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2012 American Transactions on Engineering & Applied Sciences




                                American Transactions on
                              Engineering & Applied Sciences

                   http://TuEngr.com/ATEAS,               http://Get.to/Research




                        Computer Modeling of Internal Pressure Autofrettage
                        Process of a Thick-Walled Cylinder with the
                        Bauschinger Effect
                                   a*                          a
                        Zhong Hu , and Sudhir Puttagunta

a
    Department of Mechanical Engineering, South Dakota State University, USA

ARTICLEINFO                      A B S T RA C T
Article history:                         In this paper, the internally pressure overloading autofrettage
Received January 13, 2012
Accepted January 27, 2012        process of a thick-walled cylinder has been numerically investigated.
Available online                 The corresponding axi-symmetric and plane-stress finite element
January, 28 2012                 model has been employed. The elasto-plastic material model with
Keywords:                        nonlinear strain-hardening and kinematic hardening (the Bauschinger
Thick-walled cylinder;           effect) was adopted. The residual stresses in the thick-walled cylinder
Internal Pressurize;             induced by internal autofrettage pressure have been investigated and
Autofrettage;
                                 optimized. The optimum autofrettage pressure and the maximum
Bauschinger effect;
Finite Element Analysis          reduction percentage of the von Mises stress in the autofrettaged
                                 thick-walled cylinder under the elastic-limit working pressure have
                                 been found, the differences of stress and strain distribution between
                                 adopting the Bauschinger-effect and the non-Bauschinger-effect have
                                 been compared.

                                    2012 American Transactions on Engineering & Applied Sciences.



1. Introduction 
      Thick-walled cylinders subjected to high internal pressure and/or elevated temperature are
widely used in the nuclear and chemical industries involving pressures as high as 1380 MPa and
*Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address:
Zhong.Hu@sdstate.edu.   2012. American Transactions on Engineering & Applied
Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at
                                                                                              143
http://TUENGR.COM/ATEAS/V01/143-161.pdf
temperatures of up to 300 °C, (Ford et al. 1981) especially for military applications involving
transient peak internal pressures as high as 350 MPa and temperature of up to 1500 °C inside the
gun barrel in a ballistic event. (Bundy et al. 1996) In the absence of residual stresses, cracks usually
form at the bore where the hoop stress developed by the working pressure is highest. (Daniels
1942; Zapfec 1942; Bush 1988; Masu and Graggs 1992) To prevent such failure and to increase the
pressure-carrying capacity, a common practice is autofrettage treatment of the cylinder prior to use.
Autofrettage is used to introduce advantageous favorable compressive residual hoop stress inside
wall of a cylinder and result in an increase in the fatigue lifetime of the component. There are
basically three types of autofrettage. These are carried out by hydraulic pressurization, by
mechanically pushing an oversized mandrel, or by the pressure of powder gas, (Davidson and
Kendall 1970; Malik and Khushnood 2003) in which hydraulic and powder gas pressurization are
based on the same principal and strengthening mechanism. In general, vessels under high pressure
require a strict analysis for an optimum design for reliable and secure operational performance.
Prediction of residual stress field and optimization of the autofrettage processes’ parameters are
some of the key issues in this context, which normally involve a careful evaluation of the related
modeling, simulation and experimental details. (Davidson et al. 1963; Chu and Vasilakis 1973;
Shannon 1974; Tan and Lee 1983; Gao 1992; Avitzur 1994; Kandil 1996; Lazzarin and Livieri
1997; Zhu and Yang 1998; Venter et al. 2000; Gao 2003; Iremonger and Kalsi 2003; Kihiu et al.
2003; Parker et al. 2003; Perry and Aboudi 2003; Zhao et al. 2003; Perl and Perry 2006; Bihamta et
al. 2007; Hojjati and Hassani 2007; Korsunsky 2007; Gibson 2008; Perry and Perl 2008; Ayob et
al. 2009; Darijani et al. 2009) Efforts are continually made in the regarding aspects.

    Overloading pressure autofrettage process involves the application of high pressure to the
inner surface of a cylinder, until the desired extent of plastic deformation is achieved. Analytical
solution of pressure autofrettage of a constant cross-section cylinder, subject to some end
conditions, is possible through the use of simplifying assumptions, such as choice of yield criteria
and material compressibility and, critically, material stress-strain behavior. On the other hand,
autofrettage causes large plastic strains around the inner surface of a cylinder, which noticeably
causes the early onset of non-linearity when remove the autofrettage pressure in the unloading
process – a kinematic hardening phenomenon termed the Bauschinger effect. This non-linearity
typically causes significant deviation from those material models that are often assumed. The effect
is most pronounced around the inner surface, and in turn has a significant effect on the residual
    144         Zhong Hu, and Sudhir Puttagunta
stresses developed when the autofrettage load is removed, especially as it can cause reverse
yielding to occur when it otherwise would not be expected. Research has been done on this issue
with theoretical analysis mostly based on bilinear kinematic hardening (linear elastic and linear
hardening) material model which is a good approximation for small strain. (Lazzarin and Livieri
1997; Venter et al. 2000; Kihiu et al. 2003; Parker et al. 2003; Perry and Aboudi 2003; Huang 2005;
Perl and Perry 2006; Korsunsky 2007). However, the practical material model is of nonlinear
kinematic hardening and with equivalent strain up to 0.5~1% in the autofrettage process, which
gives complexity to theoretical analysis using nonlinear kinematic hardening material model. In
this paper, the internally pressure overloading autofrettage process will be numerically
investigated. An axi-symmetric and plane stress (for open-ended cylinder) finite element model
will be presented. The elasto-plastic nonlinear material constitutive relationship will be adopted,
incorporating a nonlinear kinematic hardening (the Bauschinger effect) for which no analytical
solution exists. The effects of the autofrettage pressure on the residual stresses in a thick-walled
cylinder will be evaluated. The percentage of stress reduction by autofrettage treatment will be
calculated based on von Mises yield criterion. The optimum autofrettage pressure will be found.
The differences of stress and strain distribution between adopting the Bauschinger-effect and the
non-Bauschinger-effect will be compared.


2. Mathematical Model   
    In this work, the thick-walled cylinder is made of stainless steel AISI 304. An elasto-plastic
governing equations for material behavior with a homogeneous and isotropic hardening model is
used. The true stress – true strain behavior of the strain hardening material follows the Hooke’s law
in the elastic region, and, for comparison purpose, the power-law hardening in the plastic region,
(Hojjati and Hassani 2007)

                                                                                              (1),


where σ and     are true stress and true strain, respectively.   is the strain at the yield point. E is
the modulus of elasticity. K is a material constant equal to                           , and n is the
strain-hardening exponent of the material (0 ≤ n < 1). σy is the yield stress. Table 1 lists the basic

*Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address:
Zhong.Hu@sdstate.edu.   2012. American Transactions on Engineering & Applied
Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at
                                                                                             145
http://TUENGR.COM/ATEAS/V01/143-161.pdf
parameters and material data and derived material parameters to be used in the FEA modeling.
(Peckner and Bernstein 1977; Harvey 1982; Boyer et al. 1985; ASM International Handbook
Committee 1990). Figure 1 shows the elasto-plastic stress-strain relationship with kinematic
hardening (the Bauschinger effect). Commercially available software ANSYS has been used for
finite element modeling of the autofrettaged thick-walled cylinder. (Swanson Analysis System Inc.
2011) The finite element model is shown in Figure 2. The element PLANE183 with the capacity of
elastic   and   plastic    material   nonlinearity   and   non-linear   kinematic   hardening   (the
Bauschinger-effect) has been adopted, which is an eight-node plane-stress 2-D element with higher
accuracy quadratic shape function. In order to get reasonable accuracy, more elements are used
near inner surface and outer surface of the cylinder, see Figure 2.

