Multilinear Kinematic Plasticity created from a stress-strain graph

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peteroznewman posted this 30 October 2017

I was sent a stress-strain graph of a brittle metal tested in an Instron machine. It looked like this.

I have matlab and downloaded Grabit, a utility that makes digitizing data from a graph very easy. Here is what that looked like:

Now that I had the curve digitized, I could convert the Stress and Strain into True Stress and Plastic Strain so I could enter values in a material model using Multilinear Kinematic Plasticity. The formulas to convert are shown below:

E = 13,000 ksi

True Strain = ln(1+Strain)   where ln is the natural logarithm function.

True Stress = Stress(1+Strain)

Elastic Strain = True Stress/E

Plastic Strain = True Strain - Elastic Strain

I used a spreadsheet to correct the offset in the origin of the graph to put it back on zero.

The first two columns came from the matlab Grabit utility.

The last two columns are copy paste into the Multlinear Plasticity material model, except for the first line.

The first first line is the yield stress. This was the material model used for the thread fracture analysis.

 

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Fabricio.Urquhart posted this 2 weeks ago

Peter, do you have any information like this post for strucutre steel materials?

- ASTM A572 Steel, Grade 50 (beams and columns)

- ASTM 36 (plates)

- ASTM A325 (bolts)

I found in matweb.com. But plasticity properties I did not find.

Thank you!!

peteroznewman posted this 2 weeks ago

I can borrow the Atlas of Stress-Strain Curves from my Engineering Library.

Curve 6 is ASTM 36.

Fabricio.Urquhart posted this 2 weeks ago

Thank you, very very much Peter.

For the bolts do you have any curve?

 

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peteroznewman posted this 2 weeks ago

Fabricio,

When I search astm A325 stress strain curve on Google, it gives me this.

Figure 7 is the stress strain curve for ASTM A325.

You should try Google for yourself, it is really very good!

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Fabricio.Urquhart posted this 2 weeks ago

I have found a curve for bolts in this article whcih I have attached. If would like to add in your engineering library, I shared with you.

Fabricio.Urquhart posted this 2 weeks ago

Peter I am using multilinear isotropic hardening plasticiy, but I could not see the nonlinearity.

The model is a beam, fixed in both extreme, but I modeled only the half, and use a plane of symmetry,

I could not see the nonlinear curve in equivalent total strain neither nonlinearity in the equivalent stress:

In spite of reach the yeld and rupture stress, the curve is linear, I am not understanding why.

 

Attached Files

Fabricio.Urquhart posted this 2 weeks ago

You are right!!

Attached Files

Fabricio.Urquhart posted this 2 weeks ago

Peter, I solved. The problem was the load, which was normal to surface. So with component, the plasticity occure easier.

 

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