Inventor stress analysis: low minimum safety factor

[Anonymous]

Hello all,

I would request for guidance and suggestion on the following stress analysis case. Please bear with me I am only sharing the images of the simulation. I will try to be precise as possible. The issue here is that the minimum safety factor is low and or decreases as I do any refinements.

Case: I am trying to simulate stress analysis case in Inventor 2014 for the shown model(Fig. 1 and Fig 2 is the actual/real case-attachment). There is a belt(not included/unnecessary for the simulation here) that is lifting at the two shackles on either side. The shackle is connected to the bolts, which is connected to the support ring. This support ring is bolted/fixed to the lifting body.

Setup: I have defined a fixed constraint on the face (lifting body) as shown in the fig.3. The loads are acting on either side of the shackles(i.e.,belt lifting) with magnitude of 177 lbforce each side as shown by the display(yellow arrows) fig 4. The material defined is steel. The contacts are bonded ( chose automatic, default). Mesh settings default.Post simulation, the results are shown in figure 5 and 6 displaying min. safety factor and Von Mises stress.

Goal: To achieve min. safety factor of 1 or more.  

Real case: This part can lift good weight of more than 500 lbs. Also the support ring will produce a twisting force upon load. Now I do not know if Inventor can simulate such scenario (I am newbie to the software). I did run few cases with sliding/No Separation contacts for the bolt and the ring support but was not lucky. The min. SF kept decreasing. Tried h-refinements- no luck. Seems like a simple study but I am missing may be on the contacts or constraints if that is making all the difference. 

Any suggestions or advice would be of great help and much appreciated. Feel free to discuss.

[JDMather]

1. In your own words - what does Safety Factor mean to you and why do you want it greater than 1?

2. You could do this analysis with only the bracket - you do not need the assembly.

[Anonymous]

Many thanks for your prompt response.

1. For me, it means how much stronger the model is than it usually needs to be. Since the model has the capacity to lift the given load without any problem I am wondering why it is predicting lower SF. What do you think?

2. You mean just the bracket alone (no shackle and bolts) and apply the same conditions. Please elaborate. 

[JDMather]

SF=Sy/Smax

(calculated Smax)

S=F/A

You have a cylinder contacting whatever edge, therefore A of contact is approaching zero which means S is approaching infinity.

The material will yield along this contact area until it work-hardens and/or the area increases enough to match the Sy.

Inventor only calculates the deformation - it does not actually deform your part.

All of this is within the linear elastic range.

Your part can still deform well into the plastic deformation before failure.

Safety factor is probably a misleading term, but I have not seen anyone propose a better term.

You only need the bracket to do the analysis.  Everything else is much stronger than the bracket.

[Anonymous]

Yes you are right. I speculated same about stress reaching higher values at the edge. That is the reason stress was shooting up and I was not obtaining the desired values. 

For your info, I have attached bracket only analysis images. Hope I have simulated it right. May be the bracket does yield or deform like you mentioned. 

[JDMather]

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@Anonymous wrote:

For your info, I have attached bracket only analysis images. ..... 

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I am not familiar with doing analysis from images.

Also, keep in mind that your bracket appears to be a bent sheet metal part which means that it was bent well beyond the linear elastic limit and the bend zone is work hardened.  (unless it was heat treated after bending the material properties are not isotropic)

[Anonymous · Accepted solution]

I will consider that. Thank you JDMather for your time and thoughts.