ET techniques offer several advantages over competing inspection methods, including freedom from chemicals or liquids as well as rapid inspection rates. ET techniques can also offer low-cost, non contact evaluation of samples of widely varying temperature.
ET is used to evaluate the quality of conductive and nonconductive samples. Conductive samples may be ferrous or nonferrous. Electromagnetic testing is a catchall method, comprising several diverse techniques. ET method most often refers to magnetic flux leakage (MFL) testing, eddy current testing and microwave testing (MW), although magnetic barkhausen noise, ground penetrating radar (GPR) and others may fall within this category (Note: MFL, GPR and MW are considered separate methods.)
All ET techniques, as well as the variety of electromagnetic waves and fields that are encountered daily, are described by the set of partial differential equations know as "Maxwell`s equations" this equations interrogate electric, magnetic and electromagnetic induction theories, which, before being advanced by James Clerk Maxwell, had been considered as separate disciplines with independent constants. Vector calculus is needed to fully grasp the underlying mathematics of Maxwell`s equations, but at their core they simply describe the interdependence of time-varying electric and magnetic fields.
An electromagnetic field is a vector quantity, which means that it has both magnitude and three-dimensional direction. Fields may be described by their rotation, or curl, and the nature of their source that is, divergence. Quasistatic, time-varying conditions of eddy current, such as remote field testing (RTF) or multifrequency eddy current testing, are described by parabolic equations. When an electric field changes with very high frequency, there is another current within the specimen, which is know as displacement current. This displacement current is proportional to the frequency and the dielectric permittivity of the material.
Test Frequencies. Each technique varies in its test frequencies, transducer type and signal analysis methods employed. For example, magnetic flick leakage often uses test frequencies between 0 Hz and 60 Hz, eddy current testing uses frequencies between 100Hz and 10MHz, and microwave testing employs test frequencies as high as 300 MHz. As described by Maxwell`s equations, the underlying physical process changes with test frequency. Electromagnetic fields with a frequency below 10 MHz are said to be "quasi-static", Which means that displacement current is negligible. At higher frequencies, the probing energy propagates as waves. Sensor technologies used in ET vary; for example, MFL uses Hall effect sensors, eddy current testing uses one or more coiled wire sensors and microwave testing and ground penetrating radar use antennae.
ET variables generally vary nonlinearly with frequency; at times, the rate of change can Ary from a positive slope to a negative slope. Because of this complicated relationship between test variables, most ET applications, such as alloy sorting heat-treatment verification, hardness determination, hardness determination, or thickness measurement, require reference standards that properly match all changes that may exist in the test objects. Equipment standardization, with the proper reference standards, is important to discontinuity detection.
Electrical Conductivity- Most metals are good conductors of electricity and electrical conductivity of the test object is an important factor in many ET techniques.
For simplicity, a change in conductivity is generally associated with a change in the ability of electrons to flow. Conductivity can be anisotropic and t varies with several factors, including temperature, alloying elements and their concentrations, lattice structure or strain and the number and concentration of dislocations with the atomic lattice structure. While conductivity- the inverse of resistivity- may be described in absolute terms, in Siemens per meter, the relative international. Annealed Cooper Standard (%IACS) scale. is often used. This relative scale uses annealed, or soft, cooper with a sample temperature of 20ºC as the basis of comparison, identifying this material as 100% IACS. Other conductors are then attributed a value relative to annealed cooper.
Lattice strain and dislocation density may be modified by the thermal an mechanical history of the test object. An example of a thermal process is annealing, which reduces dislocation density and consequently the number of obstacles to the flow of conduction-band electrons. A mechanical modification may be room-temperature plastic deformation, such as cold rowing, which increases atomic lattice strain and dislocation density and changes the material`s electrical resistivity. The conductivity of aluminium alloys is of particular interest in some industries. Aluminium alloys may be thermally and/or mechanically processed using special recipes to induce a desired strength, corrosion resistance or other property. Each ductivity; therefore, ET is useful for rapid estimations of material properties in aluminium and other non ferromagnetic materials.
To obtain information about a sample, an inspector might include different test frequencies, a variety of types or configurations of probes, multiple probe orientations or different procedures. Many electromagnetic test techniques may be applied at all stages of forming, shaping and heat treating of metas and alloys, where the effectiveness of processing steps can be detected and removed from production without incurring further processing cost. There are several electromagnetic testing methods and techniques and we will briefly touch on some of the most common ones.
Test Frequencies. Each technique varies in its test frequencies, transducer type and signal analysis methods employed. For example, magnetic flick leakage often uses test frequencies between 0 Hz and 60 Hz, eddy current testing uses frequencies between 100Hz and 10MHz, and microwave testing employs test frequencies as high as 300 MHz. As described by Maxwell`s equations, the underlying physical process changes with test frequency. Electromagnetic fields with a frequency below 10 MHz are said to be "quasi-static", Which means that displacement current is negligible. At higher frequencies, the probing energy propagates as waves. Sensor technologies used in ET vary; for example, MFL uses Hall effect sensors, eddy current testing uses one or more coiled wire sensors and microwave testing and ground penetrating radar use antennae.
ET variables generally vary nonlinearly with frequency; at times, the rate of change can Ary from a positive slope to a negative slope. Because of this complicated relationship between test variables, most ET applications, such as alloy sorting heat-treatment verification, hardness determination, hardness determination, or thickness measurement, require reference standards that properly match all changes that may exist in the test objects. Equipment standardization, with the proper reference standards, is important to discontinuity detection.
Electrical Conductivity- Most metals are good conductors of electricity and electrical conductivity of the test object is an important factor in many ET techniques.
For simplicity, a change in conductivity is generally associated with a change in the ability of electrons to flow. Conductivity can be anisotropic and t varies with several factors, including temperature, alloying elements and their concentrations, lattice structure or strain and the number and concentration of dislocations with the atomic lattice structure. While conductivity- the inverse of resistivity- may be described in absolute terms, in Siemens per meter, the relative international. Annealed Cooper Standard (%IACS) scale. is often used. This relative scale uses annealed, or soft, cooper with a sample temperature of 20ºC as the basis of comparison, identifying this material as 100% IACS. Other conductors are then attributed a value relative to annealed cooper.Lattice strain and dislocation density may be modified by the thermal an mechanical history of the test object. An example of a thermal process is annealing, which reduces dislocation density and consequently the number of obstacles to the flow of conduction-band electrons. A mechanical modification may be room-temperature plastic deformation, such as cold rowing, which increases atomic lattice strain and dislocation density and changes the material`s electrical resistivity. The conductivity of aluminium alloys is of particular interest in some industries. Aluminium alloys may be thermally and/or mechanically processed using special recipes to induce a desired strength, corrosion resistance or other property. Each ductivity; therefore, ET is useful for rapid estimations of material properties in aluminium and other non ferromagnetic materials.
To obtain information about a sample, an inspector might include different test frequencies, a variety of types or configurations of probes, multiple probe orientations or different procedures. Many electromagnetic test techniques may be applied at all stages of forming, shaping and heat treating of metas and alloys, where the effectiveness of processing steps can be detected and removed from production without incurring further processing cost. There are several electromagnetic testing methods and techniques and we will briefly touch on some of the most common ones.