Long term performance of plastic pipes in gas distribution by exb38697

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									                                                    Academic Network Event, June 4th 2009




 Long-term performance of plastic pipes in
 gas distribution systems
     H.A. Vissera, M. Woltersa, R. Akkermana, T.C. Bora and L.E. Govaertb




 a                                         b




h.a.visser@ctw.utwente.nl                                                    0/20
 Brief overview of plastics in pipe networks

 • First introduced in gas/water distribution networks in mid 1950s.
 • Laboratory test running for 50 years proved the durability1.
 • 22,500 km of uPVC pipes is still in service in the Dutch gas
   distribution network2.
 • Replacement of the distribution network is very costly.




Installation of a uPVC pipe network3

1Schulte (2006) Proceedings of Plastic Pipes XIII, Washington
2Hermkens   et al. (2009) Proceedings of Plastic Pipes XIV, Budapest   Longest running burst tests on PE pipe1
3(1955) Kunststoffe 48:108


                                                                                                     1/20
Goal



  Development of a method to determine the residual lifetime of
       unplasticised PVC pipes in a non-destructive way.



                                                           Residual
      Robot                           ??
                                                           lifetime




Note: The focus will be on uPVC pipes here. However, in theory, the
      approach is applicable to other polymers as well.


                                                                      2/20
 What limits the lifetime of uPVC gas pipes?

 •    Spontaneous fracture hardly occurs4.
 •    Most failures originate from third party damage4.
 •    Brittle fracture behaviour increases the risk of (fatal) incidents.
 •    Fracture behaviour limits the service lifetime of uPVC gas
      pipes.




                             Gas explosion due to road works, Leiden, the Netherlands, 2001

4Visser   et al. (2008) Proceedings of the International Gas Union Research Conference, Paris
                                                                                                3/20
    “Measuring” the residual lifetime




                                                                             Modelling of
                                                                            ageing kinetics
                     Condition


Related to frac-                                                            Non-destructive
ture behaviour                                                               measurement


                                 Criterion

Ductile-to-brittle
                                             Time   Lifetime 1 Lifetime 2
  transition




                                                                                          4/20
 Phenomena influencing the fracture behaviour

 •    Level of gelation5
 •    Weathering6
 •    Environmental stress cracking7
 •    Physical ageing8




5Terselius et al. (1985) Plast. Rub. Proc. Appl., 5(1-7)
6Roux et al. (1981) J. Macromol. Sci. –Phys. B20(505-217)

7Breen et al. (1994) J. Mater. Sci. 29(39-46)

8Struik (1978) Physical aging in amorphous polymers and other materials, Elsevier, Amsterdam


                                                                                               5/20
What is physical ageing?

•   After extrusion the entangled polymer chain structure freezes.
•   The chains move towards their thermodynamic equilibrium.
•   This results in volume reduction and stronger secondary bonds.
•   Physical ageing is accelerated by thermal energy and stress.




                                                                6/20
 Influence of physical ageing on glassy polymers

      As manufactured (“young”)                        After annealing (aged)




 Deformation occurs on a more local scale after ageing9 leading to
 brittle behaviour on a macroscopic scale10.


9Brady   et al. (1971) J. Appl. Phys., 42(4622-4630)
10Melick  et al. (2003) Polymer, 44(3579-3591)
                                                                                7/20
Consequences for mechanical properties


                           As manufactured                                                  Aged


                      Yield                                                       3
True stress




                                                        True stress
                       3                        4                                                           4
                        Softening       Hardening

                  2                                                           2




          1                   True strain                             1                   True strain




              1               2             3       4                     1           2                 3   4
                                                                                                                8/20
 Eyring describes deformation kinetics

 Pressure modified Eyring relation11 in equivalent terms:


                               − µpν *   − ∆U        τν * 
    γ (T ,τ , p ) = γ0 ⋅ exp
                               RT  ⋅ exp RT  ⋅ sinh  RT 
                                                             
                                                          
 Plastic defor-                                             Deformation kinetics:
  mation rate                                               Temperature and stress activated
                               Hydrostatic pressure:
                               Required for proper representation of 3D loading
   Thermodynamic state:
   Depends on processing conditions, changes with physical ageing12


11Ward   (1971) J. Mater. Sci., 6(1397-1417)
12Visser  et al. (2008) Proceedings of PVC 2008, Brighton
                                                                                               9/20
   Incorporating ageing kinetics

   Ageing is accelerated by thermal energy and stress:

                                                                      c1
                                                                 
             γ0 (T ,τ , t ) = c0 ⋅  t a + ∫
                                              t       dt '
                                                                 
                                             0 a (T ) ⋅ a (T ,τ ) 
                                               T        σ        


                              Initial age:
Depends on thermal and loading history

                  Thermal acceleration factor

                               Stress acceleration factor



                                                                           10/20
                     Ageing kinetics of yield stress


                     70
                            Ta = 65°C
                            Ta = 55°C
                     65
Yield stress [MPa]




