Structural Health Monitoring - Engineering · Structural health monitoring (SHM) A structural Health Monitoring System (HMS) can be defined as a tool to continuously observe the …

Structural Health Monitoring

Ajit Mal and Sauvik BanerjeeMechanical & Aerospace Engineering Department

University of California, Los Angeles

Fabrizio RicciDipartimento di Progettazione Aeronautica

University of Naples Federico II – Italy

Frank ShihMechanical Engineering Department,

Seattle University

Structural health monitoring (SHM)

A structural Health Monitoring System (HMS) can be defined as a tool to continuously observe the degradation of Aircraft, Aerospace, Mechanical and Civil structures in service, with minimum manual intervention

The system shouldevaluate changes in critical structural parameters from baseline

assess structural integrity

recommend maintenance strategy

An autonomous SHM system

Motivation

Hidden flaws caused by aging, service loads or manufacturing processes, if left undetected, can lead to catastrophic failure of a structure.Conventional inspections/maintenance on regular basis are costly and often unnecessary. On-board autonomous health monitoring systems integrated into the design will increase the safety and reduce the maintenance cost significantly

Major features of the proposed SHM system

Analysis of data recorded by a network of distributed sensors in critical areas of structure.

Low frequency narrowband sensors to record modal response

High frequency broadband sensors to record motion due to wave propagation

Analysis of recorded data using a damage index approach

The procedure can be automated requiring minimum operator intervention

Effects of damage on the modal response of a beam

1 2 3 4 5 6 7 8

Excitation (input)

Control Points (output)

Damage LocationsA B

δ(t)

Aluminum beam

Frequency response function (FRF) as velocity square at control point #6 on the beam produced by load, δ(t). Damage location A

Damage was simulated by progressively reducing the area moment of inertia to 15 % in steps of 5 % in one element of the beam, which constitutes 2% of its entire volume.

The simulated flaw appears to have very small effects on the modal response of beam. It would be difficult if not impossible to use the modal properties directly to identify damage in the beam.

Effects of damage on the modal response of a beam (cont.)

( ){ } { }

{ } { } 02

02

22

,D

== ∗

∗=

DLi

T

DLi

DLi

T

DLiDLi

VV

VV( )

{ } { }{ } { }

02

02

22

,,D

== ∗

∗=

DLj

T

DLi

DLj

T

DLiDLji

VV

VV

DL is the damage level (0 - 3) and is the velocity-squared response vector (700 elements consisting f = 0 – 14 kHz at steps of 20 Hz) at node # i at damage level DL.

{ }DLiV 2

Damage index 1:

Damage index 2:

0.940

0.960

0.980

1.000

1.020

1.040

1.060

1.080

1.100

1.120

Dam

age

Inde

x 2

CP#4_1 CP#4_2 CP#4_3 CP#4_4 CP#4_5 CP#4_6 CP#4_7 CP#4_8

Control Point

DL = 0 (No Damage)DL = 1DL = 2DL = 3

Damage location A. Damage index 2 showing correlation of CP #4 with others

0.900

0.950

1.000

1.050

1.100

1.150

Dam

age

Inde

x 1

CP#1 CP#2 CP#3 CP#4 CP#5 CP#6 CP#7 CP#8

Control point

DL = 0 (No Damage)DL = 1DL = 2DL = 3

Damage location B. Damage index 1

Damage indices increase with the level of damage, and more importantly, the increase is pronounced at control points closer to the damage location.

Effects of damage on the modal response of a plate

Fixed ended unidirectional graphite/epoxy composite plate

200

200

75 75

75

75

25

25

37.5

37.5

2525

5.125.12 ×

SourceControl pointsDamaged area

10 % reduction of Ex , Ey- Dimensions are in mm

1 2 3

4 5 6

7 8 9

1

Fiber direction

200

200

75 75

75

75

25

25

37.5

37.5

2525

5.125.12 ×

SourceControl pointsDamaged area

10 % reduction of Ex , Ey- Dimensions are in mm

1 2 3

4 5 6

7 8 9

1

Fiber direction

A. B.

200

200

75 75

75

75

25

25

37.537.5

12.512.5

5.375.37 ×

SourceControl pointsDamaged area

25 % reduction of Ex , Ey- Dimensions are in mm

1 23

4 56

7 8 9

1

Fiber direction

200

200

75 75

75

75

25

25

37.537.5

12.512.5

5.375.37 ×

SourceControl pointsDamaged area

25 % reduction of Ex , Ey- Dimensions are in mm

1 23

4 56

7 8 9

11

Fiber directionFiber direction

A. Simulated damage over a small area B. Simulated damage over a large area

FRFs

Point 2 (left) and

Point 5 (right)

Effects of damage on the modal response of a plate (cont.) The damage index approach

Damage index

( ) { } { }{ } { } 00

,*

*1

==

−=DLi

TDLi

DLiTDLi

DLiRR

RRD

A. Damage index for small damage B. Damage index for large damage

Damage indices increase with level of damage

Indices are high at control points closer to the damage Major damage within the structure can easily be identified from thehigh values of the indices

Damage identification using wave propagation approach

( ){ } { }{ } { } impactprei

Timpactprei

impactpostiT

impactpostii

FF

FFD

−−

−−−=*

*11

A network of PZT transducers (sources and receivers) are located on the surface of the plate.The elastic waves generated by the source are acquired by receivers, pre-processed in an ultrasonic date acquisition system and stored in the computer for analysis.