                          Table 1: Model dimensions and material properties.
                        Material of the Cylinder              AISI304
                        Modulus of Elasticity E               196.0 GPa
                        Poisson's Ratio υ                     0.29
                        Yield Strength                        152.0 MPa*
                        Strain at Yield Point                 7.755×10-4
                        Strain-Hardening Exponent n           0.2510
                        Material Constant K                   917.4 MPa
                        Inner Radius a                        60 mm
                        Outer Radius b                        90 mm
                        Maximum Working Pressure pi           47.22MPa
          * corresponding to 215MPa at 0.2% offset and ultimate tensile strength of 505 MPa.




  Figure 1: The elasto-plastic stress – strain model with kinematic hardening (the Bauschinger
                                                effect).
    146           Zhong Hu, and Sudhir Puttagunta
Figure 2: Finite element model of a plane-stress thick-walled cylinder.


3. Modeling Results and Discussions   
    Consider a thick-walled cylinder having inner radius a and outer radius b and subjected to the
internal pressure pi as shown in Figure 2. The material will obey the Hooke’s law when it is within
the elastic region. This allow us to use the Lame’s equations for calculating the hoop stress,     , and
radial stress,     , along the thickness of the cylinder, when the ends of the cylinder are open and
unconstrained so that the cylinder is in a condition of plane stress. (Ugural 2008)


                    1                                                                 (2),

                    1                                                                 (3),

           0                                                                          (4),


    Therefore, the von Mises (equivalent) stress is



                                          1    3                                      (5),

    The radial displacement is

*Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address:
Zhong.Hu@sdstate.edu.   2012. American Transactions on Engineering & Applied
Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at
                                                                                             147
http://TUENGR.COM/ATEAS/V01/143-161.pdf
1             1                                                     (6),


      Obviously, the maximum von Mises stress is at r = a. Assuming von Mises yield criterion
applied, i.e.,                                , so by substituting the data from Table 1, the maximum applied working
pressure (the maximum internal pressure without causing yielding) is



                                                1   3           47.2 (MPa)                                    (7),


      Figure 3 shows the analytical and modeling results of stress components and radial
displacement along the thickness of the cylinder subjected to the maximum working pressure (pw
max   = 47.2 MPa). The modeling results are well agreed with the analytical results from Lame’s
equations, indicating the reliability of the model employed in the numerical analysis. Figure 3 also
shows that the maximum von Mises stress and hoop stress located at the inner surface of the
cylinder, and the hoop stress is the major stress component causing yield failure.


                                1.2                                                                          4.2E‐5
                                                                                       σθ/σy by Eq.(2)
                                                                                       σθ/σy by Modeling
                                 1                                                     σr/σy by Eq.(3)       4.1E‐5
                                                                                       σr/σy by Modeling



                                                                                                                      Displacement u (m)
                                0.8                                                    σi /σy by Eq.(5)      4.0E‐5
                                                                                       σi/σy by Modeling
            Stress Ratio σ/σy




                                                                                       u by Eq.(6)
                                0.6                                                                          3.9E‐5

                                0.4                                                                          3.8E‐5

                                0.2                                                                          3.7E‐5

                                 0                                                                           3.6E‐5
                                      0             20          40         60             80           100
                            ‐0.2                                                                             3.5E‐5

                            ‐0.4                                                                             3.4E‐5
                                                                 (r‐a)/(b‐a)×100 (%)

Figure 3: Analytical and modeling results of stresses and radial displacement along the thickness
            of the cylinder subjected to the maximum working pressure (pw max = 47.2 MPa).


      148                             Zhong Hu, and Sudhir Puttagunta
1.6




                 Autofrettage Pressure Ratio 
                                                1.5

                                                1.4


                          pa/pwmax
                                                1.3

                                                1.2                         pa/pw max by Eq.(8) from Refs. (Gao 
                                                                            1992 and 2003)
                                                1.1                         pa/pw max by Modeling

                                                 1
                                                      0        20          40          60             80           100
                                                              Elasto‐plastic interface (ρ‐a)/(b‐a)*100 (%)

     Figure 4: The relationship of elastic-plastic interface ρ and the autofrettage pressure pa.


    As the internal pressure increasing, yielding (plastic deformation) starts from cylinder’s inner
surface and gradually extends towards cylinder’s outer surface. This process is basically an
elastic-plastic deformation process. The relation between internal autofrettage pressure, pa, and the
radius of the elastic-plastic boundary (interface) in plane stress condition, ρ, is determined by (Gao
1992; Gao 2003)



                                                                    √
                                                                                          cos     ,
            √

                                                  /                             √                                    (8),
                                                                                                       ,
                                                  /

                                         √
            cos                                           ,

Where
                                          √
           cos                                                                                                       (9),
                                 √



    During autofrettage process, the elastic-limit pressure pe (pressure at which yielding
commences at inner surface) is obviously                                            1 and              in the given case, and the

plastic-limit pressure py (pressure at which plasticity has spread throughout the cylinder) is

*Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address:
Zhong.Hu@sdstate.edu.   2012. American Transactions on Engineering & Applied
Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at
                                                                                                                            149
http://TUENGR.COM/ATEAS/V01/143-161.pdf
1.543 and                            in the given case. Figure 4 shows the relation of elastic-plastic interface

ρ and the autofrettage pressure pa obtained by modeling and analytical approaches, they are well
agreed.


    Figures 5-7 show the autofrettage stress distributions for the thick-walled cylinder subjected to
different internal autofrettage pressure range of                                                      1~1.6 by modeling. Hoop stress and

von Mises stress in Figures 5 and 7 clearly indicate that by increasing applied autofrettage pressure,
elastic-plastic interface (the turning point of the curve corresponding to the position ρ in Figure 4)
moves towards the outer surface of the cylinder and eventually reaches the outer surface, while
radial stress in Figure 6 indicate that the radial stress in compression, with highest compressive
stress in the inner surface and zero in the outer surface, increasing as the autofrettage pressure
increasing.


                                                  1.1


                                                  1.0
          Autofrettage Hoop Stress Ratio σaθ/σy




                                                  0.9


                                                  0.8


                                                  0.7

                                                                by Modeling under pw max
                                                  0.6           by Modeling under 1.1 pw max
                                                                by Modeling under 1.2 pw max
                                                                by Modeling under 1.3 pw max
                                                  0.5           by Modeling under 1.4 pw max
                                                                by Modeling under 1.5 pw max
                                                                by Modeling under 1.6 pw max
                                                  0.4
                                                        0              20             40              60            80             100
                                                                                     (r‐a)/(b‐a)*100 (%)

      Figure 5: Autofrettage hoop stress distribution under different autofrettage pressure.




    150                                                     Zhong Hu, and Sudhir Puttagunta
(r‐a)/(b‐a)*100 (%)
                                                                0.00
                                                                         0           20             40           60            80           100
                                                               ‐0.05
                     Autofrettage Radial Stress Ratio σar/σy
                                                               ‐0.10

                                                               ‐0.15

                                                               ‐0.20

                                                               ‐0.25
                                                                                                             by Modeling by pw max
                                                               ‐0.30
                                                                                                             by Modeling under 1.1 pw max
                                                                                                             by Modeling under 1.2 pw max
                                                               ‐0.35
                                                                                                             by Modeling under 1.3 pw max
                                                               ‐0.40                                         by Modeling under 1.4 pw max
                                                                                                             by Modeling under 1.5 pw max
                                                               ‐0.45                                         by Modeling under 1.6 pw max

                                                               ‐0.50

     Figure 6: Autofrettage radial stress distribution under different autofrettage pressure.


