Chapter 4: NDT Flashcards
what is NDT?
testing of materials that will not cause the destruction on the material or impair its designed use
why is NDT used?
test piece:
- cannot be destroyed
- reused after inspection
- is in service
what is visual testing?
- viewing the material with naked eyes
- only surface defects may be seen
- reliability depends on inspector
- most common form of testing
tools to aid visual testing
1) magnifying glass / mirrors
2) borescope
3) fibrescope
what does optical inspection probes do? and what are the 2 types?
permits visual inspection of limited access areas (ducts, cavities, or pipes)
types:
- rigid
- flexible
what is hydrostatic testing used for?
- check for leakage in components subject to pressure
- water at 1.5 - 2 times working pressure is used (helium gas is used for high sensitivity)
- leakage can be detected by seepage or pressure change
what is dye penetrant testing used for?
- check for defects open to the surface, small cracks or discontinuities
- suitable for use on all homogenous (same kind) material
types of dye penetrants and how its viewed
1) visible dye - under white light
2) fluorescent dye - under UV light (aka black light)
methods of penetrant removal
1) water-washable (least sensitive)
2) post-emulsified (most sensitive, emulsifier makes dye water soluble)
3) solvent-removable (dye can be removed by using a solvent)
steps for liquid penetrant testing
1) pre-cleaning
2) penetrant application
3) excess penetrant removal
4) developer application
5) inspection & interpretation
6) post-cleaning
types of pre-cleaning cleaner for liquid penetrant testing
1) solvent: in aerosol spray cans
2) ultrasonic: used limited to small parts
3) alkaline: not for Al alloys
ways to apply penetrants
1) spraying
2) immersion/dipping
3) brushing (localised inspection, only thin layer should be applied)
advantages of using brushing to apply penetrant
- provides greater control
- thickness of penetrant layer can be regulated
- reduces chances of overspray
how to remove penetrant using water-washable remover?
water pressure not too high, spray nozzle should not be too close to surface
how to remove penetrant using solvent removable remover?
by hand wiping using a solvent-dampened cloth, wipe in 1 direction only
how to remove penetrant using post-emulsified remover?
- emulsified is applied after application of penetrant
- allow time for emulsification to take place (around 2 minutes max)
- excess dye on surface are soluble and can be removed by water
when to apply developer and what does it do?
when? - after excess penetrant has been removed
purpose:
- enhance penetrant bleed-out to form indications
- provide good contrast to the entrapped penetrant indications
types of dye penetrant developer
1) dry - fine dry powder
2) non-aqueous wet developer (solvent developer) - powder applied as a suspension in a quick-drying solvent
3) wet - powder applied as a suspension or solution in a liquid
ways to apply developer
dry:
- electrostatic spray
- dust storm cabinet
non-aqueous:
- aerosol spray can
procedure of inspection, interpretation and evaluation
inspection - part viewed under UV/white light depending on dye
interpretation - determine whether indication is relevant or not
evaluation - taking notes of category, location and size
what will appear during dye penetrant testing of defects?
- porosities will show as glowing spots
- cracks will show as fluorescent lines
why post-cleaning is needed in dye penetrant testing?
as residues from penetrant may affect future performance and developer can cause corrosion
how to post clean for water-washable and post-emulsified methods?
- water spray (big parts)
- immersion tanks (small parts)
how to post clean for solvent-removable method?
hand wiping with cloth dampened with solvent remover
pros & cons of penetrant testing
pros:
almost all materials can be tested
not affected by defect orientation
cheap & simple
cons:
only for surface defects
no information on depth of flaw
rough surface hard to test
what is magnetic particle testing used for? and for which materials?
