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Evaluating Low Frequency Eddy Current Based Technologies for Detecting Fatigue Cracks in Multi-Layer Metallic Structures 2014 ASIP Conference Dec 3, 2014 Gary Steffes AFRL/RXCA Integrity Service Excellence Acknowledgements

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SLIDE 1

Integrity  Service  Excellence

Evaluating Low Frequency Eddy Current Based Technologies for Detecting Fatigue Cracks in Multi-Layer Metallic Structures

2014 ASIP Conference Dec 3, 2014 Gary Steffes AFRL/RXCA

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Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Acknowledgements

  • Co-authors:

– USAF – David Campbell, Charles Buynak – Texas Research Institute-Austin (TRI) - Doyle Motes, David Forsyth, Mark Keiser, Michael Mazurek – Computational Tools – John Aldrin

  • AFRL Sustainment Office (sponsors)

– Stephan Wolanczyk, Stephanie Flanagan

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SLIDE 3

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Outline

  • Introduction
  • Program Overview
  • Fatigue Crack Specimens
  • Sensor Systems
  • Preliminary Results
  • Accounting for Changes in Structural

Geometries

  • Summary
  • Conclusions

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SLIDE 4

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Introduction

  • Background

– Low frequency eddy current (EC) methods used to inspect sub-surface aircraft structures – Increasing requirements for detection as fleets age

  • Smaller subsurface defects
  • Larger areas of coverage
  • Motivation

– Reached the limits of current Low Frequency EC – Need to interrogate thick, complex structures using NDE – Avoid a/c structure disassembly

  • Impacts schedule and cost
  • Possibility of maintenance induced damage
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SLIDE 5

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Program Overview

  • Program goals were:

– Collect data from candidate NDI sensor systems for detecting sub-surface fatigue cracks in complex structures

  • Fabricate samples

– Results evaluated by AFRL

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Program targeted specific inspections on aircraft structure NDI inspections of thick, complex wing structures can be time consuming and difficult

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SLIDE 6

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Fatigue Crack Specimen Sets

  • Simulate second layer

cracking

  • Four 7075-T6 Al

specimen sets

– Thicknesses (0.500”, 0.375”, 0.250”, 0.100”) – 4 - ¼” holes per specimen

  • Sixty (60) specimens

tested for each thickness

– 240 holes per set – 60 cracks per set

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0.500” 0.375” 0.250” 0.100”

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SLIDE 7

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Fully Assembled Inspection Targets

  • Specimen Setups Included:
  • Boeing (Cd plated) steel fasteners (BACB30LU) and Al nuts
  • 1st layer consisted of 0.170” and 0.250” 7075-T6 Al top sheet
  • 2nd layer contained 60 individual 7075-T6 Al specimens at

specimen set thicknesses

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SLIDE 8

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Other Sample Set Information

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0.250” Top Sheet 0.500” Specimen Set

Setup #2

0.170” Top Sheet 0.500” Specimen Set

Setup #1

Fatigue Crack Fatigue Crack

  • Crack content information
  • Cracks only in holes
  • Crack types
  • Toward adjacent fastener

holes

  • Toward edges
  • Cracks orientations occur at 12

and 6 o’clock only

  • DO NOT CONTAIN
  • Double cracked holes
  • Complete hole-to-edge or hole-

to-hole cracks

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SLIDE 9

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Sensor Systems Examined

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  • 4. Remote Field

EC Ring Probe

  • 2. Low Frequency EC

Probe

  • 1. Giant

Magnetoresistive (GMR) Array

  • 3. Conventional

EC Ring Probe

  • 5. Remote Field EC

Ring Probe on Scanner

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SLIDE 10

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

  • 1. GMR Linked to MAUS
  • Scans completed by TRI’s NDI Level III

inspector

  • Data interpretations

– Raw c-scans – Automated Defect Analysis (ADA) processing codes

  • Scan frequency range 150-300Hz
  • High false call rates for both scans
  • Issues for interpreting C-scan data

included:

– Target edge effects – Sensor sensitivity bias – Inconsistencies between scans – Effects from cracks at adjacent inspection sites

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SLIDE 11

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Example Raw C-scan Produced by GMR w/MAUS

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Edge effect (from individual specimen)

GMR C-scan – circles indicates EDM notches

Magnetized fastener Sensitivity Bias (appears as data striping) Large edge effect (end of assembled specimens) Fastener site signal

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SLIDE 12

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

  • 2. LFEC Sensor Mounted to the MAUS
  • Completed by AFRL personnel
  • Scan frequency

– 550 Hz used for thin top sheet (0.170”) – 225 Hz used for thicker top sheet (0.250”)

  • Visual interpretation difficult due to effects from adjacent holes (green

arrows)

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Called crack “Missed crack” Called crack

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SLIDE 13

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

  • 3. EC Ring Probe (Current method)
  • Scans completed by TRI’s NDI

Level III inspector

  • Method provided a baseline for

comparison

  • Inspections adapted from AF tech
  • rder procedures

– Frequency set by deepest area to be inspected – Full Screen Height deflection (%FSH) recorded for each site – Detect threshold set at a ≥ 30% FSH

  • Low false call rates and good

results with Cd-plated steel fasteners

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SLIDE 14

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

  • 4. RFEC Ring Probe
  • Tests conducted by TRI’s

NDI Level III inspector

  • NDI procedures developed

specifically for test

  • Scan frequency - 100 Hz

(exception - 0.170” top and 0.100” specimen set)

  • Effects from cracked

adjacent holes seen for longer cracks

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SLIDE 15

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

  • 5. RFEC Ring Probe Mounted to the MAUS
  • Conducted by AFRL personnel
  • Same procedures used as RFEC ring probe (100 Hz

frequency)

