Force of impact

[JohnBeach]

I have a stiff, protective, beam element type, structure that stands above equipment.

There is the possibility that a 200 kg (600 x 600 x 70 mm^3) steel plate could fall from up to 2 m onto this structure.

I do not want to protect the steel plate i.e. no energy absorption is required.

I simply want to know whether the steel structure is sufficient to deflect the steel plate and not crush or buckle. I can reinforce the structure as much as I like to make it extremely stiff; but if I do this, how can the steel plate potential energy be absorbed if there is no deflection of my structure?

Also, I can reduce the height of drop of the steel plate; but what drop height will be low enough?

What is the force transfer if a relative small object bounces off a relative massive one? I consulted a physics text regarding impulse and when it comes to quantifying a force they simply assume a small time of impact and a resulting large force.

It would be great if MES were capable of calculating all deflections throughout the structure, relating to the local forces and thus determine the energy absorbed by the structure. Apparently it does not do this.

The best idea I can come up with is to …

determine the relationship between stress and deflection of my structure. Determine the maximum deflection at which my structure goes plastic (to keep it simple). Multiply this deflection distance by the force I calculate required to cause this deflection. Compare this energy with the potential energy of the falling steel plate. Then, keep fiddling with the structure until the deflection and force to maximum stress.

Any ideas/suggestions would be much appreciated.

[John_Holtz]

Hi John,

Out of curiousity, what is the steel structure connected to? Something infinitely stiff? Something "fragile" that cannot take the force? As you alluded to, a stiff structure will simply transmit the total impact force to the supports, so they may need to be part of the design, too.

Also, can you clarify what aspect of the analysis cannot be handled by MES (Mechanical Event Simulation)?

[JohnBeach]

Hi,

The structure is connected to concrete foundations via bolted connections i.e. relatively stiff.

The aspect that I think cannot be cannot be handled by MES is to determine the energy absorbed by the structure based on the deflection of each member in the structure. Perhaps, I could apply a deflection, MES will give me the associated force, and it would be reasonable to calculated the energy absorbed via (max force x distance) / 2.

Thanks for your response.

John

[JohnBeach]

Does anyone have suggestions, please?

[John_Holtz]

Hi John,

If you really need the energy, such as required by some specification/customer or to report its load carrying capability, then you could do the following. (I am doing this off of the top of my head, so some of the steps may not be exactly correct, but this should be close.)

• Suspend the plate above the structure by using prescribe displacements.
• Turn gravity on and let the model come to equilibrium.
• Remove the prescribe displacements by using the death time (active range) capability. That is, drop the plate.
• Let surface to surface contact determine what happens during the impact.
• Display the displacement results.
• Select all of the nodes on the beams and inquire on the results.
• Save the results to a CSV file.
• Display the axial force in the elements.
• Save the results to a CSV file.
• Open both CSV files in Excel. If I remember correctly, the displacement file gives the original coordinates and the displacements. Thus, you can calculate the original length and the deformed length.
• Calculate the energy by your formula (0.5*force*displacement) for each element.
• Sum the energies for all of the elements.
• You may need additional formulas for the energy absorbed by bending and shear, but I do not know for certain.

If you just need to know that the structure will not buckle or crush, then do the above steps but do not calculate the energy ;-).