MPT is used to check surface/subsurface defects
only for ferromagnetic materials
what are considered ferromagnetic materials?
significant iron, nickel or cobalt content materials
how does magnetic particle testing work?
a ferromagnetic component is magnetised with a strong magnetic field
if component has discontinuity, magnetic lines of force will be interrupted and leakage field will occur
how to see defects in magnetic particle testing?
finely milled iron particles coated with dye pigment are applied
they are attracted to flux leakage to form an indication (bunched up)
what angles of defect orientation will be detected in magnetic particle testing and what are not?
cracks between 90° and 45° to the magnetic field lines are detected
cracks parallel to field lines are usually undetected
magnetic particle pre cleaning points
test surface should be clean and dry
contaminants may prevent particles from being attracted to the leakage fields
types of direction of magnetic field in MPT
longitudinal magnetic field: parallel to the long axis of part
circular magnetic field: circumferentially around the perimeter of part
ways to generate longitudinal magnetic field
permanent magnet
electromagnetic yoke
solenoid coil
how does solenoid coils work in MPT
solenoid coils shows longitudinal magnetic field for detection of transverse crack
ways to generate circular magnetic field
direct induction
prod contacts
types of current in MPT
direct current - flows in constant direction
alternating current - reverses direction of flow are regular freq.
rectified current - rectifies ac to full wave and half wave
what is used for detection of any flux leakage through accumulation of particles?
a medium
types of magnetic particles
dry magnetic particles
wet magnetic particles
properties required by magnetic particles
held by weak leakage field
high in permeability
low in retentivity
easily attracted
what are dry magnetic particles? and how to apply them?
they are powder form and comes in colours for high level of contrast
applied by:
hand shaker
spray bulb
air stream
what are wet magnetic particles and how to apply it?
they are particles suspended in light oil that are coloured or fluorescent
more sensitive as the liquid gives extra mobility to particles
applied by:
dipping
immersing
spraying
how to interpret MPT?
accumulation of particles on surface means discontinuity under suitable lighting conditions
why is demagnetisation required after MPT?
may have residual magnetic field that can interfere with future operation of parts
reasons:
interfere with machine operations
adhere of magnetic particles which speeds up wear
adverse effect on instruments
induces stray voltage nearby circuits
how to demagnetise a part with residual magnetism after MPT?
subjecting it to alternating magnetic field which is decreasing in strength
what equipment is used to demagnetise a part? and how to check if is demagnetised already?
demagnetisation unit that scrambles the magnetic domains and reduces strength of residual field
measured by a field indicator
magnetic flux 2 Gauss and below is good
advantages of MPT
1) most dependable & sensitive method for surface and subsurface defects
2) fast, simple & cheap
3) direct visible indication on surface
4) can inspect parts with irregular shapes easily
5) can be used on painted objects
disadvantages of MPT
1) only good for ferromagnetic materials
2) subsurface defects may not always be detected
3) magnetic field and defect line relative direction is important
4) objects must be demagnetised after testing
what is radiographic testing used for?
verify the internal structure and integrity of specimen or welding
like medical x-ray
what is radiographic testing
imaging technique by using short wavelength electromagnetic radiation
2 sources of radiation used for radiographic testing and its differences
x-radiography:
controllable
fixed power
longer λ = 0.01 - 10 Å (less penetrating)
gamma (γ) radiography:
uncontrollable
variable power
shorter λ = 0.005 - 0.1 Å (more penetrating)
how are x-rays produced?
x-ray generator system
consisting of x-ray tube head, high volt generator, control console
how are γ-rays produced?
decomposition of radioactive substance called radioisotope
what is radioactive decay?
spontaneous breakdown of an atomic nucleus resulting in the release in energy
how does energy of radiation affect penetrating power?
higher energy radiation means penetrate thicker and more dense materials
what is an x-ray film?
cellulose material with coating of photosensitive emulsion
why are intensifying screens used? and what materials are used for it?
as only 1% of x-rays are used to expose the film, it can help intensify those that have penetrated, improving film image
lead or salt screens
what is film density? and how is it determined?
degree of film darkness
determined by amount of light transmitted through the film
formula for film density
D = log10 (Io/It)
D - Density
Io - incident light intensity
It - intensity of transmitted light
formula for film density
D = log10 = (Io
/ It)
D - Density
Io - incident light intensity
It - intensity of transmitted light
formula for film density
D = log10 = (Io
/ It)
D - Density
Io - incident light intensity
It - intensity of transmitted light
formula for film density
D = log10 = (Io
/ It)
D - Density
Io - incident light intensity
It - intensity of transmitted light
formula for film density
D = log10 = (Io
/ It)
D - Density
Io - incident light intensity
It - intensity of transmitted light
formula for film density
D = log10 = (Io
/ It)
D - Density
Io - incident light intensity
It - intensity of transmitted light
what is film contrast?
differences in film densities on different parts on the radiographic image
what is film definition?
sharpness of the outline of the radiographic image
how is film definition affected?
