Overlooking the behavior of stress relaxation steel is one of the most significant, and quietest, risks a product team can take. It’s a material property that can silently compromise the integrity and safety of your components over their intended service life.
As the people behind the Torontech Isothermal Stress Relaxation Tester, we’ve dedicated ourselves to understanding this process. Here is our professional take on what stress relaxation is, why it carries major business consequences, and how to properly measure it.
So, What is Stress Relaxation in Steel, Exactly?
Think about a brand-new guitar string you’ve just tuned. It’s tight and the note is perfect.
If you leave that guitar sitting for a few months, you’ll pick it up and find the note is flat. The string has lost some of its tension all by itself, even though its length between the two fixed points never changed.
That, in a nutshell, is the relaxation of stress in steel. It’s the slow decrease of internal stress in a material that’s being held at a constant stretch. This becomes a critical issue for components like tendons in prestressed concrete, where consistent tension is the entire point.
Our work shows this process accelerates significantly with heat, turning a long-term consideration into a more immediate design problem.
The Business Cost of Ignoring It: Real-World Consequences
From a practical standpoint, failing to account for the relaxation of stress in steel can lead to significant material failures and commercial liabilities. A slow loss of preload or tension isn’t a dramatic event, but the results can be.
1. Construction & Infrastructure
For pre-stressed concrete strands in bridges and parking garages, a loss of tension can reduce load-bearing capacity below its specified limits.
This phenomenon also explains a common headache (and all you structural engineers out there know exactly what we’re talking about): high-strength bolts in steel frame construction that mysteriously loosen over time, compromising joint integrity.
2. Automotive & Aerospace
These industries are built on components that perform under intense heat and sustained loads. Stress relaxation can affect the clamping force of critical cylinder head bolts, leading to gasket failure, or weaken valve springs, causing a loss in engine performance.
In aerospace, it’s a concern for high-temperature fasteners in jet engines where structural integrity is non-negotiable.
3. Energy Sector
Components inside turbines and reactors operate under high stress for decades.
A classic example is the bolting in high-pressure pipe flanges. If these bolts lose their preload due to relaxation, it can lead to dangerous leaks.
Accurately predicting this effect isn’t just good engineering; it’s a core safety and operational requirement.
A Critical Distinction: Creep vs. Stress Relaxation
In professional discussions, we often see these two terms get tangled, but they describe two very different outcomes. Choosing the wrong test will give you data that is completely irrelevant to the problem you’re trying to solve.
Let’s put it this way: a bolt loosening over time is a stress relaxation problem. A turbine blade slowly stretching and deforming under heat is a creep problem.
- Stress Relaxation: The component’s length is held perfectly constant. You measure how its internal stress decreases over time. Your question is, “Will this bolt lose its clamping force?”
- Creep: The component is subjected to a constant, steady load. You measure how its length increases over time. Your question is, “Will this hanging part stretch out of shape?”
The testing method for each is fundamentally different, which is why a specialized machine like the Torontech Isothermal Stress Relaxation Tester is so crucial.
How to Perform a Stress Relaxation Test The Right Way
Getting reliable data isn’t just about following a manual; it’s about absolute control. Answering how to perform a stress relaxation test correctly means acknowledging the microscopic changes that happen over a very long time. The equipment has to be exceptionally precise.
1. Sample Preparation
It all starts here. The stress relaxation steel sample must be perfectly machined to the exact dimensions specified by standards like ASTM E328. There are no shortcuts. This ensures that your results are repeatable and can be compared with established material data.
2. Environmental Control
This is arguably the most critical stage. The sample is placed inside a high-stability temperature chamber. The “Isothermal” part of our tester’s name is the key—it means constant temperature. If the temperature fluctuates, the sample will expand or contract, and the machine’s sensors would read that as a change in stress, completely polluting your results.
3. Applying the Load
Once the sample is stable at the target temperature, the machine applies a precise initial force, stretching it to the exact starting strain. This is a smooth, highly controlled process designed to mimic the real-world application of force.
4. The Long Haul: Monitoring the Unseen
For hundreds or even thousands of hours, the system’s only job is to hold the sample’s length perfectly constant while measuring the tiny decrease in the pulling force. Our tester’s high-sensitivity load cell and servo-controlled drive work in a constant feedback loop to make this possible, giving you a live stream of data.
5. Turning Data into Decisions
At the end of the test, you get a curve showing how stress decays over time. The real business value comes from using this standard 1000-hour data to confidently extrapolate what will happen over the service life of the stress relaxation steel. This is the predictive data that allows you to design with confidence.
Torontech: Ensure Your Product’s Long-Term Integrity
Our entire philosophy at Torontech is that product failure is not an option. Preventing it requires access to exceptionally accurate testing equipment.
