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Understanding the Anatomy of Fatigue Failure

Interface’s specialized fatigue-rated load cells are commonly used for fatigue testing. This test and measurement application, sometimes called lifecycle testing, determines how long the product can endure while performing its normal function.

Fatigue testing validates safe and reliable product designs and structures and is also used to determine the point of destruction. By understanding the fatigue behavior of materials, components, and assemblies, engineers can design products resistant to fatigue failure.

Critical to fatigue testing is the quality of the measurement device. Understanding the load cell’s electrical, mechanical, and performance specifications is important before using it in any fatigue testing. This first step helps confirm the load cell is designed and capable of withstanding the cycling and load requirements of the test—in other words, to ensure the load cell doesn’t fail during fatigue testing.

Interface force measurement experts detail the anatomy of fatigue failure in our popular Interface Load Cell Field Guide. The ultimate load cell resource is available for download here.

Fatigue Failure Theory

It is well known that metals will fail in a statically loaded situation if the yield strength is exceeded. As load cells are structural devices stressed during their normal use, they are commonly given ultimate overload ratings to characterize the magnitude of static load they will withstand without failing structurally.

However, all metal structures, including load cells, are also subject to failure due to repetitive loadings much lower than the ultimate overload rating. This phenomenon is known as fatigue failure, and it is because the stress that a metal can withstand under cyclic loading usually becomes less and less as the number of cyclic loadings is increased.

The cause of this apparent anomaly can be explained by noting that metals are typically not perfectly homogeneous solids. They are composed of crystals, and at locations called grain boundaries, along slip planes, or in a region of a microscopic defect, minute strains under load can occur that do not completely reverse during unloading, leaving the material with a slight plastic deformation at the end of each complete cycle. This effect is highly dependent on the magnitude of the load and the number of cycles.

It is generally acknowledged that a structural fatigue failure develops in three stages:

  • Repeated cycling builds up local plastic deformation, and a microscopic crack is initiated.
  • The crack propagates, and a larger section becomes weakened.
  • Stress concentration in the cracking section increases rapidly, and continued cycling enlarges the crack until a sudden fracture occurs.

Fatigue Life Prediction

Accurate prediction of fatigue life of any structure is not a reality. Well-controlled tests on the simplest configurations of test specimens result in a wide scatter band of results. The analysis is even more complex, with the structure typical of a load cell. Theoretical analysis can produce approximations, however, which can be useful in estimating the margin of safety at which a particular load cell design is operating.

In materials science, the S-N curve is a well-known tool. It represents the load cycles required to break a specimen at peak cyclic stress levels. Thus, the fatigue life can be approximated if the stress level is known. However, some factors make fatigue life difficult to characterize.

The fatigue rating of a load cell is a distinct specification that guarantees a service life of up to 100 million fully reversed load cycles at full rated capacity. Load cells are typically integrated into testing machines or equipment.

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

Interface fatigue-rated load cells are vital for test and measurement projects in various industries, from aerospace and automotive to civil engineering and manufacturing. Manufacturers of test machines, aircraft, spacecraft, automotive components, consumer products, heavy machinery, bridges, energy production equipment, and industrial systems use our fatigue-rated load cells.

ADDITIONAL RESOURCES

Fatigue Testing with Interface Load Cells

Interface Specializes in Fatigue-Rated Load Cells

Prosthetics Load and Fatigue Testing App Note

Furniture Fatigue Cycle Testing App Note

Testing Labs Choose Interface High Accuracy Products

Specialists focused on testing applications work in a variety of testing lab environments. In each lab, technicians rely on the tools to collect and report on data that is used to make products safer, guarantee performance, ensure quality, and to meet the strict industry standards and requirements. Accuracy in testing data is dependent on the precision measurement devices and instrumentation used to capture the results.

We supply lab engineers with high-accuracy sensor technologies used to complete rigid test requirements. Interface is the top provider of test and measurement products used for structural and material testing, static and fatigue testing, torsion effects, tension tests, calibration testing, and environmental testing. Read more in Types of Force Measurement Tests 101.

Our standard high precision load cells, torque transducers, multi-axis sensors, and instrumentation are used on every continent for T&M. Based on our quality and performance, we are the chosen supplier to calibration and testing labs. We see our products used today for continuous improvement programs, advancements in smart manufacturing and new product designs.

If it must be measured, Interface has a solution. Our products are designed for small and large testing facilities, including calibration-grade load cells, load frames and test stands, along with data acquisition systems. The wide variety of our force measurement solutions designed for testing labs means we play a role in every industry that is making a physical product and the test labs that validates the products performance.

Testing Labs and Types of Testing Using Interface Solutions

General Automotive Test Labs:

  • Component and Sub-Component Level Testing
  • Suspension Testing
  • EV Battery Testing

Automotive Driveline Testing:

  • Engine Performance and Durability Tests
  • Motor Efficiency Testing
  • Power Analyzation (Electric)

Aerospace Testing:

  • Full Scale Structural Static Testing
  • Component Fatigue Test
  • High Precision Thrust Testing
  • Simulators
  • Wind Tunnel Testing

Geotechnical and Civil Testing

  • Concrete or Asphalt Core Testing
  • Soils Testing

General Structural and Component Testing

  • General Push and Pull
  • Design Proofing
  • Life Cycle Fatigue Validation

Medical Device Testing:

  • Prototyping
  • PPAP Validation and FDA Certification
  • Device Lifecycle Testing

Consumer Product Testing Labs:

  • Design Validation
  • Material Testing
  • Fatigue and Failure Tests

Interface recently highlighted testing lab applications in our Test Lab Essentials Webinar. Here you can see the lab use cases and products as they are reviewed by our applications experts.

Each of these testing types requires different force testing equipment, and our experts work directly with testing lab professionals to determine the products or systems they need for single and ongoing test requirements.

As testing technologies becomes increasingly complex, off-the-shelf products may not meet the needs of every Interface customer. We lend engineers expertise in test and measurement to support unique and custom requirements to get the right sensor, instrument, and system in place.

Since our first load cells were designed five decades ago, we have built millions upon millions of load cells and torque transducers used in testing labs around the world. Our products are built to withstand the rigor and requirements needed for high quality and reliable data collection in test and measurement. Our test customers depend on us for proving accuracy, consistency, and reliability in performance.

ADDITIONAL RESOURCES

Interface and Testing Lab Applications

Testing Lab Essentials Webinar

Engine Dynamometer App Note

Consumer Product Testing Case Study

Interface Solutions for Safety and Regulation Testing and Monitoring

Metrologists and Calibration Technicians 101

Motor Test Stand

GS-SYS04 Gold Standard® Portable E4 Machine Calibration System

Electric Vehicle Structural Battery Testing

Furniture Fatigue Cycle Testing App Note

Regular Calibration Service Maintains Load Cell Accuracy