                                                               1.4
       Autofrettage von Mises Stress Ratio σai/σy




                                                               1.2



                                                               1.0



                                                               0.8
                                                                             by Modeling under pw max
                                                                             by Modeling under 1.1 pw max
                                                                             by Modeling under 1.2 pw max
                                                               0.6           by Modeling under 1.3 pw max
                                                                             by Modeling under 1.4 pw max
                                                                             by Modeling under 1.5 pw max
                                                                             by Modeling under 1.6 pw max
                                                               0.4
                                                                     0              20             40           60             80           100

                                                                                                  (r‐a)/(b‐a)*100 (%)

   Figure 7: Autofrettage von Mises stress distribution under different autofrettage pressure.
*Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address:
Zhong.Hu@sdstate.edu.   2012. American Transactions on Engineering & Applied
Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at
                                                                                                                                                  151
http://TUENGR.COM/ATEAS/V01/143-161.pdf
When the internal autofrettage pressure is removed, the elastic deformation is trying to resume
its original shape while the plastic deformation is resisting this process so that residual stresses
within the cylinder have been induced, and the so-called pressure autofrettage process has been
accomplished. This unloading process basically is treated elastically. However, reverse yielding is
possible if earlier onset happens due to the Bauschinger effect. Figures 8-10 show the residual
stress distribution after removed autofrettage pressure. From Figure 8, it shows a compressive
residual hoop stress near the inner surface generated which is favorable to the thick-walled cylinder
when it is under internal working pressure (partially cancelling the tensile hoop stress induced by
the working pressure), and a tensile residual hoop stress near the outer surface of the cylinder.
Figure 9 shows a relative smaller but compressive residual radial stress left inside of the thick-wall,
satisfying the boundary conditions of zero radial stress on inner surface and outer surface of the
cylinder. Figure 10 shows the residual von Mises stress with higher values basically near the inner
surface and outer surface of the cylinder. The turning points of the curves in Figures 8 and 10 are
almost the same as in Figures 5 and 7 indicates that the earlier onset of the reverse yielding is very
less, not changing the elastic-plastic interface very much.


                                               0.3
           Residual Hoop Stress Ratio σrθ/σy




                                               0.2

                                               0.1

                                               0.0
                                                      0        20          40            60             80              100
                                               ‐0.1
                                                                          by Modeling with autofrettage of 1.1 pw max
                                                                          by Modeling with autofrettage of 1.2 pw max
                                               ‐0.2
                                                                          by Modeling with autofrettage of 1.3 pw max
                                                                          by Modeling with autofrettage of 1.4 pw max
                                               ‐0.3                       by Modeling with autofrettage of 1.5 pw max
                                                                          by Modeling with autofrettage of 1.6 pw max
                                               ‐0.4
                                                                           (r‐a)/(b‐a)*100 (%)

       Figure 8: Residual hoop stress distribution after removed the autofrettage pressure.




    152                                          Zhong Hu, and Sudhir Puttagunta
0.01

                                                                       (r‐a)/(b‐a)*100 (%)
                                                      0.00
       Residual Radial Stress Ratio σrr/σy
                                                              0   20     40             60              80             100

                                              ‐0.01


                                              ‐0.02


                                              ‐0.03
                                                                         by Modeling with autofrettage of 1.1 pw max
                                                                         by Modeling with autofrettage of 1.2 pw max
                                              ‐0.04                      by Modeling with autofrettage of 1.3 pw max
                                                                         by Modeling with autofrettage of 1.4 pw max
                                                                         by Modeling with autofrettage of 1.5 pw max
                                                                         by Modeling with autofrettage of 1.6 pw max
                                              ‐0.05

      Figure 9: Residual radial stress distribution after removed the autofrettage pressure.


                                                       0.40

                                                                       by Modeling with autofrettage of 1.1 pw max
             Residual von Mises Stress Ratio σri/σy




                                                       0.35
                                                                       by Modeling with autofrettage of 1.2 pw max
                                                                       by Modeling with autofrettage of 1.3 pw max
                                                       0.30            by Modeling with autofrettage of 1.4 pw max
                                                                       by Modeling with autofrettage of 1.5 pw max
                                                       0.25            by Modeling with autofrettage of 1.6 pw max

                                                       0.20

                                                       0.15

                                                       0.10

                                                       0.05

                                                       0.00
                                                              0   20     40             60              80             100
                                                                       (r‐a)/(b‐a)*100 (%)

   Figure 10: Residual von Mises stress distribution after removed the autofrettage pressure.


*Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address:
Zhong.Hu@sdstate.edu.   2012. American Transactions on Engineering & Applied
Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at
                                                                                                                             153
http://TUENGR.COM/ATEAS/V01/143-161.pdf
When an autofrettaged thick-walled cylinder is applied to an internal working pressure, this
reloading process is treated elastically, since a plastic deformation is not desired in the application
of the cylinders based on the static yield design criterion. Figures 11-13 show the stress
distributions of an autofrettaged thick-walled cylinder after reloading by an elastic-limit pressure
(i.e., the maximum working pressure                                          47.2 MPa). It shows more uniform distribution of
the stress components and von Mises stress throughout the thickness of the cylinder due to the
autofrettage treatment, which makes maximum von Mises stress in the cylinder less than that
without autofrettage treatment. However, radial stress has not been changed very much, see Figure
12. Figure 14 shows the relationship of the final maximum von Mises stress versus autofrettage
pressure for a thick-walled cylinder under the maximum working pressure (pw max = 47.2 MPa). It
clearly shows the optimal autofrettage pressure is about 1.5 times the maximum working pressure,
and the maximum von Mises stress reduction with this autofrettage pressure is about 28%.


                                      0.9
                                                                    by Eq.(2) without autofrettage
                                                                    by Modeling without autofrettage
                                                                    by Modeling with autofrettage of 1.1 pw max
                                                                    by Modeling with autofrettage of 1.2 pw max
                                      0.8
     Final Hoop Stress Ratio σfθ/σy




                                                                    by Modeling with autofrettage of 1.3 pw max
                                                                    by Modeling with autofrettage of 1.4 pw max
                                                                    by Modeling with autofrettage of 1.5 pw max
                                                                    by Modeling with autofrettage of 1.6 pw max
                                      0.7



                                      0.6



                                      0.5



                                      0.4
                                            0             20            40               60               80       100
                                                                      (r‐a)/(b‐a)×100 (%)

   Figure 11: Final hoop stress distribution of an autofrettaged thick-walled cylinder under the
                                                      maximum working pressure (pw max = 47.2 MPa).




