Experiments And Life Prediction Methods For Panels With Multiple Site Damages

Multiple site damage is a complex but important problem for damage tolerance design of aircraft structure.The resulted aviation accidents and potential threats have great impact on the design and analysis of aircraft structure.It is necessary to predict the propagation life of fatigue crack in plate with multiple site damage.Due to the location of structural damage,crack length and crack distributions of random,interaction between cracks,and which between cracks and holes is very complex.Efficient and reliable calculation of stress intensity factor for collinear hole-edge cracks is the key method of life prediction for structure with multiple site damage.At the same time,the effective fatigue crack propagation experiments of multiple site damage will also directly affect the progress of analysis and verification of multiple site damage.Therefore,the following three parts are included.The main work of this paper is divided into the following three parts:(1)Improving the crack tracking equipments.The traditional method for crack observation is complicated and inefficient.Based on the existing digital camera and stepper motor,according to the features of metal crack,using machine learning and image recognition technology,the existing crack observation system has been improved to satisfy the accuracy of the test.Implement the function of the automatic tracking and recognition for different cracks,and real-time recording of crack the length and loading times.This system has the advantages of high efficiency,automation,greatly improve the efficiency of fatigue crack growth test for multiple site damage.(2)An improved crack propagation tracking system was used to study the behavior of fatigue crack propagation for a porous multi crack plate.The results of experiments show that the interaction between the adjacent hole-edge cracks seriously reduces the propagation life of fatigue crack for the multi crack plate.For this extreme case of equal length cracks at the edge of the holes,the propagation life of cracks is 1/10 of plate containing a single center hole with hole-edge cracks.(3)An approximate solution of stress intensity factors at the tip of a plate crack at a typical porous and multi crack plate is proposed.In this paper,the Eshelby inclusion theory and the principle of phase toughening are used to determine the influence of the side hole on the stress intensity factor of the crack tip in a porous and multi crack plate.The factors that affect the stress intensity factor of the porous multi crack plate mainly include the holes and cracks.The influence of the hole mainly occurs when the crack grows from the edge of the hole and the crack is extended to the edge of the side hole.For the situation of the holeedge cracks emanating from edge of hole,this paper puts forward the method of weighted function to obtain the stress intensity factor of hole-edge cracks;when the crack gradually extended closed to the adjacent hole,the influence of hole on stress intensity factor of cracks was given quantitatevely by Eshelby inclusion theory with different length of cracks.When the cracks on both sides of the two adjacent crack holes gradually extended,the stress intensity factor is obtained by the approximate treatment of two collinear cracks when a/r>1.These approximate methods are used to predict the propagation life of fatigue cracks in porous and multi crack plates.The predicted results are in good agreement with the experimental results.It is very close to the results of fatigue crack based on the finite element method,but the efficiency of method in this paper is far higher than that of the finite element method.