This post is by Chris Stone, Environmental Test Manager at TRaC.
What’s the difference between earthquake and vibration testing?
When Engineers are told that their design will have to withstand seismic events and are faced with designing for and carrying out earthquake testing (see my previous post about how this is increasing), it is often their first experience of this test regime and at first glance, the complicated test specifications are a real cause for concern!
Most Engineers have some experience of vibration testing and understand acceleration levels and the frequency of vibration but Earthquake testing is commonly specified by Required Response Spectra (RRS) as the tests are typically simulating events that are over in 30 seconds but involve large displacements at low frequency.
I’m often asked how seismic testing differs from vibration testing – the main differences are:
- Earthquake testing is typically carried out from 0.1 to just 70Hz (although our table can reach 100Hz)
- Earthquake tests generally involve excitation in all three axes simultaneously, whereas with vibration testing the sinusoidal or random vibration is carried out in axis and then the equipment under test is remounted to test the other two axes in turn.
- Earthquake testing may comprise 6 degrees of freedom, with roll, pitch and yaw as well as the three translational axes of shaking. Vibration testing has just one linear degree of freedom.
- Earthquake testing involves excitation with a much bigger displacement (up to 300mm peak to peak). Vibration testing, using electrodynamic shakers, is typically limited to 50mm peak to peak.
- Earthquake testing is carried out to give confidence that equipment can survive and operate when subjected to a thirty second shake that may only statistically occur once in ten thousand years. Vibration testing is carried out at accelerated levels to simulate a lifetime of exposure to vibration in a much shorter timescale.
- To achieve the different characteristics, earthquake testing uses hydraulic actuation whereas shakers are typically electromagnetic devices.
- Usually earthquake testing requires the equipment to be functional during testing and may be repeated to account for different operational states e.g. all relays, switches, etc are left open, the system is tested to make sure that it will continue to operate correctly, then the process is repeated with the electromechanical parts in the opposite state.