Measurement And Study On The Fatigue Crack-tip Displacement And Strain Fields Based On The High-speed Digital Image Correlation Method

Fatigue failure is main failure mode for mechanical parts.Fatigue crack initiation and propagation occurs in material or structure while they are subjected to alternating loading,resulting in the action of fatigue.Nowadays,fatigue failure cannot be completely studied by the effective theoretical method,so,the specific materials fatigue crack propagation experimental test is a subject of considerable importance for studying fracture mechanism,It has a great important significance to improve the reliability and the service life of mechanical products.Metallic material fatigue crack propagation test is used to study the material fracture properties with pre-notch.In the present time,the metallic material fatigue crack propagation test can not reveal the evolution process and mechanism of fatigue crack initiation,growth and fracture from an overall perspective of micro and macro deformation of material.In this paper,the method based on digital image correlation(DIC)which is a new non-contact precise optical measurement mechanics for objects micro deformation and high speed photograph technology has been proposed to measure the displacement,strain fields at crack tip in fatigue crack propagation test.And the variation law of displacement and strain fields are studies in the fatigue crack tip area of the compact tension(CT)specimen during the steady crack propagation stage and under high frequency sinusoidal alternating load in the fatigue crack propagation test.The main work in the dissertation contains five aspects:1.The resonant fatigue crack propagation test system based on the high speed DIC was established,the system mainly includes the testing load control system and high-speed DIC measurement system.2.In order to research the variation law of displacement and strain fields in the fatigue crack tip area of the CT specimen under high frequency sinusoidal alternating load,a non-contact optical measurement method based on the DIC has been proposed in this paper,the principle of DIC method and the numerical calculation method are discussed in detail.Due to the displacement variation of crack tip region adjacent point is smaller and the characteristics of discontinuous region may be produced in the fatigue crack tip region,the Reliability-guided DIC method and forward additive Gauss-Newton algorithm for sub-pixel matching algorithm are applied to calculate the displacement and strain fields of fatigue crack tip region,and compiles DIC program by matlab.3.A series of digital speckle images of CT specimen under sinusoidal alternating load were collected by digital high-speed photography equipment,three degrees freedom vibration system mechanics model and the dynamic equation are established.According to kinematic analysis of the specimen under high frequency sinusoidal alternating vibration load and deformation of the specimen is the periodic sinusoidal alternating variables with the same frequency with the applied sinusoidal load within the elastic range.On this basis,the displacement and strain fields within the region of crack tip in each image were calculated by DIC.The sinusoidal changing strain curve of the feature point in the fatigue crack tip area has been obtained by the least square sine wave fitting method,and the characteristic parameters of sinusoidal strain were calculated,such as the amplitude,frequency,phase,mean load.The images of characteristic position in a stress cycle were obtained by comparing the fitted sine curve of strain with the corresponding speckle images.4.The crack tip position and stress intensity factor(SIF)were calculated on the basis of the displacement field by DIC.The displacement field,stress field and stress intensity factors at CT specimen fatigue crack tip are calculated by static finite element analysis under the same condition.The comparing results of the static finite element analysis with DIC calculation shows that the high speed DIC measurement method is accurate and the feasibility of measurement system has been verified.5.The relationship between strain amplitude values in crack tip and fatigue cycle numbers before the fatigue crack extension,and the variation laws of displacement and strain fields in crack tip area at the different lengths of fatigue crack have been studied.In this paper,both the problems of the evolution mechanism of crack in fatigue crack propagation test studied only by the macroscopic crack length measurement and angle measurement and the limitation of the crack propagation problem studied by numerical simulation and theoretical methods are resolved.It can measure deformation information in the perspective of micro from fatigue crack initiation to growth process,Not only help deepen the understanding of the nature of material fatigue deformation,but can provide theoretical and test basis for further high-speed DIC fatigue crack propagation test technology application and study on the crack propagation threshold value,crack propagation parameters and the fatigue life measurement and research.the presented method has the theoretical significance and practical value.