Stress And Stress Intensity Factor Measurement Of Structural Members Using PVDF Film

Cracks existing in structural members may affect the safety of a structure and may cause a catastrophic failure of the structure. Therefore, strength of the cracked member is important matter of concern when maintaining structures.Stress intensity factor (SIF) has often been employed for the strength evaluation of cracks induced in structures. The stress intensity factors of cracked members are usually obtained by numerical analysis methods such as finite element method (FEM) or boundary element method (BEM). However, numerical methods sometimes meet problems that cannot be easily resolved, such as the difficulties due to complicated boundary conditions and external loads applied to the structure, the complexity of structural configuration and the crack shape.In this thesis, one method to measure the stress distribution and one method to measure the SIFs of mode I and mode II cracks using PVDF films and an electrostatic voltmeter of non-contact type are proposed. Experiments were performed to validate those methods.First, the relationship between the output voltage of PVDF films and applied strains and stresses that they suffer was analytically deduced. Parameters in the relationship were identified with an experiment.Second, a kind of isotropic piezoelectric element was developed. One piece of PVDF film was tightly adhered to another one in the same size with their stretching directions perpendicular to each other. An experiment to validate the feasibility to real application of this developed isotropic piezoelectric element was carried out.Third, an experiment to measure stress distribution near a crack tip using PVDF films was performed, and FE numerical calculation was completed. The results from the experiment and numerical calculation were compared, and showed good agreement.Last, stress intensity factors of mode I and mode II cracks of a two-dimensional steel specimen were measured. Both methods with or without consideration of the second constant term in the series describing the stress distribution around a crack tip were employed. Method with consideration of the second constant term showed more sufficient accuracy than that without the consideration according to comparison to the results of numerical calculation. In addition, the comparison to numerical calculation validates the application of the propose stress intensity factors measurement method.