Testing the durability of fasteners is a crucial aspect of quality control and engineering across various industries. Fasteners, such as screws, bolts, nuts, and rivets, play a fundamental role in securing structures, machines, and products together. Ensuring the longevity and reliability of fasteners is paramount for the safety, performance, and integrity of countless applications, from aerospace and automotive engineering to construction and consumer goods manufacturing.

Fasteners endure a wide range of challenging conditions during their service life, including mechanical stresses, temperature variations, corrosive environments, and dynamic loads. Without proper testing, the reliability of fasteners in these conditions may be uncertain, potentially leading to costly failures, safety hazards, and product recalls.

This blogpost explores the significance of evaluating the long-term performance of fasteners, the various types of tests and methodologies employed to assess their durability, and the critical role these testing compliance fasteners play in ensuring the reliability and safety of products across diverse industries.

We will delve into tension and shear tests, hardness test, and corrosion resistance assessments, shedding light on how these tests enable engineers and manufacturers to make informed decisions about fastener selection and the importance of implementing premium fasteners and premium anti-corrosion coating in your projects.

Dimensional testing of fasteners is a critical quality control process that ensures fasteners are manufactured in accordance with the correct size and dimensional requirements. Proper dimensions are essential to ensure that fasteners can be correctly installed and perform their intended functions in various applications.

Here's how a Dimensional Test is conducted:

1. Measuring Equipment: High-precision measurement tools and equipment are used for dimensional testing. Common tools include micrometres, callipers, thread gauges, optical comparators, and coordinate measuring machines (CMMs).
2. Key Dimensions: The specific dimensional parameters that are measured or checked can vary based on the type and purpose of the fastener and may include: Length, Diameter, Thread Pitch, Head Diameter, Thread Length, Drive size etc
3. Measurement Process: Skilled technicians or quality control personnel use the measuring tools to obtain accurate readings of the selected dimensions. Multiple measurements may be taken at various points on each fastener to ensure consistency.
4. Tolerance Evaluation: The measured dimensions are compared to established industry standards, customer specifications, or design tolerances as specified in the technical drawing to determine if the fasteners meet the necessary dimensional requirements.
5. Quality Assessment: Fasteners that fall within the acceptable tolerance range for each dimension are considered suitable for use. Those that exceed specified tolerances shall be rejected and purged out accordingly.

Fasteners' hardness refers to their material’s ability to resist plastic deformation when subjected to indentation. Fastener Hardness Test helps evaluate the resistance ability against wear, abrasion, deformation when load is applied.

While tensile tests are the best method for verifying minimum tensile strength requirements, they are not practical for small parts. In such cases, hardness testing provides an accurate estimate of a fastener's tensile strength. Additionally, hardness testing is efficient and quick.

Hardness testing involves pushing a uniquely shaped indenter with a precisely defined force into the material. These testing methods are categorized into two groups: macro hardness testing and microhardness testing.

Macro Hardness Tests: Macro hardness testing is carried out using large indenters and substantial loads, resulting in visible indentations that can be seen by the naked eye. Conversely, Micro Hardness testing employs light loads and small indenters, creating indentations that are not visible to the naked eye.

Macro hardness testing is typically used for general-purpose assessments and encompasses various common test methods, including Rockwell B and C, Macro Vickers, and Brinell. In these tests, both the indenter's size and the applied load are relatively large, leaving discernible marks.

Micro Hardness Tests: Micro hardness testing assesses delicate or extremely thin sections, such as the outer case regions of surface-hardened fasteners. These case-hardened sections are exceptionally thin, and the forces applied in Macro Hardness Tests would be excessively harsh, causing the indenter to penetrate the area of interest, leading to inaccurate measurements.

Micro hardness testing is frequently employed in experimental research scenarios where multiple groupings of test sites within compact areas are required. Micro Hardness Tests employ light loads, and the resulting indentations will be put under magnification for analysis.

