Investigation of Mechanically Fatigued Low-Frequency Energy Harvesting Effect on Isotropic Materials

One of the most popular methods of harvesting electrical energy from mechanical stress is the utilization of piezoelectric materials. A significant amount of research has been done regarding piezoelectric material and has answered questions about what happens to the piezoelectric material during energy harvesting. However, the question that has yet to be answered is what happens to the properties of the material from which energy is being harvested. To answer this question, this study will focus on a simple energy harvesting system is designed that consists of a piezoelectric transducer, voltage regulator, rectifier, and energy storage element to collect energy. The base material from which energy is being harvested is an ASTM standard dog bone test piece of aluminum 6061, structural steel, and copper. To harvest energy from the test specimen, a low-frequency mechanical fatigue load is applied using a fatigue testing machine. Based on the SN curve estimated for the aluminum 6061, structural steel, and copper, the number of fatigue cycles for the test specimen is determined. The maximum fatigue tensile amplitude is set as half of the yield point of the aluminum. The test specimens are designed to go through three types of fatigue cycles which are 50%, 80%, and 100% of their strength. From each set of specimens, energy is harvested from a simulated time as defined in the study. The results of these experiments show a clear correlation between the harvested energy with the material strength of the mechanically fatigued isotropic structures. As the material strength decreases, the amount of energy that is extracted also decreases. Moreover, as the frequency increases, the amount of harvested energy increases exponentially. From each set of specimens, energy is harvested from half the time the experiment runs. At the end of each set of experiments, the ultimate tensile strengths are determined in order to observe if energy harvesting has negative, neutral, or positive effects by comparing the strengths of energy-harvested specimens to the non-harvested tensile strength. To understand the uniform effect of harvesting, the final strength is normalized with the amount of energy being harvested from the material. The results of these experiments show a clear correlation between the harvested energy with the final strength of the mechanically fatigued aluminum structure. As the material strength decreases, the amount of energy that is extracted also decreases. As the frequency increases, the amount of energy increases exponentially.