20 Years of Developing Representative Reference NDI Coupons with Fatigue Cracks. Marko Yanishevsky

Transcription

1 53 rd Annual ATA NDT Forum, Albuquerque, New Mexico, USA September 20-23, Years of Developing Representative Reference NDI Coupons with Fatigue Cracks Marko Yanishevsky Marcias Martinez, Catalin Mandache, Muzibur Khan. Abbas Fahr and David Backman Structures and Materials Performance Laboratory Institute for Aerospace Research

2 Outline Motivation General approach for creating fatigue cracks Case studies #1 Cracks in radii #2 Cracks in fastener holes #3 Surface cracks (flat surface) #4 Surface cracks in cylinders #5 Cracks in lugs Discussion Summary Acknowledgements

3 Motivation One of the most challenging aspects of quantifying the performance of Non Destructive Inspection (NDI) techniques is the development of representative coupons containing crack-like discontinuities for NDI technique calibration and to truly assess its detection capability and reliability. Electrical discharge machining (EDM) using shim or wire electrodes offers opportunities to develop rudimentary level discontinuities; however, these have several characteristics that differ from fatigue cracks found inservice.

4 Motivation Fatigue cracks vs EDM slots EDM slots are often triangular in an attempt to represent corner cracks or rectangular to represent surface cracks; unfortunately, these are crude approximations of the quarter-elliptical or semi-elliptical cracks found in service. Deep EDM slots also have a significantly larger width (air gap). Shallow EDM slots can behave more like a machining mark rather than a sharp discontinuity. Responses of NDI techniques such as eddy current and ultrasonics are affected.

5 General approach for creating fatigue cracks Machine coupons with the required geometry and extra material that can be slotted with a starter EDM notch. The thickness and size of the extra material can be tailored to develop the appropriate crack length versus crack depth ratio desired. Apply fatigue loading to grow a crack from the EDM slot. AFGROW crack growth prediction software can aid in this effort. Track crack development and growth. Once desired size is achieved - machine away the extra material feature to the final detail size. Reinstall the coupon into the load frame and fatigue cycle for a few extra hundred cycles to help open the crack after potential smearing of crack edge after machining. Perform additional manufacture and assembly of the coupons into required configuration for inspection.

6 Case Study #1 - Cracks in radii

7 Case Study #1 - Cracks in radii Shortly after developing the NDI reference coupons, Canadian Forces NDI specialists established that the existing UT technique that had been used for several years was not able to reliably detect cracks at and below the critical crack size. An alternate ET technique was developed and it was that technique that was used further in Probability of Detection studies using Canadian Forces NDI technicians. A mockup of the surrounding structure was created and located in the centre of a hangar, which put the NDI technicians at the same crouched position, level and ambient lighting conditions as would be experienced in-service. Since the number of NDI reference coupons was limited (28 with cracks and 2 without cracks), the coupons were inserted into the mockup using a different random order for each inspector, with many of the coupons being inspected more than once by each inspector.

8 Case Study #2 - Cracks in fastener holes Examples of wing plank to wing plank, wing plank to spar and spar to web joints.

9 Case Study #2 - Cracks in fastener holes Gripping Corner Cracks Fin EDM Notch with fatigue crack (A) countersink Final Fatigue Crack after machining (B) mid bore Mid bore Cracks Fin EDM Notch with fatigue crack (C) bottom surface Gripping Final Fatigue Crack after machining Approach to making multiple coupons with corner and mid-bore cracks in fastener holes (D) final machined hole with mid bore fatigue crack and countersunk fastener installed.

10 Case Study #2 - Cracks in fastener holes Panel undergoing fatigue cycling. (A) single coupon (top view), (B) inspection set (side view), (C) inspection sets assembled in box, (D) an inspector performing a BHET inspection.

11 Case Study #2 - Cracks in fastener holes Mid bore Cracks - on Top Surface Probe Corner Crack in Bottom Surface Probe Top Layer Crack c a 7.95 mm (0.313 ) Top Layer Bolt Hole 7.95 mm (0.313 ) Bottom Layer Bolt Hole 2.29 mm (0.090 ) Bottom Layer Crack c 2.29 mm (0.090 ) a Interface Surface Corner Crack Probe Corner Crack on Top Surface a Probe Top Layer Crack c 7.95 mm (0.313 ) Top Layer Crack c 2.29 mm ) ( Bottom Layer a 2.29 mm (0.090 ) Bottom Layer Bolt Hole 2.29 mm ) ( Four configurations: mid bore crack in top thick layer (top left); corner crack in thin bottom layer (top right); corner crack in internal faying surface; (bottom left); and corner crack in top surface (bottom right).

12 Case Study #2 - Cracks in fastener holes Lab Grown Corner Crack Size: [0.03 mm] Lab Grown Corner Crack Size: [1.28 mm] EDM Slot Size: [0.03 mm] EDM Slot Size: [1.27 mm] ET signal traces for corner cracks versus EDM slots of similar size (Staveley Nortec 2000 ET instrument with RA 3000 rotating probe).