    154                                         Zhong Hu, and Sudhir Puttagunta
0
         Final Radial Stress Ratio σfr/σy          0            20               40                 60               80               100
                                          ‐0.05

                                            ‐0.1

                                          ‐0.15
                                                                                        by Eq.(3) without autofrettage
                                            ‐0.2                                        by Modeling without autofrettage
                                                                                        by Modeling with autofrettage of 1.1 pw max
                                          ‐0.25                                         by Modeling with autofrettage of 1.2 pw max
                                                                                        by Modeling with autofrettage of 1.3 pw max
                                                                                        by Modeling with autofrettage of 1.4 pw max
                                            ‐0.3
                                                                                        by Modeling with autofrettage of 1.5 pw max
                                                                                        by Modeling with autofrettage of 1.6 pw max
                                          ‐0.35
                                                                              (r‐a)/(b‐a)×100 (%)

  Figure 12: Final radial stress distribution of an autofrettaged thick-walled cylinder under the
                                                           maximum working pressure (pw max = 47.2 MPa).
                                             1


                                            0.9
    Final von Mises Stress Ratio σfi/σy




                                            0.8


                                            0.7

                                                       by Eq.(5) without autofrettage
                                            0.6
                                                       by Modeling without autofrettage

                                            0.5        by Modeling with autofrettage of 1.1 pw 
                                                       max
                                                       by Modeling with autofrettage of 1.2 pw 
                                                       max
                                            0.4
                                                   0           20                40                60                80               100
                                                                             (r‐a)/(b‐a)×100 (%)

Figure 13: Final von Mises stress distribution of an autofrettaged thick-walled cylinder under the
                                                           maximum working pressure (pw max = 47.2 MPa).

*Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address:
Zhong.Hu@sdstate.edu.   2012. American Transactions on Engineering & Applied
Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at
                                                                                                                                      155
http://TUENGR.COM/ATEAS/V01/143-161.pdf
1.05




                                                    Reloading Max von Mises Stress 
                                                                                           1
                                                                                      0.95
                                                                                       0.9



                                                                σi /σy
                                                                                      0.85
                                                                Ratio
                                                                                       0.8
                                                                                      0.75
                                                                                       0.7
                                                                                      0.65
                                                                                       0.6
                                                                                               0.9            1.1         1.3            1.5         1.7

                                                                                                      Autofrettage Pressure Ratio pa/pw max

        Figure 14: Final maximum von Mises stress versus autofrettage pressure for a thick-walled
                   cylinder under the maximum working pressure (pw max = 47.2 MPa).

                                          0.4                                                                                                                1.4E‐3
                                                                                                 σi/σy with Bauschinger effect under 1.4 pw max
                                                                                                 σi/σy without Bauschinger effect under 1.4 pw max
                                                                                                 σi/σy with Bauschinger effect under 1.5 pw max
                                                                                                 σi/σy without Bauschinger effect under 1.5 pw max           1.2E‐3
 Residual von Mises Stress Ratio σri/σy




                                                                                                 σi/σy with Bauschinger effect under 1.6 pw max




                                                                                                                                                                      Residual Equivalent Strain εri
                                          0.3                                                    σi/σy without Bauschinger effect under 1.6 pw max
                                                                                                 εi with Bauschinger effect under 1.4 pw max                 1.0E‐3
                                                                                                 εi without Bauschinger effect under 1.4 pw max
                                                                                                 εi with Bauschinger effect under 1.5 pw max
                                                                                                 εi without Bauschinger effect under 1.5 pw max              8.0E‐4
                                                                                                 εi with Bauschinger effect under 1.6 pw max
                                          0.2                                                    εi without Bauschinger effect under 1.6 pw max
                                                                                                                                                             6.0E‐4


                                                                                                                                                             4.0E‐4
                                          0.1

                                                                                                                                                             2.0E‐4


                                          0.0                                                                                                                0.0E+0
                                                0                                     20                 40              60               80           100
                                                                                                          (r‐a)/(b‐a)×100 (%)

Figure 15: Comparison of residual von Mises stress and equivalent strain with and without the
                                    Bauschinger effect.
   156        Zhong Hu, and Sudhir Puttagunta
0.75
                                                                                                                 1.4E‐3


                                                                                                                 1.2E‐3
                                       0.70
 Final von Mises Stress Ratio σfi/σy




                                                                                                                 1.0E‐3




                                                                                                                          Final Equivalent Strain εfi
                                                                                                                 8.0E‐4
                                       0.65


                                                  σi/σy with Bauschinger effect under 1.4 pw max
                                                                                                                 6.0E‐4
                                                  σi/σy without Bauschinger effect under 1.4 pw max
                                                  σi/σy with Bauschinger effect under 1.5 pw max
                                                  σi/σy without Bauschinger effect under 1.5 pw max
                                       0.60       σi/σy with Bauschinger effect under 1.6 pw max                 4.0E‐4
                                                  σi/σy without Bauschinger effect under 1.6 pw max
                                                  εi with Bauschinger effect under 1.4 pw max
                                                  εi without Bauschinger effect under 1.4 pw max
                                                  εi with Bauschinger effect under 1.5 pw max                    2.0E‐4
                                                  εi without Bauschinger effect under 1.5 pw max
                                                  εi with Bauschinger effect under 1.6 pw max
                                                  εi without Bauschinger effect under 1.6 pw max
                                       0.55                                                                      0.0E+0
                                              0         20                40                60        80   100
                                                                           (r‐a)/(b‐a)×100 (%)

              Figure 16: Comparison of final von Mises stress and equivalent strain with and without the
                                                Bauschinger effect.

                            Figure 15 shows the difference of the residual von Mises stress and residual equivalent strain
with or without the Bauschinger effect by modeling. It shows a difference between the results by
considering the Bauschinger effect or not considering the Bauschinger effect, and an earlier onset
of the reverse yielding occurred by modeling with the Bauschinger effect (an earlier turning point)
than that without the Bauschinger effect, especially when larger autofrettage pressure applied.


                            Figure 16 shows the difference of the final von Mises stress and final equivalent strain with or
without the Bauschinger effect by modeling. It shows a clear difference between the results by
considering the Bauschinger effect or not considering the Bauschinger effect, and a earlier turning

*Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address:
Zhong.Hu@sdstate.edu.   2012. American Transactions on Engineering & Applied
Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at
                                                                                                                    157
http://TUENGR.COM/ATEAS/V01/143-161.pdf
point by modeling with the Bauschinger effect than that without the Bauschinger effect, especially
when larger autofrettage pressure applied, due to the previous generated elastic-plastic interface
during the autofrettage process. This phenomenon indicates an earlier yield may occur for an
internally pressure autofrettaged thick-walled cylinder under a working pressure by considering
the Bauschinger effect. Therefore, in a design of safety, the Bauschinger effect should not be
ignored.


4. Conclusions 
    The Internal pressure overloading autofrettage processes of a thick-walled cylinder have been
numerically investigated by finite element analysis, taking into account the elasto-plastic strain
hardening material with kinematic hardening (the Bauschinger effect) and the von Mises yield
criterion. The corresponding axi-symmetric and plane-stress model has been employed. The
residual stresses in the thick-walled cylinder induced by autofrettage pressure have been
numerically analyzed and optimized. Under the giving geometric and material condition, the
optimum autofrettage pressure has been found to be about 1.5 times the elastic-limit working
pressure, and the maximum reduction percentage of the maximum von Mises stress in the
autofrettaged thick-walled cylinder under the elastic-limit working pressure has been found to be
about 28%. The Bauschinger effect does induce an earlier onset of the reverse yielding for an
internally pressure autofrettaged thick-walled cylinder under a working pressure. Therefore, in a
design of safety, the Bauschinger effect should not be ignored.


5. Acknowledgements 
    This work was supported by the State of South Dakota and Mechanical Engineering
Department at South Dakota State University, and inspired by the Department of Defense project
(Cooperative Agreement # W15QKN-09-2-0002) by METLAB at South Dakota State University.


6. References 
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    158          Zhong Hu, and Sudhir Puttagunta
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*Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address:
Zhong.Hu@sdstate.edu.   2012. American Transactions on Engineering & Applied
Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at
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http://TUENGR.COM/ATEAS/V01/143-161.pdf
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    160           Zhong Hu, and Sudhir Puttagunta
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       443-446.



