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I was studying the design of joints and came to knuckle joints. Generally compressive strength is greater than tensile strength, but considering knuckle joints compressive strength and tensile strength are considered equal (as mentioned below). What are the reasons for assuming equal strengths?

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Here is a link to the page, shown above, in Google Books.

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  • $\begingroup$ generally compressive stress is about 2 times the tensile strength. (observed from sums in that book) $\endgroup$ – Fennekin Jan 9 '16 at 15:12
  • $\begingroup$ Is your confusion about knuckle joints or about the ratio of compression to tension? $\endgroup$ – hazzey Jan 9 '16 at 23:23
  • $\begingroup$ My doubt is why this is the situation particularly in knuckle joint? $\endgroup$ – Fennekin Jan 10 '16 at 4:01
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Assuming they're equal, it allows you to use the more commonly available tensile strength data, while also getting a conservative solution. If it turns out they're not equal, then your design will still work correctly but might not make optimal use of the materials.

Assuming uniaxial tensile and compressive strengths are equal is a common practice in engineering and is built into the popular von Mises and Tresca yield criteria which are used for all kinds of structures. If you want to take advantage of higher compressive strength, you can use less common failure criteria like Mohr-Coulomb.

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The ratio of compressive to tensile strength is dependent upon the steel. A36 structural steel has higher tension yield than compressive yield, 36kpsi vs 22kpsi, reference: http://www.matweb.com/search/datasheet.aspx?matguid=d1844977c5c8440cb9a3a967f8909c3a&ckck=1 A very brittle tool steel will have a much higher compressive strength than tensile strength.

For the cold drawn and stainless steels used commonly in machine components, usually only yield strength is published. These steels probably have yield and compressive strengths in the same range. The following test report (from 1942) that tested both compressive and tensile strength of stainless steels showed comparable results for compression and yield: http://nvlpubs.nist.gov/nistpubs/jres/28/jresv28n4p499_A1b.pdf

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