SURFACE NDE METHODS
- 1. RAMCO INSTITUTE OF TECHNOLOGY Mr.M.LAKSHMANAN Assistant Professor (Senior Grade) Department of Mechanical Engineering
- 2. UNIT II SURFACE NDE METHODS
- 3. Syllabus Liquid Penetrant Testing - Principles, types and properties of liquid penetrants, developers, advantages and limitations of various methods, Testing Procedure, Interpretation of results. Magnetic Particle Testing- Theory of magnetism, inspection materials Magnetisation methods, Interpretation and evaluation of test indications, Principles and methods of demagnetization, Residual magnetism.
- 4. Liquid Penetrant Testing • Dye penetrant inspection (DPI) • Liquid penetrant inspection (LPI) • Penetrant testing (PT) The oil and whiting method used in the railroad industry in the early 1900s was the first recognized use of the principles of penetrants to detect cracks. By the 1940s, fluorescent or visible dye was added to the oil used to penetrate test objects.
- 5. Standards • ISO 3059, Non-destructive testing - Penetration testing • ISO 3452-1, Non-destructive testing. Penetrant testing. Part 1. General principles • ISO 3452-2, Non-destructive testing - Penetrant testing - Part 2: Testing of penetrant materials • ISO 3452-3, Non-destructive testing - Penetrant testing - Part 3: Reference test blocks • ISO 3452-4, Non-destructive testing - Penetrant testing - Part 4: Equipment • ISO 3452-5, Non-destructive testing - Penetrant testing - Part 5: Penetrant testing at temperatures higher than 50 °C • ISO 3452-6, Non-destructive testing - Penetrant testing - Part 6: Penetrant testing at temperatures lower than 10 °C • ISO 10893-4: Non-destructive testing of steel tubes. Liquid penetrant inspection of seamless and welded steel tubes for the detection of surface imperfections.
- 6. ASTM International (ASTM): • ASTM E 165, Standard Practice for Liquid Penetrant Examination for General Industry • ASTM E 1417, Standard Practice for Liquid Penetrant Testing American Society of Mechanical Engineers (ASME): • ASME Boiler and Pressure Vessel Code, Section V, Art. 6, Liquid Penetrant Examination • ASME Boiler and Pressure Vessel Code, Section V, Art. 24 Standard Test Method for Liquid Penetrant Examination SE-165 (identical with ASTM E-165)
- 7. Principle DPI is based upon capillary action, where low surface tension fluid penetrates into clean and dry surface- breaking discontinuities. Penetrant may be applied to the test component by dipping, spraying, or brushing. After adequate penetration time has been allowed, the excess penetrant is removed and a developer is applied. The developer helps to draw penetrant out of the flaw so that an invisible indication becomes visible to the inspector. Inspection is performed under ultraviolet or white light, depending on the type of dye used - fluorescent or nonfluorescent (visible).
- 8. Inspection Steps Pre-cleaning Application of penetrant Excess penetrant Removal Application of Developer Inspection Post Cleaning
- 10. 1. Pre-cleaning: • The test surface is cleaned to remove any dirt, paint, oil, grease or any loose scale that could either keep penetrant out of a defect, or cause irrelevant or false indications. • The end goal of this step is a clean surface where any defects present are open to the surface, dry, and free of contamination.
- 11. 2. Application of penetrant: The penetrant is then applied to the surface of the item being tested. The penetrant is allowed "dwell time" to soak into any flaws (generally 5 to 30 minutes). The dwell time mainly depends upon the penetrant being used, material being tested and the size of flaws sought. As expected, smaller flaws require a longer penetration time.
- 12. DWELL TIME Penetrant dwell time is the total time that the penetrant is in contact with the part surface. The dwell time is important because it allows the penetrant the time necessary to seep or be drawn into a defect. Dwell times are usually recommended by the penetrant producers or required by the specification being followed.
- 13. 3. Excess Penetrant Removal: • The excess penetrant is then removed from the surface. The removal method is controlled by the type of penetrant used. • Water-washable, solvent-removable, lipophilic post- emulsifiable, or hydrophilic post-emulsifiable are the common choices. • Emulsifiers represent the highest sensitivity level, and chemically interact with the oily penetrant to make it removable with a water spray. • When using solvent remover and lint-free cloth it is important to not spray the solvent on the test surface directly, because this can remove the penetrant from the flaws. • The removal of excessive penetrant is done towards one direction either vertically or horizontally.
