Research On The Detection Method Of Weld Metal Cracking Of Metal Structure Based On PVDF

Metal structure inevitable generates crack on secular alternating loading. Thecrack will reduce the stiffness and cause great threat to the safety of the metalstructure. The emergence of the piezoelectric smart materials provides a new thoughtand method on metal structure crack identification. Due to the advantage of highsensitivity、high mechanical strength、resistance to fatigue、good performance onflexible and machining property、wide frequency response range，the PVDF iswidely used on structure health monitoring of aerospace and civil engineering.Thisthesis utilizes the piezoelectric effect of PVDF and the damage identificationalgorithm of curvature operating shape to identify the location of weld crack.In this thesis, the main research contents list as follows:(1) Analyze the piezoelectric effect and strain sensing mechanism ofpiezoelectric thin film, deduce the first type piezoelectric equation in detail. Utilizethe theory of the piezoelectric charge response of rectangular PVDF sensor bondedon a flexurally vibration beam is proportional to the integral of flexural stain over thesurface of application of the sensor, deduce the formula between the charge output ofPVDF and the structure acceleration.(2) Through theoretical analysis and mathematical deduction, get the damageidentification theory by the imaginary part of frequency response function, extractparameter from the FRF to build operating deflection shape，utilize the centraldifference approximately method to calculate the curvature operating shape, utilizethe Gapped-Smoothing method to estimated curvature operating shape of healthystructure, then make use of the difference in curvature operating shape between thedamaged and undamaged structure. Utilize the average coherence function of thegauging point to optimize the damage identification index which can reduce thenoise jamming and increase its sensitivity.(3) Build finite element modal and conduct numerical calculation by thetransient dynamics analysis module to verify the sensitivity of the damageidentification index proposed above on different length and different depth crack.Contrast the sensitivity of the damage identification index under different frequency. (4) Build dynamic signal acquisition system based on PVDF piezoelectric thinfilm to study its fundamental characteristics, including the effect of pasted place andsize of PVDF to its test result, and conduct test on a metal beam with two weldcrack,utilize PVDF piezoelectric thin film to measure ODS of the structure to detectdamage. Utilize the average coherence function of the gauging point to optimize thedamage identification index to reduce outside noise interference. At the same time.utilize the acceleration sensor to examine the accuracy of PVDF piezoelectric thinfilm.Through the above research, it is fully verified the good effect in metal structureweld crack detection by PVDF piezoelectric thin film and the damage identificationalgorithm of curvature operating shape. This research work makes a fruitful basis formetal structure weld crack detecting by PVDF piezoelectric thin film.