FINITE ELEMENT ANALYSIS OF AUTOFRETTAGE

PROCESS OF THICK- WALLED CYLINDERS

http://www.stu-library.org/uploads/1/1/0/5/110551273/2016.pdf

Dr. Lattif Shekher Jabur

Mechanical

Technics Department, Southern Technical University,

Technical Institute of Nasiriya, Nasiriyah, Iraq.

Dr. Dr. Hazim Ismael Radhi

Mechanical

Department, Engineering College, Thi – Qar University, Nasiriyah, Iraq.

Najah Rustum Mohsin

Mechanical

Technics Department, Southern Technical University,

Technical Institute of Nasiriya, Nasiriyah, Iraq.

ABSTRACT

The process of producing residual

stresses in thick_walled cylinder before

it is putin to usage is called Autofretage, which it means; a suitable large

enough pressureto cause yielding within the wall, is applied toinner surface of

a sylinder and then removed. So that

acompressive residual stresses are generated to acertain radial depth at a sylinder

wall.

The objective

ofpresent study, is to investigate the influenceof autofretage process onthe

radial, circumferential andtotal stresses using von.mises yieldcriteria. Num.simulation

carried outon ABAQUS software to investigate thestresses distribution and

calculate the autofretage radius. The results revealthat, the autofretage

processof thick_wall sylinder lead to

decrease the hoob and max.von.mises stresses and relocate them from the inner

surface of the sylinder to somewhere

along it’s thickness. The reduction in max.stresses is strongly depending on autofretage

pressure, it wasvarying from ( 3.6% at Pautofretage = 105 MPa

to 19.2% at Pautofretage =

130 MPa ) Also, it

has been found, there is no influenceof autofretage stages number on both of max.von.mises

stressand autofretage radius.

Key words: autofretage, radial, hoob and

axial stresses, von.mises yield criteria, autofretage radius, optimum autofretage

pressure.

1.

INTRODUCTION

The wide applications of

pressurized sylinder in chemical,

nuclear, armaments, fluid transmitting plants,

power plants and military equipment, in addition to the increasing scarcity and

high cost of materials lead

the designers to

concentrate their attentions to the elastic – plastic approach which offers

more efficient use of materials 1, 2.The process of producing residual

stresses in the wall of thick_walled sylinder before it is put in to usage is called autofretage, which it means; asuitable large enough

pressure to cause yielding within thewall, is applied to the inner surface of

the sylinder and then removed.

So that a compressive residual stresses are generated to a certain radial depth

at the sylinder wall. Then, duringthe

subsequent application of an operating pressure, the residual stresses will

reduce the tensile stresses generated asa result of applying operating pressure

1,3.

The influenceof

residual stresses onload-carry capacity of thick_walled sylinders have been

investigate by Ayob and Albasheer 4, using both analytical andNum.techniques.

The results of the study reveal three scenarios in the design of thick_walled sylinders.

Ayob and Elbasheer 5, used von.mises and Tresca yieldcriteria to develop a

procedure in whichthe autofretage pressure determined analytically resulting in

a reduced stress concentration. Then they compared the analytical results with

FEM results. They concluded that, the autofretage process increase the max.allowable

internal pressure but it cannot increase the max.internal pressure to case

whole thickness of the sylinder to

yield. Noraziah et al. 6 presented an analytical autofretage procedure

topredict the required autofretage pressure of different levels of allowable

pressure andthey validate their results with FEM results. They found three

cases of autofretage in design of pressurized thick_ walled sylinders.

Zhu and Yang 7, using

both yield criteria von.mises and Tresca, presented an analytical equation for

optimum radius of elastic-plastic junction in autofretage sylinder , alsothey

studied the influence of autofretage on stress distribution and load bearing

capacity. They concluded, to achieve optimum radius ofelastic – plastic

junction, an autofretage pressure a bit larger than operating pressure should

be applied before a pressure vessel is put in to use. Hu and Puttagunta 8

investigate the residual stresses in thick_ walled sylinder induced by internal autofretage pressure, also

they found the optimum autofretage pressure andthe max.reduction percentage of

the von.mises stress under elastic-limit working pressure. Md. Amin et al. 9

determined the optimum elasto_plasticradius and optimum autofretage pressure using

von.mises yield criterion , then they have been compared with Zhu and Yang’s

model 8. Also they observed that the percentage of max.von.mises stress

reduction increases as value of radius ratio (K) and working pressure

increases. F. Trieb et al. 10 discussed practical application of autofretage

on components for waterjet cutting. They reported that the life time of high

pressure components is improved by increasing autofretage depth due to

reduction of tangential stress at inner diameter, on other hand too high

pressure on outside diameter should be avoided to prevent cracks generate. In

addition to determine the optimum autofretage pressure and the optimum radius

of elastic-plastic junction , Abu Rayhan Md. et al.11 evaluated the influenceof

autofretage process in strain hardened thick_ walled pressure vessels using

equivalent von.mises stress as yield criterion. They found, the number of autofretage

stages has no influenceon max.von.mises stress and pressure capacity. Also,

they concluded that, optimum autofretage pressure depends on the working

pressure and on the ratio of outer to inner radius.