- 14. 4. Application of Developer: • After excess penetrant has been removed, a white developer is applied to the sample. • Several developer types are available, including: non-aqueous wet developer, dry powder, water- suspendable, and water-soluble. Choice of developer is governed by penetrant compatibility (one can't use water- soluble or -suspendable developer with water-washable penetrant), and by inspection conditions. • Developer should form a semi-transparent, even coating on the surface. • The developer draws penetrant from defects out onto the surface to form a visible indication, commonly known as bleed- out. Any areas that bleed out can indicate the location, orientation and possible types of defects on the surface.
- 15. 5. Inspection: The inspector will use visible light with adequate intensity (100 foot-candles ) for visible dye penetrant. Ultraviolet (UV-A) radiation of adequate intensity (1,000 micro-watts per centimeter squared is common), along with low ambient light levels (less than 2 foot-candles) for fluorescent penetrant examinations. Inspection of the test surface should take place after 10- to 30-minute development time, depends of product kind. This time delay allows the blotting action to occur.
- 16. 6. Post Cleaning: The test surface is often cleaned after inspection and recording of defects.
- 17. Common Uses of Liquid Penetrant Inspection: 1. Liquid penetrant inspection (LPI) is one of the most widely used nondestructive evaluation (NDE) methods. 2. Its popularity can be attributed to two main factors: its relative ease of use and its flexibility. 3. LPI can be used to inspect almost any material provided that its surface is not extremely rough or porous. 4.Materials that are commonly inspected using LPI include the following: • Metals (aluminum, copper, steel, titanium, etc.) Glass • Many ceramic materials • Rubber • Plastics
- 18. Advantages: • The method has high sensitivity to small surface discontinuities. • The method has few material limitations, i.e. metallic and nonmetallic, magnetic and Non magnetic, and conductive and nonconductive materials may be inspected. • Rapid inspection of large areas and volumes • Suitable for parts with complex shapes • Indications are produced directly on the surface of the part and constitute a visual representation of the flaw. • Portable (materials are available in aerosol spray cans) • Low cost (materials and associated equipment are relatively inexpensive)
- 19. Disadvantages: • Only surface breaking defects can be detected. • Precleaning is critical since contaminants can mask defects. • Metal smearing from machining and grinding must be removed prior to LPI. • The inspector must have direct access to the surface being inspected. • Surface finish and roughness can affect inspection sensitivity. • Multiple process operations must be performed and controlled. • Post cleaning of material is required. • Chemical handling and proper disposal is required.
- 20. Penetrant A colored or fluorescent liquid used to penetrate cracks, pores, and other surface defects to facilitate their detection. Penetrants are carefully formulated to produce the level of sensitivity desired by the inspector. Penetrant Materials: Penetrant Type I Fluorescent II Visible Method A Water Washable B Post emulsifiable - Lipophilic C Solvent Removable D Post emulsifiable – Hydrophilic
- 21. Characteristics of Penetrant Materials Spread easily over the surface being inspected. Be drawn into surface breaking defects by capillary action or other mechanisms. Remain in the defect but remove easily from the surface of the part. Be highly visible or fluoresce brightly to produce easy to see indications. Not be harmful to the inspector or to the material being tested.
- 22. Sensitivity Levels • Level 4 - Ultra-High Sensitivity • Level 3 - High Sensitivity • Level 2 - Medium Sensitivity • Level 1 - Low Sensitivity The properties of penetrant materials that are controlled by flash point, surface wetting capability, viscosity, color, brightness, ultraviolet stability, thermal stability, water tolerance, and removability.
- 23. Penetrant Types • Fluorescent Penetrant: It contain a dye or several dyes that fluoresce when exposed to ultraviolet radiation. • Visible Penetrant: It contain a red dye that provides high contrast against the white developer background.
- 24. Penetrant Types (Methods) • Method A - Water Washable: Penetrants can be removed from the part by rinsing with water alone. These penetrants contain an emulsifying agent (detergent) that makes it possible to wash the penetrant from the part surface with water alone. Water washable penetrants are sometimes referred to as self-emulsifying systems. • Method B - Post-Emulsifiable -Lipophilic: The penetrant is oil soluble and interacts with the oil-based emulsifier to make removal possible.
- 25. • Method C - Solvent Removable: It require the use of a solvent to remove the penetrant from the part. • Method D - Post-Emulsifiable-Hydrophilic: It use an emulsifier that is a water soluble detergent which lifts the excess penetrant from the surface of the part with a water wash.
- 26. Emulsifiable concentrate formulations are a blend of active ingredient, organic solvent, and surfactants. When the solution is diluted into water, a spontaneous milky emulsion forms with dispersed phase droplets in the size range of 1 to 25 μm.
