Study On The Damage And Fracture Of The Composite Flywheel By The Acoustic Emission Method

Based on the current demand of the high-speed flywheels for storage-energy, this thesis focuses on the systematic study of the damage and fracture of the high-speed flywheels for storage-energy. For the first time the acoustic emission method (AE) is used successfully to measure the damage of the high-speed flywheel for storage-energy. The artificial neural network technology is also used successfully to the damage-type recognition.The following issues have been studied in the research: First, the preparation of the samples of composite material and the detection of their damage; second, the design, manufacture of the simulation-structure of flywheels and the detection of their damage; third, the manufacture of flywheels, the experiment on high spin-speeded flywheels is carried out and detection of their damage; lastly, the finite element method is used on the simulation of the crack propagation of composite material and the analysis of the damage and fracture of flywheels. The major conclusions of the research and the creative ideas are as following.1. It is the first time in China that the acoustic emission technology is used to detect the damage of the structure spinning in high speed—flywheels. Based on the damage-detection on the sample and simulation-structure, a set of effective procedures are applied during the testing process so that the damage signals of flywheels can be successfully collected and distinguished.2. For the first time in China, the artificial neural network technology is used to recognize the damage pattern of flywheels. After repeated testing, we found that the artificial neural network of tatansig-logsig with one hidden layers and 29 hidden units can reach low variance (0.0192) and accurate simulation after it is trained by trainlm-method. It can accurately distinguish the different types of damages of carbon/epoxy composite material used in our testing.3. For the first time in China, the "element birth and death" in finite element method is used to simulate debonding in unidirectional laminates composite. Through continuous adjustment of the model of the simulation, finally the types of damage to defect-sample under tensile load can be simulated reasonably, thus the process of the damage can be recreated realistically.4. During the load-testing, the major types of the damage of two composite materials of glass/unsaturated polyester and carbon/epoxy, are matrix-crack, crack in the interface of fiber and matrix and fiber-fracture. The damages start when the load is much less than the real fracture-load. The early-stage damages are mostly matrix-crack, while the late-stage damages are mostly fiber-fracture.5. The different damage-stage in polyester-composite reinforced by long fiber can be distinguished by following correlation diagrams: Time-Duration, Time-Frequency, Time-Energy, Time-Counts of the character- curve in acoustic emission test.6. When fiber is fracturing dramatically, high-energy, large-counts and long-duration signals appear. And the energy, counts and duration mentioned above is one magnitude greater than those appeared with signals caused by matrix-fracture and fiber/matrix interface-crack. The critical load is determined when the fiber start to fracture dramatically.7. I designed and developed a static testing structure which simulates perfectly the damages of flywheels.8. Dye penetrant method can be used fairly well to detect the damages of flywheels. For tangential-winding flywheels, which are studied in this paper, the common types of damages are matrix-crack > fiber/matrix interface-crack.9. When steady state propagation and unstable extension of the cracks appear in flywheels, the character-curve of AE- signals have obvious characters.10. When steady state propagation and unstable extension of the cracks appear in glass/unsaturated polyester flywheels, the Felicity's ratio is less than 0.93.11. We learned from finite element analysis that both maximal radial stress and maximal tangential stress locate close to the middle of flywheels, and the location does not change with the rotate speed. The maximal radial stress and tangential stress increase with the rotation speed. The relation between maximal radial stress and rotate speed, and the relation between tangential stress and rotate speed, fit second order polynomial. According to the finite element analysis: the failure of flywheels appears when radial stress in middle of flywheels exceeds the critical strength of the material, and for glass/unsaturated polyester composite flywheels, the critical rotation speed is 21840r/min.12. The ANSYS analysis of location and style of damages to composite flywheels well agrees with experiment, but the critical rotation speed we obtained through testing is 18655~20865r/min, which is a little smaller than what the ANSYS predicts.The work of this thesis was supported by the project of national natural science foundation of China (19972063).