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ME5656 - MECHANICS OF CONTACT & LUBRICATION
TERM PROJECT
SHOT PEENING PROCESS
Baran YILDIRIM1
1Northeastern University 360 Huntington Ave Boston, MA 02115, USA
Abstract
Shot peening is a process where compressive residual stresses are generated at the surface
of the processed part by impacting small hard particles to the surface in order to increase the
fatigue resistance of the part. In this work, a general literature survey of shot peening process is
presented. Survey is mainly focused on mechanics and numerical simulation of the process.
Effects of various process parameters on resulting residual stresses and surface characteristics are
summarized. In the second part, effect of surface roughness is investigated through numerical
simulations. For this purpose, a 2D finite element model is developed in which substrate surface
is modeled as a random rough surface. Results show that surface roughness has a substantial
effect on the impact characteristics when the particle size is close to the average surface
roughness. For large enough particles the effect of roughness is limited to the region near to the
surface, but higher stresses are seen to be generated at the surface as a result of localization of
deformation.
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1. Description of shot peening process:
Shot peening is a cold working process where the surface is impacted with small
spherical hard metal or ceramic particles (also called “shot”) at velocities between 20 to 150 m/s.
The typical diameter of particles used in shot peening process ranges between 0.5 to 2.5 mm. In
shot peening, plastic deformation of the surface occurs rather than abrasion. The process consists
of multiple repeated impacts to the substrate. Each particle, when impacted to surface, acts as a
tiny hammer creating a small indent (dimple) and local plastic deformation. Deformation due to
impact forms a zone with compressive residual stresses just beneath the surface. Since cracks
generally propagate from the surface, with the compressively stressed layer, the chance of crack
propagation is reduced. Thus, resistance to fatigue and stress corrosion failure of a material can
be greatly increased. With increased resistance to failure, applying shot peening to a part can
considerable extend the part’s life [20]. Surface hardness also increases due to cold working
aspect of the process.
The effectiveness of the process is often measured by the amount of compressive residual
stresses induced in the part and the integrity of the surface. Many factors can affect the quality of
shot peening process. These can be categorized as:
Particle diameter
Particle impact velocity and impact angle
Hardness and structure (thickness, shape, etc.) of shot-peened part
Roughness of shot-peened surface
Peening time
Peening coverage (percentage of a surface that has been impacted by peening particles)
Particles used in shot peening process are generally made of hard steel, ceramic or glass.
The parts to be processed are ductile metals such as aluminum or steel. The hardness of the
impacting particles should be much higher relative to the material that is being processed. Since
the process involves high level of deformation and straining at the surface of the processed part,
brittle materials cannot be shot peened.
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2. Mechanics of shot peening process
During the impact of particles in shot peening process, contact can be considered as fully
plastic for all cases. This can be validated by a simple analysis on loading conditions at the
surface [21]. The following equation gives the equation of motion for contact:
2
2
dt
dmP z (1)
where P is the force between contacting bodies, m is the composite mass and z is the
approach of distant points found from Hertz elastic contact theory. Integrating equation (1) with
respect to z and setting 0/ dtd z , one can find the maximum compression ( *z ) value as:
5/2
*2/1
2*
16
15
ER
mViz (2)
where iV is the initial approach velocity. Then inserting equation (2) into equation (1) and
dividing by contact area found by Hertz elastic contact theory, the maximum contact pressure (
0p ) generated can be found. Plastic deformation starts beneath the surface when 0p reaches the
value Y60.1 where Y is the yield stress of the softer material. Using this 0p value, in the case of
sphere impacting on a plane surface the equation for velocity which causes plastic deformation (
YV ) can be written as:
4*2
/26 EYY
VY
(3)
For hard steel ( MPaY 1000 ), critical velocity is smVY /14.0 . Since the impact
velocities in shot peening process is well beyond this limit, one can say plastic deformation
dominates the process and significant plastic deformation is expected. Assuming the particle as
rigid, energy stored for plastic deformation in substrate ( W ) can be expressed as:
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22 12
1ri emVW (4)
where re is the coefficient of restitution (ratio between initial and rebound velocities of
impacting particle).
Approximate analytical solutions to find plastic strains and residual stresses are
developed by researchers [8, 9, 18]. But since the process is too complicated for an exact
analytical solution, current research to investigate the mechanics of the process is focused on the
numerical simulations [1-12, 16]. Dynamic procedures are used to include the inertial effects.
Impact of a uniform sphere impacting a flat plate is investigated. 3D model is used generally to
be able to simulate oblique and multiple impacts.
Due to the fact that particle material’s hardness is much higher than that of substrate,
particle is assumed as rigid or elastic in most numerical studies. For substrate, different material
models to define plastic hardening are employed. Elastic, linearly plastic models with either
isotropic or kinematic hardening are used generally. In a few studies, strain-rate effect is also
included by using different strain-rate dependent models such as Johnson-Cook plasticity model
[6] and Cowper Symonds material model [1]. Also it is mentioned in [19] that due to the
relatively short times and high plastic strains occurring in the process, a considerable amount of
temperature rise can be seen at the substrate very near the surface.
