ASM Metals HandBook Vol. 17 - Nondestructive Evaluation and Quality Control
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Microwave Inspection
William L. Rollwitz, Southwest Research Institute
Introduction
MICROWAVES (or radar waves) are a form of electromagnetic radiation located in the electromagnetic spectrum at the
frequencies listed in Table 1. Major subintervals of the microwave frequency band are designated by various letters; these
are listed in Table 2. The microwave frequency region is between 300 MHz and 325 GHz. This frequency range
corresponds to wavelengths in free space between 1000 cm and 1 mm (40 and 0.04 in.).
Table 1 Divisions of radiation, frequencies, wavelengths, and photon energies of the electromagnetic
spectrum
Photon energy Division of radiation Frequency, Hz
Wavelength, m
J eV
Radio waves (FM and TV)
3 × 10
8
1 1.6 × 10
-25
10
-6
3 × 10
9
10
-1
1.6 × 10
-24
10
-5
3 × 10
10
10
-2
1.6 × 10
-23
10
-4
Microwaves
3 × 10
11
10
-3
1.6 × 10
-22
10
-3
3 × 10
12
10
-4
1.6 × 10
-21
10
-2
Infrared
3 × 10
13
10
-5
1.6 × 10
-20
10
-1
Visible light 3 × 10
14
10
-6
1.6 × 10
-19
1
3 × 10
15
10
-7
1.6 × 10
-18
10 Ultraviolet light
3 × 10
16
10
-8
1.6 × 10
-17
10
2
3 × 10
17
10
-9
1.6 × 10
-16
10
3
3 × 10
18
10
-10
1.6 × 10
-15
10
4
3 × 10
19
10
-11
1.6 × 10
-14
10
5
X-ray and -ray radiation
3 × 10
20
10
-12
1.6 × 10
-13
10
6
3 × 10
21
10
-13
1.6 × 10
-12
10
7
Cosmic ray radiation 3 × 10
22
10
-14
1.6 × 10
-11
10
8
Table 2 Microwave frequency bands
Band designator
Frequency range, GHz
UHF
0.30-1
p
0.23-1
L
1-2
S
2-4
C
4-8
X
8-12.5
K
u
12.5-18
K
18-26.5
K
a
26.5-40
Q
33-50
U
40-60
V
50-75
E
60-90
W
75-110
F
90-140
D
110-170
G
140-220
Y
170-260
J 220-325
Source: Ref 1
Although the general nature of microwaves has been known since the time of Maxwell, not until World War II did
microwave generators and receivers useful for the inspection of material become available. One of the first important uses
of microwaves was for radar. Their first use in nondestructive evaluation (NDE) was for components such as waveguides,
attenuators, cavities, antennas, and antenna covers (radomes). The interaction of microwave electromagnetic energy with
a material involves the effect of the material on the electric and magnetic fields that constitute the electromagnetic wave,
that is, the interaction of the electric and magnetic fields with the conductivity, permittivity, and permeability of the
material. Microwaves behave much like light waves in that they travel in straight lines until they are reflected, refracted,
diffracted, or scattered. Because microwaves have wavelengths that are 10
4
to 10
5
times longer than those of light waves,
microwaves penetrate deeply into materials, with the depth of penetration dependent on the conductivity, permittivity, and
permeability of the materials. Microwaves are also reflected from any internal boundaries and interact with the molecules
that constitute the material. For example, it was found that the best source for the thickness and voids in radomes was the
microwaves generated within the radomes. Both continuous and pulsed incident waves were used in these tests, and either
reflected or transmitted waves were measured.
Reference
1.
Electromagnetic Testing, Vol 4, 2nd ed., Nondestructive Testing Handbook,
American Society for
Nondestructive Testing, 1986
Microwave Inspection
William L. Rollwitz, Southwest Research Institute
Microwave Inspection Applications
The use of microwaves for evaluating material properties and discontinuities in materials other than radomes began with
the evaluation of the concentration of moisture in dielectric materials. Microwaves of an appropriate wavelength were
found to be strongly absorbed and scattered by water molecules. When the dry host material is essentially transparent to
the microwaves, the moisture measurement is readily made.
