THERMOGRAPHY AND EDDY CURRENT TESTING (ET)
- 1. RAMCO INSTITUTE OF TECHNOLOGY Mr.M.LAKSHMANAN Assistant Professor (Senior Grade) Department of Mechanical Engineering
- 2. UNIT III THERMOGRAPHYAND EDDY CURRENT TESTING (ET)
- 3. Syllabus Thermography- Principles, Contact and non contact inspection methods, Techniques for applying liquid crystals, Advantages and limitation - infrared radiation and infrared detectors, Instrumentations and methods, applications. Eddy Current Testing- Generation of eddy currents, Properties of eddy currents, Eddy current sensing elements, Probes, Instrumentation, Types of arrangement, Applications, advantages, Limitations, Interpretation/Evaluation.
- 4. INTRODUCTION Infrared thermography is equipment or method, which detects infrared energy emitted from object, converts it to temperature, and displays image of temperature distribution. The image of temperature distribution is called infrared thermograph and the method to be called as infrared thermography.
- 5. • Infrared thermography is a proactive troubleshooting and predictive maintenance tool. • It is the process of acquisition and analysis of thermal information from non-contact thermal imaging devices. • Thermal or infrared energy = Light which is not visible because its wavelength is too long to be detected by the human eye: it's the part of the electromagnetic spectrum we perceive as "heat". • Thermography = The use of an infrared imaging and measurement device to "see" and "measure thermal energy emitted from an object. • Infrared testing is essentially non-destructive inspection process that uses thermography cameras. • These devices gather temperature signatures that lie far beyond the range of visible light.
- 6. HISTORY OF INFRARED It was discovered by a British astronomer, Herschel, in 1800. When dispersing sunlight using a prism, Herschel accidentally found that there was an invisible light on the outside of red light when increases the temperature of an object. It is an electromagnetic wave.
- 7. PRINCIPLE • Every object whose surface temperature is above absolute zero (0 K) radiates energy at a wavelength corresponding to its surface temperature. Utilizing our highly sensitive infrared cameras, it is possible to convert this radiated energy into a thermal image of the object being surveyed . • After survey findings is provided through the use of two different types of images. Colour Thermo grams (photographs of the infrared image) and Control Photos are provided of problem areas uncovered during the inspection.
- 8. ELECTROMAGNETIC SPECTRUM • The electromagnetic spectrum is the distribution of electromagnetic radiation according to energy. It is the range of all possible frequencies of electromagnetic radiation.
- 9. THERMAL SCIENCE • Heat is thermal energy associated with temperature- dependent motion of particles • Photography can be described as writing with Light • Thermography can be described as writing with Heat Heat Transfer Modes: Conduction Convection Radiation EMISSIVITY Emissivity is a term representing a material's ability to emit thermal radiation Conduction Radiation Convection
- 10. Visual Image Thermal image Thermal Imaging with an infrared camera. “ Paints a different picture.” These two glasses visually appear the same.
- 12. THERMOGRAPHIC CAMERA • The camera converts radiated heat energy into an electrical signal which is then displayed on the monitor as a real-time heat image of the object being scanned. • The camera assigns black colour for the coolest area and white colour for the hottest area.
- 15. COMPONENTS FOR IR IMAGER Detector: • There are two options for a thermal imaging detector, cooled and uncooled, both types of detector absorb infrared energy which in turn affect the detectors electrical properties to produce an image. • These detectors use narrow gap semiconductors to offer high sensitivity to infra-red radiation. These semiconductors have to be housed in a vacuum sealed case and cryogenically cooled (typically to below a temperature of 110K).
- 16. Lens: The Lens is not made out of glass like the lens of a normal CCTV camera. The properties of glass mean that it does not transmit infrared radiation very well and would be an impractical lens material for thermal imagers. Instead Germanium is used which is a naturally occurring chemical element that is transparent to IR Radiation (that is that it will allow IR Radiation to pass through it).
