Why use the Frequency Domain?

We are often asked why should I use the frequency domain when doing fatigue analysis. Please let us give you a quick overview of why.

Below is a hypothetical example of damage calculations done using RLD (road load data) from automotive testing at a proving ground.  This comparison also applies to any process that uses time domain data as the input loading to their components. 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 direction.  So, if we have accelerometers at 4 locations on the vehicle, which are measuring accelerations in 3 directions, than we have 12 channels (RLD time histories) of information being collected at the same time as we drive the vehicle over a planned route for some period of time.  All of the channel information is combined into a single proving ground EVENT and in many cases, automotive testing collects data from multiple events throughout the proving ground with different loading conditions at different speeds.

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.

To calculate damage using the FREQUENCY DOMAIN, we need to run a SINGLE stress analysis and use those results as a multiplier between the collected road load data and the output response of damage. However, we need one additional 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 using Nastran as the stress solver.  As mentioned, the key difference is the number of Nastran SOL112 runs required in the Time Domain when the testing duty cycle contains numerous proving ground events.

This is not the case with the Frequency Domain where only one SOL111 run 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.

*CAEfatigue Limited (www.caefatigue.com) is a privately owned company with it’s world headquarters located in London, England.   CAEfatigue Limited is dedicate to developing random response and fatigue evaluation software for dynamic mechanical systems that is easy to use for the average Engineer or Designer.   The CAEfatigue Limited suite of products are in use across multiple industries throughout the world and have become the industry standard for use in the frequency domain. Share with:

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