Research On Crack Location Method Of Composite Laminate Based On Lamb Wave Group Velocity Direction

Wind turbine blade are mainly made of glass fiber composite materials.Crack damage often occurs in the process of production and operation,which will cause major safety hazards for the operation of wind turbines.Lamb wave is considered the most has the application prospect of composite detection due to its long propagation distance and high sensitivity to small damage.Similar to “strain rosette” structure,using the directional piezoelectric fiber sensor can identify the principal strain direction of Lamb wave propagation.Theoretically,the intersection point of the Lamb wave propagation principal strain directions determined by two groups of piezoelectric fiber sensors is the damage location,and then the damage location is realized.Compared with the traditional damage location method based on time of flight,the principal strain direction identification method will be more simple and accurate.However,in the anisotropic composite material,Lamb wave principal strain direction is different from the group velocity direction(propagation direction),there is a certain deviation.Therefore,thesis conducted the analysis of Lamb wave propagation characteristics and crack in composite material plate scattering characteristics;Established the mapping relationship between Lamb wave principal strain direction in composite material plate with group velocity direction,and mastered the Lamb wave scattering characteristics in different incident wave direction and crack length;Proposed a crack localization method based on Lamb wave group velocity direction recognition of piezoelectric fiber sensors,using the piezoelectric fiber sensor array to accurately locate the crack in composite laminate,which provides a basic theory for the defect detection in large areas of complex structures.The main research contents of this paper include the following three aspects:(1)The second order Mindlin plate theory is used to derive the Lamb wave dispersion equation in the composite laminate.Based on the finite element simulation analysis,the effects of ply stacking sequence on Lamb group velocity,amplitude response and principal strain direction are studied,and the variation law between the principal strain direction and group velocity direction of Lamb wave under different layer modes is obtained,which provides a theoretical basis for the identification of Lamb wave group velocity direction of composite laminate.(2)Considering the direction of the crack,the effects of different incident wave directions and crack lengths on the scattering of Lamb wave were simulated and analyzed.The scattering characteristics of Lamb wave and different crack forms were mastered,which provided a theoretical basis for the layout of the sensor array.Based on the matching pursuing algorithm,the five-period sine wave atomic function modulated by hanning window was used to construct the dictionary library,and the scattering characteristic signals of Lamb wave interaction with crack were extracted effectively,which provided a useful analysis tool for the experiment of crack location.(3)Based on piezoelectric equation,the response characteristics of a single piezoelectric fiber are derived.Based on the principle of "strain rosette",the calculation formula of the principal strain direction is derived by using the voltage response amplitude of the three fibers in the piezoelectric fiber sensor.According to the mapping relationship between Lamb wave principal strain direction and group velocity direction in the composite laminate,the Lamb wave group velocity direction is obtained by modifying the principal strain direction,the crack position is determined by the intersection of the group velocity direction,and then the crack can be accurately located by piezoelectric fiber sensor array.