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. Considering the cost of raw materials, non-ferrous metals should be considered only when a special application is required.
Tensile strength is the mechanical property most widely associated with standard threaded fasteners. Tensile strength is the maximum tension-applied load the fastener can support prior to fracture.
|P = St x As|
|P = tensile load (lbs., N)||St = tensile strength (psi, MPa)||As = tensile stress area (sq. in, sq. mm)|
|Applied to a 3/4-10 x 7" SAE J429 Grade 5 HCS|
|P = ?||St = 120,000 psi||As = 0.3340 sq. in|
|P = 120,000 psi x 0.3340 sq. in|
|P = 40,080 lbs.|
The tensile load a fastener can withstand is determined by the formula P = St x As.
- P = Tensile load - a direct measurement of clamp load (lbs., N)
- St = Tensile strength - a generic measurement of the material's strength (psi, MPa)
- As = Tensile stress area for fastener or area of material (in², mm²)
To find the tensile strength of a particular bolt, you will need to refer to Mechanical Properties of Externally Threaded Fasteners chart in the Fastenal Technical Reference Guide. To find the tensile stress area, refer to the Thread Stress Areas chart also in the Guide.
For this relationship, 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 (as this is characteristically its smallest and therefore weakest 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.
As the fastener approaches the maximum strength of the threaded portion, it will permanently deform. To avoid this risk, most carbon or alloy steel bolts have a defined proof load, which represents the usable strength range for that particular fastener. 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.
The relationship between tension and bolt stretch can be observed on a Tensile Stress-Strain Diagram. To the left is the stress-elongation curve. Steel possesses a certain amount of elasticity as it is stretched. Thus, a bolt that is properly tensioned should be functioning in the elastic range (as viewed on the Diagram). If the load is removed and the fastener is still within the elastic range, the fastener will always return to its original shape.
However, if the load applied causes the fastener to exceed its yield point, it enters the plastic range. At this point, 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 a specified amount of permanent deformation 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 down and elongate further with a reduction in stress. Additional stretching will ultimately cause the fastener to break at the tensile point.
Harder, higher tensile strength fasteners, such as the A574 tend to be less ductile than the softer lower strength fasteners. Although they have higher tensile strength, the overall length of the strain curve is often decreased.
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.
For most standard threaded fasteners, shear strength is not specified even though the fastener may be commonly used in shear applications. While shear testing of blind rivets is a well-standardized procedure that calls for a single shear test fixture, the shear testing technique for 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 (i.e., the variations in test procedures produce non-standard results).
Double Shear Through Threads(1/2-13 SAE J429 Grade 8)
Double Shear Through Body(1/2-13 SAE J429 Grade 8)
|1/2-13 Thread Root Area: 0.126 sq-in||Minimum Body Area 0.191 sq-in|
|60% of Tensile Strength: 90,000 PSI||60% of Tensile Strength: 90,000 PSI|
|Double Shear = 2 x 0.126 sq-in x 90,000 PSI||Double Shear = 2 x 0.191 sq-in x 90,000 PSI|
|Double Shear = 22,680 lbs.||Double Shear = 34,380 lbs.|
To determine the shear strength of the fastener, the total cross-sectional area of the shear plane is important. For shear planes through the threads, we could use the thread root area. There are two possibilities for applied shear load (as illustrated below). One possibility is that the shear plane occurs in the threaded portion of the bolt. Since shear strength is directly related to the net sectional area (i.e. the amount of solid bolt material in the diameter), a narrower area will result in lower bolt shear strength. To take full advantage of strength properties the shank of the bolt body should be within the shear planes. To illustrate, consider the difference in shear strength between two Grade 8 bolts: one with the threads in the shear plane, the other with the shank in the shear plane.
When no shear strength is given for common carbon steels with hardness up to 40 HRC, 60% of the ultimate tensile strength of the bolt is typically used as acceptable shear strength. Note: the shear strength must fall within the constraints of a suitable safety factor. This formula should only be used as an estimation.
A fastener subjected to repeated cyclic loads can break suddenly and unexpectedly, even if the loads are well below the strength of the material. The fastener fails in fatigue. 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. Self-tapping screws and socket set screws require a torsional test to ensure that the screw head can withstand the required tightening torque.
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 (occasionally Vickers). For more information about these hardness tests and their corresponding scales (e.g. HRC, HRB, etc.) see the glossary.
Stainless steel is an example of a very ductile metal. When placed under enough stress, it will elongate significantly before it fractures.
Ductility 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 (e.g. SHCS). 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 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 requires impact testing on certain grades.