The Plastic Strain at First Yield Must Be Zero

[PutrandaXY]

Hi there.

I am validating an experiment test using ABAQUS.
However, the only data available from the experiment is YIELD STRENGTH which is equal to 350 MPa.
According to Eurocode, at this point, the YIELD STRAIN = 0.02

So I entered the data on ABAQUS as it is (YIELD STRESS=350, YIELD STRAIN=0.02)

But I receive this message

"THE PLASTIC STRAIN AT FIRST YIELD MUST BE ZERO"

I don't understand how this means.

Can somebody please explain to me. Thank you.

 

[burningDiesel]

The format you need to enter is "stress / plastic strain". At yield, plastic strain is 0.
The 0.02 you mention is total strain. And you probably mean 0.2% or 0.002 (the offset yield strength).
You'd better use the proportionality limit or elastic limit for the plastic strain starting point.

 

[PutrandaXY]

I am not 100% sure what you mean. This is what I have done so far. I hope you can check this for me.

ELASTIC

Elastic Modulus Poisson ratio
210000 0.3

PLASTIC

Stress Strain Plastic Strain
350 0.0017 0.0000
350 0.0018 0.0001
350 0.0020 0.0003
350 0.0040 0.0023
350 0.0060 0.0043
350 0.0080 0.0063
350 0.0100 0.0083
350 0.0120 0.0103
350 0.0140 0.0123
350 0.0160 0.0143
350 0.0180 0.0163
350 0.0200 0.0183

Please note I used "Stress" and "Plastic Strain" values on the table above to define the material properties in ABAQUS.

The "strain" part was calculated using formula (STRESS/YOUNG'S MODULUS).
To obtain plastic strain, I substract strain with the elastic strain (0.0017). For example, on the second row, the plastic strain was calculate as (0.0018-0.0017=0.0001).

I calculated the plastic strain until it reaches its yield strain (0.02).

 

[burningDiesel]

Why are you considering a perfectly flat yield? An FE software will not be able to handle this (local instability), you'll get hit by negative eigenvalues etc, and it is not realistic anyway.
If you have data on UTS and strain at UTS, you can make a simple bilinear curve.
Also, now you have your yield stress of 350 start at 0.0017, while it is the offset yield at 0.002
If you use a bilinear curve, you can interpolate from the second 'yield' curve to the elastic one.
Finally, you are still mixing up 0.02 (2%) and 0.002 (0.2%)

 

[PutrandaXY]

What is UTS? The only data I have from the experiment is the yield strength which 350 MPa. In order to define the material properties in ABAQUS, I use Eurocode model as suggested by my corresponding researchers.

Actually I'm doing thermal analysis. And the reason it is perfectly yield is because according to Eurocode, the reduction factor at ambient temperature for strength at proportional limit and yield strength is 1.00.

However, these reduction factors differ as temperatures elevates. For example at 200C, the stress and strain are below:

Stress Strain Plastic Strain
282.45 0.001494 0.0000
288.57 0.001600 0.0001
293.63 0.001800 0.0003
297.14 0.002000 0.0005
315.74 0.004000 0.0025
326.17 0.006000 0.0045
333.56 0.008000 0.0065
339.08 0.010000 0.0085
343.23 0.012000 0.0105
346.28 0.014000 0.0125
348.37 0.016000 0.0145
349.60 0.018000 0.0165
350.00 0.020000 0.0185

I have a set of data like this for 20C to 1100C.

From this data I can form a bilinear curve starting from the elastic point to the plastic point.
In the Eurocode, the yield strain suggested is 0.02 and the yield strength remains constant until it reaches 0.15 (limiting strain for yield strength) before it goes down to its ultimate strain at 0.20 (strength = 0)

For better understanding, I uploaded the Eurocode model for you.