Threaded fasteners are the preferred choice for joining two or more components together as compared to welding or adhesives. ln addition to providing a firm connection, fasteners allow quick and easy dismantling if required, particularly if we need to replace components or carry out maintenance operations.
It is important to select the right fastener material based on the tact that a fastener is a mechanical clamp which utilizes the elastic properties of a metal. Under applied loads, a fastener will stretch within limits and return to its original length when the load is removed, however, if the load keeps on increasing, the fastener material will stretch beyond its elastic limit and get permanently deformed. Applying further load will result in the fastener ultimately fracturing and break off.
Tensile Strength: Over 95% of fasteners are manufactured from either carbon steel, alloy steel or stainless steel and the mechanical property most associated with fasteners is Tensile Strength. It is an expression of the maximum load (load in tension) that a fastener can support prior to, or coincident with its fracture. Tensile strength in the inch system is expressed in pound force per square inch (lbf/in2 or psi) and in the metric system in Newtons per square millimeter (N/mm2) or Megapascals (MPa). 1 N/mm2 = 1 MPa.
Tensile Load: is the maximum load (load in tension) that can be applied to the particular fastener prior to or coinciding with its fracture. For Inch fasteners tensile load is denoted in pound force (lbf) whilst Metric fasteners, it is expressed in kilonewtons(kN).
P = St x As
where P = tensile load (lbf or kN)
St = tensile strength of the material (psi or MPa)
As = tensile stress area (in2 or mm2)
Page 37 rajouter les dessins
As= o.7854 (o- o.9~43r
where As = Tensile Stress Area
D = nominal thread diameter
n = threads per inch
As= 0 7854(D-0.9382P)2
where As = Tensile Stress Area
D = nominal thread diameter
P = thread pitch
Example 1 (INCH): To calculate the tensile load of a 1/2 -13 UNC Grade 5 Bolt
(Grade 5 bolts have a tensile strength (St) of 120,000 psi and a tensile stress area(As)of 0.1419in2).
If P = StxAs – therefore P = 120000×0.1419
= 170281bf tensile load.
Example 2 (METRIC): To calculate the tensile load of a M12x 1. 75 Glass 8.8 Bolt
(Class 8.8 bolts have a tensile strength (St) of 600MPa and a tensile stress area (As) of 64.30 mm2).
If P = St x As – therefore P = 800 x 64.30 = 67 440 Newtons
= 67.44 kN tensile load.
Ultimate Tensile Stress: is the theoretical minimum point at which a fastener will fracture.
Elongation : it is a linear stretching of a fastener caused by a tensile load due either to tightening or to an external load.
Proof load: is the load (load intension) that the fastener must support without evidence of permanent deformation. For testing purposes, proof load is an absolute value, and for application design, it is taken as a maximum.
Pre-load: is a measure of the axial load imparted on a fastener. lt’s importance in the fastener industry cannot be underestimated, because almost all critical fastener joints are assigned a pre-load value in order to achieve the correct tension in the fastener. For inch fasteners pre-load is expressed in pounds per square inch (psi) or Newton per square millimeter (N/mm2) in metric terms and is commonly accepted to be 75% of proof load.
Shear Load: is the load that the fastener can support prior to fracture, when applied at right angles to the fastener axis. A load occurring in one transverse plane is know as single shear which attempts to cut the fastener in two pieces. Double shear is the load applied in two planes which attempts to cut the fastener into three pieces. Shear load will vary depending upon whether the shear plane is a cross the threaded or unthreaded portion of the fastener.
Torsional Strength: is the torque a fastener can withstand prior to its failure by being twisted off about its axis. Torsional strength test are usually carried out on self-tapping screws,socket head screws and other fasteners which are tightened (torqued) by the head.
Yield Strength: is the load (load in tension) per unit are a that can be applied to a fastener (specimen) at which point the fastener specimen experiences a specific amount of permanent deformation (0.2%) and moves beyond its elastic limit and into the plastic zone. ln the inch system, yield strength is expressed in lbf/in2 and MPa in the metric system.
Yield Point: is the point at which a fastener in tension moves from the elastic range ta the plastic range.
Fatigue Strength: A fastener is subjected to a variety of mechanical and thermal stresses when torqued in a fastened joint. During this process, it will constantly stretch and return to its original shape. These stretch-and-return actions are called cycles and can happen many thousands or even millions of times a day. Depending upon the intensity of the stresses, the fastener will develop cracks at its most vulnerable point (maximum stress concentration point) and it will ultimately fracture from « fastener fatigue ». Fatigue strength is therefore the maximum stress a fastener can withstand for a specific number of cycles, prior to its failure.
Creep Strength: At elevated temperatures, fasteners, under stress gradually elongate and this stretch increases overtime. Changing to heat resistant materials will often eliminate this problem.
Hardness: is a relative term used to compare the ability of one material to scratch another, without itself being scratched by that material. Hardness tests are used on fasteners because the test is quick, easy and nondestructive and because there is a close corelation between hardness and tensile strength particularly in fasteners manufactured from carbon steels.
Most fastener class standards specify a maximum hardness range. Exceeding the maximum specified hardness couId result in brittleness and hence place the fastener at risk of failure.
Vickers Hardness: (HV) is a test performed on a material to ascertain its resistance to penetration by the test body. The advantage of the HV test is that the entire hard ness range encountered in the fastener range is covered by this method. lt is expressed as HV followed by 3 numbers eg. HV130.
Rockwell Hardness: (HR) testing is somewhat similar to the Vickers Hardness method, in so much as, it too uses the penetration test, however, the results are displayed in scales followed by 2 numbers eg, HRB95, HRC32 etc.
Brinell Hardness: (HB) is expressed by HB followed by 3 numbers eg. HB 124
The hardess Comparasion Table is intented as a guide only. It is applicable to fasteners that are manufatures from Carbon Steel, Cast Steel & Low Alloy Steel.
There may be considerable differences for High Alloy Steel such as A1,A2,A3 & A4 and col formed steels used in grades 4.8 ,5.8 &8.8
The vicker testing machine method is extensively used in fastener because it is applicable over a wide hardness range and is the preferred mehod according to ISO 898-1. Rockwell C is used for hard steels, Rockwell B for soft steel.
|Brinell HB||Rockwell||Tensile Strenght|