Download (III) Laser Detection of Wave Motion in Solids

Laser-based Non-destructive Evaluation and Monitoring
John S. Popovics
Summary
Lasers are devices that create a high intensity, directed light beam. The light beam
can travel in air or through fiber optic cables. Lasers can be used to generate waves and
detect stretching or motion in a solid material without direct contact with the material.
Thus lasers are useful in many important engineering applications that require remote
non-destructive evaluation (NDE) and sensing such as laser ultrasonics, “smart”
structures and vibration sensing. In this lesson, necessary basic concepts about light,
lasers, fiber optics and wave propagation are introduced. The basis of wave generation
and sensing with lasers is given. Finally specific engineering applications that make use
laser-based techniques are described.
(I) Introductory Concepts
(A) Harmonic motion and wave propagation
Many natural phenomena arise from simple harmonic motion. For example,
consider a mass hanging vertically from the end of a spring. At first the mass is at rest. If
the mass is slightly displaced downward, it will move up and down for some time under
the downward action of gravity and the upward restoring action of the spring. A plot of
the vertical position of the mass with respect to time shows a general sinusoidal form, and
this type of motion is termed “harmonic.” The maximum value gives the “displacement
amplitude” of the motion. The frequency of the wave motion () is defined as
 = 1/T
(1)
where T is the period of the wave motion. Harmonic motion that has a low value of T
therefore has a high value of , and vice versa. T is measured in units of seconds (s), and
thus  in units of 1/s. Usually the Hertz (Hz) is used as the name for the unit of frequency
instead of 1/s, although they have the same meaning.
1.5
T
D is place me nt
1
0.5
0
-0.5
-1
phase delay
-1.5
0
20
40
60
80
100
T ime
Harmonic motion shows a general sinusoidal form. The maximum value of the signal
gives the “displacement amplitude” of the motion. The period of the wave motion (T) is
the time required for the mass to perform an entire round trip of motion. For example, T
is the time spacing between two peaks in the plot. If one harmonic signal is slightly
delayed behind another identical signal, the time lag is called the “phase delay” or “phase
angle” between the two signals. The value of the phase delay ranges between 0 and T. If
the phase angle between two signals is either 0 or T, the signals are perfectly aligned and
are said to be “in phase.”
Propagating waves also cause harmonic motion at a sensed point. Propagating
waves are disturbances that travel through a medium with a certain velocity (V). The type
of disturbance depends on the type of propagating wave. It is important to understand that
no mass is transported by wave itself. Rather, the wave is a passing disturbance that
causes motion (or some other disturbance) at a single location in the medium. It is this
disturbance at a point, caused by the passing wave, which exhibits harmonic motion.
Usually, the resulting motion is described by a combination of individual harmonic
motions each with different frequency and amplitude. Audible sound (disturbance of air
pressure), ultrasound, RADAR and visible light all exhibit properties of propagating
harmonic waves, even though the specific phenomena for each are very different. As an
example, consider a piano that is recorded by a nearby microphone. The output of the
microphone is electrical voltage that is directly related to the variation in pressure of the
air near the front of the microphone. If we play a single key on the left-hand side of the
piano keyboard, we hear sound with low pitch. Until the sound dies out, the output of the
microphone will look like harmonic wave motion with characteristic values T and . (As
a reference, the middle “A” key on the piano has a frequency () of 440 Hz and a period
(T) of 2.27x10-3 s.) If we now play a single key on the right-hand side of the keyboard,
we hear sound with high pitch. The output of the microphone again looks like harmonic
wave motion, but now T is much smaller and therefore  much higher. Thus frequency is
interpreted as the pitch in the case of audible sound. If we now play a chord on the piano
(several keys played together), the microphone output is a complicated signal that does
not look like harmonic motion. However, this complicated signal can be broken down
into three individual harmonic signals, each with specific values of T and .
All harmonic propagating waves obey the fundamental frequency-wavelength
relation and exhibit reflection, refraction and interference behaviors. Equation 2 relates
the frequency of the wave to the wavelength () in terms of the wave propagation
velocity (V)
V=
(2)
Consider again the piano and the microphone. It is known that the velocity of sound
waves in air is approximately 330 m/s. If the microphone is 1 m away from the piano, the
sound waves will reach the microphone 3.03x10-3 s after the moment that the key (let’s
say the middle “A” key) is played. After the wave-front reaches the microphone, the
characteristic frequency ( = 440 Hz) of the harmonic wave motion can be determined
from the harmonic signal. The wavelength of the resulting sound in air can be computed
(using equation 2) to be 0.75 m. It can be seen that  has units of length and can be
visualized as the distance in space (in this case air) between two points of the same phase
of the harmonic wave. Assuming constant wave velocity,  is inversely related to .
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). Another
important property that harmonic waves display is interference. If two identical harmonic
waves (same frequency and amplitude) are combined together, the amplitude of the
resulting combined wave depends on the alignment (phase delay) of the two individual
waves. The amplitude of the combined wave will be maximal if the individual waves are
perfectly aligned; this is called constructive interference and the individual waves are “in
phase.” If one wave is shifted with respect to the other, the amplitude of the combined
wave will be lower than that obtained from constructive interference. In one special case,
called destructive interference, the combined signal has zero amplitude. This occurs when
one wave signal has a phase delay of T/2 with respect to the other.