  • Edge effects and adjacent cracks were found to be less

pronounced at 100Hz

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SLIDE 16

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Caveats to Preliminary Results

  • Specimens – aircraft not typically constructed with 2.4”x

2.4” squares (edge effects)

  • Procedures were developed specifically for test
  • Mean crack lengths
  • Definition of a false call
  • POD calculations –

– Results collected by two people

  • Insufficient number
  • Non-aircraft inspectors

– Fastener head diameter (0.469”)

  • Warning: Extrapolating results for similar geometries can

be bad (i.e. part specific)

Specimen Set Thickness (in) Mean Crack Length (in) 0.100 0.175 0.250 0.200 0.375 0.252 0.500 0.352

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SLIDE 17

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Results Table

(0.250” top x 0.500” bottom)

Inspection Method Correct (%) False Call (%) a90 (in) a90/95 (in)

  • 1a. GMR ImagIn

13% 5% > 1.000 > 1.500

  • 1a. GMR ImagIn Adj

20% 3% > 1.000 > 1.500

  • 1b. GMR ADA

80% 17% 0.370 0.471

  • 1b. GMR ADA

62% 12% 0.480 0.584

  • 2. LFEC

52% 6% NA NA

  • 3. Conventional EC ring

75% 0% 0.310 0.336

  • 4. RFEC Ring Probe

90% 0% 0.212 0.220

  • 5. RFEC Ring Probe MAUS

93% 1% 0.206 0.217 Best Results RFEC MAUS Tie RFEC MAUS RFEC MAUS

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SLIDE 18

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Summary of Results

  • Sensitivities drop off with increasing depth for all sensor

systems.

  • EC ring probe results are better than the GMR Array MAUS

and the LFEC MAUS for all inspections described in this work.

  • RFEC averaged a better correct call rate than EC ring

probe by 17%

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Best a90 Results 0.170” Top Sheet 0.250” Top Sheet 0.100” Specimen Set RFEC Ring Probe RFEC Ring Probe 0.250” Specimen Set RFEC Ring Probe MAUS RFEC Ring Probe 0.375” Specimen Set Conventional EC Ring Probe / RFEC Ring Probe RFEC Ring Probe MAUS 0.500” Specimen Set RFEC Ring Probe MAUS RFEC Ring Probe MAUS

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SLIDE 19

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Other Evaluations

  • Motivation – Account for changes in structural geometry

present in aircraft inspections

– Tapers and steps – Changes in fastener material

  • Evaluated signal response due to ET frequency per

thickness

  • Difference in signal response for structures using steel
  • vs. aluminum fasteners
  • RFEC Ring Probe system used
  • Setup

– Configuration 1: 0.170” top x 0.100” bottom – Configuration 2: 0.170” top x 0.500” bottom – Same inspection procedures used in all cases

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SLIDE 20

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Signal Effects Due to Frequency

0.170” Top x 0.100” Bottom

10 20 30 40 50 60 70 80 90 100 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Steel Fasteners 50Hz Steel Fasteners 100Hz Steel Fasteners 200Hz Steel Fasteners 300Hz Steel Fasteners 400Hz

Crack Length (in) Signal Response Screen Height (%FSH)

Crack No crack

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SLIDE 21

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Signal Effects Due to Frequency

0.170” Top x 0.500” Bottom

10 20 30 40 50 60 70 80 90 100 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Steel Fasteners 50Hz Steel Fasteners 100Hz Steel Fasteners 200Hz Steel Fasteners 300Hz Steel Fasteners 400Hz

Crack Length (in) Signal Response Screen Height (%FSH)

Crack No crack

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SLIDE 22

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Signal effects due to Steel vs. Al fasteners

10 20 30 40 50 60 70 80 90 100 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Steel Fasteners 50Hz Steel Fasteners 100Hz Steel Fasteners 200Hz Steel Fasteners 300Hz Steel Fasteners 400Hz Aluminum Fasteners 200Hz Aluminum Fasteners 300Hz Aluminum Fasteners 400Hz Aluminum Fasteners 500Hz

Crack Length (in) Signal Response Screen Height (%FSH)

Crack No crack

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SLIDE 23

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Conclusions

  • Low frequencies and large probes produce diffuse fields

affected by local geometry and fatigue cracks.

– Edge effects are important and aircraft structures are full of them – Depending on the distance between fasteners, signal may be affected by adjacent anomalies

  • C-scan interpretations are difficult and can be subjective
  • Addressing deep sub-surface defect detection must

account for many factors

– Structures - tapers, steps, repairs, mods, 50+ years of in-service – Speed – especially with large inspection areas

  • More work must be done with LFEC technologies

– RFEC (good detectability) – MR Sensor Arrays (high throughput)

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SLIDE 24

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Summary

  • Background - increasing demand for

structural sub-surface inspection req’ts

  • Fatigue crack specimens
  • 5 Low Frequency ET sensor systems
  • Preliminary results
  • Addressing structural variables for in-

service NDI inspections

  • Conclusions

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SLIDE 25

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Other Acknowledgements

  • USAF – Steve West, Walter Matulewicz, Richard Duin,

Ward Fong, Tommy Mullis

  • UTC – Bob Cochoy
  • Sfhire – Floyd Spencer
  • AP/ES – Craig Brooks, Tom Mills, Kyle Honeycutt, Scott

Prost-Domasky

  • Boeing – Nancy Wood, Tom McGehee, Don Palmer
  • IMTT – Yushi Sun, Changhong Sun

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SLIDE 26

Distribution Statement A: Approved for public release; distribution is unlimited. Case Number: 88ABW 2014-5495.

Questions

  • ???

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