13 Corner Cracks EDM vs Fatigue Surfaces # (0.15 mm) depth / (0.74 mm) length EDM remelt material Fin with EDM slot # (0.71 mm) depth / (1.24 mm) length EDM corner slot (shim) (0.74 mm) depth / (0.99 mm) length Fin with EDM slot

14 Mid-Bore Cracks # (0.15 mm) depth / (0.69 mm) width # (0.36 mm) depth / (0.89 mm) width

15 Case Study #3 Surface cracks Single rib with single crack Multiple ribs with multiple cracks

16 Case Study #3 Surface cracks Strain gauge (on patch) Length 2 = 1 in Primary side 2 in Crack Length 1 = in 5.51 in Substrate: Al 2024-T3 Patch: Carbon/Epoxy Adhesive: FM73 15 º (3 locations) Number of plies = 6 Taper angle 1 = 45º 0.25 in Secondary side Strain gauge (on patch) 5.51 in 12 in Composite patch repair with disbond UT and CDDT monitoring

17 Case Study #3 Surface cracks TSA measurements of normalized stress showing damage progression at patch disbond (from top) and induced fatigue crack under the patch (centre) from left to right (a) 145K (b) 300K and (c) 345K fatigue cycles.

18 Case Study #3 Surface cracks Multi frequency eddy current results: (a) surface crack before bonding a patch (b) same crack after bonding of the repair patch (c) surface crack after the coupon with the repair patch was fatigued (d) surface crack with the repair patch removed, (over 2x original size (e) pulsed eddy current showing the crack extremities

19 Case Study #3 Surface cracks CDDT versus cycle count. TSA results.

20 Case Study #4 Surface cracks in cylinders Compressor impellor (inner cylindrical bore). Top and side views schematic of the cylindrical specimens.

21 Case Study #5 Cracks in lugs Pylon hinge lug Folding helicopter tail.

22 Case Study #5 Cracks in lugs

23 Case Study #5 Cracks in lugs

24 Discussion Often there are few service parts with representative damage and fatigue cracking that can be used to help develop NDI techniques and evaluate their performance. The critical crack sizes for many aerospace applications are quite small, so their detection at an early stage when they can be repaired is the preferred option. Reference NDI coupons are developed with artificial damage, such as EDM slots to simulate the service scenario. Unfortunately, EDM slots are not always representative of the in-service experience and may give the inspectors / engineers / maintainers a false sense of confidence that the damage can indeed be found reliably and in time. As such, it is a major challenge to develop crack-like discontinuities representative of the in-service fatigue cracking experience to assess the true inspection capability, particularly if the geometry is complex.

25 Discussion To aid in the prediction of load levels and number of fatigue cycles required to generate cracks from EDM slots, the crack growth software program AFGROW was used in many of the case studies presented. When conducting Probability of Detection, technique detectability and reliability studies, it is important to actually break open at least a portion of the cracks in your reference coupons to accurately measure and obtain more confidence in the actual sizes and shapes of the cracks that cannot be measured and only predicted from surface measurements while the coupons are loaded in a load frame and the cracks are opened. This is particularly important for establishing the aspect ratio of surface length to depth of surface cracks.

26 Summary Five cases were presented for which NDI reference coupons were developed, each with its own challenges and nuances, requiring innovative thinking and methodologies for introducing fatigue cracks of requisite size into fatigue prone locations. Using these approaches one is able to create NDI reference coupons that can be used both as aids for developing new inspection techniques, verifying the performance of established techniques, and for further use in Probability of Detection / Probability of Miss evaluation of the in-service experience. The reference NDI coupons can also be implemented in recreated service inspection mock-ups that would be representative of the environment in terms of location, lighting, accessibility and noise, where the inspection would be typically conducted.

27 Acknowledgements Case studies 1, 4 and 5 were conducted with support from engineering and technical staff of the Quality Engineering Test Establishment (QETE) of the Canadian Department of National Defence (DND). Case studies 2, 3 and 4 were conducted with support from research and technical staff at the National Research Council Canada Institute for Aerospace Research Structures and Materials Performance Laboratory. Support and funding from DND for conducting these case studies over the last 20 years are gratefully acknowledged.

28 QUESTIONS? Next Generation Canadian Stealth Fighter

29 Acknowledgements Case studies 1, 4 and 5 were conducted with support from engineering and technical staff of the Quality Engineering Test Establishment (QETE) of the Canadian Department of National Defence (DND). Case studies 2, 3 and 4 were conducted with support from research and technical staff at the National Research Council Canada Institute for Aerospace Research Structures and Materials Performance Laboratory. Support and funding from DND for conducting these case studies over the last 20 years are gratefully acknowledged.