BMFB 4283 NDT & FAILURE ANALYSIS

1. BMFB 4283NDT & FAILURE ANALYSIS Lectures for Week 3 Prof. QumrulAhsan, PhD Department of Engineering Materials Faculty of Manufacturing Engineering

2. Issues to address 3.0 Magnetic Particle Testing 3.1 Introduction 3.2 Theory 3.3 Techniques and Equipment 3.4 Inspection and Application

3. MAGNETIC PARTICLE TESTING - Outline • Magnetism and Ferromagnetic Materials • Introduction of Magnetic Particle Inspection • Basic Procedure and Important Considerations • Magnetizing methods and apparatus • The detecting medium • Examples of MPI Indications • Reporting Indications

4. Introduction • This module is intended to present information on the widely used method of magnetic particle inspection. • Magnetic particle inspection can detect both production discontinuities (seams, laps, grinding cracks and quenching cracks) and in-service damage (fatigue and overload cracks).

5. Magnetic Particle Testing MT MT Surface Defect Subsurface Internal Ferromagnetic Material • Test method for the detection of surface and slightly sub-surface indications in ferromagnetic materials CANNOT BE DETECTED BY Magnetic Particle Testing

6. Introduction to Magnetism Magnetism is the ability of matter to attract other matter to itself. Objects that possess the property of magnetism are said to be magnetic or magnetized and magnetic lines of force can be found in and around the objects. A magnetic pole is a point where the a magnetic line of force exits or enters a material. • Magnetic field lines: • Form complete loops. • Do not cross. • Follow the path of least resistance. • All have the same strength. • Have a direction such that they cause poles to attract or repel. Magnetic lines of force around a bar magnet Opposite poles attracting Similar poles repelling

7. Domain Theory UNMAGNETISED STATE DOMAINS RANDOMLY ORIENTATED MAGNETIZED STATE. DOMAINS ORIENTATED IN EXTERNAL MAGNETIC FIELD FIELD SATURATED STATE DOMAINS ORIENTATED IN STRONG EXTERNAL FIELD FIELD

8. Domain Theory RESIDUAL STATE. DOMAIN REMAINING ORIENTATED DEMAGNETISED STATE. DOMAINS RANDOMLY ORIENTATED IN OPPOSING CURVE FIELD

9. How Does Magnetic Particle Inspection Work? A ferromagnetic test specimen is magnetized with a strong magnetic field created by a magnet or special equipment.If the specimen has a discontinuity, the discontinuity will interrupt the magnetic field flowing through the specimen and a leakage field will occur.

10. How Does Magnetic Particle Inspection Work? (Cont.) Finely milled iron particles coated with a dye pigment are applied to the test specimen. These particles are attracted to leakage fields and will cluster to form an indication directly over the discontinuity. This indication can be visually detected under proper lighting conditions.

11. Flux Leakage Ring Magnet Ring Magnet Principle of MT : Flux Leakage Ferromagnetic Particles N S Attracted at poles Magnetic field is Fully contained: No Poles Flux Leakage occurs: Poles created

12. Principle of MPI : Flux Leakage No Defect Defect Flux Leakage N S N S The change in permeability causes flux leakage

13. N S Principle of MPI : Flux Leakage STEEL µ= 1000 No Flux Leakage because No change in permeability

14. N S Principle of MPI : Flux Leakage Flux Leakage AIR µ= 1 STEEL µ= 1000 The change in permeability causes flux leakage

15. Factors Affecting Flux Leakage • Depth of defect • Orientation of defect shape of defect • Size of defect • Permeability of material • Amount of flux available

16. Factors Affecting Flux Leakage Depth below surface N S N S • Depth of defect • Orientation of defect shape of defect • Size of defect • Permeability of material • Amount of flux available

17. Basic Procedure Basic steps involved: • Component pre-cleaning • Introduction of magnetic field • Application of magnetic media • Interpretation of magnetic particle indications

18. Pre-cleaning When inspecting a test part with the magnetic particle method it is essential for the particles to have an unimpeded path for migration to both strong and weak leakage fields alike. The part’s surface should be clean and dry before inspection. Contaminants such as oil, grease, or scale may not only prevent particles from being attracted to leakage fields, they may also interfere with interpretation of indications.

19. Introduction of the Magnetic Field The required magnetic field can be introduced into a component in a number of different ways. • Using a permanent magnet or an electromagnet that contacts the test piece • Flowing an electrical current through the specimen • Flowing an electrical current through a coil of wire around the part or through a central conductor running near the part.

