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PowerPoint Presentation - Predictive Engineering


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									August 2007   LS-DYNA Cargo Net Simulation                                                                                       1 of 10

                                    Industry standard cargo nets. The nets are made from 1.75” wide nylon or polyester webbing with
                                    nominal thicknesses of 0.062”. The webbing is connected together via steel rings. The net is
                                    attached to the aluminum pallet via steel hooks. Cargo weights can reach 10,000 lbfs.
August 2007   LS-DYNA Cargo Net Simulation                                                                                         2 of 10

                                    The cargo net model was built completely with plate elements. A quarter-symmetry section was
                                    created using the surfacing capabilities within Femap V9.3.1. These parasolid surfaces were then
                                    sized to create a 100% cuad mesh and reflected to create the complete FEA model.
August 2007   LS-DYNA Cargo Net Simulation                                                                                           3 of 10

                               Wave speed (explicit time step) was calculated using a custom API program created by Predictive
                               Engineering. Anyone who has used LS-DYNA will appreciate how useful this capability is as one is sizing
                               your FEA model for an explicit analysis. The minimum LS-DYNA time step is 1.43 s for this model.
August 2007   LS-DYNA Cargo Net Simulation                                                                                            4 of 10

                               The meshed model was exported from Femap and directly imported to LSTC’s LS-PREPOST v2.2. At this
                               stage, contact definitions were defined, mass scaling enabled and all of the other analysis controls
                               (stiffness damping, D3Plot export controls, etc.).
August 2007   LS-DYNA Cargo Net Simulation                                                                                             5 of 10

                              The load case for the cargo net simulation is a transient acceleration pulse. The graphic on the upper left-
                              hand corner shows the stresses for the main side straps during the impact event.
August 2007   LS-DYNA Cargo Net Simulation                                                                                             6 of 10

                              Another version of a cargo net is shown above. This type of net is attached to a plastic pallet and the straps
                              are wrapped around steel tubes at the base of the pallet.
August 2007   LS-DYNA Cargo Net Simulation                                                                                             7 of 10

                              The velocity profile of the cargo is shown in the upper left-hand corner. The cargo reaches it peak velocity
                              at about 0.2 sec and then reverses direction after 0.25 seconds. Below this chart is the displacement profile.
                              The cargo moves a peak displacement of about 35 inches. The data point locations for these curves is
                              shown on the cargo model graphic in the lower right-hand corner.
August 2007   LS-DYNA Cargo Net Simulation                                            8 of 10

                              Side view of how the cargo pushes against the straps.
August 2007   LS-DYNA Cargo Net Simulation                                                                                            9 of 10

                              Dynamic behavior of the cargo as it pushes against the straps under the transient acceleration pulse.
August 2007   LS-DYNA Cargo Net Simulation                                                                                            10 of 10

                              Sometimes….the straps just break. LS-DYNA fully captures the failure response of the nylon straps as their
                              breaking strength is exceeded.
                 LS-DYNA Cargo Net Simulation                                                                    11 of 10

              LS-DYNA Cargo Net Simulation
              Cargo nets are rather mundane industrial things. They are manufactured from simple nylon or
              polyester webbing with steel rings and hooks sewn into attachment points. We see cargo nets
              in the back of pickup trips, thrown over cargo on large trucks and, if we are passing by a
              military airport where cargo is being transported, cargo nets are ubiquitous. In the military
              world, aircraft often carry both cargo and personnel and the last thing that one wants is for
              the cargo to go flying around during a severe landing event (a.k.a., crash).
              Thus, we have a crash test requirement that the specified cargo net can withstand a 8g
              deceleration with a 10,000 lbf cargo load. Experimentally, a go / no go result can be
              obtained by impacting the cargo net system at a fixed velocity against a rigid wall (think
              crash testing of automobiles). The downside to this approach is the lack of good engineering
              information about the mechanical behavior of the straps.
              To design the “better cargo net” – the cargo net system was turned into a complete finite
              element analysis simulation. This project turned into an insanely difficult challenge to capture
              the contact behavior between the nylon straps, steel rings and hooks and the semi-
              elastic/plastic cargo boxes. Nylon straps are basically membrane type structures but they do
              have some bending strength. Getting this behavior modeling correctly required the use of an
              overlay technique within LS-DYNA of membrane plates on one layer and then another layer of
              fully integrated plates meshed over the same nodal locations.
              Analysis results were compared against experimental data and…almost somewhat
              unbelievably…good correlation was found. In fact, it was determined that the experimental
              test was being performed under some false assumptions. As I am fond of saying…the model is
August 2007

              always right it is just a matter of understanding of what the model is telling you!

              Software Tools: Femap V9.3.1 and LS-DYNA (ls971_s_7600.1116_winx64_p)

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