The Math of Random Signals, Fatigue Damage, Vibration Tests

Description

In the first part of this course, the mathematics of fatigue damage is addressed. The most important equations are derived, and this will serve as a guide to illustrate the main applications of the theory in the second part.

This course is not only aimed at those students who are interested in vibration tests and reliability engineering, but also at those who want to see the beautiful mathematics applied to engineering. In this case the theory contains: stochastic processes, probability distributions (and how they arise in nature/real-life applications), as well as single-degree-of-freedom systems, the probability density of the maxima of a random process, and much more. In this regard, the mathematical theory of damage is quite advanced mathematically; for instance, we will have to make use of the Gamma and the Bessel functions. Anyway, the concepts will be developed intuitively throughout the course, and the connection with real-life applications will always be highlighted.

Most of the results shown in this course are described in the instructor’s PhD dissertation: “Advanced Mission Synthesis Algorithms for Vibration-based Accelerated Life-testing” and the references contained therein. This work will be public as of February 2023 after a two-year “embargo”, and it will be available for consultation. If you would like to consult this dissertation and you cannot find it online, just check the attachment to the first lecture of this course, you should find the dissertation attached.

This theory was exploited to create Graphical User Interfaces (GUI), which were the result of the collaboration between the instructor and companies interested in the PhD project. We will use these GUI in the course to shed light on how the equations can be implemented to generate vibratory signals.

Let's briefly contextualize the theory of fatigue damage in the field of vibration tests. In real-life applications, components are often subjected to stochastic loads that might lead to a premature failure; therefore, experimental tests are needed to check the components’ resistance to environmental vibrations.

The type of tests related to fatigue-life estimation aims to reproduce the entire fatigue damage experienced by the component during its operational life, but in a shorter amount of time. These tests are usually referred to as fatigue-life tests or durability tests, and the common practice is to tailor them to the specific application. The signals which are used during the tests have the same "damage potential" as the environmental conditions (to which the components are subjected).

The signals are created starting from a Power Spectral Density (PSD), which is used by vibrating tables or shakers to generate a vibratory motion.

In the course, a method of relating a PSD to the fatigue damage will be shown. This will help create a (Gaussian) signal to be used during tests.

The Gaussian distribution appears very frequently in nature and practical applications. In this course, it was deemed necessary to shed light on the importance of this distribution, by deriving the results previously derived in another course on the Central Limit Theorem (although in a different way).

However, since there are signals measured in real applications that show a deviation from the Gaussian distribution (due to the presence of peaks and bursts in the signals), the synthesis of non-Gaussian signals will also be discussed.

Who this course is for:

• Reliability engineers
• Mechanical engineers
• Mathematicians
• Engineering or math students who are interested in the mathematical modeling of Fatigue Damage
• Students who want to see the connections between mathematical models and practical applications
• Students who already have a good mastery of mathematics and want to learn some of its beautiful applications to Random Processes
• Engineering and technical managers
• Quality Engineers