II. Pressure Limits and Stress

distribution in non – autofrettaged sylinder

2.1. Pressure limits of non – autofretage

sylinder

According to Von._Mises yield

criterion, Both of the internal pressure requires to yield the inner surface of

the sylinder ( i.e. partial autofretage ),

PYi , and that to yield the whole wall of the sylinder ( i.e. completely autofretage ), PYo

, can be calculated from equations ( 1& 2 )4, 7

PYi

=

……………………. ( 1 )

PYo

=

……………………. ( 2 )

2.2. Stress distribution of

non – autofretage sylinder

The

radial stress ?r, circumferential ( hoop

) stress ?? and axial stress ?z,

distributions in non _autofretage sylinder subjected to an operating pressure, Pi,

are given by Lame’s formulations which is available in 3, 4, 5, 6, 7 . As

shown in Fig. ( 1 ), it is obvious that the

tensile hoob, ??,

compressive radial , ?r,

and max. Von._Mises stresses have

their max. values at the inner surface of the sylinder . The hoop stress has

always positive value which represents

as tensile stress while the stress in the radial direction is always

compressive. Also the hoop tensile stress’s value is greater than radial

compressive stress’s value.

Fig.

1: Stress distribution on non-autofretage thick-walled sylinder subjected to operating pressure.

Fig. 2: Geometry of investigated model.

III. Finite Element

Analysis and Materials of Num.Simulation

Models

Fig. ( 2 )

illustrates the geometry of investigated sylinder that is made up of carbon steel with young’s

modulus of ( 203 GPa ), Poisson’s ratio of ( 0.33 ) and yield stress of ( 325

MPa ) 12 . It subjected to internal pressure ( Pi ). The material is assumed

homogeneous and isotropic. To compute the required results, Num.simulation is

carried out on ABAQUS ver.6.9 13. The investigated cases are consider as 2D –

planar problem and quadratic element have been used ( CPS8R–8– nodes )

IV.

Validation of Num.Simulation

In the

present study, the validation of software has been done by comparing the

analytical calculation results which obtained by solutions of equations are

available in literatures 3, 4, 5, 6 7, with results of Num.solution using

ABAQUS ver.6.9.

From Fig.

( 3 ) , it is obvious that, the theor. and Num.calculations of circumferential, radial and max. Von._Mises

stresses for different internal pressure are very closed and overlap each

other. It means, a good agreement is found between the results, and the static

analysis shows that, the percentage of errors between the result of analytical and Num.solution are les than

0.5%. This low percentage of errors affirm, there are no significsnt

differences between the theor. results and those obtained by simulation.

Consequently, FE modeling using ABAQUS software can be used to study the influenceof

autofretage process on the stress distribution and location of autofretage radius

( Ra ) of thick_walled sylinder subjected to operating pressure.

a

b

Fig. 3 : Validation of Num.solution results with theor.

results at different operating pressure; a – operating pressure = 80 MPa, b –

operating pressure = 100 MPa.

V. Results and Discussions

5.1. Min.. Autofretage Pressure

By calaculating the min.. pressure

that needed to yield the inner surface of the tested sylinder ( PYi ) from equation (1) , it was

found equal to ( 104.243 Mpa ). That is mean, the influenceof autofretage pressure

will start at (104.243 MPa), then the plastic deformation spreads through the sylinder

thickness. Fig. (4) shows that, the

simulation solution of influenceof autofretage pressure on max. Von._Mises stress

for different operating pressure, it is obvious that , there is no influenceof autofretage

pressure on max. Von._Mises stress generating in the sylinder due to the operating pressure as long as it is

less than ( 104 Mpa ) for both value of operating pressure.Then , when it is

exceed ( Pautofretage ? 104

MPa ) the maximunm Von._Mises stress decreases depending on the autofretage pressure,

the bigger value of autofretage pressure, the lower of max. Von._Mises stress.

In addition to that , it has been

observed from Table 1 that, the max. Von._Mises stress decreases with

increasing the autofretage pressure even Pautofretage reache value

of about ( 130 MPa ) then starts increasing, which it means, this value of autofretage

pressure represents the optimum autofretage pressure 5,6. This results agree

with result was found by 1, 9, 11.

Fig.

4 : Simulation solution results of autofretage pressures’ influenceon Max.

vonMises stress at different operating pressure.

Table 1 : FEM results of influenceof Autofretage Pressure

on Max. Von._Mises Stress

No.

