FIELD OF THE INVENTION
This invention relates to the development of fixture for carrying out robotic laser
peening on round sample for fatigue testing that helps eliminate bending of the
cantilever sample that may result due to the effect of laser peening. More
particularly, the invention relates to an improved stand-alone system for fatigue
testing of round samples during robotic laser peening process with minimum
bending of the samples.
BACKGROUND OF THE INVENTION
The known process of laser peening on the sample occurs in the form of a burst
of laser energy (pulse) of pulse width lasting typically 10-50 ns and spot size of
laser typically ranging from 1-5 mm. The time between two pulses used for laser
peening range typically from 0.2 to 2 seconds. With each pulse, the area of
affected metal approximately the size of the laser spot gets locally plastically
deformed and develops a net compressive stress which is beneficial for
enhancing fatigue life.
The laser peening of fatigue samples is carried out robotically wherein the laser
peening process and the sample movement are integrated. Since the sample is
of the free standing cantilever type, it tends to get deformed with each laser
pulse and hence loses the centering that is required of carrying out axial fatigue
testing. Figure 1.
This in turn causes early fatigue failure during fatigue testing since there is an
additional component of stress involved alongwith axial stress.

Fatigue testing of round samples is carried out to determine life of metals under
fatigue loading conditions. Fatigue failures almost invariably originate from the
sample surface and hence surface enhancement can improve the fatigue lives of
metallic components. Laser peening is a surface enhancement technique that
introduces compressive residual stresses in the surface thereby improving fatigue
life.
Laser shock peening process exploits laser-generated shock waves to introduce
high level of compressive stresses into a substrate surface. The process involves
irradiation of the substrate with high-energy short laser pulses causing
instantaneous vaporization of the surface layer into a high-temperature high-
pressure plasma. Rapid expansion of the resultant plasma from the surface
generates a high-pressure shock wave, which propagates into the substrate.
When peak pressure of the shock wave exceeds the dynamic yield stress of the
substrate (Huguniot elastic limit), the metal is plastically deformed, thereby
generating compressive residual stress on the surface of the substrate.
The process of laser peening involves use of a protective absorptive layer that
absorbs the laser energy and ablates, causing the rapid expansion and
generation of plasma. To enhance the duration of the shock wave pulse, a
tamping medium of high dielectric constant, typically a water film of thickness
more than 2 mm is made to flow over the sample. This ensures that the rapidly
expanding plasma is contained within the film and all the laser energy is
transferred as a shock wave to the sample thereby resulting in its plasticization.
The arrangement of the water flow over the sample depends greatly on the
geometry of the sample and hence requires customized holding devices for every
new sample geometry.

The fatigue properties of a metal are measured by making round dog-bone
samples that are then subjected to axial fatigue loading till failure to determine
their lives at given stress values. In order to robotically carry out laser peening
on round samples, a stand-alone assembly is used, which causes a bending of
the sample thereby making it unsuitable for fatigue testing. Hence a suitable
holding device is needed to generate step wise laser shots in the sample test
area with adequate support form the free end of stand alone arrangement to
minimize sample bending.
United States Patent7, 906,745 by Tenaglia, et al. March 15, 2011 shows a bend
bar for use in a quality control test for testing for a consistency of residual stress
effects in a particular material using a given a laser peening process. The bar is
composed of the particular material to be tested and has a bar length and a bar
thickness. The particular material has a characteristic maximum stress
penetration depth for compressive residual stresses that can be formed in using
the given laser peening process. The bar thickness is chosen so as to be at least
twice the characteristic maximum stress penetration depth. The bar has a test
surface that extends parallel to the bar length and perpendicular to the bar
thickness. After forming a spot pattern on the test surface using the given laser
peening process, the deflection generated in the bar due to the compressive
residual stresses induced by laser peening can then be measured and used as a
quality control measurement.
This prior art indicates that bending is caused by the laser peening process, but
does not suggest any means for minimizing the bending in fatigue samples.

United States Patent7, 816,622 by Deaton, Jr. , et al. October 19, 2010 teaches
to a work holding fixture configured to hold the workpiece; a laser source
configured to emit a plurality of laser beam pulses on the workpiece; a
transducer disposed on the work holding fixture and configured to detect
acoustic signals emitted from the workpiece for each laser beam pulse; a data
acquisition system configured to: measure a total acoustic energy represented in
the acoustic signals detected by the transducer for each laser beam pulse.
This prior art involves arrangement for attaching fixture for measuring and
logging acoustic signals to determine an efficiency of coupling of energy for each
laser beam pulse associated with the workpiece based on the total acoustic
energy associated with each laser beam pulse.
United States Patent 6483076 by O’Loughlin, et al. Nov 19, 2002 discloses a
method of positioning the workpiece at a current processing position indicated by
a hard-coded part program and then collecting position data which defines the
positional arrangement of a current target area of the workpiece. The collected
position data is processed by comparing it to reference position information that
represents the positional arrangement of the same target area in an ideal
workpiece employed in the development of the part program. The position of the
workpiece (and hence the target area) is adjusted in accordance with the
comparison results. A laser shock processing operation is performed on the
workpiece at the current target area following the position adjustment step.
This prior art relates to positioning of the work piece for collecting data regarding
its position prior to laser shock peening.

