FIELD OF INVENTION
The present invention generally relates to evaluation of stresses in high carbon
steel wires. More particularly, the present invention relates to a non-destructive
method of determining stresses in high carbon steel wires by eddy current
measurement.
BACKGROUND OF THE INVENTION
The residual stresses negatively influence the mechanical behavior of the wires
and their durability by reducing the service life of the produced wires due to
stress corrosion cracking or fatigue. The residual stresses further influence the
Bauschinger effect in cold-drawn wires. In the case of prestressed concrete rods,
the residual stress reduces the elastic limit and increase the losses during the
stress relaxation tests. This, in turn, may make the material unsuitable in respect
of pre-stressing standards. It is a matter of great significance that the wires
when subjected to manufacturing processes, such as the stranding tire cords, the
pre-stresses concrete wires or the coiling springs, the wires undergo further
plastic deformation.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a non-destructive
method to determine stresses in high carbon steel wires by eddy current
measurement which adapts a correlation between the tensile stresses and the
eddy current output voltage in high carbon steel wires.
Another object of the present invention is to propose a non-destructive method
to determine stresses in high carbon steel wires by eddy current measurement
which enables achieving desired stresses in coils made of high carbon steel
wires.
A still another object of the present invention is to propose a non-destructive
method to determine stresses in high carbon steel wires by eddy current
measurement which enables determination of the stresses in a large volume of
coils.
A further object of the present invention is to propose a non-destructive method
to determine stresses in high carbon steel wires by eddy current measurement
which ensures a better quality control of the product.
A still further object of the present invention is to propose a non-destructive
method to determine stresses in high carbon steel wires by eddy current
measurement which ensures reliability of the products produced out of such high
carbon steel wires.
SUMMARY OF THE INVENTION
In the present invention, a non-destructive method using eddy current principle
has been developed to evaluate the high carbon steel wires with respect to its
tensile stresses. A good correlation was also developed between the tensile
stresses and the eddy current output voltage which can be utilized to measure
the tensile stresses in such wires on line.
If the measurements of important properties like tensile stresses of high carbon
steel are made on line by non-destructive method like eddy current, the entire
coils can be tested for their tensile stresses and decisions on the coils can be
made in a more realistic way. Before entering for field trial, it is necessary to
verify this concept in laboratory stage by developing a correlation between the
eddy current parameters like output voltage and the tensile stresses in high
carbon steel wires.
In the present invention, eddy current testing was performed on high carbon
steel wires of commercial known Prestressed Concrete (PC) with different
stresses.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1- Graphically shows a correlation between eddy current frequency and
penetration depth in Prestressed concrete (PC) 115 wire.
Fig. 2- shows a Correlation between stress relieving temperature and eddy
current out put voltage in PC wire of dia. 4.25 mm
Fig. 3- shows a correlation between eddy current out put voltage and tensile
stresses during tensile stress testing.
Fig. 4- shows a correlation between eddy current phase angle tensile stresses
during tensile stress testing.
Fig. 5- shows a correlation between measured stresses and predicted stresses
using an empirical equation illustrated in Fig. 3.
Fig. 6- shows a correlation between measured stresses and predicted stresses
using an empirical equation illustrated in Fig. 4.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Samples of cold drawn wires of 4.25 mm diameter were taken from a bulk
sample of commercially known PC 115 wire. The chemical composition of these
wires is indicated in Table 1.

These wire samples were taken from the cold drawn wires which were tested for
torsional properties as per IS: 1717-1985. Nondestructive eddy current output
voltage in these samples was measured using a known Eddy Current Tester. The
eddy current measurements were taken by inserting these wires into an
encircling coil of internal diameter (ID) 5.5 mm.
Principle adapted in measurement of eddv current
The induced output voltage (V) in a sensing eddy current coil carrying current (I)
and impedance (Z) is given by the formula:

The phase angle e , between the output voltage (v) and the current (I) is given
by the formula:

where, XL is the inductive reactance and is given by the formula XL = 2 n.f.L------
—(3) (f is the frequency of the current and L is the property of coil known as
inductance).
Depth of penetration s of the induced eddy current in a test specimen is given
by the formula:

The output voltage (V) of the eddy current depends on magnetic permeability,
electrical conductivity (increase in electrical resistivity) and increase in frequency
of the current. The variables of the test material for example, microstructural
constituents, type of elements, discontinuities, residual stresses etc. affect the
magnetic permeability and electrical conductivity of the samples vis-a-vis the
output voltage of the eddy current.
EXAMPLE
In a complete stress relieved wire sample of diameter 4.25 mm, the standard
depth of penetration of eddy current for different test frequencies was calculated
and plotted as shown in Fig. 1.
A Stress relieving treatment of the said wire samples was imparted at
temperatures 150, 200,250,300,350 and 400 °C in a known Muffle Furnace. The
test samples were subjected to withstand these temperatures for at least ten
minutes, and then air cooled. Eddy current out-put voltage in these samples was
measured using a 5.5 mm dia encircling coil. Fig. 2 shows a correlation between
stress relieving temperatures and eddy current out put voltage. The graph clearly
shows the evidence of correlation between the released stresses and eddy
current out put voltage.
To quantify the stresses and to correlate the tensile stresses in the LRPC wire
rods with eddy current out put voltage, a 225 mm long and 10 mm diameter test
piece was prepared from 13.13 mm diameter wire rod. During the tensile
loading, for different loads/stresses before its yield point, corresponding to eddy
current out put voltage including the phase angles were recorded. Fig. 3 shows a
graph plotted between the eddy current out put voltage and the stresses
measured in tensile loading. Similarly, Fig. 4 shows a graph plotted between the
eddy current phase angle and the stresses measured in tensile loading. Fig. 5
shows a correlation between the stresses measured during tensile loading and
those predicted using an empirical equation derived from the eddy current out
put voltage measurements. Similarly Fig. 6 shows a correlation between the
stresses measured during tensile loading and those predicted using an empirical
equation derived from the eddy current phase angle measurements.
We Claim:
1. A non-destructive method to determine tensile stresses in high carbon
steel wires/wire rods by eddy current measurements, the method
comprising the steps of:
a) providing a completely stress-relieved sample of high carbon steel
wire/wire rod of identical diameter, surface condition and grade
with manufacturing history and microstructure in which the tensile
stresses are to be measured for the sample;
b) preparing a standard calibration graph taking into consideration the
eddy current out put voltage/phase angle, and tensile stresses
before its yield point during the testing;
c) deriving empirical equations correlating eddy current out put
voltage/phase angle with tensile stresses; and
d) determining the tensile stresses in the samples using the empirical
equation derived at step ( C).
2. A non-destructive method to determine tensile stresses in high carbon
steel wires/wire rods by eddy current measurements, the method as
substantially described and illustrated herein with reference to the
accompanying drawings.

The invention relates to a non-destructive method to determine tensile stresses
in high carbon steel wires/wire rods by eddy current measurements, the method
comprising the steps of: a) providing a completely stress-relieved sample of high
carbon steel wire/wire rod of identical diameter, surface condition and grade with
manufacturing history and microstructure in which the tensile stresses are to be
measured for the sample; b) preparing a standard calibration graph taking into
consideration the eddy current out put voltage/phase angle, and tensile stresses
before its yield point during. the testing; c) deriving empirical equations
correlating eddy current out put voltage/phase angle with tensile stresses; and
d) determining the tensile stresses in the samples using the empirical equation
derived at step ( C).