Steel fasteners come in a variety of materials, shapes, sizes and costs. Understanding their mechanical properties is useful to determine the best fastener for a specific application. We think it's an interesting topic, and we hope you do too. At Bolt-In Co on the Sunshine Coast, we have a large range of high-tensile, stainless steel and galvanised fasteners. Join us for this brief review of the the mechanical properties of fasteners, and don't hesitate to contact us if you have any related enquiries.

Tensile Strength

Tensile strength is an important property to begin with. It refers to the maximum amount of stress that can be withstood by a fastener before failure when axial tension is applied. This refers to the type of tension where the fastener is pulled in two opposite directions along its length. The tensile strength is how much tension it can take before it breaks. Tensile strength is measured in PSI (Pounds per Square Inch) or Mpa (Megapascals), depending on where you are in the world. To get an idea of the forces at work, a low strength plastic fastener may have a tensile strength of only 1,000 to 5,000 psi. A high-tensile grade 12.9 fastener (exceptionally strong) may have a tensile strength of up to 180,000 psi.

Yield Strength

Whereas tensile strength (above) refers to the maximum stress or load a material can withstand before breaking (when subjected to axial tension), yield strength measures the amount of stress that the fastener will tolerate before it permanently deforms to a specific degree. Deformation has two stages: elastic (the fastener will return to its original shape after the load is removed) or plastic (permanently changed). Yield strength indicates the load limit before plastic deformation occurs. Like tensile strength, yield strength is measured in psi or mpa. Yield strength may range from 100 psi (plastics, rubbers) to 150,000 psi for some advanced steel alloys.

Shear Strength

Shear strength is another important mechanical property of fasteners. It refers to the maximum amount of stress a fastener can withstand when subjected to oppositional parallel forces applied to the cross-sectional area of the fastener. The shear strength is calculated by dividing the force at which the fastener fails by the cross-sectional area of the fastener being tested. Shear strength may range from 100psi (plastics, rubbers) to 50,000 psi or higher for high-strength steel allows used in structural and automotive applications.

Hardness

Fastener hardness is measured with a variety of different tests, such as Rockwell Hardness Testing, Brinell Hardness Testing, Vickers Hardness Testing and others. You would think hardness would always be useful, but too much hardness isn't always a good thing. Materials that are too hard may be brittle, whereas materials that are too soft may lack the required strength. Like all things with fasteners, it's about an optimal compromise between properties, applications, costs and codes. 

Fatigue Strength

Fatigue strength is used to measure the performance of fasteners under repeated loading and unloading cycles. In this case, the test is concerned with understanding how well the bolt will do in applications of variable stress. Examples include fasteners for aircraft or automobiles. In fatigue strength testing, loads are applied, removed and re-applied in a controlled manner to create a fatigue curve, expressing the applied stress and the number of cycles to communicate the fatigue strength of the fastener.

Ductility

Ductility is used to describe the ability of a bolt to undergo plastic deformation without fracturing - in other words, how much a bolt can deform plastically when subjected to tensile loads. Ductility is expressed as a percentage of elongation or reduction in the area of the material being tested. This is different to yield strength (above) which measures the amount of load a fastener can take before it undergoes plastic deformation.

Corrosion Resistance

Corrosion resistance is an important consideration when assessing fasteners. We're located on the Sunshine Coast which includes marine environments where the ability of nuts and bolts to withstand corrosion is vital. Various coatings and materials can enhance a bolt's resistance to corrosion. For example, stainless steel 316 has a high degree of corrosion resistance, making it suitable for marine applications.

Modulus of Elasticity

The modulus of elasticity (Young's Modulus) measures a fastener's stiffness or ability to deform elastically in response to an applied stress.

Environmental Resistance

Some fasteners are designed to withstand extreme environmental conditions, such as high or low temperatures, exposure to chemicals, or UV radiation.

Torsional Strength

Torsional strength is the ability of a fastener to resist twisting forces without deforming or breaking. This property is essential for threaded fasteners like bolts and screws.

Vibration Resistance

Fasteners used in applications prone to vibration should have the ability to resist loosening or self-loosening over time.

Ease of Installation

Ease of installation includes properties like thread engagement and the availability of self-locking features to prevent loosening under load. 

What's the Most Important Mechanical Property if a Steel Bolt?

The mechanical properties of fasteners create a composite picture of the capabilities of a given fastener. The most important mechanical property is the one required for the application.  For example, high tensile strength may not be as relevant in carpentry as it is in aircraft or automobile manufacture.

For assistance with your nuts and bolts on the Sunshine Coast QLD, please contact our team for more information and assistance with high-tensile, stainless steel, and galvanised fasteners.

Bolt grades serve the important purpose of communicating the mechanical properties of bolts. In this way, properties of fasteners such as strength, safety, reliability, consistency, and cost-effectiveness can be matched to required applications to achieve an optimal result. Like all things with nuts and bolts, things can be a little complex! In this article, we review bolt grade systems - what they are, how they work and how they differ from each other. Let's check it out!

Bolt Grades Overview

Bolts come in many different shapes and sizes. They're made of a variety of materials for a range of purposes. To understand their capabilities and determine the applications they are suitable for, they are assigned grades, which are like ratings that explain their properties.

The grades they are assigned come from standards (or systems). These standards are not identical - they are developed by different nations and industries to evaluate and present information about fasteners. Examples of different standards include:

ISO (International Organization for Standardization)
ASTM (American Society for Testing and Materials)
AS (Australian Standards)
DIN (Deutsches Institut für Normung)
BS (British Standards)

When you're talking about bolt grades, it's important also understand which standard it relates to, in order to properly understand what it means.

What do Grades Indicate?

Bolt standards help engineers and builders make informed decisions about which fasteners to use in different situations. Different standards provide different sets of information about fasteners, but generally they are concerned with strength, tensile strength, yield strength, materials, application suitability, performance under load, durability compatibility, and safety.

Which Grade is Best?

With fasteners, it's important to understand that suitability is more important than cost. In other words, you don't need the most expensive fastener - you only need the fastener which is mechanically suitable for the application. Since grades describe the properties of fasteners, the best grade is relative to the application.

Which Standard Should I Use?

Standards are not uniform in their conclusions. They are all essentially trying to resolve the same problem (the correct fastener to use, and how to use it) in ways that reflect the local (or industry-specific) practices and regulations, and heritage of practice. Accordingly, it's important to refer to the most current and relevant standards to ensure proper selection, installation, and performance of fasteners - it will depend on what you're working on and where you are working on it. If you're not sure, simply ask our team. We are here to help!