Residual stress measurements in metal objects using four coils

A device for measuring residual stress in ferromagnetic and non-ferromagnetic metal objects. The device having four electrically identical induction coils which form a four terminal alternating current bridge circuit. There is a fine wire shield formed of fingers for shielding the coils from stray capacitance. The bridge having four coil terminals. Two diagonally opposite coil terminals are connected to a variable frequency constant voltage generator. The other two diagonally opposite coil terminals are connected to a low noise broad band preamplifier. The preamplifier amplifies any unbalance in the bridge. There is a double pole double throw switch connected to two coil terminals which are diagonally opposite each other. Connected to the preamplifier is an amplifier. The amplifier is connected to a phase detector which is connected to a computer. The phase detector detects in phase and quadrature component signals. The computer has software for determining changes in differential resistivity as a function of frequency, and for converting resistivity differences into residual stress using an algorithm.

TECHNICAL FIELD 
The present invention relates to the generation of eddy currents and more 
particularly to a sensitive test method and equipment for determining the 
residual stress in both ferromagnetic and non-ferromagnetic metal objects 
using eddy currents. 
BACKGROUND OF THE INVENTION 
In industrial situations some metals, e.g., titanium, aluminum, steel 
alloys and stainless steel, are shot-peened in order to increase the 
residual stress in the metal object. As a non-destructive test procedure, 
eddy current measurements are employed to determine whether the entire 
surface of the metal object was shot peened and whether the residual 
stress has the correct distribution with respect to the depth in the metal 
object. In other instances eddy currents are used to determine surface and 
sub-surface cracks or flaws in metal objects. Such defects produce 
relatively large changes in the induced currents and common eddy current 
equipment easily detects these changes. The SmartEddy.TM. system, 
manufactured by SES Corporation, Menlo Park, Calif., uses a two resistor, 
two coil bridge to detect amplitude and phase shifts produced by treated 
metal objects compared to a test standard. U.S. Pat. No. 5,610,515 
modified the SmartEddy.TM. system to improve its sensitivity in 
determining residual stress in treated non-ferromagnetic metal objects by 
reducing the sensitivity of the apparatus to vibration and temperature 
changes. 
Current induced into a metal object by an external coil is called eddy 
current. The use of eddy current technique to characterize the properties 
of metal objects is based on the relationship between a conductive metal 
object structure and electromagnetic properties. More specifically, eddy 
current testing relies on electromagnetic interaction between the coil 
driven by an alternating electrical current and the metal object under 
test. In order to measure applied or residual stresses with a conventional 
eddy current approach, the application of stress must change the 
electrical conductivity such that a detectable change in the test coil 
impedance will occur. 
Presently, current commercial eddy current equipment is not sensitive 
enough to detect extremely small changes in conductivity in a treated 
metal object. Nor is present commercial eddy current equipment sensitive 
enough to detect changes in conductivity at the near-surface of a treated 
metal object, i.e., within 0.020 inch. 
It is an object of the present invention to provide a method and equipment 
to detect very small changes in conductivity between a treated metal 
object and an untreated metal object sample used as a reference. 
It is another object of the present invention to be operated at a wide 
range of frequencies thereby checking the sample at various depths. 
It is another object of the present invention to be operated at high 
frequency in order to measure conductivity at the near-surface of a 
treated metal object. 
It is a further object of the present invention to determine the residual 
stress of a metal object that has been subject to machining, shot-peening 
or other chemical and mechanical treatments. 
It is a still further object of the present invention to avoid ambiguous 
results due to mechanical vibration and lift-off as the test coil is 
brought close to the sample. 
These and further important objects of the present invention will become 
more apparent upon considering the following detailed description of the 
present invention. 
SUMMARY OF THE INVENTION 
The method of this invention for determining residual stress in both 
ferromagnetic and non-ferromagnetic metal objects comprises the steps of: 
contacting simultaneously the surfaces of a treated and untreated metal 
object with four spaced apart identical induction coils, two coils for 
each metal object, the four coils form a four terminal balanced 
alternating current bridge circuit having two diagonals; connecting a 
double pole double throw switch between two diagonally opposite coil 
terminals for interchanging two of the coils of the bridge by exchanging a 
coil on the treated metal object with a coil on an untreated metal object 
to produce a maximum asymmetry in the bridge configuration; connecting two 
diagonally opposite corners of the bridge to a variable frequency constant 
voltage generator; connecting the other diagonally opposite corners of the 
bridge to a low noise broad band preamplifier, wherein the preamplifier 
amplifies any unbalance in the bridge; connecting the broad band 
preamplifier to an amplifier; connecting the amplifier to a phase 
detector; energizing the bridge in the frequency range of 10 kHz to 200 
MHz causing current flow in the coils; detecting an in phase component 
signal and a quadrature component signal when the bridge is in nearly 
symmetric configuration; interchanging two of the coils of the bridge 
using the double pole double throw switch; energizing the bridge in the 
frequency range of from 10 kHz to 200 MHz causing current flow in said 
induction coils; detecting in phase component and quadrature component of 
the unbalanced signals when the bridge is in an unsymmetric configuration; 
subtracting the signals in the nearly symmetric configuration from the 
signals in the asymmetric configuration to determine a difference as a 
function of frequency, determining changes in differential resistivity as 
a function of frequency wherein low frequency penetrates said treated and 
untreated metal objects more deeply than a higher frequency; and 
converting a resistivity difference between the treated and untreated 
metal objects into residual stress created in the treated metal object 
using an algorithm. 
