Downloaded from https://www.everyspec.com AMCP 70&619 The magnitude of tie voltage gives an indica- a defect whose length Is comparable to the tion of the size of the flaw. This method is coil length (roughly 0.1 in, minimum) and very useful when the entirt object to be in- whose depth is 5% or more of material wall 4pected (pipes, wire, etc.) can be piassed thickness can be detected. The probes are through a coil, A third method takes advant- awo sensitive to the displaced volume of age of the Hall effect, Hall effect probes metal and are sensitive to defects on the order usually are made of a semiconductor material of the probe diameter.
One such probe, the and are passed over the surface of the mag- Probolog, developed by Shell Development netized specimen. Variations In the magnetic Company, can detect cracks or seams 0.005 in. field, due to defects and discontinuities result deep by 0.5 in. long, The probe also can de- hn a variation in the Hall voltage of the probe. tect 1% thickness changes in a 0.5 in. length, The sensitivity of this method depends on 8.7 ULTRASONICS the strength of the magnetic field. Defects of any consequence can usually be detected Ultrasonic waves are acoustic waves above down to 0.060 in. below the surface. Defects the audible range.
They are employed in down to 0.100 in, deep will show under ideal NDE to detect and locate flaws In composite conditions (Ret'. 1). A number of other materials and nonbonded areas in bonded factors such as sharpness, direction, and materials. The impedance to ultrasonic orientation of the defects also aftect the sensi- propagation is different for a flaw or anomaly tivity of the method, than for the basic material.
Therefore, a portion of the induced ultrasonic energy is Eddy current testing is based on the princi- reflected by a flaw, just as It is by a boundary pie that when a coil carrying a high frequency of the material, Measurement of the re- alternating current is brought into the vicinity flected portion or the unreflected portion is of an electrical conductor, currents are in- the basis for employing ultrasonics in NDE, duced. These currents, in turn, induce a magnetic field about the conductor. The in- There are three methods of ultrasonic duced currents and the magnetic field are af- testing in general use:
pulse echo, trans- fected by the permeability of the material, mission, and resonance. Eddy currents can be used to test and measure hardness, alloy content, uniformity of heat. 8.7.1 PULSE ECHO METHOD treatment, as well as for flaw detection. Flaws perturb and distort the magnetic field In the pulse echo method, an applied produced by the eddy currents, pulse travels through the material and is re- flected from flaws and material boundaries. Two general types of probes are used in As the surface of the material is scanned, eddy current testing, One is an encircling the appearance of a "defect pulse" locates coil which surrounds the specimen and In- the surfuce position of a flaw.
The energy of vestigates everything within the coil geometry, this pulse is related to flaw size, but is The other is a point probe which inspects usually difficult to correlate precisely. By only the area beneath it. monitoring the time relationship of the Initial pulse, the "defect pulse", and the echo pulse, The coil type detector is affected by all the defect can be located in depth. Many the metal enclosed by the coil, so statements techniques have been used in the pulse echo about sensitivity are difficult to mcke, e.g,, method. For example, by introducing the a long shallow crack may give the same output initial pulse at an angle to the material as a short deep one.
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