Getting tired: potential fatigue failure in load cells
Load cells are extremely hard-working and robust tech, designed to be in continual use for extended periods of time, with the minimum of maintenance and care required. With regular recalibration, your load cells can last for years even in challenging conditions. However, even load cells can become fatigued through rapid and repetitive loading and unloading, due to their metal construction.
Metal fatigue
All metals are subject to fatigue when they are subject to a load that is more than their intrinsic yield strength. That’s why you can’t snap a wire coat-hanger in half in one go, but you can break the wire at a single point through repeated and rapid bending back and forth through 180 degrees.
To help you account for this, all load cells have an overload rating, so you know how much load they can withstand. For example, our 800 Series low capacity Cantilever Beam load cells are single point cells designed for process weighing applications. Their over load characteristics are 150% Safe Overload and 200% Ultimate Overload.
Rapid reversals
Repetitive and rapid loading and release can also cause metal to fail, even if a single load does not exceed the ultimate overload rating figure. Metal fatigue can happen in load cells when a force is applied and reversed rapidly over a considerable period of time. Also, the load that already fatigued metal can withstand usually decreases with each new cycle of loading and unloading, weakening it further.
This ‘fatigue failure’ is because even the highest grade metals are not a ‘completely’ uniform structure. The crystals within a metal can have tiny defects or weakness at a slip plain. Repetitive loading and unloading puts pressure on these defects, which cause the metal not to return fully to its unloaded state once a load is removed.
Over time, this area can develop a plastic deformation. These deformations can result in a microscopic crack, which spreads as more stress is applied. This can build into an unexpected, sudden fracture. Signs that your load cell may have exceeded its fatigue life may include:
- zero point signal drift
- interrupted signals
- external signs of damage or wear
Concerned your load cell might be suffering from fatigue? Need to replace a multiple load cells? Call us to discuss your requirements.
The S-N curve
An S-N curve can be used to predict the number of load cycles a load cell may endure, depending on its structure and type of metal used in its construction. It’s important to remember that an S-N curve is based on cycles that are ‘fully’ reversed over a period of time, such as in endurance testing. A load cell under unidirectional loading is less likely to suffer from fatigue failure than other types.
For example, our 340 High Capacity Series load cells is designed for compression only applications. The Over Load Characteristics are 150% Safe Overload and 300% Ultimate Overload.
Testing for fatigue
We manufacture our load cells to the highest possible standards to reduce the risk of fatigue. In turn, our load cells can be used for fatigue testing in a vast range of materials and components including:
Need a long-lasting load cell?
Call us to discuss your specific requirements. We design and manufacture our own loads cells right here in the UK, and the person you speak to on the phone will be part of the team here too. There’s no lengthy wait for load cells to arrive from abroad or clear customs, just normal UK delivery times from our base in Reading Berkshire to you!