Next, the thickness of thin metallic coatings on nonmetallic substrates and of dielectric slabs was measured. In this case,
incident and reflected waves were allowed to combine to form a standing wave. Measurements were then made on the
standing wave because it provided a scale sensitive to the material thickness.
The measurement of thickness was followed by the determination of voids, delaminations, macroporosity, inclusions, and
other flaws in plastic or ceramic materials. Microwave techniques were also used to detect flaws in bonded honeycomb
structures and in fiber-wound and laminar composite materials. For most measurements, the reflected wave was found to
be most useful, and the use of frequency modulation provided the necessary depth sensitivity. Success in these
measurements also indicated that microwave techniques could give information related to changes in chemical or
molecular structure that affect the dielectric constant and dissipation of energy at microwave frequencies. Some of the
properties measured include polymerization, oxidation, esterification, distillation, and vulcanization.
Advantages. In comparison with ultrasonic inspection and x-ray radiographic inspection, the advantages of inspection
with microwaves are as follows:
• Broadband frequency response of the coupling antennas
• Efficient coupling through air from the antennas to the material
• No material contamination problem caused by the coupling
• Microwaves readily propagate through air, so successive reflections are not obscured by the first one
• Information concerning the amplitude and phase of propagating microwaves is readily obtainable
•
No physical contact is required between the measuring device and the material being measured;
therefore, the surface can be surveyed rapidly without contact
•
The surface can be scanned in strips merely by moving the surface or by scanning the surface with
antennas
• No changes are caused in the material; therefore, the measurement is nondestructive
• The complete microwave system can be made from solid-
state components so that it will be small,
rugged, and reliable
•
Microwaves can be used for locating and sizing cracks in materials if the following considerations are
followed. First, the skin depth at microwave frequencie
s is very small (a few micrometers), and the
crack is detected most sensitively when the crack breaks through the surface. Second, when the crack is
not through the surface, the position of the crack is indicated by a detection of the high stresses in the
surface right about the subsurface crack. Finally, microwave crack detection is very sensitive to crack
opening and to the frequency used. Higher frequencies are needed for the smaller cracks. If the
frequency is increased sufficiently, the incident wave c
an propagate into the crack, and the response is
then sensitive to crack depth
Limitations. The use of microwaves is in some cases limited by their inability to penetrate deeply into conductors or
metals. This means that nonmetallic materials inside a metallic container cannot be easily inspected through the container.
Another limitation of the lower-frequency microwaves is their comparatively low power for resolving localized flaws. If a
receiving antenna of practical size is used, a flaw whose effective dimension is significantly smaller than the wavelength
of the microwaves used cannot be completely resolved (that is, distinguished as a separate, distinct flaw). The shortest
wavelengths for which practical present-day microwave apparatus exists are of the order of 1 mm (0.04 in.). However, the
development of microwave sources with wavelengths of 0.1 mm (0.004 in.) are proceeding rapidly. Consequently,
microwave inspection for the detection of very small flaws is not suited for applications in which flaws are equal to or
smaller than 0.1 mm (0.004 in.). Subsurface cracks can be detected by measuring the surface stress, which should be
much higher in the surface above the subsurface crack.
Microwave Inspection
William L. Rollwitz, Southwest Research Institute
Physical Principles of Microwaves
In free space, an electromagnetic wave is transverse; that is, the oscillating electric and magnetic fields that constitute it
are transverse to the direction of travel of the wave. The relative directions of these two fields and the direction of
propagation of the wave are shown schematically in Fig. 1. As the wave travels along the z-axis, the electric and magnetic
field intensities at an arbitrary fixed location in space vary in magnitude. A particularly simple form of a propagating
electromagnetic wave is the linearly polarized, sinusoidally varying, plane electromagnetic wave illustrated in Fig. 2. The
magnitude of the velocity, v, at which a wave front travels along the z-axis is given by the relation v = f λ, where f is
frequency and is wavelength. In free space, this velocity is the speed of light, which has the value 2.998 × 10
8
m/s and
is usually designated by the letter c.