There are many different types of propagating waves, several of which are
classified as “mechanical waves.” These waves propagate a mechanical disturbance in a
medium, such as pressure or motion. For example audible sound, ultrasound and
vibrations all are mechanical wave phenomena. In fluids and gasses, the mechanical wave
propagates a disturbance of pressure. Only this type of disturbance is possible, and this
wave is often called an “acoustic” wave. In solid materials, however, more than one
mechanical wave mode can exist. In solids, the wave propagates as a disturbance in
motion (displacement). The type of the wave is defined by the direction of local particle
motion with respect to the direction of wave propagation. “Longitudinal” waves (Lwaves) have particle motion coincident with the propagation direction, while “transverse”
waves (T-waves) have particle motion perpendicular to the propagation direction. Lwaves and T-waves propagate throughout a solid. Another type of wave, the Rayleigh
surface wave (R-wave), propagates only along the free surface of a solid, and the particle
motion is a combination of parallel and perpendicular motions. Each of these wave types
travel with different velocity defined by the mechanical properties of the medium through
which they propagate. For a given medium, L-waves have the highest velocity and Rwaves the lowest.
Mechanical waves exhibit reflection and refraction when they impinge on an interface.
The magnitude of the reflection is primarily controlled by the differences in mechanical
properties between the two media (for example material stiffness and density.) If the two
materials have very different mechanical properties, the amount of reflection will be very
high. Thus most mechanical wave energy is reflected at the interface between a solid and
air. As a result most sound energy (acoustic wave in air) will reflect back from a distant
solid wall (an echo). Similarly, most ultrasonic wave energy traveling in a solid will
reflect from an air-filled crack in the solid material.
(B) Optics
Light is a form of electromagnetic radiation, which behaves as a propagating
harmonic wave. All electromagnetic radiation propagates through vacuum with a velocity
of approximately 3.0x108 m/s, which is much higher than that for mechanical waves. The
constant velocity of electromagnetic waves in vacuum is commonly referred to as “c.” In
actual materials such as air, the velocity of propagation is slightly slower, depending on
the refractive index of the material. The refractive index of a material is the propagation
velocity of electromagnetic waves in vacuum divided by that in the material, and is
greater than or equal to 1.0. The refractive index of air at atmospheric pressure is very
close to 1.0, so we can assume that the wave velocity air is effectively the same as that in
vacuum. Electromagnetic radiation is a disturbance in the electric and magnetic fields,
which propagates through a given medium. During propagation, the wave energy is
transferred between electric and magnetic fields, which are normally perpendicular to
each other and to the direction of propagation. The plane that contains both the direction
of the electric field and the direction of wave propagation is called the plane of
polarization. Light is unusual in that it can behave both as a wave and also as a collection
of discrete packets (photons) of energy. Like ordinary wave propagation, light waves
obey the fundamental relation between wavelength () and frequency () (related by the
wave velocity c). The full electromagnetic spectrum covers a wide range of frequencies:
104 Hz (long radio waves) to 1021 Hz (high-energy gamma rays). The optical spectrum
occupies a small portion of the full spectrum, ranging from 1x1014 Hz (high end of infrared spectrum) to 5 x1015 Hz (low end of ultraviolet spectrum.) Further, visible light
occupies a range of frequencies within the optical spectrum, bounded by the infra-red and
ultraviolet spectra. In the case of visible light, the frequency of the wave is interpreted as
the color of the light. For example, red light has a frequency of 4x1014 Hz while violet
light has 8x1014 Hz. Assuming that waves travel with a velocity c, the frequency limits of
the visible light spectrum correspond to wavelengths of 700 x10-9 m and 400x10-9 m,
respectively.
In addition light reflects and refracts at an interface. For light, the nature of
reflection and refraction are controlled by the refractive indices of the two materials.
Unlike ordinary wave propagation however, photons of light can absorb or emit a finite
amount of energy (a quantum). The amount of energy is related to the wave frequency
through Planck’s constant (h)
E=h
(3)
where E is the change in energy (typically reported in units of electron volts or eV), and
h= 4.14 eV s. Atomic and molecular systems have many discrete energy states in which
they can exist. The lowest energy state is termed the “ground state” and the multiple
higher energy states are collectively termed “excited states.” Quantum theory states that
the energy change associated with transition from the ground state to a specific excited
state (E) is associated with a specific frequency, as seen in equation 3.
(C) Lasers
High amplitude waves that are made up of a single frequency and forced into a
highly-directed beam are often needed for engineering applications. This situation is
relatively easy to achieve with mechanical waves and some electromagnetic waves such
as RADAR. However it is not easy to achieve with visible light using standard
technology. The advent of lasers now allows single frequency light amplification and
beam confinement. Actually, the word laser is an acronym of Light Amplification by the
Stimulated Emission of Radiation. From the previous section, we know that light is
emitted when something (for example an electron in an atom) drops from a higher energy
state to a lower one and that this change in energy is associated with a specific frequency.
