# Why Use the Frequency Domain for Fatigue Analysis?

We at CAEfatigue Limited are often asked why one should use the Frequency Domain when doing fatigue analysis. Please let us give you a quick overview of why.

By way of an automotive example, let’s look at the damage calculations that can be done using RLD (road load data) from automotive testing at a proving ground. For those not familiar with proving ground automotive vehicle testing, the workflow is fairly straight forward. A vehicle is instrumented with accelerometers at various locations on the vehicle.  Most often each accelerometer is measuring data in the X, Y and Z directions.  So, if we have accelerometers at 4 locations on the vehicle, which are measuring accelerations in 3 directions, then we have 12 channels (RLD time histories) of information being collected as we drive the vehicle over a planned route for some period of time.  Once complete, all channel information is combined into a single proving ground EVENT and in many cases, testing will collect this sort of data from multiple events throughout the proving ground with different loading conditions at different speeds for different periods of time.

To calculate damage using the TIME DOMAIN, we need to apply the collected road load data to an FEA model within a stress solver for EVERY event and then sum up the damages from the multiple events to calculate the total damage to the vehicle.  This is extremely time consuming and requires a significant amount of solver effort and storage memory.

To calculate damage using the FREQUENCY DOMAIN, we need to run a SINGLE stress analysis and use the frequency response results as a multiplier between the collected road load data and the output response of damage. However, before doing this we need a precursor step, which is to convert the road load data time histories into frequency-based power spectral densities (PSD’s).  We do this within CAEfatigue VIBRATION (CFV) using a manual conversion tool called TIME2PSD or we can also use our automated conversion tool called CAEfatigue CONDITIONING (CFC).

The image above graphically represents the differences between a Time Domain and a Frequency Domain fatigue analysis approach using Optistruct as the stress solver.  As mentioned, the key difference is the number Optistruct modal transient runs required in the Time Domain analysis when the fatigue analysis contains numerous proving ground events.

This is not the case with the Frequency Domain where only ONE frequency response analysis is required.  This difference can result in significant time savings with the added benefit of better output parameters captured by the Frequency Domain approach.

CAEfatigue Limited have work with numerous companies to benchmark the frequency domain process against the time domain process for models directly relevant to their needs.  On all occasions, we have been able to prove the correlation between the two processes and show why it makes sense to switch to the Frequency Domain.

This guest contribution on Innovation Intelligence is written by Stuart Kerr, President & COO at CAEfatigue Limited, developers of CAEfatigue VIBRATIONCAEfatigue Limited is a member of the Altair Partner Alliance.

Latest posts by Altair Partner Alliance (see all)