Electromagnetic Testing (ET)

Electromagnetic Testing (ET)


Electromagnetic testing is a general test category that includes Eddy Current testing, Alternating Current Field Measurement (ACFM) and Remote Field testing. While magnetic particle testing is also an electromagnetic test, due to its widespread use it is considered a stand-alone test method rather as than an electromagnetic testing technique. All of these techniques use the induction of an electric current or magnetic field into a conductive part, then the resulting effects are recorded and evaluated.

ET Techniques

1. Eddy Current Testing

Pic 1

Eddy Current Testing uses the fact that when a an alternating current coil induces an electromagnetic field into a conductive test piece, a small current is created around the magnetic flux field, much like a magnetic field is generated around an electric current. The flow pattern of this secondary current, called an "eddy" current, will be affected when it encounters a discontinuity in the test piece, and the change in the eddy current density can be detected and used to characterize the discontinuity causing that change. A simplified schematic of eddy currents generated by an alternating current coil ("probe") is shown in Pic 1. By varying the type of coil, this test method can be applied to flat surfaces or tubular products. This technique works best on smooth surfaces and has limited penetration, usually less than ¼".

Pic 2

Encircling coils (Pic 2) are used to test tubular and bar-shaped products. The tube or bar can be fed through the coil at a relatively high speed, allowing the full cross-section of the test object to be interrogated. However, due to the direction of the flux lines, circumferentially oriented discontinuities may not be detected with this application.


2. Alternating Current Field Measurement

Alternating Current Field Measurement (ACFM) uses a specialized probe that introduces an alternating current into the surface of the test piece, creating a magnetic field. In parts with no discontinuities this field will be uniform, but if there is a discontinuity open to the surface, the magnetic field will flow around and under the discontinuity, causing a disruption of the field that can be detected by sensors within the probe. The resulting feedback can then be fed to software that can determine the length and depth of the discontinuity. ACFM provides better results on rough surfaces than Eddy Current and can be used through many surface coatings.

3. Remote Field Testing (RFT)

Remote Field Testing (RFT) is most commonly used to inspect ferromagnetic tubing due to the presence of a strong skin effect found in such tubes. Compared to standard eddy current techniques, remote field testing provides better results throughout the thickness of the tube, having approximately equal sensitivity at both the ID and OD surfaces of the tube. For non-ferromagnetic tubes, eddy current tends to provide more sensitivity.

Vibration Analysis (VA)

Vibration analysis refers to the process of monitoring the vibration signatures specific to a piece of rotating machinery and analyzing that information to determine the condition of that equipment. Three types of sensors are commonly used: displacement sensors, velocity sensors and accelerometers.

Displacement sensors uses eddy current to detect vertical and/or horizontal motion (depending on whether one or two sensors are used) and are well suited to detect shaft motion and changes in clearance tolerances.

Basic velocity sensors use a spring-mounted magnet that moves through a coil of wire, with the outer case of the sensor attached to the part being inspected. The coil of wire moves through the magnetic field, generating an electrical signal that is sent back to a receiver and recorded for analysis. Newer model vibration sensors use time-of-flight technology and improved analysis software. Velocity sensors are commonly used in handheld sensors.

Basic accelerometers use a piezoelectric crystal (that converts sound waves to electrical impulses and back) attached to a mass that vibrates due to the motion of the part to which the sensor casing is attached. As the mass and crystal vibrate, a low voltage current is generated which is passed through a pre-amplifier and sent to the recording device. Accelerometers are very effective for detecting the high frequencies created by high speed turbine blades, gears and ball and roller bearings that travel at much greater speeds than the shafts to which they are attached.

Thermal/Infrared Testing (IR)

Thermal/Infrared Testing, or infrared thermography, is used to measure or map surface temperatures based on the infrared radiation given off by an object as heat flows through, to or from that object. The majority of infrared radiation is longer in wavelength than visible light but can be detected using thermal imaging devices, commonly called "infrared cameras." For accurate IR testing, the part(s) being investigated should be in direct line of sight with the camera, i.e., should not be done with panel covers closed as the covers will diffuse the heat and can result in false readings. Used properly, thermal imaging can be used to detect corrosion damage, delaminations, disbonds, voids, inclusions as well as many other detrimental conditions.

Neutron Radiographic Testing (NR)

Neutron radiography uses an intense beam of low energy neutrons as a penetrating medium rather than the gamma- or x-radiation used in conventional radiography. Generated by linear accelerators, betatrons and other sources, neutrons penetrate most metallic materials, rendering them transparent, but are attenuated by most organic materials (including water, due to its high hydrogen content) which allows those materials to be seen within the component being inspected. When used with conventional radiography, both the structural and internal components of a test piece can be viewed.