Fig. 1 Relative directions of the electric field intensity (E), the magnetic field intensity (H
), and the direction of
propagation (z) for a linearly polarized, plane electromagnetic wave
Fig. 2 Diagram of a
linearly polarized, sinusoidally varying, plane electromagnetic wave propagating in empty
space. , wavelength; z, direction of wave propagation; E, amplitude of electric field; H
, amplitude of magnetic
field
In microwave inspection, a homogeneous material medium can be characterized in terms of a magnetic permeability, μ; a
dielectric coefficient, ε; and an electrical conductivity, σ. In general, these quantities are themselves functions of the
frequency, f. Moreover, μ and must usually be treated as complex quantities, rather than as purely real ones, to account
for certain dissipative effects. However, a wide variety of applications occur in which μ and ε can be regarded as mainly
real and constant in value. The magnetic permeability, μ, usually differs only slightly from its value in vacuum, while the
dielectric coefficient, ε, usually varies between 1 and 100 times its value in vacuum. The electrical conductivity, σ, ranges
in value from practically zero (10
-16
Ω· mm) for good insulators to approximately 10
7
Ω· mm for good conductors such as
copper.
For an electromagnetic wave incident upon a material, a part of the incident wave is transmitted through the surface and
into the material, and a part of it is reflected. The sum of the reflected energy and refracted energy (transmitted into the
material) equals the incident energy. If the reflected wave is subtracted in both amplitude and phase from the incident
wave, the transmitted wave can be determined. When the reflected wave is compared, in both amplitude and phase, with
the incident wave, information about the surface impedance of the material can be obtained.
Plane electromagnetic waves propagating through a conductive medium diminish in amplitude as they propagate, falling
to 37% of their amplitude at a reference position in distance, referred to as the skin depth, measured along the direction of
propagation. The skin depth, , in a good conductor ( , where is the angular frequency) is given by the relation
= (2/ )
1/2
. The velocity, v, of an electromagnetic wave propagating in a nonconductor is given by the relation v =
1/( )
1/2
. This velocity can be expressed relative to the velocity of electromagnetic waves in vacuum, the ratio being the
index of refraction, n, where n = c/v = ( /
0 0
)
1/2
with
0
and
0
being the magnetic permeability and dielectric
coefficient of free space. The phase velocity of a plane harmonic electromagnetic wave in a conductive medium can be
given as v = = (2 / )
1/2
, where is the skin depth. Therefore, the velocity, v, depends strongly on the frequency,
, even if magnetic permeability, , and electrical conductivity, , do not themselves depend on . As a result, a
conductive medium is said to be highly dispersive, because a wave packet, which comprises sinusoidal components of
many different frequencies, disperses (spreads) as it propagates. In conductive media, the magnetic component of an
electromagnetic wave does not propagate in phase with the electric component. Assuming | | | |, the surface
impedance of a material is (Ref 2):
(Eq 1)
where is the complex permeability, is the conductivity, is the complex permittivity, is the angular frequency, and j
= .
The skin depth, , of an electromagnetic wave propagating in a weakly conductive medium ( ) is given
approximately by the relation 2/ . In this case, the wavelength is approximately given by:
(Eq 2)
where
0
is the wavelength of the wave in vacuum, n is the index of refraction of the medium. Equation 1 is sufficiently
accurate for most materials having electrical conductivity low enough for practical microwave inspection involving
transmission. The criterion for its validity is that the nonattenuative wavelength,
0
/n, be short compared to the skin
depth, .
Reflection and Refraction. The laws of reflection and refraction of microwaves at interfaces between mediums of
differing, electromagnetic properties are essentially the same as those for the reflection and refraction of visible light. The
angle of refraction is determined by Snell's law, n
2
sin = n
1
sin , where n
1
and n
2
are, respectively, the indexes of
refraction of the two media; is the angle of incidence; and is the angle of refraction. The medium in which the incident
ray occurs is arbitrarily designated by the subscript 1, and the other medium by the subscript 2.
