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- Acoustic Emission
- When a flaw is stressed properly, it grows. This growth
releases energy in the form of mechanical vibrations called acoustic
emissions (AE). These AE signals propagate through out the
material and can be detected by sensors placed on the surface of the
structure under evaluation. The detected AE signals are then
processed by computer based hardware. By analyzing several of
the signals characteristics, the flaw severity can be
determined. If the sensor location and the AE wave velocity is
known, the computer can calculate the location of the flaw. AE
is a global inspection technique that primarily determines the
location of a flaw. Flaw orientation is not critical.
However, the flaw must grow in order to be detected. A
different NDT method is needed to accurately size the flaw.
- Eddy Current
- Eddy Current (EC) inspection instruments are used to detect
surface and near sub-surface flaws. The EC instruments
generate an alternating magnetic field. The field is coupled
from the EC probe into the part under test. In turn, the
alternating magnetic field generates eddy current flow in the
part. Any flaws at, or near the surface, will disrupt the
current flow, which is detected by the instrument. EC
inspections are used to size flaws. Best results are obtained
when the flaw is no greater than 0.050" deep, perpendicular to
the parts surface and located in non-ferrous material.
- NDT Accessories
- Penetrant
- Penetrant (PT) inspection is rapid means to locate flaws open to
the surface material under test. A penetrating liquid is
sprayed onto the part under test. The liquid is allow to
penetrate the surface cracks. The surface of the part is then
cleaned and a developer is strayed on. The developer draws the
penetrating fluid out of the flaws, thereby indicating their
location. By monitoring the size of the 'bleed spot', a
relative indication of the flaws depth is obtained.
- Radiography
- Radiography (RT) inspection uses high energy radiation (x-ray,
gamma, neutron, etc.) to detect cracks parallel to the radiation
beam. The radiation passes through the part and exposes film
on the opposite side of the part under test. Flaws exposed to
the radiation leave darker areas on the film. Radiography will
provide flaw width, but not depth, information.
- Systems
- An inspection system maybe required based on a parts complexity,
inspection volume or inspection hazards. Inspection systems
primarily manipulate the part to keep the inspection surface in the
proper orientation. All NDT techniques are candidates for an
inspection system. However, the systems are primarily
associated with EC, PT RT and UT inspections.
- Ultrasonic
- Ultrasonic (UT) inspection used a transducer to transmit a
mechanical vibration (UT wave) into the part under evaluation.
When the wave strikes a flaw perpendicular to the direction of the
wave, a portion of the wave is reflected back to the
transducer. By measuring the time between the
transmitted and received wave, the depth of the flaw is
determined. The strength of the returned signal relates to the
size of the flaw.
- Vibration
- Vibration (VIT) monitoring uses pizo-electric transducers to
measure mechanical displacement generated by vibrating
machinery. By analyzing the displacement signals, problems
within the rotating system can be detected.
- Visual
- Visual (VT) inspections encompass a variety of techniques that use
ads to assist in the visual inspection of the part under test.
Rigid, flexible and videoscopes, along with television system, are
typical of the equipment used in VT.
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