How does Nastran InCAD calculates thickness for a Midsurface?

Anonymous — Wed, 17 Apr 2019 13:15:49 GMT

How does Nastran InCAD calculates the Thickness of a Solid when using Find Thin Bodies?

I think it uses the area of the solid divided by the length of the midsurfaces created.

For example, I have the HEB1000 beam(attachment.jpg). I Calculated this manually and compared it to the thickness given by Nastran InCAD:

HEB1000 with fillets (left side picture):

Length midsurfaces= 300 x 2 + 868 = 1.468 mm

Area =40.004 mm2

Average thickness = 40.004/1468 = 27,25 mm

Thickness given by NASTRAN InCAD = 25,9482 mm

I did the same calculation, but this time removing the fillets.

HEB1000 without fillets (right side picture).

Length midsurfaces = 300 x 2 + 928 = 1.528 mm

Area = 39.232 mm2

Average thickness = 39.232 / 1.528 = 25,68 mm

Thickness given by NASTRAN InCAD = 25,9482 mm

This small difference I can’t explain. How does Nastran InCAD calculate the average thickness it gives to the Midsurface?

A follow-up question:How does Nastran InCAD calculate the length of the Midsurface? If the fillet is removed the length of the Midsurfaces changes.

Re: How does Nastran InCAD calculates thickness for a Midsurface?

Mike.Fiedler — Thu, 23 May 2019 18:17:58 GMT

We have discussed this as a support case, however, I am sharing the information here in response as well - in the event someone finds this posting and could also be helped by the information;

We use the following approach to compute the average thickness:
1. Loop through all the faces and get the max and min thicknesses and also area of the face.
2. For each face we compute the average thickness as follows:

averThickness [face] = area of the face * (max thickness + min thickness )/2;

3. averThickness [part] = averThickness [face sum] computed above / total area of all the faces;

***

Going to an HEB 1000 section (1000mm height, 300mm flange width, 19mm web width, 36 mm flange thickness, 10000 mm in length), and reference the attached image (Beam Areas.png) for areas of regions as extracted by Inventor's measure tool. Our approximate equivalent hand calculation for the average thickness would be as follows;

3,000,000mm^2 * 36 mm = 108,000,000

1,571,226.9mm^2 * 36 mm = 56,564,168.9

9,280,000mm^2 * 19mm = 176,320,000

Finally, the sum of the right side yields 794,896,675 and we divide by the sum of the areas on the left (30,844,907) and arrive at a calculated result of around 25.77mm. The thickness that Autodesk Nastran InCAD arrives at from the find this body is 25.68mm, for a difference of around 0.3%.

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How does Nastran InCAD calculates thickness for a Midsurface?

Re: How does Nastran InCAD calculates thickness for a Midsurface?