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how can the plastic strain be greater than unity?

Does anyone know how can the plastic strain be greater than unity? Such as in the benchmark manual 3.2.10 Indentation of a crushable foam plate: 

*CRUSHABLE FOAM HARDENING

 0.2000E6, 0.0000

 0.2577E6, 0.0094

 0.2760E6, 0.0258

 0.3053E6, 0.0452

 0.3267E6, 0.0655

 0.3623E6, 0.1084

 0.3891E6, 0.1540

 0.4250E6, 0.2405

 0.4568E6, 0.3812

 0.4738E6, 0.4600

 0.5170E6, 0.6391

 0.5862E6, 0.8570

 0.6503E6, 0.9857

 0.7470E6, 1.1324

 0.9820E6, 1.2965

 1.4702E6, 1.4808

 2.7262E6, 1.6609

 5.3911E6, 1.9000

 

As I Know that the formula for True strain is 

True Strain = ln(1+nominal strain)

and if we consider a compression process then nominal strain should not be greater than 1 and thus max true strain will be 

True Strain = ln(1+1)=ln(2)= 0.69

I know I am confusing somethings here, Kindly correct me

Comments

Ruobing Bai's picture

Hello,

To answer this purely from mathematics:

" if we consider a compression process then nominal strain should not be greater than 1"

A compression process will lead to a negative value of the elongation, which means the nominal strain is negative in this case. The above sentence should be modified to

"then nominal strain is negative, and its absolute value should not be greater than 1". And this obviously makes sense.

However if you really consider an elongation process, especially elongation of a metal rod, regardless of necking effect, the strain can then be greater than 1, either the nominal or true strain. A simple model people sometimes use is called "ideal plastic", see https://www.princeton.edu/~maelabs/mae324/glos324/idealplastic.htm , from where you can see ideally the material is like fluid, and the strain can be infinitely large with a finite stress.

 

Hi,

But in the mentioned table for foam hardening, plastic strain is greater then unity and its a compression data.

How could this comes ?

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