Skip to main content
G
gkemner1-Visitor
1-Visitor
August 5, 2005
Question

Brittle Material Stress Analysis

  • August 5, 2005
  • 7 replies
  • 2468 views
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!

    7 replies

    R
    randyspeed1-Visitor
    1-Visitor
    August 5, 2005
    Greg,

    There is no place to handle a Brittleness property in MECHANICA, other than
    to enter an allowable stress. Just be aware of the fatigue stress limits
    (S/N curve) of the material you are analyzing.

    Randy Speed
    President and CEO
    Speed Consulting, LLC - "MECHANICA Experts"
    (972) 938-0490 ph (972) 937-2319 fax
    www.speedconsulting.com
    J
    jholst-21-Visitor
    1-Visitor
    August 5, 2005
    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.

    ---Jim Holst
    P
    ptc-4426731-Visitor
    1-Visitor
    August 6, 2005
    Greg,

    some additional points:

    * 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

    Stefan Reul




    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!
    >
    D
    david.reid1-Visitor
    1-Visitor
    August 9, 2005
    Surely it necessary to consider vibration?

    After all brittle materials will mostly break under impact or impulse load.
    Can be very strong otherwise.

    I can think of a case when someone did a stunt that involved balancing a
    steam roller on 4 pint glasses, and they survived.





    ---
    David Reid
    Optima Design Services Ltd
    davidr@optima-design.co.uk
    R
    randyspeed1-Visitor
    1-Visitor
    August 9, 2005
    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
    R
    r1chardgreen1-Visitor
    1-Visitor
    August 9, 2005
    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.
    consult@finglow.co.uk
    Tel: +44(0)1992 550 700
    Fax: +44(0)1992 550 843
    D
    doug.bittner1-Visitor
    1-Visitor
    August 9, 2005
    Hi All,

    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.

    Yours,
    Doug


    Doug Bittner
    Staff Mechanical Engineer
    Danaher Precision Systems
    Danaher Motion
    200 Flanders Road
    Westborough, MA 01581-0200
    Terms & ConditionsAccessibility statement