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Simulation results seem unrealistic compared to real test

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Simulation results seem unrealistic compared to real test

Using a mechanical testing machine I have determined that the bending strength of a hollow circular shaft with brittle material to be about 18 KN at the height of 1.47m. The force was applied perpendicular to the shaft axis at the top end. The attached image shows the simulation setup similar to the real test.

ID and OD of the cross section of the shaft are 0.13m and 0.18 respectively. The breaking stress due to bending has been calculated (momentum/section modulus) as 63 MPa at the critical section where it gets constraint . I have been using this value to compare the results from simulations.

 

Simulating this bending test in Creo, I find quite different results:

 

1- Using solid body mesh, the stress is about (Max Principal) 150 MPa at the critical section

2- Simulation Live in Creo Parametric shows Max Principal to be 82 MPa

2- Using mid-surface shell, the max principal is about 108 MPa at the critical section

3- When using shell it is possible to exclude membrane and shear, so the stress due to bending becomes 54 MPa

 

Here are my questions:

 

1- Why should we see such high gaps among results from real test, simulkation live, and simulations using shells and or solids?

 

2- Why the membrane stress is so high (about 57 MPa) in this setting when using shells? (please note the shear stress was quite small and could be neglected and also the mass of the component is 95 Kg) 

 

3- We use the component in a structure that should be tested against shock after finding mode shapes. Surprisingly, when using shell elements it is not possible to exclude membrane after performing the shock study. Is this a software limitation?

A screenshot of the results configuration window for a shock study is attached.

1.jpg2.jpg

 

28 REPLIES 28

Re: Simulation results seem unrealistic compared to real test

pdf att.

Re: Simulation results seem unrealistic compared to real test

plot2.JPG

Re: Simulation results seem unrealistic compared to real test

example att.

mesh.JPG

Re: Simulation results seem unrealistic compared to real test

Thank you for so much for your comments and providing the example model. I reviewed the model and run it; the result is quite close. I have tried mapped meshing on the entire shaft (not mirrored) but again no success in getting close to the expected result. I am reluctant to use mirror symmetry constraint because the shaft will be used in a larger non-symmetrical assembly where forces from all directions will be acting on the components (a shock study with accelerations on x, y, and z axes, and a static study with forces on x, y, and z directions). 

 

More info: the larger assembly is a vertical structure that includes solid components on both ends of the shaft. The shaft is located in the middle of the structure acting as a pillar. I use structural bolts to mount the bottom component to a plate (acting as ground). 

 

 

 

Re: Simulation results seem unrealistic compared to real test

"... I use structural bolts to mount the bottom component to a plate (acting as ground). ..." not relevant

 

dyn.JPG

please observe measures!

3D-example att.

Re: Simulation results seem unrealistic compared to real test

Thank you again for your reply. Could you please take a look at the attached assembly and help me find the reason(s) why I couldn't achieve 63 MPa at the critical section?

 

I know it creates singularities at the edge and I have tried different tricks such as creating volume and surface regions, etc. but no luck. Only creating top surface shell gave me 63 Mpa, but I wouldn't trust it. 

 

Based on some instructions, I usually use a plate at the bottom of the assembly and constraint the bottom surface of the plate in order to search for resonant frequencies and subsequently perform a shock study.  Using "Spring to ground" feature in the model you provided was very interesting. I haven't seen any useful reads about the feature on PTC web support. Are you aware of any good online resources about this feature?  

 

 

 

 

 

Re: Simulation results seem unrealistic compared to real test

sorry, you use creo 6,
my release is creo 2 m080.

Re: Simulation results seem unrealistic compared to real test

I see. I have enclosed the STEP files. 

 

Re: Simulation results seem unrealistic compared to real test

what is the middle part?

 

middle_part.JPG