If this energy change occurs without any outside influence, it is called “spontaneous
emission.” However if we can somehow influence many similar energy drops in a
medium at once, we can generate high amplitude light that is made up of primarily one
frequency. This is “stimulated emission,” and it can be promoted in a laser medium. For
example, imagine that a photon of light energy is passing through a laser medium and
then interacts with an atom (electron) that happens to be in a higher energy state. If the
original photon has just the right amount of energy, it can induce the atom to reduce to a
lower energy. The result is a second identical photon. Since there are now two photons
instead of one, the light is amplified.
Stimulated emission by itself may not be sufficient for engineering application
purposes since the light is not confined, but is emitted in all directions similar to the light
emitted by a bulb, and has low intensity. In order to generate a beam of intense light, we
Stimulated emission is promoted
in a laser medium. In order for this
Pumping
be
to
emission
stimulated
p = (E2-E0)/h
effective, we need more atoms (or
electrons) to be in a higher energy
E1
state than in the ground state, since
electrons in the ground state will
E0
actually absorb light that is emitted
by other energy drops. The excitation process is called “pumping.” Imagine that we pump
a medium so that the material is excited from the ground state (E 0) to an excited energy
state (E2). This absorption of energy is associated with a specific frequency p (the
pumping frequency) by equation 3. Stimulated emission can now occur in the pumped
medium in response to incident radiation at another frequency, the “lasing” frequency L,
thus lowering the energy of the material to an intermediate state (E1). Since the energy
change from the highest energy level to the intermediate is less than that from the ground
state to the highest level, the pumping frequency is necessarily greater than the lasing
frequency. There are two primary techniques to pump a laser system: the laser medium is
illuminated with an intense light source of a given pumping frequency (for example by a
discharge tube) or an electrical discharge is passed through a gaseous medium.
E2
Lasing
L = (E2-E1)/h
must amplify the light and confine it to one propagation direction. Suppose that we are
able to guide light along one narrow column of a certain length in a laser medium. (For
example, we can confine light using the concept of total internal reflection, as described
in the following section.) Since this material along that light path can be manipulated to
provide stimulated emission (being a laser medium), the light is amplified along the path.
If we now add mirrors to each end of the confined light path, the light along that defined
path will continue to propagate and progressively amplify as the light travels back and
forth along the path. Emitted light rays that travel in other directions simply leak away
and have low amplitude. In essence a light “resonator” is set up, and with each pass the
internally reflected light stimulates further emission thereby increasing the intensity of
the light along that path. If one of the end mirrors allows a portion of the built-up light
energy to pass through, then the amplified light will emanate from the end and propagate
outward along a confined beam. That is, a laser beam will be generated containing
directed light made up of primarily one frequency. (Note that this described resonant
cavity scheme is just one of many possible light amplification and confinement schemes.)
The described system is capable of generating a laser beam continuously, provided that
pumping (see inset above) is continuously applied. Such systems are called continuouswave (CW) lasers, and are important in some engineering applications. In other
engineering applications however intermittent laser light is needed, and pulsed laser
systems are used to provide laser beams with short duration. There are several different
schemes used to obtained laser pulses.
Laser light is characterized by the frequency content (or alternatively wavelength
content), power, directionality and coherence length of the beam. These characteristics
are primarily controlled by the laser system used to generate the light. Lasers have highly
controlled beam directionality compared to conventional light sources. A laser beam can
be visualized as a cone with an extremely small angle of divergence, typically a few
milliradians. Thus the beam is column-like (typically a few millimeters wide) and can
propagate long distances through air (several meters) without appreciable signal losses
due to beam spreading. Coherence describes how well a light beam maintains classic
harmonic form and is an important characteristic for CW lasers. Light is completely
incoherent if there is no predictable phase behavior with respect to either space or time.
Laser light starts out coherent but soon becomes incoherent as it propagates. The
propagation distance within which the light is reasonably coherent is called the coherence
length. The coherence length is essentially controlled by the frequency content of the
light: the more monochromatic the light, the larger the coherence length. For example, a
fairly monochromatic He-Ne laser has a coherence length of the order of 200 mm.
Laser, Mode
He-Ne, CW
CO2, pulsed
YAG, CW
YAG, pulsed
Ruby, pulsed
Ga-As, CW
Type
Wavelength (m)
gas
0.633
gas
10.6
solid state
1.06
solid state
1.06
solid state
0.694
semi-conductor
0.815
Application
interferometry
wave generation
interferometry
wave generation
wave generation
fiber optics
Laser light characteristics are primarily controlled by the laser system used to generate
the light. Lasers (both CW and pulsed) can be classified into the gas, solid state or semiconductor families. Common gas lasers are Helium-Neon (He-Ne) and carbon dioxide
(CO2) lasers. Solid state lasers typically use glass or crystal with small amounts of special
atoms mixed in to provide a laser medium, such as yttrium aluminum garnet (YAG) and
ruby. Diode lasers make use of gallium arsenide (Ga-As) semi-conductor systems. Diode
lasers provide low power, less column-like laser beams than those generated by gas and
solid state lasers.
