Page 45 - MetalForming Magazine August 2022
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Metal Matters
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Modulus During Loading
Modulus
During Unloading
Increasing Part Strain
Fig. 2—The elastic modulus decreases with increasing strain and material movement through draw beads and die radii. Values shown illustrate the potential magnitude of the decrease for some steel grades.
steel and aluminum alloys, the modulus can vary by as much as 10 percent for some grades, depending on the orientation relative to rolling direction. Complicating matters: The magnitude of variation as a function of ori- entation relative to the rolling direction is not constant between all grades.
The bending-unbending sequence as sheet metal pass- es over draw beads, die radii and draw walls leads to what is known as the Bauschinger effect. Among other implications, the Bauschinger effect reduces the elastic modulus each time the sheet undergoes the tension- compression associated with each bend-unbend. In addi- tion, increasing strains from material flow occurring as the part reaches the designed shape further lower the elastic modulus.
In a tensile test, a comparison of the modulus measured during loading versus when it is measured during unload- ing reflects these differences. One study on a dual-phase steel with a tensile strength of 780 MPa showed that areas with 11-percent strain experienced a 28-percent decrease
in the elastic modulus (Fig. 2). Accurate springback sim- ulation requires capturing this variation in elastic modu- lus. MF
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Apparent Modulus (GPa)
