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derivative of stress

Posted 20.05.2014, 00:02 GMT-4 Structural Mechanics Version 4.3b 5 Replies

Huashun Wen
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Hi,

I have got the stress distribution of a structure, shown in attached Fig. 1. I want to calculate the derivative of stress along y, namely, d(solid.mises,y). But the results I got are zero, shown in Fig. 2. Is there any wrong? Thanks so much.

Huashun

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5 Replies Last Post 21.05.2014, 03:24 GMT-4
Henrik Sönnerlind COMSOL Employee

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Posted: 1 decade ago 20.05.2014, 02:08 GMT-4
Hi,

Try using d(solid.mises,Y) instead.

The Solid Mechanics interface is formulated on the material frame, so you need to take derivatives with respect to the material coordinates (X,Y,Z) rather than the spatial coordinates (x,y,z).

In addition, it is necessary that at least quadratic shape functions are used, since derivatives of stresses are second derivatives of displacements. Since the default is quadratic shape functions, this should not be a problem. As you are interested in the derivatives of stresses, it could however be a good idea to raise the shape function order to cubic.

Regards,
Henrik
Hi, Try using d(solid.mises,Y) instead. The Solid Mechanics interface is formulated on the material frame, so you need to take derivatives with respect to the material coordinates (X,Y,Z) rather than the spatial coordinates (x,y,z). In addition, it is necessary that at least quadratic shape functions are used, since derivatives of stresses are second derivatives of displacements. Since the default is quadratic shape functions, this should not be a problem. As you are interested in the derivatives of stresses, it could however be a good idea to raise the shape function order to cubic. Regards, Henrik
Huashun Wen
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Posted: 1 decade ago 20.05.2014, 02:39 GMT-4
Hi Henrik,

Thanks so much for your helpful answer. I tried d(solid.mises,Y), and it works. Now I got the stress gradient along y axis, shown in attached Fig.3.

I want ask it is possible to calculate summation of stress gradient of the whole surface? Tack.

Huashun
Hi Henrik, Thanks so much for your helpful answer. I tried d(solid.mises,Y), and it works. Now I got the stress gradient along y axis, shown in attached Fig.3. I want ask it is possible to calculate summation of stress gradient of the whole surface? Tack. Huashun

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Pieter Westerik
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Posted: 1 decade ago 20.05.2014, 10:58 GMT-4
In the 'Results' node, 'Derived Values' subnode, you can put a surface integral.
In the 'Results' node, 'Derived Values' subnode, you can put a surface integral.
Huashun Wen
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Posted: 1 decade ago 20.05.2014, 14:00 GMT-4
Hi Pieter,

Thanks for your answer. For my calculation, surface integral is different from summation. Because the units of summation and integral are N/m3, N/m, respectively. What I want to do is calculate summation of stress gradient for whole surface, in other word, add all individual values of stress gradient. Thanks.

Kind regards,

Huashun
Hi Pieter, Thanks for your answer. For my calculation, surface integral is different from summation. Because the units of summation and integral are N/m3, N/m, respectively. What I want to do is calculate summation of stress gradient for whole surface, in other word, add all individual values of stress gradient. Thanks. Kind regards, Huashun
Pieter Westerik
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Posted: 1 decade ago 21.05.2014, 03:24 GMT-4
You can integrate and then divide by the total surface area (or the other way around). Summation I do not know. What points would you choose for the summation?
You can integrate and then divide by the total surface area (or the other way around). Summation I do not know. What points would you choose for the summation?

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