(D) Optical fibers
We now know that a highly column-like light beam can be generated using a
laser. As a result, we can control the direction of the light beam and transmit the light
over a large distance. However, we cannot transmit the light through physical barriers,
such as walls, and we cannot bend the light beam in air to go around obstacles. However,
it is possible to transmit light across great distances and around obstacles if we can guide
the light along some sort of light “pipe.” Thin transparent glass fibers can be designed to
pipe light. More generally, these are called optical fibers. Optical fibers work because
they make use of the concept of total internal reflection, which was introduced earlier. If
total internal reflection is achieved in a thin fiber, the light can travel along the fiber
without losing intensity simply because no light is allowed to leak out to the material
outside of the fiber. Optical fiber (core and cladding) is usually made of very pure glass
(because it is very transparent) with a typical diameter of about 0.125 mm – about the
size of monofilament fishing line. A plastic coating is often added to the outer surface to
Optical fibers make use of total
internal reflection. If total internal
leakage
fiber cladding
reflection is achieved in a thin fiber,
fiber core
the light can travel along the fiber
without losing intensity simply
critical
because no light is allowed to “leak”
angle
out to the material outside of the
acceptance
fiber. Recall that when light waves
angle
impinge onto an interface between
two different materials, a portion of
total internal
the energy is reflected and the
reflection
remaining energy is transmitted
through to the second material. The amount of light that is reflected depends on the angle
of incidence, i, and the properties (refractive indices) of the two materials. Total internal
reflection occurs if t > i and i is greater than the critical angle. Imagine that we have a
thin transparent glass fiber (the core) coated with another material (the cladding). In order
to satisfy the first requirement for total internal reflection, the refractive index of the
cladding material should be lower than that of the core material. The second condition is
satisfied if a column-like light source is directed axially along length of the fiber. Typical
refractive index values for the core and cladding are 1.5 and 1.485, respectively.
Obviously the difference in the refractive indices is small, but it is enough to give a
critical angle value of about 81o. In other words, the fiber will transmit the light within an
acceptance angle of 18o.
ease handling and protect the outer surface from becoming scratched. Mechanically, the
fibers are stiff but flexible to some degree. Because a controlled and column-like light
source is needed to transmit light through the fiber effectively, low power diode lasers are
commonly used. A properly designed clad fiber can transmit light across great distances
and around obstacles. Optical fibers can be used to transmit digital data, in the form of
light signals, very well. In fact optical fibers are now used in local and long distance
telephone systems in place of traditional electrical wires.
(E) NDE
Destructive tests provide direct and accurate information about the material under
inspection. For example, we may be interested in finding the location and extent of cracks
or other flaws within the material. We can drill a hole into the material and look for such
internal defects. However destructive tests, by their very nature, cannot be carried out at
many locations on the structure. Thus, a large part of the structure is not inspected. Nondestructive evaluation (NDE) tests, however, can be carried out at many locations since
the tests do not cause any damage. For example, doctors use X-ray images to determine,
in a non-destructive fashion, if a bone contains cracks or other damage. In fact the
doctors can use many X-ray images of the same bone (say from different directions) since
the X-ray itself does no further damage to the bone. However, NDE tests do not provide
direct information. Rather, NDE tests provide indirect information, which is then
interpreted to assess the condition. For example, an X-ray image directly shows variation
in density through the thickness of the inspected material, which may (in some but not all
cases) be interpreted to estimate the size and location of cracks (low density) within the
bone material (higher density). Ultrasonic waves are also used successfully to detect,
locate and characterize defects, such as cracks within solid materials. Reflected wave
echoes are used to locate and size the air-filled defects (cracks or voids). However, we
need to send and receive mechanical wave energy to use ultrasound effectively in a solid.
Embedded or remote sensors can also be used to provide valuable information about a
structure or material in a non-destructive manner.
(II) Laser Generation of Wave Motion in Solids
Lasers can be used to generate mechanical waves in solids, in non-contact
fashion. Several physical processes occur when a solid material is illuminated with a laser
beam. At low powers, the solid material is locally heated because of absorbed
electromagnetic radiation by the material, while at high powers surface material may be
vaporized (ablated) and plasma formed as a result. In fact, three regimes for production of
mechanical waves in solids using lasers have been identified and are described next. All
three regimes use laser light in the near infrared and visible region of the electromagnetic
spectrum, but each makes use of different power densities. Pulsed laser sources are
usually used.
(A) Thermoelastic regime
The thermoelastic wave source makes use of lower power densities, usually <107
W/cm2 for metals. All wave modes are generated. The mechanical waves are generated
through a thermal mechanism, which makes use of the thermoelastic effect. The
thermoelastic effect means that a material will increase in size when heated and contract
when cooled. This source generates waves (longitudinal, shear and surface) that
propagate in all directions symmetrically about the point of excitation. Within the
material, however, the amplitudes of the various wave modes are highly dependent on
direction. In general, shear waves and surface waves are most readily generated, although
longitudinal waves are also excited. A medium high energy pulsed laser source is
appropriate for this application (for example a pulsed ruby laser).