US patent no 6759626 by Clauer; Allan H. (Worthington, OH), Lahrman; David
F. (Powell, OH), Dulaney; Jeff L. (Dublin, OH), Toller; Steve M. (Dublin, OH),
have proposed various laser shock processing systems to establish selective
compressive residual stress distribution profiles within a workpiece. An
asymmetrical stress profile may be formed through the thickness of a thin
section of a gas turbine engine airfoil. One system is configured to
simultaneously irradiate a workpiece with a set of laser beams to form a
corresponding set of adjacent non-overlapping laser shock peened surfaces,
enabling the shockwaves to encounter one another. Another system irradiates
opposite sides of the workpiece at different times to form opposing laser shock
peened surfaces, enabling the shockwaves to meet at a location apart from the
mid-plane. Another system simultaneously irradiates opposite sides of the
workpiece using laser beams having different pulse lengths to form opposing
laser shock peened surfaces. Another system simultaneously irradiates opposite
sides of the workpiece to form a set of laterally offset laser shock peened
surfaces.
This prior patent gives different configurations of the laser shock processing
systems for various applications.
OBJECT OF THE INVENTION
It is therefore an object of the invention to propose an improved stand-alone
system for fatigue testing of round samples during robotic laser peening process
with minimum bending of the samples.

SUMMARY OF THE INVENTION
Accordingly, there is provided an improved stand-alone system for fatigue testing
of round samples during robotic laser peening process with minimum bending of
the samples, the round sample provided with a protective layer that absorbs the
laser energy to restrict rapid expansion including generation of plasma, a
tamping film of high dielectric constant such as a water film of high thickness
allowed to flow-over the sample to generate a plasticising effect on the sample
due to transfer of the laser energy in the form of pulses with pre-determined
pulse widths, contained within the water film in the form of shock waves, the
system being attachable to the robotic laser peening machine, the improvement
is characterized in that a holding device is attached at the sixth axis of the robot,
and comprising : a bearing means to permit stepwise rotation of the sample in
correspondence to the laser peening pulse frequency; and a spring loaded
support member provided at the free standing and; -a connecting rod to restrict
rotation of the device including locking of fixture movement with the robot still
allowing rotation of sample on 6th axis with bearing arrangement for continuous
laser-peening.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 Free standing round fatigue sample for laser peening according to
prior art.
Figure 2 Device for laser peening round fatigue sample according to the
invention.

DETAIL DESCRIPTION OF THE INVENTION
Accordingly, the invention provides an improved system for fatigue testing of
round samples during robotic laser peening process with minimum bending of
the samples. The improved system allows an increase in the stiffness of the
fatigue sample during laser peening and thereby minimize bending resulting from
the effects of high pressures generated during the laser process. The device is
attached to the 6th axis of the robot and is capable of rotation along with the
sample. It consists of a bearing mechanism to permit step-wise rotation of the
sample to match with the laser peening pulse frequency. The device also
comprises a spring-loaded support provided at the free standing end of the
fatigue sample while permitting robotically controlled laser peening on the
sample test area. To prevent the fixture rotating itself, an arrangement for
locking it with the robot using a connecting rod is made. This ensures
undisturbed flow of water on the test area from a nozzle that can be kept close
to the fatigue sample for better water confinement that is mandatory for good
laser peening of the sample. The wieght of the fixture is not more than 15 kg
and can easily be manipulated with the robot.
ADVANTAGES OF THE INVENTION
The invention allows carrying out uniform laser peening on the test area of round
fatigue samples robotically of M16 thread diameter. The distortion due to
bending caused by the high pressure shock pulses generated as a result of laser
peening in the fatigue sample is reduced significantly by incorporating this
improvement of the invention. The resultant reduced distortion has been
measured for sample eccentricity using dial gauge and the effectiveness

of this invention. The invention is operative in course of carrying out axial fatigue
testing of the sample after laser peening surface treatment. The disclosed
improvement in the system enhance the stiffness of round fatigue samples for
laser peening making use of bearing assembly for rotation and a spring
mechanism for supporting free standing sample. The device ensures coordinated
rotation of sample with laser pulse frequency to ensure complete laser peening
on the curved test area of the fatigue sample. Further, the device ensures an un-
interrupted flow of water on the sample as required for the laser peening process
by having a locking arrangement by a connecting rod and attached to the robot
which will permit fatigue sample rotation while arresting rotation of the device.
The device of the invention exhibits enhanced reliability and consistency in the
fatigue results.

WE CLAIM :
1. An improved stand-alone system for fatigue testing of round samples
during robotic laser peening process with minimum bending of the
samples, the round sample provided with a protective layer that absorbs
the laser energy to restrict rapid expansion including generation of
plasma, a tamping film of high dielectric constant such as a water film of
high thickness allowed to flow-over the sample to generate a plasticising
effect on the sample due to transfer of the laser energy in the form of
pulses with pre-determined pulse widths, contained within the water film
in the form of shock waves, the system being attachable to the robotic
laser peening machine, the improvement is characterized in that a holding
device is attached at the sixth axis of the robot, and comprising :
a bearing mechanism to permit stepwise rotation of the sample in
correspondence to the laser peening pulse frequency; and
a spring loaded support member provided at the free standing and;
a connecting rod to restrict rotation of the device including locking of
fixture movement with the robot still allowing rotation of sample on 6th
axis with bearing arrangement for continuous laser-peening.
2. The improved system as claimed in claim 1, wherein the device is enabled
to allow an un-interrupted water flow from the system from a nozzle to
the test location of the sample.