For linearly polarized plane waves incident perpendicularly on an interface separating two dielectric mediums, the
amplitudes of the reflected and transmitted waves, respectively, are given by:
E
max, reflected
= [(n
2
- n
1
)/(n
1
+ n
2
)] E
max, incident
(Eq 3a)
E
max, transmitted
= [2n
1
/(n
1
+ n
2
)] E
max, transmitted
(Eq 3b)
Analogous relations hold for the amplitudes of the magnetic field. For angles of incidence other than zero, the
corresponding relations are more complicated and will not be quoted here. The amplitudes of reflected and refracted
waves vary as a function of the angle of incidence for a typical choice of the ratio n
1
/n
2
, as shown in Fig. 3. The shapes of
the curves vary somewhat with the dielectric constant (Ref 3).
Fig. 3
Representative reflection of a linearly polarized plane electromagnetic wave at a dielectric interface with
electric field parallel or perpendicular to the plane of incidence.
(a) Wave entering dielectric. (b) Wave leaving
dielectric
The curves in Fig. 3 show the amplitude reflection factor as a function of the angle of incidence when the polarization
factor (direction of the electric field vector) is either parallel or perpendicular to the plane of the interface. The curves in
Fig. 3(a) represent the conditions in which the microwaves are entering the material, and the curves in Fig. 3(b) are for
waves leaving the material. In Fig. 3(a), the reflection factor increases steadily to unity at 90° for the perpendicular
polarization factor. The reflection factor for the parallel polarization decreases to zero at the Brewster angle and then
increases to total reflection as a function of increasing angle of incidence. In Fig. 3(b), the same occurrence takes place
except that the unity reflection factor for both polarization factors occurs at the critical angle of incidence rather than at
90°. The critical angle for reflection, from Snell's law, is equal to arc sin (1/
1/2
), where is the dielectric constant of the
material. The Brewster angle for the case of reflection is equal to the arc tan (1/
1/2
). The scattering cross section
oscillates with decreasing magnitude as the ratio of the circumference to wavelength varies from 0.3 to 10. Ratios from
0.3 to 10 are in the resonance region. The maximum scattering occurs when the circumference equals the wavelength. In
the optical region, the scatter cross section equals the physical cross section, and / r
2
1, where r is the radius of the
sphere. Thus, scattering is proportional to physical cross section ( r
2
).
Absorption and Dispersion of Microwaves. Microwaves are also affected during their propagation through a
homogeneous nonmetallic material, primarily by the interaction of the electric field with the dielectric (molecular)
properties of the nonmetallic material. The storage and dissipation of the electric field energy by the polarization and
conduction behavior of the material are the basic factors under consideration.
Polarization and conduction are the cumulative results of molecular-charge-carrier movement in the nonmetallic material.
Polarization involves the action of bound charges in the form of permanent or induced dipoles. Conduction refers to
whatever small amount of free charge carriers (electrons) is present. As a microwave passes through the material, the
dipoles oscillate because of the cyclic nature of the force on them from the electric field. The dipole oscillation alternately
stores and dissipates the electric field energy. Conduction currents only dissipate the energy, that is, convert it to heat.
The dielectric properties of a nonmetallic material are normally expressed in terms of the dielectric constant (permittivity)
and loss factor. The dielectric constant is related to the amount of electric field energy that the dipoles in a material
temporarily store and release during each half cycle of the electric field change. Materials with a high dielectric constant
have a great storage capacity. This reduces the electric field strength, the velocity, and the wavelength. The loss factor, or
loss tangent, expresses the dissipation of energy caused by both conduction and dipole oscillation losses. In other words,
the dielectric constant measures the energy storage, while the loss factor measures the dissipation of electromagnetic
energy by nonmetallics.
Standing Waves. Interference conditions usually prevail when microwaves are used for nondestructive inspection
because of the wavelengths and velocities involved, the coherent nature of the microwaves used, the high transparency of
most nonmetallic materials, and the fact that the thicknesses of the nonmetallic materials are within several wavelengths.
The familiar standing wave is the usual wave pattern resulting during nondestructive inspection. A standing wave (Fig. 4)
is produced when two waves of the same frequency are propagating in opposite directions and interfere with each other.
The result is the formation of a total field whose maximum and minimum points remain in a fixed or standing position.