            Dr. Zhong Hu is an Associate Professor of Mechanical Engineering at South Dakota State University.
            He received his BS and Ph.D. in Mechanical Engineering from Tsinghua University. He has worked for
            railway manufacturing industry as a senior engineer, Tsinghua University as a professor, Japan
            National Laboratory as a fellow, Cornell University, Penn State University and Southern Methodist
            University as a research associate. He has authored about 70 publications in the journals and
            conferences in the areas of nanotechnology and nanoscale modeling by quantum mechanical/molecular
            dynamics (QM/MD); development of renewable energy related materials; mechanical strength
            evaluation and failure prediction by finite element analysis (FEA) and nondestructive engineering
            (NDE); design and optimization of advanced materials (such as biomaterials, carbon nanotube,
            polymer and composites).

            Sudhir Puttagunta is a graduate student in the Department of Mechanical Engineering at South
            Dakota State University. He holds a BS in Mechanical Engineering from Nagarjuna University, India.
            He is interested in mechanical design, strength analysis and computer modeling.




Peer Review: This article has been internationally peer-reviewed and accepted for publication
               according to the guidelines given at the journal’s website.


*Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address:
Zhong.Hu@sdstate.edu.   2012. American Transactions on Engineering & Applied
Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at
                                                                                                    161
http://TUENGR.COM/ATEAS/V01/143-161.pdf

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Computer Modeling of Internal Pressure Autofrettage Process of a Thick-Walled Cylinder with the Bauschinger Effect

