Nationaal Lucht- en Ruimtevaartlaboratorium – National Aerospace Laboratory NLR

Structural Health Monitoring (SHM)

Jaap HeidaAerospace Vehicles Division

NANDTB, NDT Expert Day – 18 November 2009

2

National Aerospace Laboratory NLR

Staff of 700Turnover ~ 80 MEuro/yr

3

NDI expertise at NLR

Evaluation of NDI techniques

Investigation of NDI reliability - POD, false calls

NDI of hybrid/composite materials- Glare, CFRP, RTM

Full scale tear-down and inspection- EPAF TDI of RNLAF F-16A aircraft J-239- LHS wing TDI of Lockheed P-3C Orion - Airbus A380 Megaliner Barrel TDI

Structural health monitoring (SHM)

Level III assistance, training and examination of NDI personnel

Modelling of NDI (CIVAnde)

4

Contents

Structural health monitoring (SHM) Principle and objectives of SHM NLR experience with SHM SHM techniques/sensors Applicability of SHM

5

Structural health monitoring (SHM)

Long-term and in-service monitoring of the condition and damage state of vehicle systems in operation using advanced sensors that are permanently attached to the structure

Main objectives:

- Introduce condition-based structural maintenance (replacing scheduled maintenance)

- Reduce the cost of ownership (inspection and maintenance)

- Improve the system operational availability while maintaining current safety levels

Additional objectives:Solve problems of poor accessibility and remove the human factor of inspector fatigue

6

Conventional scheduled maintenance

UT C-scan

Shearography

Manual UTNH-90 tail fatigue test

Pulsed thermography

7

Structural health monitoring (SHM)

Kumar, Acellent Technologies

8

SHM features

• Part of on-board systems (centralised or distributed) with low-profile sensors that are permanently attached to the structure

• Operation on-line during the flight (vehicle in operation) or off-line on the ground

• Static system that interrogate the structure at predetermined intervals (active measurement), or dynamic system that require continuous, reliable monitoring (passive measurement)

• Global inspection of large surface areas, or local inspection of highly critical areas (hot spots)

9

NLR experience with SHM

• EU projects• MONITOR (Monitoring On-line Integrated Technologies for

Operational Reliability)• AHMOS I + II (Active Health Monitoring System)• CESAR (Cost Effective Small Aircraft)• FP7?

• NIVR SRP projects• SHM system for composites based on optical FBG sensors

(embedded and surface mounted)

• Dutch Space project• SHM applications for hot metallic structures

• RNLAF project• Optical FBG sensors for strain measurement on metallic

structure

10

Active Health Monitoring System

Prototype demonstration of modular structural health monitoring system for military platforms

NLR participation:

- Instrumented NLR test bench

- Central computer for data acquisition

- Eddy current array inspection

- Evaluation of AHMOS project

AHMOSACTIVE HEALTH MONITORING SYSTEM

11

AHMOS-II flight test

Hawk MK1A jet with flight test pod

2 metallic test specimens

12

AHMOS data presentation, level 1

13

AHMOS data presentation, level 2

14

AHMOS data presentation, level 3

15

SHM techniques

Piezoelectric sensor

Fibre optic sensor

Eddy current sensor

Comparative vacuum monitoring (CVM)

Other sensors• Corrosion sensor, strain gauge, accelerometer,

temperature sensor, heat flux sensor, leak detection, MEMS, etc.

16

Piezoelectric sensor

Dynamic system (passive mode) System that requires continuous, reliable monitoring- Acoustic emission

Static system (active mode)System that interrogates the structure at predetermined intervals- Phased array- Lamb wave- SMART layer

17

Acoustic emission

• PE sensors in the passive mode• Real time and on-line detection of

defect initiation and defect growth• Global monitoring of large specimens

(Localisation of defects may be difficult)

18

Acoustic emission

Monitoring of superalloy specimens with TBC during creep

CM186 SX specimen at 950 °C and 160 MPa

19

UT phased array inspection

20

UT phased array (SWISS sensor)

21

Lamb wave inspection

• Combination of transverse and longitudinal waves in plate-like structures. Propagation over long distances

• Transmission measurements with sensors in pitch- catch configuration

• Localisation and sizing of damage is difficult

Disbond detection in an Al stringer assemblya) undamaged structure and b) disbond

AHMOS pod

22

UT SMART layer

23

UT SMART layer

Detection of crack growth and disbond growth of a bonded repair on F-16 aircraft structure

