Shot Peening Technology Center
Schools of Engineering and Technology
Purdue University
Introduction
Shot peening is a process in which small spherical media called
shot, bombard the surface of a metal. This shot striking the surface
cold works the material and induces residual stresses, both of
which can significantly improve the fatigue behavior of the component.
The size, mass and velocity of the shot
are important parameter in the process, however, the current nondestructive
methods for measuring the process effectiveness and relating it
to the fatigue properties are insufficient, making it very difficult
to control the process. Furthermore, better understanding of the
micromechanics of the deformation
mechanisms and the generation of residual stresses are needed
to develop predictive models that can provide the fundamental
basis for the process control. Improvement to the process will
lead to better fatigue performance and provide a better shot peened
product. Improved technology for nondestructive measurements of
material properties and residual stress are paramount to quantifying
the process and improving the product.
Project Title: In-process measurement of residual stress by
an ultrasonic technique
Residual stress in one of the most important process conditions
in shot peening processes. The ability to monitor residual stresses
from parts during a shot peening process is very desired since
it provides means to design an optimal shot peening cycle and
to control the process. Despite the importance and need for measuring
residual stresses, currently there is no effective way of measuring
residual stresses other than x-ray diffraction techniques, which
involve a rather expensive device and long processing time.
Therefore, a more efficient and economic approach to measure residual
stress is proposed. The proposed research will develop an ultrasonic
technique that will fill the existing need to perform real-time,
in-process measurement of residual stress and provide feedback
for control of the shot peening process. Preliminary work on this
has been already completed and the proposed technique has proven
to show a good correlation between the measured ultrasonic data
and bulk stress of a part. Since the residual stresses produced
by the shot peening process is compressive in nature and is developed
primarily in the subsurface, Raleigh wave which will propagate
along the surface will be utilized for the measurement. The technique
has potential of measuring residual stresses from both straight
and curved surfaces.
Prof. LeRoy F. Silva
SHOT PEENING RELATED RESEARCH INTERESTS
Non-destructive evaluation of shot peening efficacy is a key issue
in compressive stress inducement of metal workpiece using shot
peening methodology. Current practice utilizes indirect calibration
of the shot peening process with occasional destructive evaluation
or laboratory evaluation of the workpiece to verify the process
parameters. However, the intrinsic nature of contemporary shot
peening technology introduces variation in the process that could
be controlled and minimized in an inexpensive, real-time non-destructive
evaluation method were available. To date, eddy current decay
mensuration, x-ray, and ultrasonic techniques have been studied
as possibilities to achieve the desired end.
I am interested in a microwave technique that might prove useful
in compressive stress measurement in metal workpieces. If there
is a repeatable variable of electrical conductivity, magnetic
permeability or electric permittivity as a function of stress
(compressive or tensile), then a microwave technique may prove
applicable.
In my past research [1], the electrical properties of soils, as
a function of depth using a microwave technique, were measured.
A microwave-emitting antenna was placed on the surface of the
soil and a separate receiving antenna was moved along the soil
surface to measure the portion of the wave reflected from the
soil subsurface. The result of the measurement is the microwave
interference pattern at the soil surface. A mathematical technique
for interpreting this interference pattern in terms of the electrical
properties of the soils was developed.
I believe that the technique would be extendible to metal work
pieces, especially alloys, assuming a definable relationship exists
between the stresses in a metal and its electrical properties.
The starting point would be to study these electrical relationships
and then develop the instrumentation protocol if the electrical
properties research show feasibility.
[1] L.F.Silva, F.V.Schultz and J.T.Zalusky, "Electrical Methods
of Determining Soil Moisture Content", LARS, Information
Note 112174, Purdue University, May 1985.
