Download 5.0 Background Information and Eddy Current Theory

5.0 Background Information and Eddy Current Theory
5.1 Anodize and Alodine Coatings
The purpose of conversion coating aluminum prior to painting is to prime the
surface for paint adhesion and protect against corrosion. Table 1 summarizes the
properties of anodized and alodined coatings. Anodizing is an electrochemical
process which converts surface aluminum to aluminum oxide [7]. The process
results in a non-conductive coating. The anodic finishing on the 737 rivets uses
coating per MIL-A-8625. All anodized rivets are Class 1, Type I, IB or II. Type I is
a chromic acid anodize, Type IB is a low-voltage chromic anodize, and Type II is
a sulfuric anodize [8]. Anodized coatings provide better corrosion resistance and
harder surface finish; they are less expensive than alodined coatings. Anodized
coatings consist of a solution that varies with the type. For example, Type I and
IB are chromate solutions, and Type II is sulfuric. A cathode is connected to the
negative terminal of a voltage source and placed in the solution. An aluminum
component is connected to the positive terminal of the voltage source and placed
in the solution. When the circuit is on, the oxygen in the anodizing solution
separates from the water molecules and combines with the aluminum on the part
forming an aluminum oxide coating [9].
Alodined coatings, often referred to as chromate conversion coatings, are
electrically conductive and clear. Most alodined coatings contain 18-20%
Chromium, 5% Aluminum, 15-17% Phosphate and up to 2% Fluoride [10]. The
alodined finish on the 737 rivets use coating per MIL-C-5541E [3]. The
application of alodined coatings is easier than that of anodized coatings. The
application of alodined coatings involves immersing the material in strong acid
chromate solutions [11]. Upon heating, the coatings lose 40% of their weight [10].
The result is favorable because dehydration creates desirable corrosion
resistance.
Table 1. Properties of Conversion Coatings [7, 8, 9, 10, 11]
FINISH
ANODIZED RIVETS
ALODINED RIVETS
Coating Description
Electrolytic Coating
Chromate Conversion
Coating
Conductivity
Non-Conductive
Electrically Conductive
Military Specification
MIL-A-8625F (Class I,
Type I, IB, or II)
MIL-C-5541E (Class 1A,
Colorless)
Corrosion Resistance
Excellent
Good
Bonding Properties
Excellent
Excellent
Cost
Inexpensive
More Inexpensive
Application Time
Time Consuming
Quick Application
Hardness
Harder Surface Finish
Hard Surface Finish
Coating Thickness
0.002 in ± 20%
0.006 – 0.009 in
From the table one can see that the properties of alodined and anodized coatings
do not greatly vary. The greatest property difference is the electrical conductivity
of alodined coatings.
5.2 Boeing’s Rivet Change
One of the most important factors in the design of an aircraft is proper grounding.
Improper grounding may result in unreliable system operation--e.g., EMI,
electrostatic discharge damage to sensitive electronics, personnel shock hazard,
or damage from lightning strike [5]. Grounding is the process of electrically
connecting conductive objects to either a conductive structure or some other
conductive return path for the purpose of safely completing either a normal or
fault circuit [5]. Changing from anodized rivets to alodined rivets enables the
fuselage to act like a channel because alodined rivets are electrically conductive
and anodized rivets are not. As a result, physical damage to the aircraft during
lightning storms is minimal. For this reason, Boeing mandated that all new lap
joint rivets be alodined instead of anodized.
5.3 How Eddy Currents Work
Eddy current inspections are the most common non-destructive testing method
for detecting surface and near-surface defects in metals that are electrically
conductive [12]. There are two types of eddy current testing: high-frequency eddy
current (HFEC) and low-frequency eddy current (LFEC). HFEC detects surface
cracks, porosity, and corrosion. LFEC detects corrosion but is best at detecting
subsurface cracks. This method passes alternating current through a coil that
produces a magnetic field. When the coil is near an electrically conductive
surface, the changing magnetic field induces current flow in the surface material.
The changing magnetic field induces its measurement current flow in the
materials being inspected. After its measurement, the flow detects flaws and
characterizes material properties [12]. If the rivet-to-skin interface is nonconductive (if the lap joints contain anodized rivets), the receiving signal shown
on the inspection instrument’s display reveals defects. However, lap joints that
contain alodined rivets, and therefore contain a conductive rivet-to-skin interface,
result in degraded signals on the instrument display. Because alodined rivets are
electrically conductive, the current induced in the lap joint is able to flow through
the fastener. For this reason, the current density underneath and around the
rivet head is reduced and therefore causes a degradation in eddy current signal.
If a defect is present, the signal may not reach the required threshold for
rejection.
5.4 Eddy Current Detection
As mentioned earlier, alodined rivets conduct electricity therefore allowing current
to easily flow into the fastener, reducing the current density underneath the rivet
head and therefore resulting in a degradation of the signal. Current is emitted
into the inspection surface via a transmitting coil [13]. The quantity of emitted
current is defined by flux, Φ, which is described by equation 2.