(B) Plasma regime
When higher power densities are applied to a metallic material, usually >107 W/cm2 for
metals, the thermoelastic source is supplemented by the vaporization (ablation) of a layer
of the material at the illuminated point. The stress field of the resulting waves is very
similar to that owing to the application of a normal point force at the surface. This source
generates all wave modes that propagate in all directions symmetrically about the point of
excitation, but the generation of longitudinal and Rayleigh surface waves are enhanced.
thermoelastic
source
plasma
source
laser
beam
heated area
vaporized
plasma
resulting
forces
Two different laser-based wave generation mechanisms are possible: the thermoelastic
source and the plasma source. Physically, the thermoelastic source results from a rapid
local heating at a surface point with subsequent rapid expansions (owing to the
thermoelastic effect) followed by a much slower rate of cooling and contraction. The
heated area can be visualized as a thin disc extending into the solid material to a depth
determined by the heat diffusion characteristics of the material and the duration of the
incident laser pulse. The diameter of the heated area is defined by the diameter of the
incident laser pulse itself. The mechanical waves (for example ultrasound) are generated
from this heated zone by sudden strain, owing to the thermoelastic expansion, acting
radially outward from the beam center in the plane of the surface of the material. The
plasma source is the thermoelastic source supplemented by the vaporization (ablation) of
a layer of the material at the illuminated point. Vaporization occurs when the laser energy
is increased. The ablated material forms an ionized gas containing positive ions and
electrons (known as plasma) that immediately expands away from the surface of the
metal. The resulting momentum pulse from the plasma expansion is transmitted back into
the solid, thus enhancing the compression wave generation. As a result of the ablation, a
small pit (about 5x10-6 m deep) is formed on the surface of the material; this damage may
render the plasma source unsuitable for some applications.
High energy pulsed laser sources are appropriate in this application (for example a pulsed
YAG laser).
(C) Constrained surface regime
If the surface of the test material has some sort of coating, the resulting
mechanical wave generation by lasers will be affected. Certain coatings act to enhance
the mechanical wave generation significantly. The coating acts as a buried wave source at
the coating-surface boundary, and the amplitude of all wave modes is enhanced. Coatings
such as oil, resin, grease and a transparent constraining layer have been successfully used.
The nature of this source most closely resembles that of the plasma source. In fact, the
coating layer may be ablated, forming plasma, at higher power densities, although power
densities in both regimes may be used.
(III) Laser Detection of Wave Motion in Solids
Surface motion owing to propagating mechanical waves and vibration in solids
can be detected in non-contact fashion using a laser beam. However, the basis of the
approach to detect waves (generally called interferometry) is very different than that used
to generate mechanical waves in a solid using lasers. Continuous wave laser sources are
usually used for interferometry.
(A) Interferometry Overview
Optical interferometry is a sensitive way to measure surface displacement or
motion at all frequencies in a solid. Interferometers are useful because they are noncontact and also provide a flat, broadband response. However, interferometry requires a
highly monochromatic (light wave comprised of a single frequency) and directed light
source with a reasonably large coherence length. Thus, the use of CW lasers is essential
for optical interferometry. Interferometry makes use of the concept of wave interference,
described earlier, which is a natural characteristic of harmonic wave motion. Many
different interferometry schemes have been developed over the years, but all use laser
light reflected from or scattered by the surface of the tested material. Essentially all
interferometry schemes can be divided into two types. In the first type, reflected or
scattered light is made to interfere with a reference light beam. The phase difference
between the two coherent light beams, inferred from the intensity of the combined light
beam, gives direct measurement of out-of-plane displacement at the surface. The second
type of interferometer detects changes in the frequency of reflected or scattered light. The
output is dependent on the out-of-plane motion (velocity) at the surface. Interferometry
schemes can be applied using laser beams propagating through air and also laser light
propagating in a fiber optic cable.
(B) Michelson Interferometry
Interferometers of the first type are usually variations of the classic Michelson
interferometry scheme. Very small displacements (of the order of the wavelength of the
light) such as that caused by passing waves can be detected with this scheme. For this
scheme to work effectively however a sufficient amount of light must be reflected back
from the sensed surface. In other words, the tested surface should be shiny. Thus,
Michelson interferometry may not work for rough surfaces that do not reflect light
reference
mirror
beam
splitter
laser
source
moving
surface
photodetector
output
The Michelson scheme is the most basic interferometer. A laser beam is passed through a
splitter, which divides the light beam and sends it in two different directions. One of the
split beams reflects from a stationary reference mirror and is sent back to the splitter. The
other beam reflects from the moving surface, and the reflected light is also sent back to
the splitter. The two coherent beams are aligned and recombined at the splitter and this
light is sent to a light intensity sensor (also called a photodetector). The relative phase
difference between the two combined beams at the photodetector depends on the
difference between the path lengths of the individual beams. If this path difference is an
integral number of wavelengths, the beams interfere constructively and the intensity of
the combined beam is at a maximum. If the path difference is an integral number of
wavelengths  ½, the beams interfere destructively and the intensity of the combined
beam is at a minimum. Thus as the measured surface moves toward or away from the
splitter, the intensity of the combined beam at the photodetector varies sinusoidally
between the maximum and minimum values.
effectively. He-Ne lasers are most commonly used with this scheme because of the good
properties of the light (monochromatic light with good coherence and small wavelength).
