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Condition-based maintenance: Non-destructive testing for the CBM course

Classnotes

Prepared for

Investmech (Pty) Ltd

Copyright ©2018, University of Pretoria.

All rights reserved. The copyright in any original material contained in this work vests in the

owner/author/publisher. Permission to reproduce any additional material has been obtained.

DIRECTORS:

M Heyns Pr.Eng., Ph.D., (Managing)

CJ Botha B.Eng(Hons): Industrial

Document No:

Revision:

Date:

IM-TR000

0.0

January 2018

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Table of Contents

1. SUMMARY ...................................................................................................................................... 3 2. Liquid dye penetrant testing (PT) .................................................................................................... 3

2.1. Remote fluorescent penetrant replica testing of internal surfaces ......................................... 3 3. Magnetic particle inspection (MT) ................................................................................................... 4 4. Ultrasonic testing (UT) .................................................................................................................... 5 5. Acoustic emission ........................................................................................................................... 5 6. Potential crack depth probes .......................................................................................................... 6 7. Coupon testing ................................................................................................................................ 6 8. Radiography .................................................................................................................................... 6 9. Boroscopes ..................................................................................................................................... 8 10. Fiberscope .................................................................................................................................. 8 11. Shaft and silo inspections ........................................................................................................... 8 12. Concrete crack monitors............................................................................................................. 9 13. Eddy current testing (ECT) ......................................................................................................... 9 14. Magnetostrictive techniques ....................................................................................................... 9 15. Air of Water pressure testing ...................................................................................................... 9 16. Replica testing .......................................................................................................................... 10 17. Telltale & Sentinel holes ........................................................................................................... 10

17.1. Telltale holes ........................................................................................................................ 10 17.2. Sentinel holes ....................................................................................................................... 12

18. Etch inspection ......................................................................................................................... 12 19. Other inspections ...................................................................................................................... 12 20. Welds and NDT ........................................................................................................................ 13 21. Hardness testing ....................................................................................................................... 13 22. Conclusion ................................................................................................................................ 13

List of Tables Table 1: Some non-destructive testing techniques ................................................................................ 3

List of Figures Figure 1: Remote fluorescent penetrant replica testing of internal surfaces .......................................... 4 Figure 2: Magnetic particle testing ......................................................................................................... 4 Figure 3: Ultrasonic testing ..................................................................................................................... 5 Figure 4: Potential crack depth measurement........................................................................................ 6 Figure 5: Radiography results ................................................................................................................ 7 Figure 6: Comparing Neutron diffraction and X-ray radiography ........................................................... 7 Figure 7: Image of the Investmech silo/shaft inspection rig MKI ............................................................ 8 Figure 8: Concrete crack monitors ......................................................................................................... 9 Figure 9: Replica testing of etched surface .......................................................................................... 10 Figure 10: Example of a telltale hole .................................................................................................... 11 Figure 11: Telltale holes indicated on drawings ................................................................................... 11 Figure 12: Sentinel holes ...................................................................................................................... 12

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1. SUMMARY

Some techniques in non-destructive testing is summarised in Table 1.

Table 1: Some non-destructive testing techniques

Acoustic emission Liquid (dye) penetrant

Remote fluorescent penetrant replica testing of internal surfaces

Electrostatic fluorescent penetrant

Magnetic particle Strippable magnetic film

Ultrasonic techniques:

1. Pulse-echo

2. Transmission

3. Resonance

4. Frequency modulation

Coupon

Air or water pressure testing Etch inspection

Eddy current X-ray radiography

X-ray radiography fluoroscopy Rigid boroscope

Cold light probes Deep-probe endoscope

Pan-view fiberscopes Electron fractography

Strain gauging Crack gauging

2. Liquid dye penetrant testing (PT)

In summary:

• Surface discontinuities or cracks due to fatigue, wear, surface shrinkage, grinding, heat-treatment, corrosion fatigue, corrosion stress, hydrogen embrittlement, etc.

• Ferrous and non-ferrous materials

• Electrostatic Fluorescent penetrant

– Opposing electrostatic polarity induced between workpiece and testing materials

– Ensures more complete and even deposition of penetrant

– Greater sensitivity

Please study:

1. https://www.youtube.com/watch?v=xEK-c1pkTUI

2. http://www.callingtonhaven.com/_assets/non_destructive_testing/manual_visible_dye_penetrant.pdf

2.1. Remote fluorescent penetrant replica testing of internal surfaces

In summary:

• Internal section cleaned and exposed to fluorescent penetrant fluid

• After exposure time, surplus penetrant liquid drained off

• Area washed and dried

• Cracks/flaws are filled with penetrant fluid at this time

• Special type of plastic pressed against internal surface by remote operation

• Penetrant fluid in cracks/flaws migrates onto plastic

• Remove & examine under ultra-violet light

• Crack clearly visible on plastic material

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Source: http://www.force.dk/NR/rdonlyres/38E6C555-334E-4D4B-9020-61CBF28AE8A0/387/25591en.pdf

Figure 1: Remote fluorescent penetrant replica testing of internal surfaces

3. Magnetic particle inspection (MT)

In summary:

• Surface and near-surface cracks & discontinuities caused by fatigue, wear, laminations, inclusions, surface shrinkage, grinding, heat treatment, hydrogen embrittlement, laps, seams, corrosion fatigue, stress corrosion cracking, corrosion stress, etc.