   B  nˆ da
(2)

where B = flux density
and
nˆ  da = inspection area over which current is transmitted
Because the magnetizing coil within an eddy current probe is generally held
close to the inspection area, a flux,  p , is generated and is a function of the
coiler parameters and the primary excitation current I p as stated in equation 3.
I p  I o sin( t )
where I o = maximum current
ω = angular frequency
and t= time [13]
The angular frequency is calculated in equation 4.
(3)
  2f
(4)
where f = frequency
Equations 2 and 3 are related in equation 5.
 p  N pI p
(5)
Flux flowing through the inducting coil is oscillatory in nature and therefore
induces a current in the object undergoing inspections [14]. The induced eddy
currents are distributed into the rivet or other inspection media. Because eddy
currents are circulatory, they also produce a secondary flux,  s which opposes
the primary flux,  p . When the eddy current probe is not near a ferromagnetic
material, the flux in the coil is the primary flux only. As the probe is moved
toward the inspection area, the secondary flux is induced, causing the net flux
within the coil to change[14].
According to Faraday’s Law, electrical current is generated by placing a
conductor in a changing magnetic field. This process is referred to as induction
because current is “induced in the conductor by the magnetic field” [13]. The
amount of current induced in a conductor is dependent on the rate at which the
magnetic field changes. In an eddy current probe, AC current is passed through
an inducting coil into a conductive material. It should be noted that since it is the
changing magnetic field that is responsible for inductance, it is only present in AC
circuits and that high frequency AC will result in greater inductive reactance since
the magnetic field is changing more rapidly. As the current in the coil changes,
the properties of the magnetic field also change in both magnitude and direction
[14]. Voltage is also applied as the magnetic field changes and is proportional to
the change in current with time. Voltage is defined in equation 6.
VI  L
(6)
di
dt
where V I = induced voltage
L = impedance
and
di
= rate of change of current
dt
The induced voltage, V I , opposes further changes in the applied voltage. If AC
current is driven through an inductor, the voltage across the inductor will be
maximum when the rate of change of current is highest. For a sinusoid wave,
this occurs where the actual current is zero. Therefore the voltage applied to an
inductor is at it’s maximum value a quarter-cycle (90°) before the current reached
it’s maximum value [13]. In other words, the voltage leads the current by 90
degrees.
The voltage and current values are calculated as follows
V  I *XL
(7)
where X L = inductive reactance
Inductive reactance is defined in equation 8.
X L  2 * f * L
(8)
where L = inductance
f = frequency in hertz
Impedance is directly related to resistance and inductance as follows
Z  ( X L2  R 2 )
(9)
1
 is
 sin
( X L / Z ) between phase angle and current as described in equation
the relationship
10.
  sin 1 ( X L / Z )
(10)
Material conductivity and eddy current flow have a direct relationship. Materials
with larger conductivities have a greater flow of eddy currents on the surface.
Eddy current density is greatest on the surface of a material and decreases with
depth. Therefore a “standard depth of penetration” is defined and is the depth at
which the eddy current is 1/e (37%) of it’s surface value [14]. The standard
depth of penetration is
  sin 1 ( X L / Z )
  50  / f * r
where
(10)
(9)
 r = resistivity
f = frequency in hertz
Several trends are able to be noted with measurement of delta. Depth of
penetration decreases as frequency increases and also as conductivity increase.
“Effective depth of penetration” is defined as three times the standard depth. At
this depth, eddy current density is reduced to approximately 3% of its surface
value [14]. When a crack is present in the inspection media, the eddy current
flow will decrease, causing a decrease in loading on the coil and therefore
increasing the effective impedance. By monitoring the voltage changes across
the coil, defects in the inspection material are noted [14].
5.5 Cause of Error in Inspections
After the rivet process change, degradation in eddy current signals occurred
during routine lap joint inspections. Later, mechanics discovered that alodined
rivets caused the degraded signal. Because alodined rivets conduct electricity,
current flows easily into the fastener; thus, the current density underneath the
rivet head diminishes and results in a degradation of the signal. Figure 3
illustrates the current path around a small crack of both an anodized and an
alodined rivet head. From the figure, one can visualize the deviation of the
current’s path through the alodined fastener and the degrading effect it has on
the received signal. Even though a crack exists, the strength of the alodined
rivet’s signal is not strong enough to reach threshold level. Instead, the
instrument screen will display a false “accept indication.” Signal degradation
causes false readings to occur and impairs the detection of premature cracks.
Figure 3. Current Problem Associated With Alodined Rivets [15]
It is important to note that when eddy current testing occurs, anodized rivets
consistently create long rivet signals, and alodined rivets produce inconsistent,
short rivet signals that have a phase change in the position versus impedance
display [15].
5.6 Current Process for Rivet Finish Detection
Boeing’s current solution for determining the lap joint rivet finish uses lowfrequency eddy current. The procedure uses a sliding probe and a measuring
instrument with an impedance display. The method is inefficient because it is
essential that the probe be aligned with the fasteners’ centerline. This requires
the use of a probe guide or a second member of maintenance to monitor the
probe’s alignment. The instrument calibration is both time-consuming and
complicated. Furthermore, this eddy current method is not able to detect all
alodined rivets. Boeing’s 737 NDT Manual states that alodined rivets having
poor contact with the countersink may cause large signals and are not
identifiable by the procedure [3]. The procedure requires that an additional
internal inspection be conducted if the first inspection reveals alodined rivets at
four or more tear strap locations or if it reveals twenty or more alodined rivets in
one lap joint section [5].