Both component waves are still traveling, and only the resultant wave pattern is fixed. A simple way to form standing
waves is to transmit a coherent wave normal to a surface. The incident and reflected waves interfere and cause a standing
wave. The standing wave wavelength and peak amplitude change along the standing wave pattern and are related to the
velocity and attenuation experienced by the wave in a given medium. The technique by which standing waves are formed
with microwave radiation is used to make accurate measurements of thickness where conventional caliper methods are
especially difficult. The technique involved is discussed in the section "Thickness Gaging" in this article.
Fig. 4 Pattern of a standing wave formed by interference of an incident wave and a reflected wave
Scattering of Microwaves. Microwaves reflect from inhomogeneities by a process known as scattering. The
scattering is generally used to describe wave interaction with small particles or inhomogeneities. The term reflection is
generally used to describe wave interaction with surfaces that are large compared to wavelength. When the surface is not
smooth on a scale commensurate with the wavelength of the microwaves used, the reflected wave is not a simple single
wave, but is essentially a composite of many such waves of various relative amplitudes, phases, and directions of
propagation. This effect is greatest when the wavelength is comparable to the dimensions of the irregularities. Under these
circumstances, the radiation is said to be scattered. The scattering behavior of metal spheres having differing ratios of
circumference to the wavelength of incident microwave radiation is shown in Fig. 5. The scattering cross section is
proportional to the amplitude of the wave scattered in the direction back toward the source of the incident wave.
Fig. 5 Microwave scattering by metal spheres of various sizes
The scattering is small for values of the ratio of the circumference to the wavelength less than 0.3, and scattering varies as
the fourth power of this ratio. Ratios below 0.3 lie in what is known as the Rayleigh region.
Mathematically, the dielectric constant and loss factor are expressed in combined forms as a complex permittivity:
* = ' - j ''
(Eq 4)
where * is the complex permittivity or complex dielectric constant, ' is the permittivity or dielectric constant, j is the
phasor operator [(-1)
1/2
], and '' is the loss factor. Equation 4 shows that the dielectric constant is 90° out of phase with
respect to the loss factor. The loss tangent is equal to the ratio of ''/ '. Wave velocity and attenuation are related to
dielectric properties and therefore serve as a means of measurement.
The dielectric properties of a nonmetallic material are frequently affected by other material properties of industrial
importance. The degree of correlation depends on the frequency of the electromagnetic wave, and sensitive measurements
can often be made with microwaves.
References cited in this section
2.
H.E. Bussey, Standards and Measurements of Microwave S
urface Impedance, Skin Depth, Conductivity, and
Q, IRE Trans. Instrum., Vol 1-9, Sept 1960, p 171-175
3.
A. Harvey, Microwave Engineering, Academic Press, 1963
Microwave Inspection
William L. Rollwitz, Southwest Research Institute
Special Techniques of Microwave Inspection
The following general approaches have been used in the development of microwave nondestructive inspection:
• Fixed-frequency continuous-wave transmission
• Swept-frequency continuous-wave transmission
• Pulse-modulated transmission
• Fixed-frequency continuous-wave reflection
• Swept-frequency continuous-wave reflection
• Pulse-modulated reflection
• Fixed-frequency standing waves
• Fixed-frequency reflection scattering
• Microwave holography
• Microwave surface impedance
• Microwave detection of stress corrosion
Each of these techniques uses one or more of the several processes by which materials can interact with microwaves,
namely, reflection, refraction, scattering, absorption, and dispersion. These techniques will be briefly described from the
standpoint of instrumentation and are grouped to four areas:
• Transmission
• Reflection
• Standing wave
• Scattering
Transmission Techniques
The basic components of the transmission technique are shown schematically in Fig. 6. A microwave generator feeds both
a transmitting antenna and a phase-sensitive detector (or comparator). The transmitting antenna produces the
electromagnetic wave that is incident on one face of the material to be inspected. At the surface, the incident wave is split
into a reflected wave and a transmitted or refracted wave. The transmitted wave goes through the material into the
receiving antenna. All of the transmitted wave will not pass through the second face of the material because some of it
will be reflected at the second surface. The microwave signal from the receiving antenna can be compared in amplitude
and phase with the reference signal taken directly from the microwave generator.
Fig. 6 Diagram of the basic components of the transmission technique used for microwave inspection