- 17. SENSOR: • The ML8540 is a medium-resolution (approximately 2,000 pixels) infrared image sensor that allows noncontact temperature measurement of an object and easy obtaining of thermal images. • The ML8540 receives infrared rays from an object at 48×47 (2,256) pixels resolution, converts them into voltage through a thermopile, and selects pixels in accordance with external CPU clocks to output signals.
- 19. ADVANTAGES It is a non-contact type technique. A large surface area can be scanned in no time. Presented in visual & digital form. Software back-up for image processing and analysis. Requires very little skill for monitoring
- 20. DISADVANTAGES Cost of instrument is relatively high. Unable to detect the inside temperature if the medium is separated by glass/polythene material etc. Difficult to interpret even with experience
- 21. APPLICATIONS Printed circuit board evaluation and troubleshooting. Circuit board component evaluation Production-type inspection of bonded structures Inspection of solder joints In medical thermography Level detection Industrial roof moisture detection
- 22. Types of Thermography • Passive Thermography • Active Thermography
- 23. Passive Thermography Passive Thermography directly measures the surface temperature for evaluation, since the interest region will have abnormal hot-spot as compared with the surroundings. Abnormal temperature profile indicates a potential problem, where the key word is the temperature difference with respect to the surrounding, as referred as extra hot spot. The features of interest are naturally at a high or low temperature than the background.
- 24. Active Thermography • Active Thermography uses an external source for measured object excitation, that means introducing an energy into the object. The excitation sources can be classified by the principles: • optical radiation or microwaves absorption, • electromagnetic induction, • elastic waves transformation (e.g. ultrasound), • convection (e.g. hot air), • plastic deformation transformation (thermoplastic effect during mechanical loading). • Various excitation sources can be used for the active Thermography and non destructive testing, for example laser heating, flash lamps, halogen lamps, electrical heating, ultrasonic excitation, eddy currents, microwaves, and others.
- 43. Review
- 45. EDDY CURRENT TESTING (ET)
- 46. Introduction • Non-destructive techniques are used widely in the metal industry in order to control the quality of materials. • Eddy current testing is one of the most extensively used non-destructive techniques for inspecting electrically conductive materials at very high speeds that does not require any contact between the test piece and the sensor. • Recent advances in complex models towards solving crack-sensor interaction, developments in instrumentation due to advances in electronic devices, and the evolution of data processing suggest that eddy current testing systems will be increasingly used in the future.
- 47. • The principle of the eddy current technique is based on the interaction between a magnetic field source and the test material. This interaction induces eddy currents in the test piece. • Evaluator can detect the presence of very small cracks by monitoring changes in the eddy current flow.
- 48. Eddy current testing is based on the physics of electromagnetic induction. In an eddy current probe, an alternating current flows through a coil and generates an oscillating magnetic field - Ampere’s Law If the probe and its magnetic field are brought close to a conductive material like a metal test piece, an electric magnetic field (emf) is induced resulting in induced currents called eddy currents (like swirling water in a stream) - Faraday’s Law
- 49. EC testing works on the principles of electromagnetic induction. In this technique, a coil (also called probe or sensor) is excited with sinusoidal alternating current (frequency, f, ~ 50 Hz-5 MHz, ~ 100 mA). Following the Ampere’s law, this current generates primary magnetic field in the vicinity of the coil. When an electrically conducting material is brought close to this coil, eddy currents are induced in the material according to the Faraday’s law
- 52. The eddy currents have very unique and interesting properties such as: • They are induced currents that exist only in electrically conducting materials. • They are always in closed loops, usually parallel to the coil winding. • They are distorted by defects such as cracks and corrosion wall loss and by discontinuities such as edge-effect and end-effect. • They attenuate with depth (also axially or laterally) • Their intensity depends on material properties, electromagnetic coupling and excitation frequency, but maximum on the surface.
- 53. Uses of Eddy current Testing: • Conductivity variations • Detection of discontinuities • Spacing between test coil and test material (Lift-off distance) • Material thickness • Spacing between conductive layers
- 54. Eddy Current Probes • Three major types of probe are surface, outside diameter and inside diameter. • These three configurations, as well as some cross over designs, are used for most flaw detection applications. • Absolute probes have single coil design and give an 'absolute' reading at the flaw. • Differential probes use two coils to check for flaws in different areas or to differentiate between two variables.
