Surface Crack Depth Evaluation Using PVDF Film And FE Method

The crack size is crucial for structure fatigue design, fracture mechanics analysis, judgment criterion of fracture and fracture control. A method to evaluate depths of surface breaking cracks using PVDF film and FE method is proposed based on the relationship between the ratio of crack opening displacement to nominal strain and the crack depth in the thesis.The analytical solution to the super singularity integral function for 3-D elastic body shows that there is a simple proportional relationship between the displacement difference and the crack depth. The influence of boundary conditions on the analytical solution was studied by FE calculation, which shows that an elastic body can be regarded as an infinite one in case that the geometric scale of the body is 7 times or larger than that of the crack. A surface-influence parameter was used to modify the relationship to obtain an empirical formulation for semi-infinite elastic body. A calculation example was presented to illustrate its applicability.A series of FE calculations were made to determine the influence of plate breadth, plate thickness and load form on the proportional relationship for plate structures, which was one of the most popular structure forms in engineering practice. Different crack shapes, crack depths and crack lengths were taken into account. The ratio of crack mouth opening displacement (CMOD) to nominal strain was used to eliminate the loading effect. Calculation results prove that the relationship keeps in spite of different boundary conditions of plate structures. An algorithm to determine the coefficient was given for practical application.The feasibility to measure CMOD of surface breaking cracks using PVDF piezoelectric film was discussed. It is shown that the structural deformation near crack mouth is mainly caused by the existence of the crack, and there is a "small-strain" area at each side of the crack mouth. Therefore, a relative larger PVDF film can be used to measure the CMOD without inducing much error. For PVDF film, the strain caused by the CMOD is limited to a small area just up the crack mouth. The suit size of PVDF film and electric circuit for measurement were decided.At last a technique was presented to evaluate crack depth. Three experiments were carried out to validate the proposed method. Good accordance between the results of experiment and the real ones. The evaluation of the maximum crack depth has higher precision than that of the crack depth of other parts. The method has proved practical in the experiments.