- 27. Developers The role of the developer is to pull the trapped penetrant material out of defects and spread it out on the surface of the part so it can be seen by an inspector. Developers used with visible penetrants create a white background so there is a greater degree of contrast between the indication and the surrounding background.
- 28. Classification of Developers • Form a - Dry Powder • Form b - Water Soluble • Form c - Water Suspendable • Form d - Nonaqueous Type 1: Fluorescent (Solvent Based) • Form e - Nonaqueous Type 2: Visible Dye (Solvent Based) • Form f - Special Applications
- 29. Dry Powder: Dry powder developers are generally considered to be the least sensitive but they are inexpensive to use and easy to apply. Dry developers are white, fluffy powders that can be applied to a thoroughly dry surface in a number of ways; By dipping parts in a container of developer, By using a puffer to dust parts with the developer, or placing parts in a dust cabinet where the developer is blown around.
- 30. Water Soluble: • As the name implies, water soluble developers consist of a group of chemicals that are dissolved in water and form a developer layer when the water is evaporated away. • The best method for applying water soluble developers is by spraying it on the part. The part can be wet or dry. • Dipping, pouring, or brushing the solution on to the surface is sometimes used but these methods are less desirable. • Drying is achieved by placing the wet, but well drained part, in a recirculating warm air dryer with a temperature of 21°C. • Properly developed parts will have an even, light white coating over the entire surface.
- 31. Water Suspendable: • Water suspendable developers consist of insoluble developer particles suspended in water. • Water suspendable developers require frequent stirring or agitation to keep the particles from settling out of suspension. • Water suspendable developers are applied to parts in the same manner as water soluble developers then the parts are dried using warm air.
- 32. Nonaqueous: • Nonaqueous developers suspend the developer in a volatile solvent and are typically applied with a spray gun. • Nonaqueous developers are commonly distributed in aerosol spray cans for portability. • The solvent tends to pull penetrant from the indications by solvent action. • Since the solvent is highly volatile, forced drying is not required.
- 33. Special Applications: Plastic or lacquer developers are special developers that are primarily used when a permanent record of the inspection is required.
- 34. Penetrant Removal Process The penetrant removal procedure must effectively remove the penetrant from the surface of the part without removing an appreciable amount of entrapped penetrant from the discontinuity. Removal Method: • Method A: Water-Washable • Method B: Post-Emulsifiable - Lipophilic • Method C: Solvent Removable • Method D: Post-Emulsifiable - Hydrophilic
- 35. The relative sensitivities of developers and application techniques as ranked in Volume II of the Nondestructive Testing Handbook Ranking Developer Form Method of Application 1 Nonaqueous, Wet Solvent Spray 2 Plastic Film Spray 3 Water-Soluble Spray 4 Water-Suspendable Spray 5 Water-Soluble Immersion 6 Water-Suspendable Immersion 7 Dry Dust Cloud (Electrostatic) 8 Dry Fluidized Bed 9 Dry Dust Cloud (Air Agitation) 10 Dry Immersion (Dip)
- 36. Developer Advantages Disadvantages Dry Indications tend to remain brighter and more distinct over time Easy to apply Does not form contrast background so cannot be used with visible systems Difficult to assure entire part surface has been coated Soluble Ease of coating entire part Coating is translucent and provides poor contrast (not recommended for visable systems) Indications for water washable systems are dim and blurred Suspendable Ease of coating entire part Indications are bright and sharp White coating of good contrast can be produced which work well for both visible and fluorescent systems Indications weaken and become diffused after time
- 37. Nonaqueous • Very portable • Easy to apply to readily accessible surfaces • White coating of good contrast can be produced which work well for both visible and fluorescent systems • Indications show-up rapidly and are well defined • Provides highest sensitivity Difficult to apply evenly to all surfaces More difficult to clean part after inspection
- 38. Lighting Quality Control Lighting for Visible Dye Penetrant Inspections: • When using a visible penetrant, the intensity of the white light is of principal importance. Inspections can be conducted using natural lighting or artificial lighting. However, since natural daylight changes from time to time, the use of artificial lighting is recommended to get better uniformity. • Artificial lighting should be white whenever possible (halogen lamps are most commonly used). The light intensity is required to be 100 foot-candles (1076 lux) at the surface being inspected.
- 39. Lighting for Fluorescent Penetrant Inspections: • Fluorescent penetrant dyes are excited by UV light of 365nm wavelength and emit visible light somewhere in the green-yellow range between 520 and 580nm. • The lamps emit many wavelengths and a filter is used to remove all but the UV and a small amount of visible light between 310 and 410nm.
