Fundamentals of Fatigue Life Prediction

Fatigue failure is a leading cause of structural damage in materials and components, making fatigue life prediction a vital part of design, validation, and maintenance across many industries.

When dealing with force measurement, load cells are indispensable tools for accurately quantifying the forces applied during fatigue testing. It is important to understand what fatigue life is, why it’s difficult to characterize, and how tools like the S-N curve are used to estimate the operational lifespan of load cells and the structures they measure.

Fatigue refers to the weakening of material caused by repeatedly applied loads. Failure occurs after a certain number of cycles, even when the stresses are well below the material’s ultimate tensile strength.

Fatigue testing is performed for several critical reasons:

• Validation and liability by generating data to prove a product meets life-cycle expectations.
• Model confirmation in validating Finite Element Analysis (FEA) models for structural integrity.
• Safety assurance by demonstrating the safety of structures susceptible to fatigue damage.
• Maintenance planning: in predictive and proven maintenance and inspection schedules.

Fatigue is a non-linear process, meaning damage accumulation is complex and difficult to characterize. Even in controlled tests on simple samples, results show a wide scatter band. With complex structures like a load cell, this analysis becomes even more intricate.

While an accurate prediction of fatigue life is not a reality, theoretical analysis and empirical data allow for useful approximations to estimate the margin of safety for a given design.

The Role of Load Cells in Fatigue Testing

Load cells are crucial for accurately measuring the forces applied to materials during fatigue testing. They are a necessity wherever materials, parts, or assemblies are tested for life cycle verification or destruction.

The load level and cycle count can categorize fatigue failure. High cycle fatigue is defined as having millions of relatively low-stress loads leading to failure. Low cycle fatigue is considered thousands of high-stress load cycles lead to failure.

When conducting fatigue tests, it is essential to understand the limitations of the mechanical setup, including the load cell’s capacity and resolution, machine stiffness, and environmental factors.

Interface offers a variety of high-performance fatigue-rated sensors, including specialized LowProfile® and Mini Load Cells, designed with materials and processes that balance the propensity of these two failure modes to ensure reliable, long-term performance. The following is a list of Interface load cells commonly used in fatigue testing applications.

Interface Load Cells for Testing Fatigue

• 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
• 1700 Flange LowProfile Load Cell
• MBI Overload Protected Miniature Beam Load Cell
• SSMF Fatigue Rated S-Type Load Cell

Load Cell Components Subject to Failure

There are two primary metal components in a load cell that must be considered in fatigue analysis: the flexure (spring element) and the strain gage (sensor).

Flexure failure is structural. The flexure bears the physical load; therefore, its failure is always structural. Strain gage failure is electrical. Since the gages’ function is the electrical measurement of minute deflections, failure is typically non-structural. It is noted by a shift in resistance or gage factor, compromising measurement accuracy.

Predicting Life with the S-N Curve

The S-N curve (Stress-Number of Cycles) is the most well-known tool in materials science for graphically estimating fatigue life. What is an S-N Curve? An S-N curve plots the allowable number of load cycles versus the cyclic stress levels required to break a specimen. It helps predict material fatigue life and is key for designing components to operate safely. By knowing the maximum stress level a component will experience, you can use the S-N curve for that material to approximate its operational fatigue life.

The Endurance Limit

A key characteristic of steel is the endurance limit, the stress level at which the S-N curve becomes essentially flat.  If a steel component operates below this stress level, it is theoretically predicted to endure an infinite number of load cycles without fatigue failure.

Nonferrous metals, such as aluminum alloy, generally do not exhibit an endurance limit. Their S-N curves continue with a small, continuous slope, meaning that they will eventually fail regardless of the low stress level applied.

Strain-N Curves

For analyzing the fatigue characteristics of the strain gages, it is more convenient to use a Strain-N curve, which plots the number of cycles against strain (a dimensionless quantity). Strain is typically measured in microstrain. Stress and strain are related by the material’s modulus of elasticity, allowing for comparison between S-N and Strain-N curves.

By leveraging accurate force measurements from fatigue-rated load cells and utilizing data interpretation methods like S-N curves, engineers can make informed decisions to maximize the service life and ensure the safety of critical structures. Be sure to download our Load Cell Field Guide to learn more about fatigue testing theory and predicting fatigue.