• Work only on Ferromagnetic materials

• Strippable magnetic film can be generated by pouring self-curing silicon rubber containing fine iron oxide particles on the surface

Source: http://www.nationalboard.org/images/site_img/large/mpe4.jpg

Figure 2: Magnetic particle testing

Please watch:

1. https://www.youtube.com/watch?v=qpgcD5k1494

2. https://www.youtube.com/watch?v=AEBOrri2cB0

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4. Ultrasonic testing (UT)

The following techniques are used:

1. Pulse-echo

2. Transmission

3. Resonance

4. Frequency modulation

Please watch:

1. https://www.youtube.com/watch?v=1MERDVlRtDo

2. https://www.youtube.com/watch?v=nN24Gy0ZcI0

Source: https://en.wikipedia.org/wiki/Ultrasonic_testing

Figure 3: Ultrasonic testing

5. Acoustic emission

In summary:

• Acoustic emission (AE):

– Radiation of elastic waves in solids

– Occurs when material undergoes irreversible changes in its internal structure

• Cracking

• Plastic deformation due to aging, temperature gradient, external mechanical forces

• Short impulsive stress waves are generated, indicating the presence of active flaws

• Measurement of acoustic emission (AE) activity is valuable NDT method to detect and locate faults at an early stage in mechanical structures, pressure vessels, pipes, welds, etc

Reading/watching:

1. http://www.tms.org/pubs/journals/jom/9811/huang/huang-9811.html

2. https://www.nde-ed.org/EducationResources/CommunityCollege/Other%20Methods/AE/AE_DateDisplay.php

3. http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.491.2897&rep=rep1&type=pdf

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6. Potential crack depth probes

In summary:

1. A technique used after the detection of a crack (by magnetic testing, penetrant testing, etc.)

2. Used to measure the depth of the surface cracks

3. Measures potential drop between two probes with alternating current

Source: http://www.karldeutsch.de/PDF/Prospekte/P%204015%20E%2004_07.pdf

Figure 4: Potential crack depth measurement

Read:

1. http://www.karldeutsch.de/PDF/Prospekte/P%204015%20E%2004_07.pdf

7. Coupon testing

ASTM G4 is a standard used for the corrosion coupon field testing of materials. The coupon is made of the same material as the pipe wall. It is then installed in the stream for fixed period of time. Accurate measurement of weight and dimensional changes from before to after the test are then used to determine the effects of wear, corrosion, etc.

8. Radiography

In summary:

• Conditions monitored

o Surface and sub-surface discontinuities caused by fatigue, stress, inclusions, lack of penetration in welds, gas porosity, intergranular corrosion, stress corrosion

o Semi-conductor discontinuities such as loose wires, etc

• X-Ray radiography

• X-ray radiographic fluoroscopy

X- or gamma rays are passed through optically opaque materials. Darkest where object is thinnest. Crack, inclusion or void observed as dark patch as shown in Figure 5.

Neutron diffraction radiography, offered by the Safari reactor, can offer more variation in the image as shown in Figure 6.

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Source: http://www.sandia.gov/materials/science/capabilities/failure-analysis.html

Figure 5: Radiography results

Figure 6: Comparing Neutron diffraction and X-ray radiography

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9. Boroscopes

Boroscopes can be rigid or flexible, depending on requirements. In summary:

1. It is an instrument used where direct line-of-sight observation not feasible or possible

2. Optical device consisting of rigid or flexible tube with eyepiece on one end, objective lens on other end linked by a relay optical system in between

3. Optical system surrounded by optical fibres for illumination

4. Protected by rigid or flexible outer sheath

Boroscopes can be very long, up to 21 m, and longer. With imaging technology today, a camera can be positioned at the one end and Investmech inspects silos and mine shafts of depths of 100ths of m.

10. Fiberscope

In summary:

1. Flexible fiber optic bundle with eyepiece at one end, lens at other

2. Similar to boroscope, except it can be very long & flexible

a. Quartz fiberscopes reach length of 100 m

3. To examine small components in tightly packed equipment

4. Used in:

a. Medicine, machining, computer repair, espionage, locksmithing, safecracking, etc.

