AMT 1090 NDT
Non-Destructive
Testing
Chapter 11
Section “A”
Non-Destructive Testing

What is NDT? (also called NDI)

Inspection techniques developed to allow
inspection without disassembly or
destruction of the part.
Non-Destructive Testing

Why do we perform NDT?

Because airlines could not afford to
ground an aircraft for long periods of time
for maintenance or inspection, new ways
of inspection had to be developed. NDI
saves time and money!
Non-Destructive Testing
We are going to discuss various methods
of NDT, including; visual, liquid penetrant,
magnetic particle, eddy current, ultrasonic,
and radiographic (x-ray) inspections.
Visual Inspections

Basic Inspections:
 Most
fundamental method of inspecting
aircraft structures.
 The part must be made visible…
Visual Inspections

Basic tools required are:
 Good
light
 Mirror
 Magnifying
Glass
Visual Inspections

Borescope: enlarges small objects like a
telescope
 has
a small lens mounted on a shaft with a
built-in light source.
 typically used to inspect the inside of engines
using the spark plug hole for access.
Fiber-optic Scope
Similar to a borescope, but has a flexible
articulated probe that can bend around
corners
Video Scope
Another type of borescope similar to a
fiber-optic scope, except it has a TV to
look at the part.
Weld Inspection

Many ways to NDI welds
 most
common and most used is visual
inspection.
 good weld is uniform in width ~ even ripples
that taper off into the base metal ~ no burn
marks ~ free of gas pockets ~ porosity and
inclusions.
Weld Inspection
Bad welds: too much acetylene causes
bumps along the center and craters on the
edge ~ Cold weld has irregular edges and
considerable variation in depth of
penetration. ~ Too much heat causes
pitting on edges, and long pointed ripples.
If cooled to quickly, cracks form.
Weld Inspection
If weld has any of these defects, all the old
weld needs to be removed and the joint
rewelded.
Examples of welds
<~~Types of welds
<~~ Good weld
Liquid Penetrant Inspection
method of NDI used to find cracks,
porosity, or other types of faults open to
the surface.
 is
based on the principle of capillary
attraction
Liquid Penetrant Inspection

Preparation:
 Surface
should be free of grease, dirt and oil.
 Clean surface is ONLY way that penetrant
can get into the fault.
 Best method is using a volatile petroleumbased solvent. (if material can not be hurt by
it)
 Also clean by scrubbing with a solvent or a
strong detergent.
Liquid Penetrant Inspection

Preparation cont.
 Do
not clean by abrasive blasting, scraping,
or heavy blasting.
 Those methods tend to close surface faults
that could have been detected.
 Rinse and dry thoroughly
Liquid Penetrant Inspection
A: Spray penetrant on
surface to be inspected.
Allow it to dwell the
correct amount of time.
B: Rinse excess
penetrant off part, being
careful not to rinse too
much.
C: Developer is sprayed
on. The penetrant is
soaked up and the fault
is seen as a bright line
against the white
developer.
Liquid Penetrant Inspection

Penetrant Application:
 Typically
applied by immersing in liquid or by
swabbing of brushing penetrant onto part.
 Also sprayed on from spray cans for small
areas.
 Allow penetrant to cure the proper amount of
time (called DWELL TIME)
Liquid Penetrant Inspection

Penetrant application cont.
 Dwell
time is determined by the fault being
looked at.
 small thin cracks require more time than
larger more open cracks.
 Dwell time is decreased if part is heated.
 Penetrant evaporates if too hot.
Liquid Penetrant Inspection

Removal of surface penetrant:
 Liquid
penetrants are typically removed using
water or an emulsifying agent.
 Water-soluble penetrants
 easiest to remove
 flushed w/water at pressure of 30 – 40 psi
with an adjustable spray nozzle
 hold at 45 degree angle to avoid rinsing
penetrant out of fault.
Liquid Penetrant Inspection

Removal of surface penetrant cont:
 Post-emulsifying
penetrants;
 are NOT water soluble
 must be treated with an emulsifying agent
before they can be removed from the parts
surface.
 allows you to control the amount of penetrant
removed prior to cleaning.
Liquid Penetrant Inspection

