3D FINITE ELEMENT SIMULATION OF THE ELECTROFUSION PROCESS OF POLYETHYLENE FITTINGS

Transcription

1 GERG Academic Network Meeting Brussels, June 14th 15th 3D FINITE ELEMENT SIMULATION OF THE ELECTROFUSION PROCESS OF POLYETHYLENE FITTINGS Chebbo Ziad Industrial supervisor Dominique GUEUGNAUT Adil BOUJLAL Academic supervisor Michel VINCENT Yannick TILLIER

2 Context GDF SUEZ operates more than 137,000 Km of Polyethylene (PE) gas mains and more than 6 millions services Polyethylene pipes have the advantage of being assembled by fusion techniques (Electrofusion, butt welding) The Electrofusion is the main used technique Recently: identified malfunctions, resulting in poor quality welds Necessity for a better understanding of the thermomechanical mechanisms during the EF process in order to optimize the welding conditions What is Electrofusion? 2

3 Electrofusion process Junction of a pipe with an Electrofusion fitting Four important steps: scraping, cleaning, positioning, assembly Fittings are made of PE and contain a metal coil Metal coil provides required heating energy for melting through Joule effect 3

4 Electrofusion process Different phases exists during the EF process Heating Melting Cooling Temperature C Time (s)

5 Objectives and strategy Welding parameters Fitting Design: coil winding, thickness Material characteristics: PE, metal of coil Electric parameters: supplied power Welding conditions: heating time, initial gap Experimental & Numerical output Temperature Interdiffusion Melted area Displacement Pressure 5

6 Outlines Numerical modeling (Finite Elements) New EF numerical and experimental model Numerical results Current work Conclusion and prospects 6

7 Numerical modeling Thermal Heat transfer equation. C p. dt dt dx c H K. T dt FE Mechanical Physicochemical Elasto-visco-plastic Fusion and crystallization kinetics. (DSC analysis) 1 n K Ozawa( T) X c( t, T) 1exp. t T 1 n K Ozawa ( T ) A 1 n T n exp( AT B ) n Interdiffusion A 2 t 2 l exp k. D0 t1 # E RT. dt Interdiffusion law: based on the molecular reptation theory of De Gennes et al.(1971) Saint-Royre et al. (1989): interpretation of the welding mechanisms based on the theory of self-diffusion 7

8 Numerical modeling Interdiffusion law A 2 t 2 l exp k. D0 t1 # E RT. dt 2 l t k D E R 0 # average depth of interdiffusion time of interdiffusion proportionality coefficient self diffusion coefficient of material activation energy of diffusion perfect gas constant Good welding occurs when A exceeds a certain value which depends on the materials to be welded together 8 Finite Element simulation Based on Forge, a finite element software initially developed for metal forming Parallel and sequential computing Application to : Simplified Electrofusion geometry Real Electrofusion fitting

9 Outlines Numerical modeling (Finite Elements) New EF numerical and experimental model Numerical results Current work Conclusion and prospects 9

10 New EF numerical and experimental model Parametric model Hot-plate Heating wire Bottom plate Symmetry plane l b b c b e l Ф 0.52 b Symmetry plane 10 L 0.5

11 PE New EF numerical and experimental model Modeling and Mesh The geometry is modeled with Gmsh A conform mesh at the interfaces (Hot-plate/Bottom-plate, wire/hot-plate) Wire: refined anisotropic mesh (stretched elements) (270,000 elements) Hot-plate: refined anisotropic mesh (stretched elements) (1.4 million elements) Wire 11

12 New EF numerical and experimental model Material properties Hot-plate and bottom plate : PE Wire: Copper alloy Thermal conductivity: 0.34 W/m.K Specific Heat = 435 J/Kg.K Density and specific heat (thermal dependence) Density = 8100 Kg/m 3 (960 kg/m 3 & 1920 J/Kg.K at 23 C) Conductivity = 109 W/m.K Boundary conditions Input data The welding machine records intensity and imposed voltage One can deduce the dissipated power in the wire Thermal resistance contact Hot-plate/Wire (between and m²k/w) -4 Hot-plate/Bottom-plate (about m²k/w) h=10 W/m 2.K, T=23 C Q=0 S2 S1 S1 Hot-plate Bottom-plate 12 h=10 W/m 2.K, T=23 C h=10 W/m 2.K, T=23 C

13 Outlines Numerical modeling (Finite Elements) New EF numerical and experimental model Numerical results Current work Conclusion and prospects 13

14 Numerical results Temperature evolution Input power 280W, heating time 50s, initial temperature 23 C Upper face Temperature ( C) Near the wire 1-Near the wire 2-Middle of the interface 3-upper face Time (s) 3 Sensors

15 Temperature evolution Numerical results Lower temperature s Temperature ( C) s Middle 2-Corner Time(s)

16 Numerical results Melted area Computed melted area during the EF process (Xc) 20 s 30 s 40 s 50s (end of the heating) s 500 s

17 Numerical results Interdiffusion Computed interdiffusion parameter at the interface 50 s end of heating 60 s 250 Temperature ( C) s 500s 17 0 Middle of the interface Time (s)

18 Numerical results Interdiffusion Computed interdiffusion parameter at the interface symmetry Welded area Heating time 63 s 56% of the total surface Density of welded area % W 280 W Heating time (s) After 90 sec, the density of the welded area becomes almost the same for 240 W and 280 W 18

19 Outlines Numerical modeling (Finite Elements) New EF numerical and experimental model Numerical results Current work Conclusion and prospects 19

20 Current work The study is currently completed with an experimental work in order to validate the numerical model Injection of the insert groove machining wire introduction hot-plate over injection Welded area Temperature measurements T-Peel tests on welded plates (micro-macro samples) microscopic evaluation of the surfaces A thermo-mechanical model is developed and coupled with the physicochemical model 20 Bottom plate

21 Conclusion and prospects This work will be extended and applied to real fittings Welded area 2.3 millions elements The comparison with experimental data will allow to validate the numerical model The optimization of the geometrical parameters is permitted due to the parameterized model The optimal welding conditions will be identified by inverse analysis, taking into account the different material characteristics Further work will aim at defining a methodology to assess for weld quality for a wide range of fittings 21

22 Thank You For Your Attention! 22 For further information: