Study On The Resonance Fatigue Crack Growing Test System Based On The Machine Vision Technology

The propagation experiment of fatigue crack is an important test of metal material fatigue performance for researching fatigue fracture characteristics by measuring in real time the fatigue crack length under the action of alternating load, it is one of the tests for the materials used in energy, aviation, aerospace, and other important equipment. Electromagnetic resonant fatigue test system is the equipment to test the fatigue properties of metal materials based on the resonance principle, its advantage has high testing frequency, low energy consumption, short testing time. To maintain the accuracy of the test results, fatigue crack propagation test needs not only accurate measurement of crack length, but also accurate control of test load. At present, the main problems of the resonant fatigue crack propagation test system are:the lack of a precise visual fatigue crack length of online measurement and high precise and stable test load control strategy during the process of crack propagation. On this basis, the online measurement method of fatigue crack based on machine vision technology and the test load control method based on the online measurement of crack were proposed, and the resonant fatigue crack propagation test for machine vision system was established.The paper’s main research work are as follows:1. The resonant fatigue crack propagation test system based on machine vision was established, the system mainly includes the testing load control system and the online measurement system of crack size. The online measurement system of crack size includes the high-resolution image sensor(CCD), the image collection system composed of high-definition microscopic magnification optical lens, light source and high-resolution image acquisition card, the camera movement control system consisted of the camera PTZ and camera motion controller, the computer system equipped with crack image processing software module. The test load control system is host and lower computer distributed structure, the lower computer is the digital load scene controller based on DSP and FPGA technology, the host computer is the computer installed the load control program.2. To achieve precise control of test load in the process of fatigue crack propagation, the dynamic characteristics of resonant fatigue crack propagation test system was studied. Take the international standard specimen C(T) compact tension specimen and electromagnetic resonant fatigue test machine model representative mainstream—Hongshan PLG-100 series as the research object, the mathematical model of vibration system of the electromagnetic resonant fatigue crack propagation was established, to establish, the calculation model of the C (T) standard specimen, the main vibration and excitation spring stiffness was built by the finite element analysis method, the specimen stiffness change rule along with the crack length was analyzed, the natural frequency of vibration system, the amplitude-frequency curve, the resonance amplitude with the changing rule of the specimen stiffness and crack length were studied, further, according to the data of different time measured by the fatigue crack online detection system, the system intrinsic frequency and the resonance amplitude variation law with time were researched.3. The fatigue crack online measurement method based on machine vision technology was Studied, the crack edge detection method based on fuzzy logic theory and connection method was put forward based on the analysis of the crack morphology and grayscale distribution features of image, the pixel-level vertex position of the edge of the crack and crack extension was got, on this basis, the subpixel edge location technology is used to detect the precise location of crack extension points.4. The test load control method based on online measurement of cracks was proposed, at first, the independent tracking technology of system resonance frequency was studied, including the modified climbing method online measurement of crack and the tracking step adaptive inherent frequency tracking technology based on the crack online measurement and fuzzy control theory, and the fuzzy control technology of test load amplitude when the system worked in resonance zone was studied, and finally the united control strategy of the resonant frequency tracking of stable amplitude and vibration load was put forward, the joint control was realized by using two level fuzzy control structure of MIMO supervision.5. The system performance test and fatigue crack propagation test were operated based on the built test system, the experimental results show that: The crack length measurement accuracy based on machine vision can reach 0.05 mm, the measuring system of high precision, strong anti-interference ability, stable and reliable data, can meet the requirements of fatigue crack propagation test of international standards; Using the presented control strategy of the test load control system, the crack propagation can be kept within 1% of the dynamic load fluctuation degree, effectively guarantee the reliability of fatigue crack propagation test results.