Development of control system in LabVIEW environment for the study of the random fatigue behavior of metallic materials

Nowadays alloy materials are often used in mechanical designs. This leads to the need to known how they respond to vibration excitation. This is an important research topic of a branch of the applied mechanics, called ‘structural mechanics’. This discipline investigates deformations, stresses, and strains in solid materials. During the years it was of great interest to the development of techniques based on modal parameters measurement. An open topic is about how these parameters change when the materials react to fatigue stress. Fatigue is considered the most common cause of structural failures of mechanical systems. This thesis aims to develop a control firmware, to allow studies on alloy materials. We choose to develop it in the LabVIEW enviroment, to run it on a National Instrument cRIO computer. It is in charge of a test bench with an electromechanical actuation system called modal shaker. The choice of the NI Reconfigurable Input/Output as the platform is related to its particular architecture. These systems are composed of a custom system specifically designed to allow developers to deploy their software to both real-time computers and FPGAs, alongside with different analog conditioning devices suitable for the sensors needed by this study case, like extensometers, strain gauges, and load cells. The developed software can generate both sinusoidal and white noise signals required to control the shaker, and acquire of the signals coming from sensors collocated on the alloy specimen. The system is also capable to show the acquired information on a host computer, used by the bench operator, and logging them to perform further off-line analysis. The main challenges of this system are the strict timing requirements needed to obtain a laboratory-grade quality data. The test bench performs two different kinds of operation on the specimens. The first is a modal analysis. Thanks to the data provided by accelerometers when the specimen is subjected to a frequency swipe, it is possible to find its resonance frequency. Once this frequency has been obtained, the system starts the second operation, the generation of a filtered white noise stress signal to simulate a fatigue life cycle stress. The filter is set based on the resonance frequency found in the modal analysis. After we stressed the specimen for a time corresponding to a known time in the real application, the process is repeated over and over again to evaluate changing on its modal frequency. The test ends when the specimen breaks down.