An Enhanced Fluorescent Liquid Penetrant Inspection Technique for Measurement of Surface CracksMarko Yanishevsky, P.Eng.
Structural Integrity and Damage Tolerance Specialist
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The Canadian Forces (CF) and the Royal Australian Air Force (RAAF) are conducting an International Follow-On Structural Test Program (IFOSTP), a joint mid-life full scale structural life reassessment of the CF188 fighter aircraft, commonly known as the F18 Hornet. A proposed repair for a critical design detail was to enhance its durability by inducing compressive residual stresses by shot peening. To determine the effectiveness and the appropriate implementation time for this repair, coupons were developed, simulating the geometry and stress concentration for accelerated fatigue testing to the CF188 service usage spectrum. During testing it was important to establish the onset of crack formation and to monitor fatigue crack progression. This program provided an opportunity to determine the level of crack detection effectiveness of the Eddy Current method used to inspect this design detail on the aircraft, for both baseline as-machined and shot peen repaired coupons.
This paper describes how an unconventional enhanced Fluorescent Liquid Penetrant Inspection (LPI) technique was developed to improve quantitative measurement of crack lengths in order to determine the effectiveness of the Eddy Current (EC) technique.
A test coupon was designed by the CF188 repair and maintenance contractor duplicating the geometry and stress concentration at a fatigue prone area of the aircraft structure. The coupons were manufactured from 7050-T7451 aluminium plate, with the same grain orientation as on the manufactured aircraft component.
Fig 1: Experimental setup showing the test coupon mounted in the load frame, video microscope and ultra violet illumination.
Fig 2: Coupon PL50 location #3 (ring light). Coupon loaded to 50% of the maximum test load.
The simulation coupon was fatigue loaded in a computer automated servo hydraulic material test load frame. During fatigue testing, a video microscope system was used to monitor and record crack development and progression, as shown in Figure 1.
Inspections of the design details were conducted at regular intervals and images were recorded using a video microscope. The fatigue prone areas were then inspected using the EC method, this latter method being the one presently used to inspect these locations on the CF188 aircraft fleet. The equipment was calibrated using a 7075-T6 aluminium alloy test block with electron discharge machining (EDM) slots with approximate depths of 0.2, 0.5 and 1.0 mm. The EC setup, with a full screen of 6 graticule divisions, provided responses of 1 division, 4 divisions and off screen respectively.
The fatigue prone areas were initially monitored using a ring light and a 20x to 100x video microscope and a set of grid lines, where 10 lines = 2.56 mm. An example inspection of surface cracks using this setup is shown in Figure 2. (Note the quality of the images has been deteriorated by the digitization and reduction processes for this paper).
Inspections were performed at the completion of each block of the CF188 IFOSTP fatigue spectrum, which contained 325 simulated Structural Flight Hours (SFH), and took approximately 1.25 hours to reproduce in the load frame.
During testing of the as-machined baseline coupons (surface finish, Ra ~65 (in), it was determined that visual inspection using a 20x magnification video camera was not effective to determine the onset of crack formation, particularly since multiple fatigue cracks would often form at different sites on different planes, and these would become visible only after they became several mm in surface length.
LPI inspection according to ASTM Standard Practice E 1417 was then conducted. Though this method improved crack detection, as shown in Figure 3, it was deemed inadequate as a real-time laboratory crack measurement tool.
For the purposes of the fatigue test program, the conventional LPI technique had several drawbacks: tedious effort to clean surfaces; inspections typically took 60 minutes each (from test interruption to test restart); and it was often difficult to illuminate the tiny indications from developing cracks to be bright enough for video microscope recording. As well, due to the time lag between development of the crack indication and capture on video, the penetrant would often bleed out, thereby affecting the accuracy of the crack length measurements.
|Fig 3: Coupon PL50 location #3 (conventional LPI). No load. Indications (arrows) had been allowed to bleed out to improve video capture.||Fig 4: Coupon PL50 location #3 (enhanced LPI). Coupon loaded to 50% of the maximum test load. Cracks are indicated by the arrows.|
It was important to improve the illumination of cracks for video recording and to reduce the inspection time, since the time spent inspecting was comparable to the time spent fatigue testing. After several iterations, the following technique was developed. The procedure required only one good surface preparation cleaning at the beginning of the fatigue test. To highlight the areas of interest, a cotton swab dampened with penetrant was found to work satisfactorily. During inspection, excess penetrant was removed from the areas of interest using fresh cotton swabs, leaving a thin layer of penetrant. The areas near the cracks, were then illuminated with ultra violet (black) light. A load of 50% maximum test load was found to adequately open cracks for the as-machined coupons, as shown in Figure 4.
