Fastener Material Selection
There is no fastener material that is applicable for every environment. Making the right choice from a wide variety of materials is an important concern. Numerous factors, such as environmental conditions, presence of corrosive elements, physical stress requirements, and overall structural stability can similarly influence material effectiveness.
Most fastener applications are designed to support or transmit some form of externally applied load. More than 90% of all fasteners are made of carbon steel. In general, taking into account the prices for raw materials, non-ferrousone should be considered only when a special application is required.
Another mechanical property associated with standard threaded fasteners is tensile strength. Tensile strength is the maximum tension-applied load a fastener can support prior to or coinciding with its fracture.
Due to this significant consideration must be given to the definition of the tensile stress area. When a standard threaded fastener fails in pure tension, it typically fractures through the threaded portion (this is characteristically its smallest area). For this reason, the tensile stress area is calculated by an empirical formula involving the nominal diameter of the fastener and the thread pitch.
Proof load represents the acceptable strength range for certain standard fasteners. Proof load is an applied tensile load that a fastener must support without permanent deformation. In other words, a bolt returns to its original shape once the load is removed.
Steel possesses a certain amount of elasticity as it is stretched. Proof load is defined as the maximum tensile force that can be applied to the bolt that will not result in plastic deformation. In other words, the material must remain in its elastic region when loaded up to its proof load. Proof load is typically between 85-95% of the yield strength.Yield strength can be defined as the tensile force that will produce a specified amount of permanent deformation (most commonly 0.2%) within a specific fastener.If we continue to apply the load, we will reach the point of maximum stress known as the ultimate tensile strength. After this stage 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. The 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 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 requires a single shear test fixture, the testing technique of threaded fasteners is not as well designed. In most procedures a double shear fixture is used, 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 can use the equivalent tensile stress area.
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 get full advantage of strength properties, the preferred design would be to position the full shank body in the shear planes.
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.
Fatigue is the most common form of fracture of fasteners, accounting for up to 80% of all costs associated with fracture. Fatigue crack initiation and growth occurs when cyclic stresses exceed the fatigue strength of local material for a sufficient number of loading cycles.Fastener material, geometry, stress amplitude,mean stress, and assembly parameters all affect fatigue performance. 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 measure of the ability of a fastener to withstand a twisting load. It is the ultimate strength of a material subjected to torsional loading, and the maximum torsional stress that a material sustains before rupture.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 undergoes 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 is 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.