Laser-based
Non-destructive Evaluation
John S. Popovics
Objective:
Learn the basis and application
of laser-based NDE techniques
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
Laser-based NDE
Lecture outline
Basic concepts
•
waves
•
optics
•
lasers
•
optical fibers
Techniques
•
wave generation
•
wave detection
•
fiber sensing
Applications
•
smart structures
•
laser-based ultrasound
Laser-based NDE
What are waves?
Propagation of a disturbance through a medium
(mass is not transported in propagation direction)
Excitation
Direction of Travel
Direction of Particle Disturbance
of Wave Front
Laser-based NDE
Types of waves
Mechanical waves - motion or pressure
sound, ultrasound, vibration, etc.
(can propagate in solids and fluids, but not vacuum)
Electromagnetic waves – electric or magnetic field
light, radio waves, RADAR, microwaves, etc.
(can propagate in all media – very fast)
Laser-based NDE
Harmonic wave propagation
1.5
T
D is place me nt
1
0.5
0
-0.5
m
-1
phase delay
-1.5
0
20
40
60
80
T ime
100
position of
mass on a spring
• Many phenomena arise from harmonic motion
• Propagating waves cause harmonic motion at a
sensed spot.
Laser-based NDE
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.
Laser-based NDE
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
Laser-based NDE
Wave interference
+
+
=
(T/2 phase
delay)
Constructive
Interference
Laser-based NDE
=
Destructive
Interference
Combine waves
Combine waves
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.
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).
Laser-based NDE
Wave reflection & refraction
Reflected angle
equals the
incident angle
Note: t > i
Laser-based NDE
Transmitted
angle depends
on the incident
angle and the
properties of
the two media
Total internal reflection
incident
reflected
wave
wave

i

r
M edium 1
Incident angle is
greater than the
“critical” angle:
no refraction, all
reflection
M edium 2
Note: t > 90o
Laser-based NDE
Refracted angle
must be greater
than incident
Optical spectrum
Ultra-violet
Infra-red
Wave length
(in air)
700 nm
550 nm
400 nm
5.45x1014 Hz
7.50x1014 Hz
Frequency
4.28x1014 Hz
Laser-based NDE
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 Planck’s constant (h):
E = h 
where h = 14.4 eV s
Laser-based NDE
Lasers
Lasers are devices that amplify and direct
a light beam.
Flash light
Laser
low intensity,
broad range of frequencies,
large beam divergence.
high intensity,
narrow range of frequencies
(single frequency),
small beam divergence.
Laser-based NDE
Lasers
Rapid decay
Excited states
Light
amplification
E2
(at a single
meta-stable
E1
frequency) achieved
Lasing
Pumping
by “pumping” and
L = (E1-E0)/h
P = (E2-E0)/h
“lasing” atoms in a
E0
special medium. A
stimulated emission
Ground state
transition between
two energy states is
Pumping frequency p is
associated with
greater than lasing
absorption or
frequency L
emission of light at
a certain frequency
Laser-based NDE
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
Laser-based NDE
Lasers
Beam “collimation” (tight directivity)
achieved by a light “resonator”.
Collimated laser beam
Stimulated laser light
Fully-reflecting
mirror
1 to 4 mm
wide
Partially-reflecting
mirror
Laser-based NDE
Laser systems
Laser, Mode
m m)
Type
Wavelength (
He - Ne, CW
gas
0.633
interferometry
CO , pulsed
gas
10.6
wave generation
2
Application
YAG, CW
solid state
1.06
interferometry
YAG, pulsed
solid state
1.06
wave generation
Ruby, pulsed
solid state
0.694
wave generation
semi -conductor
0.815
Ga - As, CW
fiber optics
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
Laser-based NDE
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 < ncore
i > crit
t > i
leakage
fiber cladding
fiber core
light
critical
angle
acceptance
angle
total internal
reflection
Laser-based NDE
Lasers are
a good
light source
for optical
fiber
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
Laser-based NDE
Ultrasonic NDE
Flaw detection: wave echoes from airfilled defects such as cracks and voids
wave
path
Signal amplitude
Back surface echoes
wave
source
Time
Signal amplitude
crack
crack echoes
Time
Laser-based NDE
Laser wave generation
thermoelastic
source
plasma
source
laser
beam
heated area
vaporized
plasma
resulting
forces
Lasers can generate mechanical waves in solids without contact
Laser-based NDE
Wave sensing with Lasers
reference
mirror
beam
splitter
laser
source
moving
surface
photodetector
output
Michelson interferometer
Laser-based NDE
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
Wave sensing with Lasers
incident light
surface
motion
higher frequency
incident light
surface
motion
lower frequency
Heterodyne interferometer
or “vibrometer”
Laser-based NDE
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.
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
Laser-based NDE
stretching
“Smart” structures
“Smart” or “adaptive” materials contain
embedded fiber optic sensors combined with
distributed actuator systems.
Capable of sensing the state of the material
and then modifying accordingly
“smart”
Hmmm..
Laser-based NDE
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
Laser-based NDE
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
Laser-based NDE
Application: vibration monitoring
Surface motion map
for van provided by
scanning laser vibrometer
Full-field displacement
monitoring can rapidly
provide information
about dynamic motion
of a structure. Here areas
of large surface motion
(vibration) are indicated
by “+”.
Laser-based NDE
Application: metals processing
Ultrasonic signal
Back-wall echoes
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)
Laser-based NDE
Application: train rail inspection
(Center Nondestructive Evaluation at The Johns Hopkins University)
Power
Supply
M1
IR
Pulse
Laser
Oscilloscope
Charge
Amplifier
M2
Capacitive
Air-Coupled Transducer
Testing set up
Laser-based NDE
A hybrid
Laser scanning
system allows
rapid non-contact
inspection of
rail in place.
Small fatigue
cracks in the
rail head should
be located
Analysis approach
Images from CNDE at The Johns Hopkins University
Crack
No Crack
Laser-based NDE
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.
Laser-based NDE