Removal of surface penetrant
 Post-emulsifying
penetrants cont.
 By varying the emulsifier dwell time, surface
penetrant can be emulsified while the
penetrant in the cracks is untouched.
 excess penetrant is rinsed away but the
penetrant in the fault remains to expose the
defect.
Liquid Penetrant Inspection

Removal of surface penetrant cont.
 Solvent-removable
penetrants.
 excess penetrant is removed with an
absorbent towel.
 part should not be sprayed or immersed in the
solvent, this will wash out the penetrant from
the faults.
Liquid Penetrant Inspection

Application of Developer:
3
types of developers used

Dry, wet, non-aqueous.
Liquid Penetrant Inspection

Dry Developers:
 is
a loose powder material like talcum that
adheres to the penetrating liquid, acts as a
blotter to draw the penetrant out of the faults.
 usually placed in a bin of loose developer or
applied with a soft brush or blown on.
Liquid Penetrant Inspection

Dry Developers:
 applied
as soon as surface penetrant
removed
 excess removed with low pressure air flow.
 developer used is usually treated with a
fluorescent dye, are typically examined under
a black light .
 faults appear as a green indication.
 or colored dye, usually red
Liquid Penetrant Inspection

Wet developer:
 applied
as soon as surface penetrant
removed
 ~ white powder mixed with water
 either flowed over a surface or is immersed in
it and air dried
 inspected like dry developer
 either treated with fluorescent or colored dye
Liquid Penetrant Inspection

Non-Aqueous Developer
 is
most common developer used for field
maintenance
 consists of a chalk like powder suspended in
a solvent
 normally applied from a spray can or spray
gun
Liquid Penetrant Inspection

Non-Aqueous Developer cont.
 part
must be thoroughly dry
 apply a thin moist coat
 developer dries fast and pulls out any
penetrant that exists in any fault.
 comes in fluorescent and colored dyes.
Magnetic Particle Inspection

NDI method most often used for parts
made of iron.
 part
is magnetized, then a fluid containing
magnetic particles is poured over the part.
 faults on or near the surface can be detected.
Magnetic Particle Inspection
Magnetic Particle Inspection
Magnetic Particle Inspection
a discontinuity is a disruption in a parts
normal physical structure that may or may
not affect the usefulness of the part.
 the magnetic particles align with the fault.

Magnetic Particle Inspection

is useful for detecting
 cracks,
splits, seams, and voids that forms
when metal ruptures.
 some types of subsurface discontinuities do
not produce sharp enough poles to form a
good indication of the fault
Magnetic Particle Inspection

Principles of Magnetic Inspection:
 When
a material containing large amounts of
iron is subjected to a strong magnetic field the
magnetic domains within the material
becomes magnetized
Magnetic Particle Inspection
Magnetic Particle Inspection
therefore, when conducting a magnetic
particle inspection these poles attract the
magnetic particles in the solution thereby
giving you and indication of the break.
Magnetic Particle Inspection

Magnetic Orientation
 in
order to detect a crack the part must be
magnetized so that the lines of flux are
perpendicular to the fault.
 parallel to the lines of flux causes a minimal
disruption in magnetic field.
Magnetic Particle Inspection
a defect that is perpendicular to the fault
creates a large disruption of the magnetic
field.
 makes fault easy to detect.

Magnetic Particle Inspection
To ensure that the flux lines are nearly
perpendicular to a flaw, the part should be
magnetized both longitudinally and
circularly.
Magnetic Particle Inspection

Circular Magnetism: When current flows
through ANY conductor (a part) lines of
magnetic flux encircle the conductor (part)
 when
this occurs, flaws or faults located along
the material are magnetized and attract
magnetic particles.
Magnetic Particle Inspection
Current is sent through the part by
placing it between the heads of the
magnetizing machine.
~~
Magnetic Particle Inspection
if part is tubular it is slipped over a
conductive rod that is placed between the
heads of the machine.
Magnetic Particle Inspection
large flat objects are
circularly magnetized
by using test probes
that are held firmly
against the surface
with current flowing
through them.
Magnetic Particle Inspection

Longitudinal Magnetism:
 Current
flows through a coil in which the part
is placed (yokes). The magnetic field is
oriented along the material so that magnetic
fields form on either side of a fault located
across the material.
Magnetic Particle Inspection
Longitudinal Magnetism: The lines of flux
flow AROUND the part, the part is
magnetized lengthwise.
Magnetic Particle Inspection

