The present invention relates to a system for use in measurement of sub-surface
residual stress in rails and a method for measurement of sub-surface residual stress in
rails. The invention is applicable especially for railway track design as well as for design
for structural such as bridges, trusses, etc.
Presently it is known to measure residual stress using the "Blind Hole Drilling Method".
Such a method is carried out by first rubbing the surface of the rail sample gently with
fine emery paper. Special care is taken while rubbing so that the surface of the sample
should not become hot (to avoid thermal stress). The working area is cleaned with acidic
and alkaline solution to remove rust and other kind of dirt. Then it is cleaned with
neutralizing agent. Strain gauge (rosette type with three similar strain gauge placed at
45° & 90°) is fixed on rail surface. Connecting wires are soldered with strain gauges and
terminals.
The strain is measured by using "Switch and Balance Unit" and "Strain Indicator". The
measurement taken is treated as initial reading. Now incremental hole is drilled on the
rail sample in the center of strain gauge rosette. The drilling increment is kept very low
so that no thermal stress is generated on the surface. The relaxed strain at each cut is
noted down along with depth. Apparatus used for this is known as "Blind Hole Drilling
Machine" which is shown in accompanying figure 1.
There are several other known method of measurement of stress in rails like X-ray
method, Laser method, Barkhousen noise method. However, all the above methods
have limitation of depth of measurement.
Such known devices/methods have limitations in measurement of residual stress at
depth of upto 2.4 millimeter only.
It is thus the basic object of the present invention to provide for a system and method for
measurement of sub-surface residual stress on rails which would avoid the limitations of
measurement of stress upto a depth of only 2.4 mm and can even measures stress at
depth of upto 10 mm.
Another object of the present invention is to provide for a system and method for
measurement of sub-surface residual stress on rails which would be reliable and
convenient and easy to use and carry out.
Yet further object of the present invention is directed to provide for a system and method
for measurement of sub-surface residual stress in rails which would help in assessing
the quality of rails thereby reducing the chances of accident due to rail failure.
Thus according to one aspect of the present invention there is provided a system for use
in measurement of sub-surface residual stress in rails comprising
means for cleaning of the rail surface ;
strain gauge adapted for fixing on rail surface ;
said strain gauge operatively connected with the respective terminals
switch and balance unit
strain indicator;
means for cutting narrow slit in the rail in steps ;
means for measuring the depth and the released stress at each cut.
In the above system of the invention the said strain gauge is operatively connected with
said respective terminals by connecting wires which are soldered with strain gauge and
the terminals.
The means for cleaning the rail surface comprise emery paper, acidic and alkaline
solutions and neutralizing agents.
The means for cutting narrow slits in the rail in steps comprise precision sharper
machine having specially developed cutting tool.
According to another aspect the present invention provides a method of measurement of
sub-surface residual stress in rails comprising
cleaning the surface of the rail to be measured ;
fixing strain gauge on rail surface ;
operatively connecting the strain gauge and the respective terminals;
measuring the initial strain using said switch and balance unit and strain indicator;
cutting incremental narrow slits in the rail surface in steps;
measuring the relaxed strain at each cut along the depth and obtaining therefrom the total residual stress
at each cut.
The above disclosed method of stress measurement of the invention may be called as "Incremental Slip
cutting method". Surface of the rail sample is gently rubbed with fine emery paper. Special care is taken
while rubbing so that the surface of the sample should not become hot ( to avoid thermal stress). The
working area is cleaned with acidic and alkaline solution to remove rust and other kind of dirt. Then it is
cleaned with neutralizing agent. Strain gauge is fixed on rail surface in longitudinal direction. Connecting
wires are soldered with strain gauges and terminals.
Strain is measured by using Switch and Balance Unit" and "Strain Indicator". This is shown in
photograph. The measurement taken is treated as initial reading. Now incremental narrow slit is cut on
the rail sample very close to strain gauge in a precision shaper machine with the specially designed
cutting tool. The cutting increment is very low so that no thermal stress is generated on the surface.
The relaxed strain at each cut is noted down along with depth. Total strain at any increment (ez total) is
converted into total residual stress in that increment (sz total) using the relation :
sz total = CfE ez total
"wherein
Cf is calibration factor for cutting method
E is the modulus of elasticity of rail steel (~ 2.06*105 N/mm2)
As regards explanation of stress measurement, present system uses calibration factors Cf which has been
determined by a large number of experiments done in laboratory for rail steel. These calibration factors
at different depths have been determined by generating known residual stress in rail steel samples and
comparing it with the residual stress measured using this method. Validation of calibration factors has
indicated that these values are accurate for measurement of sub-surface residual stress in rails up to a
depth of 10mm.
The system and method of measurement of sub-surface residual stress in rails of the invention its objects
and advantages are explained hereunder in greater detail in relation to non-limiting exemplary
embodiments as per the accompanying figures wherein
Fig. 2a is a photographic illustration of a "strain indicator" used in the system ;
Fig. 2b is a photographic illustration of a "switch and balance unit" used in the system of
the invention ;
Fig. 3 substantially illustrates the connection of the strain indicator and switch & balance
unit to the stress gauge pasted on the rail;
Fig. 4a is an illustration of the measurement of longitudinal residual stress at rail foot
obtained following the system and method of the invention ;
Fig. 4b is an illustration of the measurement of longitudinal residual stress at rail head
obtained following the system and method of the invention.