Some common Hardness Testing Methods includes:

1. Rockwell Hardness: Within the Rockwell scales, ranging from A to G, steel fasteners are exclusively examined using either the Rockwell B or Rockwell C scales. The Rockwell B scale employs a 1/16" diameter round ball and a 100kgf load, while the Rockwell C scale utilizes a 120-degree Diamond Cone and a 150kgf load. Naturally, higher loads are more effective at indenting harder components, leading to more precise hardness measurements. Consequently, the Rockwell B scale is suitable for annealed and untreated parts, while the Rockwell C scale is employed for heat-treated components.
2. Vickers Hardness: The Vickers Hardness Test is known for its simplicity since the calculations are not influenced by the indenter's dimensions. Remarkably, the same pyramid-shaped diamond indenter can be universally applied, regardless of the material's hardness. The Vickers Test Results yields a Vickers pyramid number (HV), commonly employed with lighter loads akin to micro hardness testing, although it can accommodate more substantial loads, spanning from 1gf to 100kgf, in macro-scale testing.
3. Knoop Hardness: The Knoop hardness is a microhardness test that employs a pyramid-shaped diamond indenter and applies a 100gf load. Due to its indenter's specific geometry, the Knoop test is particularly well-suited for assessing extremely brittle materials or thin layers. Similar to the Vickers test, the Knoop test measures the indentation's dimensions; however, unlike Vickers, which considers both diagonals, Knoop calculations rely solely on the long side of the indentation.
4. Brinnell Test: The Brinell indenter features a significantly large 10mm diameter ball and is subjected to a 3000kgf load. Subsequently, the resultant indentation is quantified and processed through an equation to determine the resulting Brinell Number.

Hardness Testing is typically utilized either as an in-process step during heat treatment of parts or as a final inspection tool, encompassing heat treatment processes like strengthening quench and temper, case hardening, or annealing. Quench and tempered parts will typically be assessed with the Rockwell C or macro-Vickers testing, while case-hardened fasteners will require microhardness methods such as Knoop or micro-Vickers. Annealed fasteners, on the other hand, are commonly assessed using the Rockwell B test.

Tensile test, also known as tension test, is a fundamental mechanical test used to determine the mechanical properties of materials, particularly their behaviour under axial loading (stretching or pulling).

The primary purpose of a tensile strength test is to measure how a material responds to an applied force and to gather information about the fasteners’ strength, ductility, and other mechanical characteristics. This information is essential for evaluating the fastener’s strength and quality control, ensuring the fasteners meet specific standards and performance requirements for their intended applications.

Here's how a typical tensile test is conducted:

1. Mounting: The fastener is securely mounted in a testing machine, such as a universal testing machine (UTM). The UTM has two grips, one at each end, to hold the fastener.
2. Loading: The machine applies an axial load or force to the specimen, typically in the form of tension, pulling the specimen in opposite directions. The force is gradually increased at a constant rate.
3. Recording Data: During the test, various parameters are recorded, including the applied force (load) and the corresponding deformation or elongation of the fastener. This information is used to create a stress-strain curve.
4. Analysis: The test generates a stress-strain curve, which shows how the stress (force per unit area) applied to the material changes as the strain (deformation) increases. From this curve, several mechanical properties can be determined, including:
   • Yield Strength: The stress at which the material begins to deform plastically.
   • Ultimate Tensile Strength: The maximum stress the material can withstand before breaking.
   • Elongation: The percentage increase in length of the specimen at the point of failure.
   • Young's Modulus: A measure of the material's stiffness or elasticity.
   • Fracture Point: The point on the curve where the material eventually fails.

Tensile testing is widely used in engineering and various other fields to assess the mechanical properties of various materials. Tensile strength is a fundamental property of fasteners, measuring their ability to resist axial forces.

In this test, fasteners are subjected to increasing axial loads until failure occurs. Understanding the point of failure helps determine the load a fastener can withstand without breaking, aiding in material selection and design. Tensile testing is a basic mechanical measurement to detect how a material resists loaded tensile forces until it fractures.

Shear forces can be equally as important as axial loads in some applications. Shear strength tests assess a fastener's resistance to forces that would cause it to slide or shear along its axis. This is crucial in structures where lateral forces are prevalent, such as in trusses or joints.

