Dynamic Analysis to determine Impact Forces?

[Kisskha]

Hello,

I wish to reach out to know if a dynamic analysis, as described below, is within the capabilities of PreProMax? I have viewed online tutorials and could not find an analogue. I have past FEA experience but only with static analysis.

In the fist accompanying picture “Complete Assembly” the item in black is a hydraulically actuated cylinder with an internal spring return, these items have failed in the past with the cast steel cap being ejected along with the spring. The cap is shown alone, in the image “Body Hidden, Cap Visible”; a safety guard prototype has been designed, the threaded rod and stainless steel plate(s) that sandwich the guard. The combined spring and hydraulic pressure generate a maximum force of 7000N, the weight of the cap with spring (the projectile) is 0.717 kg.

Static analysis on the components is satisfactory, with a minimum factor of safety of 2.7 ( yield) and prototype testing has been completed with no visible deflections anywhere at 1.25x max force.

The matter at hand is that it is desired to have a 5mm gap between the guard and the cap, to enable quick visual detection of a failed condition, for removal from service. Hand calculations have been conducted, in the most basic form F*d = ½ m v^2, where F is 7000N, d is 5mm, m is 0.717kg, resulting in a velocity at impact of 9.88 m/s.

The issue is the deceleration time or collision distance to calculate impact forces. Assuming deceleration times to calculate force, with arbitrary values of 0.1s and 0.05s gives very low force values, 141N and 283 N respectively. Attempting to model deflection in the total system, from the static model at 7000N, using the threaded rods and maximum deflection in the guard components summed to derive a deceleration distance however yields very low displacement numbers; the threaded rods deflect only 0.0923mm under load (5/16-18 B7), maximum deflection in the guard closest to the cap is only 0.05mm. With a total deflection of 0.18mm entered as a collision distance the forces become enormous: 388,831 N.

Impact Energy Calculator | Impact Force

I was hoping that FEA software could better model the actual interaction between the components. Please let me know if you have any thoughts or suggestions. Otherwise I intend to have no gap and eliminate the dynamic analysis.

Thank You,
Frank

[FEAnalyst]

PrePoMax supports CalculiX’s dynamic analyses - implicit or explicit dynamic step. The latter is normally better for impact simulations but it can be difficult to make it work properly because of its not-so-good implementation in CalculiX. You can try both step types though, possibly using a simple model first. Of course, you will have to define contact between the parts that are supposed to collide.

PrePoMax (CalculiX FEA) - Tutorial 38 - Explicit dynamics - bar impact

OpenRadioss is much better for explicit dynamics (simulations involving impact/crash) but it doesn’t have easy to use preprocessor.

[Paul18fr]

@FEAnalyst

OpenRadioss is much better for explicit dynamics (simulations involving impact/crash) but it doesn’t have easy to use preprocessor.

I’ve few experience on the topic, but Open Radioss can read Ls-Dyna ascii input file which can be made using [Ls-prepost](https://lsdyna.ansys.com/knowledge-base/ls-prepost/ which is free of charge (for now). For the results, they can be exported to Paraview format.

There is also the “old” way using the .rad ascii file ; for the mesh, Radioss team developped an export format from gmsh …

(for your information)

[Matej]

@Paul18fr

Ls-Dyna ascii input file which can be made using [Ls-prepost]

I think I found somewhere that it is free for non-commercial use only, but I might be wrong.