Stress Strain Data for Elastic Plastic Analysis 1

[struclearner]

To define the non linear portion of the material after yield stresses, Engineering Stress continuous to increase up to the Ultimate Tensile stress and then stress value starts to decrease until the fracture of the material occurs, it means the Tangent Modulus (E) becomes negative after the Ultimate Tensile Stress.

While True stresses calculation based upon the current/decreased cross section area continuous to increase after the Ultimate Tensile Stress value is reached until the fracture occurs, it means the Tangent Modulus (E) does not become negative after the ultimate tensile stress value.

What Stress Strain curve is used for Elastic Plastic analysis in ANSYS, if Engineering Stress Strain data is used with negative Tangent Modulus (E), does the analyis continuous and solution converges after Tangent Modulus (E) becoming zero or True Stress Srain curve based upon the True Stress is used.

Thanks a lot for your help and sharing the knowledge.

 

[L_K]

Ansys assumes the stress-strain curve you have defined is in the form of true stress/strain.

 

[NRP99]

Some More points to add-
1) Ansys uses true stress-true plastic strain data for non-linear material behavior.
2) After the true ultimate stress, Ansys considers curve as flat assuming the material is perfectly plastic beyond true ultimate stress. What it signifies is the material can not sustain the loads beyond the true ultimate stress which is limit used for design purpose. In actual case after ultimate stress and beginning of necking, the material still exhibits strain hardening which is not generally measured.

 

[struclearner]

A few questions to continue the discussion,
- How the Fracture stress of a material can be estimated from recommended Yield, Tensile strength, percentage elongation and percentage reduction in area at fracture values.
- To what stress value, the analysis should be continued after yielding, if interested to see the yielding/plasticity of the whole section, should the stress strain curve be continued up to fracture stress for a true stress strain curve, in the case, the fracture stress is higher than tensile stress.
- The strain at fracture from percentage elongation and percentage reduction in area can be estimated analytically, how the strain values could be interpolated between strain at yield and at fracture stress values respectively or extrapolated to the extent to see the whole section yielding/plasticity, if one need to create the stress strain curve analytically in the absence of tensile test data.

 

[NRP99]

- How the Fracture stress of a material can be estimated from recommended Yield, Tensile strength, percentage elongation and percentage reduction in area at fracture values.

-True Fracture stress= Fracture Load/Area at fracture. Refer Mechanical Metallurgy by Dieter, Chapter 8-Section 8.2-True Fracture Stress, for concerns in calculating true fracture stress.

- - To what stress value, the analysis should be continued after yielding, if interested to see the yielding/plasticity of the whole section, should the stress strain curve be continued up to fracture stress for a true stress strain curve, in the case, the fracture stress is higher than tensile stress

-It is sufficient to use true ultimate stress as limiting point since you are not interested in checking the necking behavior. If load is sufficiently high, you will get the total section yielding. If necking needs to be followed, then go up to fracture stress and fracture strain.

- The strain at fracture from percentage elongation and percentage reduction in area can be estimated analytically, how the strain values could be interpolated between strain at yield and at fracture stress values respectively or extrapolated to the extent to see the whole section yielding/plasticity, if one need to create the stress strain curve analytically in the absence of tensile test data.

-Check above replies. Just check whether values of the stresses are above the yield in total section. Another option is to check the total section plastic strain. If not then the load is not inducing total section yielding.

EDIT: You can anyway draw a graphical stress-strain plot using available end points and check the interpolated values. Another option is to use Ramberg-Osgood Equation and calculated the true stress-strain curve.