Art Illustration
For a test engineer, ability to tailor test procedure, to represent the actual field condition, bringing the actual environment into lab scale test, has become one of their major challenges.
For some test engineers, this question may not be so relevant, since some vibration test are done by following certain test standard. But there are some cases, can be in automotive, aerospace, defence, or even railway, when test engineers need to tailor their own specific test procedure.
When they find out some specific cases where the test procedure from the test standard has been subjected to their product does not represent the actual environmental condition, in specific local area. When the failure happens during service operational period was not observed during prototype testing period, this is when the question regarding type of vibration test need to be used will become relevant. For a test engineer, ability to tailor test procedure, to represent the actual field condition, bringing the actual environment into lab scale test, has become one of their major challenges.
Sine Test
SINE TEST
This type of vibration test is considered as one of the simple one (if not the simplest). It is considered simple since the vibration signal comes in sinusoidal type of waveform. Sinusoidal waveform consists of two major parameters: amplitude and frequency. Sine test is used when the product experience vibration environment mainly due to rotating equipment/rotating engine. When the product is located/installed near a rotating equipment (motor, turbine, compressor, helicopter rotor, etc), then likely it will experience vibration environment that can be represent by using sine test.
RANDOM TEST
This type of vibration test is used when the product experience vibration environment which has random type of waveform in nature. This type of vibration environment includes air turbulence, off road, sea wave, railroad, etc. Unlike the sine test which can be represent by two major parameters and can be analytically calculated, random test needs to be calculated more in statistical approach. Power Spectrum Density (PSD) is the type of graph that normally used to analyse a random vibration signal. The same PSD graph is needed to run vibration testing, either using Electrodynamic Shaker or Hydraulic Shaker.
Air Turbulence Illustration
SHOCK TEST
This type of vibration test is developed to represent single peak (pulse) in short duration type of signal. Unlike sine test or random test which always spend relatively long test duration (10 minutes or more) to be completed, shock test is done based on how many pulses. Each pulse is normally less than 1 second duration. Shock test is normally used to represent impact load that experienced by the product during transportation or during service life.
By looking at the characteristic of each type of test, it seems not difficult to choose the type of vibration test to be used for representing the vibration environment experienced by our product. Though, when we perform data acquisition for vibration environment of our product, very rarely that we will see only one type of signal in our waveform. Most of the cases, we will see combination of sine and random signal or shock and random signal, or even sine and random and shock all together.
The capability of test engineer and vibration analyst to determine which part of vibration signal is majorly contributing to the damage or failure mode of the product will determine how much the test procedure will represent the actual vibration environment. Without carefully running the vibration analysis, it may lead to over testing or under testing the product.
