Load Cells Built for Stress Testing

Stress testing with load cells is an integral part of research, design, and manufacturing processes for various products and components. It helps to ensure that material, equipment, and final products can withstand the stresses they will be subjected to in regular use.

Stress testing with load cells involves applying a known load to a test specimen and measuring the resulting strain. The strain is then used to calculate the stress, which measures the force per unit area.

For destructive stress testing, the test specimen is loaded to failure. The failure load is then used to calculate the ultimate tensile strength (UTS) of the material. In non-destructive testing, the test specimen is loaded to a predetermined stress level and then unloaded. The stress-strain curve is then plotted to determine Young’s modulus and the yield strength of the material.

Selecting the right load cell for any stress testing protocol is important. A detailed review of the sensor’s performance specifications is where to start. Consider the quality of the load cell, along with the materials used to build the testing device and the strain gages.

In designing and building load cells, material composition and build quality play a critical role in the quality, accuracy, and overall lifetime of a load cell. This is especially true when testing involves long, stress-test cycle testing. Interface load cells are designed for optimum fatigue life.

Built for Stress

When looking for a load cell that needs to go the distance over long periods, it’s essential to understand the difference between sensors built for stress and those not. In materials science, the S-N curve is a well-known tool. It is a graphical representation of the number of load cycles required to break a specimen at the range of peak cyclic stress levels.  S-N curves for the high-quality materials used in Interface load cells determine the stress level.

Commonly selected load cells used for high-stress level testing are known as fatigue-rated. Fatigue-rated load cells are designed explicitly for component durability and fatigue test machines where highly cyclical loading is present. These quality load cells resist extraneous bending and side-loading forces.

The table below outlines a load cell strain and safety factor comparison chart, which shows how Interface load cells, including our  1000 Fatigue-Rated Universal LowProfile® Load Cell and 1000 High Capacity Fatigue-Rated Universal LowProfile® Load Cell stack up against generic competitive load cells.

This table compares actual strain levels in Interface LowProfile Load Cells versus generic load cells. The safety factors are a means of visualizing the merit of the various designs. The value of fatigue-rated load cells for fatigue applications is evident from the safety factor data. It is also apparent that Interface load cells with 4 mV/V output have lower stress levels and, therefore, more fatigue resistance than other cells, even though their output is only 3 mV/V or less.

Lower Stress by User Limits

Note that the tests in the safety factor comparison are based on fully reversed load cycles. This type of loading cycle is considerably more stringent than unidirectional loading, which is the more common application of load cells. Suppose a fatigue load cell is repeatedly loaded in only one direction. In that case, the Goodman Law predicts that it can be loaded to about 133% of the bidirectional fatigue-rated capacity with no degradation of its fatigue rating. Conversely, unidirectional loading to a fatigue cell’s rated capacity is much less stressful on the cell than bidirectional. It can be expected to yield a fatigue life well beyond the number of cycles that could be reasonably and economically applied in a verification test program. For additional information on this topic, please refer to Interface’s Load Cell Field Guide under Fatigue Theory.


Fatigue Testing with Interface Load Cells

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Fatigue Testing with Interface Load Cells

Engineers rely on fatigue testing to ensure the safety and reliability of their product designs and structures. By understanding how materials behave under repeated loading, engineers can design components resistant to fatigue failure.

Fatigue testing requires accurate and reliable force measurement. Interface uses ‘fatigue-rated’ as an exact specification that defines a special class of load cell design and construction. Interface fatigue-rated load cells are designed to withstand the rigors of repeated loading, which makes them ideal for even the most demanding high cycle count fatigue testing applications.

In a typical fatigue testing setup, Interface fatigue-rated load cells are attached to the test specimen or the test machine, and the cyclic loading is applied according to the test protocol. The load cells continuously record the applied forces or stresses, allowing engineers and researchers to monitor how the material responds to repeated loading.

By analyzing the data from Interface load cells, researchers and material engineers can determine the material’s endurance limit, fatigue life, and stress-strain behavior. This information is invaluable for optimizing material selection, design, and manufacturing processes to enhance product performance and reliability while identifying fatigue and potential failure risks.

The use of fatigue-rated load cells and data logging instrumentation is necessary for most test and measurement applications, particularly when materials, parts, or assemblies are tested for destruction. This is true because an accurate record of the forces at every moment of the tests is the only way an engineer can analyze the stresses that occurred in the moments just before the ultimate failure. Read more about fatigue testing in our Interface’s Technical Library.

Interface Fatigue-Rated Load Cells

1000 Fatigue-Rated LowProfile® Load Cell

1000 High Capacity Fatigue-Rated LowProfile® Load Cell

1500 Low Capacity LowProfile® Load Cell

1208 Flange Standard Precision LowProfile® Load Cell

Profile of a Fatigue-Rated Load Cell

  • Design stress levels in the flexures are about one-half as high as in a standard LowProfile load cell.
  • Internal high-stress points, such as sharp corners and edges, are specially polished to avoid crack propagation.
  • Extraneous load sensitivity is specified and adjusted to a lower level than in a standard LowProfile load cell.
  • All Interface fatigue-rated load cells have a specified service life of 100 million fully reversed, full-capacity loading cycles.

