Structure Strain And Damage Detection Using PVDF Films

Polyvinylidene fluoride (PVDF) film is a type of intelligent sensor. It is widely used in the field of structural damage inspection for its good performance. Some research work on the various performance of the film as strain sensor has been done in this thesis. In addition, series of experiments were carried out to validate the accuracy of strain measurement.First, the relationship between the measured potential on the PVDF film and the strain components on the structural surface was deduced from the piezoelectric constitutive law under the non-contact condition. Then calibrating experiments of the PVDF film are carried out.Secondly, with the aim to improve the application of polyvinylidene fluoride (PVDF) films for surface-strain sensing of ship and marine structures, we develop an experiment method for the strain sensing by using an electrostatic voltmeter of non-contact type to detect inductive charges on the PVDF film which is covered by paint coating of the structure. Then, according to the measurement theory of the probe connected to the electrostatic voltmeter, we analyzed the field-nulling technique between the probe and the detected film surface, and doped out that it was possible to measure the inductive charges on the PVDF film even covered by the paint coating of the structure. An Experiment was carried out, in which PVDF films were adhered upon a smooth plate specimen under known axial uniform tension, to obtain two parameters in the deduced potential-strain relationship. The same measurement was also performed after the PVDF films were sprayed with the paint. The experiment results conclude that it has little effect on the detected electric charge whether there is paint or not. The different thickness of the paint also has no influence to the results. Our research shows that the proposed method is feasible and has obvious significance for the strain sensing of engineering structural surface.Furthermore, in order to simplify the operation and solve some practice problems in stress measurement of real structures, a method of making combined PVDF film is also proposed based on the piezoelectric constitutive law. The combined PVDF film has more advantages than ordinary PVDF film and can greatly decrease the cost of the stress measurement.Lastly, an experimental method for the measurement of surface crack depth using polyvinylidene fluoride (PVDF) films is developed. First, the relationship between the surface crack opening displacement and the crack depth is deduced, based on the analytical solution of the displacement jumps across an crack surface embedded into the infinite three-dimensional homogeneous elastic solid.