- 41. Introduction • This method uses magnetic fields and small magnetic particles (i.e.iron filings) to detect flaws in components. • Magnetic particle inspection can detect both production discontinuities (seams, laps, grinding cracks and quenching cracks) and in-service damage (fatigue and overload cracks). • Many different industries use magnetic particle inspection such as structural steel, automotive, petrochemical, power generation, and aerospace industries.
- 42. 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.
- 43. 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
- 44. Ferromagnetic Materials • A material is considered ferromagnetic if it can be magnetized. Materials with a significant Iron, nickel or cobalt content are generally ferromagnetic. • Ferromagnetic materials are made up of many regions in which the magnetic fields of atoms are aligned. These regions are call magnetic domains. • Magnetic domains point randomly in demagnetized material, but can be aligned using electrical current or an external magnetic field to magnetize the material. Demagnetized Magnetized
- 45. Magnetic Particle Inspection (MPI) 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.
- 46. 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.
- 47. Basic Procedure Basic steps involved: 1. Component pre-cleaning 2. Introduction of magnetic field 3. Application of magnetic media 4. Interpretation of magnetic particle indications
- 48. 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.
- 49. Introduction of the Magnetic Field The required magnetic field can be introduced into a component in a number of different ways. 1. Using a permanent magnet or an electromagnet that contacts the test piece 2. Flowing an electrical current through the specimen 3. Flowing an electrical current through a coil of wire around the part or through a central conductor running near the part.
- 50. 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. A circular magnetic field has magnetic lines of force that run circumferentially around the perimeter of a part.
- 51. 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.
- 52. Flux Leakage No Flux Leakage
- 53. Question • From the previous slide regarding the optimum test sensitivity, which kinds of defect are easily found in the images below? Longitudinal (along the axis) Transverse (perpendicular the axis)
- 54. Magnetization of Ferromagnetic Materials • Magnetization Using Direct Induction (Direct Magnetization) With direct magnetization, current is passed directly through the component. The flow of current causes a circular magnetic field to form in and around the conductor. When using the direct magnetization method, care must be taken to ensure that good electrical contact is established and maintained between the test equipment and the test component to avoid damage of the the component (due to arcing or overheating at high resistance points).
- 55. • One way involves clamping the component between two electrical contacts in a special piece of equipment. • Current is passed through the component and a circular magnetic field is established in and around the component. • When the magnetizing current is stopped, a residual magnetic field will remain within the component. • The strength of the induced magnetic field is proportional to the amount of current passed through the component.
- 56. • The “direct” technique is defined as one in which the magnetizing current flows through the part, thereby completing the electrical circuit. This is accomplished by placing the part between the “heads” of a stationary unit.
- 57. • A second technique involves using clamps or prods, which are attached or placed in contact with the component. Electrical current flows through the component from contact to contact. The current sets up a circular magnetic field around the path of the current.
- 58. Magnetization Using Indirect Induction (Indirect Magnetization) The “indirect” or magnetic flow technique is defined as one in which the test part becomes part of the magnetic circuit by bridging the path between the poles of a permanent magnet or electromagnet. Indirect magnetization is accomplished by using a strong external magnetic field to establish a magnetic field within the component. As with direct magnetization, there are several ways that indirect magnetization can be accomplished.
- 59. • The use of permanent magnets is a low cost method of establishing a magnetic field. However, their use is limited due to lack of control of the field strength and the difficulty of placing and removing strong permanent magnets from the component.
- 60. • Electromagnets in the form of an adjustable horseshoe magnet (called a yoke) eliminate the problems associated with permanent magnets and are used extensively in industry. Electromagnets only exhibit a magnetic flux when electric current is flowing around the soft iron core. When the magnet is placed on the component, a magnetic field is established between the north and south poles of the magnet.
- 61. • Another way of indirectly inducting a magnetic field in a material is by using the magnetic field of a current carrying conductor. • A circular magnetic field can be established in cylindrical components by using a central conductor. • Typically, one or more cylindrical components are hung from a solid copper bar running through the inside diameter. • Current is passed through the copper bar and the resulting circular magnetic field establishes a magnetic field within the test components.
- 62. • The use of coils and solenoids is a third method of indirect magnetization. • When the length of a component is several times larger than its diameter, a longitudinal magnetic field can be established in the component. • The component is placed longitudinally in the concentrated magnetic field that fills the center of a coil or solenoid. • This magnetization technique is often referred to as a "coil shot".
