Baran YILDIRIM 11/24/2009smuftu/docs/2009/ME5656_Term_Project_Shot_Pe… · Baran YILDIRIM...

Baran YILDIRIM 11/24/2009

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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

[email protected]

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:


4

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.

Baran YIL

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

11/24/

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

/2009

nt for

with

higher

mpact

mage

ded in

mplex

o, it is

e best

scale

to be

article

mains

Baran YIL

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]

11/24/

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.

/2009

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

Baran YIL

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

11/24/

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

/2009

mesh

ively.

mm.

ferent =1080 s (50,

alized

een at

cle is

th the

Baran YIL

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

11/24/

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

/2009

states esses as

sized

at the

all the

tween

erated

. Von

same

ighest

ose in

on the

Baran YIL

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

11/24/

ocities and surfas rough surface.

and surface condh surface. (d) 15

/2009

ce

ditions. 0 m/s


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References:

[1] Baragetti, S. Shot peening optimization by means of `DoE': Numerical simulation and

choice of treatment parameters. International Journal of Materials and Product Technology,

1997, 12(2-3), 83-109.

[2] Helling, C.D.G. and Schiffner, K. Numerical simulation of shot peening. pp. 303-311

(WITPress, Seville, Spain, 2001).

[3] Hong, T., Ooi, J.Y. and Shaw, B. A numerical simulation to relate the shot peening

parameters to the induced residual stresses. Engineering Failure Analysis, 2008, 15(8),

1097-1110.

[4] Baragetti, S., Guagliano, M. and Vergani, L. Numerical procedure for shot peening

optimization by means of non-dimensional factors. International Journal of Materials and

Product Technology, 2000, 15(1), 91-103.

[5] Baragetti, S. and Terranova, A. Non-dimensional analysis of shot peening by means of

DoE. International Journal of Materials and Product Technology, 2000, 15(1), 131-141.

[6] Eltobgy, M.S., Ng, E. and Elbestawi, M.A. Three-dimensional elastoplastic finite element

model for residual stresses in the shot peening process. Proceedings of the Institution of

Mechanical Engineers, Part B: Journal of Engineering Manufacture, 2004, 218(11), 1471-

1481.

[7] Frija, M., Hassine, T., Fathallah, R., Bouraoui, C. and Dogui, A. Finite element modelling

of shot peening process: Prediction of the compressive residual stresses, the plastic

deformations and the surface integrity. Materials Science and Engineering A, 2006, 426(1-

2), 173-180.

[8] Shen, S. and Atluri, S.N. An analytical model for shot-peening induced residual stresses.

Computers, Materials and Continua, 2006, 4(2), 75-85.

[9] Franchim, A.S., de Campos, V.S., Travessa, D.N. and de Moura Neto, C. Analytical

modelling for residual stresses produced by shot peening. Materials & Design, 2009,

30(5), 1556-1560.


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[10] Rouhaud, E. and Deslaef, D. Influence of shots' material on shot peening, a finite element

model. pp. 153-158 (Trans Tech Publications, Switzerland, 2002).

[11] Meguid, S.A., Shagal, G. and Stranart, J.C. Development and validation of novel FE

models for 3D analysis of peening of strain-rate sensitive materials. Journal of Engineering

Materials and Technology, Transactions of the ASME, 2007, 129(2), 271-283.

[12] Meguid, S.A., Shagal, G., Stranart, J.C. and Daly, J. Three-dimensional dynamic finite

element analysis of shot-peening induced residual stresses. Finite elements in analysis and

design, 1999, 31(3), 179-191.

[13] Adamovic D., Babic M., Jeremic B. Shot peening influence on tribological characteristics

of surfaces, Conf Proc: ICSP-7, 1999, 350-358.

[14] Tosha K. Characteristics of shot peened surfaces and surface layers, Asia-Pacific Forum on

Precision Surface Finishing and Deburring Technology, 2001, 193-201.

[15] Vaxevanidis N.M., Sideris I., Mourlas A., Antoniou S.S. The effect of shot peening on

surface integrity and tribological behavior of tool steels, International Conference on

Tribology, 2006

[16] Barrios D.B., Angelo E., Goncalves E. Finite element shot peening simulation for residual

stress. Analysis and comparison with experimental results, Mecánica Computacional

Volume XXIV, 2005, 413-422.

[17] Schwarzer J., Schulze V., Vöhringer O., Finite Element Simulation of Shot Peening - A

Method to Evaluate the Influence of Peening Parameters on Surface Characteristics,

Proceedings from International Conference of shot peening nr 8 (ICSP-8) Munich,

Germany, 2002, 507-515

[18] Al-Hassani, S. T. S. Mechanical aspects of residual stress development in shot peening.

ICSP1, 1981, 583-603.

[19] http://www.shotpeener.com/library/spc/2007028.htm, Kirk D. Review of Shot Peened

Surface Properties

[20] www.shotpeener.com/learning/spo.pdf, Champaigne J. Shot Peening Overview

[21] Johnson, K. L. Contact Mechanics. Cambridge [Cambridge shire]: Cambridge University

Press, 1985.


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[22] Johnson, G.R. and Cook, W.H. A constitutive model and data for metals subjected to large

strains, high strain rates and high temperature. Proceedings of Seventh International

Symposium on Ballistics, 1983, 541-547.

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