Has anyone used Mechanical to analyze the stress in a brittle material? Such as glass, crystal, ceramic or something similar. Any problems or special considerations that I need to take into account to get accurate results?
Make sure you do not have any non-tangent inside corners in your model. Include all fillets even if they are very small. You may want to add some mesh control in the fillets. In real life, brittle materials will fail in these areas and are not as forgiving as ductile material. Stresses calculated in these area should not be ignored. Other than that there is not anything special about FEA and brittle materials.
* use for evaluation max. principal stresses or Tresca (2 times max. principal shear stresses) * crystals and ceramics often have anisotropic behavior; use orthotropic material definition (and orientation)
Be aware of the big scattering of the max. allowable stresses of such materials (e.g. if you use Si or GaAs the average allowable stress is e.g. 100 MPa, but to get a - bad - 10 % crack probability you should allow only about 10 to 20 MPa !).
With best regards
Kemner, Greg (Space Technology) schrieb:
>Has anyone used Mechanical to analyze the stress in a brittle material? Such as glass, crystal, ceramic or something similar. Any problems or special considerations that I need to take into account to get accurate results? > >Thanks! >
I was thinking of that same thing this morning - I believe you are right David. Fatigue is less important than just keeping stress down to an absolute minimum.
Someone mentioned it earlier and was right on (didn't keep the emails) - sharp (reentrant) corners are extremely bad. Must maintain very generous fillets. Reminds me of a glass top I had made for my desk. I initially had a rectangular penetration for cables and such (very small fillets). After replacing it two times, each one getting larger fillets, I eventually just made a half moon slot on the edge of the part.
Randy Speed President and CEO Speed Consulting, LLC - "MECHANICA Experts" (972) 938-0490 ph (972) 937-2319 fax www.speedconsulting.com
I've read the views expressed on brittle materials with interest and, with apologies for stating the obvious, would like to add the following to the discussion:
- Brittleness in a material is effectively the absence of the ability of deform plastically. - Brittle materials, when subject to tensile stress, will generally fracture at the elastic limit or after a very small amount of strain has developed beyond this point. - Brittle materials are often much stronger in compression than in tension (four pint glasses and a steam roller !! I would like to have seen that David). - The maximum principal stress theory is the most appropriate criterion to use for predicting failure. (This explains the need to avoid sharp corners Randy). - The wide range of allowable stresses quoted for brittle materials can be directly related to the "quality" of the material. (Type "glass allowable stress" into Google and you'll see what I mean !!)
It is clear that great care should be taken to seek out all potential stress raisers in the component under consideration and to use appropriate mesh refinement to ensure that stresses in these areas are predicted as accurately as possible. It follows that "defeaturing" should be avoided at all costs!! The selection of an allowable stress which takes account of the specific grade or class of material used is clearly also very important.
Richard Green Technical Director Finglow Consultants Ltd. firstname.lastname@example.org Tel: +44(0)1992 550 700 Fax: +44(0)1992 550 843
Might I suggest, rather than maximum principle stress theory, you might consider modified Mohr theory, or perhaps Coulomb-Mohr theory. See most any Strength of Materials textbook. Joseph E. Shigley gives a good explanation in his text, Mechanical Engineering Design (page 254 in the Fifth Edition), McGraw-Hill, 1989.
The Failure Index, of the Mechanica Results Display, will handle the modified Mohr theory. Just be sure to enter the correct failure criteria in the material properties set.
Doug Bittner Staff Mechanical Engineer Danaher Precision Systems Danaher Motion 200 Flanders Road Westborough, MA 01581-0200