The Relationship Between Fatigue and Durability Modeling and Testing

With advances in computer modeling, designs can be validated on the desktop before anything is manufactured, and so the need for physical testing is diminishing.

Or is it?

Actually, no. The role of the lab is changing so that it can support the designers, reduce the time to market, and ensure that developed products are even safer and more reliable than before.

Models are hungry for data in order for them to be accurate; they need data on inputs such as materials, performance of non-linear components such as elastomers, information on the accuracy of assumptions, such as boundary conditions, or statistics on variables such as production variability. None of these can be modeled directly, and so the test lab is moving from a "necessary evil" validation role at the end of development to an integral and essential player in the design process. Here are a few examples:

The Test Lab provides material and component properties

When a designer builds a model on the desktop, s/he needs to assign properties to materials and "grey box" components. These properties are pulled from a database that is created and maintained by a test lab. The data falls into these general categories:

• Monotonic data for static strength and stiffness
• Calibration of Hyperelesticity, Hyperplasticy, etc.
• Cyclic strain and/or stress life data with various surface finishes and manufacturing processes for more accurate fatigue life prediction
• Multi-axial non-linear dynamic behavior of components for "grey-box" modeling (e.g. tires)

Kinematic and Dynamic Models Predict Loads

Component performance data is used to build kinematic and Multi-Body Dynamic (MBD) models using Adams or equivalent. These models can then be exposed to usage inputs so that loads can be predicted. The loads can be used to test components, assemblies and full systems

Hybrid Tests In the Lab

Physical components and assemblies are combined with analytic dynamic models, and a test is performed where the model provides inputs to the test, and the test feeds-back loads to the model, in real time. This type of test can be used in place of traditional "mule" tests, where the model can be a substitute for the vehicle dynamics when testing a suspension for example. Also, this can be a Hardware-In-the-Loop (HIL) test for active components, such as Electric Power Assisted Steering (EPAS).

FEA Fatigue Predicts Critical Locations

FEA based Fatigue Analysis can now be used to find the locations where cracks will initiate under multi-axial loading. Once the designs have been refined, the loads can then be reduced using Fatigue-Sensitive Editing (FSE) to only the cycles that create damage. 

Test Lab Verifies Model Predictions

Life predictions can be verified in the test lab using a simplified test, with only the damaging loads focussed on the critical locations for fast turnaround. By using physical parts that have been manufactured, the effects of variability in residual stress, surface finish and manufacturing precesses can be evaluated.

These are just a few examples. I am sure you can think of more...

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