# Ansys material models

## Main Question or Discussion Point

Hello,
I am doing a non-linear analysis in ANSYS.
I have curve for the strain,stress curve for the material in tension, however, in the problem I'm working on I'm compressing the material. Does anyone know, if ANSYS just mirrors the material curve about origo as I have not given it data for compression but only tension?

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Mech_Engineer
Gold Member
It depends on the kind of material you're analyzing. Some materials show very different properties in tension vs compression (ceramics, composites). Most ductile metals have similar performance in compression and tension, and so are not typically modeled with tension/compression material models.

https://en.wikipedia.org/wiki/Compressive_strength#Comparison_of_compressive_and_tensile_strengths

Wikiepdia said:
Concrete and ceramics typically have much higher compressive strengths than tensile strengths. Composite materials, such as glass fiber epoxy matrix composite, tend to have higher tensile strengths than compressive strengths. Metals tend to have tensile and compressive strengths that are very similar.

JBA
Gold Member
You must be careful not to confuse "strength" and "modulus" for materials. The above statements are related to such things as "yield strength" or "compressive strength" which are related to the load/stress point at which a material will either transform from elastic to plastic deformation or suffer a tensile or compressive failure but do not necessarily imply or indicate that the tensile and compressive modulus (strain vs stress) is different.

Mech_Engineer
Gold Member
but do not necessarily imply or indicate that the tensile and compressive modulus (strain vs stress) is different
I think what you're describing is an isotropic vs. orthotropic material yes? ANSYS and other FEA packages do support both isotropic and several orthotropic material models. However, stress transformation techniques are typically used for modeling tension/compression in a ductile material, such that Young's modulus and Poisson's Ratio are enough to model most isotropic materials. Compression forces (and in some cases torsional moments) will manifest as shear and out-of-plane stresses in this case.

Wikiversity.org said:
For an isotropic material we have to deal with three material properties of which only two are independent.

These material properties are
1. The Young's modulus (also called the modulus of elasticity).
2. The Poisson's ratio.
3. The shear modulus (also called the modulus of rigidity). 