ductile fracture or shear fracture in Abaqus of a hair using only stress strain data from lab?

[mech_engineer]

Hello everyone,
I am trying to model the damage evolution of a hair in Abaqus with element deletion option. I have the stress-strain data for the hair but not sure how to extract the required inputs necessary to model the damage or the fracture Below is a short explanation for how I did the modeling:
Material - mechanical - damage for ductile metals - shear damage. Then, a window opened asking to enter the material parameter (ks), fracture strain, shear stress ratio, and strain rate.
After that in order to model the damage evolution, I selected sub-option and it asked me for the displacement at failure.
I do not know how or from where I can get these values. I tried to read about the damage mechanics but could not get how to obtain these values. Are there any resources that can help or any examples or explanations?

Or ideas on how else to model the semi ductile failure of a hair by ductile fracture or shear fracture in Abaqus?
Thank you,

 

[mronlinetutor]

If you have the stress strain curve of the hair, you can tell whether it is elastic or non-elastic. Elastic being a straight line and non-elastic being a curve. From the origin of the stress strain curve to the proportional limit is the elastic region. From the proportional limit to the plateau where E(non-elastic, instant)=0 is the strain-hardening of the hair. This is the ultimate stress. Area of the stress strain curve from the ultimate stress to fracture is the ductility of the hair. By looking at the stress strain curve, you can tell whether it is a ductile failure. If not, it is a fracturing.

The computer program usually asks you whether it is a elastic or non-elastic analysis. After telling the computer whether what type of problem it is, you can find the answer by inputting the data and the boundary conditions.

If the hair is stiff, it might be a elastic problem.

If the hair creeps while straining, it is a non-elastic problem. Creep usually deals with strain rate which suppose to be data from the experiment.

If the hair is ductile,it might become visco-elastic and then plastic and a plastic hinge is formed. If a plastic hinge is formed, the hair will fail if a further load is applied.

But if the hair fractures suddenly, that is with little elasticity and ductility, then it is a fracture problem.

disclaimer: all calculations and comments must be checked by senior engineers before they are taken to be acceptable.

 

[mech_engineer]

I am posting an example of the data for the hair. This hair itself fractures, however it is not a clean break. It fails like a mix of ductile and brittle.

 

[swimfar]

mronlinetutor, I don't agree with a lot of what you said.

1. A variable stiffness does not necessarily mean that the problem is elastic-plastic. There are elastic materials with non-linear behavior. The correct way to determine if there is plastic strain accumulation is to unload the material and see if the strain returns to zero.
2. The stiffness of a material won't tell you whether the problem is elastic or elastic-plastic.
3. I think it's a little confusing when you mention creep. That information doesn't come from a typical tensile test (although you could run a creep test on the same test machine). And it doesn't sound like Gabby_optical_mech is testing for this behavior.
4. Plastic hinges occur during bending, not during a tensile test. You may have meant to say "necking".

Gabby_optical_mech, many damage models in FEA code were created for metals. Hair has a very different micro-structure so it might not make sense to use these models. However, using a simple piecewise elastic-plastic material model with a stress-strain curve and an ultimate strength might work. Otherwise, it might make sense to look at some composite material models.

 

[mronlinetutor]

Gabby, I have made some modification to my earlier post. From the look of the stress strain curve of your hair, it looks like brittle material similar to cast iron. From the origin to the proportional limit is the elastic curve. From the proportional limit to the next point is the ultimate stress. The area from the proportional limit to the ultimate stress is strain hardening. You can approximate it with the two straight lines. The first ultimate stress has a drop like in the proportional limit of the elastic region and further hard straining until it reaches the next highest point. the next highest point is properly a plastic hinge which "Swimfar" refer to as necking. a plastic hinge is very similar to necking. a plastic hinge is total necking. the stress strain curve does show some ductility before it is fracture.

Since a hair is a non standard material, you have to draw the stress strain curve for the material property and that is where parameter is required. At the ultimate stress, the instant stress and strain ratio is zero. You can calculate the instant stress and strain ratio which is the instant modulus of elasticity which in term is the slope of the stress strain curve. Strain rate has little effect before ductility and it can take any number. Strain rate after ductility is very large so takes some large number.

The hair might be twisted and the shear strain is equal to tan(feta) where feta is the angle twisted.

An alternative way to study the stress strain curve of a hair is by considering the true engineering strain using optical techniques.

To my understanding, As long as the material is homogeneous, you don't have to know what material it is. All you have to look at is the stress strain curve. Stress strain curve is almost a finger print of material properties.

You have to verify the assumptions after the computer runs.

disclaimer: all calculations and comments must be checked by senior engineers before they are taken to be acceptable.