Fatigue Damage Spectrum FAQs

Technical Notes

Author Jade Vande Kamp


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In this technical note, discover the answers to frequently asked questions about the fatigue damage spectrum (FDS).

What is the Purpose of FDS?

The fatigue damage spectrum (FDS) software is a tool that allows engineers to create a random test profile that is the damage equivalent to multiple weighted time-history files from the operational environment. Engineers can use the FDS to compare product failure runs, compare specifications to real-world data, and determine if a current testing method is valid or over/under-testing.

Is it Possible to Combine Multiple Time-history Files?

The FDS software can create a power spectral density (PSD) test profile from multiple time-history files. These time-history files can be weighted individually by time or cycles to generate a target life for the product. When combining the data, the software uses the same m, Q, and frequency range to create an equal relationship between each imported file. The output PSD will be the damage equivalent of the imported files based on the target life for each waveform and the FDS settings.

Does the FDS Include the Test Item’s Resonances?

If the engineer uses recorded field data, then the acceleration waveform will include information about the item resonances, and the FDS will include them. Otherwise, the acceleration waveform doesn’t include information about the item’s resonances.

To What Degree Can a Test be Accelerated? Is There a Recommended Ratio?

An accelerated test is not equivalent to running the test for the full time. Generally, the less the test is accelerated, the more accurately it reflects the real world. At a minimum, the test cannot exceed the product’s instantaneous stress limit, such as yield stress or ultimate stress (assuming the product fails due to fatigue damage). The instantaneous stress limit varies depending on product construction.

Additionally, the FDS uses the stress-strain curve to determine the acceleration amount, so the test acceleration is only as accurate as the m value. As the test acceleration level increases, the more important the m accuracy becomes. Some composite materials may make it difficult to estimate the m value.

According to MIL-STD-810G, test engineers should not accelerate tests by applying excessive amplitudes “simply to achieve short test durations. Such excessive amplitudes may lead to wholly unrepresentative failures and cause suppliers to design material to withstand arbitrary tests rather than the in-service conditions” (p. 237).

The military standard also defines an exaggeration factor. “It is recommended that the exaggeration factor be kept to a minimum value consistent with the constraints of in-service time and desired time, and should generally not exceed values of 1.414^(S1/S2) or 2^(W(f)1/W(f)2).” According to this statement, for a material with m=7.5, the maximum ratio between the in-service time and test time can be 1.414^(7.5) ≈ 13.4.

How Can I Create a Time-history File from PSDs?

In VibrationVIEW, the simplest method to produce a time-history file from a PSD is to use the Random Import function. Import the PSD, run the test, and record the Ch1 time waveform. You can use demonstration mode or loop the drive to Ch1 with a short BNC cable. Connection to a shaker is unnecessary because a PSD is not uniquely associated with a time waveform. The FDS is a time-based algorithm; if you run the same PSD repeatedly, you will not get the same FDS but a good approximation.

import of acceleration time-history file

Random Import settings in VibrationVIEW.

A second option is to assume random phases using a tool like MATLAB, but the results would not be significantly different.

Is it Possible to Determine How Much Damage a PSD Causes?

Not by itself. A PSD is only associated with an acceleration waveform. The FDS requires an m value and acceleration waveform. If you know the product’s m and Q values and import the time-history data, you can generate an FDS.

Can the FDS Determine Which PSD Causes More Damage?

To compare two PSDs using the FDS, the m and Q value must be the same. From this perspective, the FDS is an identical transformation applied to both PSDs, and the FDS generation process does not provide any new information. The comparison would provide a direct comparison of the PSDs in the frequency domain.

An example of a useful FDS comparison uses field time-history data and a PSD based on a test specification. The two FDSs should align closely if the standard PSD accurately reflects the combination of field environments. This comparison can validate a PSD and assure the engineer that a product is not being over/under-tested.

What m and Q Values should I Use? Is it Possible to Determine a Product’s m and Q?

See the m and Q Values for FDS technical note for information.

How Does Kurtosis Factor Into PSD Import?

Measuring the acceleration waveform in the field includes the measurement of kurtosis. When we calculate the FDS, an increased kurtosis will result in more damage at nearly all frequencies (if performed correctly). However, if we convert the FDS back into acceleration spectral density (ASD), the ASD will be Gaussian, where the kurtosis equals 3. The damage due to the time-history file’s kurtosis is calculated into the FDS but spread throughout the ASD during PSD import.

The Kurtosion® software can output an FDS with a higher kurtosis. A higher value may be useful for lowering the GRMS requirement of the test. The GRMS will be decreased, but the peak requirement will increase. Lowering the GRMS may be beneficial when a system cannot handle the sustained load of the test but can reach higher peaks.

A higher kurtosis FDS can also create peaks equal to the input waveform. With or without added kurtosis, the output PSD from the FDS will be the damage equivalent to the combined input time-history files.

For instructions on creating an FDS test profile, see the tech note on How to Create a Fatigue Damage Spectrum Test.

Learn More

If you still have questions after browsing these fatigue damage spectrum FAQs, you can download the software case study or reach out to the Vibration Research support team.

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