(C) Heterodyne Interferometry
There are several kinds of the second type of interferometer, with heterodyne
interferometry perhaps being the most common. Essentially, the heterodyne approach
makes use of Doppler shift phenomenon to measure surface velocity (as opposed to
surface displacement with the Michelson scheme) of a material. The well-known Doppler
shift phenomenon states that the frequency of a wave (for example laser light) that
reflects from a surface will be slightly shifted if, at the moment of reflection, the surface
is moving with a specific velocity. If the reflecting surface is moving towards the wave
source the frequency will shift upwards, while if moving away the frequency will shift
downwards. Like Michelson interferometry, a laser beam is split into a reference beam
and test beam and then recombined after reflection from the surface of the test material.
Instead of monitoring the relative phase difference in the combined signal however, the
frequency is monitored; this process is called “heterodyning.” If the recombined beams
have different frequency, intensity fluctuations, known as “beats,” are setup at a specific
frequency. The beat frequency is a function of the difference in light frequency of the two
beams. In order to simplify the signal processing, the frequency of the reference beam is
first slightly shifted a known amount (usually 10x106 Hz to 40x106 Hz increase) before
recombining with the test beam. An acousto-optic device, such as a Bragg cell, can be
used effectively to shift the frequency of light passing through it. The resulting beat
frequency is monitored with a basic frequency tracking procedure. The beat frequency of
the combined signal is a result of the known shift of the test signal plus or minus
(depending on the direction of surface motion at that moment) that owing to the Doppler
phenomenon in the test beam. The frequency tracking procedure gives the instantaneous
beat frequency, but is independent of the signal amplitude and often precludes the
detection of higher frequency wave motion. This scheme is appropriate for relatively
large amplitude surface motions (notably larger than the wavelength of the laser light)
and can be applied more readily to rough surfaces since frequency shifts, as opposed to
intensity of reflected light, are monitored. As a result, heterodyne interferometry is often
used to detect lower frequency, high amplitude wave motion such as that caused by
vibration. In fact such a test scheme is commonly referred to as “vibrometry.” CW He-Ne
lasers are commonly used in this scheme.
(IV) Application of Technology
Several engineering applications of the described technology are now presented.
(A) Laser-based ultrasonics
With the use of lasers, we can generate and detect mechanical wave energy
(ultrasound) in a solid material without direct physical contact with that solid. A noncontact sensing technique allows remote and speedy monitoring (for example
determination of dimensional properties, presence of defects, monitoring of material
composition, etc.) that does not disturb the process under investigation and is not
influenced by varying sensor contact and coupling conditions. Furthermore, a non-contact
sensing technique allows inspection in environmental conditions that preclude the use of
ordinary contact sensors. Laser ultrasonics allow the combination of a mature and
effective inspection technique, ultrasonic inspection, with non-contact sensing capability.
(1) metals
Laser ultrasonics has been successfully used to monitor the quality of steel
components during the manufacturing process. The temperature of steel during the hotrolling manufacturing process is approximately 1000o C, so conventional NDE
measurements with contact sensors are not possible. However, flaw detection,
dimensional sizing and material characterization are possible with laser ultrasonics. For
example, consider a laser wave generator acting at a point on the surface of a hot-rolled
steel plate and a laser detector sensing the same location on the opposite surface. The
presence of planar air-filled in the plate is indicated by a loss of signal between the wave
sender (pulsed thermoelastic or plasma source) and receiver (CW interferometer) because
of intensive reflection at the defect. A laser system can also be used to determine the
dimensions of continuous hot steel elements during the manufacturing process. Finally,
Non-contact sensing techniques
such as laser ultrasonics allow
inspection in environmental
conditions (for example high
temperature) that preclude the
use of ordinary contact sensors.
For example, a laser system can
be used to determine the
dimensions of hot (1000o C)
steel tube elements during the
manufacturing process. A high
energy pulsed laser is used to
generate longitudinal waves from a plasma source on the surface of the hot tube. The
plasma source is appropriate for this application since the vaporized material is a surface
oxide, leaving the base steel material unaffected, so the source is actually a constrained
surface ablation source. A laser interferometer is focused at the same point on the surface
so to detect motion owing to wave pulses reflected from the back wall of the tube. (see
upper inset figure) This configuration simulates ultrasonic pulse echo testing, where the
time required for wave echoes to travel through the wall thickness to the far surface and
back is monitored. (see lower inset figure) The thickness of the tube is then deduced
knowing the longitudinal wave velocity in the metal at the elevated temperature
(approximately 4900 m/s). In the signal below is generated by laser ultrasonic system on
a hot steel tube. The time delay between two echoes is approximately 4.5x10-6 seconds.
