Solidworks COSMOS - How to read results

[assafwei]

Hi,

First of all I must say, I am a bit ashamed asking this question...

Since its been a long time since I studied mechanics of materials and since I never used it in my line of work, I am trying to analyze a problem regarding this subject using COSMOS in solidworks, I have a model and I figured out the boundary conditions and the forces acting in the system. The analysis seems ok, but i have difficulties trying to remember what the numbers say...
Assuming small displacements, I am using the VON MISSES criteria, as far as I remember this criteria predicts yielding of ductile materials in a good manner, first of all, is this correct?
Second - how does the other criteria (shear and normal stresses in all directions) should be interpreted, and how can they be correlated to the yield strength?

Thanks.

 

[Nick Bruno]

1. yes von misses is a good approximation assuming isotroptic materials.
2. 2sigy^2 = (sig1-sig2)^2 + (sig2-sig3)^2 + (sig1-sig3)^2

where
sigy = yield stress of isotropic matieral
sig1 = largest principle stress (from your mohrs circle)
sig2 = second principle
sig3 = third principle

principle stress => no shear

sigy = Fy/A

The reason this equation works is because it takes the principle stresses into consideration in all directions.

We kno G = E/2*(1+eta) = only usable on isotropic materials

where eta is poissons ratio, G = shear modulus and E = mod of elasticity.

remember how when you strain somthing it either expans or shrinks in a different direction, von misses takes those aspects into consideration.

I don't know if I did well at answering your question... but what do you mean by "Second - how does the other criteria (shear and normal stresses in all directions) should be interpreted, and how can they be correlated to the yield strength?"

other criteria in the equation? or other stress criterions?

*Von misses stress = maximum distortion energy theory = maximum octahedral shear stress criterion

For orthotropic, and anisotropic materials other stress criterion are used if memory serves me correct? This would exclude composites from von-misses stress prerequisites. For example, E1 does not equal E2 and v12/E1 = v21/E2

 

[assafwei]

Thanks for the reply,

but what do you mean by "Second - how does the other criteria (shear and normal stresses in all directions) should be interpreted, and how can they be correlated to the yield strength?

I meant the other stress criterions, Since I know shear is the critical parameter, I check all three shear parameters, but than how should I consider the normal stresses in the points where the shear is at maximum, do they increase or decrease the stresses on the material?

BTW - if von mises only adherds to the stress invariants and not the shear stresses, how is shear taken into account in the von mises criterion?

 

[xxChrisxx]

Von Mises does take into account shear stress. As Nick said its basically a 'lumped' stress of all principle stresses, and generally asumed to ast in the direction of the largest principle stress.

Priniciple stresses are not 'real' per se. Its a combination of he normal and shear acting in a certain direction. (I think this is where you are going wrong, but I guess you already knew it, so sorry if it sounded patronising)

Other stress plots arent really useful if you only want to determine failure or factor of safety, as Von Mises allows you to to directly compare it to the yield or tensile strength.

But the shear plot is useful if you know that a certin shear stress is unacceptable even if the von mises is within acceptable limits. (you wouldn't compare this to yield stress but he shear limit)

The normal stresses are the same as above only can be compared to the yield strength. Both are basically a breakdown of what goes into the calculating the principle stresses.

 

[assafwei]

I returned to the textbook and realized I was really wrong.
Thanks for the replies it really helped.