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difference in simulated stress and strain

Posted 18.10.2012, 04:41 GMT-4 Materials, Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.2 3 Replies

Yinjie Ding
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Hi,

I simulated stress for a 2D Silicon structure using "thermal stress" in COMSOL, the simulated stress values are quite consistent with my experimental results.

however, when I look at the simulation results in strain by typing "ts.eX" in the expression, the strain values do not make sense to me and they are not consistent with the simulated stress values. The strain values are much lower than the expected values that I converted from stress using Young's modulus.

I just entered value for the Young's modulus and Poison's ration for Silicon, and typed "ts.eX" in the expression instead of "ts.sx" when I look at the results in stress. Is there any other thing that I need to enter for the simulator to calculate the strain correctly?

Thanks!
3 Replies Last Post 19.10.2012, 02:38 GMT-4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 18.10.2012, 07:16 GMT-4
Hi

you need to use the correct stress tensor referred to the correct frame, x and X refer to two different frames, the material and spatial, check the doc ;)

I can only recommend the book of E.B. Tadmor et co, "Continuum Mechanics and Thermodynamics", Cambridge/CUP, 2012, ISBN-978-1-107-00826-7. it has the detailed explanations and an notation close to COMSOLs (note there might be a sign convention difference, to be checked closer)

--
Good luck
Ivar
Hi you need to use the correct stress tensor referred to the correct frame, x and X refer to two different frames, the material and spatial, check the doc ;) I can only recommend the book of E.B. Tadmor et co, "Continuum Mechanics and Thermodynamics", Cambridge/CUP, 2012, ISBN-978-1-107-00826-7. it has the detailed explanations and an notation close to COMSOLs (note there might be a sign convention difference, to be checked closer) -- Good luck Ivar
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago 18.10.2012, 13:34 GMT-4
Relating stress to strain via Young’s modulus only is valid only for uniaxial loading. In general 2D or 3D loading you have to relate the stress vector (tensor to be precise) to the strain vector through the stress-strain matrix (tensor again to be precise).

Nagi Elabbasi
Veryst Engineering
Relating stress to strain via Young’s modulus only is valid only for uniaxial loading. In general 2D or 3D loading you have to relate the stress vector (tensor to be precise) to the strain vector through the stress-strain matrix (tensor again to be precise). Nagi Elabbasi Veryst Engineering
Yinjie Ding
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Posted: 1 decade ago 19.10.2012, 02:38 GMT-4
Hi,
Thanks for the reply.

Even when I look at the simulated stress (ts.SY) and strain (ts.eY) at the same tensor, the stress shows a compressive with -ve values but strain shows tensile with +ve values!

Hi, Thanks for the reply. Even when I look at the simulated stress (ts.SY) and strain (ts.eY) at the same tensor, the stress shows a compressive with -ve values but strain shows tensile with +ve values!

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