Operating Pressure, MPa

Autofretage Pressure, MPa

Max. vonMises Stress, MPa

1.

90

120

247.00

2.

90

125

241.40

3.

90

130

238.8

4.

90

131

240.20

5.

90

132

241.40

6.

100

120

273.10

7.

100

125

265.20

8.

100

130

260.00

9.

100

131

260.80

10.

100

132

261.00

5.2. Influenceof Autofretage process

on stress distribution

Fig.s ( 5, 6 & 7 ) demonstrates the influenceof

autofretage process on stress distribution of thicked–walled sylinder subjected to operating pressure of ( 100 MPa

). It is obvious, the autofretage process leads to decrease the value of max. Von._Mises

stress and relocated the compressive circumferential & max. Von._Mises stresses

from the inner surface of the sylinder to somewhere through it’s thickness. This new

location of max. Von._Mises stress called Autofretage radius, Ra

. It does not depend on operating pressure while it is strongly affected by autofretage

pressure as shown in Table 2, which shows the values of autofretage radius, Ra

, with different values

of autofretage pressure. Also, it is found , the reduction in max. Von._Mises stresses

varying from ( 3.6 % at Pautofretage =105 MPa ) to ( 19.2% at Pautofretage

=130 MPa ). It is vital to see that , there is no significant influenceof

autofretage process on radial stress as that seen on the circumferential

stress.

Fig. 5 :Influenceof Autofretage Pr. on hoob & Radial stresses at

operating Pressure = 100 MPa.

Fig. 6 : Influenceof Autofretage Pr. on max. Von._Mises stress at operating Pressure = 100 MPa.

Table 2 : FEM results of influenceof Autofretage Pressure

on Max. Von._Mises Stress

No.

Operating Pressure, MPa

Autofretage Pressure, MPa

Max. Von._Mises Stress, MPa

Autofretage Radius, mm

Reduction in Max. Von._Mises stress %

1.

90

without

290.00

100

—

2.

90

105

278.975

101.99836

3.8 %

3.

90

110

264.108

103.99686

8.9 %

4.

90

120

246.88

111.9915

14.8 %

5.

90

130

238.792

125.9761

17.65 %

6.

100

without

321.836

100

—

7.

100

105

310.00

101.99836

3.6 %

8.

100

110

294.020

103.99686

8.6 %

9.

100

120

273.116

111.9915

15.2 %

10.

100

130

259.992

125.9761

19.2 %

a

b

c

d

Fig. 7 : FE analysis of influenceof

autofretage Pressure on max. Von._Mises stress and location of autofretage radius

at operating Pressure = 100 MPa ; a- without autofrettage, b- Pautofretage = 110 MPa, c –

Pautofretage = 120 MPa, d –

Pautofretage = 130 MPa.

5.3. Influenceof Autofretage stages

on max. Von._Mises stress

To investigate the influenceof autofretage

stages on max. Von._Mises stress,

the investigated sylinder was subjected

to ( 100 MPa ) as operating pressure and autofretage pressures of ( 110, 120 and 130 MPa ) are done by

two steps, at first step,the autofretage pressure has been applied in one stage, while at

second step it was done by three loading stages ( see Table 3 ). As can be noticed clearly in Table 3

and Fig. ( 7 ), the Num.results confirm there is no influenceof autofretage stages

on the max. Von._Mises stress generated in the sylinder due to operating pressure. This results are

very close to the with results have been

found by 3.

Tabe 3 : FEM results of influenceof Autofretage stages

on Max. Von._Mises Stress

No. of case

Autofretage pressure, MPa

First

stage

Unloading

step

MPa

Autofretage

pressure, MPa

second stage

Unloading step

MPa

Loading of Operating Pressure, MPa

Max. Von._Mises Stress,

MPa

Case I

110

0

–

–

100

294.020

Case II

120

0

–

–

100

273.116

Case III

130

0

–

–

100

259.992

Case IV

105

0

110

0

100

294.033

Case V

105

0

120

0

100

273.05

Case VI

105

0

130

0

100

260.254

Fig. 7 : Num.solution results of influenceof autofretage

stagse on Max. von_mises

stresses and autofretage radius at operating Pressure = 100 Mpa

VI. Conclusion

The results of present

investigation can be summarized as :-

1. The autofretage process

on thick_walled sylinder leads to

decrease the circumferential and max. Von._Mises stresses and relocate them from the inner surface of the sylinder

to somewhere along it’s thickness, which

called as, autofretage radius, Ra .

2. The autofretage radius, Ra ,

is strongly affected by autofretage pressure

while it does not depend on the operating pressure..

3. There is no influenceof autoffrettage

stages on max. Von._Mises stress developed in the sylinder subjected to an operating pressure.

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