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Is my Model is safe or Not....?

Dear all,

I have created a model using Solid edge ST7 and I done FEA through Simulaton Tool .At one Particular Point the Factor of Safety is Showing Below 1.But All Other Portion of the Model is having Factor of Safety Greater than 2 ....What Is the Problem In my Model ........Whether it is correct are not...For your Refernce I have Attached The Screen Shot of my Model...Any one please help......if you need Any Further Details Please feel free to Contact me @dineshkumar.karthikeyan@petronash.com Phone:+91 8220873367

 

 

Regard 

DineshKarthi

Designer

7 REPLIES

Re: Is my Model is safe or Not....?

Factor of safety below 1 means that at your stated load, the part is going to fail. The high stress is due to a combination of too little material (or low material properties) and a load that's too high. The easy thing to do might be to add material on the outside of the part where it is showing the high stress.

 

Part of simulation is being able to interpret the results.

Retired Community Manager for Solid Edge. This account is no longer active.

Re: Is my Model is safe or Not....?

Using FEA software is not fool proof.  You still need to understand the FEA method so that you can make an educated guess as to whether the software got accurate results, as simply having too course of a mesh or using the wrong type of contraint and/or load can make your results invalid.


Thanks,
Ken

Production: ST9 MP7
Testing: ST10

Re: Is my Model is safe or Not....?

Don't forget to do some hand calculations also, so that you can see if the stresses and strains are anywhere near what they should be.

Re: Is my Model is safe or Not....?

[ Edited ]

in general......NO, not safe. A structural PE will want 3:1 SF or your chart maxing at 0.33

 

My first question would be, can a person be hurt if the part fails? if the answer is yes, I would consider 0.33 max. if the answer is no, then 0.5 would be ok if there are no fatuge or tempurature issues.

 

Factors of safety for FEA.

 

I have debated this topic working under many PE's (I'm not a PE, but do have a BSME). previous posts are correct. FEA does not constitute code compliance. However, if you can confirm the boundary condition vs. test data in some fashion, then the following logic is as good as it gets in situation where hand calculations are not practical.

 

Most codes and FEA can be related through the stress level of the materials.

 

Let's say you are working with A36, that also happens to have the capacity for 36KSI. If the situation is tension, you can go to 36KSI for 1 second of the materials life. As soon as you consider cycles, fatigue, abuse, fit up and welding , most PE's will not accept a safety factor of less than 3:1.

 

Also, having worked with AISC and ASME extensively. Any steel stress over 12KSI is a potential problem. Anything over 18KSI (or 2:1 SF) is BAD, and that is only for very predictable and low cycle situations.

 

The largest loads most steel will see is the moment is it put into place assuming there are not extreme temperatures (outside -20 to 350 F) involved.

 

Playing devils advocate (like your shop guys will do) Reality is that shop people know exactly where things fail, and they tend to work with SF of under one, because they know they can get away with it for a one day set up to get things in place. This is very dangerous, but is actually done every day.

 

If you have any legal liability or risk to human life involved with a product, 12KSI stress should be the limit for mild steel.

 

If you are working with columns, things get complex. When stuff buckles is very difficult to be accurate with.

 

Another issue to bring up is deflection. If you are getting any deflections over 1/16" for any single part, people standing on stuff can feel very uncomfortable.

 

This is all assuming a scale of 1' to 40' cube in object size.

 

Most people at this point will ask. Why is the steel reated at 36KSI if you want to design for 12KSI. The answer is many fold: What if you need to calculated bending forces. What if you need a part to fail at a specific load, there are 50 other questions that need to be answered without a factor of safty.

 

 

 

Re: Is my Model is safe or Not....?

Even though i added material it showing the same issue .....can u solve it if i give the Part file in which i am facing the issue ........if u need the part file please reply with ur email id or Contact no

Re: Is my Model is safe or Not....?

I have done my best to explain the basic of stress analysis using FEA. Learning to correctly apply boundary condition is a steep learning curve that most PE's will argu and contest with until that user has 1000(s) of hours of experience in FEA that I do not have. I just happen to know a bit about code compliance.

 

I'm using the fundimental version of SE and don't use it's FEA.

Re: Is my Model is safe or Not....?

FEA is one of the most beautiful programs but on the other side can be the real PITA many many times.

I did start to use it around the 1987-1988, as Skyjack Work Platforms chief engineer and later very often since 1992 for my own company.

Just because the portion of the model has stress over the yield point does not neccesary mean catastrophic failure.

There is such a thing as the plastic design ( components of the structure under the load may go plastic, but the structure is still holding due to its design). Structure can be designed with the stress somewhere higher than yield point. Still remembering the professor at my university around 1976 talking about it.

The best sample is the simple steel nail. When you hammer it to the piece of wood it will definitely go plastic due to its contact with the hammer. Nail head will show pernament plastic deformation. Nail will go thru the wood, doing its job framing the house, but will it shatter itself?

 

It will not!

 

One has to be very carefull. 

 

Milan Wendl, P. Eng, MEngSc

www.aaaengineering.com

ST7, SW2015 Platinum

 

 

Milan Wendl, MEngSc, P.Eng.
AAA Engineering, Canada
ST10, SolidWorks Premium 2017