How to Work Out Tensile Ratings on Metric Bolts

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The markings on the head of a bolt provide information about the strength of that bolt. For metric bolts, the tensile strength ratings can are in the form of a number with a single digit after the decimal, such as "8.8" or similar. These markings are usually found on the top of the head but can also be found along the side. Other marks, such as an underline beneath the strength marking, can give you additional information, such as the type of metal used to manufacture the bolt.

Skill level:
Moderately Easy

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  1. 1

    Locate the tensile strength marking and determine the ultimate tensile strength. The digit before the decimal indicates the ultimate tensile strength in hundreds of megapascals (MPa). A metric bolt with a rating of 8.8 would have an ultimate tensile strength of 800 MPa.

  2. 2

    Determine the yield strength using the number after the decimal place. The second number is converted to a percentage that is multiplied by the ultimate strength to determine the yield strength. A metric bolt with a rating of 8.8 would have a yield strength that is 80 percent, or 0.8, of the ultimate strength, which is 800 MPa.

    0.8*800 = 640 MPa

  3. 3

    Convert the ultimate and yield strength from MPa to pounds per square inch (psi) by multiplying each by 145. Values in psi are more common than MPa in the US.

    Ultimate tensile strength = 800*145 = 116,00 psi

    Yield tensile strength = 640*145 = 92,800 psi

Tips and warnings

  • Yield strength is the stress at which the bolt will be permanently deformed, while ultimate strength is the stress at which the bolt will break.

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