Von Mises & Concrete
Von Mises & Concrete
(OP)
Does the Von Mises stress have an application in concrete? I am specifically looking at a notch in a vertical wall of a cylindrical tank. The intent of this tank is that it be watertight. Its easy to reinforce in both principle directions within a few inches of the edge of the notch. Obviously I am more concerned with tensile stresses than compressive stresses. Is it reasonable to assume that anywhere your Von Mises stress exceeds the tensile stress of the concrete you will form a crack? I am only looking at plane stresses and not including normal stresses.





RE: Von Mises & Concrete
RE: Von Mises & Concrete
Rick Fischer
Principal Engineer
Argonne National Laboratory
RE: Von Mises & Concrete
Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis
http://www.engissol.com/cross-section-analysis-des...
RE: Von Mises & Concrete
RE: Von Mises & Concrete
No, isotropy is only one key assumption of the von Mises Yield Criterion - other critical assumptions are that the it applies to linearly elastic, ductile materials, with a defined yield point, with essentially the same yield strength in tension and compression. Concrete is not ductile, and its tensile strength is MUCH lower than its compressive strength, so it violates two of most fundamental assumptions inherent in the formulation. (And reinforced concrete is not isotropic anyway, as its tensile strength is locally concentrated in the rebar.)
Look at the shape of the von Mises Yield Surface here: http://en.wikipedia.org/wiki/File:Tresca_stress_2D...
That is NOT the shape of the stress-strain curve that we usually apply to concrete!
Even putting aside the theory for a moment (which I would NOT advise!), in practical senses, von Mises Stresses can be quite misleading for concrete. Consider a typical symmetric reinforced concrete beam in pure bending - the post-processor will show von Mises stresses are the same value on the top and bottom faces (suggesting that the top and bottom faces are equally critically loaded), but the bottom (tensile) face will be heavily cracked long before the top face is anywhere near its compressive strength.
As others have stated, I suggest you use the Maximum Principal Stress to indicate where tensile stresses are significant, and the Minimum Principal Stress to indicate where the concrete matrix is heavily stressed.
http://julianh72.blogspot.com
RE: Von Mises & Concrete
A stress is NOT a stress. Yes, all materials have a "stress" at which they fail, but different materials fail by widely different mechanisms, and so the measure of that mechanism is different. Do you really believe that concrete, mild steel, high density polyethyene and carbon fiber composite, for instance, all behave the same and can be analyzed in the same way? The con Mises stress is not a real stress. It is a stress convention. It is a quantity made up from real stresses that is used as a criteria for a specific type of failure. You might want to take a look at "Failure of Materials in Mechanical Design" by Collins. He deals with faiure criteria and how and when to apply them in chapter 6.
Rick Fischer
Principal Engineer
Argonne National Laboratory