Von Mises Yield Criterion

[eftyss]

Hi,

I've calculated the safety factor for the Von Mises Yield Criterion compared to the yield strength of the material. I got a safety factor of 1.075 which is very close to 1. I know that the material is about to fail.

However, is there a range of values that the safety factor can have? I found from an online source that the factor of safety should be in the range of 1.25-4. Is this true?

Is there anything we can do to increase the safety factor so that it is ensured that the material is not going to fail?

I will appreciate any help

Thank you

 

[KootK]

What is it that you're designing?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[jrbaus]

Most FEA and working stress checks I have seen/undertaken on steel structures compare the Von Mises stress to Fy/FOS where Fy = yield strength and FOS=1.5.

The FOS of 1.5 came from the old (Australian) steel design code before ultimate limit state was adopted.

A FOS of 1.075 is very low. That said, if you are comparing stresses from an FE model you need to look at the location of the stresses as you will get stress rises at T intersections, poor meshes, load locations & boundaries that may not be representative of the actual stress state.

If your stresses are not driven by thermal effects, extra material thickness should see stresses reduce.

 

[WARose]

I don't know what kind of material you are using but the SF for Von Mises depends on the material. I've typically used a SF of about 1.3-1.5 for structural steel. My basis for that is an old AISC Journal article (third quarter 1986; 'Combined Shear and Tension Stresses') where interaction curves were developed based on Von Mises for the interaction of shear and tension. A SF of 1.67 yielded results a bit too conservative while the range I mentioned was a bit better.

 

[bkal]

Have you applied any safety factors on loads?

 

[eftyss]

Thank you all guys for your reply.

The problem is that I haven't assumed a safety factor but I have derived it.

I have designed a shear wall made of reinforced concrete and I had to check if a particular point in the wall is going to fail. I was given the yield stress of the material (100 MPa) as well as the tensor matrix for that particular point. Based on the tensor matrix I have calculated the principal stresses so that the von mises stress can be obtained. The value of the von mises stress was 93 MPa. I have divided those two values to get the safety factor (100/93) and it gave me 1.075. What does that says to me?

 

[KootK]

This is entirely the wrong way to go about designing a concrete shear wall. None of the Von Mises stuff applies. You need to base your design on established methods for RC concrete design. A good textbook and the governing concrete code should be consulted.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[WARose]

Yeah, KootK is right: for reinforced concrete, ACI code (and practical experience) should lead the way. With a shear wall, the calculations are fairly straight forward in any concrete text.

Von Mises failure criteria is more for a ductile material (like steel).

 

[Brad805]

Von Mises does not apply, but there has been some research in this field that allows one to use the FEM design for solids. We have been investigating this for some atypical precast elements using solidworks. If you are dealing with a solid element I agree with the others. Stick to the code methods.

 

[bkal]

There is also a useful 'state-of-the-art report' by CEB-FIPB: Practitioners’ guide to finite element modelling
of reinforced concrete structures, report No. 45, 2008.

 

[Brad805]

bkal, thanks for mentioning that reference.