  • 1. 2012 American Transactions on Engineering & Applied Sciences American Transactions on Engineering & Applied Sciences http://TuEngr.com/ATEAS, http://Get.to/Research Computer Modeling of Internal Pressure Autofrettage Process of a Thick-Walled Cylinder with the Bauschinger Effect a* a Zhong Hu , and Sudhir Puttagunta a Department of Mechanical Engineering, South Dakota State University, USA ARTICLEINFO A B S T RA C T Article history: In this paper, the internally pressure overloading autofrettage Received January 13, 2012 Accepted January 27, 2012 process of a thick-walled cylinder has been numerically investigated. Available online The corresponding axi-symmetric and plane-stress finite element January, 28 2012 model has been employed. The elasto-plastic material model with Keywords: nonlinear strain-hardening and kinematic hardening (the Bauschinger Thick-walled cylinder; effect) was adopted. The residual stresses in the thick-walled cylinder Internal Pressurize; induced by internal autofrettage pressure have been investigated and Autofrettage; optimized. The optimum autofrettage pressure and the maximum Bauschinger effect; Finite Element Analysis reduction percentage of the von Mises stress in the autofrettaged thick-walled cylinder under the elastic-limit working pressure have been found, the differences of stress and strain distribution between adopting the Bauschinger-effect and the non-Bauschinger-effect have been compared. 2012 American Transactions on Engineering & Applied Sciences. 1. Introduction  Thick-walled cylinders subjected to high internal pressure and/or elevated temperature are widely used in the nuclear and chemical industries involving pressures as high as 1380 MPa and *Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address: Zhong.Hu@sdstate.edu. 2012. American Transactions on Engineering & Applied Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at 143 http://TUENGR.COM/ATEAS/V01/143-161.pdf
  • 2. temperatures of up to 300 °C, (Ford et al. 1981) especially for military applications involving transient peak internal pressures as high as 350 MPa and temperature of up to 1500 °C inside the gun barrel in a ballistic event. (Bundy et al. 1996) In the absence of residual stresses, cracks usually form at the bore where the hoop stress developed by the working pressure is highest. (Daniels 1942; Zapfec 1942; Bush 1988; Masu and Graggs 1992) To prevent such failure and to increase the pressure-carrying capacity, a common practice is autofrettage treatment of the cylinder prior to use. Autofrettage is used to introduce advantageous favorable compressive residual hoop stress inside wall of a cylinder and result in an increase in the fatigue lifetime of the component. There are basically three types of autofrettage. These are carried out by hydraulic pressurization, by mechanically pushing an oversized mandrel, or by the pressure of powder gas, (Davidson and Kendall 1970; Malik and Khushnood 2003) in which hydraulic and powder gas pressurization are based on the same principal and strengthening mechanism. In general, vessels under high pressure require a strict analysis for an optimum design for reliable and secure operational performance. Prediction of residual stress field and optimization of the autofrettage processes’ parameters are some of the key issues in this context, which normally involve a careful evaluation of the related modeling, simulation and experimental details. (Davidson et al. 1963; Chu and Vasilakis 1973; Shannon 1974; Tan and Lee 1983; Gao 1992; Avitzur 1994; Kandil 1996; Lazzarin and Livieri 1997; Zhu and Yang 1998; Venter et al. 2000; Gao 2003; Iremonger and Kalsi 2003; Kihiu et al. 2003; Parker et al. 2003; Perry and Aboudi 2003; Zhao et al. 2003; Perl and Perry 2006; Bihamta et al. 2007; Hojjati and Hassani 2007; Korsunsky 2007; Gibson 2008; Perry and Perl 2008; Ayob et al. 2009; Darijani et al. 2009) Efforts are continually made in the regarding aspects. Overloading pressure autofrettage process involves the application of high pressure to the inner surface of a cylinder, until the desired extent of plastic deformation is achieved. Analytical solution of pressure autofrettage of a constant cross-section cylinder, subject to some end conditions, is possible through the use of simplifying assumptions, such as choice of yield criteria and material compressibility and, critically, material stress-strain behavior. On the other hand, autofrettage causes large plastic strains around the inner surface of a cylinder, which noticeably causes the early onset of non-linearity when remove the autofrettage pressure in the unloading process – a kinematic hardening phenomenon termed the Bauschinger effect. This non-linearity typically causes significant deviation from those material models that are often assumed. The effect is most pronounced around the inner surface, and in turn has a significant effect on the residual 144 Zhong Hu, and Sudhir Puttagunta
  • 3. stresses developed when the autofrettage load is removed, especially as it can cause reverse yielding to occur when it otherwise would not be expected. Research has been done on this issue with theoretical analysis mostly based on bilinear kinematic hardening (linear elastic and linear hardening) material model which is a good approximation for small strain. (Lazzarin and Livieri 1997; Venter et al. 2000; Kihiu et al. 2003; Parker et al. 2003; Perry and Aboudi 2003; Huang 2005; Perl and Perry 2006; Korsunsky 2007). However, the practical material model is of nonlinear kinematic hardening and with equivalent strain up to 0.5~1% in the autofrettage process, which gives complexity to theoretical analysis using nonlinear kinematic hardening material model. In this paper, the internally pressure overloading autofrettage process will be numerically investigated. An axi-symmetric and plane stress (for open-ended cylinder) finite element model will be presented. The elasto-plastic nonlinear material constitutive relationship will be adopted, incorporating a nonlinear kinematic hardening (the Bauschinger effect) for which no analytical solution exists. The effects of the autofrettage pressure on the residual stresses in a thick-walled cylinder will be evaluated. The percentage of stress reduction by autofrettage treatment will be calculated based on von Mises yield criterion. The optimum autofrettage pressure will be found. The differences of stress and strain distribution between adopting the Bauschinger-effect and the non-Bauschinger-effect will be compared. 2. Mathematical Model    In this work, the thick-walled cylinder is made of stainless steel AISI 304. An elasto-plastic governing equations for material behavior with a homogeneous and isotropic hardening model is used. The true stress – true strain behavior of the strain hardening material follows the Hooke’s law in the elastic region, and, for comparison purpose, the power-law hardening in the plastic region, (Hojjati and Hassani 2007) (1), where σ and are true stress and true strain, respectively. is the strain at the yield point. E is the modulus of elasticity. K is a material constant equal to , and n is the strain-hardening exponent of the material (0 ≤ n < 1). σy is the yield stress. Table 1 lists the basic *Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address: Zhong.Hu@sdstate.edu. 2012. American Transactions on Engineering & Applied Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at 145 http://TUENGR.COM/ATEAS/V01/143-161.pdf
  • 4. parameters and material data and derived material parameters to be used in the FEA modeling. (Peckner and Bernstein 1977; Harvey 1982; Boyer et al. 1985; ASM International Handbook Committee 1990). Figure 1 shows the elasto-plastic stress-strain relationship with kinematic hardening (the Bauschinger effect). Commercially available software ANSYS has been used for finite element modeling of the autofrettaged thick-walled cylinder. (Swanson Analysis System Inc. 2011) The finite element model is shown in Figure 2. The element PLANE183 with the capacity of elastic and plastic material nonlinearity and non-linear kinematic hardening (the Bauschinger-effect) has been adopted, which is an eight-node plane-stress 2-D element with higher accuracy quadratic shape function. In order to get reasonable accuracy, more elements are used near inner surface and outer surface of the cylinder, see Figure 2. Table 1: Model dimensions and material properties. Material of the Cylinder AISI304 Modulus of Elasticity E 196.0 GPa Poisson's Ratio υ 0.29 Yield Strength 152.0 MPa* Strain at Yield Point 7.755×10-4 Strain-Hardening Exponent n 0.2510 Material Constant K 917.4 MPa Inner Radius a 60 mm Outer Radius b 90 mm Maximum Working Pressure pi 47.22MPa * corresponding to 215MPa at 0.2% offset and ultimate tensile strength of 505 MPa. Figure 1: The elasto-plastic stress – strain model with kinematic hardening (the Bauschinger effect). 146 Zhong Hu, and Sudhir Puttagunta
  • 5. Figure 2: Finite element model of a plane-stress thick-walled cylinder. 3. Modeling Results and Discussions    Consider a thick-walled cylinder having inner radius a and outer radius b and subjected to the internal pressure pi as shown in Figure 2. The material will obey the Hooke’s law when it is within the elastic region. This allow us to use the Lame’s equations for calculating the hoop stress, , and radial stress, , along the thickness of the cylinder, when the ends of the cylinder are open and unconstrained so that the cylinder is in a condition of plane stress. (Ugural 2008) 1 (2), 1 (3), 0 (4), Therefore, the von Mises (equivalent) stress is 1 3 (5), The radial displacement is *Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address: Zhong.Hu@sdstate.edu. 2012. American Transactions on Engineering & Applied Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at 147 http://TUENGR.COM/ATEAS/V01/143-161.pdf