Reference is first invited to Fig. 2a and 2b which show the strain indicator and the switch
and balance unit used in the system of the invention. Such strain indicator and switch
and balance unit are well known in the art and have been used in the system of the
invention.
Reference is now invited to Fig. 3 which schematically illustrates the arrangement of the
system on the rail surface which is to be measured. As shown in said figure the strain
gauge is glued on the cleaned surface aligned in longitudinal direction of the rail sample.
Measurement of strain is done by wheat stone bridge method. The slot is cut on the rail
sample in steps. Measurement of cut depth and strain relieved is carried out as detailed
hereunder.
Initial strain is noted down first. Then slot is cut in steps and strain relieved is noted
down. Slot cutting is very slowly carried out so that there is no significant temperature
rise. Measured strain is converted into stress by well known relation c = Es.
where a = Stress
e = Measured strain
E = Modulus of elasticity
Cutting of slots relieves the residual stress of the sample under test. It is possible by way
of this invention to know the magnitude of residual stress at different depth from the
surface. Thus the cutting of the slot in steps is important.
Reference is now invited to Fig. 4a and 4b showing depth from the surface Vs. residual
stress as measured. Fig. 4a shows that in first step slot depth is 0.4 mm and at this
depth residual stress is very close to 100 Mpa. Similarly in second step slot depth is 0.8
mm and at this depth residual stress is close to 150 MPa.
It is thus possible by way of the present invention to provide for a system and method of
measurement of sub-surface residual stress on rails which would be capable of
measuring the stress at depths beyond 2.4 mm and to a depth of upto about 10 mm. The
method is reliable and also cost-effective.
We claim :
1) A system for use in measurement of sub-surface residual stress in rails
comprising
means for cleaning of the rail surface ;
strain gauge adapted for fixing on rail surface ;
said strain gauge operatively connected with the respective terminals
switch and balance unit ;
strain indicator ;
means for cutting narrow slit in the rail in steps ;
means for measuring the depth and the released stress at each cut.
2) A system as claimed in claim 1 wherein said strain gauge is operatively
connected with said respective terminals by connecting wires.
3) A system as claimed in any one of claims 1 or 2, wherein said means for
cleaning the rail surface comprise emery paper, acidic and alkaline
solutions and neutralizing agents.
4) A system as claimed in anyone of claims 1 to 3, wherein the connecting
wires are soldered with strain gauge and the terminals.
5) A system as claimed in anyone of claims 1 to 4 wherein the means for
cutting narrow slits in the rail in steps comprise precision sharper machine.
6) A system as claimed in claim 5, wherein the precision sharper machine
comprise specially developed cutting tool.
7) A method of measurement of sub-surface residual stress in rails comprising
cleaning the surface of the rail to be measured ;
fixing strain gauge on rail surface ;
operatively connecting the strain gauge and the respective terminals ;
measuring the initial strain using said switch and balance unit and strain
indicator ;
cutting incremental narrow slits in the rail surface in steps ;
measuring the relaxed strain at each cut along the depth and obtaining
therefrom the total residual stress at each cut.
8) A method as claimed in claim 7, wherein said strain gauge is fixed in rail
surface in a longitudinal direction.
9) A method as claimed in anyone of claims 7 or 8, wherein said cleaning of
the surface of the rail comprises gently rubbing the surface with emery
paper without generating any thermal stress ; cleaning the working area
with acidic and alkaline solutions followed by cleaning with neutralizing
agent.
10) A method as claimed in anyone of claims 7 to 9, wherein said operative
connection of strain gauge to the terminals is achieved by soldering the
wires to the strain gauge and terminals.
11) A method as claimed in anyone of claims 7 to 10 wherein the said
incremental narrow slits are cut in steps using a precision sharper
machine.
12) A method as claimed in anyone of claims 7 to 11 wherein the cutting
increment is very low such that there is no thermal stress on the surface.
13) A method as claimed in anyone of claims 7 to 12, wherein the total strain at
each increment (sz total) is converted into total residual stress in that
increment (az total) using the relation
sz total = CfE ez total
wherein Cf is calibration factors for cutting method
E is the modulus of elasticity of rail stress (~ 2.06*105 N/mm2)
14) A system for use in measurement of sub-surface residual stress in rails and
a method of measurement of sub-surface residual stress in rails
substantially as hereindescribed and illustrated with reference to the
accompanying examples and figures.

A system for use in measurement of sub-surface residual stress in rails. The system has
means for cleaning of the rail surface comprise selected from emery paper, acidic and
alkaline solutions and neutralizing agents. It also has a strain gauge adapted for fixing
on rail surface, the strain gauge operatively connected with the respective terminals. The
strain gauge is operatively connected with said respective terminals by connecting wires.
There are also provided switch and balance unit, a strain indicator, means for cutting
narrow slit in the rail in steps and means for measuring the depth and the released
stress at each cut.