Rob LaPointe | President & CEO
AIM Testing Laboratory
Laboratory analysis for fasteners includes a variety of tests that can be grouped into four broad areas: mechanical, metallographic, chemical, and nondestructive. Nearly all fastener testing required by drawings and manufacturing specifications can be categorized into one of these four major areas.
Unless dealing directly with procurement or quality at your company, you might not clearly understand the common fastener tests that qualify your products to be sold in the marketplace. Of these four areas, mechanical is the most fundamental and will be the focus of this article.
Mechanical properties of a fastener include hardness, ultimate tensile strength, yield strength, torsional strength, and structural performance. The most fundamental property of a fastener is hardness. The hardness of the metal used to make a fastener influences nearly all the mechanical performance characteristics of that fastener.
Hardness is a material’s ability to resist being scratched by another object. It’s a measure of the stiffness of the structures (molecules, crystals, and grains) that make up the metal used to manufacture screws and bolts. Generally, smaller grain means harder metal.
Hardness is measured by pushing a very hard object, typically a diamond, into the metal under controlled conditions and seeing how much the metal being tested deforms.
- More deformation means the metal has less resistance to changing shape and lower hardness.
- Less deformation means the metal has more resistance to changing shape and higher hardness.
The most common scale used to measure hardness in metals is the Rockwell C scale. Typical HRC (Hardness Rockwell C) values range from 20 HRC for soft metal, such as low-carbon steel, to 60 HRC for high-carbon or tool steel.
Tensile strength and yield strength are intimately related. They’re both measured when a fastener is pulled along its length, reproducing what happens to it when tightened into an assembly. The fastener is gripped by its head and thread and pulled in a tensile frame machine.
The tensile frame measures the amount of force applied to the fastener and the amount of stretch the metal experiences as a result of the tensile (pulling) force. As the metal is pulled, it stretches like a rubber band. If the tensile force is released, the metal relaxes and returns to its original shape. This is known as elastic or non-permanent stretch.
At a certain load, depending on the type and hardness of the metal, the material deforms permanently. In other words, if the tensile force is released after this critical load, the metal will no longer return to its original shape. The load value that causes the metal to stretch permanently is known as the yield strength. Yield is achieved because at the yield strength, molecular bonds in the metal begin to rip apart and relocate, forming new bonds to other adjacent molecules.
If the load on the fastener is allowed to increase continually, the bonds between the molecules will continue to break and rearrange as the material gets thinner. Eventually, it will reach its ultimate thinness and new bonds will not be able to form, resulting in complete separation of the material. The force at which this happens is known as ultimate tensile strength.
Think of stretching a piece of taffy. The taffy will become thinner and thinner as the pulling force is applied until it has reached its thinnest possible state, then it will separate into two pieces for you to share with a friend — it’s much the same as a bolt, but tastier.
Torsional strength is the fastener’s ability to withstand being twisted. Most fasteners are tightened by twisting them. The thread on a fastener is like an inclined plane or ramp, wrapped around a solid bar. By twisting a threaded fastener into a corresponding threaded hole, the fastener becomes tensioned by running the ramp of one thread up the ramp of the other thread.
A fastener’s ability to gain increased tension is related, in part, to its torsional strength and tensile strength. Torsional strength is measured by holding the fastener firmly, preventing it from rotating and applying a torque (force to rotate an object) until the fastener eventually yields and breaks by twisting it in two.
The torsional strength of a fastener is most critical at the head-body junction and in the portion of the fastener between the head and the first engaged thread. These are the two areas that fasteners would typically fail due to insufficient torsional strength. Torsional strength is measured in units of torque such as the pound-inch (lb-in) or Newton-meter (Nm).
Structural performance is the least common mechanical test for fasteners. It could include head ductility and recess torque. Head ductility tests the ability of the head-body junction to withstand bending without failure. Typically, a fastener should be able to withstand a 10-degree bend between the body and the under-head surface without cracking or failure.
Recess torque is the limiting torque that can be applied to the drive recess of a screw. Common drive recesses are sockets (hex), six-lobe (Torx), and cruciform (Philips). In this test, the screw is prevented from turning and a measured torque is applied with an end-load pushing the drive bit into the drive recess.
The drive recess must handle a minimum torque value without stripping or deforming to satisfy the requirement of the standard.
The mechanical tests outlined here are the most common tests for fasteners. There are many more tests that analyze other mechanical performance features, but most fastener drawings or manufacturing standards require, at least, hardness and tensile verification.
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