Characterizing the Performance of Lamb Wave Based SHM ......Transducers Total focusing ray paths (beamforming) Potential defects (hole, delamination, corrosion, cracks etc.) SHM-POD

5th European-American Workshop on Reliability of NDE – Lecture 28

1 License: http://creativecommons.org/licenses/by/3.0/

Characterizing the Performance of Lamb Wave Based SHM Systems – A Two-Step Approach Based on Simulation-Supported

POD and Reliability Aspects

Frank SCHUBERT *, Bernd FRANKENSTEIN *, Mike RÖLLIG *, Lars SCHUBERT *, Georg LAUTENSCHLÄGER *

* Fraunhofer Institute for Nondestructive Testing, Dresden Branch, IZFP-D, Dresden, Germany

Abstract

The performance of conventional NDE systems with one or a few sensors can be adequately described by classical POD concepts. In the case of Lamb wave based structural health monitoring (SHM) the situation is more complex. In SHM a larger number of permanently installed sensors arranged in a network configuration are often used. For this problem the conventional POD concept needs to be adapted since POD becomes a function of position or more general, a function of the specific network configuration. Moreover, the long-term behavior of sensors and electronics plays an important role since their degradation directly affects the POD of the SHM system over time. In the present paper a two-step approach to characterize the performance of Lamb wave based SHM systems based on POD and reliability aspects is presented. In a first step we introduce a new POD concept for Lamb wave based sensor networks where the effective network aperture is used as a virtual scanning device. Therefore the conventional POD methods already known from classical UT can be directly transferred to SHM. The only exception is the location-dependent POD determined by the specific network configuration. For this purpose a simulation platform is presented that allows the calculation of location-dependent POD values and additionally offers the possibility to optimize the network configuration if the statistical distribution of flaws is empirically known. In a second step we address the age-dependent performance of sensors and electronics by presenting results of an extensive experimental and simulation-supported reliability study on CFRP integrated sensor systems. Environmental tests have been done to investigate the reliability of ultrasonic transducers and electronic components in the longterm. The tested sensor package concept has been specifically designed with respect to functionality (high efficiency of Lamb wave coupling) and reliability of hardware components. We discuss how these aspects can be incorporated in the classical POD framework. As a final result for the description of SHM system reliability we propose a time-dependent or age-related POD, i.e. a POD time curve, rather than a single POD value.

http://creativecommons.org/licenses/by/3.0/

DRESDEN

Characterizing the performance of Lamb wave based SHM systems – A two-step approach based on simulation-supported POD and reliability aspects

F. Schubert, M. Röllig, L. Schubert, B. Frankenstein

Fraunhofer Institute for Nondestructive TestingDresden Branch (IZFP-D)Dresden, Germany

Motivation

An emerging trend in modern structure design is the combination of structures and sensors for monitoring purposes (SHM)

One possible SHM approach is based on guided ultrasonic waves (Lamb waves)

Sensor nodes typically consist of small piezoelectric transducers and sensor-near electronics for signal processing, power supply and – if possible –wireless communication

In the past the sensor nodes were directly attached to the surface of the structure (e.g. by gluing) and electrically connected with cables. This might be sufficient for laboratory measurements but will not be accepted in real aircraft components

2

Motivation

Current development targets:

To increase the TRL of SHM systems by improved integration concepts

To develop an embedded system to be used in the CFRP fuselage of the Airbus A350

To ensure

a high efficiency of ultrasonic wave excitation and detection

an autoclave process integration

a long lasting field operation (improved reliability)

Laboratory system

Reliable structure integrated field system

Layer stacking

CFRP

GFRP

Sensor / Electronic

Air Evacuation

Vacuum bag

Air

Lamination

Challenge 1: Lamination process integration

0 200 400 600 800 1.0000

100

200

300

400

500

Am

plitu

de [

Oh

m]

Frequency(KHz)

Initial state GFRP-Integration CFRP-Integration

3

Take offground Cruise flight Landing

# Factor Value Range

3 Temperaturecycles

-55/ +85°C

4 Max. strain(uniaxial)

0,3%

0,2% - cyclic

5 Vibration 0,2g²/Hz

6 Moisture til 90% r.H.

7 Mechan. shock 6-10g at 90Hz

Challenge 2: Loading Conditions During Flight Material Degradation over Time (Structure and SHM system!)

Different flight stages Environmental loadings during flight

The reliability of conventional NDE systems with one or a few moving sensors can be adequately described by classical POD concepts (e.g. Berens, POD as a function of flaw size).

In the case of Lamb wave based structural health monitoring (SHM) the situation is more complex.

In SHM a larger number of permanently installed sensors arranged in a network configuration are often used. For this problem the conventional POD concept needs to be extended since POD becomes a function of position or more general, a function of the specific network configuration.

Moreover, the long-term behavior of structure, sensors and electronics plays an important role since their degradation directly affects the POD of the SHM system over time

Challenge 3: How to Develop an Appropriate POD Approach for embedded SHM Systems ?