- 63. Demagnetization The reduction of the magnetic moment in an object; that is the reverse of magnetization. Its commonly encountered as a procedure for eliminating the inadvertent magnetization of iron (Ferromagnetic) parts of a sensitive mechanical device that would otherwise result in a malfunction. A suitably intense magnetic field applied in a direction opposite to that of the existing magnetization will serve to reduce or destroy that magnetization.
- 64. Removal of a field may be accomplished in several ways. The most effective way to demagnetize a material is by heating the material above its curie temperature (for instance, the curie temperature for a low carbon steel is 770°C). When steel is heated above its curie temperature then it is cooled back down, the orientation of the magnetic domains of the individual grains will become randomized again and thus the component will contain no residual magnetic field. The material should also be placed with its long axis in an east-west orientation to avoid any influence of the Earth's magnetic field.
- 65. • A field meter is often used to verify that the residual flux has been removed from a component. • Industry standards usually require that the magnetic flux be reduced to less than 3 Gauss (3x10-4 Tesla) after completing a magnetic particle inspection.
- 66. Measuring Magnetic Fields • When performing a magnetic particle inspection, it is very important to be able to determine the direction and intensity of the magnetic field. • The field intensity must be high enough to cause an indication to form, but not too high to cause nonrelevant indications to mask relevant indications. • Also, after magnetic inspection it is often needed to measure the level of residual magnetism.
- 67. • Since it is impractical to measure the actual field strength within the material, all the devices measure the magnetic field that is outside of the material. • The two devices commonly used for quantitative measurement of magnetic fields n magnetic particle inspection are the field indicator and the Hall-effect meter, which is also called a Gauss meter.
- 68. Field Indicators • Field indicators are small mechanical devices that utilize a soft iron vane that is deflected by a magnetic field. • The vane is attached to a needle that rotates and moves the pointer on the scale. • Field indicators can be adjusted and calibrated so that quantitative information can be obtained. • However, the measurement range of field indicators is usually small due to the mechanics of the device (range from plus 20 to minus 20 Gauss). • This limited range makes them best suited for measuring the residual magnetic field after demagnetization.
- 69. Hall-Effect (Gauss/Tesla) Meter • A Hall-effect meter is an electronic device that provides a digital readout of the magnetic field strength in Gauss or Tesla units. • The meter uses a very small conductor or semiconductor element at the tip of the probe. Electric current is passed through the conductor. • In a magnetic field, a force is exerted on the moving electrons which tends to push them to one side of the conductor.
- 70. • A buildup of charge at the sides of the conductors will balance this magnetic influence, producing a measurable voltage between the two sides of the conductor. • The probe is placed in the magnetic field such that the magnetic lines of force intersect the major dimensions of the sensing element at a right angle.
- 71. Hall Effect The Hall effect is the production of a voltage difference (the Hall voltage) across an electrical conductor, transverse to an electric current in the conductor and a magnetic field perpendicular to the current. It was discovered by Edwin Hall in 1879.
- 72. Magnetization Equipment for Magnetic Particle Testing • Portable Equipments Permanent Magnets Electromagnetic Yokes Prods Portable Coils and Conductive Cables Portable Power Supplies • Stationery Equipment
- 73. Permanent Magnets Permanent magnets can be used for magnetic particle inspection as the source of magnetism (bar magnets or horseshoe magnets). The use of industrial magnets is not popular because they are very strong (they require significant strength to remove them from the surface, about 250 N for some magnets) and thus they are difficult and sometimes dangerous to handle.
- 74. However, permanent magnets are sometimes used by divers for inspection in underwater environments or other areas, such as explosive environments, where electromagnets cannot be used. Permanent magnets can also be made small enough to fit into tight areas where electromagnets might not fit.
- 75. Electromagnetic Yokes • An electromagnetic yoke is a very common piece of equipment that is used to establish a magnetic field. • A switch is included in the electrical circuit so that the current and, therefore, the magnetic field can be turned on and off. • They can be powered with AC from a wall socket or by DC from a battery pack. • This type of magnet generates a very strong magnetic field in a local area where the poles of the magnet touch the part being inspected. • Some yokes can lift weights in excess of 40 pounds.
- 76. Prods • Prods are handheld electrodes that are pressed against the surface of the component being inspected to make contact for passing electrical current (AC or DC) through the metal. • Prods are typically made from copper and have an insulated handle to help protect the operator. • One of the prods has a trigger switch so that the current can be quickly and easily turned on and off.
- 77. • Sometimes the two prods are connected by any insulator, as shown in the image, to facilitate one hand operation. • This is referred to as a dual prod and is commonly used for weld inspections.