Download ASNT Level III Study Guide Electromagnetic [Yasser Tawfik].pdf PDF

TitleASNT Level III Study Guide Electromagnetic [Yasser Tawfik].pdf
TagsElectric Current Magnetic Field Inductor Inductance Electromagnetic Induction
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Total Pages82
Table of Contents
                            ASNT Level III Study Guide Electromagnetic Testing
BW Cover
Table of Contents
Chapter 1 - Principles of Eddy Current Testing
Chapter 1 Review Questions
Chapter 2 - Test Coil Arrangements
Chapter 2 Review Questions
Chapter 3 - Test Coil Design
Chapter 3 Review Questions
Chapter 4 - Effects of Test Object on Test Coil
Chapter 4 Review Questions
Chapter 5 - Selection of Test Frequency
Chapter 5 Review Questions
Chapter 6 - Instrument Systems
Chapter 6 Review Questions
Chapter 7 - Eddy Current Applications
Chapter 7 Review Questions
Chapter 8 - Other Electromagnetic Techniques
Chapter 8 Review Questions
Chapter 9 - Eddy Current Procedures, Standards and Specifications
Chapter 9 Review Questions
Answers to Review Questions
Document Text Contents
Page 1

Level III
Study Guide
ElectroDlag etic
second edition

The American Society for Nond estructive Testing

Page 41

C =

B =
S/N =

Equation 29

rate of information transmitted in bits
per second
bandwidth of the signal
signal-ta-noise power ratio

This is known as the Shannon-Hartley theorem.
Another approach to multiparameter techniques

is to use a multiplexing process (12). The
multiplexing process places one frequency at a time
on the test coil. This results in zero crosstalk
between frequencies and eliminates the need for
channel specific bandpass filters. The major
advantages of a multiplex system, in addition to the
crosstalk reduction issues, are lower cost and
greater flexibility in frequency selection.

If the multiplex switching rate is sufficiently
high, both broadband and multiplex systems have
essentially the same results. The characterization of
eddy current signals by their phase angle and
amplirude is a common practice and provides a
basis for signal mixing to suppress unwanted
signals from test data (12). Two frequencies are
required to remove each unwanted variable.

Practica l multipara meter frequency selection can
be demonstrated by the following example:

Problem: Eddy current inspection of installed
thin-wall non ferromagnetic heat exchanger tubing.
Tubing is structu rally supported by ferromagnetic
tube supports at several locations. It is desired to
remove the tu be support response signal from tube
wall data.
Solution: Apply a ffiultiparameter technique to
supprt.'Ss the tube support signal response.


First, a frequency is selected to give optimum
phase and amplitude information about the tube
wall. This is ca lled the prime frequency. At the prime
frequency, the response to the tube support and to a
calibration through wall hole are about equal in
amplitude. They may also ha\'e about the same
phase angle.

A second frequency called the sub/rador frequency
is selected on the basis of the phase angle of the
tube support response. Because the tube support
surrounds the outside diameter of the tube, a lower
frequency is selected . At the subtractor frequency
the tube support signal response is about 10 times
greater than the calibration through wall hole. The
phase difference between the support signal and the
through wall hole in this lower frequency will be
about 90 degrees. Parameter separation limitations
are greatest for those parameters producing nearly
similar Signals, such as dents.

If the prime and subtractor channels have been
selected properly then Signal subtraction algorithms
should be able to suppress the tube support signal
leaving only slightly attenuated prime data
(discontinuity) information. For suppression of
inside or near surface Signals, a higher subtractor
frequency would be chosen.

A combination of prime, low and high subtractor
frequencies is often used to suppress both near and
far surface signals, leaving only data pertaining to
the part thickness and its condition. Bandwidth of
the coil is of prime importance when operation over
a wide frequency range is required in
multifrequency I multiparameter testing.

Optimization of a test frequency (or frequencies)
will therefore depend on the desired measurement
or parameter(s) of interest (11, 12, 4).

Page 42

Chapter 5
Review Questions

Q.'s.l Wh<l t frequency is required to establish a
standard depth of penetration of 7.6mm
(0.1 in.) in Zirconium?
A. 19.6 kHz
B. 196 Hz
C. 3.4 kHl
D. 340 H:t.

Q.5.2 To reduce effects of far surface indications,
the test frequency:
A. must be m ixed .
B. must be raised .
C. must bt:' lowered.
D. has no effect.

Q.5.3 The frequency required to establish the
Bessel function argument A cqUelJ to 1 is
A. an optimum frequency.
B. a fesonant frequency.
C. "limit f<equency.
D. a penetration frequency.

Q.5.4 Calculate the limit frequency for a copper
bar (q = 50.6 meter /ohm mm2) 1 em in
diameter. The correct limit freq uency is:
A. 50 kHz.
B. 50.6 Hz.
C. 100 Hz .
D. loa kHz.

Q.S.5 Using the example in Question 5.4, what is
the fifg ratio if the test frequency is 60kHz?
A. 1.2
B. 110
C. 60
D. 600


Q.5.6 In Figure 3.1(b) the value roLsG equaling 1.4
'would be indicative of:
A. a high resisti\' ity material.
B. a high conductivity material.
C. a low conductivity material.
D. a nonconductor.

Q.5.7 Primary resistance is subtracted from
Figure 3.1 (b ) because:
A. rcsistJIlCe is always constan t
B. resistance is not frequency dependent.
C. resistance does not add to the

D. None of the above.

Q.5.8 The reference qu.,n tity is d ifferent for solid
cy linder and thin-wall tube in Figure 5.2
b ecause:
A. the frequency is different.
B. the conductivity is different.
e. the skin effect is no longer negligible.
D. the thin-w<lll tube has not been


Q.5.9 A 25% dcep crack open to the ncaT su rface
gives a n,-,sponse times greater
than the same crack 3.3% of diameter under
the surface. (Refer to Figure 5.4. )
A. 10
B. 3
C. 2
D. 5

Q.5.10 When using multifrequency systems, low
subtractor frequencies are used to suppress:
A. conducti vity changes.
B. far surf<lce signals.
C. ncar 1:iurface signals.
D. permeability changes.

Page 82

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