aramid fibre honeycomb core, faintly visible on the left figure) and a 0.5 fibre glass inner skin. The overall dimensions of this specimen in addition to its 7.0 mm thickness were 190 mm length and 115 mm width. The shearogram depicted in figure 8 was obtained by the system s capability of performing a phase stepped routine with artificial colour introduced for dramatic effect, with the shearing of the images purposely set at a quite large dimension which makes the system very sensitive. It is suggested here that the position of each individual defect (triangles cut in the inner or bottom skin of the specimen) is located between the centres of their double bull s eye image and their size can be approximated by their single separate image. (note the white triangle pointed by the black arrow superimposed on the bottom left of the right image). The specimen was heated for very short period of time (about a second or two) with hot air from a hair drier and carefully observed while it was allowed to cool to the extent that there were no more overall surface gradients across its surface hence the absence of 6

7 formation of zebra like stripes superimposed on the specimen s image. In many instances dramatic indications of the location of defects are obtainable (as in the example shown in this case) if sufficient time is allowed for a specimen (obviously being of the composite type) to almost return to its equilibrium condition while the operator performing the test samples continuously the dynamic nature of surface deformation. In this case it took approximately about 5 minutes of observation and sampling of the images to reach the level indicated in figure 8 (right). Figure 8: Sandwich type composite with defects simulated by cutting away the inner skin in the shape of equilateral triangles of sides 20, 10, and 5 mm Test number 4 The results from test number 4 serve to reinforce what has been presented above with regard to the results from test number 3. In this experiment the test specimen was a simple three ply laminate of 2 mm thick transparent Plexiglas sheet glued together, 320 mm long and 120 mm wide, with three 20 mm square thin Teflon tape patches sandwiched between two sheets, thus creating an internal disbond in the laminate. The laminate was painted with grey primer to make it opaque. The specimen was subjected to a couple of seconds of hot air from the hair drier and immediately reacted in the expected manner of whole body distortion characterized by rapid moving fringes which slowed down as the specimen cooled down and approached its former equilibrium condition with the environment. Figure 9 (left) demonstrates that the disbond (the double bull s eye) is visible during the early stages of cooling of the specimen, while being observed with the phase stepped routine, when the fringes were still fairly active but no longer unduly obscuring the location of the disbond, however still fairly noisy to necessitate a filtering step. On the right we have the image of the same specimen (smaller shearing distance) when sufficient time of cooling for the specimen was allowed still using the phase stepped technique, however now the only activity observed was over the location of the disbond devoid of noise and hence not requiring filtering. Either image provides enough evidence of the presence of a defect for UT to move in and attempt to locate its position/depth below the testing surface. Figure 9: Three-ply Plexi-Glass laminate with Teflon manufactured internal disbonds detected by Digital Shearography; (left -phase stepped (early) filtered image) and (right- noise free phase stepped (late) image) 7

8 2.1.5 Test number 5 For this experiment the specimen manufactured consisted of three sheets of thickness (2.5 mm near surface, 2.5 mm middle, 2.0 mm far side) of hardboard bonded together forming a 7.0 mm. thick laminate of 147 mm dimensions square. The three holes that were manufactured in the middle ply were of 48, 24 and 11 mm diameters (as seen penciled on the near surface of the specimen in figure 10(left), and thus emulate large voids. This particular specimen presented us with a challenge in that it was difficult to obtain an image when all three defects were depicted reasonably well. After a few attempts the image in figure 10 (right) was obtained and deemed sufficiently descriptive or informative for the purpose. Figure 10: The three ply hardboard specimen with three internal circular voids manufactured (left) and its intensity shearogram (right) Test number 6 Specimen no. 6 consisted of a beam 300 x 40 x 12 mm sandwich type composite, with 1 mm thick E fiberglass skins and 10 mm thick foam core. The specimen as depicted in the middle of figure 11 was manufactured with a defect (40 x 40 mm delamination/void) between the skin and the core simply by inserting a thin blade and separating the skin from the core. Figure 11: The sandwich composite beam (in the middle) with its phase stepped shearographic images. Top image is indicating the defect in the far skin and bottom image is indicating the presence of the defect under the skin of the near surface. The specimen used in this test was subjected to a couple of seconds of warming up with hot air from a hair drier and the defect was detected both under the near and the far skins. Of course as expected, it is plainly clear that the indication is more dramatic when the defect is immediately under the skin of the surface that is being inspected (near surface). 8

9 2.2 Ultrasonic thicknesses testing A portable digital ultrasonic flaw detector (SIUI CTS-9009) was employed in order to determine the depth (or thickness of material from the surface) at which the defects were located, as indicated or detected by the shearographic testing, within the same composite material specimens that were used during the tests described in section 2.1 above. The UT instrument is fairly user friendly and capable of measuring/determining the speed of sound of a large range of materials (when required to be tested): one of its functions in A scan mode enables the gating of the echo from a reflector (defect) to establish its depth from the surface where the UT probe is located. The instrument is also capable of one button measurement in B scan mode. Table 1.0 below was constructed as a summary of the results (average of 3 readings for each test) obtained using the ultrasonic flaw detector with probes in the 1.0 to 4.0 MHz frequency range. Table1.0 - Summary of results of UT measurements Specimen 1 Position of Defect from Instrument indication Near side Far side Near side Far side 1.5mm GFRP Comments 43mm Nomex Flat hole No indication n/a No echo obtainable mm Flat hole An echo from 6mm detected n/a Overall thickness achieved and an echo from the flat bottom hole mm Flat hole No indication n/a No echo obtainable mm 2.0 mm 3.90 mm 2.20 mm mm 2.0mm 2.53 mm 2.07 mm Defect detected/quantified from both sides of the specimen Defect detected/quantified from both sides of the specimen mm 11.0 mm No indication No indication No echo obtainable 3.0 Conclusion The indications from this feasibility study are promising in that some positive results were obtained from specimens of the laminate type composites (2, 4, 5). It was possible to obtain indications of an echo from the defects from either side of the laminate specimens 4 and 5. The material thickness above the flat bottom hole of specimen no. 2 was also obtained. Therefore UT was successful in quantifying the defects indicated by Shearography for laminates of same or homogeneous material composition. In the case of the sandwich type composite specimens the echo technique failed to produce any results possibly because the materials involved cause high sound attenuation. This problem may be overcome by using low frequency ultrasonic probes in the range of 0.4 to 0.8 MHz. as has been suggested in [5] which certainly will be the subject of further research work in the near future. Acknowledgements The authors wish to thank Mr. Yusuf Patel, the Managing Director of Cape Town s branch of DE-TECT Unit Inspection, for the assistance provided in the ultrasonic testing of the composite specimens. 9

10 References 1. Hung YY: Shearography: a new optical method for strain measurement and nondestructive testing: Opt. Eng. 21 No.3, (1982) 2. Gryzagoridis J, Findeis D, Musonda V Using Digital Shearography to Visualizeand Quantify Defects in Composite Aerospace Components - CD -Proceedings of the 8th South African Conference on Computational and Applied Mechanics- SACAM08 - Cape Town, March Galela D: FAA Inspection Research Activities for Composite Materials The 2006 Composite Damage Tolerance & Maintenance Workshop, ATO-P, July 20, (2006) 4. Prototypes (last accessed August 2012) 5. Hillger, W. : Ultrasonic imaging of defects in Sandwich Composites from laboratory research to infield inspections, 7 th European Conference on Non- Destructive Testing Copenhagen, May 1998, Conf. Proc., pp