Revised IF container passes FEA drop test

IDAC has confirmed that the design of a revised IF container, designed and manufactured by Nuvia, is safe.

IDAC subjected the IF container to a drop test using finite element analysis (FEA) and studied the structural responses.

The analysis had to be set up to simulate a fully loaded IF container being dropped in an upright position from a height of 6.5m.

The 3D model provided to IDAC in Autodesk Inventor format consists of a lid, a seal plug, a quick-release coupler, a top section, a mid section and a base.

Ansys Designmodeler was used on the Inventor geometry for further defeaturing and preparation of an FEA.

Earlier tests had shown that the deformation and stresses in the region of interest (around the base) were highly symmetrical about the axis of the container.

As a result, the smallest symmetric sector of 30 degrees was used as the base geometry in the modelling.

For continuity of the structure, three representative weld geometries were also added to connect the seal plug to the lid, the top section to the mid section and the mid section to the base.

With the model finalised within Ansys Designmodeler, it was then imported into Ansys LS-DYNA for the transient dynamic analysis.

All the parts/volumes within the container were 'glued' together so that adjacent volumes shared common nodes at the boundaries and no bonded contact element was needed across the interfaces.

The 3D solids were meshed using 3D explicit solid elements within Ansys LS-DYNA.

Regular, sweepable volumes were meshed using predominantly hexahedral and wedge elements (eight-noded Solid164).

Irregular volumes such as the seal plug and the lid were meshed using tetrahedral elements.

For the drop test simulation, a bilinear elastic-plastic material model and a de-rated Young Modulus were used for the IF container.

In addition, all materials were assumed to have a stiffness (Beta) damping coefficient of 0.2.

The advantage of using this type of damping is that it cancels out oscillatory motion at high frequencies, such as ringing.

At the start of the simulation, the bottom surfaces of the fully loaded IF container were placed 1mm above the rigid floor element.

A drop velocity of 11,292mm/s was specified, which was calculated using a drop distance of 6,499mm with a zero initial velocity.

With the existing mesh and standard gravity, an appropriate time step of 90ns was used to resolve the shock waves propagating through the structure.

The event was simulated up to a total time of 0.025 seconds, starting at this drop position.

In determining the validity of the design, equivalent (von Mises) stress was calculated as this provided an indication of how close the material within the structure was to the yield limit.

The IF container design was revised with an increased wall thickness at the container base.

The resultant contour plots show the deflection during the transient run.

While the walls do undergo deflection under the drop test, the analysis shows that the new design still fairs better than its predecessor.

Following the accidental drop event, the nominal plastic strain in the lower wall section was less than eight per cent.

The peak plastic strain in the base - where it was hit by the payload - was 14 per cent, which was still well within the assumed breaking strain of 26 per cent at UTS.

The thickened wall was only deforming with a maximum value of 1.24mm outwards in the most critical location during impact.

The equivalent stress at the end of the simulation was also found to be within the operable design limits of the assembly.

The aim of the analysis was to determine whether an accidental drop of the assembly would cause a loss of containment of the payload from the system.

According to the results, there would be no leakage in a real-world situation similar to the boundary conditions.

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