Assuming a wave velocity of 4900 m/s, this corresponds to a tube wall thickness of 11
mm. With such a system, tube
thickness in the range of 10 to 25
mm can be accurately measured
while the tube material is
traveling
past
the
sensor
configuration at a rate of 2 m/s.
The sensors can be as far away as
5 m from the hot material, thus
isolating it from the high
temperatures.
(Images
from
Scruby and Drain.)
laser ultrasonic systems can be used to monitor changing material properties as a function
of temperature. The material properties are surmised from the longitudinal wave velocity.
For example, the material properties of plutonium-gallium alloy across the temperature
range of 40 to 500o C can be measured with a laser system. Such remote measurements
were needed because of the high temperature of the metal and also the toxicity of
plutonium, which is housed in a sealed container.
(2) other materials
Laser-based ultrasonic systems are effective in rapidly scanning large areas of
panels made of fiber-reinforced composite material. Area scan rates of over 10 m2 per
hour are achieved. These types of structures (composite panels) are becoming more
common in civilian and military aircraft structures because of their low weight and high
strength, but may have complex shapes and contours that make conventional inspection
difficult. To assure the structural integrity of the component, it is important to detect the
damage in the panels during the manufacture of the material and throughout the life of
the aircraft. Recently, laser ultrasonic measurements have been applied to monitor paper
Laser based ultrasonic systems are effective in rapidly scanning large areas of panels
made of fiber-reinforced composite. Usually, a pulsed CO 2 laser is used to generate the
waves in the constrained layer mode. This source generates lower frequency waves,
which is appropriate for inhomogeneous materials such as fiber composites. An
interferometer is used to detect the signals, at a point on either the same side (as
illustrated above) or the far side of the panel, thereby simulating conventional ultrasonic
testing. A special interferometer is used that can detect signals from non-reflective
surfaces. The light beams are scanned across the large specimen using precisely
controlled mirrors. Thus the laser does not move at all during the inspection process and
the laser system is located up to 1.5 m away from the panel. Variation in panel thickness
and the location and depth of damage or defects is determined. Because of the large
amount of data generated by the scanning system, images that indicate the location of
damage in the panel can be created. In the time-of-flight image shown above, air-filled
delaminations within the material are clearly indicated as dark regions. (Image from
Thomson and Chimenti)
quality during the manufacturing process. The elastic stiffness of paper sheet, along
various directions, are important characteristics that should be monitored during the
manufacturing process to ensure quality. The measurements can be carried out while the
paper is moving; linear scan rates of 400 m per minute have been achieved. A pulsed
YAG laser is used to generate waves at a point with a thermoelastic (non-damaging)
source, and the resulting waves are detected with a Michelson interferometer using a CW
Argon laser. The waves are detected at a point on the same surface of the paper but some
known distance away from the point of wave generation. The time required for the wave
disturbance to travel the known distance along the surface between sending and receiving
points is used to infer elastic stiffness in that direction.
(B) “Smart” structures
If sensors are embedded within a material or inside of a structure, the internal
conditions can be monitored throughout the life of the component. For example, sensors
could monitor the stretching or compressing of a component continuously or monitor the
existence and propagation of cracks inside the component. The sensors provide a
feedback mechanism about the performance internal integrity of the structure, so these
types of structures are called “smart” structures.
(1) fiber optic sensors
Optical fibers may be embedded within or glued onto a structure. After placement
the fibers can detect minute variation in structural conditions and thus act as sensors.
External conditions such as stretching (strain), pressure or temperature variations induce
changes in the phase, intensity or wavelength (frequency) of the light traveling in the
optical fiber. By monitoring the changes in the light, variation in the conditions causing
the change can be inferred. An added benefit of using optical fiber in large structures is
that the light signals can travel great distances inside the optical fiber. Therefore, the laser
source need not be near the inspected area, and the data can be collected efficiently at a
centralized location. At present, fiber optic sensing technology is used primarily to
monitor the amount of stretching or compressing (strain) that occurs inside the material
as a result of external loads. However, it can also be used to monitor the progression of
cracking inside the material. The principle is based on the assumption that the optical
fiber that is firmly embedded in a solid material will itself fracture if a crack propagating
in the material interacts with the fiber. As soon as the fiber breaks, the intensity of the
light propagating in the cable drops to zero. The most basic sensor is the intensity-type,
where the change in the intensity of the light, generated by a diode laser, passing through
the optical fiber correlates to the amount of strain applied to the cable. Other types of
optical fibers (for example Bragg grating sensors) correlate the applied strain to a shift in
the frequency of the light passing through. Interferometry techniques can also be applied
using fiber optics, where the phase of light passing through strained optical fiber is
compared to that passing through an unstrained (reference) cable.