4

Referencestate

Actualstate

SHM-POD as a Function of Position:Total Focusing or Synthetic Aperture Concept

Actuator1

Actuator2

Actuator3

Actuator4

. . .


5

Symmetric in a linear system!

Pulse-echodata

Pitch-catchdata


Total focusing ray paths (beamforming)Transducers

Potential defects(hole, delamination,corrosion, cracks etc.)


SHM sensor network in total focusing mode can be considered as a specialcase of an ultrasonic scanning device!

However, the POD becomes a function of position (of the focal point relative to the sensor network)

6


POD Landscapes: Local height represents the maximum amplitude that can beobtained by the total focusing method.

(Needs to be corrected by the flaw orientation for non-axisymmetric defects).

Mechanical Tests

Temperature Cycle Tests

Humidity Exposure Tests

85

-55

25

Tem

pe

ratu

re[°

C]

Time [min]0 6030

SHM-POD as a Function of Life Time:Quantification by Fatigue & Environmental Tests

7

0 125 250 500 1000 1690 2000 30000

1

2

3

4

5

6

7

Ca

pa

city

[n

F]

Cycles0 200 400 600 800 1000

-90

-88

-86

-84

-82

-80

-78

-76

-74

-72

-70

Pha

se [

°]

Frequency [kHz]

Initial state C4P8 1000 temperature cycles 2000 temperature cycles 3000 temperature cycles

0 200 400 600 800 1000-90

-88

-86

-84

-82

-80

-78

-76

-74

-72

-70

Pha

se [

°]

Frequency [kHz]

Initial state C3P7 1000h humidity exposure 2000h humidity exposure 3000h humidity exposure 4000h humidity exposure

0 168 504 1000150020002500300040000

1

2

3

4

5

6

7

8

C

apac

ity [

nF]

Cycles [h]


Temperature cycle testof piezo sensor(-55°C to 85°C)

Damp/Heat testof piezo sensor(85°C at 85% rel. H.)


Temperature cycle testof electronics (DC1 & 2) (-55°C to 85°C)

Damp/Heat testof electronics (DC1 & 2)(85°C at 85% rel. H.)

012

525

050

010

0016

9020

0030

0040

00

0,0

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0,2

0,3

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1,0

Re

sist

an

ce D

C1

[Oh

m]

Cycles

012

525

050

010

0016

9020

0030

0040

00

0,0

0,1

0,2

0,3

0,4

0,5

0,6

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0,8

0,9

1,0

Re

sist

an

ce D

C2

[Oh

m]

Cycles

016

850

410

0015

0020

0025

0030

0040

0050

00

0,0

0,1

0,2

0,3

0,4

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0,9

1,0

Res

ista

nce

DC

1 [

Oh

m]

Cycles [h]

016

850

410

0015

0020

0025

0030

0040

0050

00

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

Res

ista

nce

DC

2 [

Oh

m]

Cycles [h]

8

100 1.000 10.000 100.000 1.000.000 1E70,05

0,1

0,2

0,3

0,4

0,5

DC1 DC2S

trai

n [%

]

Cycles [-]


to failure in dynamic tensile test

Fatigue Life Evaluation byS-N (Wöhler) curves

Microscopic failure analysis

Test Requirement Test conditions Load in hours

RTCA DO‐160Fsuccessfully

compl.

High‐temp.test 110°C,24h / 180°C,3,5h 180 ± 5°C 24

Low‐temp.test ‐55°C, 24h ‐70 ± 1°C 24

Damp/Heat 55°C,95%r.H., approx. 120h 85°C / 85%r.H. 4000

TCT ‐55 ‐ 85°C 10K/min. 2Cyc ‐55/85°C (1Cyc.=1h) 3000

Cycles

successfullycompl.

stat. Pulltest Ɛ=1% Ok

dyn. Pulltest Ɛ=0,15‐0,3% * Ɛ=0,08% 5.000.000Ɛ=0,12% 198.000

Ɛ=0,15% 212.000

Ɛ=0,2% 44.000

Ɛ=0,3% 2.000


Equip SHM system withadditional temperature,humidity & strain sensors

Determine fatigue lifeand loss of performanceof SHM system

Correct POD frominitial state value

9

Summary The initial POD of an embedded SHM system can be determined

after the lamination process

A Lamb wave sensor network in TFM mode can be considered as a special case of a mechanical ultrasound scanner. Thus, the conventional Berens approach is applicable in general.

Since POD becomes a function of position the Berens approach needs to be corrected by the concept of POD landscapes

Since POD also becomes a function of time the results of fatigue, environmental and functionality tests together with online sensor data of temperature, humidity and strain can be used to determine the current POD of the SHM system (< initial POD)

Break-Out-Session on Reliability of SHM

10

Documents

Characterizing the Performance of Lamb Wave Based SHM ......Transducers Total focusing ray paths (beamforming) Potential defects (hole, delamination, corrosion, cracks etc.) SHM-POD