testing testing materials testing and structural testing destructive or
non-destructive the difference is clear-cut destructive testing leaves you
without a serviceable component or test specimen once the testing is over the
sharply test the vend test the tensile test this chassis box section this pipe
section and this car they're all destroyed in the testing process
well that's no use of your structure is in daily service so that's where
non-destructive testing comes in
in this short introduction to non-destructive testing we're going to
be looking at half a dozen of the most common types of NDT and also take a
glance at a few of the less traditional newcomers to the field well let's get
started with magnetic particle inspection we use MPI as it's known to
find surface and just beneath the surface discontinuities in ferromagnetic
materials and we do it by introducing a magnetic flux into the material an ink
containing extremely fine ferrous particles is applied to the surface of a
ferromagnetic material whilst a magnetizing field is applied floors
cause leakage of magnetic flux which creates new magnetic poles so the
ferrous ink particles are drawn to the site of the flux leakage field
disturbance and the inspector sees it there are a couple of types of MPI color
contrast and fluorescent with color contrast a very thin layer of white
contrast paint is applied magnetic force is also applied and black ink spray on
the surface below the excess away and any defects or discontinuities show up
black against white with the fluorescent technique we coat the component under
inspection with a fluorescent medium while the part is magnetized and
a fluorescent lamp discontinuities or defects show up vividly as bright green
it's a more sensitive technique than the color contrast method now liquid
penetrant inspection works on non ferromagnetic materials for locating
surface defects only clean and degrease the component then apply a brightly
colored penetrant which in due course is drawn into the surface breaking defects
after a specified period the excess penetrant is wiped off and a powder
developer applied it works rather like blotting paper drawing the penetrant out
of the flaws so that they stand out in sharp contrast to the surrounding
material after a specified bleed time the defects are recorded now radiography
is an entirely different type of non-destructive testing
x-rays are produced by directing a stream of high velocity electrons across
an evacuated chamber against an angled target material the x-ray is passed
through almost every material and can be detected by a sensitive film on the
other side however a high density material will absorb x-rays while low
density materials allow a greater proportion of them to pass through for
radiographic inspection there's no need for surface preparation before testing
but you do need to have access to both sides of the component the film which is
highly sensitive to x-rays is placed on one side and the x-ray source is
carefully located opposite the x-ray is passed through the component and create
a latent image on the film on the other side any volumetric flaws or
discontinuities in the component allow more of the radiation onto the film and
this creates a darker image on the radiograph in recent years wet
radiography which uses photographic films and traditional darkroom
technology has been overtaken by digital radiography now digital radiography
relies on the ability of certain substances to fluoresce when they are
exposed to x-radiation the fluorescence that's emitted from the substance is
made to create an analog electronic signal which can be converted to a
digital signal by means of an analog to digital converter
during testing a fluorescent coated surface is placed behind the test
specimen as the radiation passes through the specimen it causes different levels
of fluorescence to occur on the coated surface depending on the dosage received
and this is controlled by the specimen geometry this is then converted to a
digital signal and displayed on a screen
microfocus test equipment differs from standard
radiography kit in that the focal spot size is extremely small typically in the
micron range the benefit is that it produces radiographs with extremely high
definition the equipment lends itself well to producing large magnification
images while retaining good definition of tiny defects some of the benefits of
digital radiography over wet radiography are obvious there's no need for chemical
processing the exposure to image viewing time is shorter the images are easily
stored on a hard drive and they can be easily manipulated and enhanced on
computer well now let's look at ultrasonic testing UT as it's known is
actually a generic title for several different methods of generating and
observing ultrasonic signals they're all different but use the same basic
principle they all use a signal above the audible frequencies hence the word
ultra sonic this is passed from a probe into the component under investigation
the signal bounces or is reflected when it comes across a discontinuity if we
can measure how long the signal takes to return
knowing the speed at which it travels we can tell where the discontinuity is
time-of-flight diffraction another type of UT is used mainly for sizing plane
our floors orientated perpendicular to the surface
however it's less reliable when there's a high density of defects when the
material contains scattered inclusions and also with coarser grained materials
Toft is widely used in the petrochemical and nuclear industries for the
inspection of butt wells in pressure vessels and process pipe work it's often
used to provide critical floor sizing data for input into engineering critical
assessments phased array systems work quite differently they offer the
possibility of performing inspections with ultrasonic beams of various angles
and focal lengths using a single array of transducers so what's it used for
well finding the same manufacturing flaws like lack of sidewall fusion lack
of root penetration lack of root fusion and porosity in service floors like
fatigue cracking stress corrosion cracking
corrosion and erosion and parent material flaws like
inclusions and lamellar tearing only it does it quicker and with a reduced
amount of scanning a specific application of phased arrays lies in
pipe girth weld inspection particularly in installation of long pipe lines where
it replaces conventional systems with many probes this technique is also used
for the rapid inspection of thick section pressure vessel welds turbine
disks shafts and blade routes Filat worlds and
welds in coarse-grained materials so now to surface ultrasonics it tends to be
used for the detection of environmental cracking in petrochemical plant and also
fatigue cracks in civil engineering structures like bridges as the name
suggests surface waves travel along the surface of components penetrating to a
depth of only one ultrasonic wavelength it's used to find Plano floors that
break the surface and lie at 90 degrees to the scanned surface such as fatigue
cracks in Weld toes and also stress corrosion cracking finally long-range
guided wave ultrasonics make you search low-frequency guided waves to detect
corrosion and erosion in pipe work the main advantage of the technique is that
it provides 100% initial screening coverage of a long length of pipe and
only requires local access to the pipe surface at those points where the
transducer array is attached it's suitable for use on pipe diameters above
50 millimetres and on wall thicknesses up to 40 millimetres
it's used specifically to find both internal and external corrosion and also
erosion in thermally insulated coated and buried pipe work corrosion under
pipe supports and hidden welded joints so it finds its niche in petrochemical
process pipe work oil and gas transmission lines gas manifolds
offshore risers jetty lines and power station boiler tubes now to eddy current
testing in standard eddy current testing a circular coil carrying an alternating
current is placed close to an electrically conductive specimen the
alternating current generates a changing magnetic field which interacts with the
test object and induces eddy currents standard eddy current testing is
essentially a near-surface technique it's useful for detecting surface
breaking or near surface cracking and variations in material composition eddy
current methods are used for surface inspection of both non ferritic and
ferritic materials eddy current penetration is deeper on non ferritic
materials so it's also used for special applications with thin components such
as in the inspection of multiple layer airframe components for cracking or
corrosion thinning lastly to thermography
thermography is the simplest of all thermal inspection techniques it
involves using an infrared camera to look for abnormally hot or cold areas on
a component operating under normal conditions most modern thermal cameras
are able to detect temperature differences of less than a quarter of a
degree Celsius and they enable thermal images to be recorded on video or
computer disk for future analysis thermography is a remote technique for
use in most areas of plant but is particularly suited to equipment
operating at elevated temperatures active thermography where heat is
applied directly to the component is used for inspection of low conductivity
materials like composites well thermography just about winds up
this brief introduction to the popular non-destructive testing techniques to
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