The method of this invention may be carried out using a device for 
measuring residual stress in a treated metal object which comprises four 
spaced apart coils of identical inductance which form a four terminal 
alternating current bridge circuit and suitable for operation in a 
frequency range of from 10 kHz to 200 MHz, a double pole double throw 
switch between two diagonally opposite coil terminals, wherein the switch 
interchanges two of the coils of the bridge, and a variable frequency 
constant voltage generator. The variable frequency constant voltage 
generator is connected to diagonally opposite coil terminals. The device 
further includes a low noise broad band preamplifier, wherein the 
preamplifier amplifies an unbalance in the bridge. The low noise broad 
band preamplifier is connected to the other diagonally opposite coil 
terminals. The device further includes a means for detecting in phase and 
quadrature components, a means for determining changes in differential 
resistivity as a function of frequency, and a means for converting 
resistivity differences into residual stress using an algorithm. 
Another embodiment of the device for measuring residual stress in a treated 
metal object comprises four flat identical induction coils positioned on a 
substrate, wherein the four coils are spaced apart so as to prevent any 
mutual inductance effects and wherein the four coils form a four terminal 
balanced alternating current bridge circuit having two diagonals and 
suitable for operation in a frequency range of from about 10 kHz to 200 
MHz. The four coils are shielded from stray capacitance by a fine wire 
shield formed of fingers. The device having a double pole double throw 
switch between two diagonally opposite coil terminals, and a variable 
frequency constant voltage generator which is connected to diagonally 
opposite coil terminals. There is a low noise broad band preamplifier, 
wherein the preamplifier amplifies an unbalance in the bridge and the 
preamplifier is connected to two other diagonally opposite coil terminals. 
The device also including an amplifier and a phase detector. The device 
further including a means for detecting in phase and quadrature 
components, a means for determining changes in differential resistivity as 
a function of frequency, and a means for converting differences into 
residual stress using an algorithm. 
The various features and advantages of the invention will become more 
apparent from the detailed description of a preferred embodiment of the 
invention when considered along with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to the drawings in greater detail, and first to FIG. 1, the 
invention is incorporated in a device for determining residual stress in 
both ferromagnetic and non-ferromagnetic metal objects, generally 
designated 10, which includes four spaced apart flat identical induction 
coils, 12, 14, 16, 18. One embodiment has the coils loose and the other 
embodiment has the coils positioned on a substrate 20 (FIG. 2). The 
substrate can be any plastic, ceramic or other suitable material that is 
flat with a low dielectric loss, preferably Teflon.RTM.. The four coils 
are spaced far enough apart to prevent any mutual inductance effects or 
interacting magnetic fields. The coils have an inductance of about 8 
microhenries to about 1 microhenry depending upon the chosen frequency 
range. The coils can be electrically similar and geometrically dissimilar 
in order to permit testing of the metal object if it has different 
contours. The four coils form a four terminal balanced alternating current 
bridge circuit, 22, 24-26, 28, 30-32. A treated metal object 34 is placed 
on two coils, e.g., 12 and 16, and an untreated metal object 36 is placed 
on the other two coils, e.g., 14 and 18. The metal objects can be 
aluminum, steel, stainless steel, nickel-cobalt-steel and titanium. The 
device will be sensitive to very small changes in conductivity. The 
treated metal object can be stressed to a minimum depth in order to 
strengthen the metal object, e.g., to 0.020 inches. The metal object can 
be stressed by machining, shot preening or other mechanical means. Eddy 
currents can also detect changes in the electrical conductivity of the 
surface due to carburizing, nitriding, and heat treatment. In a 
ferromagnetic metal object the alternating current is held to a few 
milliamperes to produce a field of about 1.times.10-4 Tesla or less. The 
bridge 38 has two pairs of diagonal coil terminals. Two coil terminals 
e.g., 22 and 28 are connected to a variable frequency constant voltage 
generator 40. The generator 40 generates a frequency range of 10 kHz to 
200 MHz. The lower frequencies penetrate a depth greater than higher 
frequencies. It is desirable to sample the surface of the metal object at 
various depths. The two coil terminals are diagonally opposite each other. 
