Plastic material input from a tensile test

[JohnnyCC]

Hello everyone!

I have testdata from a tensile test which I will use as material input for a nonlinear analysis in Abaqus CAE. At this stage I transformed the nominal stress-strain to true stress strain and calculated the plastic strain.

My question is, since the measurement was made with an extensometer of 61mm, this gives an average strain over these 61 mm. What about the maximum strain obtained? Is there anyway to calculate this?

Can anyone give me any clearance in this matter? Is it correct to take an average value for the strain as input to the FE-model?

Thank you in advance

/Andreas

 

[gurmeet2003]

JohnnyCC,

In a uniaxial tensile test why should the max strain be different from the average strain? Both should be the same. The extensometer is mounted on the gage section of the specimen which has a uniform diameter.

The exception to this would occur after necking. In the past I have usually ignored the data after necking.

Gurmeet

 

[JohnnyCC]

Hello again,

I will reformulate my question abit. I am trying to capture the behavior after necking at localized stress-points. I want to find these large plastic strains that occur. Do you know how to find these very localized points? I didn't manage to find equivalent plastic strains larger than about 22%.

This has been accomplished in a previous project using the FE-program DIANA.

A simple calculation using the necked area, A (127.4 mm2) and the original cross-sections area, A0 (312 mm2) shows that the average plastic strain at failure is about 89%. eF=LN(A0/Af)

Has anyone modelled anything like this before? My model is a simple rectangular specimen loaded in tension. I'm using 3D solid elements.

Regards,

Andreas

 

[RonDana]

I have carried out several coupon tests (rectangular coupons similar to those of ASTM E8). Some of these tests have kind of weird behavior. The problem arises when I simulate results with ABAQUS. It goes pretty well until a few steps after necking. Test and ABAQUS results match exactly until several steps before fracture. After this point, there is a discrepancy between results and my attempts in capturing the observed behavior was unsuccessful. (around 10% or more plastic strain).
The FORCE-DISPLACEMENT curve from experimental result show a more plateau type curve after necking, with a sharp decrease in force right before fracture. I am unable to capture this part. I think it can be attributed to the fact that stress, in experimental test, remains uniform and no stress concentration or necking takes place until a sudden necking followed by fracture.
In my models, I either have necking take place, say right on time, or no such necking takes place and strain remains uniform and entire length of the specimen takes part in elongation.

I would appreciate any suggestion.

Thanks.
Ron, PhD

 

[gurmeet2003]

Once necking starts subsequent behavior is unstable. I am not sure but perhaps Riks method may give a better answer.

Gurmeet

 

[williwang]

surface finish and material quality play a part in this test too.

I believe it is unstable.

Forever Young.....

 

[gatre]

You can you trial-and error method like YingBao wrote in his thesis. After necking point, we can iterate the simulation a couple of times.
I use his method with two iterates and got amazing curve in Abaqus. The simulated curve coincide completely experiment result.