  • 6. 1 1 (6), Obviously, the maximum von Mises stress is at r = a. Assuming von Mises yield criterion applied, i.e., , so by substituting the data from Table 1, the maximum applied working pressure (the maximum internal pressure without causing yielding) is 1 3 47.2 (MPa) (7), Figure 3 shows the analytical and modeling results of stress components and radial displacement along the thickness of the cylinder subjected to the maximum working pressure (pw max = 47.2 MPa). The modeling results are well agreed with the analytical results from Lame’s equations, indicating the reliability of the model employed in the numerical analysis. Figure 3 also shows that the maximum von Mises stress and hoop stress located at the inner surface of the cylinder, and the hoop stress is the major stress component causing yield failure. 1.2 4.2E‐5 σθ/σy by Eq.(2) σθ/σy by Modeling 1 σr/σy by Eq.(3) 4.1E‐5 σr/σy by Modeling Displacement u (m) 0.8 σi /σy by Eq.(5) 4.0E‐5 σi/σy by Modeling Stress Ratio σ/σy u by Eq.(6) 0.6 3.9E‐5 0.4 3.8E‐5 0.2 3.7E‐5 0 3.6E‐5 0 20 40 60 80 100 ‐0.2 3.5E‐5 ‐0.4 3.4E‐5 (r‐a)/(b‐a)×100 (%) Figure 3: Analytical and modeling results of stresses and radial displacement along the thickness of the cylinder subjected to the maximum working pressure (pw max = 47.2 MPa). 148 Zhong Hu, and Sudhir Puttagunta
  • 7. 1.6 Autofrettage Pressure Ratio  1.5 1.4 pa/pwmax 1.3 1.2 pa/pw max by Eq.(8) from Refs. (Gao  1992 and 2003) 1.1 pa/pw max by Modeling 1 0 20 40 60 80 100 Elasto‐plastic interface (ρ‐a)/(b‐a)*100 (%) Figure 4: The relationship of elastic-plastic interface ρ and the autofrettage pressure pa. As the internal pressure increasing, yielding (plastic deformation) starts from cylinder’s inner surface and gradually extends towards cylinder’s outer surface. This process is basically an elastic-plastic deformation process. The relation between internal autofrettage pressure, pa, and the radius of the elastic-plastic boundary (interface) in plane stress condition, ρ, is determined by (Gao 1992; Gao 2003) √ cos , √ / √ (8), , / √ cos , Where √ cos (9), √ During autofrettage process, the elastic-limit pressure pe (pressure at which yielding commences at inner surface) is obviously 1 and in the given case, and the plastic-limit pressure py (pressure at which plasticity has spread throughout the cylinder) is *Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address: Zhong.Hu@sdstate.edu. 2012. American Transactions on Engineering & Applied Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at 149 http://TUENGR.COM/ATEAS/V01/143-161.pdf
  • 8. 1.543 and in the given case. Figure 4 shows the relation of elastic-plastic interface ρ and the autofrettage pressure pa obtained by modeling and analytical approaches, they are well agreed. Figures 5-7 show the autofrettage stress distributions for the thick-walled cylinder subjected to different internal autofrettage pressure range of 1~1.6 by modeling. Hoop stress and von Mises stress in Figures 5 and 7 clearly indicate that by increasing applied autofrettage pressure, elastic-plastic interface (the turning point of the curve corresponding to the position ρ in Figure 4) moves towards the outer surface of the cylinder and eventually reaches the outer surface, while radial stress in Figure 6 indicate that the radial stress in compression, with highest compressive stress in the inner surface and zero in the outer surface, increasing as the autofrettage pressure increasing. 1.1 1.0 Autofrettage Hoop Stress Ratio σaθ/σy 0.9 0.8 0.7 by Modeling under pw max 0.6 by Modeling under 1.1 pw max by Modeling under 1.2 pw max by Modeling under 1.3 pw max 0.5 by Modeling under 1.4 pw max by Modeling under 1.5 pw max by Modeling under 1.6 pw max 0.4 0 20 40 60 80 100 (r‐a)/(b‐a)*100 (%) Figure 5: Autofrettage hoop stress distribution under different autofrettage pressure. 150 Zhong Hu, and Sudhir Puttagunta
  • 9. (r‐a)/(b‐a)*100 (%) 0.00 0 20 40 60 80 100 ‐0.05 Autofrettage Radial Stress Ratio σar/σy ‐0.10 ‐0.15 ‐0.20 ‐0.25 by Modeling by pw max ‐0.30 by Modeling under 1.1 pw max by Modeling under 1.2 pw max ‐0.35 by Modeling under 1.3 pw max ‐0.40 by Modeling under 1.4 pw max by Modeling under 1.5 pw max ‐0.45 by Modeling under 1.6 pw max ‐0.50 Figure 6: Autofrettage radial stress distribution under different autofrettage pressure. 1.4 Autofrettage von Mises Stress Ratio σai/σy 1.2 1.0 0.8 by Modeling under pw max by Modeling under 1.1 pw max by Modeling under 1.2 pw max 0.6 by Modeling under 1.3 pw max by Modeling under 1.4 pw max by Modeling under 1.5 pw max by Modeling under 1.6 pw max 0.4 0 20 40 60 80 100 (r‐a)/(b‐a)*100 (%) Figure 7: Autofrettage von Mises stress distribution under different autofrettage pressure. *Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address: Zhong.Hu@sdstate.edu. 2012. American Transactions on Engineering & Applied Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at 151 http://TUENGR.COM/ATEAS/V01/143-161.pdf
  • 10. When the internal autofrettage pressure is removed, the elastic deformation is trying to resume its original shape while the plastic deformation is resisting this process so that residual stresses within the cylinder have been induced, and the so-called pressure autofrettage process has been accomplished. This unloading process basically is treated elastically. However, reverse yielding is possible if earlier onset happens due to the Bauschinger effect. Figures 8-10 show the residual stress distribution after removed autofrettage pressure. From Figure 8, it shows a compressive residual hoop stress near the inner surface generated which is favorable to the thick-walled cylinder when it is under internal working pressure (partially cancelling the tensile hoop stress induced by the working pressure), and a tensile residual hoop stress near the outer surface of the cylinder. Figure 9 shows a relative smaller but compressive residual radial stress left inside of the thick-wall, satisfying the boundary conditions of zero radial stress on inner surface and outer surface of the cylinder. Figure 10 shows the residual von Mises stress with higher values basically near the inner surface and outer surface of the cylinder. The turning points of the curves in Figures 8 and 10 are almost the same as in Figures 5 and 7 indicates that the earlier onset of the reverse yielding is very less, not changing the elastic-plastic interface very much. 0.3 Residual Hoop Stress Ratio σrθ/σy 0.2 0.1 0.0 0 20 40 60 80 100 ‐0.1 by Modeling with autofrettage of 1.1 pw max by Modeling with autofrettage of 1.2 pw max ‐0.2 by Modeling with autofrettage of 1.3 pw max by Modeling with autofrettage of 1.4 pw max ‐0.3 by Modeling with autofrettage of 1.5 pw max by Modeling with autofrettage of 1.6 pw max ‐0.4 (r‐a)/(b‐a)*100 (%) Figure 8: Residual hoop stress distribution after removed the autofrettage pressure. 152 Zhong Hu, and Sudhir Puttagunta
  • 11. 0.01 (r‐a)/(b‐a)*100 (%) 0.00 Residual Radial Stress Ratio σrr/σy 0 20 40 60 80 100 ‐0.01 ‐0.02 ‐0.03 by Modeling with autofrettage of 1.1 pw max by Modeling with autofrettage of 1.2 pw max ‐0.04 by Modeling with autofrettage of 1.3 pw max by Modeling with autofrettage of 1.4 pw max by Modeling with autofrettage of 1.5 pw max by Modeling with autofrettage of 1.6 pw max ‐0.05 Figure 9: Residual radial stress distribution after removed the autofrettage pressure. 0.40 by Modeling with autofrettage of 1.1 pw max Residual von Mises Stress Ratio σri/σy 0.35 by Modeling with autofrettage of 1.2 pw max by Modeling with autofrettage of 1.3 pw max 0.30 by Modeling with autofrettage of 1.4 pw max by Modeling with autofrettage of 1.5 pw max 0.25 by Modeling with autofrettage of 1.6 pw max 0.20 0.15 0.10 0.05 0.00 0 20 40 60 80 100 (r‐a)/(b‐a)*100 (%) Figure 10: Residual von Mises stress distribution after removed the autofrettage pressure. *Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address: Zhong.Hu@sdstate.edu. 2012. American Transactions on Engineering & Applied Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at 153 http://TUENGR.COM/ATEAS/V01/143-161.pdf
  • 12. When an autofrettaged thick-walled cylinder is applied to an internal working pressure, this reloading process is treated elastically, since a plastic deformation is not desired in the application of the cylinders based on the static yield design criterion. Figures 11-13 show the stress distributions of an autofrettaged thick-walled cylinder after reloading by an elastic-limit pressure (i.e., the maximum working pressure 47.2 MPa). It shows more uniform distribution of the stress components and von Mises stress throughout the thickness of the cylinder due to the autofrettage treatment, which makes maximum von Mises stress in the cylinder less than that without autofrettage treatment. However, radial stress has not been changed very much, see Figure 12. Figure 14 shows the relationship of the final maximum von Mises stress versus autofrettage pressure for a thick-walled cylinder under the maximum working pressure (pw max = 47.2 MPa). It clearly shows the optimal autofrettage pressure is about 1.5 times the maximum working pressure, and the maximum von Mises stress reduction with this autofrettage pressure is about 28%. 0.9 by Eq.(2) without autofrettage by Modeling without autofrettage by Modeling with autofrettage of 1.1 pw max by Modeling with autofrettage of 1.2 pw max 0.8 Final Hoop Stress Ratio σfθ/σy by Modeling with autofrettage of 1.3 pw max by Modeling with autofrettage of 1.4 pw max by Modeling with autofrettage of 1.5 pw max by Modeling with autofrettage of 1.6 pw max 0.7 0.6 0.5 0.4 0 20 40 60 80 100 (r‐a)/(b‐a)×100 (%) Figure 11: Final hoop stress distribution of an autofrettaged thick-walled cylinder under the maximum working pressure (pw max = 47.2 MPa). 154 Zhong Hu, and Sudhir Puttagunta