Compared to experimental findings, FE simulations have found to be effective in
describing the mechanics of the process. Thus FE simulations are considered to be an effective
way to optimize the process parameters for different conditions.
3. Effect of shot peening parameters on subsurface residual stresses
As mentioned before, impact of a particle onto substrate surface generates subsurface
compressive residual stresses in the substrate. Figure 1 shows a typical residual stress
distribution obtained by shot peening process. General shape of the curve is preserved but factors
such as impact velocity, impact angle, particle diameter and surface hardness can have an effect
on the residual stress parameters shown in Figure 1.
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3.1. Imp
M
different
increasin
velocity i
velocities
develops
3.2. Imp
[3] that n
parts suc
importan
surface q
3.3. Pa
Z
change [
used sho
size shou
in the int
DIRIM
pact veloc
Maximum co
particle im
ng impact v
impacts [12]
s up to ~30
on the surfa
pact angle
cmax and 0Z
normal impa
h as gears, p
nt to investig
quality.
rticle diam
maxZ and 0Z
17]. cmax do
uld be selec
uld be chose
terior of the p
Figure 1: Typic
ity:
ompressive
mpact veloci
velocity [17]
]. In [3] it is
m/s. Impac
ace after the
e:
decreases w
acts have th
particles are
gate the effec
meter:
has found to
oes not chan
cted carefully
n such that t
part.
cal residual stres
residual str
ities but the
]. Also, the
shown that
ct velocity s
process.
with the incr
e most bene
bound to be
ct of impact
o increase lin
ge significan
y especially
the region u
5
s profile and res
ress ( cmax )
e depth at w
e compressiv
impact veloc
should be ad
rease of off-
eficial effect
e impacted to
angle even
nearly with p
ntly with par
for shot pee
under tensile
sidual stress para
is seen to
which it oc
ve layer ( Z
city has an e
djusted such
f-normal imp
t. However,
o surface wit
though norm
particle diam
rticle diamet
ening of thin
loading (be
ameters [6]
remain nea
ccurs ( maxZ )
0Z ) gets dee
effect on cm
h that no sig
pact angle. I
in shot pee
th off-norma
mal impacts
meter, mainly
ter. Size of t
n and compl
elow 0Z in F
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arly constan
), increases
eper with h
max for low im
gnificant dam
It is conclud
ening of com
al angles. So
produce the
y due to the
the particles
lex parts. Pa
Figure 1) rem
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nt for
with
higher
mpact
mage
ded in
mplex
o, it is
e best
scale
to be
article
mains
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3.4. Eff
In
by peeni
increasin
%50. To
coverage
of the sur
4. Effec
S
transfer
importan
roughnes
is shown
attributed
coefficien
wear rate
DIRIM
fect of cove
n shot peenin
ing particles
ng coverage
achieve a g
e is somewha
rface becaus
ct of shot p
tudying sur
properties,
nt for certain
ss is mostly e
n in Figure 2
d to low init
nt of frictio
e is seen in [
erage:
ng, coverage
s. Accordin
up to %50
good shot pee
at close to %
se of the dam
peening on
rface roughn
electrical co
n applicatio
experimenta
2. An increas
tial roughnes
n is observe
15] also. Th
Figure 2:
e is defined a
ng to FE sim
is seen. cm
ened surface
%100. On the
mage done by
n surface r
ness is imp
onductance
ons. The wo
al. Surface pr
se in roughn
ss and relativ
ed in the sa
is can be con
: Surface profile
6
as the percen
mulations m
max remain al
e, peening ti
e other hand
y impacting
roughness
ortant becau
and abrasiv
ork done on
rofiles befor
ness of the su
vely large si
ame work. A
nsidered as a
es before and afte
ntage of a su
made in [2]
lmost consta
ime should b
d excessive p
particles [19
s
use it has a
veness of th
n the effect
re and after s
urface is see
ize of partic
A decrease i
a beneficial
er shot peening [
urface that h
, an increa
ant for cove
be adjusted s
peening decr
9].
a substantia
he surface
of shot pee
shot peening
en upon shot
les [13]. A s
in coefficien
effect of sho
[13]
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has been imp
se in cmax
erage higher
such that pee
reases the qu
al effect on
which migh
ening on su
g obtained in
t peening. T
slight decrea
nt of friction
ot peening.
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pacted
with
r than
ening
uality
heat
ht be
urface
n [13]
his is
ase in
n and
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The proportionality of arithmetic average ( aR ) of the surface to the process parameters
are found in [14] through experiments. aR is expressed as:
2/1 HVDkR iRa (5)
where D is the particle diameter, H is the hardness and Rk is a coefficient. It can be seen that
with increasing particle diameter and impact velocity, surface roughness also increases. However
surface roughness after shot peening is less of harder materials.
5. Numerical Simulation of a Single Shot on a Rough Surface
Numerical simulations of shot peening process in literature do not include the effect of
surface roughness of the part that is being processed. But as the particle size gets closer to the
average roughness of the substrate surface, the assumption of flat surface in substrate may give
inaccurate results in terms of subsurface stresses and surface damage. Therefore it is
advantageous to study the roughness effects.
