Laser-based Non-destructive Evaluation

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Laser-basedNon-destructive Evaluation

• John S. Popovics

Objective Learn the basis and application of
laser-based NDE techniques
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Topic significance
Lasers are important in many engineering
applications
Remote or embedded sensing Manufacturing process
monitoring Medical procedures Communication
systems
The application of lasers to engineering
tasks will become more widespread in the future
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Lecture outline

• Basic concepts
• waves
• optics
• lasers
• optical fibers
• Techniques
• wave generation
• wave detection
• fiber sensing
• Applications
• smart structures
• laser-based ultrasound

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What are waves?
Propagation of a disturbance through a
medium (mass is not transported in propagation
direction)
Excitation
Direction of Travel of Wave Front
Direction of Particle Disturbance
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Types of waves
Mechanical waves - motion or pressure Electrom
agnetic waves electric or magnetic field
sound, ultrasound, vibration, etc.
(can propagate in solids and fluids, but not
vacuum)
light, radio waves, RADAR, microwaves, etc.
(can propagate in all media very fast)
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Harmonic wave propagation
position of mass on a spring

• Many phenomena arise from harmonic motion
• Propagating waves cause harmonic motion at a
• sensed spot.

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What is wave frequency?
The period (T) is the time required for wave
motion to complete a round trip (measured in
seconds)
The frequency (?) is the inverse of T (measured
in 1/seconds or Hertz) In audible sound,
frequency is interpreted as the pitch In visible
light, frequency is interpreted as the color.
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Harmonic wave behavior
All waves obey fundamental principles
Frequency-wavelength relation V ? ? A the
frequency (?) of a propagating wave is related
to its wavelength (?) by the propagation velocity
(V)
Sound waves in air
V 330 m/s
For ? 440 Hz (middle A on a piano), then ?
0.75 m
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Wave interference
If two harmonic waves are combined together, the
amplitude of the resulting combined wave depends
on the alignment (phase delay) of the two
individual waves.


Combine waves


(T/2 phase delay)
Destructive Interference
Constructive Interference
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Reflection refraction
When a propagating wave traveling in a medium
impinges on an interface with another
medium, wave reflection and refraction occurs.
A portion of the incident wave energy will
propagate back into the original
medium (reflection) while the remaining wave
energy will propagate through into the second
medium (refraction).
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Wave reflection refraction
Reflected angle equals the incident angle
Transmitted angle depends on the incident angle
and the properties of the two media
Note ?t gt ?i
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Total internal reflection
Incident angle is greater than the critical
angle no refraction, all reflection


Refracted angle must be greater than incident
Note ?t gt 90o
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Optical spectrum
Wave length (in air) Frequency
550 nm
400 nm
700 nm
5.45x1014 Hz
4.28x1014 Hz
7.50x1014 Hz
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Quantum behavior of light
Light behaves both as a wave and also as a
collection of discrete packets (photons)
of energy. Light can absorb or emit a
finite amount of energy a quantum. The amount
of energy is related to the frequency of the
light (?) Through Plancks constant (h)
?E h ?
where h 14.4 eV s
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Lasers
Lasers are devices that amplify and direct a
light beam.
Flash light
Laser
high intensity, narrow range of
frequencies (single frequency), small beam
divergence.
low intensity, broad range of frequencies, large
beam divergence.
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Lasers
Excited states
Light amplification (at a single frequency)
achieved by pumping and lasing atoms in
a special medium. A transition between two energy
states is associated with absorption or emission
of light at a certain frequency
meta-stable
Ground state
stimulated emission
Pumping frequency ?p is greater than
lasing frequency ?L
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Lasers
Incident photon
Output photons
Laser material
Pump energy
Pumping laser medium illuminated with an intense
light source with ?P or an electrical discharge
is passed through a gaseous medium. Stimulated
emission a photon of light passes through an
excited later medium and induces an excited atom
to reduce back to the ground state. Thus we
generate a second identical photon, and the
initial light is amplified. Light amplification
by stimulated emission of radiation
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Lasers
Beam collimation (tight directivity) achieved
by a light resonator.
Collimated laser beam
1 to 4 mm wide
Stimulated laser light
Partially-reflecting mirror
Fully-reflecting mirror
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Laser systems
Properties of the light controlled by the laser
generating system gas, solid state or
semi-conductor
Lasers can generate either a continuous beam
(CW) or repeated short bursts (pulsed) of light
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Thin (0.1 mm) optical fibers
Light can travel great distances within a
transparent fiber if total internal reflection is
achieved incident light within acceptance angle
and ncladding lt ncore
?i gt ?crit
?t gt ?i
leakage
leakage
fiber cladding
fiber cladding
fiber core
fiber core
critical
critical
critical
light
angle
angle
angle
acceptance
Lasers are a good light source for optical fiber
angle
total internal
total internal
reflection
reflection

