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Roger Casas

Mechanical properties question

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Hi engineers!

I have been watching this forum many time ago and this is the first time I write something. I hope to have more things to discuss since now :-)

In the image attahced, you will see a table of the mechanical properties of diferent types of steel. Is the usual table of any catalogue.

And there is one think that I don't understand. If you have a look at how either tensile strength or the yield strength changes according to the nominal thickness, you can see while the thickness increases, the value of its N/mm2 decreases. This fact seem's to be impossible because any bar or any object, the bigger it is, the higher stress it can sustain, but in the table tends to be lower everytime. 

For example, the steel S235JR, with a thick of less than 3 mm can hold (tensile stress) between 360 and 510 MPa (N/mm2), and if it its thickness is between 3 and 100 mm it can hold between 340 and 470 MPa. Both intervals cover at some range, but the thicker one has a lower stress sustain. The same happens with the yield strength.

Might somebody help me with this question? 

Thanks a lot!

Roger Casas

20180604_205640.jpg

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Thin materials (sheet) have higher strength (albeit minor) than same thicker ones (rod or bulk).

Strength is stress/pressure, i.e. force per area. You thin material is stronger stress-wise but actually weaker force-wise.

Depends on what's your material (sheet, rod or bulk), you make design based on the listed value.

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Yield strength reduces with increasing plate or section thickness because thinner material is worked more than thick material and working increases the strength. The reason is due to the manufacturing process for that type of steel. If you draw out strands of metal then the drawing process eliminates many of the dislocations in the laticce of the steel. Many working processes also reduce dislocations. So material properties are much dependent on manufacturing processes less dislocations higher strength. For hot rolled, thinner section tend to have finer grain which slightly increases yield.

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Ok, I understand.

As you say Henry, is interesting how to distinguish between a stress material selection and a force analyzing situation. It was a point that I didn't see.

And understanding howw effects the manufacturing to the material, as Michal explains, is what makes everything clearer.

Thanks a lot for your inputs.

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If anyone is interested, it is due to how homogeneous the material is (roughly translated as 'same throughout'). With thinner materials, the final product is generally reasonably homogeneous, but as the plate thickens, it starts to develop more and more intrinsic flaws within the material. Basically, in order to mitigate for this, the quoted strengths have been reduced.  

Beware of complacency though...these 'reduced' figures are not a "fudge" but verified from tests and can be taken as true for the materials in question.

Michal is exactly right, but for those not so technically minded (or those without the materials knowledge) this explanation  may help.

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