# Frequency response of Transient Strucutral analysis

Good day

I'm conducting a transient analysis with the intent of extracting signal data which would help in identifying loose bolts in a structure. Currently I have an output of the acceleration vs time data from the resulting vibration of the system , I would prefer the acceleration vs frequency plot as an output (FRF).

Is there a way to plot this result in ansys?

Note that the excitation of the structure is due to an impulse hammer, therefore varying force with respect to time. This is why I've opted to use the transient structural analysis rather than the harmonic analysis.

The force and time data is treated as a constant input to the simulation, the variance in the simulation is from the adjustment of the bolt pretension forces.

FIg 2: Force vs Time data for modal / impulse hammer

FIg 3: Analysis settings for first step

FIg 4: Analysis settings for second step

FIg 5: Directional Acceleration in y direction

• I don't think a time step of 5e-3 seconds is nearly small enough to capture rattling bolts.

I don't think an input force pulse that lasts 0.1 s has nearly enough high frequency content to rattle a bolt.

What is the mass m of a nut and bolt in kg?

What is the spring rate k of a bolt head pressing into a steel plate in N/m?

Do a hand calculation of the natural frequency f of that mass-spring system, 1/(2Pi)*Sqrt(k/m) in Hz.

A good time step to capture the vibration of that mass-spring system is t = 1/(20f) in seconds.

A good force pulse time duration is t = 1/(2f) in seconds to excite that vibration.

• Thank you for your response .

The force-time data used to simulate my geometry was an arbitrary force-time data, to get my simulation to work, that I found online. The actual force- time data will be measured during experimental testing of the beam geometry.

I will complete the calculations nevertheless, would the spring rate K, be calculated similar to the attached figure 1?

Fig 1. Spring stiffness calculation

• Yes, a spring rate like this will do.