Further information:

1. http://www.sasrad.com/products/ultimate_fiberscope/ultimate_fiberscopes.php

11. Shaft and silo inspections

In summary:

• For inside:

– Cameras positioned on a frame with sufficient lighting

– Lowered in the silo/shaft/etc

– With sufficient lighting, further zooming is possible

• For outside:

– Drone with mounted camera

Figure 7: Image of the Investmech silo/shaft inspection rig MKI

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12. Concrete crack monitors

Instruments mounted over a crack to monitor movement over the crack. For more information on these devices read:

– https://www.humboldtmfg.com/concrete-crack-monitor-crack-gauge.html

– http://www.globalgilson.com/concrete-crack-monitors

Source:

http://www.globalgilson.com/scratch-a-

track-motion-monitor

Source: https://www.humboldtmfg.com/standard-

crack-monitor.html

Figure 8: Concrete crack monitors

13. Eddy current testing (ECT)

In summary:

1. Make use electromagnetic induction

a. Coil excited with alternating electrical current

b. Magnetic field oscillates at same frequency as alternating current

c. In proximity of conductive materials, currents opposed to the ones in coil are induced in the material – called eddy currents

d. Electrical conductivity, magnetic permeability of test object & presence of defects cause change in eddy current, and, corresponding change in phase & amplitude

e. Detected by measurement of impedance changes in the coil

2. Detects surface and sub-surface flaws

3. Only limited to conductive materials

Further information:

1. http://www-pub.iaea.org/MTCD/publications/PDF/TCS-48_web.pdf

14. Magnetostrictive techniques

Magnetostrictive sensors measure displacement. They are small and because of their size used to measure gap changes in cracks in concrete bridges, beams, etc.

15. Air of Water pressure testing

In summary:

• Also called leak testing

• Done with gasses or liquids

• Voids through which gasses or liquids escape from component = gross (large) or fine leaks

• Extremely small gas leaks measured in PPM (parts per million)

– Require Mass Spectrometer to sniff for tracer gasses

• Soap water forms soap bells – easy to detect and is recognised test in piping industry

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16. Replica testing

In summary:

• Replica (Image of material structure)

– First, careful cleaning (grinding)

– Polish (~ 1 cm2) surface to be smooth as mirror

– Etch surface by acid solution

– Mount label of plastic material softened by spirit/chemical

• Plastic strip absorb exact “image” of material structure

– Take to lab and analyze in electron microscope

• Can also be used to remove and analyze particles on tape on rough surface containing particles

Source: http://www.safecontrol.eu/replicatesting.html

Figure 9: Replica testing of etched surface

17. Telltale & Sentinel holes

17.1. Telltale holes

In summary:

1. Telltale holes in tank reinforcing pads are used to detect leaks.

2. According to ASME B31.3, Paragraph 328.5.4:

a. “A vent hole shall be provided at the side (not the crotch) of any pad or saddle to reveal leakage in the weld between branch and run and to allow venting during welding and heat treatment.”

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Figure 10: Example of a telltale hole

Source: ASME B31.3

Figure 11: Telltale holes indicated on drawings

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17.2. Sentinel holes

Sentinel holes are drilled to a controlled depth at various locations on pipework. When the internal surface corrodes, the hole starts leaking after certain remaining thickness is reached. Fracture Mechanics & Fatigue analysis is done to determine the minimum required thickness.

Source: http://www.ndt.net/article/v04n02/tij4n3/fig1.gif

Figure 12: Sentinel holes

18. Etch inspection

In summary:

• To evaluate effects of poorly controlled secondary manufacturing processes on metallic components

– Machining, grinding, heat treatment

• Applicable to from steels to copper alloys

• Use dilute acidic solutions

– 2 to 4% Nitric Acid in Alcohol (Ethanol or Methanol)

– Examine etched surface visually

– Untempered martensite due to overheating during improper machining visible

– Also known as “Temper Etch”

19. Other inspections

In summary, but, you can add to the list:

• Thickness measurement

• Impulse response technique

– Transfer function calculation – acceleration

– Vibration type analysis

• Strain and crack gauges

• Many more like insulation testing on electrical wires in electric motors, etc

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20. Welds and NDT

Standard used: AWS D1.1M

Common weld defects:

• Porosity

• Undercutting

• Rollover or ‘Cold Lap’

• Slag inclusion

• Poor fusion

• Poor penetration in partial penetration joints

• Voids

• Hydrogen embrittlement

Common methods used are:

• Visual inspection (VT)

o Most common method

o Reveals:

▪ Spatter, excessive build-up, incomplete slag removal, heat distortion, cracks, undercutting, poor penetration (although not always)

o Tools used:

▪ Fillet gauges

▪ Magnifying glasses

▪ Flashlights

▪ Tape measures

▪ Callipers

• Magnetic particle inspection (MT)

o For cracks and flaws located at or near the surface

o Can only be used on material that can be magnetized

• Liquid (dye) penetrant inspection (PT)

o Only shown surface flaws

• X-ray inspection (RT)

o Used for quality inspection purposes

o Can reveal flaws deep in the component

• Ultrasonic testing (UT)

o Flaws can be detected inside the part

• Air or Water pressure testing (LT)

o For through cracks and leaks

• Eddy current testing (ECT)

o Can be used on electrically conductive material

21. Hardness testing

See the following websites:

1. https://www.rohloff.co.za/

2. https://www.rohloff.co.za/images/Hardness_testing/sonodur2.pdf

22. Conclusion

Non-destructive testing can be made part of a condition-based maintenance program by:

1. Trending the parameter measured

2. Define the limits of the parameter using fracture mechanics

3. Compile a fracture control programme

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a. Monitor the parameter

b. Act according to the results.