What does yield, tensile, reduction of area, proof load, hardness, elongation, and shear mean in the context of fastener strength?
The following terms are measurements of strength that are commonly used in the fastener industry. All ASTM and SAE graded fasteners possess requirements for some or all of these strength characteristics. Our Strength Requirements by Grade Chart will provide a list of these strength requirements for each specification.
Yield
The yield strength or yield point of a material is defined as the stress at which a material begins to deform plastically. Prior to the yield point the material will deform elastically and will return to its original shape when the applied stress is removed. Once the yield point is passed, some fraction of the deformation will be permanent and non-reversible.
Tensile
The tensile strength of a material is defined as a stress which is measured as force per unit area, or the maximum tension-applied load the material can support prior to or coincident with its fracture or rupture. Tensile and hardness are very similar, as chemical composition is a large factor.
Reduction of Area
This is defined as the percent of stretching that is reported in a tensile test. It is the difference between the original cross-sectional area of the a tensile specimen and the smallest area after rupture, and is expressed as a percentage of the original area.
Proof Load
This defined as a tension applied load that a material must support without any evidence of permanent deformation. It represents the useable range of a material before it goes into its “plastic range” where it cannot return to its original size and shape.
Hardness
The hardness as it relates to materials is defined as the resistance to deformation, indentation or penetration by means such as abrasion, drilling, impact, scratching and/or wear. These dynamics are measured by hardness testing to evaluate the link between hardness and chemical composition. Tensile strength and hardness are strongly related.
Elongation
Is defined as the distance a material will stretch lengthwise before rupture and is expressed as a percentage of the original length. It is a measure of the ductility of a material determined in a tensile test.
Shear
Shear is very important for engineering the size and material grade to be used for a fastener. Shear is a force that tends to produce a sliding failure on a material along a plane that is parallel to the direction of the force, or, put another way, it is the maximum load applied to a fasteners axis, that can be supported prior to fracture.