Detection Of Crack Propagation In Metal Materials Based On Micro-cavity Array Fiber

With the development trend of modern industry more and more mechanization and equipment large-scale,many mechanical equipment and large structures often face long and overloaded working conditions.Fracture and failure of mechanical structures have gradually become a major threat to the safety,reliability and service life of equipment structures.According to surveys,more and more engineering accidents are caused by mechanical structure fractures caused by fatigue cracks.Therefore,the crack detection and crack propagation research of the structure is very important to the safety of the structure.In large mechanical equipment and metal structures,the internal components of the structure are in a "fatigue" state for a long time due to the long-term impact of alternating loads such as external temperature and stress.It is inevitable to cause component wear,cracks,etc.,and even cause structural fracture.It poses a serious threat to industrial production and public safety.For this reason,many scholars and researchers in related fields have begun to focus on the formation and development of structural cracks,and have carried out in-depth research on this.In practical applications,it needs to have the characteristics of large range,multiple points,high accuracy,and large capacity.Traditional sensing methods cannot meet the above requirements.Micro-cavity array(MCA)fiber based on dense ultra-weak(DUS)FBG arrays can realize capacity.In a micro-cavity array,each pair of adjacent FBGs forms a micro-cavity(MC).A large number of micro-cavities are used as sensing elements to form a micro-cavity array.Local cracks in metal are typical structural damage caused by stress concentration.Therefore,compared with traditional back-scattering technology and dense ultra-short FBG array,MC has higher signal-to-noise ratio(SNR)and interference spectrum in crack detection.In this paper,micro-cavity array sensing technology based on dense ultra-short and weak grating arrays is used to realize crack detection and crack propagation research of metal materials.Acquisition and demodulation of sensor signals through optical frequency domain reflection technology(OFDR).Random wavelength and interval random methods effectively suppress the multiple reflections and spectral shadow effects of the micro-cavity array,and improve the reuse capacity and signal-to-noise ratio of the entire micro-cavity array.The in-depth study of single crack detection and crack propagation was realized by modeling and simulation of metal plates and experimental analysis.The main research contents are as follows:(1)Research on the sensing mechanism and multiplexing capacity of micro-cavity arrays.Based on the FBG coupling mode equation,the mathematical expression of the micro-cavity spectrum is derived,and its sensing mechanism is analyzed.Investigate the mechanism of multiple reflection effect and spectral shadow effect.The effects of the above two conditions on the multiplexing capacity of micro-cavity array fibers are analyzed.It is proposed that the use of sensor arrays with random grating spacing and random grating center wavelength distribution can effectively suppress multiple reflection effects and spectral shadow effects.(2)Crack detection and crack propagation simulation analysis.In the engineering application of the online crack detection system,it is necessary to build a theoretical model of the application object.By analyzing the fracture theory of applied materials and combining with model experiments,the signal analysis of tensile,crack and expansion transient strains is collected and analyzed.Non-uniform strain detection with high spatial resolution is achieved through micro-cavity array fibers.(3)Crack detection and crack propagation system integration and testing.In practical applications,it is necessary not only to realize the online monitoring of cracks,but also to judge and analyze the length and development trend of cracks.First,a limited detection of cracks is required.Based on this,the change of crack length and strain and the corresponding position of the crack are simulated.Summarize the change law and development trend of cracks,compare and analyze through experiments and simulations,and draw conclusions.