Our Isothermal Stress Relaxation Tester is engineered to deliver the precise, repeatable measurements that global standards demand. We’ve incorporated features that produce better data, like a self-aligning specimen anchorage system and that high-response servo-drive.
What sets us apart is our commitment to making this level of equipment accessible. We provide cost-effective solutions because we believe every company focused on quality should have the tools to achieve it. This machine is built for durability and straightforward operation, ensuring a low total cost of ownership.
At the end of the day, what company can truly afford the cost and reputational damage of a recall caused by material failure? If component reliability is a priority for your team, our experts are available to help. Contact us today for a direct consultation and to review the product specifications. Let’s ensure your products are built to perform for the long haul.
Need Accurate Stress Relaxation Data?
References:
- Rao, G., Gupta, O., & Pradhan, B. (2011). Application of stress relaxation testing in evaluation of creep strength of a tungsten-alloyed 10% Cr cast steel. International Journal of Pressure Vessels and Piping, 88, 65-74. https://doi.org/10.1016/J.IJPVP.2011.02.005
- Pagliarello, A., & Beddoes, J. (2009). The stress relaxation and creep behaviour of a manganese-stabilized austenitic stainless steel. The Journal of Strain Analysis for Engineering Design, 44, 201 – 209. https://doi.org/10.1243/03093247JSA476
- Zhang, S., Chen, W., , K., Jiang, L., Ye, J., & Xu, L. (2024). Creep and stress relaxation experiments of 1960-grade high-strength steel wire at elevated temperatures. Structures. https://doi.org/10.1016/j.istruc.2023.105682
- Guo, J., Zhang, W., & Sun, X. (2012). Stress Relaxation Continuum Damage Constitutive Equations for Relaxation Performance Prediction. Advanced Materials Research, 455-456, 1434 – 1437. https://doi.org/10.4028/www.scientific.net/AMR.455-456.1434
- Zheng, W., Zhu, J., & Yuan, W. (2023). Tempering stress relaxation behavior and microstructure evolution of 300 M steel. Materials Characterization. https://doi.org/10.1016/j.matchar.2023.112688
- Khayatzadeh, S., Tanner, D., Truman, C., Flewitt, P., & Smith, D. (2017). Creep deformation and stress relaxation of a martensitic P92 steel at 650 °C. Engineering Fracture Mechanics, 175, 57-71. https://doi.org/10.1016/J.ENGFRACMECH.2017.02.008
- Pollock, J., Barton, S., & Clissold, R. (1981). The derivation of creep properties from stress relaxation in steels and non-ferrous metals. Materials Science and Engineering, 49, 155-171. https://doi.org/10.1016/0025-5416(81)90150-6
Frequently Asked Questions
1. What is the main cause of stress relaxation?
Stress relaxation is primarily caused by a microscopic process called dislocation climb. At a molecular level, when a steel component is held under a constant stretch (strain), internal defects in the crystal structure begin to slowly move and rearrange themselves to find a lower-energy state. This internal movement effectively reduces the overall stress required to hold that stretch, especially at elevated temperatures where this atomic movement is more active.
2. How long does a stress relaxation test take?
A standard stress relaxation test is a long-duration procedure, typically lasting for 1000 hours. This extended timeframe is necessary because the relaxation of stress in steel is a very slow, time-dependent process. Running the test for this long allows engineers to collect enough data to create an accurate performance curve, which can then be used to reliably predict the material’s behavior over a service life of many years or even decades.
3. Is stress relaxation always a bad thing?
While it is often a critical factor to design against in structural applications, stress relaxation is not always undesirable. In some manufacturing processes, like the heat treatment of bolts or springs, a controlled amount of stress relaxation is intentionally induced. This can help stabilize the component’s internal stresses after forming, reducing the risk of premature failure and improving its overall dimensional stability and long-term performance.
4. What is the difference between stress relaxation and creep?
The key difference between stress relaxation and creep lies in what is held constant. In a stress relaxation scenario, the material’s length (strain) is held constant, and you measure the decrease in internal stress over time, like a guitar string losing tension. In a creep scenario, the load (stress) is held constant, and you measure the increase in the material’s length over time, like a heavy weight slowly stretching a wire.
5. Which materials are most affected by stress relaxation?
While many materials exhibit this behavior, stress relaxation is most pronounced in metals at elevated temperatures, which is why it’s a major concern for steel, aluminum, and nickel alloys used in engines, power plants, and high-temperature industrial equipment. It is also a significant factor in polymers (plastics) and viscoelastic materials, where the effect can be observed even at room temperature. For steel, high-strength prestressing strands and bolts are particularly susceptible.
