Fastener Material Selection
There is no one fastener material that is right for every environment. Selecting the right fastener material
from the vast array of materials available can appear to be a daunting task. Careful consideration may need
to be given to strength, temperature, corrosion, vibration, fatigue and many other variables. However, with some basic knowledge and understanding, a well thought out evaluation can be made.
Mechanical Properties
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.
Tensile Strength
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.
Proof Load
Figure 1 Tensile Stress-Strain Diagram
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
1
further with a reduction in stress. Additional stretching will ultimately cause the fastener to break at the tensile point.