1) film unsharpness (Uf)
2) geometric unsharpness (Ug)
how can Ug be minimised?
1) reduce FF’
2) reduce t (specimen to film distance)
3) increase d (source to specimen distance)
4) rays should be directed perpendicular to the film surface
5) film and specimen should lie in parallel planes
what is radiographic sensitivity? and how it is measured?
measures the smallest defect that can be detected on the radiographic film
measured by image quality indicators (IQI) or penetrameters
types of image quality indicators (IQI)
1) wire IQI
2) step hole IQI
radiographic sensitivity formula
sensitivity = (thinnest diameter observed / specimen thickness) * 100%
advantages of radiography
1) can be used on most materials
2) provide permanent record of inspection
3) reveal internal nature of material
4) sensitive to change in thickness, corrosion, flaws
disadvantages of radiography
1) expensive
2) high level of skill required for interpretation
3) safety considerations by X & gamma rays
4) specimen must have 2-side accessibility
what is ultrasonic testing
use of high-frequency sound energy to examine parts and make measurements
what device is used to generate and receive ultrasound?
ultrasound transducer (probe)
how is ultrasound generated? and what are materials used?
by piezo-electric materials, it converts electrical energy to mechanical energy and vice-versa
piezo-electric material:
quartz crystal
barium titanate
lithium sulphate
Types of ultrasonic transducers
1) normal beam contact (flaws parallel to surface)
2) angle beam contact (flaws not parallel to surface)
types of ultrasonic testing
1) pulse-echo - 1 probe acts as transmitter and receiver
2) through transmission - 2 probes are used, 1 as transmitter and 1 as receiver
pulse-echo vs through transmission ultrasonic method
pulse-echo: has a crack echo in the middle of the graph if there is a defect (exact location determined)
through transmission: pick up signal (end signal) will be lower in amplitude if there is defect present (no depth information)
advantages of ultrasonic testing
1) sensitive to small defects on both surface or internal
2) equipment is highly portable or automated
3) depth of penetration for detection is superior than other methods
4) high accuracy in determining defect position
limitations of ultrasonic testing
1) surface must be accessible to transmit ultrasound
2) required coupling medium to provide transfer of sound energy into specimen
3) linear defects oriented parallel to the sound beam may go undetected
4) high level of skill needed
what is eddy current testing used for?
it make use of electromagnetic induction to detect and characterise surface and subsurface flaws in conductive materials with smooth surface
what are eddy currents?
induced electrical currents that flows in a circular path
how does defects affect eddy currents?
path of eddy currents will be distorted by it, which will change the apparent impedance of the coil
depth of penetration by eddy currents
strongest at the surface and decreases exponentially below the surface (skin effect)
what is the standard depth of penetration?
1/e or 37% (skin depth)
factors affecting eddy current
1) magnitude and frequency of AC current
2) conductivity, permeability & shape of specimen
3) relative position of coil to specimen
4) presence of defects
equipments needed for eddy current testing
1) instrumentation
2) test coils/probes
3) calibration block
probes classification for eddy current testing
1) concentric / encircling probe
2) point probe / surface coil
3) internal bobbin type coil
probes used for different specimen
concentric probe: round bars, rods, wires, tubes
point probe: confined areas (small)
internal bobbin probe: boiler, heat exchanger, steam generator tubes or oil pipes
application of eddy current testing
1) crack detection
2) material & coating thickness measurements
3) conductivity measurements for:
material identification
heat damage determination
case depth demetermination
advantages of eddy current testing
1) sensitive to small cracks and defects
2) detects surface and subsurface defects
3) equipment is portable
4) part preparation is minimum
disadvantages of eddy current testing
1) only for conductive materials
2) surface must be accessible to probe
3) more skill needed
4) surface must be quite smooth
5) flaws parallel to scan direction will be undetected
