Presented by: Vishaal Bhana Supervisor: Prof. Stephan Heyns

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ONLINE DAMAGE DETECTION ON SHAFTS

USING TORSIONAL AND UNDERSAMPLING

MEASUREMENT TECHNIQUES

Presented by: Vishaal Bhana

Supervisor: Prof. Stephan Heyns

PUBLIC DEFENCE OF MASTERS DEGREE

DATE: 13TH FEBRUARY 2013

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• Turbogenerator shafts needs to function

continuously

• Shafts subjected to complex torsional

loadings

• Often leads to failure due to propagation of

cracks

• Catastrophic and dangerous

• Failure results in downtime

• High repair costs

Introduction

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• High costs for repair

• Negative effect on industry and public

• Literature has shown various failure cases

• Continuous online monitoring is essential

Blackouts

Downtime for repair

High strain on remaining generators

Potential failures

Failures High repair

cost

Cyclic loading,

cracknitiation

High repair cost

Cyclic loading,

crack initiation

Introduction

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Introduction

• Constantly monitor the system

• Changes in the signal are further investigated

• Problem is isolated

• Machine stopped and repaired/replaced if necessary

• Pre-plan maintenance programs/ cost effective

Online monitoring

system

Vibration signature normal?

Machine operating constantly

No

Yes

Maintenance done

Pre-plan maintenance

schedule

Investigate irregularity

• Lateral and torsional vibration have been successful

• More focus on torsional methods recently

• Introduction of fibre-optic sensors and torsional laser vibrometers and

non-contact measurement methods

• Considerable research by Maynard and M. Trethewey with fibre-optic

sensors

• Found damage in shafts and blades

• DIC methods used for strain visualisation by taking discrete images

• Impact tests done by Lall

• Vibration and mode shape measurements by Helfrick

• Strain gauges ideal for measuring high frequency content

• Yam investigated strain and displacement modes

• Various modelling methods implemented (Analytical, Timoshenko

beams, commercial software)

• Different methods used for vibration and specifically torsional vibration

but no comparison

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Literature study

Introduction

• Turbogenerator shafts needs to function continuously

• Shafts subjected to complex torsional loadings

• Often leads to failure due to propagation of cracks

• Cracks develop in shafts due to fatigue


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Investigate the torsional dynamics of a shaft for

undamaged and damaged shaft

Comparison study of different torsional measurement

techniques

Investigation of undersampling for damage detection

Numerical analysis, FEM-model built through use of Patran and

the Nastran solver, Normal modes analysis done

Verify using lumped spring-mass system

Conduct an experimental analysis on a rotor with increasing

damage

Measurements done using Telemetry strain system, fibre optic

sensor, Digital Image Correlation (DIC) system

Objectives

Introduction


• Effects on the public


• Offline monitoring results in downtime

• Pre-plan maintenance programs

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System setup

• Two disks fixed onto a shaft

• Coupled to a 3kW DC motor

• Self-aligning bearings

• Free-end on the other side

Introduction






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FEM

• Study of the dynamics of the rotor investigating

the natural frequencies

• Sensitivity analysis with damage

• Key parameters

Geometry- Basic structure that would resemble

manufactured setup

Mesh- Ease of modelling shaft and damage

Boundary conditions- representation of motor

connection and bearings

Material Properties

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FEM- Model Setup

• Modelled as a single unit

• Surface Quad 8 elements that is extruded

• Control over meshing geometry and modelling the damage (non

equivalenced nodes

• Fixed motor end using MPC

• Node BC applied at bearing location (x and y direction)

• Mild steel properties used

Introduction






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Torsional mode Lumped Patran % Difference

1 136.04 133.02 2.27

2 420.17 408.95 2.74

Lumped model

FEM- Convergence and verification

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FEM- Analysis

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FEM- Mode Shape

• 1st mode shows high rotational response at free-end

• Strain comparison showed higher strain response at the motor-end

• Location of test equipment determined

Introduction






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Experimental setup

• Rotor manufactured consisting of a shaft with two discs

• Damage introduced by cutting a slot in shaft

• Rigid coupling to prevent torsional damping

• Overall setup with telemetry, Fibre-optic sensor and DIC system

• Brake setup for calibration

Fibre-optic sensor

Telemetry

Introduction






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Techniques

• Fibre-optic sensor Black-White coded tape (Square

wave output)

