Learning Forum

[Sokpanha Mao]

Dear everyone,

I have been working on finding damage results for RC structures. However, I saw all research papers using "effective plastic strain" results in LS-Prepost to represent crack patterns/damage of concrete. What is the meaning of "effective plastic strain" and why is it used to represent crack or damage patterns?

Best Regards,
Panha

[Alex R.]

Hello Sokpanha, 

Effective plastic strain is a measure used in finite element analysis to represent the accumulation of irreversible deformation in a material. When a material undergoes plastic deformation, it means that the material has been stressed beyond its elastic limit and will not return to its original shape when the load is removed. In the context of RC (reinforced concrete) structures, effective plastic strain is particularly useful for representing crack patterns and damage because it can indicate areas where the concrete has yielded and potentially cracked.

The reason effective plastic strain is used to represent crack or damage patterns in concrete is that it provides a way to quantify the extent of damage in a numerical simulation. As concrete is a brittle material, it does not exhibit significant plastic deformation before cracking. Therefore, the effective plastic strain can be used as an indicator of the initiation and propagation of cracks within the concrete. By analyzing the distribution of plastic strain, engineers can predict the locations where cracks are likely to form and assess the overall damage to the structure.

Please find article (formula)  about effective plastic strain on dynasupport:  https://www.dynasupport.com/howtos/general/effective-plastic-strain

Thank you,
Alex 

[Sokpanha Mao]

Thanks, Alex for your response,

For concrete material, normally there is no plastic stage for the tensile strength. Therefore, for effective plastic strain result here, does it consider both compressive and tensile plastic strain (crushing and spalling) or only failure because of compression?

Best regards,
Panha

[Peter Yip]

Panha,

Bare in mind that effective plastic strain and equivalent plastic strain are different strain measures. Their definitions are very different, but that wasn't too intuitive for me so I will share a slide I put together for a presentation a few months ago. As you can see in the image, the Z-Strain represents the single degree of freedom compression and expansion. The difference is that as you return back to its originally shape via expansion, the effective plastic strain returns to zero and will only accumulate plastic strain as it is expanded beyond it's original shape. However, the equivalent plastic strain continuously accumulates plastic strain even when the element begins expanding by definition since anything beyond yield is considered plastic strain.

So, now you should be equipped to answer why brittle ceramics tend to use effective plastic strain vs other strain measures to quantify damage in their respective simulations. 

All information I have gathered is from online and found here (https://ftp.lstc.com/anonymous/outgoing/support/FAQ/effective_plastic_strain#:~:text=%5BThus%5D%20effective%20plastic%20strain%20and,they%20mean%20the%20same%20thing.&text=Effective%20plastic%20strain%20is%20a,the%20rate%20of%20deformation%20tensor.).