Using this latter technique, cracks would appear as dark indications against a bright fluorescent background, (compared to tiny fluorescent dots against a black background using the conventional LPI technique). This enhanced the viewing, video recording and measurement of cracks. As well, this technique did not require coupon removal from the load frame, and after making the video recording and EC inspections, the fatigue test could be immediately restarted. The permanent video recordings of the LPI inspections allowed crack length measurements to be conducted off-line by either viewing the tapes between inspections or at the completion of the fatigue test.
It was determined that this enhanced LPI technique was more sensitive than EC, thereby making it possible to measure and monitor crack formation and crack growth prior to reliable detection by EC. Two coupons were sacrificed to measure the sizes of cracks once they were detected by EC. The crack sizes were measured by sectioning the coupons and exposing the crack surfaces by breaking them open. The measured crack sizes are shown in Table 1.
|Coupon / Site# (EC Reading)||Surface Length (2a)||Depth (c)||2a/c Ratio|
|PL3 #1 (EC=1/4 div)||0.45 mm (.018")||0.21 mm (0.008")||2.14|
|PL3 #4 (EC=1 div)||0.90 mm (0.035")||0.58 mm (0.023")||1.55|
|PL20 #4 (EC=1/4 div)||0.56 mm (0.022")||0.30 mm (0.012")||1.87|
From the video records of these and other cracks, it was determined that in the awkward radius of the inspection sites, the EC technique was reliable for detecting cracks in excess of ~0.5 mm surface length (~0.25 mm surface depth).
The superiority of the enhanced LPI technique was particularly evident when trying to inspect coupons which had been repaired by shot peening. The shot peening process was performed by the repair and maintenance contractor using conditions which simulated the location height and access difficulty on the aircraft, achieving an Almen intensity of 0.008 ± 0.001. X Ray diffraction residual stress measurements indicated that typically, the depth of the imparted compressive residual stresses was 0.1 mm (0.004"), with the compressive residual stresses ranging from 50 to 65% of the yield strength for 7050-T7451 aluminium alloy.
|Fig 5: Coupon PL19 location #1 (white light). Coupon loaded to 80% of the maximum test load. Crack not visible under these conditions.|
It was found that the inspectability of cracks was affected by the shot peening. Inspection using the 20x magnification video microscope with ring light was again unreliable to detect cracks several mm in length, as shown in Figure 5.
Figure 6 exhibits the same location shown in Figure 5 using conventional LPI.
The compressive residual stresses imparted to the surface by shot peening made it very difficult for penetrant to enter cracks as they were forming. As well, the dimples on the surface from shot peen processing, made it very difficult to properly clean the surfaces. To remove the excess penetrant, extreme wetting of the surface was required, which also removed penetrant from the cracks. It was also difficult to develop and illuminate these indications for the purposes of video recording.
The enhanced LPI technique as shown in Figure 7, was found to be superior, particularly on the shot peen repaired coupons. Cracks with surface lengths of 0.3 mm could be reliably measured using this technique. To overcome the surface compressive residual stresses, tensile loads of at least 75% of the maximum test load were necessary to open up these cracks.
Though there were a few instances of the EC threshold of inspectability for cracks with surface lengths of ~1.0 mm (~0.5 mm depth), consistent reliable indications were found for cracks typically of 1.5 mm surface length, ~3 times the size reliably detected on as-machined coupons.
|Fig 6: Coupon PL19 location #1 (conventional LPI). No load. Arrow points to LPI indication.||Fig 7: Coupon PL19 location #1 (same site as in Figures 5 and 6 using enhanced LPI). Coupon loaded to 80% of the maximum test load. Arrow points to crack.|
From this study, it was determined that shot peening significantly influences the reliable threshold of inspectability using the EC inspection method. For as-machined coupons, cracks of approximately 0.5 mm surface length x 0.25 mm depth were found to be reliably inspectable by EC. For coupons which had been shot peened, EC became reliable for cracks of 1.5 mm surface length x 0.75 mm depth.
It was very difficult to find cracks using the conventional LPI technique on shot peened coupons, since the dimples produced by shot peening made it difficult to properly clean the areas without flooding, thereby removing all penetrant, including what little was able to actually enter the cracks. The surface compressive residual stresses made it difficult for penetrant to enter cracks and indications were often unclear even for cracks of several mm in surface length. With the enhanced LPI technique, cracks with surface lengths of 0.3 mm were found to be readily inspectable.
I would like to particularly thank the QETE NDT Specialists, Mr. Larry Boone, Sgt. Michel Montambeault, and Mr. Orville Francescone, for providing the EC and conventional LPI inspections. Their professionalism, dedicated efforts, time and frustrations providing ideal quality LPI inspection sites inspired me to develop the enhanced LPI technique as an alternative for our laboratory experiments.
I would also like to acknowledge Mr. Sylvain Forgues and his colleagues at Canadair/Bombardier Military Aircraft Division for designing the test coupon, providing the CF188 IFOSTP fatigue test spectrum and for the shot peen processing.