| Welding structure has been widely used in industrial production,and the main failure mode of welding structure is fatigue fracture.Once the fatigue crack is extended to a certain size,the welding structure will be brittle fracture,and the result is serious,therefore the detection of fatigue crack of welding structure is very important.The detection accuracy of existing technology is satisfied,but it takes a long time and needs downtime detection.Therefore,the vibration acoustic modulation technology is used to detect the weld structure and evaluate the fatigue damage degree and the crack size of the weld online in this paper.In this project,an experimental platform was setted up independently,prepare the sample,select the appropriate hardware equipment to connect,ensure the signal can be transmitted and received,and transfer data to the computer terminal.Then the Lab VIEW program is edited to get the time-frequency diagram by digital filtering,synchronous demodulation and short time Fourier transform.A preliminary test was carried out on the experimental platform.According to the experimental results,the modulation index was calculated by using synchronous demodulation and the first four low frequency harmonic amplitudes after STFT,and the parameters selected by the three point bending fatigue test were determined by experiment.The intrinsic modulation index of the specimen was studied and detected,and the change law of the natural frequency with the fatigue times was obtained.The change of modulation index in the fatigue damage process is studied.The results show that the peak of modulation index peak is stable in the early stage,but due to the instability in the later period,using the peak of modulation index as the basis for judging the magnitude of fatigue damage will have greater error.With the accumulation of fatigue damage,the average modulation index has a stable growth rule in the experimental process,which can be used as a criterion for detecting fatigue damage.The correlation between modulation index and fatigue crack length was established.The detection method used as the fatigue crack length is discussed,and the crack specimen is detected on-line by the vibration acoustic modulation technology,the multi group of detection images are obtained.Using the modulation peak and the average modulation index extracted the data,get the scatter plot,and then refer to the goodness of fit,finally was the relationship between the modulation index and the crack length by the way of fitting the peak of the modulation index of the two function.A method for on-line detection of fatigue crack length is given. |
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