Briefly explain the limitations of visual
inspection and microscopic techniques.
The limitations of visual inspection are that it is not always possible
to visually see a fracture or damage as it may be beneath the surface.
Furthermore even when the damage is on the components surface it may be too
small to be seen by the human eye.
Limitations of the microscopic inspection technique include the
microscopes zoom restrictions, most microscopes used for NDT can only zoom 2, 3
or 4 times, so similarly to the visual inspection, the damage or fault may
still not be visible. The damage might also again be beneath the surface which
would not be visible under a microscopic examination, therefore it would not be
possible to detect internal imperfections. Microscopes also focus on a very
small area of a component so depending on the size of the part being examining
it using this technique may be very time and cost inefficient.
List the steps in dye penetrant technique.
Cleaning: Use a cleaning fluid such as a degreaser to remove any dirt,
oil or substance that could hinder the accuracy of the dye penetrant.
Application: Apply the white powder to give a contrast on the components
surface. Spray the penetrant on the component, removing any excess and allow to
Rinse: Rinse the test piece and allow to dry.
Developer: Apply the developer to the component and wait.
Inspection: The powder pulls out the liquid dye that would have soaked
into any voids or cracks so they are visible. Liquid then evaporates off and as
it turns to a powder the ink is drawn out.
In ultrasonic testing, how can you tell there is
a flaw in the material and how can you determine the location and size of
A flaw is determined in a material using ultrasonic testing when a part
of the sound energy that is propagated through the material reflects back from
the flaws surface. This causes acoustic abnormalities which are reflected to
the transducer, the data is then shown on the machines display as a pulse echo
on an intensity-time plot.
The location and size of the flaw can be determined by numerous signals
detected at different angles and their travel times. The ultrasonic machine is
highly accurate at generating the reflector position of the sound waves and
locating fractures in 3D on x, y and z axis.
Can you use magnetic particles technique to
inspect aluminium castings? Explain.
You cannot use the magnetic particles technique to inspect aluminium
castings as this method is restricted to ferromagnetic materials. Aluminium is
not a ferromagnetic material so it would not be possible to induce an
electromagnetic field around a component made from this material, which is what
this technique of NDT is based upon.
Briefly explain what Eddy current is and how it
can be used in NDT.
Eddy current operates on the fundamentals of electromagnetic induction,
where current loops are induced by an alternating magnetic field in a material
that is metallic.
Eddy current can be used in NDT as when an initial magnetic field
induces and eddy current into a material a secondary is generated in the polar
direction. The coil impedance changes are monitored and demonstrate any defects
in the material. Often an alarm on the machine is sounded when a defect is
The upper sections of the Blackpool Tower need
inspection to detect any possible surface and internal flaws developed
over the years. How can this inspection be done and which NDT technique
would you suggest and why?
The best NDT technique to detect any possible surface or internal flaws
would be ultrasonic testing. The structure itself is made from iron and steel
which both conduct sound which is suitable for this form of testing,
furthermore the ultrasonic vibration is capable of travelling long distances in
It would also allow the testing of the upper sections of the tower to be
conducted from the ground in a simple manner that is not heavily impacted by
the environmental factors. Although background noise would need to be taken
into consideration when interpreting the data ultrasonic testing is capable of
examining large scale areas with accurate size and locational results.
surface crack of 10 mm long is detected by dye penetrant testing in an
airplane component made of an aluminum alloy having a plain strain
fracture toughness of 25MPa. Is it safe to use the component
if the maximum applied stress is 100 MPa?
it is not safe to use the component as as K < . If K ? the crack would grow and eventually a fracture would happen. To calculate K: Where; K= stress intensity factor Y= 1.1 (if the crack is at the surface) s= maximum applies stress = 100 MPa = length of the surface crack = 10mm Therefore; K = 19.5 MPa