This chapter presents the modeling approach for the numerical simulations and results
obtained. To investigate the effect of surface roughness on impact behavior and stress and strain
distributions, a finite element model is developed using ABAQUS/Explicit software. 2D plane
strain assumption is made to decrease complexity of the rough surface generation and to reduce
size of the problem in terms of number of degree of freedoms. Heat generation due to plastic
straining and heat conduction is included in the model.
Particle and substrate materials are hardened steel and Armco iron respectively. Particle
is assumed to be elastic. Plastic response of the substrate is modeled with Johnson-Cook
plasticity model [22] which defines yield stress as a function of strain, strain-rate and
temperature. Detailed information regarding this model and material properties used can be
found in Ref. [22].
Rough surface with 200 data points is generated in MATLAB using autocorrelation
length method. Then these data points are fed into ABAQUS and mesh is generated using free
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meshing
structure
Figure 3
Imlocationsμm) diam100, 150
It
within a p
the surfa
strongly
average s
(a) dp =
DIRIM
algorithm av
. Standard d
3: Rough surface
mpact of a ps on the surfmeter particl
m/s).
F
t is seen from
peak or sum
ace of the su
dependent o
surface roug
= 5σ
vailable in A
deviation (σ)
e with mesh stru
particle havface with anle (Figure 4b
Figure 4: Investi
m simulatio
mmit dependi
ubstrate. Fig
on the locati
ghness.
ABAQUS. F
and length o
cture. Standard d
ving a diamen initial velob) is impacte
gated particle si
ons with 5σ
ing on the im
gure 5 also
ion of impa
8
Figure 3 show
of the surfac
deviation of the
eter (dp) of ocity of 100 ed to the sur
zes with respect
particles tha
mpact locatio
shows that
ct for partic
(b) dp
ws the gener
ce (l) are 43.2
surface: σ = 43.
5σ (= 216 m/s is inve
rface with d
t to roughness of
at the deform
on (Figure 5)
the rebound
cles that hav
p = 25σ
rated rough s
2 μm and 2 m
2 μm. Length of
μm) (Figureestigated. Aldifferent initi
f the surface
mation is ge
). Highest st
d direction
ve comparab
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surface and
mm respecti
f surface: l = 2 m
e 4a) to difflso a 25σ (=ial velocities
enerally loca
tresses are se
of the partic
ble sizes wit
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mesh
ively.
mm.
ferent =1080 s (50,
alized
een at
cle is
th the
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Figure 5: Swith particle
F
particle w
substrate
peaks. N
an impac
von Mise
Mises str
S
condition
compress
literature
subsurfac
DIRIM
Subsurface resides approaching t
a
igure 6b, c
with differen
also increas
earby peaks
ct to perfectl
es stresses ne
resses beneat
imilar to F
ns. As the im
sive stresses
e. Compariso
ce pressure s
dual stresses (vonto the surface wiresult of impact
and d show
nt initial vel
ses. The sur
are seen to
ly flat surfac
ear the surfa
th the surfac
Figure 6, Fi
mpact veloci
s occur just
on of Figure
stresses.
n Mises) for 3 diith 100 m/s. (d), t. (von Mises stre
ws the residu
locities. As
rface at the i
remain stres
ce and a rou
ace which ca
ce are seen to
igure 7 giv
ity increases
below the
7a and c rev
9
ifferent impact lo(e) and (f) are thess contours are
ual von Mis
the impact
mpact regio
ss-free. Figu
ugh surface.
an damage th
o be similar
ves the pres
s, compressiv
surface. The
veals that su
ocations. (a), (b)he correspondingscaled between
ses stresses
velocity inc
on takes a ro
ure 6a and c
Roughness
he surface ca
in both case
ssure stress
ve stressed r
ese finding
urface rough
) and (c) give theg deformations a0-500 MPa)
for an imp
creases, defo
ounded shape
gives a com
greatly affe
ausing crack
es.
distribution
region expan
are consiste
hness has a l
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e before-impact and residual stre
pact of 25σ
ormed area a
e crushing a
mparison bet
ects the gene
ks to initiate.
ns for the
nds. Also hi
ent with tho
ittle effect o
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states esses as
sized
at the
all the
tween
erated
. Von
same
ighest
ose in
on the
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Figure cond
Figure 7: SReboundin
DIRIM
6: Subsurface reditions. (a) 100 m
Subsurface residung particles are a
esidual stresses (m/s flat surface. (
ual pressure strealso shown. (a) 1
(von Mises) afte(b) 50 m/s rough
sses after impac100 m/s flat surf
ro
10
er impact for diffh surface. (c) 100
t for different inface. (b) 50 m/s rough surface.
ferent initial part0 m/s rough surf
nitial particle imprough surface. (c
ticle impact veloface. (d) 150 m/s
pact velocities ac) 100 m/s rough
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ocities and surfas rough surface.
and surface condh surface. (d) 15
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ce
ditions. 0 m/s
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