Stanford University, CA

24

UT SMART layer

Detection of crack growth and disbond growth of a bonded repair on F-16 aircraft structure

25

Fibre optic sensor (FOS) technology

Main applications:

• Structural damage detection- environmental, accidental

• Strain/temperature/vibration monitoring

• Process monitoring of composite materials- state of cure during fabrication- internal strain after fabrication

Best prospect for SHM exploitation of FOS technology: Fibre Bragg Gratings (FBG)

26

Fibre optic Bragg grating sensor

λB = 2n.ΛλB

27

Fibre optic Bragg grating sensor

CASA composite inter-tank demonstrator with embedded and surface mounted optical fibres

(McKenzie, ESA, 2005)

28

Fibre optic Bragg grating sensor

Advantages:

• Small diameter, light weight, flexible, non-electrical and EMI immunity, low signal loss

• High sensitivity (~ 1 pm/με)

• Multiplexing of multiple FBG’s per fibre

• Possibility of embedded sensors in composites

Limitations

• Vulnerability of the sensor system

• Sensor/connector integration into the structure

29

Eddy current inspection

30

Eddy current array sensor - AHMOS project -

Local detection of cracks, monitoring of limited number of critical areas (‘hot spots’) in metallic aircraft structure

High-frequency (800 kHz), flexible, absolute sensor

Low-frequency (1-20 kHz), ring-shaped, reflection sensor

31

Eddy current array sensor

Evaluation of EC sensors using specimens with artificial defects and real fatigue cracks. TRL4 demonstrated

High-frequency absolute sensor for detection of surface cracks. Flexible sensor, reliable signals for radii down to 7 mm

Low-frequence reflection sensor for detection of sub-surface cracks in fastener holes. Practical detection depth in Al about 4 mm

32

Comparative vacuum monitoring (CVM)

Vacuum technique developed by SMS Ltd. (Australia)

sensor

vliegtuigcomponent

dwarsdoorsnede

sensor

vliegtuigcomponent

dwarsdoorsnede

naar SIM8

bovenaanzicht

naar SIM8

bovenaanzicht

oppervlaktescheuroppervlaktescheursurface crack

top viewcross-section

to SIM8

sensor

component

polymer sensor

33

Comparative vacuum monitoring (CVM)

CVM lab kit

CVM portable version

CVM recording

34

CVM measurements on FSW panels

0

5000

10000

15000

20000

25000

11:36:00 11:37:26 11:38:53 11:40:19 11:41:46 11:43:12 11:44:38 11:46:05 11:47:31

Time [hrs:min:sec]

CVM

read

ing

delta

P [P

a]

CVM

35

Accelerometer

Impact detection system (IDS) for WLE of NASA Space Shuttle (result of Columbia crash, 2003)

On aft surface of each WLE spar:- 66 accelerometers - 22 temperature sensors

(Studor, NASA, 2007)

36

Multifunction sensor

(Hunter, NASA GRC, 2005) (Wrbanek, NASA GRC, 2001)

37

Applicability of SHM techniques- hot metallic structures -

++ primary method, + secondary method, 0 method applicable but not practical or with limitation, - method not applicable

SHM

technique

Defect type

Impact event

Fatigue crack

Fastener damage

Seal leakage

Bondline failure

PE passive (AE) ++ + 0 - +PE active

- Phased array- Lamb wave- SMART Layer

--+

++++

-+++

---

+++

Optical FBG + - - + +Strain gauge 0 - - - -Accelerometer ++ - - - -Temp. sensor - - - ++ -Heat flux sensor - - - + -Leak detection - - - + -

38

SHM application

Slow transition from laboratory to field application

• Optimised sensor layout (small-scale damage to be detected in large-scale structures)

• Self-diagnostics (damage to the structure or damage to the sensor itself)

• Environmental compensation

• Probability of detection

• Damage quantification

• Robustness and redundancy of the SHM system(Beard and Farrar, 2007)

39

Near-term and far-term application of SHM

Near-term application

• Retrofitting existing air vehicles with SHM systems to detect well-defined damage in known ‘hot spots’

Far-term application

• Additional maturation of current SHM technologies that will be ‘designed-in’ to the structure.

• Result: continuous, autonomous, real-time, in-service monitoring of the entire vehicle structure

(Derriso, AFRL, WP AFB, 2007)

40

AFRL SHM Vision

Sensors for:- strain- temperature- corrosion

41

Structural health monitoring (SHM)

Kumar, Acellent Technologies

42

End of presentation

Questions?