Author: JJ Blenkhorn, application engineer, powerful application, INSTRON®

Fasteners such as bolts, rivets, and washers play a key role in maintaining the structural integrity of consumer products, vehicles, and structures. Due to the importance of fasteners to the automotive, construction and aerospace industries, ASTM (American Society for Testing and Materials) and ISO (International Organization for Standardization) have established intensive standards to ensure that fastener products meet expected expectations.

Testing the material properties according to ASTM and ISO standards is an excellent indicator of product quality assurance and consumer safety. ASTM and ISO standards are developed and updated in response to market needs and trends in related industries. Both organizations are composed of global experts participating in specific committees designed to solve the problems required by the standard. These experts meet regularly to discuss market needs and whether standards need to be changed or updated to keep up with evolving trends, technology, and quality requirements. 

Two more relevant tensile and shear testing standards for fasteners and fixtures are ASTM F606 and ISO 898. ASTM F606 includes methods and procedures for testing external and internal threaded fasteners, washers, direct tension indicators, and rivets. Similarly, ISO 898 tests the mechanical properties of fasteners made of carbon steel and alloy steel.

Both ASTM and ISO are internationally recognized, but it is important to understand that similar standards between the two organizations may have different requirements. For example, usually two organizations may define different sample sizes or have different tolerances for the pass or fail criteria for any given material.

The mechanical properties of fasteners can be obtained in many ways. However, most of the results were obtained by using a universal testing machine (UTM). By applying tensile, compressive, or shear forces to the specimen, these machines allow us to obtain useful data about the material's maximum force, yield strength, strain, and many other properties.

However, the testing of these standards is not without challenges. This is the result of the extensive requirements outlined by the standards committee. These challenges may include factors such as complex verification load procedures, various test types and settings, and recording test results. 

One of the biggest challenges in testing fasteners is the test specimen verification load. The guaranteed load of the specimen is the force required for the specimen to yield (permanent deformation). Every material has so-called "elastic" regions and "plastic" regions. Before the specimen yields, the material remains in the elastic zone, which means it will spring back to its original length when the load is removed. However, after the specimen yields, it enters the plastic zone, where the specimen undergoes permanent deformation and cannot return to its original position. This can be seen in the stress/strain curve in Figure 1.

  For many industries, it is important to understand how much force the material can withstand before deformation, because the behavior of the specimen after yielding may be unpredictable and easily fractured. Testing to ensure that this verification load is a necessary requirement because it ensures that industrial products can withstand the rigors of their intended applications. By using a tensile testing system and software package that can run a series of controlled ramps, the challenges of verification load testing can be alleviated. 

Both ASTM F606 and ISO 898 require that fasteners must be loaded to the specified verification value and maintained for a period of time. After this, the specimen must be unloaded and verified to ensure that no deformation occurs. To improve efficiency, strain recording equipment can be used in conjunction with software to automatically determine the verification load and record the pass/fail criteria in the exported test report.

Another challenge posed by fastener testing is the various test types required by these standards. For example, ASTM F606 and ISO 898 both have profile tensile and shear tests, in which shear can be divided into single shear or double shear tests. Fortunately, there are some techniques that can improve the efficiency of testing with various fixtures. First of all, it is important to have test fixtures that are compatible with the machine and can be easily interchanged. Another option is to use a dual space system, in which tension clamps and compression/shear clamps can be used at the same time. 

This is possible because the system has multiple crossheads, usually called upper and lower test spaces. Since a large number of samples need to be tested to confirm that the samples comply with international standards, the versatility of the fixture and the dual-space test system are essential for fastener testing. The ability to swap fixtures and/or utilize multiple test space machines can greatly reduce setup time and increase laboratory optimization.

Finally, ASTM F606 and ISO 898 both require a large number of test results documents. All modern general-purpose test systems are capable of recording and exporting the mechanical properties of the tests being performed, but the challenge posed by the documentation requirements is that unique sample information is often required in the report. Some examples of this additional information include manufacturing date, coating characteristics, preparation steps, and many other possible data points. Although historically this has been included in each sample as a separate document or done manually, advanced testing software is able to add custom inputs to the test program. Advanced software can also include the ability to create link calculations based on user input. This feature helps alleviate the need to maintain a large library of methods for each dimension of fasteners.

In addition to verifying loads, fixations, and documentation, there are many other challenges that arise when fasteners are properly tested in accordance with ASTM and ISO standards. In addition, depending on the intended application, other types of testing may also be required. However, it is precisely because of these challenges and requirements that products can be recognized internationally as meeting the required standards to ensure their quality.  

Will joined Fastener + Fixing Magazine in 2007. In the past 14 years, he has experienced all aspects of the fastener industry-interviewing key figures in the industry, visiting leading companies and exhibitions around the world.

Will manages the content strategy of all platforms and is the guardian of the magazine's prestigious high editorial standards.

No account? Sign up

Signing up for fasterandfixing.com enables you to manage your account details.