Localized Flexibility Analysis And Measurement Of Pipe Cracks

The crack in the structure can be described by a local flexibility,and the local flexibility changes with the crack propagation,this change can effectively reflect the damage degree of the structure.Therefore,local flexibility is of great significance in the study of structure dynamics and crack detection.In this paper,based on the principle of strain energy release,the mechanical model of the cracked pipe is established,and the local flexibility of pipe crack with arbitrary crack orientation angle is derived.Then,two kinds of methods for measuring the local flexibility of pipe cracks are proposed.The local flexibility measurement technique of pipe crack is studied from two aspects of experimental study and numerical simulation.The main work is as follows:(1)According to the theory of linear elastic fracture mechanics,the pipe is decomposed into a series of thin walled tube is calculated by using the radius from small to large,the local flexibility of crack is obtained by using the stress intensity factor formula of thin-walled tube,and the crack model of pipe considering the change of direction of crack is established;Then the crack and crack breathing in pure bending under the calculation equation of the local flexibility of pipe crack tip crack direction and the vertical axis of an arbitrary angle tube.(2)A vibration based method for measuring local flexibility of pipe crack is proposed.By using the pipe dynamic model and vibration test phase,characteristics of the inherent relationships between the local flexibility and pipe frequencies for the different crack positions of the crack is established pipe fault database,drawing the natural frequency influence surface.And then test for pipe,obtaining the first two order natural frequencies as input parameters to capture natural frequency affect the surface,drawing the corresponding effect of local flexibility crack front two order frequencycurve to the crack position,using the intersection measurement of local flexibility crack.The validity of the method is verified by comparing the experimental and numerical results.In the process of pipe modal parameter testing,the sensor can be installed in the structure which is not suitable for people,and the process of vibration signal extraction does not affect the normal use of the structure,suitable for on-line measurement.(3)Based on the study of the relationship between the equivalent stiffness and the deflection of the crack,a new method for measuring the local flexibility of pipe crack is proposed.Based on the principle of strain energy,the relationship between the strain energy,bending moment and deflection of the crack region is derived,the expression of the equivalent stiffness of the pipe at the crack location is established to obtain the local flexibility of the crack.In order to verify the validity of the local compliance crack measurement method,finite element analysis software to establish a cracked pipe model using ANSYS,and finite element calculation results and measurement results were verified,consistent.The validity of the method for measuring the local flexibility of pipe cracks based on static deformation is verified.Because the parameters such as the deflection of the pipeline and the position of the crack are easy to measure,the requirement of the instrument is low,so this method is easy to operate and reliable.