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Fatigue Life Prediction and Test-CAE Correlation

Three virtual vibration fatigue scenarios you should know about

Vibrations on a physical part can be a source of fatigue.  These vibrations are described in the frequency domain and are usually represented as random power spectral densities (PSDs), harmonics (sine tones and sweeps), or some combination of the two.  In the real world, these vibrations can come from multiple directions (e.g., X, Y, and Z) at the same time.

Electrodynamic shaker tables are commonly used to replicate the vibration environment in a test scenario.  However, tests conducted on an electrodynamic shaker are often limited to a single axis of excitation.

The test can be run as a duty cycle to address this limitation. With this approach, the physical part is repositioned each event in the duty cycle to expose the part to vibrations in each direction.  However, each direction is run independently and sequentially.  Thus, any interaction between the different directions is not captured.

Fortunately, nCode DesignLife can perform virtual vibration fatigue analysis in all the scenarios described below. 

1. Single-axis, random excitation

 

The simplest vibration fatigue analysis within DesignLife uses a single-axis random PSD to describe the loading.  In addition to the PSD loading, the analysis requires a frequency response function (FRF) from finite element analysis (FEA). This consists of a set of real and imaginary stresses calculated at various frequencies and driven by a unit load.

This vibration-based analysis results in stress cycles with a zero mean stress.  DesignLife is also capable of superimposing the vibration loading onto a static offset such as a gravitational force or preload.

Additionally, thermal effects can be considered using multitemperature fatigue curves.  Temperatures can be constant across the whole model or they can vary if they were solved for in FEA.

2. Multi-axis, sequential, random excitation

 

Multiple single-axis random PSDs can also be run as a series of vibration fatigue analyses in DesignLife.  First the X-axis is excited for a prescribed time, then the Y-axis, then the Z-axis. This simulates the common electrodynamic shaker test where each direction is excited individually and sequentially. 

This virtual vibration fatigue analysis is also useful when evaluating a multi-PSD test conducted on a single axis. For example, it may be used to evaluate sequential tests of a single axis under different conditions.

3. Multi-axis, simultaneous, random excitation

 

Finally, simultaneous excitations from multiple directions can be analyzed by DesignLife.  This type of loading is often found in real world applications.  Just like the two scenarios above, a random PSD is used to describe the excitation of each direction.  Additionally, cross spectral densities (CSDs) are used to describe how each excitation interacts with the others. 

Thus, measured loads are first recorded in the time domain then converted into the frequency domain.

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