20 Years of Developing Representative Reference NDI ... · PDF file20 Years of Developing...

20 Years of Developing Representative Reference NDI Coupons with Fatigue Cracks

Marko YanishevskyMarcias Martinez, Catalin Mandache, Muzibur Khan.

Abbas Fahr and David Backman

Structures and Materials Performance LaboratoryInstitute for Aerospace Research

53rd Annual ATA NDT Forum, Albuquerque, New Mexico, USA September 20-23, 2010.

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

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 in-service.

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.

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.

Case Study #1 -Cracks in radii

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.

Case Study #2 -Cracks in fastener holes

Examples of wing plank to wing plank, wing plank to spar and spar to web joints.


Gripping

Gripping

EDM Notch with fatigue crack

Final Fatigue Crack after machining

EDM Notch with fatigue crack

Final Fatigue Crack after machining

Corner Cracks Fin

FinMid‐bore Cracks

Approach to making multiple coupons with corner and mid-bore cracks in fastener holes

(A) countersink

(B) mid‐bore

(C) bottom surface

(D) final machined hole with mid‐bore fatigue crack and countersunk fastener installed.

(A) single coupon (top view), (B) inspection set (side view), (C) inspection sets assembled in box, (D) an inspector performing a BHET inspection.


Panel undergoing fatigue cycling.

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).


Bottom Layer

Mid -bore Cracks on Top Surface

a 7.95 mm (0.313”)Top Layer

2.29 mm (0.090”)

cCrack

Probe

BoltHole

a

Corner Crack in Bottom Surface

c

7.95 mm (0.313”)Top Layer

2.29 mm (0.090”) Bottom LayerCrack

Probe

BoltHole

Top Layer

Corner Crack on Top Surface

c 2.29 mm (0.090”)

2.29 mm (0.090”)Bottom Layer

a

Crack

Probe

BoltHole

Bottom Layer

Interface Surface Corner Crack

c

7.95 mm (0.313”) Top Layer

2.29 mm (0.090”)a

Crack

Probe

Lab Grown Corner Crack Size: 0.012” [0.03 mm]

EDM Slot‐Size: 0.012” [0.03 mm]

Lab Grown Corner Crack Size: 0.051” [1.28 mm]

EDM Slot Size: 0.050” [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).


Corner CracksEDM vs Fatigue Surfaces

EDM remelt material

EDM corner slot (shim)0.029” (0.74 mm) depth / 0.039” (0.99 mm) length

#2050 0.028” (0.71 mm) depth / 0.049” (1.24 mm) length

#2006 0.006”(0.15 mm) depth / 0.029” (0.74 mm) length

Fin with EDM slot

Fin with EDM slot

Mid-Bore Cracks

#4006 0.006” (0.15 mm) depth / 0.027” (0.69 mm) width

#4018 .014” (0.36 mm) depth / 0.035” (0.89 mm) width

Case Study #3 –Surface cracks

Single rib with single crack

Multiple ribs with multiple cracks

2 in

0.25 in

Length 1 = 0.125 in

12 in

5.51 in

Length 2 = 1 in

15 º (3 locations)Taper angle 1 = 45º

Number of plies = 6

Substrate: Al 2024-T3Patch: Carbon/EpoxyAdhesive: FM73

5.51 in

Primary side

Secondary sideStrain gauge (on patch)

Strain gauge (on patch)

Crack

Composite patch repair with disbond UT and CDDT monitoring


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.


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


CDDT versus cycle count.

TSA results.


Case Study #4 –Surface cracks in cylinders

Top and side views schematic of the cylindrical specimens.

Compressor impellor (inner cylindrical bore).

Case Study #5 –Cracks in lugs

Folding helicopter tail.

Pylon hinge lug

Case Study #5 –Cracks in lugs

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.

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.

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.

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.

QUESTIONS?

Next Generation Canadian Stealth Fighter

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.

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