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, reactors to 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 material
to be inspected and measured without damaging them. Because it allows
inspection without interfering with a product's final use, NDT provides
an excellent balance between quality control and cost-effectiveness. Generally
speaking, NDT applies to industrial inspections. The technologies that are used
in NDT are similar to those used in the medical industry, but
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, an 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.
Many people are already familiar with some of the technologies
that are used in NDT and NDE from their uses in the medical industry.
Most people have also had an X-ray taken and many mothers have had ultrasound
used by doctors to give their baby a checkup while still in the womb.
X-rays and ultrasound are only a few of the technologies used in the field
of NDT/NDE. The number of inspection methods seems to grow daily, but
a quick summary of the most commonly used methods is provided below.
Visual and Optical Testing (VT)
The most basic NDT method is visual examination. Visual examiners follow
procedures that range from simply looking at a part to see if surface
imperfections are visible, to using computer controlled camera systems
to automatically recognize and measure features of a component.
RT involves using penetrating gamma- or X-radiation on
materials and products to look for defects or examine internal or hidden features. An X-ray generator
or radioactive isotope is used as the source of radiation. Radiation is
directed through a part and onto film or other detector. The resulting
shadowgraph shows the internal features and soundness of the part. Material
thickness and density changes are indicated as lighter or darker areas
on the film or detector. The darker areas in the radiograph below represent internal
voids in the component.
Magnetic Particle Testing (MT)
NDT 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 flaws disrupt the flow of the magnetic field within the part and force some of the field to leak out at the surface. Iron particles
are attracted and concentrated at sites of the magnetic flux leakages. This produces a visible indication of
defect on the surface of the material. The images above demonstrate
a component before and after inspection using dry magnetic particles.
Ultrasonic Testing (UT)
ultrasonic testing, high-frequency sound waves are transmitted into
a material to detect imperfections or to locate changes in material
properties. The most commonly used ultrasonic testing technique is pulse
echo, whereby sound is introduced into a test object and reflections
(echoes) from internal imperfections or the part's geometrical surfaces
are returned to a receiver. Below is an example of shear wave
weld inspection. Notice the indication extending to the upper limits
of the screen. This indication is produced by sound reflected from a
defect within the weld.
Penetrant Testing (PT)
With this testing method, the
test object is coated with a solution that contains a visible or fluorescent
dye. Excess solution is then removed from the surface of the object
but is left in surface breaking defects. A developer is then applied
to draw the penetrant out of the defects. With fluorescent dyes, ultraviolet
light is used to make the bleedout fluoresce brightly, thus allowing
imperfections to be readily seen. With visible dyes, a vivid color
contrast between the penetrant and developer makes the bleedout
easy to see. The red indications in the image represent a defect
in this component.
Electromagnetic Testing (ET)
There are a number of electromagnetic testing methods but the focus here will be on eddy current testing. In eddy current testing, electrical
currents (eddy currents) are generated in a conductive material by a
changing magnetic field. The strength of these eddy currents can be
measured. Material defects cause interruptions in the flow of the eddy
currents which alert the inspector to the presence of a defect or other change in the material. Eddy
currents are also affected by the electrical conductivity and magnetic
permeability of a material, which makes it possible to sort some materials
based on these properties. The technician in the image is inspecting an aircraft
wing for defects.
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, or simple soap-bubble tests.
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 and arrival time to collect information (such as their location) about the sources
of the energy.