No one can accurately predict exactly when the failure will occur, nor which part of an assembly will be the weakest link that eventually will fail. This is why high cycle count testing is the best way to measure fatigue life. To read more about fatigue testing and fatigue theory, consult Interface’s Load Cell Field Guide.

Fatigue Testing Applications

Interface fatigue-rated load cells are used in various industries, including aerospace, automotive, civil engineering, and manufacturing. They are used to test various products, from aircraft wings and landing gear to furniture and industrial machinery.

How Interface fatigue-rated load cells are used in fatigue testing:

  • Aerospace: Interface fatigue-rated load cells test the durability of aircraft wings, landing gear, and other aerospace components. This helps to ensure that aircraft can withstand the rigors of repeated takeoffs, landings, and flights. These load cells test the materials used for structures and even rockets.
  • Automotive: Interface fatigue-rated load cells test the fatigue life of engine components, chassis, and suspension systems. This helps to ensure that vehicles are safe and reliable and that they can withstand the stresses of everyday driving.
  • Civil engineering: Interface fatigue-rated load cells test the fatigue resistance of bridges, buildings, and critical infrastructure. This helps to ensure that these structures can withstand the loads they are designed to carry and are safe for the public.
  • Manufacturing: Interface fatigue-rated load cells test the fatigue life of industrial machinery, tools, and consumer products. This helps to ensure that these products are reliable and can withstand the demands of everyday use.

Watch how Interface load cells are used in this bike frame testing application.

Interface has specialized in fatigue-rated load cells and their applications since our founding in 1968. Our LowProfile® fatigue-rated load cells provide up to 100 million duty cycles, and the gaged sensors in every load cell are individually inspected, tested, and certified to meet our rigid performance standards.

It is imperative to choose the right load cell for your fatigue testing application. Load cells come in various sizes and capacities, so it is vital to choose one that is right for your fatigue testing application. Ensure you know the maximum load that will be applied to the load cell, the type of loading, the accuracy requirement, and the environmental conditions for testing. Consult with Interface application engineers to find the suitable load cell for your testing requirements.


CPG Bike Handlebar Fatigue Testing

Interface Specializes in Fatigue-Rated Load Cells

Prosthetics Load and Fatigue Testing App Note

Furniture Fatigue Cycle Testing App Note

Aircraft Wing Fatigue App Note


S-Type Load Cells 101

There are many different types of devices used in test and measurement from load cells to torque transducers and tension links to multi-axis sensors. In addition, there are sub-categories in each of these product types that are based on various specifications, capabilities, capacities, and application requirements.

Discussing load cells specifically, there are different models and configurations depending on the use case, the amount of force measurement or weighing requirements for a particular load, dimensions, and even test environment considerations. No matter what our customers need, we have standard and custom load cells up to the task. In our 101 series, we are highlighting the innovative miniature load cell sub-category of Interface S-Type Load Cells.

What Is an S-Type Load Cell and What Is It Used For?

S-type load cells, sometimes called s-beam, gets its nomenclature from the “S” looking model of the load cell. It is shaped this way because it is designed to measure well-controlled tension and compression forces. There are preferred by engineers and testing labs for the precision, size, material, and ability to fit in limited spaces. They are often used for weighing, in test machines as well as product designs for ongoing performance measurement by OEMs.

An s-type load cell will often be used within a system designed to stress test products in a controlled environment for fatigue and product testing to measure the way the product stands up to force over long periods of time. The benefit of Interface S-Type Load Cells is that they are very cost-effective, highly accurate, easy to mount, and offer flexibility because it can be used universally for tension and compression testing. They are also smaller than typical load cells, providing major benefits when there are limitations in space or for smaller test product dimensions.

Interface has a wide range of specialized miniature s-type load cells including sealed, micro-size, fatigue-rated, high-temperature ratings, low height, overload protected and intrinsically safe to meet all types of testing protocols and plans. You can see all the s-type models here.

An s-type load cell is generally used with eyebolts or rod-ends when used in tension and this can cause binding or the associated hardware to unthread. These uniquely designed load cells should not be used when weighing an object that can sway or rotate. Additionally, an s-type load cell is not recommended when the load cell will be used for both tension and compression, where accuracy in compression is critical. In this case we’d recommend a shear type of load cell.

