One Structural bolted joints rarely fail because the screw is not strong enough.. In many cases, the root cause lies elsewhere: insufficient preload, excessive variation in friction, inconsistent tightening conditions, or a discrepancy between the torque applied during assembly and the actual tightening force achieved.
This difference is becoming increasingly relevant as manufacturers, contractors and certification bodies place greater emphasis on reliability of the connections, Traceability and compliance with international standards.
For decades, Tests of mechanical properties They have formed the basis for ensuring the quality of the fixing elements. The tensile strength, the load test, the elastic limit and the elongation remain essential indicators of the product quality. However, these values only describe the behavior of the fixing element as an individual component.
In service, the screws do not work in isolation. They function as part of a complete assembly consisting of the screw, nut, washer, coating, lubrication, and installation method. It is the behavior of this assembly—and not just the screw—that determines whether The union will fulfill its function throughout its entire lifespan.
The hidden variable: the friction
One of the major challenges in designing bolted connections is that the applied torque does not directly correspond to the preload obtained.
A large part of the torque applied during tightening is consumed by overcoming the friction between the threads and the support surfaces. Only the remaining portion transforms into useful tightening force.
As a consequence, two fasteners with identical mechanical properties can generate very different preload levels when tightened with the same torque value. Small variations in lubrication, surface coating, the condition of the threads, or the quality of the connected components can significantly influence the relationship between torque and tension.
From an engineering perspective, this means that achieving the specified tightening torque does not necessarily guarantee that the desired preload has been achieved. In critical applications, this uncertainty can have significant consequences.
Insufficient preload can cause the joint to loosen, fatigue-related damage and vibrations-induced failures. Excessive preload can cause plastic deformation, damage to the threads, or premature failure of the components.
In both cases, the fastener element can fully meet the specifications of the material while the assembled joint does not provide the expected performance.
Why the standards go beyond the testing of the material
This challenge is reflected in the modern standards that regulate structural bolted joints. The standard EN 14399, For example, it requires verifying the overall behavior rather than relying solely on the mechanical properties of the individual components.
The standard recognizes that the behavior of the preload depends on the interaction between all the elements of the joint and therefore requires validating the par–preload ratio under controlled conditions.
The objective is simple: to demonstrate that a given assembly is capable of repeatedly generating the required tightening force when installed in accordance with specifications.
This represents a change in approach within the fastener industry: moving from component verification to system characterization.
Instead of asking whether a screw meets tensile strength requirements, manufacturers need to increasingly demonstrate how the entire assembly behaves during assembly.
Understanding what happens during the tightening process
To correctly assess the behavior of the ensemble, lEngineers need more than just a simple result of being fit or not fit.. The most valuable information is obtained by simultaneously measuring the torque, the tightening force, and the rotation angle during the entire tightening process.
This approach generates a complete torque–angle–tension curve that shows how the assembly behaves from the start of engagement until reaching the final preload.
The data obtained allows us to analyze aspects such as:
- The behavior of the friction during tightening.
- The uniformity of the thread engagement of the threads.
- The development of preload throughout the rotation.
- The onset of flow and plastic deformation.
- The repeatability between different samples.
- The efficiency with which the applied torque is converted into tightening force.
For manufacturers that They supply critical sectors such as construction, infrastructure, energy or heavy industry, This information strengthens both quality assurance and customer confidence. Additionally, it provides traceable evidence for certification programs and technical documentation.
The growing importance of trackable data
As quality requirements increase, traceability has become as important as the test itself.
Many laboratories continue to use reliable mechanical equipment that has been in service for more than a decade. However, the control systems and the reporting software They can become a limitation when customers require complete digital records, historical traceability, and documented evidence of compliance with applicable standards.
When a claim arises, the ability to prove exactly How a group performed during their qualification test It can significantly reduce the time of research and uncertainty.
In the same way, for manufacturers that supply projects where Compliance with EN 14399 is mandatory, Having complete and traceable records is often an indispensable requirement for approval.
Therefore, the industry is increasingly placing importance on testing systems capable of recording the complete behavior of the assembly and not just the maximum values achieved.
Characterizing the actual behavior of a bolted joint
To meet this need, specialized systems have been developed. torsion test and preload that allow measuring the interaction between the pair, the tension and the angular displacement under controlled laboratory conditions.
An example it is the torsion testing system MT 5000 N/m, Made by Servosis in Madrid (Spain). Designed for structural fixing elements from M12 to M36, the system simultaneously records torque, tightening force and rotation angle during the entire test, generating a complete characterization of the assembly’s behavior.
The objective is not simply to determine whether a fastening element passes the test, but to understand how the assembly behaves and verify that it can repeatedly achieve the preload required under specified conditions.
For manufacturers operating in increasingly demanding markets, this level of knowledge constitutes a valuable competitive advantage.
Looking to the future
As structural applications require higher levels of reliability and documentation, the industry is moving away from the traditional view of testing fixing elements.
Mechanical properties remain fundamental, but they are no longer sufficient on their own.
The key question is no longer solely whether the screw meets the specifications.
The real question is whether the complete assembly will be able to provide the required preload, maintain the integrity of the joint, and offer consistent performance throughout its entire lifespan.
Answering that question requires testing the whole system, not just the screw.
About the author
Servosis designs and manufactures testing equipment for fastener manufacturers, certification bodies and quality laboratories worldwide. With more than three decades of experience in material testing systems and fastener elements, the company helps its customers meet increasingly demanding requirements in terms of quality, certification and traceability.
For more information about the pre-load tests, compliance with the EN 14399 standard or the torsion testing system Servosis MT 5000 N/m, contact us.
