The field of Nondestructive Testing (NDT) is a very
broad, interdisciplinary field that plays a critical role in assuring
that structural components and systems perform their function in
a reliable and cost effective fashion. NDT technicians and engineers
define and implement tests that locate and characterize material
conditions and flaws that might otherwise cause planes to crash,
fail, trains to derail, pipelines to burst, and a variety of less
visible, but equally troubling events. These tests are performed
in a manner that does not affect the future usefulness of the object
or material. In other words, NDT allows parts and materials to be
inspected and measured without damaging them. Because it allows inspection
without interfering with a product's final use, NDT provides an
balance between quality control and cost-effectiveness. Generally
speaking, NDT applies to industrial inspections. While technologies
are used in NDT that are similar to those used in the medical industry,
typically nonliving objects are the subjects of the inspections.
Nondestructive Evaluation (NDE) is a term that is often
used interchangeably with NDT. However, technically, NDE is used to
describe measurements that are more quantitative in nature. For example,
a NDE method would not only locate a defect, but it would also be
used to measure something about that defect such as its size, shape,
and orientation. NDE may be used to determine material properties
such as fracture toughness, formability, and other physical characteristics.
Take this link to learn
about the background of NDT and NDE
The number of NDT methods that can be used to inspect
components and make measurements is large and continues to grow. Researchers
continue to find new ways of applying physics and other scientific
disciplines to develop better NDT methods. However, there are six
NDT methods that are used most often. These methods are visual inspection,
penetrant testing, magnetic particle testing, electromagnetic or eddy
current testing, radiography, and ultrasonic testing. These methods
and a few others are briefly described below.
Visual and Optical Testing
Visual inspection involves using an inspector's eyes to look for defects.
The inspector may also use special tools such as magnifying glasses,
mirrors, or borescopes to gain access and more closely inspect the
subject area. Visual examiners follow procedures that range from simple
to very complex.
Test objects are coated with visible or fluorescent dye solution.
Excess dye is then removed from the surface, and a developer is applied.
The developer acts as blotter, drawing trapped penetrant out of imperfections
open to the surface. With visible dyes, vivid color contrasts between
the penetrant and developer make "bleedout" easy to see.
With fluorescent dyes, ultraviolet light is used to make the bleedout
fluoresce brightly, thus allowing imperfections to be readily seen.
This NDE method is accomplished by inducing a magnetic field in a
ferromagnetic material and then dusting the surface with iron particles
(either dry or suspended in liquid). Surface and near-surface imperfections
distort the magnetic field and concentrate iron particles near imperfections,
previewing a visual indication of the flaw.
Testing (ET) or Eddy Current Testing
Electrical currents are generated in a conductive material by an
induced alternating magnetic field. The electrical currents are
currents because they flow in circles at and just below the surface
of the material. Interruptions in the flow of eddy currents, caused
by imperfections, dimensional changes, or changes in the material's
conductive and permeability properties, can be detected with the
Radiography involves the use of penetrating gamma or X-radiation to
examine parts and products for imperfections. An X-ray generator or
radioactive isotope is used as a source of radiation. Radiation is
directed through a part and onto film or other imaging media. The
resulting shadowgraph shows the dimensional features of the part.
Possible imperfections are indicated as density changes on the film
in the same manner as a medical X-ray shows broken bones.
Ultrasonics use transmission of high-frequency sound waves into a
material to detect imperfections or to locate changes in material
properties. The most commonly used ultrasonic testing technique is
pulse echo, wherein sound is introduced into a test object and reflections
(echoes) are returned to a receiver from internal imperfections or
from the part's geometrical surfaces.
Acoustic Emission Testing (AE)
When a solid material is stressed, imperfections within the material
emit short bursts of acoustic energy called "emissions."
As in ultrasonic testing, acoustic emissions can be detected by special
receivers. Emission sources can be evaluated through the study of
their intensity, rate, and location.
Leak Testing (LT)
Several techniques are used to detect and locate leaks in pressure
containment parts, pressure vessels, and structures. Leaks can be
detected by using electronic listening devices, pressure gauge measurements,
liquid and gas penetrant techniques, and/or a simple soap-bubble test.