A shear strength test is a mechanical test used to determine the shear strength of a material, which is a measure of its resistance to forces that cause one layer of the material to slide parallel to an adjacent layer. This type of test is particularly important in engineering and materials science to assess the ability of a material to withstand shearing or sliding forces, which are common in various structural and mechanical applications.

The torque applied during fastener installation can significantly affect its performance. These tests measure the relationship between torque and axial tension to ensure consistent preload and prevent over-tightening, which can lead to fatigue failure.

A torsional test, also known as a torque test, is a mechanical test used to assess the torsional strength and behaviour of fasteners such as screws, and bolts. This test is particularly important in applications where fasteners are subjected to twisting or rotational forces, as it helps ensure the reliability and performance of these components.

Here's how a typical torsional test is conducted:

1. Fixture Setup: The fastener is placed securely in a testing fixture that allows one end of the fastener to rotate while the other end is held stationary. The fixture is designed to apply a controlled and precise amount of torque to the fastener.
2. Torque Application: A twisting or rotational force (torque) is then applied to the fastener. The torque is applied gradually and at a constant rate until the fastener either reaches a specified limit (e.g., a predetermined torque level) or fails.
3. Analysis: The primary objective of the test is to determine the fastener's torsional strength, which is the maximum torque it can withstand before failure

Torsional testing is essential for fasteners used in applications where twisting or rotational forces are significant, such as in machinery, automotive components, and aerospace structures. It helps engineers and manufacturers select fasteners that can safely transmit torque and rotational loads without experiencing premature failure or deformation.

These tests assess a fastener's resistance to being pulled out of a substrate. Pull-out tests provide insights into fastener performance when subjected to forces perpendicular to their axis. A pull-out test for a fastener, such as a screw or anchor, is a mechanical test used to assess the fastener's ability to resist axial loads and remain securely anchored within a substrate, typically concrete, wood, or a similar substrate.

This test helps determine the fastener's pull-out strength, which is essential for structural integrity and safety in construction and engineering applications.

Here's how a pull-out test for fasteners is conducted:

1. Preparation of the Substrate: Depending on the material in which the fastener will be anchored (e.g., concrete, wood), the substrate is prepared by pre-drilling holes or creating slots at the desired depth and diameter to accommodate to the fasteners.
2. Installation of Fasteners: The fasteners are secured into the substrate through a pre-drilled hole or drilled through into the substrate, making sure they are installed correctly and securely. The depth of insertion should match the intended use or design specifications.
3. Testing Machine Setup: The load cell or appropriate grips are attached to the fastener and the testing machine.
4. Loading: Tension is gradually applied to the fastener in an axial direction, simulating the force the fastener would experience during its intended application. The load is applied at a constant rate until the fastener either pulls out of the substrate or fails in some other way.
5. Data Collection: During the test, the data is monitored and recorded, including the applied force (load) and the corresponding displacement (movement) of the fastener. This information is used to create a load-displacement curve.
6. Analysis: The primary objective of the test is to determine the pull-out strength of the fastener. This is typically defined as the maximum load that the fastener can withstand before it fails or pulls out of the substrate.
7. Quality Assessment: Compare the test results to industry standards, design specifications, or safety requirements to determine if the fasteners meet the necessary strength criteria. This helps ensure that the fasteners will perform reliably in their intended applications.

Pull-out tests for fasteners are essential in construction, engineering, and various industries where fasteners are used to secure loads and structures. The results of these tests help engineers and builders make informed decisions about fastener selection and installation methods, ultimately contributing to the safety and stability of structures and systems.

Fastener tests are an essential process in the engineering and manufacturing industries to assess the quality, performance, and reliability of fasteners like screws, bolts, and nuts. These tests serve several critical purposes, like Quality Control, Safety, Performance Assessment, Durability, and Compliance. Fastener tests help ensure that fasteners meet specific standards and quality criteria, reducing the risk of product failures due to subpar components.