- 55. • Reflection probes have a primary coil being supplied by the oscillator and at least one coil from the measurement circuit. Reflection probes can be either absolute or differential
- 59. • Changes in metal thickness or defects like near- surface cracking will interrupt or alter the amplitude and pattern of the eddy current and the resulting magnetic field. This in turn affects the (inductance and resistance) impedance of the coil. • Eddy current density is highest near the surface of the part, so that is the region of highest test resolution.
- 60. Inductance • Alternating current running through a coil creates a magnetic field in and around the coil that is building and collapsing as the current changes. • As current increases, the coil becomes more magnetic and induces circulating (eddy) currents in conductive material that is near the coil. • The amplitude and phase of the eddy currents will change the loading of the coil and its impedance. • If a surface or sub-surface discontinuity exists in a conductive material, the eddy currents will be interrupted and the flow can be measured by UniWest's instruments.
- 61. Testing Methods • Coil with single winding: The most simple coil comprises a ferrite rod with several turns of wire wound at one end and which is positioned close to the surface of the product to be tested. When a crack, for example, occurs in the product surface the eddy currents must travel farther around the crack and this is detected by the impedance change.
- 63. • Coil with Two windings (Differential Probe) Coils can also be used in pairs, generally called a driven pair, and this arrangement can be used with the coils connected differentially. In this way ‘lift off’ (distance of the probe from the surface) signals can be enhanced.
- 65. • Transformer type coil with Three Windings Coils can also be used in a transformer type configuration where one coil winding is a primary and one (or two) coil windings are used for the secondaries.
- 67. Advantages of Eddy Current Testing • Suitable for the determination of a wide range of conditions of conducting material, such as defect detection, composition, hardness, conductivity, permeability etc. in a wide variety of engineering metals. • Information can be provided in simple terms: often go/no go. Phase display electronic units can be used to obtain much greater product information. • Extremely compact and portable units are available. • No consumables (except probes ) • Suitable for total automation.
- 68. Disadvantages of Eddy Current Testing • The wide range of parameters which affect the eddy current responses means that the signal from a desired material characteristic. • Careful selection of probe and electronics will be needed in some applications. • Applicability to only electrically conducting (metallic) materials. • Inability to identify circumferential location of a defect when encircling or bobbin coils are used. • Difficulty in detection of a small defect under a large defect Inability to detect defects at the centre of rods using encircling coils. • Need for skilled personnel for interpretation of signals and results.
- 69. Applications Position Measurement/Sensing: • Eddy-Current sensors are basically position measuring devices. Their outputs always indicate the size of the gap between the sensor's probe and the target. When the probe is stationary, any changes in the output are directly interpreted as changes in position of the target. This is useful in: • Automation requiring precise location • Machine tool monitoring • Final assembly of precision equipment such as disk drives • Precision stage positioning
- 70. Dynamic Motion Measuring the dynamics of a continuously moving target, such as a vibrating element, requires some form of noncontact measurement. Eddy-Current sensors are useful whether the environment is clean or dirty and the motions are relatively small. It also have high frequency response (up to 80 kHz) to accommodate high- speed motion. • Drive shaft monitoring • Vibration measurements
- 71. Material Thickness Measurement • Thickness measurements are possible with eddy current inspection within certain limitations. • Only a certain amount of eddy currents can form in a given volume of material. • Therefore, thicker materials will support more eddy currents than thinner materials. • The strength (amount) of eddy currents can be measured and related to the material thickness. Eddy Currents Magnetic Field From Probe Test Material
- 72. • Eddy current inspection is often used in the aviation industries to detect material loss due to corrosion and erosion.
- 73. • Eddy current inspection is used extensively to inspect tubing at power generation and petrochemical facilities for corrosion and erosion.
