What really counts in judging component failure.( Max. Prince. Stress, von mieses.) 1

[Timoko]

Hallo dear Friends,

I have a FEM model which should fullfil the technical Standard sent from OEMs.
. Component stifness should be Not less than 200 N/mm under static loading.

The question as Follow:

1. Who can explain me which type of Stress Evaluation really counts. And what is the idea behind Max.princible stress (i am Not Looking for book definition).
2. How to find the stifness k=F/d Analytically for Component having complex Geometry?
I dont want to be a guys Who just bild a FE-models without having a check B .(analytical approach).

I will be thankfull for Any help.

 

[corus]

What really counts in the evaluation of stress is what is defined in design standards. A component can fail from yielding, fatigue, buckling, creep, and probably other reasons. Each have their own criteria for assessment. The maximum principal stress is usually (but not necessarily) the maximum tensile stress. Unfortunately you'll have to read a book to find out more.
To find the stiffness of something you apply a force and calculate the displacement and divide the two.

 

[Timoko]

Thanks cours,
Regarding to my little experince, static loading is very deterministic. If part fail at ex. 10 MPa under static loading. I guess failure Progression ocours earlier in dynamic loading. The idea is to Start with static stifness and i would like to know if Anyone knows a method where geometrically complex parts can be idealized and an approximate analytical solution is Found.the idea is a Bit far from simple be am under converting load. Summing up, Which stress creteria is important under static loading and what to Check for?

 

[rb1957]

"Which stress creteria is important under static loading and what to Check for?" ... there isn't a "one size fits all" answer; and none would help with your original requirement (for a stiffness of 200N/mm).

another day in paradise, or is paradise one day closer ?

 

[inline6]

from what i have come across von mises is used for yielding, and principal stress fatigue this is for ductile metallic components from steel/stainless/alumimium etc

as for how to do analytical hand calc for deflection and hence stiffness of complex part. you need an approximation for the geometry and then dig out a handbook with a similar case

 

[Henry26]

Use a random force, run analysis, and after that measure the displacement then use this equation:

k=F/x
Where:
F: Force applied
x: displacement
k: stiffness

Tips:
- If it's a non-linear results desconsider all I said
- The displacement should be along to your model and not resultant component
- The final results will be the stifness

As far as I'm aware von mises is used to ductile material and maximum principal for fragile material and fatigue calculation.
Maximum Principal > traction
Minimum Principal > compression

 

[stressebookllc]

Your max principal stress is a good indicator of maximum tensile stresses (important for fatigue stress amplitude).

But it is only suitable as the critical failure stress indicator for brittle materials such as cast iron, concrete etc.

In general, Von Mises is used for ductile yield failure prediction. About complex geometry stiffness calculations the boys have spoken already above.

In aerospace, its more of a combined stress interaction equation.

There are many good videos on failure criteria on youtube, you should check those out. They go into high detail on different failure theories.

http://www.stressebook.com

Stressing Stresslessly!

 

[inline6]

Henry, i think he wanted to know how to analytically calculate (hand calc) for the stiffness/displacement as a means to compare to the FE results