Fiber optic sensors have been incorporated in a range of smart structures,
including complicated components such as panels and wingbox structures in aircraft and
large structures such as bridges. In the case of aircraft structures, the sensed material is
usually fiber-reinforced composites. The optical fiber is incorporated into the fiber lay-up
of the composite, thus providing an active and continuous internal strain sensor without
disrupting the continuity of the material itself. In bridge structures, optical fibers have
been used to monitor strains in concrete girders and also in steel reinforcing bars placed
in concrete. Fiber optic sensors have been incorporated into several new structures for the
purpose of long term strain monitoring, and the experience to date have been generally
favorable. For example, fiber optic cables are embedded in the triple span
Schiessbergstrasse bridge in Germany and the double-span Beddington train bridge in
Canada to monitor the long term compressive pre-stressing force applied to cables
embedded in the concrete bridge girders. In the case of the Canadian bridge, the sensors
provided continuous dynamic and static strain monitoring for a period of two years and
internal displacement measurements with a precision of 5x10-6 m were achieved.
In a recent study, optical fibers were used to monitor strain in steel reinforcing bars
placed in concrete beams that were subjected to bending loads. The optical fibers are
embedded inside the beams; the fibers are glued to the reinforcing bars before being cast
in concrete. Conventional strain gauges were also attached to the bars. As illustrated in
the graph above, the fiber optic strain sensors give comparable results to that from
conventional externally mounted strain gauges. In the figure, the amount of bending load
(moment ratio) is plotted against bending deformation (strain). Fiber optic sensors are
more rugged than conventional strain gauges and can provide continuous strain
information for long periods of time, possibly throughout the life of the structure. (Image
from Ansari)
As the performance of fiber optic sensors improves, they will be used on a much
larger scale and for a wide range of measurements. For example, new applications of
optical fibers have been recently proposed to monitor the condition of bridge
components. These new sensors provide information on bridge deck vibration, expansion
joint travel, bolt clamp load, cable tension and metal fatigue. These tests would provide a
comprehensive monitoring approach to bridges. The performance of these test schemes in
actual bridges will be evaluated in the near future.
(2) remote vibration or strain monitoring
Internal damage in a large structure introduces local stiffness losses, which in turn
is manifested in the dynamic response of the structure. Thus, the natural vibration
response of a structure (for example the frequency and mode shape of the vibration) is
affected by damage within the structure. Thus, monitoring the dynamic response of a
large structure can provide clues about the condition of the structure itself. These
vibration measurements are usually carried out with the use of sensors mounted directly
on the structure. However, the amount of data that can be collected from the structure is
restricted because of the limited number of test points and the long-term aggressive
environmental conditions to which the sensors are subjected. These problems are
remedied by the use of remote, non-contact vibration monitoring with a laser heterodyne
interferometer (vibrometer). The velocity of the surface motion, caused by natural
vibration (for example that caused by nearby machinery, wind or traffic loading) is
monitored. From this response, the natural frequencies of vibration or extent of surface
motion at a point can be determined.
In some cases, it is
important to monitor the
extent of surface motion
of structures owing to
vibration. Scanning laser
vibrometers can perform
multiple
vibration
measurements from one
test
head
location,
allowing
complete
inspection of a structure
in a relatively short
amount of time. A full
field vibration image of a
large object may be
created if enough data is
collected. In the inset image, the surface motion data from the side panel of a van during
engine operation is collected using a laser vibrometer. Different magnitudes of surface
motion are separated by the contour lines superimposed on a photograph of the van.
Areas of high surface velocity are marked by “+”. (Image from Scruby and Drain)
Laser vibrometers have been used successfully to monitor the cable force in
cable-stayed bridge structures. In these structures, cable tension is one of the most
important structural parameters. The amount of tension in the cable is directly related to
the natural vibration frequency of the cable, analogous to a guitar string. The technique
was successfully applied to the Weirton-Stubenville bridge in West Virginia, which has
52 cables in the structure. Vibration in the cables was excited naturally by wind. All
cables were monitored from several different vibrometer stations and were inspected
within two days. Vibrations were accurately monitored from distances up to 100 m, and
the resulting computed cable tensions matched those obtained by conventional methods.
The conventional method to monitor cable tension is expensive and time consuming.
(V) References
State of the art in the applications of fiber optic sensors to cementitious composites. F.
Ansari in Cement and Concrete Composites, Volume 19, pages 3-19. 1997.
Laser-generated ultrasound: its properties, mechanisms and multifarious applications.
S.J. Davies, C. Edward, G.S. Taylor and S.B. Palmer in Journal of Physics D: Applied
Physics, Volume 26, pages 329-348. 1993.
Understanding Fiber Optics, Third Edition. J. Hecht. Prentice-Hall Inc., Upper Saddle
River, NJ. 1993.
Laser Ultrasonics – Techniques and Applications. C.B. Scruby and L.E. Drain. Adam
Hilger Ltd., Bristol, UK. 1990.
Review of Progress in Quantitative Nondestructive Evaluation. Volumes 12 and 17. D.O.
Thompson and D.E. Chimenti, editors. American Institute of Physics, Melville, NY.
1999.