The other two diagonal coil terminals, 26-30 or 24-32 connect to those 
terminals either in this order or the reverse order and are connected to a 
low noise broad band preamplifier 42. Each coil is shielded from stray 
capacitance by a fine wire shield formed of fingers 44, 46, 48, 50. 
Between two pairs of diagonally opposite coil terminals is a double pole 
double throw reversing switch 52. The switch connects terminals 26-30 to 
terminals 24-32 or to terminals 32-24 depending on the position of the 
switch. The preamplifier 42 is connected to an amplifier 54. The amplifier 
54 is connected to a phase detector 56. The phase detector 56 is connected 
to a computer 58. The phase detector 56 detects in phase and quadrature 
component signals. The computer 58 has several different software 
programs. Using the in phase and quadrature component signals, one program 
determines changes in differential resistivity as a function of frequency. 
The second software program converts resistivity differences into residual 
stress using an algorithm. 
To determine the residual stress of a treated ferromagnetic or 
non-ferromagnetic metal object, a user will simultaneously contact the 
surface of the-treated metal object 34 and the untreated metal object 36 
with 4 induction coils, 12, 14, 16 and 18. Two coils are on the surface of 
each metal object, e.g., 12 and 16, 14 and 18. The coils are arranged 
geometrically to have non-interacting magnetic fields. The coils will form 
a four terminal 22, 24-26, 28 and 30-32 balanced alternating bridge 
circuit, 38. There will be two diagonally opposite coil terminals e.g., 22 
and 28, 24-32 and 26-30 as shown in FIG. 1. A double pole double throw 
reversing switch 52 is connected to two diagonally opposite coil 
terminals, e.g., 24-26 and 30-32. Two diagonally opposite coil terminals, 
e.g., 22 and 28, are connected to a variable frequency constant voltage 
generator, 40. Placed over each coil is a fine wire shield formed of 
fingers, 44, 46, 48 and 50. Two of the diagonally opposite coil terminals, 
e.g., 26 and 30 or equivalently 24 and 32, are connected to a low noise 
broad band preamplifier 42. The preamplifier, 42, amplifies any unbalance 
in the bridge. The preamplifier, 42, is connected to an amplifier, 54, 
which is connected to a phase detector, 56. The phase detector, 56, is 
connected to a computer, 58, having several software programs. After all 
these connections are made, the bridge is energized in the frequency range 
of 10 kHz to 200 MHz causing current to flow in the coils, i.e., 22 to 14 
to 24 to 26 to 18 to 28 and 22 to 12 to 32 to 30 to 16 to 28. Lower 
frequencies penetrate the metal object more deeply than higher 
frequencies. The current is held to a few milliamperes to produce a field 
of 10-4 Tesla or less for any ferromagnetic metal objects. The phase 
detector 56 detects an in phase component signal and a quadrature 
component signal when the bridge is in a nearly symmetrical configuration. 
If all the circuit elements and the samples were ideal the bridge would be 
perfectly balanced. In practice a small error signal is detected due to 
slight differences in the test coil and/or sight point to point variations 
in the samples. Next the coils are interchanged using the double pole 
double throw reversing switch 52. The interchanging of the coils by 
exchanging a coil on a treated metal object with a coil on the untreated 
metal object produces a maximum asymmetry in the bridge configuration. 
After the interchange of the coils, the bridge is energized in a frequency 
range of 10 kHz to 200 MHz causing current to flow in the coils, i.e., 22 
to 14 to 24 to 30 to 16 to 28 and 22 to 12 to 32 to 26 to 18 to 28 as 
shown is FIG. 1. The next step is detecting the in phase component and 
quadrature component of the unbalanced signals when the bridge is in the 
asymmetrical configuration. The values secured for the in phase and 
quadrature components from energizing the coils while in the nearly 
symmetric configuration are subtracted from the value secured while the 
coils are in the asymmetric configuration. These values are a function of 
specific frequencies. Software is used to determine the changes in 
differential resistivity, and convert the resistivity changes between the 
treated and untreated metal objects into residual stress in the treated 
metal object using an algorithm. This method can be used on the entire 
surface of the metal object, point by point to determine whether the metal 
object has been properly treated and whether the residual stress has the 
correct distribution with respect to the depth in the metal object over 
the entire surface. 
The present invention has been described with a degree of particularity. It 
is the intent, however, that the invention include all modification and 
alterations from the disclosed embodiments falling within the spirit or 
scope of the appended claims.