Definition of Mechanical Properties for Screws

Tensile strength determines how much axial load a screw must withstand without breaking. If full-sized screws are being tested, the yield strength can only be approximately established. Under ISO 898 Part 1, the exact yield strength and elongation after fracture can be determined using machined specimens. Exceptions are stainless steel screws A1 to A4 (ISO 3506).

Yield strength is the amount of resistance of a material to plastic deformation. In general terms, yield strength determines how much stress a screw (specimen) must withstand without being permanently elongated. This applies to relatively soft materials.

The yield point of somewhat harder materials is not sharply pronounced. It is then replaced by the stress at which the permanent elongation is 0.2%. In practice, neither the working load of the fastener nor the stress from tightening should exceed the yield strength or the 0.2% limit.

Elongation at fracture A [%] is the permanent elongation, as measured on a fractured specimen, in relation to the original measured length. Exceptions are screws A1 to A4, where this is measured on full-sized screws (ISO 3506).

Hardness is generally the resistance of the material to penetration by a test body. There are different tests to determine the hardness of screws:

Vickers hardness HV: ISO 6507

Test body: pyramid (encompasses the complete hardness range usual for screws)

Brinell hardness HB: ISO 6506

Test body: ball

Rockwell hardness HRC: ISO 6508

Test body: cone

The advantage of the Vickers hardness test is that the entire hardness range encountered in the screw is covered by the method. For details, see ISO 898 Part 1.

Want to convert hardness values? Our online hardness conversion tool helps you convert hardness-to-hardness or hardness-to-tensile-strength values for unalloyed and low-alloy steels and cast iron.

Notch impact energy is the impact energy consumed during notch impact testing. A notched sample is taken from the screw near the surface. This sample is broken in a pendulum impact tester with a single stroke. It gives information on microstructure, the steel-making process, inclusion content, etc. The values cannot be used for calculations.

Decarburization is a loss of carbon at the surface of ferrous materials (steels). For details, see ISO 898 Part 1.