There are several methodologies and techniques to use in order to undertake vibration tests. The choice depends on what the expected outcomes or results are after performing these tests on your products or unit loads. However, the majority of vibration tests in the industry fall into two categories: random vibration testing and the sine testing. Keep reading to find out the differences between random vibration testing vs sine vibration.

All products or unit loads to be tested have critical elements that can resonate and therefore be damaged by vibrations. This event happens if the frequency of the vibration test matches the natural frequency of that critical element during a particular time interval.

One of the main purposes of vibration testing is to detect those natural frequencies in the first place, in order to evaluate the element’s endurance when the vibration test’s frequency is the same as the element’s natural frequency. 

The vibration analysis, detection and endurance can be determined by performing both a sine vibration test or a random vibration test.

What is random vibration testing?

Random vibration testing is a type of testing consisting of a varying waveform where its energy level is defined by a Power Spectral Density (usually called PSD), where sinusoidal vibrations occur simultaneously at diverse frequencies over a specific frequency range.

All the frequencies inside the specific frequency range of the random vibration test are reproduced with a certain level of energy. This, in turn, means that all the critical elements of the product or unit load that is tested that have their natural frequencies inside the specific frequency range of the PSD resonate at the same time.

This resonance can be detected in many ways. For example, it’s possible to place accelerometers on the product to detect critical elements, or evaluate the fatigue endurance of all the product’s critical elements at the same time during a certain test duration.

 

What is sine vibration testing?

Sine vibration testing produces an acceleration sine waveform with a certain amplitude level of acceleration and displacement with a determined frequency. During the sine vibration test,  amplitude levels and frequency can be varied or kept constant.

When the vibration’s frequency is kept constant, the amplitude of the acceleration or the amplitude of the displacement can be kept constant or modified in order to change the energy level of the test. 

This is known as the Sine Dwell Test, where a critical element that matches the frequency of the vibration test is subjected to resonance during a period of time in order to evaluate its fatigue endurance.

When the vibration’s frequency is changed, the amplitude of the acceleration and displacement can be changed usually by keeping the same constant amplitude of the acceleration which makes the amplitude of displacement to change in relation to the change of the frequency. 

It’s also possible to keep the amplitude of the displacement that makes the amplitude of acceleration to change in relation to the change of the frequency. 

This is known as the Sine Sweep Test at constant acceleration or Sine Sweep Test at constant displacement. In this case, a product is subjected to an incremental/decremental frequency sine vibration in order to detect the natural frequencies of critical elements of the product or unit load that is being tested. 

Another parameter to take into account for the sine sweep test is the velocity of changing frequency, which is usually defined in octave/min.

What is sine on random vibration?

Sine on random vibration testing is a test mode in which both sine and random vibrations are undertaken at the same time, in order to simulate life cycle environments where both forces are present.

In this case, random vibrations can be less dominant than the sine vibrations, but there are other instances where the contrary is true. Sine vibrations on the random vibration testing can be one or several sine waves of fixed frequencies and different acceleration amplitudes, or even sine sweep in a narrow frequency band.

 

Random vibration testing vs Sine vibration: how to pick

As we’ve seen, sine vibration testing is mainly useful to determine the critical elements that resonate in the product or unit load to be tested. 

This helps engineers and product development teams to perform a vibration analysis in order to understand the way mechanical vibrations spread through an element of the product or unit load. Thus, they are able to apply changes on these critical elements, making them stiffer or introducing damping materials that can reduce the probability of damage by fatigue.

But sine testing can also be used to simulate the fatigue stress of a product or unit load during its lifetime. This can be done by undertaking a sine sweep testing that covers all the frequency ranges measured in the distribution and life cycle during a determined test duration and higher amplitude. 

However, if the purpose is to subject the product to the most damaging stress fatigue, a sine dwell test can be performed that matches the natural frequency of the worst critical element detected, subjecting the product or unit load to the highest displacements and accelerations.

Because vibration environments where products or unit loads are distributed or placed are mainly characterized by random vibrations, random vibration testing is the most used method in order to validate the performance of a product or unit load that is subjected to vibrations during its life cycle.

PSD profiles and energy levels published on research articles or those that are inside standards can be used for random vibration testing. However, those PSD have been acknowledged to not always produce results that correlate with actual field performance. 

In order to improve that correlation, the most effective way is to first use PSD obtained from actual measurements of the particular environment of the product or unit load to test. Then, other techniques can be used to improve random vibration tests, taking into account the fundamental “character” of random vibration in the real world: real-life vibrations are non-Gaussian, not stationary, they are not isolated but superimposed by shocks or sines, and are presented in different directions. 

In order to simulate these real-life events and vibrations, a new era of vibration testing equipment has begun. Systems like the Vertical Vibration + Pitch & Roll are able to perform simulations that are closer to what real-life conditions look like in the distribution environment. 

Want to know more about random vibration testing vs sine vibration and how to perform reliable transport simulation? At Safe Load Testing Technologies we help companies optimize their cargo by using innovative and intuitive simulation systems. Get in touch with us and let’s discuss how we can help you. 

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