Week-6 Calculate the Stretch Ratio by comparing the ELFORM (-2,-1,1,2) with Ogden_Material Model.

CALCULATE THE STRETCH RATIO BY COMPARING ELFORM 1, 2, -1, -2 USING OGDEN MATERIAL (077)

OBJECTIVES

            In this assignment we have to stimulate Tensile test using Ls-Dyna and calculate Engineering stress, strain and Stretch ratio.

PROCEDURE

• k file in ls-prepost which has mat-077 ogden rubber material.

• Then create cube with 10x10x10 mm solid mesh using mesh – shape.

• Then create section using model – keyword – section – solid, in this key word only we have to change the ELFORM formulation.

• Then in the part keyword assign the material and section to the cube.

• Then we have to create spc by model – content – boundary – spc. One side nodes of the cube is fully selected and constrained to x – axis, because we stretch the cube in that direction.

• Then middle vertical nodes are selected and constrained at Z axis and horizontal nodes are selected and constrained at y axis so that cube compress evenly in all direction because it a rubber material.

• The displacement curve is defined by model – keywords – define – curve so that the stretch ratio will be approx 5.

• Then prescribed motion is defined in opposite side of the spc nodes and assign the direction and curve.

• We need to added *Database_Extend_Binary card and activate the STRFLG to get the strain data.

• The stimulation should be run in implicit hence we create implicit_auto and implicit_general by model – keywords – control.

• Then give the control termination time as 1ms.

• Then run the stimulation in Ls-Dyna solver. (ensure the solver is double precision to do implicit analysis)

RESULT

• From the above stimulation we will get true stress and true strain graph as a result.
• With the resultant data we have to calculate engineering stress and engineering strain by formula.

Ɛ_true = ln ( 1 + Ɛ_eng)

Ɛ_eng   = EXP Ɛ_true – 1

      σ _true  = σ_eng (1+ Ɛ_eng )

Ɛ_true      = true strain

Ɛ_eng   = Engineering Strain

     σ _true   = True stress

     σ_eng      =  Engineering stress

ELFORM1

It is a constant stress solid element (default), it has hourglass control for large deformation. It is accurate and has low computational cost than others. 

ELFORM-1

Fully integrated solid intended solid for elements with poor aspect ratio with efficient formulation.

ELFORM2

Fully integrated solid elements which reduce the stiffness matrix.

ELFORM-2

Fully integrated solid intended solid for elements with poor aspect ratio with accurate formulation.

CONCLUSION

The result between Engineering Stress and Stretch Ratio is ploted for different formulation there is no significant changes except the computational time, the tensile test of cube is very small stimulation the difference might be noted in big problem eith small aspect ratio.

From the stimulation ELFORM1 has low computational time followed by ELFORM2, where ELFORM-2 has more computational time than ELFORM-1.