July 30, 2021
Most fastener applications are designed to support or transmit some form of externally applied load. If the strength of the fastener is the only concern, there is usually no need to look beyond carbon steel. Over
90% of all fasteners are made of carbon steel. In general, considering the cost of raw materials, non- ferrous should be considered only when a special application is required.
The most widely associated mechanical property associated with standard threaded fasteners is tensile strength. Tensile strength is the maximum tension-applied load the fastener can support prior to or coinciding with its fracture (see figure 1).
Tensile load a fastener can withstand is determined by the formula
P = St x As Example (see appendix for St and As values)
where 3/4-10 x 7” SAE J429 Grade 5 HCS
P = tensile load (lb., N) St = 120,000 psi
St = tensile strength (psi, MPa) As = 0.3340 sq. in
As = tensile stress area (sq. in, sq. mm) P = 120,000 psi x 0.3340 sq. in
P = 40,080 lb.
For this relationship, a significant consideration must be given to the definition of the tensile stress area, As. When a standard threaded fastener fails in pure tension, it typically fractures through the threaded portion (this is characteristically it’s smallest area). For this reason, the tensile stress area is calculated
through an empirical formula involving the nominal diameter of the fastener and the thread pitch. Tables stating this area are provided for you in the appendix.
The proof load represents the usable strength range for certain standard fasteners. By definition, the proof load is an applied tensile load that the fastener must support without permanent deformation. In other
words, the bolt returns to its original shape once the load is removed.
Figure 1 illustrates a typical stress-strain relationship of a bolt as a tension load is applied. The steel possesses a certain amount of elasticity as it is stretched. If the load is removed and the fastener is still within the elastic range, the fastener will always return to its original shape. If, however, the load applied causes the fastener to be brought past its yield point, it now enters the plastic range. Here, the steel is no longer able to return to its original shape if the load is removed. The yield strength is the point at which permanent elongation occurs. If we would continue to apply a load, we would reach a point of maximum
stress known as the ultimate tensile strength. Past this point, the fastener continues to “neck” and elongate
further with a reduction in stress. Additional stretching will ultimately cause the fastener to break at the tensile point.
Shear strength is defined as the maximum load that can be supported prior to fracture, when applied at a right angle to the fastener’s axis. A load occurring in one transverse plane is known as single shear.
Double shear is a load applied in two planes where the fastener could be cut into three pieces. Figure 2 is
an example of double shear.
For most standard threaded fasteners, shear strength is not a specification even though the fastener may be commonly used in shear applications. While shear testing of blind rivets is a well-standardized procedure which calls for a single shear test fixture, the testing technique of threaded fasteners is not as well
designed. Most procedures use a double shear fixture, but variations in the test fixture designs cause a wide scatter in measured shear strengths.
To determine the shear strength of the material, the total cross-sectional area of the shear plane is important. For shear planes through the threads, we could use the equivalent tensile stress area (As).
Figure 2 illustrates two possibilities for the applied shear load. One has the shear plane corresponding with the threaded portion of the bolt. Since shear strength is directly related to the net sectional area, a smaller
area will result in lower bolt shear strength. To take full advantage of strength properties, the preferred design would be to position the full shank body in the shear planes as illustrated with the joint on the right.
When no shear strength is given for common carbon steels with hardness up to 40 HRC, 60 % of their ultimate tensile strength is often used once given a suitable safety factor. This should only be used as an estimation.
A fastener subjected to repeated cyclic loads can suddenly and unexpectedly break, even if the loads are
well below the strength of the material. The fastener fails in fatigue. The fatigue strength is the maximum stress a fastener can withstand for a specified number of repeated cycles prior to its failure.
Torsional strength is a load usually expressed in terms of torque, at which the fastener fails by being twisted off about its axis. Tapping screws and socket set screws require a torsional test.
Other Mechanical Properties
Hardness is a measure of a material’s ability to resist abrasion and indentation. For carbon steels, Brinell and Rockwell hardness testing can be used to estimate tensile strength properties of the fastener.
Ductility is a measure of the degree of plastic deformation that has been sustained at fracture. In other words, it is the ability of a material to deform before it fractures. A material that experiences very little or
no plastic deformation upon fracture is considered brittle. A reasonable indication of a fastener’s ductility
is the ratio of its specified minimum yield strength to the minimum tensile strength. The lower this ratio the more ductile the fastener will be.
Toughness is defined as a material’s ability to absorb impact or shock loading. Impact strength toughness
is rarely a specification requirement. Besides various aerospace industry fasteners, ASTM A320
Specification for Alloy Steel Bolting Materials for Low-Temperature Service is one of the few specifications that require impact testing on certain grades.