NDT vibration HUMS Flashcards
what is the definition of NDT
its a method of testing, examination or inspection that does not render, the material, equipment, system or structure unserviceable
aims of NDT
to improve flight safety by ensuring the structural integrity of the aircraft
to increase aircraft and equipment availability
to reduce servicing cost
what methods of NDT are there
visual and visually assisted
penetrant testing
eddy current testing
ultrasonic testing
radiographic testing
magnetic particle testing
where are NDT reports written up?
rendered on an MF 762 IAW map - 01
whenever a fault id found using an NDT L2 WI
at each radiographic inspection (xray)
when specifically required by a work instruction in the relevent aircraft topic 5G or equivalent publication
on first use of a provisional WI
when any change is found whilst monitoring an existing fault indication
what faults will NDT testing find
fatigue cracking
overload failure
sub surface flaws
corrosion
loose articles
incorrect assembly
secondary impact damage
residue build up on internal bores e.g. carbon
what are the action on finding a fault?
if a fault is found with an NDT technique IAW 5G - examination standard in the technique will state the acceptable fault size if any
fault found as casual inspection - topic 6 should be consulted for repair information, advice from station NDT team should be sought,
what are the two types of visual aids used for inspection
magnifiers (maginifiers can be used in accessible areas to find surface breaking faults)
remote viewing instruments
what is the construction / make up of an endoprobe
they consist of a complex optical system surrounded by a fibre light guide,
contained in a tube of plated brass/stainless steel
the viewing end of the probe is fitted with an eye piece cup and a right angled sleeve to accept the end fitting of the light guide
at the objective end are the light outlet and objective lenses
individual probes give one angle of view and are reliable to give forward, oblique, lateral or retrograde view
fibrescope construction
fully flexible instruments used where complexity of shape precludes the use of rigid endoprobes,
they consist of two bundles of glass fibre containing tens of thousands of strands,
the light guide bundle of non- coherent strands transmit light from a light source box to the object being viewed
the image guide is a coherent bundle, which transmits the image of the viewing eyepiece,
the distal end can be angled with operation of the angular controls
what is the basic PT(penetrant testing) procedure
clean
apply penetrant and allow contat time
remove excess penetrant
apply developer and allow development time
examine indications
record
clean and restore surface protection
what are the advantages of PT
carried out by squadron/unit personnel after further specialist trainng (TEM)
results available after specified time
PT disadvantages
surface finish has to be removed
liquids used are de-fatting agents
liquids used are flammable
this method can only detect surface breaking flaws
can only be used in accessible areas
time consuming
what are the principles of magnetism
if a piece of ferrous material has been magnatised there is an area of energy surrounding the material known as a magnetic field,
this field is made up of a series of lines of force that flow from one pole of the magnet to another
they are characterised as being elastic in nature and tend to contract
they never cross and are defined by other lines of force
they form a complete magnetic circuit running from north poles to south poles outside and south to north inside the magnet
what is the theory of magnetic flaw detection
if a magnetic force is applied to a closed circuit of magnetic material the lines of force will flow in parallel lines from one side to the other.
if a flaw occurs cutting the lines of force, it will form “free poles”
magnetic particles applied to the surface of the components will follow the lines of force and will concentrate around the srongest area of attraction, the area of free poles, the gathering of particles will indicate flaws
what is the detecting media used in magnetic detection
minute particles of oxide iron suspended in kerosene
why must demagnatisation of material being tested be carried out and when
components must be demagnetised before, intermediately (when changing the direction of magnetic flux) and after testing
initial and intermediate testing is carried out to rid of any stray residual magnetism that would give spurious readings
final de-magnatisation is necessary so not to cause a reduction in accuracy of aircraft instruments
stray voltages dont induce into adjacent electrical circuits
adherence of metalic partical to moving parts that could cause excessive damge and wear
what is the test procedure of maganetic detection
clean and degrease components
fully demagnitise the components
magnetise using the selected method
apply detecting ink and view for flaw indications
demagnetise
if serviceable, prepare the component for service
what are the advantages of ultrasonic testing
ultrasonic is a very sensitive detection method
can be used on both metallic and non-metalic materials, penetrating to many feet in steel and many metres in aluminium
briefley describe the concept of UT operation
the pulsed echo method is uded to detect flaws within a component
a focused pulsed beam of high energy sound is transmitted into dense material
ultrasound attenuates in air this will cause the wavefront to be refelcted back
pulses of highh frequency (0.5-20MHz) are introduced into a material
the signal will reflect back from the boundary of the material and any flaws and will show up on the display
what couplants (UT) are used and why do we use them?
ultrasound cannot travel through air so we use couplants to aid this
fluorescent orange - for metals (thixotropic)
blue gel - for composites
bright orange - thicknes gauging applications
list the correct applications of ultrasonic testing
major structures
large forgings/casting
bolts
around bolt holes
describe the theory of eddy currents
an alternating current is passed through a coil this causes a magnetic field to form in the vicinity of the AC coil,
the polarity of the magnetic field will reverse at the same frequency as the AC that produces it,
when the probe is placed near a conducting medium the primary field and medium couple (magnetic coupling)
this causes a magnetic field to flow at 90º to the primary field, causing circulating induced magnetic currents (eddy currents)
the eddy currents produce a secondary magnetic field that opposes and modifies the primary field
how is the fault interpreted when using eddy currents testing
as the probe is moved towards the material being tested, any flaw will cause the eddy current to be distort, this will alter the strength of the secondary field, causing the strength of the primary field to alter
any changes in the primary field cause the AC to change in the coil and show an indication on the instrument
state the principles of eddy current testing
in aerospace application the coil is normally incorporated into the tip of a probe,
connected to a suitable instrument to balance the primary and secondary magnetic fields,
this will cause the probe when moved to indicate any changes in eddy currents due to flaws to indicate an imbalance in magnitude of the magnetic fields
these changes can then me displayed electronically