  • 13. 0 Final Radial Stress Ratio σfr/σy 0 20 40 60 80 100 ‐0.05 ‐0.1 ‐0.15 by Eq.(3) without autofrettage ‐0.2 by Modeling without autofrettage by Modeling with autofrettage of 1.1 pw max ‐0.25 by Modeling with autofrettage of 1.2 pw max by Modeling with autofrettage of 1.3 pw max by Modeling with autofrettage of 1.4 pw max ‐0.3 by Modeling with autofrettage of 1.5 pw max by Modeling with autofrettage of 1.6 pw max ‐0.35 (r‐a)/(b‐a)×100 (%) Figure 12: Final radial stress distribution of an autofrettaged thick-walled cylinder under the maximum working pressure (pw max = 47.2 MPa). 1 0.9 Final von Mises Stress Ratio σfi/σy 0.8 0.7 by Eq.(5) without autofrettage 0.6 by Modeling without autofrettage 0.5 by Modeling with autofrettage of 1.1 pw  max by Modeling with autofrettage of 1.2 pw  max 0.4 0 20 40 60 80 100 (r‐a)/(b‐a)×100 (%) Figure 13: Final von Mises stress distribution of an autofrettaged thick-walled cylinder under the maximum working pressure (pw max = 47.2 MPa). *Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address: Zhong.Hu@sdstate.edu. 2012. American Transactions on Engineering & Applied Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at 155 http://TUENGR.COM/ATEAS/V01/143-161.pdf
  • 14. 1.05 Reloading Max von Mises Stress  1 0.95 0.9 σi /σy 0.85 Ratio 0.8 0.75 0.7 0.65 0.6 0.9 1.1 1.3 1.5 1.7 Autofrettage Pressure Ratio pa/pw max Figure 14: Final maximum von Mises stress versus autofrettage pressure for a thick-walled cylinder under the maximum working pressure (pw max = 47.2 MPa). 0.4 1.4E‐3 σi/σy with Bauschinger effect under 1.4 pw max σi/σy without Bauschinger effect under 1.4 pw max σi/σy with Bauschinger effect under 1.5 pw max σi/σy without Bauschinger effect under 1.5 pw max 1.2E‐3 Residual von Mises Stress Ratio σri/σy σi/σy with Bauschinger effect under 1.6 pw max Residual Equivalent Strain εri 0.3 σi/σy without Bauschinger effect under 1.6 pw max εi with Bauschinger effect under 1.4 pw max 1.0E‐3 εi without Bauschinger effect under 1.4 pw max εi with Bauschinger effect under 1.5 pw max εi without Bauschinger effect under 1.5 pw max 8.0E‐4 εi with Bauschinger effect under 1.6 pw max 0.2 εi without Bauschinger effect under 1.6 pw max 6.0E‐4 4.0E‐4 0.1 2.0E‐4 0.0 0.0E+0 0 20 40 60 80 100 (r‐a)/(b‐a)×100 (%) Figure 15: Comparison of residual von Mises stress and equivalent strain with and without the Bauschinger effect. 156 Zhong Hu, and Sudhir Puttagunta
  • 15. 0.75 1.4E‐3 1.2E‐3 0.70 Final von Mises Stress Ratio σfi/σy 1.0E‐3 Final Equivalent Strain εfi 8.0E‐4 0.65 σi/σy with Bauschinger effect under 1.4 pw max 6.0E‐4 σi/σy without Bauschinger effect under 1.4 pw max σi/σy with Bauschinger effect under 1.5 pw max σi/σy without Bauschinger effect under 1.5 pw max 0.60 σi/σy with Bauschinger effect under 1.6 pw max 4.0E‐4 σi/σy without Bauschinger effect under 1.6 pw max εi with Bauschinger effect under 1.4 pw max εi without Bauschinger effect under 1.4 pw max εi with Bauschinger effect under 1.5 pw max 2.0E‐4 εi without Bauschinger effect under 1.5 pw max εi with Bauschinger effect under 1.6 pw max εi without Bauschinger effect under 1.6 pw max 0.55 0.0E+0 0 20 40 60 80 100 (r‐a)/(b‐a)×100 (%) Figure 16: Comparison of final von Mises stress and equivalent strain with and without the Bauschinger effect. Figure 15 shows the difference of the residual von Mises stress and residual equivalent strain with or without the Bauschinger effect by modeling. It shows a difference between the results by considering the Bauschinger effect or not considering the Bauschinger effect, and an earlier onset of the reverse yielding occurred by modeling with the Bauschinger effect (an earlier turning point) than that without the Bauschinger effect, especially when larger autofrettage pressure applied. Figure 16 shows the difference of the final von Mises stress and final equivalent strain with or without the Bauschinger effect by modeling. It shows a clear difference between the results by considering the Bauschinger effect or not considering the Bauschinger effect, and a earlier turning *Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address: Zhong.Hu@sdstate.edu. 2012. American Transactions on Engineering & Applied Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at 157 http://TUENGR.COM/ATEAS/V01/143-161.pdf
  • 16. point by modeling with the Bauschinger effect than that without the Bauschinger effect, especially when larger autofrettage pressure applied, due to the previous generated elastic-plastic interface during the autofrettage process. This phenomenon indicates an earlier yield may occur for an internally pressure autofrettaged thick-walled cylinder under a working pressure by considering the Bauschinger effect. Therefore, in a design of safety, the Bauschinger effect should not be ignored. 4. Conclusions  The Internal pressure overloading autofrettage processes of a thick-walled cylinder have been numerically investigated by finite element analysis, taking into account the elasto-plastic strain hardening material with kinematic hardening (the Bauschinger effect) and the von Mises yield criterion. The corresponding axi-symmetric and plane-stress model has been employed. The residual stresses in the thick-walled cylinder induced by autofrettage pressure have been numerically analyzed and optimized. Under the giving geometric and material condition, the optimum autofrettage pressure has been found to be about 1.5 times the elastic-limit working pressure, and the maximum reduction percentage of the maximum von Mises stress in the autofrettaged thick-walled cylinder under the elastic-limit working pressure has been found to be about 28%. The Bauschinger effect does induce an earlier onset of the reverse yielding for an internally pressure autofrettaged thick-walled cylinder under a working pressure. Therefore, in a design of safety, the Bauschinger effect should not be ignored. 5. Acknowledgements  This work was supported by the State of South Dakota and Mechanical Engineering Department at South Dakota State University, and inspired by the Department of Defense project (Cooperative Agreement # W15QKN-09-2-0002) by METLAB at South Dakota State University. 6. References  ASM International Handbook Committee. (1990). Metals Handbook, 10th ed., vol. 1, , ASM International, Materials Park, OH. Avitzur, B. (1994). Autofrettage – stress distribution under load and retained stresses after depressurization. Int. J. Pres. Ves. & Piping, 57, 271-287. 158 Zhong Hu, and Sudhir Puttagunta
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  • 19. autofrettage. Journal of Pressure Vessel Technology, Transactions of the ASME, 130, 041211-1-6. Shannon, R. W. E. (1974). Stress intensity factors for thick-walled cylinders. Int. J. Pres. Ves. & Piping, 2, 19-29. Swanson Analysis System Inc. (2011). ANSYS 13.0 User’s Manual, Houston. Tan, C. L. and K. H. Lee. (1983). Elastic-plastic stress analysis of a cracked thick-walled cylinder. Journal of Strain Analysis, 18(4), 253-260. Ugural, A. C. (2008). Mechanics of Materials, John Wiley & Sons. Inc, NJ. Venter, A. M., R. R. de Swardt, and S. Kyriacon. (2000). Comparative measurements on autofrettaged cylinders with large Bauschinger reverse yield zones. Journal of Strain Analysis, 35(6), 459-469. Zapfec, C. A. (1942). Boiler embrittlement. Transactions of ASME, 66(2), 81–126. Zhao, W., R. Reshadri, and R. N. Dubey. (2003). On thick-walled cylinder under internal pressure. Journal of Pressure Vessel Technology, 125, 267-273. Zhu, R. and J. Yang. (1998). Autofrettage of thick cylinders, Int. J. Pres. Ves. & Piping, 75, 443-446. Dr. Zhong Hu is an Associate Professor of Mechanical Engineering at South Dakota State University. He received his BS and Ph.D. in Mechanical Engineering from Tsinghua University. He has worked for railway manufacturing industry as a senior engineer, Tsinghua University as a professor, Japan National Laboratory as a fellow, Cornell University, Penn State University and Southern Methodist University as a research associate. He has authored about 70 publications in the journals and conferences in the areas of nanotechnology and nanoscale modeling by quantum mechanical/molecular dynamics (QM/MD); development of renewable energy related materials; mechanical strength evaluation and failure prediction by finite element analysis (FEA) and nondestructive engineering (NDE); design and optimization of advanced materials (such as biomaterials, carbon nanotube, polymer and composites). Sudhir Puttagunta is a graduate student in the Department of Mechanical Engineering at South Dakota State University. He holds a BS in Mechanical Engineering from Nagarjuna University, India. He is interested in mechanical design, strength analysis and computer modeling. Peer Review: This article has been internationally peer-reviewed and accepted for publication according to the guidelines given at the journal’s website. *Corresponding author (Z.Hu). Tel/Fax: +1-605-688-4817. E-mail address: Zhong.Hu@sdstate.edu. 2012. American Transactions on Engineering & Applied Sciences. Volume 1 No.2 ISSN 2229-1652 eISSN 2229-1660. Online Available at 161 http://TUENGR.COM/ATEAS/V01/143-161.pdf