S-Type Load Cell Applications

Prosthetic Load and Fatigue Testing

Prosthetic limbs must be tested for extreme loading that can occur during falls, accidents, and sports movements. Fatigue testing of prosthetic components determines the expected lifespan of the components under normal usage. Interface suggested a static load test apparatus using SSMF Fatigue Rated S-Type Load Cell attached to hydraulic actuators to apply and measure loads. The fatigue testing machine uses SSMF Fatigue Rated S-type Load Cell to apply and measure cyclic loads. During the fatigue test, the actuator repeatedly applies and removes the force to simulate activity such as walking. Tilt tables may also be used to apply forces at various angles to simulate the heel-to-toe movement of walking or running. Using this solution, engineers can determine whether prosthetic materials and designs will withstand the rigors of daily use and occasional high load situations. Read more here.

Furniture Fatigue Cycle Testing

To meet safety protocols in relation to the manufacturing of various furniture products, fatigue testing, shock testing, and proof testing must be rigorously performed before diffusion into the marketplace. Force testing simulations on furniture products are critical in determining the posted max loads to protect manufacturers from liability due to damages that might result from the misuse of those products and overloading. Using an Interface Model SSMF Fatigue Rated S-Type Load Cell along with Interface Model 9890 Strain Gage, Load Cell, & mV/V Indicator provides a solution that measures the force being applied in fatigue cycle testing of a furniture product, in this case testing the rocking mechanism in an office chair. Unlike other similar load cells, the Model SSMF is fatigue rated making it highly suitable for fatigue testing. No fatigue failure of any fatigue-rated Interface load cell, used within its ratings, has ever been reported. The furniture manufacturer was able to obtain accurate data about the rocking mechanism the office chair as it was fatigue cycled into failure. Adjustments were made to the design to improve the safety and life of the furniture, ensuring product quality and protecting the manufacturer from future liability. Read more here.

Interface S-Type Load Cells are highly effective, accurate and flexible products used for a wide variety of applications needing compression and tension force testing. To learn more about Interface’s S-Type Load Cells, you can also visit here or call us today to speak to an application engineer who can help you select the right product for your next project at 480-948-5555.

Considerations for Fatigue-Rated Load Cells

There are many products that are made to serve a single or limited use. Needs for these types of products, often consumable, are immediate and buying decisions usually require limited research or comparisons. On the other end of the spectrum, there are products that need to be extremely reliable, durable, and built to last a long time. The buying decisions for these types of products often require more research to make certain they are highly rated, safe, and made from sturdy and reliable materials.

How do original equipment manufacturers (OEMs) determine how long a product should last with consistent use? What design decisions need to be made to improve product reliability? The answer is often found in the defined fatigue requirements of a product. In the test and measurement and design phase of a product, answers to these questions frequently require product and component testing with fatigue-rated load cells.

Fatigue is the weakening of a material caused by repeatedly applied loads. Similar to if you repeatedly bend a paper clip, it will eventually weaken and then break. The only way to test fatigue is to put prototypes of the product through stress tests for long periods of time or use an apparatus that tests the lifetime “wear and tear” in a few hours. The tool that reports the data gathered from stress tests is the load cell.

With Interface fatigue-rated load cells, designers and engineers can predict the time and force it will take for wear and tear to take its toll on their products. They can then create safety instructions for customers or redesign the product for better results.

Interface Model 1000 series is a portfolio of fatigue-rated low profile load cells with 300% safe overload, extremely low moment sensitivity of 0.1%/in and higher fatigue life. If you need to measure 1 pound (4.45N) or 1 million pounds (4500kN) of force, Interface fatigue-rated load cells are built for this type of testing.

Interface fatigue-rated load cells are designed for an operational life of in excess of 100 million fully reversed cycles.

It’s also important to note that Interface fatigue-rated load cells are based on fully reversed load cycles, which means they are rated for two directions. This type of loading cycle is considerably more stringent than unidirectional loading. It is the more common application of load cells. If a fatigue load cell is repeatedly loaded in only one direction, it can be loaded to about 133% of the bidirectional fatigue-rated capacity with no degradation of its fatigue rating.

Interface works with customers to provide our fatigue-rated load cells for products that can’t fail. It is our mission to ensure precision products are safe, reliable and durable. If they do fail, it could result in significant harm.

Two examples of products dependent on fatigue-rated load cells for testing are aircraft wings and furniture stability. In aircraft wing testing, load cells test the materials used to build the wings to ensure they are strong and lightweight. The load cells are also used in wind tunnels to test the stress of high winds on the wings over time. Safety protocols manufacturing for furniture products require fatigue testing, seat testing, shock testing, and proof testing. These tests must be rigorously performed before entering the marketplace.

Force testing simulations using fatigue-rated load cells help the manufacturer define max loads in order to protect manufacturers from liability due to damages resulting in the misuse or overloading of their products. These types of applications require the most accurate data available provided by our load cells to ensure that testing results produce a quality product that lasts.

Fatigue-rated load cells are one of the many Interface force measurement tools available to help customers develop quality products. For information on all of the load cells we provide, please visit

Contributor:  Jay Bradley, Sr. Electrical Engineer, Interface