- 74. Crack Detection • Crack detection is one of the primary uses of eddy current inspection. Cracks cause a disruption in the circular flow patterns of the eddy currents and weaken their strength. This change in strength at the crack location can be detected. Magnetic Field From Test Coil Magnetic Field From Eddy Currents Eddy Currents Crack
- 75. • Eddy current inspection is exceptionally well suited for the detection of cracks, with an especially high sensitivity to detection of surface breaking cracks.
- 76. • Eddy current inspection of “bead seat” area on aircraft wheel for cracks using special probe that conforms to the shape of the rim.
- 77. Nonconductive Coating Measurement • The coating displaces the eddy current probe from the conductive base material and this weaken the strength of the eddy currents. • This reduction in strength can be measured and related to coating thickness. Conductive Base Metal Nonconductive Coating Eddy Currents
- 78. • The photo to the left shows an aircraft panel paint thickness inspection. On the right, the display of a digital eddy current inspection instrument shows the different signals obtained by measuring eight different thicknesses of paint on aluminum. Increasing paint thickness
- 79. Conductivity Measurements • Boeing employees in Philadelphia were given the privilege of evaluating the Liberty Bell for damage using NDT techniques. Eddy current methods were used to measure the electrical conductivity of the Bell's bronze casing at a various points to evaluate its uniformity.
- 80. Instrumentation - Meters The two general categories of meters are digital and analog.
- 81. Digital Meters • Digital meters are typically designed to examine one specific attribute of a test component such as conductivity or nonconductive coating thickness. These meters tend to have slightly higher accuracy than analog devices.
- 82. Analog Meters • Analog meters can be used for many different inspection applications such as crack detection, material thickness measurements, nonconductive coating measurements or conductive coating measurements.
- 84. • Portable eddy scopes are another category of instrumentation and they present the inspection data in the form of an impedance plane diagram. •On the impedance diagram, the total impedance is displayed by plotting its resistance component and inductive reactance component at 90 degrees to each other. •This is beneficial for both separation and identification of test variables that can effect inspection results.
- 85. • Modern eddy scopes are usually digital based instruments which can often be purchased as either a single or dual frequency tester. Dual frequency instruments are capable of sequentially driving a probe at two different inspection frequencies.
- 86. • Digital scopes often have an RS232 (serial) connection for interfacing with a serial printer or computer as well as provisions for output of signals to recording devices such as a strip-chart recorder. • In addition, these instruments contain a small amount of RAM so that equipment settings as well as screen presentations can be stored for later reference.
- 87. Multi-Frequency Eddy Current Instruments • Multi-Frequency instruments usually refer to equipment that can drive inspection coils at more than two frequencies either sequentially (multiplexing) or simultaneously. • This type of instrumentation is used extensively for tubing inspection in the power generation, chemical and petrochemical industries. • These instruments are often capable of being computer networked and may have as many as four probes attached to them at one time.
- 89. Eddy Current Probes • Probes selection is critical to acquiring adequate inspection data. • Several factors to consider include: Material penetration requirements (surface vs. subsurface) Sensitivity requirements Type of probe connections on eddy current instrument (many variations) Probe and instrument impedance matching (will probe work with instrument) Probe size (smaller probes penetrate less) Probe type (absolute, differential, reflection or hybrid)
- 90. • Due the large variety of probes in eddy current testing there are many different systems of classification. • Three of the most common classifications are: – Surface probes – Inside Diameter (I.D.) or Bobbin Probes – Outside Diameter (O.D.) or Encircling probes
- 91. Surface probes • Surface probes are coils that are typically mounted close to one end of a plastic housing. As the name implies, the technician moves the coil end of the probe over the surface of the test component.
- 92. Inside Diameter (I.D.) probes • Inside Diameter (I.D.) probes, also known as bobbin probes, are coils that are usually wound circumferentially around a plastic housing. These probes are primarily designed for inspection inside of tubular materials.
- 93. Outside Diameter (O.D.) probes • Outside Diameter (O.D.) probes are coils that are wound the circumference of a hollow fixture. The coil is designed such that the test part is ran through the middle of the coil. These probes can be used to inspect bars, rods as well as tubes.