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7LS-DYNA Cargo Net Simulation

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.

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7LS-DYNA Cargo Net Simulation

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.

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7LS-DYNA Cargo Net Simulation

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.

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7LS-DYNA Cargo Net Simulation

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.).

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7LS-DYNA Cargo Net Simulation

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.

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7LS-DYNA Cargo Net Simulation

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.

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7LS-DYNA Cargo Net Simulation

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.

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7LS-DYNA Cargo Net Simulation

Side view of how the cargo pushes against the straps.

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7LS-DYNA Cargo Net Simulation

Dynamic behavior of the cargo as it pushes against the straps under the transient acceleration pulse.

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7LS-DYNA Cargo Net Simulation

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

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7LS-DYNA Cargo Net Simulation

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 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)