Specific flaws or features within a mechanical system provide specific acoustic or vibrational responses. If a break, deformation or other failure occurs in a piece of equipment, sensitive acoustic emission sensors can detect the high frequency burst given off during the event. The location and severity can also be determined by monitoring the acoustic emissions. The NDT technique is particularly useful in determining the structural adequacy of tanks and pressure vessels.
The absorption of Beta particles is used to measure the thickness of materials or coatings. For nonmetallic materials such as paper or plastic films or webs, radiation is transmitted through the material and a radiation or Geiger-Muller detector is located on the other side measures radiation levels. On coated metallic materials, the radiation or Geiger-Muller detector is located on the same side and backscattered radiation is measured.
Eddy current, ground penetrating radar (GPR) and other electromagnetic techniques used to detect or measure flaws, bond or weld integrity, thickness, electrical conductivity, detect the presence of rebar or metals.
The eddy current method 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.
In the GPR method, a signal of high frequency electromagnetic waves (RADAR) is directed into the material. Layer boundaries, objects or features in the material that have contrasting electrical properties to the surrounding base material cause partial reflection of the waves. 2D and 3D images are created using the reflected waves that return to the surface. GPR methods can detect metal and non-metal objects or discontinuities when changes in dielectric constant occur in the material through which the signal travels. With this ability, GPR can detect and map natural or man-made sub-surface boundaries in earth or concrete structures such as:
Electrical lines or cables
Plastic or metal pipes
Pipeline leaks
Rebar reinforcement in concrete slabs or columns
Sinkhole formations
Underground storage tanks (metal or plastic)
Voids, cracks or other anomalies in concrete columns and beams
Current flow or an external magnet magnetizes the part. Magnetic poles created at flaws, cracks or other discontinuities attract magnetic particles. The magnetic particles are fine iron oxide particles (0.125 to 60 microns) with a high permeability (easily magnetized) and low retentivity (ability to stay magnetized). Three methods are typically applied: dry non-fluorescent, wet non-fluorescent and wet fluorescent.
Optical based instruments or methods such as fiberscopes, videoscopes, microscopes, borescopes, laser shearography, magneto-optical, holographic interferometry and other optical or visual inspection techniques to detect flaws, inspect internal surfaces, residual stress or measure thickness.
In penetrant testing, penetrant is applied to part by spray or immersion. The penetrant is pulled into surface flaws by capillary action. Next, a cleaner is used to remove residual penetrant from the surface of the part so the liquid penetrant only remains in the surface flaws or cracks that are present. Penetrant systems are available with varying sensitivity levels depending on the material and flaw type being inspected. Red dye penetrants are visible under normal light. Fluorescent penetrants or penetrant systems may require UV or backlight illumination. Developers or fluorescent powders may be required to enhance visibility of the residual or crack entrapped penetrant.
Equipment using penetrating X-ray or gamma radiation to capture images of the internal structure or a part or finished product. The density and composition of the internal features will alter the intensity or density of these features in the X-ray image. Densitometers are used to quantify the density variations in the X-ray image. Penetrameters or other X-ray opaque gage references are located with the part during imaging for sizing of internal cracks, pores, defects or other features.
Ultrasonic technology (UT) inspection techniques are used to detect surface and sub-surface flaws or to measure thickness. Beams of high frequency acoustic energy are introduced into the material and subsequently retrieved. Distance calculations are based on the speed of sound through the material being evaluated. The most widely used of all UT techniques is the pulse-echo technique. Flaws are detected and sizes estimated by comparing the amplitude of a reflected echo from an interface (flaw or back surface) with that of a reference interface of known size.
Other nondestructive techniques or technologies not listed.
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