Learning Forum

[Mohankumar]

Hello,

How can I find / measure the maximum force or pressure a 3D solid model can withstand (before going into deformation) with a given Youngs modulus and Poisson ratio in Ansys Workbench (version 18)?

 

Thank you.

[peteroznewman]

Hello Mohankumar,

If you mean elastic deformation, that happens as soon as any force or pressure is applied, so the answer is zero.

If you mean plastic (permanent) deformation, that depends on the yield strength of the material. If the material is ductile, like most metals, then the von Mises equivalent stress is the quantity to compare with the yield strength to determine if the model is above or below the point of permanent deformation.

Regards,

Peter

[sujaysanjiv]

Hello Mohankumar,

My structural analysis professor once made a statement, "All things are springs" and we could not agree with him more. What he wanted us to understand, which he explained later, was that just like springs deform as soon as a load is applied to them, materials too exhibit the same behavior. The only reason this is not so obvious is that the strains we come across in common engineering materials subjected to service loads are of an order of magnitude too small to detect with the unaided human eye. For example, consider a metal frame bed that a person weighing 150 lbs sleeps on. The frame is designed to have adequate flexural stiffness so it does not bow too much and adequate strength so it does not break. The vertical deflection is impossible to detect with unaided eyes so we are under the impression that the bed will not 'deform' until load reaches a crititcal value. That is WRONG! As peteroznewman said, deformation happens immediately and it is the regime of deformation that decides whether the material will be able to recover (within elastic regime) or not (plastic regime). Hope this helps!

[Mohankumar]

Thank you Mr. Peter and Mr. Sujay. I got a better understanding. Hence, I would like to re-frame the question.

How can I find the maximum force or pressure required by a 3D solid model (non ductile and brittle) to reach the break point or fracture point with a given Youngs modulus and Poisson ratio in Ansys Workbench (version 18)?

[peteroznewman]

Mohankumar,

Brittle materials are interesting to try to predict when they will fracture.

The load that a given structure will fail with a fracture depends on two things.

One is the size of a flaw in the material or an initial surface crack. Every real structure has built in flaws in the material or small surface cracks. The other thing is a material property called Fracture Toughness.

The classic example is a glass rod. Take a quarter inch smooth glass rod and hang weights from it until it fractures. It will carry a huge load.  Now take another glass rod and some fine grit sandpaper and sand that rod a little and apply the load. The sanded rod will fracture at a much lower load.

With the Fracture Toughness material property and the depth of the initial crack, you can calculate the load at which that initial crack will suddenly fracture across the whole part.

ANSYS allows you to insert a Fracture branch in your model that will allow you to put a crack in your geometry, and a Fracture Tool to insert in the Solution branch to analyze the results of the load on that crack to calculate the Stress Intensity Factor (SIF) and compare it with the Fracture Toughness. 

I recommend you study Fracture Mechanics before you use these tools.

What if you don't know the Fracture Toughness of the material?

If you only know the Ultimate Tensile Strength of the material, then a simple method is to plot the Maximum Principal Stress. If the Maximum Principal Stress is greater than the Ultimate Tensile Strength, then you predict that fracture has occurred, if it is less, you have not. 

Regards,

Peter

 

[Mohankumar]

Thank you Mr. Peter. Much appreciated.