Magnetic Flux Leakage (MFL)

Pic 1 : Magnetic Flux Leakage

Magnetic Flux Leakage detects anomalies in normal flux patterns created by discontinuities in ferrous material saturated by a magnetic field. This technique can be used for piping and tubing inspection, tank floor inspection and other applications. In tubular applications, the inspection head contain is made up of drive and sensor coils and a position transducer that are connected by cable back to the power source and signal processing computer. This head is placed around the pipe or tube to be inspected and the drive coil is energized, creating a magnetic field in the part. As the head travels along the length of the part, variations in the wall thickness due to corrosion, erosion, pitting etc., will cause a change in the magnetic flux density can be picked up by the sensor and sent back to the computer. The location of this signal is sent by the position transducer so that the area detected can be marked for further evaluation. This technique can be done without removing the insulation, resulting in a fast, economic way to inspect long runs of pipe or tubing.

Tank floor inspection applies the same principle, but uses a series of magnetic field generators ("bridges") and sensors (as shown in Pic 1) located side by side across the front of a vacuum sweeper-like machine. The bridges generate a magnetic field that saturates the tank floor, and any reduction in thickness or loss of material due to pitting or corrosion will cause the field to "leak" upwards out of the floor material where it can be picked up by the sensors. On very basic machines, each sensor will be connected to an audio and/or visual display that lets the operator know there is an indication; more advanced machines can have both visual displays and recording capability so that the results can be stored, analyzed and compared to earlier results to monitor discontinuity growth.

Acoustic Emission Testing (AE)

Acoustic Emission Testing is performed by applying a localized external force such as an abrupt mechanical load or rapid temperature or pressure change to the part being tested. The resulting stress waves in turn generate short-lived, high frequency elastic waves in the form of small material displacements, or plastic deformation, on the part surface that are detected by sensors that have been attached to the part surface. When multiple sensors are used, the resulting data can be evaluated to locate discontinuities in the part.

Guided Wave Testing (GW)

Guided wave testing

Guided wave testingon piping uses controlled excitation of one or more ultrasonic waveforms that travel along the length of the pipe, reflecting from changes in the pipe stiffness or cross sectional area. A transducer ring or exciter coil assembly is used to introduce the guided wave into the pipe and each transducer/exciter . The control and analysis software can be installed on a laptop computer to drive the transducer ring/exciter and to analyze the results. The transducer ring/exciter setup is designed specifically for the diameter of the pipe being tested, and the system has the advantage of being able to inspect the pipe wall volume over long distances without having to remove coatings or insulation. Guided wave testing can locate both ID and OD discontinuities but cannot differentiate between them.

Laser Testing Methods (LM)

Laser Testing includes three techniques, Holography, Shearography and Profilometry. As the method name implies, all three techniques user lasers to perform the inspections.

LM Techniques

Holographic Testing

Holographic Testing uses a laser to detect changes to the surface of a part as it deforms under induced stress which can be applied as mechanical stress, heat, pressure, or vibrational energy. The laser beam scans across the surface of the part and reflects back to sensors that record the differences in the surface created by that stress. The resulting image will be a topographical map-like presentation that can reveal surface deformations in the order of 0.05 to 0.005 microns without damage to the part. By comparing the test results with an undamaged reference sample, holographic testing can be used to locate and evaluate cracks, delaminations, disbonds, voids and residual stresses.

Laser Profilometry

Laser Profilometry uses a high-speed rotating laser light source, miniature optics and a computer with high-speed digital signal processing software. The ID surface of a tube is scanned in two dimensions and the reflected light is passed through a lens that focuses that light onto a photo-detector, generating a signal that is proportional to the spot's position in its image plane. As the distance from the laser to the ID surface changes, the position of the focal spot on the photo-detector changes due to parallax, generating a high resolution three-dimensional image of the part surface that represents the surface topography of the part. This technique can be used to detect corrosion, pitting, erosion and cracks in pipes and tubes.

Laser Shearography

Laser Shearography applies laser light to the surface of the part being tested with the part at rest (non-stressed) and the resulting image is picked up by a charge-coupled device (CCD) and stored on a computer. The surface is then stressed and a new image is generated, recorded and stored. The computer then superimposes the two patterns and if defects such as voids or disbonds are present, the defect can be revealed by the patterns developed. Discontinuities as small as a few micrometers in size can be detected in this manner.