×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Fracture of teeth
4

Fracture of teeth

Fracture of teeth

(OP)
Hello Group,

I am trying to predict fracture of teeth.

The approach has been to 3d scan a tooth and recreate the geometry in ceramic material by rapid prototyping techniques.
The sample teeth are then tested on a load test machine.

Also material samples were tensile tested and i got a nonlinear stress strain curve used in the FE analysis.

Anyway the basic problem is that when i recreate the actual failure load in the FEmodel the stress is way higher than the UTS of the tensile sample.

I have looked at vaious failue theories - max pinicipal, mises, tresca and looked at strains too...

There is a very sharp corner where the crack initiates when the test samples fail and this is where i get my peak stress but it is way higher (around 600Mpa) than the UTS from the tensile test (the UTS is only around 60Mpa)...

I have a good mesh and loads of elements around the stress concentration and no abrupt changes in stress across elements...  Everyting seems correct but stress is just too high...?

Does anyone have any advice?

Thanks,

 

RE: Fracture of teeth

BTW these are the biomechanical teeth you are talking about, not teeth of a mechine?

Tensile testing of ceramic material(maybe its composite) is unusual as it is affected by the initial flaw size. Wouldn`t a bending and or compression test more appropriately model the material.

 

RE: Fracture of teeth

Yes...I think you have a stress-strain curve problem.  First, you have a non-linear stress-strain curve on a rigid, brittle material...doesn't sound right.

Secondly, your compression modulus and tensile modulus can be significantly different.

Third, do you have any idea what Poisson's ratio for the material is?  You can troubleshoot your data, looking to see if all those ratios are in sinc or not.

Also, when you are molding your specimens for material properties testing, is there a chance that they are not representative of the molding process for the actual teeth, such as density, uniformity, or inclusions.

RE: Fracture of teeth

(OP)
Hi,

Sorry its not really a true ceramic though it is supposed to behave in a similar manner - the material is bluestone - data sheet here     http://www.3dsystems.com/products/datafiles/datasheets/SLA/DS_Accura_Bluestone_US.pdf

It is more a kind of plastic but it is brittle with no real plastic deformation.

The stress strain curve is also pretty straight - I just included the small nonlinearity in case it made a difference.  It diddn't...

Thanks you your replies,
 

RE: Fracture of teeth

I did a bunch of work on teeth about 40 years ago... had to do with steel pins and amalgams failing.  High tensile stresses due to the difference in materials.  At the root of the pins, you have high tensile stresses created.  The amalgam is like concrete; it has a high compressive strength and a low tensile strength, hence the fatigue tensile failure.

The compressive forces generated by your jaw muscles can be significant and can cause similar problems if there is not an 'exact' match for the biting surface.

Dik

RE: Fracture of teeth

(OP)
To Rons queries,  

I don't have a measured value to possions ratio - so i will try some variance of the value here and see if i can get better correlation between the test data.

The specimiens should be fairly free from inclusions etc as the SLA process is fairly consistent however the parts are built in layers so this may cause some issues...

I would have been less surprised if the physical test samples failed earlier than predicted in FE but it is the reverse - the FE says they should fail at perhaps 1/10 of the actual load they can withstand...

 

RE: Fracture of teeth

It sounds like the problem could be in how you've modeled the tooth.  Finite element models produce infinite stresses at sharp corners.  Sharp corners create a mathematical singularity.  Try meshing your model into smaller pieces and watch the stresses jump even higher.  The finer the mesh the higher the stresses.  This is not what would happen in reality.

You can avoid this by putting a small fillet on the corner.  I ran into the same problem designing a rectangular hole in a steel plate.

RE: Fracture of teeth

(OP)
To advice from DCBII,

Thanks for your thoughts..

The geometry i am using is completely smooth and tanget - there is a very small radius as opposed to a sharp corner and it IS radiused and has a ton of elemenets across it - It may be that the prototyped samples are not quite the same (although they are produced directly from the 3d file) and i will look into this.... but i doen't believe this is the smoking gun as to why it doesn't tie up...

Thanks...,

RE: Fracture of teeth

You need to look at the wedging effect in the tooth dynamics here.  Where the food gets trapped and compression is applied, particularly hard foods, a wedging action happens, imparting a splitting and spreading force to the tooth matrix, ie, trension stresses.  If the tooth is properly shaped, this should not happen, but due to wear, can with time.  

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources