Improving Random Tests
A random test is a standard selection for vibration testing. However, its distribution of data presents a shortcoming when simulating a real-world environment. Present-day methods of random testing assume a Gaussian mode of distribution where the kurtosis value equals three. However, not all data fit Gaussian distribution, and this assumption can result in under-testing.
A better method is to adjust the distribution of data to reflect the field environment, which involves the adjustment of kurtosis. Kurtosion® is a closed-loop method of kurtosis control developed by Vibration Research. It allows for the adjustment of kurtosis levels while maintaining the test profile and spectrum attributes.
Add Kurtosion to a Random vibration test when:
- The vibration test needs to be more realistic to real-life situations
- You want to break a product quickly without using more energy
Spend More Time at Peak Levels
The greatest damage potential of your product is typically at peak acceleration levels. When the kurtosis value of a signal is increased, the time spent at peak levels is also increased. With kurtosis control, your test will better reflect what occurs in the real world.
What is Kurtosis?
Kurtosis is a dimensionless measurement pertaining to the average deviation of a signal from its mean value. Present-day random testing assumes Gaussian distribution of random data with a kurtosis value of 3. However, there are data that do not fit the Gaussian distribution. In such instances, a kurtosis value of 3 may omit higher peak values.
What is Gaussian Distribution?
The Gaussian distribution function is the most widely used probability distribution in statistical analysis. The distribution is symmetrical; the skewness equals zero and the kurtosis equals three. Traditional random control assumes a Gaussian distribution of data, which is highly concentrated near the mean value of zero. Therefore, traditional random test acceleration is near zero most of the time. In most environments, significantly more time is spent at peak levels than what is produced by a traditional random test.
Our latest innovation in random testing is a patented method of kurtosis control. With Kurtosion, you can control the RMS and kurtosis of the random waveform to have more control over the probability distribution and more closely match your test lab to the real world.
- Full RMS Control: Acceleration is moved from the mean towards peak levels, resulting in no change of GRMS.
- Full Dynamic Range: The kurtosis of the acceleration is controlled without any reduction in dynamic range.
Kurtosion Shaker Capacity Calculator
Increasing the kurtosis value on a shaker for specific tests may decrease the manufacturer’s random lb-f rating. Engineers can use the relationship between kurtosis and the crest factor to calculate the shaker capacity for a random Kurtosion test.
Download Calculator (Excel)
Getting the Kurtosis into the Resonances
“Test results obtained at VRC with their newly developed kurtosis control technique clearly show that non-Gaussian distributions can indeed be created at product resonances, regardless of the Papoulis Rule.” Read more.
Kurtosis – The Missing Dashboard Knob.
“Many test personnel have recognized that random testing, while good, has short-comings when it comes to simulating the real-world environment. As a result, over the years, there have been many vibration testing method modifications to address these shortcomings.” Read more.
The Fatigue Damage Spectrum and Kurtosis Control
“The Fatigue Damage Spectrum shows the increasing fatigue damage across all frequencies that accompany the increased kurtosis levels that cause the faster product failure. The FDS is another great tool that clearly illustrates the effectiveness of Kurtosis Control.” Read more.
Using Kurtosion® to Accelerate Structural Life Testing
“Now kurtosis control allows such tests to be conducted in a fraction of the time required for a Gaussian drive signal to precipitate failures. However, the kurtosis control needs to be properly implemented to circumvent interference from the Central Limit Theorem. A unique feature within the Vibration Research Corporation (VRC) Kurtosion® process allows resonant fatigue, as well as simple static failure tests, to be accelerated.” Read more.