20. Direction of the Magnetic Field Two general types of magnetic fields (longitudinal and circular) may be established within the specimen. The type of magnetic field established is determined by the method used to magnetize the specimen. • A longitudinal magnetic field has magnetic lines of force that run parallel to the long axis of the part. • External solenoidal coil • With yoke • A circular magnetic field has magnetic lines of force that run circumferentially around the perimeter of a part. • Head Shot • Central conductor • Prod

21. Flux Leakage No Flux Leakage Importance of Magnetic Field Direction • Being able to magnetize the part in two directions is important because the best detection of defects occurs when the lines of magnetic force are established at right angles to the longest dimension of the defect. • This orientation creates the largest disruption of the magnetic field within the part and the greatest flux leakage at the surface of the part. • An orientation of 45 to 90 degrees between the magnetic field and the defect is necessary to form an indication. • Since defects may occur in various and unknown directions, each part is normally magnetized in two directions at right angles to each other.

22. Defect Orientation Defect at 90 degrees to flux : maximum indication

23. Defect Orientation >60 Degrees to Flux: Acceptable indication

24. Defect Orientation <60 Degrees to Flux : Weak indication

25. Longitudinal (along the axis) Transverse (perpendicular the axis) Question ? From the previous slide regarding the optimum test sensitivity, which kinds of defect are easily found in the images below?

26. Induction methods • Threaded bar • Hollow object must have access both ends • Conductor carrying current is threaded through bore passed current through it. • Produced circular field • Flexible cables • Used for a variety component shape • Place flexible cable on or around specimen • Current passed through coil induce magnetic field

27. Producing a Longitudinal Magnetic Field Using a Coil A longitudinal magnetic field is usually established by placing the part near the inside or a coil’s annulus. This produces magnetic lines of force that are parallel to the long axis of the test part. Coil on Wet Horizontal Inspection Unit Portable Coil

28. Magnetizing methods – Induction methods • Methods: • Encircling coils • Placing specimen inside coils • Low voltage, high amperage current is passed • Creates longitudinal magnetic field • Current values: • NI = K/[L/D] • N = number of turns in the coils • I = current in amperes • L = the component length • D = the component diameter • L/D = ratio of geometrical information of component • K = source constant ( K= 32 for DC, 22 for AC, 11 for mean value)

29. Flexible cable technique • Advantages: • AC or DC field • Large areas can be inspected • No poles to attract magnetic particles • Filed strength can be altered • Predictable field strength • Disadvantages: • Cumbersome long heavy cable required • Longer setting time • Heavy transformer required for large amperage

30. Producing a Longitudinal Field Using Permanent or Electromagnetic Magnets Permanent magnets and electro-magnetic yokes are also often used to produce a longitudinal magnetic field. The magnetic lines of force run from one pole to the other, and the poles are positioned such that any flaws present run normal to these lines of force. • Yokes: • Highly permeable, low retentive steel • Laminated to reduce induction and prevent the yoke from permanently magnetized

31. Electromagnetic yokes • Magnetism: • Encircling yoke with coil through which current is passed • Strength of field is varied by: • Adjusting the current (amperage) flowing through the yoke • Varying the distance between the pole pieces • The field produced is longitudinal • Depth of field depends on type of current • Require source of electrical energy (AC or DC) • Surface discontinuities using AC • Sub-surface defect using DC

32. Electromagnetic yokes • Advantages: • AC or rectified DC operation • Controllable field strength • Can be switched on/off as required • No damage done to test piece • Relatively lightweight • Disadvantages: • Requires power supply • Only small area can be examined • Leaves only one hand free

33. Magnetizing methods - Permanent magnet • Permanent magnet: • able to maintain a magnetic field in surrounding space • Field strength can vary considerably, depends on flux density in magnet and shape • Magnetic bar: • A piece of ferromagnetic material with a magnetic pole at each end • Placed into a closed loop: • create magnetic field within closed circuit and no external field would exist • If defect present in the loop, flux leakage occur • Provide magnetic flow in the specimen and produce longitudinal magnetic field between poles

34. Permanent magnets • Advantages: • No power supply required • Inexpensive • No damage to the test piece from arcing • Relatively lightweight (portable) • Disadvantages: • Deterioration with wear • Have to be pulled from the test surface • Magnetic particles attracted to poles • Limited application on awkward shapes

35. Magnetic Field Electric Current Circular Magnetic Fields Circular magnetic fields are produced by passing current through the part or by placing the part in a strong circular magnet field. A headshot on a wet horizontal test unit and the use of prods are several common methods of injecting current in a part to produce a circular magnetic field. Placing parts on a central conductors carrying high current is another way to produce the field.

