Ed's Tech Tip: Definition of Fastener Strength

Within the Tech Tips Archives you will find a previous article on The Different Strengths of Hex Cap Screws. In this article I would like to give you the exact definitions of tensile, yield, and proof strengths.

These definitions are from the Industrial Fasteners Institute, Fastener Standards book, sixth edition.

Tensile Strength: Tensile strength is the maximum tension applied load a fastener can support prior to, or coincident with, its fracture. (Force or tension required to break the part when pulled in straight tension) Tensile strengths are normally expressed in terms of stress, pounds per square inch (psi). To convert stress to a load value expressed in pounds , multiply the stress (psi) by the stress area of the thread (sq. in.)

Yield Strength: Yield strength is the tension-applied load at which a fastener experiences a specified amount of permanent deformation. In other words, the material has entered its plastic zone.

Proof Load: Proof load is a tension-applied load that the fastener must support without evidence of permanent deformation. Proof load is an absolute value, not a maximum or minimum. Proof loads are established at approximately 90 to 93 percent of the expected minimum yield strength of the fastener material.

Of course this is only one of the simpler areas that we are reviewing. There can be many problems associated with installation torque and materials being used. I have written these articles to be brief and informative. Every single application must be reviewed individually and tested accordingly.

If you are having problems associated with thread cutting fasteners, I would invite you to call or Email C.I.S. I am sure that we could be of some assistance to you.

Note: All material for this article has been referenced from the I.F.1 Fastener Standards Book, Sixth edition. If you would like to read more on this subject, please refer to pages H-1 to H-78 in the Fastener Standards Book.

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