Methods of magnetization:
 ferrous
metals can be magnetized in a variety
of ways.
 simply striking a piece of iron can induce a
weak magnetic field.
Magnetic Particle Inspection

Methods of magnetization:
 for
NDI we need to have the magnetic field
precisely controlled. Therefore Magnetic
particle inspection employs:
 Direct current (DC) magnetization
 Half-wave rectified DC magnetization
 Alternating Current (AC) magnetization
Magnetic Particle Inspection

DC
 DC
at voltages from 110 to 440 has excellent
penetrating qualities
 is suitable for magnetizing parts in coils or
with yokes
 Disadvantage; can be difficult to change its
value for inspecting objects of different sizes
Magnetic Particle Inspection

Half-wave rectified DC
 Pure
DC is not readily available in most
shops.
 AC is and can be rectified (changed) to DC
with a half-wave rectifier.
 has same penetrating qualities as straight DC.
 pulsating nature helps distribute the magnetic
particles so they arrange themselves over any
fault.
Magnetic Particle Inspection

AC:
 domain
alignment reverses each cycle
 that changes the magnetic polarity
 differs from DC in that the field strength is
almost totally concentrated on the surface of
the part.
Magnetic Particle Inspection

Testing Medium:
 is
ferromagnetic
 meaning it is finely divided, has high
permeability and low retentivity.
 for operator safety it is also nontoxic.
 in general these materials are extremely fine
iron oxides that are dyed gray, black, red, or
treated with a dye that causes them to
fluoresce under a black light.
Magnetic Particle Inspection

Definitions: (page 3-10)
 Retentivity;
as soon as a magnetizing force is
removed the domains lose their alignment
and the iron loses its magnetism.
 Permeability; The measure of ease with which
lines of flux travel through a material.
Magnetic Particle Inspection
iron oxides are often used dry but can be
mixed with kerosene or some other light oil
and sprayed onto surface.
 dry particles have no special preparations
so are good for field use.
 dry particles typically applied with hand
shaker, spray bulbs or powder guns.

Magnetic Particle Inspection
Magnetic Particle Inspection

wet particles are flowed over a part as a
bath
 typically
used with stationary equipment
 particle concentrations must be checked each
use.
 measuring concentrations is done by
collecting a sample and letting it settle
 follow manufactures recommendations
Magnetic Particle Inspection

Testing Methods:
 Residual
Magnetism; when the part is
magnetized and the magnetizing force is
removed before the testing medium is
applied.
 Relies on the parts residual or permanent
magnetism.
 only used on steels that are heat-treated for
stressed applications.
Magnetic Particle Inspection

Continuous Magnetism; requires that a
part be subjected to the magnetizing force
when the testing medium is applied.
 is
most often used to locate invisible defects
since it provides greater sensitivity in locating
subsurface discontinuities then does residual
magnetism.
Magnetic Particle Inspection
Continuous Magnetism
Magnetic Particle Inspection

Inspection:
 the
color of the dye used determines the type of light
used
 gray, black or red dye inspection is done in white
light, i.e. normal lighting
 if fluorescent dye used then a black light in a dark
booth is used.
 skill and experience of the operator is a critical factor
in determining the effectiveness of the inspection.
Magnetic Particle Inspection

Fatigue cracks:
 give
sharp, clear patterns, generally uniform
and unbroken throughout their length.
 often jagged in appearance as compared with
the straight indications of a seam
Magnetic Particle Inspection
are only found in parts that were in
service.
usually in high stressed areas of a part where
a stress concentration exists.
it is important to realize that even small
fatigue cracks indicates that failure of the part
is in progress
Magnetic Particle Inspection

Heat-treat cracks:
 have
smooth outline and are usually less
clear.
 on thin sections like cylinder barrel walls
cracks may give heavy patterns
 have a characteristic form consisting of short
jagged lines grouped together.
Magnetic Particle Inspection

Shrink cracks:
 have
a sharp clear pattern and the line is
usually very close together
 their indications generally build up to less
extent than indications of fatigue cracks.
Magnetic Particle Inspection

Grinding cracks:
 fine
sharp and seldom have build-up because
of their limited depths
 generally related to the direction of the
grinding
Magnetic Particle Inspection

Seams:
 typically
straight, sharp and fine.
 often intermittent and sometimes have very
little build-up
Magnetic Particle Inspection

Hairline cracks:
 very
fine seams in which the faces are forced
very close together during fabrication
 indications are very fine and sharp with very
little build-up
Magnetic Particle Inspection

Inclusions:
 are
nonmetallic materials that have been
trapped in the solidifying process
 include slag materials and chemical
compounds
 shows up as a broad and fuzzy indication.
Magnetic Particle Inspection

Demagnetization:
2
types;
AC
 DC

Magnetic Particle Inspection

In order to demagnetize a part the
magnetic domains must be disorganized.
 part
is subjected to a magnetizing force that is
opposite of the force used to magnetize it.
Magnetic Particle Inspection

AC demagnetization:
 slowly
remove the part from the magnetic field
while the current is flowing
 reversing action becomes weaker
 domains are left with random orientation and
the part is demagnetized.
Magnetic Particle Inspection

DC Demagnetization:
 part
is placed in a coil and more current is
used than was used to magnetize the part.
 then the current flow is reversed and
decreased until lowest value of current is
reached.
 check for residual magnetism with a magnet
strength indicator.
Non-Destructive
Testing
Chapter 11
Section B
Electronic Inspection

In section A we learned some different types of visual
inspection. They are good at detecting surface and some
subsurface flaws. Some aircraft components are nonferrous materials and must be checked for internal
imperfections. To do this we use several different types
of inspection methods.




Eddy current
Ultrasonic
Radiographic
Piezoelectricity
Electronic Inspection

Eddy Current Inspection:


requires little or no part prep
can detect surface or subsurface flaws
 can differentiate between different metals
 based on the principle of current acceptance in other
words, it determines the ease with which a material
accepts induced current and is determined by 4
properties




conductivity (varies with alloy type)
permeability (ability to accept lines of flux)
mass
presence of voids or faults
Electronic Inspection

Eddy current inspection cont.

Absolute method


identifies materials characteristics by measuring
the amount of current that flows when current is
induced in the test specimen.
Comparison method

indicates the difference in characteristics between
the material under the reference probe and that
under the test probe.
Electronic Inspection
Eddy current machine.
Electronic Inspection
Electronic Inspection

Ultrasonic Inspection:

can be used on plastics and ceramics and
most medals
 uses sound waves to determine faults in a
material
 vary in freq from 200 khz to 25mhz
 can be used in solid or liquids
 used for NDI, sonar, ultrasonic cleaning and
medicine.
Electronic Inspection
Ultrasound
probe
Electronic Inspection
Electronic Inspection

Radiographic Inspection:

One of the most important methods of
nondestructive inspection
 allows a photographic view inside a structure
 2 types, X-ray and Gamma rays
Electronic Inspection

Radiographic safety:

ANY form of radiation is HARMFUL to the human
body.
 radiation produces changes in all matter through it
passes, including living tissue, sometimes only
dislodge a few electrons. However, an excess of
these effects causes irreparable harm.
 if the whole body is exposed to a very large dose of
radiation death can occur.
 layer of lead is best defense
 if working around x-ray or gamma rays you should
wear a monitoring film badge.
Electronic Inspection
Inspecting Composites

Composite structures contain materials
that sometimes make NDT difficult.

many honeycomb structures are metal
backed making x-ray ineffective
 dye penetrant is generally ineffective
because the dye sometimes absorbs into the
laminations or weave.
Inspecting Composites

Coin tap test:

one of the simplest tests available is also one of the
most effective.
 works on laminated, bonded, and honeycomb
materials
 tap the edge of a coin lightly along an area you
suspect is damage.
 Undamaged material produces a solid ringing sound
while a damage area make a hollow thud
Inspecting Composites

Thermography:

locates flaws by measuring temperature
variation at the parts surface.
 uses infrared camera to measure temps.
 requires a knowledge of the test materials
thermal conductivity which is then compared
to a reference standard
Inspecting Composites

Radiography:

x-rays are not effective on certain bonded
structures
 can detect surface cracks and internal
damage on many composites
 can detect water inside honeycomb core
cells.
Inspecting Composites

Laser holography:

heat part then photograph using a laser light
source and a special camera
 can detect disbonds, entrapped water and
impact damage