36. Magnetizing methods – Current flow • Produce circular magnetic field by passing current through test piece • Prod technique • Current is introduced using electrical contacts (prod) • Prod induce circular magnetic field within specimen using current values • Correct positioning is essential to ensure all possible defects are detected • Ideally the prods should be in line parallel to or on the same axis as the defect

37. Current flow - Prods • Advantages: • AC or DC fields • Low voltage output • No poles to attract magnetic particles • Variable field strength • Can be used in confined spaces • Disadvantages: • Risk of creating arc strikes • Heavy transformer required • Contacts and small test items can be overheated • Careful positioning and spacing of prods are required

38. Longitudinal vs. Circular Magnetic Field

39. Types of Magnetization Current • DC, AC, HWDC, 1FWDC, 3FWDC • Type of current used depends primarily on the depth of the defect from the surface, not the crack size • AC provides a highly concentrating field at the parts surface ( detecting surface and very near surface defects) • Alternating magnetic field increases particle mobility and particles attract to leakage field(dry powder with HWDC) • A time varying magnetic field induces eddy current within the material. Eddy currents cause the magnetic field to decay exponentially . • The depth  is the skin depth, where the magnetic field is 37% of its maximum value. = 1/πµf • DC penetrates deeper, provides both moderate surfaces and subsurface sensitivity.

40. Magnetic Particles • Fine magnetic particles that create an indication at a leakage field caused by a flaw in the magnetized sample • Depending on its characteristics and application • Type of particle (dry or wet) • Viewing method (color contrast or fluorescent) • Method of application (continuous or residual)

41. Requirement of magnetic particles • Fine grains to reduce the gravitational effect. The maximum size (BS 4069): 200m for powder and 100 m for inks • Particles are chemically treated iron oxide particles, small in size and varying in shape (spherical to irregular  sensitive to flaw) • Elongated shape for easier polarization. Spherical particles are also needed to ensure dispersal over the surface • High magnetic permeability and low retentivity (avoid to become permanent magnet) • High permeability for magnetization in weak flux leakage fields • Low retentivity if particles are to be removed after the test • High contrast against the background of the test surface

42. Dry Magnetic Particles • Magnetic particles come in a variety of colors. A color that produces a high level of contrast against the background should be used. • May be black, grey, red, orange, fluorescent • Usually applied to a surface by means of puffer device: • They should be floated, not blasted on to the • area under test • the particles are lightly dusted on to the • surface • Should be ideally be used with ac or dc current • Because of extra mobility of the current • impart onto powder • Must be used when MPI is being carried out on • hot and rough surfaces • Inks are not suitable • The dry method is more portable

43. Wet Magnetic Particles Wet particles are typically supplied as visible or fluorescent. Visible particles are viewed under normal white light and fluorescent particles are viewed under black light.

44. Wet Magnetic Particles • According to BS 4069, the composition of inks shall be: • Ferromagnetic particles in non-fluorescent inks should not be less than 1.25% and not more than 3.5% by volume • Ferromagnetic particles in fluorescent inks should not be less than 0.1% and not more than 0.3% by volume • Carrier fluid may be oil based: paraffin or water to make up volume by 100% • If water is used, additives shall be added to prevent corrosion on the surface or the particles and improve the wetting action With the wet method, the part is flooded with a solution carrying the particles. The wet method is generally more sensitive since the liquid carrier gives the magnetic particles additional mobility.

45. Viewing conditions • Non-fluorescent inks and powders: • Area under inspection must be evenly illuminated • Min illumination level of 500 lux (daylight/artificial light) • Fluorescent inks and powders: • Min UV-A irradiance level: 800 W/cm2, min background intensity 10 lux • Degrade with exposure to ordinary light over a period of time and high temperature • UV intensity increases, the amount of fluorescent increases too

46. UV-A light • Generated by mercury vapor lamps • Mercury is vaporized inside a quartz capsule by small low current arc from auxiliary electrode • After 5 minutes, there is sufficient mercury vapor in the capsule to initiate between main electrode • The lamp should be used after 15 minutes to allow sufficient time to attain full working intensity

47. Safety • UV light operate with wavelengths between 320 – 400 nm • Shorter wavelength cause injury to the eyes • Filter must be used which cut wavelength under 320 nm to prevent injury • Looking into UV-A cause temporary clouding vision •  fluid in the eyeball fluorescing •  will normalize with no permanent effects after few seconds • Prolonged exposure may cause cataract

48. Health • Flammability: • Read container labels for flash points. Materials cannot be used under flash points • Asthmatic: • Do not use in confined space without masks or adequate ventilation • Skin hazard: • Use protective clothing

49. Methods of Application • Continuous method : apply particles to all surfaces of the part while the part is being magnetized • Rather easy for dry powder • For wet powder parts are magnetised several time with short duration (0.5 sec) • Residual Method: apply the particle after magnetizing has been removed • Residual force must be large • Suitable for wet part and defects under plating or coating • less sensitive

50. Relative Penetration Sensitivity • Current Type vs. Wet or Dry Method