Discussion Forum

Hi

1) for the mesh error it is most probably related to your details of the model
2) for the loads, it all depends on what you want to model:

- body loads are gravity effects, but also rotary centrifugal force effects acting on the volume (on each dv=dx*dy*dz)
- while pressure loads (or forces/area) are the forces acting from the cutting process, here I'm not sure how you want to express that

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Good luck
Ivar

I have a drill bit system that was used to drill a well and it fractured in the hole. I want to model the tool in the hole and find the stress fracture in psi that caused the fracture. How can you help in modeling this in COMSOL ? Have the drill bit in a pipe and it will drill through the pipe, however it broke and i only want to find the fracture stress. What do you use as fixed constraints and boundary load/body load as torque is applied to drill bit and bending stresses are alos acting on drill bit
Thanks for your help

Hi

I would say that is a delicate multi-physics issue.

Certainly you have the centrifugal forces from the drill rotation => a body force, easy to implement as rho * centrifugal acceleration, depending on R and on the speed of rotation

But that force is probably peanuts compared to the friction and cutting forces (apart if you were testing at Mrpm speed), then you need to add a surface load (pressure) on the cutting edges, but I have no idea where to find reasonable experimental values, related to the materials and cutting speed, and temperature and ...

Almost last, you have the heat generation and the non-linear material properties from the local temperature elevation, due to friction

Finally, there are the fatigue issues, which are of probabilistic essence, hence not obvious to model only with FEM, it would require some matlab or other Monte-Carlo programming to be fully covered

The only way out, that I see, is to study case by case and then add them one after the other

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Good luck
Ivar

I agree with Ivar, this is a complicated problem. The main forces/moments in drilling, if there is perfect alignment, are the torque (resulting from cutting and friction) and the axial force. Transverse forces and bending moments will develop due to misalignment and can be severe. Centrifugal forces should be insignificant in this problem.

If you examine the failure surfaces you will find clues about the nature of the failure (fatigue, fracture, etc.) and the loads that caused it (bending or torsion). It is important to use material properties at the right temperature experienced during operation. While temperature can be estimated using FEA, there are so many unknowns involved. It is better in my opinion to measure temperature experimentally and use it an input to your model.

Nagi Elabbasi
Veryst Engineering

HI,

I was wondering if someone can help me with my work. I need to find out the forces acting on a drill bit when it is doing the drilling, and find the stress value. Looking forwards for some help.

Thanks & Regards
Dinesh