                            Ta = 45°C
                            Ta = 25°C
                     60


                     55


                     50


                     45 2            4        6          8        10
                      10        10       10         10       10
                                   Ageing time [s]
                               Effective ageing time [s]



                     Time-temperature superposition with Arrhenius:
                                                                                     ∆U     1   1 
                                                                       aT (T ) = exp    a
                                                                                              −      
                                                                                     R       T Tref  
                                                                                                    
                                                                                                     11/20
Stress induced ageing kinetics


                                       70
                                              32.5 MPa
                                              25 MPa
                                       65



                  Yield stress [MPa]
                                                                            σa
                                       60

                                       55

                                       50

                                       45

                                       40 2           4             6            8        10
                                        10       10            10           10       10
                                                          Ageing time [s]



Shifts with Eyring type time-stress superposition:
                                                                                                                 −1
                                                                                           τν a        τν a 
                                                                        aσ (T ,τ ) =              sinh     
                                                                                               RT       RT 
                                                                                                           12/20
Including ageing in deformation kinetics


                                         70
                                                an
                                                  ne
                                                    a le



                  Applied stress [MPa]
                                         60             d

                                         50 as m
                                                 an
                                                    ufa
                                                       ctu
                                                           red
                                         40


                                         30


                                         20 0               2               4              6        8
                                          10           10              10             10       10
                                                                Time-to-failure [s]



The model predicts the influence of physical ageing on the defor-
mation kinetics accurately. When does the pipe becomes brittle?



                                                                                                        13/20
How to determine the failure criterion




                                         14/20
                        Influence of ageing on the ductile-to-brittle transition


                                       As manufactured                                                                                 Aged
                        100                                                                                    100
                              Predominantly ductile                                                                  Predominantly ductile

                         80                                                                                     80
Ductile fractures [%]




                                                                                       Ductile fractures [%]
                         60                                                                                     60


                         40                                                                                     40


                         20                                                                                     20

                                                          Predominantly brittle                                                                  Predominantly brittle
                         0                                                                                      0
                         -10           -5             0             5             10                            -10           -5             0             5             10
                                            Temperature [°C]                                                                       Temperature [°C]



                        The transition temperature increases as a result of physical
                        ageing.



                                                                                                                                                                    15/20
 Ageing kinetics of transition temperature


                                                                8




                                 Transition temperature [ °C]
                                                                6

                                                                4

                                                                2

                                                                0

                                                                -2                               As received

                                                                -4 0            1            2                  3
                                                                 10        10           10                 10
                                                                       Ageing time @ 10°C [year]



 The prediction is based upon a critical yield stress11, using the
 model for deformation kinetics.

11Engels   et al. (2008) Polymer, submitted
                                                                                                                    16/20
Non-destructive measurement of yield stress

• Non-destructive and yielding are conflicting.
• Solution: minimise the yield zone using micro-indentation.
• Result: indentation with a mark of about 0.05 mm deep.




                  Spherical diamond tip
                                    r=200 μm




               uPVC



                                                               17/20
 Micro-indentation measurements on uPVC


                                                210
                                                               Ta= 60°C
                                                               Ta= 40°C
                                                200            Ta= 23°C

                               Hardness [MPa]
                                                190


                                                180


                                                170        0           2        4        6        8        10
                                                      10          10       10       10       10       10
                                                                      Ageing time [s]
                                                                  Effective ageing time [s]



 The measured hardness data shifts with the same activation
 energy as the yield stress. The yield stress and thus the
 thermodynamic state can be determined with micro-indentation13.
13Breemen   et al. (2009) Phil. Mag., 89(677 – 696)
                                                                                                                18/20
 Determining the residual lifetime in practice


                                                                                                                                        8




                                                                                                         Transition temperature [ °C]
                                                                                                                                        6

                                                                                                                                        4

                                                                                                                                        2

                                                                                                                                        0

                                                                                                                                        -2                               As received

                                                                                                                                        -4 0            1            2                  3
                                                                                                                                         10        10           10                 10
                                                                                                                                               Ageing time @ 10°C [year]




            Micro-                                                                                                                              Fracture                                                    Residual
Robot                                          Yield stress
          indentation                                                                                                                          behaviour                                                    lifetime


                                         210
                                                        Ta= 60°C
                                                        Ta= 40°C
                                         200            Ta= 23°C
                        Hardness [MPa]




                                                                                                                                                                                            Condition
                                         190


                                         180
                                                                                                                                                                                                        Criterion

                                         170        0           2        4        6        8        10
                                               10          10       10       10       10       10
                                                                                                                                                                                                                    Time   Lifetime 1 Lifetime 2
                                                           Effective ageing time [s]




                                                                                                                                                                                                                      19/20
Acknowledgements

The financial support of the following Dutch gas network operators
                   is gratefully acknowledged:




Gratitude goes out to Tom Engels and Laurent Warnet for the help
           on the tensile and impact tests, respectively.




                                                               20/20

								
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