Impact test is performed using an instrumented drop weight test frame Instron/Dynatup 8250.

After the plate has been impacted, wave propagation tests are repeated using the same transducer configuration as in the pre-impact tests.

Pre-impact wave propagation test

Impact test

Post-impact wave propagation test

Evaluation of damage index

Fi = response vector

The wave propagation and impact experiments

Waveform generator

Any one of the transducers can be used as a source to send specific signal using waveform generator

Data acquisition system for the ultrasonic wave propagation test Schematics of the Dynatup 8250

for impact test

Damage identification in a composite plateAcoustic emission (AE) waves from low velocity impact

External appearance (61 lb)14 lb (no damage)

-100 0 100 200 300 400 500-0.40

-0.20

0.00

0.20

0.40A

Am

plitu

de

Time (µs)

-100 0 100 200 300 400 500-0.30

-0.15

0.00

0.15

0.30A

Am

plitu

de (V

)

Time (µs)

-100 0 100 200 300 400 500-0.40

-0.20

0.00

0.20

0.40A

Am

plitu

de (V

)

Time (µs)-100 0 100 200 300 400 500

-0.40

-0.20

0.00

0.20

0.40A

Am

plitu

deTime (µs)

Theory

61 lb (delamination)

Ultrasonic C-scan (61 lb)

Theory

Waveforms recorded on [0/90]8s cross-ply graphite epoxy composite plates. Impactor was dropped

from a height of 225 mm.

Damage identification in a composite plate (cont.)Wavelet transforms of AE waves

No damage Damage

Damage identification in a composite plate (cont.)Typical recorded waveforms

Source4.2 mm thick [0/90]8s graphite/epoxy plate

30 mm 30 mm

30 m

m30

mm

15 m

m

15 mm

dela

min

atio

nD

amag

e ex

tens

ion

Source

Receiver

1 2 3

54 6

7 8 9

Ultrasonic C-scan of the damaged plate showing the hidden defects. Sources,

receivers and damaged area

Recorded signals at receiver #6

Damage identification in a composite plate (cont.)The damage index approach

Damage index at the control pointsFrequency spectra of the recorded signals at #6

Delamination modifies the elastic waves propagating between the source and the receivers.

The influence is pronounced at points 3 and 6, near damage – and can belocalized successfully

Damage identification in a composite plate (contd.)24

2

SourceReceiverImpact Point

1 2 3 4 5

6 7 8 9 10

596

Signal frequency content (receiver)

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

0.E+00 2.E+05 4.E+05 6.E+05 8.E+05 1.E+06 1.E+06 1.E+06 2.E+06 2.E+06 2.E+06

Frequency [Hz]

Am

plitu

de [V

]

Pre impact

Post impact

Signal time history (receiver)

-6.00E-03

-4.00E-03

-2.00E-03

0.00E+00

2.00E-03

4.00E-03

6.00E-03

8.00E-03

0.00E+00 1.00E-05 2.00E-05 3.00E-05 4.00E-05 5.00E-05 6.00E-05 7.00E-05

Time [s]

Am

plitu

de [V

]

Pre impact

Post impact

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

D1

1 2 3 4 5 6 7 8 9 10

Point (receiver)

Typical recorded signal and its frequency spectrum; damage indices

Damage identification in a composite plate (cont.)

100 mm

1 2

34

80 m

m

20 m

m

35 m

m25 mm

40 mm

3 mm dia. hole

7 m

m d

ia. h

ole

Damage index set Si; i is the source location.

Sets S1, S3 and S4 show the highest index at the control point 2, which is closer to the 7 mm dia. hole.

For set S2, the damage index is highest at control point 4, since the hole falls in the path of the waves from 2 to 4.

Some insight about the presence of the smaller hole can be obtained when indices at locations 3 and 4 are considered from set S4 and S3, respectively.

Onset of damage within a region can be predicted with some confidence.

Any one of the transducers is used as a source and the others receive the signals.

S4S2 S3S1

Concluding remarks

The approach outlined here can be used for the characterization of materials degradation and the development of health monitoring systems for aircraft, aerospace and other advanced structures.

For complex structures under realistic service conditions, the vibrationaldata are expected to provide information on the existence and the general location of major defects only (e.g., widespread damage).

The wave based approach yields more detailed information on the location and nature of small hidden defects.

The computer assisted automatic analysis of data should improve the reliability and practical applicability of the detection system to defects-critical structures.