Eddy current, penetrating radar and other electromagnetic techniques are used to detect or measure flaws, bond or weld integrity, thickness, electrical conductivity, detect the presence of rebar or metals. Eddy current is the most widely applied electromagnetic NDT technique. The eddy current method is also useful in sorting alloys and verifying heat treatment. Eddy current testing uses an electromagnet to induce an eddy current in a conductive sample. The response of the material to the induced current is sensed. Since the probe does not have to contact the work surface, eddy current testing is useful on rough surfaces or surfaces with wet films or coatings.
Eddy Current Testing
In standard eddy current testing, a circular coil carrying an AC current is placed in close proximity to an electrically conductive specimen. The alternating current in the coil generates a changing magnetic field, which interacts with the test object and induces eddy currents. Variations in the phase and magnitude of these eddy currents can be monitored using a second ’search’ coil, or by measuring changes to the current flowing in the primary ‘excitation’ coil. Variations in the electrical conductivity or magnetic permeability of the test object, or the presence of any flaws, will cause a change in eddy current flow and a corresponding change in the phase and amplitude of the measured current. This is the basis of standard (flat coil) eddy current inspection, the most widely used eddy current technique.
Barkhausen Noise Analysis (BNA)Barkhausen Noise Analysis (BNA) method, also referred to as the Magnetoelastic or the Micromagnetic method is based on a concept of inductive measurement of a noise-like signal, generated when magnetic field is applied to a ferromagnetic sample. After a German scientist Professor Heinrich Barkhausen who explained the nature of this phenomenon already in 1919, this signal is called Barkhausen noise.
Ground Penetrating Radar (GPR) Ground penetrating radar is a nondestructive geophysical method that produces a continuous cross-sectional profile or record of subsurface features, without drilling, probing, or digging. Ground penetrating radar (GPR) profiles are used for evaluating the location and depth of buried objects and to investigate the presence and continuity of natural subsurface conditions and features.Ground penetrating radar operates by transmitting pulses of ultra high frequency radio waves (microwave electromagnetic energy) down into the ground through a transducer or antenna. The transmitted energy is reflected from various buried objects or distinct contacts between different earth materials. The antenna then receives the reflected waves and stores them in the digital control unit.
Magnetic Resonance Imaging (MRI) Magnetic resonance imaging (MRI) is an imaging technique used primarily in medical settings to produce high quality images of the inside of the human body. MRI is based on the principles of nuclear magnetic resonance (NMR), a spectroscopic technique used by scientists to obtain microscopic chemical and physical information about molecules.
Microwave InspectionMicrowave (or short-pulse radar) inspection techniques involve the transmission and reflection of relatively low frequency (often around 1 GHz) electromagnetic (EM) waves in various materials. The term ground penetrating radar (GPR) is often used to describe microwave inspection systems for locating utility lines below ground and mild steel rebar in concrete decks/pavements. Microwave inspection exploits the principle that dielectric properties of various materials affect the transmission and reflection of EM waves in those materials.
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