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ENERGY TECHNOLOGIES | NONLINEAR STRUCTURAL MECHANICS

FIGURE 2. A simulation showing the total displacement (mm) of a pipe squeeze-off (left) and the
full simulation of the squeeze-off process (right).

   Funded by Operations Technology        allowed the team to accurately analyze                 FIGURE 3. Deformation at the line of
Development (OTD), a technology           the large deformations in the pipe. The                squeeze-off at different time steps, just before
development partnership of natural gas    simulation accounted for the different                 and at maximum squeeze displacement.
distribution companies, GTI researchers   stages of the squeeze-off procedure
Oren Lever and Ernest Lever took a        shown in Figure 2: pressurization of the               material parameters to the stress-strain
closer look to see if the twelve-inch     pipe, squeeze-off, hold, release,                      response of PE, and nally implement in
distance is really necessary for smaller  and relaxation.                                        COMSOL the set of ordinary differential
pipes. Their goal was to see how close                                                           equations (ODEs) needed for the custom
the squeeze-off of the pipe could get        Special attention had to be given                   material model.
to a fused joint before it exceeded       to the meshing of the pipe under the
industry-accepted levels of strain        squeeze bars to enable the simulation                     The material tests have been
and increases in stress concentration.    to analyze the very large deformations                 carried out by GTI on medium-density
To get answers, the team turned to        encountered when the pipe is fully                     polyethylene (MDPE) pipe materials
the structural mechanics modeling         squeezed off (see Figure 3). Thanks to                 and included tension and compression
capabilities of the                       the meshing capabilities in COMSOL                     tests at different temperatures, strain
COMSOL Multiphysics® software.            and its parametric nature this particular              rates, and strains, especially high strains,
                                          meshing was easily scaled to different                 as well as loading and unloading tests
ð STRESS AND STRAIN                       pipe sizes. While the stresses and strains             (see Figure 4). The team at Veryst found
ANALYSIS OF THE                           in this region were not the focus of this              actual values of material parameters
POLYETHYLENE PIPES                        project, they are of interest regarding                that t the experimental data. To
                                          the general effect of squeeze-off on the               calibrate, Veryst used MCalibration, an
The GTI team set up a fully parametric    lifetime of the pipe. The COMSOL model
time-dependent model using the            will allow the GTI team to conduct
Structural Mechanics Module and the       further squeeze-off investigations.
Nonlinear Structural Materials Module
in COMSOL. As Oren Lever of GTI says,     ð HIGHLY NONLINEAR
their team "trusts the nite element       MATERIALS CALL FOR
method implementation in COMSOL®          COLLABORATION
software for getting good results as
proven by previous testing." They         To capture the unique behavior of
de ned the mechanical and numerical       polyethylene, GTI needed a custom
properties of two sets of contacts to     viscoelastic-plastic constitutive
model the internal pipe-to-pipe and the   model. For this, they turned to Veryst
external pipe-to-squeeze-off-mechanism    Engineering, a COMSOL Certi ed
structural contact, respectively.         Consultant, for help in implementing
                                          the chosen material model in
   They used the meshing capabilities of  COMSOL Multiphysics. To do this,
the COMSOL software to create a hybrid    explained Nagi Elabbasi from Veryst,
mesh by integrating structured and        they rst selected the experimental
unstructured meshes. This approach,       material tests needed to calibrate
together with a custom constitutive       the material law typically used for
model implemented in the software,        thermoplastics such as PE, then t the

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