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What is NDE?
Non-destructive evaluation
Detect defects, dimensions, etc. without damaging
the material

• Ultrasonic sound waves
• X-ray / CAT scans
• Magnetic techniques
• RADAR scanning
• Thermal imaging

X-ray radiograph
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Ultrasonic NDE
Flaw detection wave echoes from air-filled
defects such as cracks and voids
wave path
Back surface echoes
wave source
crack
crack echoes
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Laser wave generation
Lasers can generate mechanical waves in solids
without contact
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Wave sensing with Lasers
Very small surface motion (on the order of the
optical wavelength) can be detected. Two split
Beams are recombined. The light intensity of the
combined beam varies because of phase
interference as the path length changes owing to
surface displacement. Max intensity in
phase Min intensity out of phase
Michelson interferometer
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Wave sensing with Lasers
incident light
Larger surface motion (vibration) can be
detected. Frequency shifts in reflected laser
light are monitored. Shifts are related to
relative surface velocity resulting from Doppler
shift phenomenon.
higher frequency
surface motion
incident light
lower frequency
Heterodyne interferometer or vibrometer
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Optical fiber sensors
Optical fibers act as internal condition sensors
when they are attached to a structure or
material. Minute changes in stretching,
pressure and temperature induce changes in the
intensity, phase, polarization and wavelength of
the light traveling in the fiber. By monitoring
changes in the light, changes in internal
condition can be inferred.
Optical fiber
Light in
Light out
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Smart structures
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Application smart structures
Image from AMS lab at MIT
Material checks and corrects unwanted vibration
or deviations from pre-assigned shape in aircraft
and helicopter structures
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Application embedded sensors
Embedded optical fiber serve as rugged strain
sensors in concrete. Here fiber optic sensors
provide data during a test, which agree
with conventional sensors at the surface
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Application vibration monitoring
Full-field displacement monitoring can rapidly
provide information about dynamic motion of a
structure. Here areas of large surface motion
(vibration) are indicated by .
Surface motion map for van provided by scanning
laser vibrometer
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Application metals processing
Ultrasonic signal
Lasers can perform non-contact ultrasonic
inspection. Here a laser ultrasonic system
monitors the thickness of hot-rolled (1000oC)
steel tubes during processing from a safe
distance (up to 5 m away)
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Application train rail inspection
(Center Nondestructive Evaluation at The Johns
Hopkins University)
A hybrid Laser scanning system allows rapid
non-contact inspection of rail in place. Small
fatigue cracks in the rail head should be located
Power Supply
M1
Oscilloscope
IR Pulse Laser
Charge Amplifier
M2
Capacitive Air-Coupled Transducer
Testing set up
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Analysis approach
Images from CNDE at The Johns Hopkins University
Crack
No Crack
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Summary

• Lasers are devices that create a high intensity,
  directed
• light beam.
• Lasers can be used to generate waves and detect
• stretching or motion in a solid material
  without
• direct contact with the material.
• Lasers are useful in many important engineering
• applications, including NDE
• laser ultrasonics, smart structures
  and
• vibration sensing.
• The use of lasers will become more common
• in the future.