Study On Method For Calculating Stress Intensity Factor Of Structural Surface Crack

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. It is one of the basis factors to get the SIFs for the growth of the crack and the analysis of the residue strength. The SIFs of cracked members are usually obtained by numerical analysis methods. 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 and so on.In this paper a new method of structural surface crack detection for the measurement of surface crack depth and stress intensity factor is developed using PVDF films as strain sensor.First, based on the analytical solution of the displacement jumps across a crack surface embedded into the infinite three-dimensional homogeneous elastic solid, the relationship between the surface crack opening displacement and the crack depth is deduced, through boundary effect processing from crack embedded into the infinite elastic solid to the surface crack with the finite element numerical calculation. And finite element analysis for some cracked models was also performed to check the relationship.Then the relationship between the crack depth and the SIF is deduced, based on the analytical solution of the stress intensity factor embedded into the infinite three-dimensional homogeneous elastic solid and the finite element method.Finally, the validity of this method is demonstrated by the experimental results. First, a piece of PVDF film is adhered onto the surface crack directly. The surface crack opening displacement can be calculated by the measured electric potential on the polarized PVDF film, when the film has deformation caused by the opening displacement. Then get the depth and the SIF of the crack from the measured crack opening displacement. Experiments were carried out on a plate specimen with a surface semi-elliptical crack in order to verify the feasibility and accuracy of the proposed experimental method.