Non Destructive Testing Flashcards
(30 cards)
What are examples of destructive testing?
Tensile testing
Compression testing
Shear testing
Impact testing
Hardness testing
When are defects looked for?
When its a raw material, a component (off a production line) or in service
Where are defects seen and at what stage of the material processing?
Manufacture, raw materials, castings - Shrinkage porosity, Gas porosity, slag inclusions, segregation
Manufacture - Machining faults, heat treatment defects, welding defects, residual stress defects
Assembly - Missing parts, incorrect assembly, welding defects, stress cracking
Service - Fatigue, corrosion (+corrosion fatigue), stress
corrosion cracking, wear, creep, thermal instability etc…
What are defects that can be seen in welds?
Cracks, slag inclusions (oxide pulled into material), lack of fusion, porosity (not a proper flux or shield gas used), undercut(not complete filling on the top), poor penetration(not correct filling on the bottom), burn through, under fill, excess reinforcement, spatter(the material moves on either side of the weld), overlap, root undercut
How can cracks in welds be categorised?
Types I–III mainly associated with the fabrication of a joint
(occasionally long-term
creep). Type II not seen on surface.
Type IV(not seen on surface) almost exclusively creep related.
What are the main stages of inspection?
1) Visual
a) Naked eye - can see broken components can assess why, see the fracture surface
b) Stereo microscope
c) Probes
How to assess which NDT technique to use?
- Types of flaws
- Efficiency of detecting flaws?
- Are cracks < critical detectable?
- Monitor crack growth
- Periodic
- Continuous
- Margin of safety?
- Quality of inspection
- Cost?
What are the principles of dye penetrant inspection?
Using 3 spray cans where 1 is a cleaning fluid, the next is the dye itself and the third is a developer (powder spray e.g chalk/talc).
The process:
1. using a cloth and the cleaning fluid to clean surface
2. apply dye
3. wait for dye to penetrate capillary action
4. wash surface of dye (water) with cloth. make sure the dye is not removed from crack.
5. Apply the developer so that
6. bleed back of dye into developer on the surface
7. observation - maybe with strong light or UV if it’s an active dye
*if crack is all the way through do dye on one side and the developer on the opposite side to see the dye come out the other side
What are advantages of dye penetrant testing?
ADVANTAGES
* Easy to use technique
* Cheap
* Used on all non-porous surfaces
* More sensitive than magnetic particle inspection
* No shape problems
* In-situ testing at any fabrication stage
What are disadvantages of dye penetrant testing?
DISADVANTAGES
* Surface defects only
* Surface roughness/porosity problems
What is magnetic particle inspection?
Detects surface defects in Ferro-magnetic components. When the field is applied
magnetic discontinuities that lie in the directions ~ perpendicular to the field will
results in a strong LEAKAGE FIELD.
Field can be detected by using fine magnetic particles, dry or wet.
electro magnets are preferred as the direction can be controlled.
Collect at the point of the discontinuity forming a magnetic bridge which indicates
the location, size and shape of the defect.
Can use either a permanent magnet for electro magnets or by passing electrical
current through/around the component.
What is the application method for magnetic particle inspection?
Dry – cloud from ‘blower’ e.g. magnetite Fe3O4
Wet – ‘stationary’ process usually petroleum based carrier
How does magnetic particle inspection work?
You have a bar sample, the current is passed down the centre of the par to create a circular magnetic field around the part(circular). If the coil is around the outside of the bar then you get a linear magnetic field through the centre(longitudinal).
This spots crack as the electric field would go around the crack so there is a field leakage both inside and to the outside (surface) of the bar as it splits around it.
What is the difference between a circular magnetic field and a longitudinal magnetic field?
Any cracks in the same direction of the magnetic field will not be picked up by a circular magnetic field. If it is a wigglyer line but in same direction is might be picked up. the strongest showing one will be perpendicular to direction of field.
It is the opposite for a logitudinal as this is the opposite direction but same principles apply. Same direction, no show, opposite direction (90 deg) big show.
what are the different types of electrical current ? and what is there benefit?
- Alternating (AC) – magnetic field strongest on surface.
Surface defect sensitive. - Pulsed direct current (HVDC) – rectified single phase AC. Gives a better depth of that current.
Magnetised in depth - Direct current (DC) – penetrating. Large stationary tests. even deeper depth.
What are the advantage of magnetic particle inspection?
ADVANTAGES
* Dependable/sensitive
* Simple/Rapid
* Inexpensive
* Sub-surface flaws may be detected
* Flaws directly visible
* Unaffected by ‘contaminated’ flaws
* No special surface preparation
What are the disadvantages of magnetic particle inspection?
DISADVANTAGES
* Ferromagnetic materials only
* Deep defects not always highlighted
* Direction of magnetic fields
* Burn scars
* Interpretation of results
What is the process of magnetic particle inspection?
- clean sample (solvents, wire brush, sand blasting etc)
- demagnetise the material (depending if already been or is too magnetised )
- add contrast dye (pre paint with a contrasting paint or suspension of ferromagnetic particles) (depending if already been if so not this step )
- magnetise this component - consider the direction of the defect
How can eddy currents be used to detect defects?
Eddy currents look at how the defect in the material affect the magnetic field in the probe.
Eddy currents and/or magnetic effects are induced within the material of component and from this deductions can be made of the nature and condition of
the test piece.
What can eddy currents be used to detect?
Surface and sub-surface defects
Thickness of coatings
Information on structural features i.e. grain size, heat treatment condition
Physical properties inc. electrical conductivity, magnetic permeability, hardness
what are main problems with eddy currents?
It is not material or shape/geometry independent and so relationships for
particular shapes and defect etc.
Usually used as a quality assurance technique in industry
How does the eddy current NDT method work?
A coil carrying alternating current is placed in proximity to a conductive sample
and a secondary or eddy current is induced in the sample. If there is a crack/defect in the material the distribution of eddy currents will change by an interaction of the fields causing a back emf in the coil which can be used to
detect the flaw. The impedance of the coil can be determined by measuring the voltage across it. Changes can be indicated on a meter, chart recorded or display screen. This can also measure the depth and thickness(penetration depth) of the crack dependent on both the frequency of the voltage and the material.
What is ultrasonic testing?
Sound with a frequency much higher than audible sound = ultrasound
-Widely used technique
* Internal defects and small surface cracks
* Quality control and in-service testing
SOUND
Audible : 20Hz – 20kHZ
Ultrasonic (NDT) : 0.5 MHz – 20MHz
Wavelength
λ = v/f
v = velocity f = frequency
Sound waves only reflected if :
Object in their path is larger (or equal to) their wavelength
How can ultrasound be generated?
Piezo electric material (transducer crystal) + alternating current
Electromechanical transducers (disc):
- quartz
- barium titanate
- lead metaniobate
- lead zirconate
Disc generates compression wave normal to the disc in the surrounding
medium. Wave is reflected back to the receiver (transmitter as well)
Ultrasound generated as extremely short pulses.
Disc can also act as a receiver – sound waves stimulate alternating
current across crystal faces.