Changes in widths represent

torsional vibration

• Telemetry Analog strain data

Gauges mounted 180o apart

• DIC 2 cameras create 3D representation

White markers are picked up and

their position analysed

Strobe lights needed for dynamic

testing

Order content

• Harmonics of rotational frequency present

• Causes natural frequency to be hidden

• Constant time Constant angle sampling

• FFT to order domain

• Remove orders

• Order removed frequency domain

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Processing

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Undersampling

• Nyquist theorem , if not then aliasing occurs

• DIC strobe lights operate at 15 Hz

max2sf f

sin(2 10 ) sin(2 80 )y t t

Processing

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• Fan-paper method

• Fold to base-band frequency

• Frequency of interest band-limited

• Fold causes reverse readings on odd folds

• Base-band 4.5Hz

Processing cont’d

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• Application of Fan-paper method

Processing

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• 3 test runs conducted

• Runup test, 2 constant speed tests- 1100Rpm 1640Rpm

• 2 min constant speed runs 12 averages taken

• Frequency resolution 0.1Hz

• Recording of all 3 devices simultaneously

• Undamaged and damaged case- 5%-66% damage introduced

• Measure time domain and post-processing in frequency

domain

• Order removal done

• Investigation of undersampling methods

• Waterfall plot for runup test

• View results in frequency domain

Test procedure

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• Waterfall plot shows structural response at B

• Averaged 2D plot, Response at 120.6Hz

A B

Post Processing-Fibre-optic sensor

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• Waterfall and averaged 2D plot- Nothing distinct even

after order removal

• Line frequency harmonics

Post Processing- Telemetry

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• Capture markers at undersampled 9Hz

• Continuous marker capture. Create “components”

• Images form 3D representation of setup with displacement

and location information

Post Processing- DIC

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• Some images contain no markers- “Blind side”

• Create continuous time signal

• Undersampled FFT- rotational frequency content

• Expected frequency 3.6Hz

Post Processing- DIC

• Geometry- Include brake, coupling and damage modelling

• Material properties- Apply manual updating by adjusting

parameters within its range

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FEM Update

• Fibre-optic sensor revealed change in natural frequencies with

damage

• Comparison showed that fibre-optic method most ideal

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Final Results

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Final Results- FEM Comparison

• Close comparison between FEM and experimental results

• Change in frequency ratio shows good correlation

• Error range from 0-3.2%

• Undersampled fibre-optic sensor

• Revealed changes

• Pre-knowledge required for location

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Damage % Actual frequency Base-band equivalent Undersampled data

0 120.6 3.6 3.59

5 120.6 3.6 3.55

8.3 120.5 3.5 3.48

19.3 120.1 3.1 3.12

31.6 119.1 2.1 2.13

41.6 118.1 1.1 0.98

53.3 115.7 1.3 1.36

66.6 109.1 1.6 1.13

Final Results- Undersampling

• The fibre-optic sensor proved to be the successful torsional

measuring equipment for online monitoring and damage

detection.

• The strain gauge provided order content but nothing about

dynamics

• Further investigations showed that high excitation was required

• The DIC also revealed order content which was successfully

obtained through undersampling techniques

• Undersampling was successfully used with the fibre-optic sensor

results to obtain changes in the dynamics for a damaged shaft.

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Conclusions

• Strain modal analysis may be investigated by applying external excitation or

loading on the system to excite the modes of interest

• By creating a setup with a low enough dynamic frequency and having a high

enough strobe light available, one can investigate systems without undersampling

techniques.

• The DIC system in this study only looked at the markers while they were in camera

view. Methods by which the markers may be studied during the full rotation and

thereby giving a better full scale representation of what happens during each

revolution. The implementation of such a setup together with the external loading

for higher strain could prove useful.

• It would be interesting to apply them on a system with an actual crack. Future

analysis could be done by growing a crack onto the shaft.

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Recommendations

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Thank you

Acknowledgements

• Prof. P.S. Heyns- Supervisor

• Sasol Labs and DSG at U.P

• George Breytenbach and Herman Booysen- U.P

• Gerrit Visser and Hennie Klopper- Esteq

• SANHARP

Presented by: Vishaal Bhana Supervisor: Prof. Stephan Heyns

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