Research On Natural Frequency Fatigue Degradation Law Of Composites Under Tension-Tension Cyclic Loading

With the development of science and technology,composite materials are widely used in the main bearing parts of aerospace,wind power,automotive and other industries.The fatigue problem of composite structures is becoming more and more prominent due to the increasingly complex loading conditions and the ever-increasing use requirements.How to quickly and effectively identify and locate the damage that has occurred in the structural system and quantitatively evaluate the residual life of the damaged structure become particularly important.Because the dynamic parameters of structures can be easily obtained from the modal test,and the performance and integrity of the structure can not be destroyed during the monitoring process,it is important to study the relationship between the modal parameters and the fatigue life of composites.In this paper,experimental modal analysis is used as a nondestructive monitoring method to characterize and quantify the fatigue behavior of composite laminates by investigating the response of natural frequency to the changes of microstructures during fatigue process of laminates.Specific research contents are mainly divided into the following parts:(1)The method of calculating natural frequency of laminates are elaborated from computational and experimental modal analysis respectively.The effects of fiber structure,matrix,temperature and damage on natural frequency of laminates are discussed,and mainly discuss the effects of damage on natural frequency from the point of view of local stiffness attenuation.(2)The damage accumulation is specific to the fatigue process of laminates.From the perspective of the degradation of the material properties and the change of geometric properties during the fatigue process of the laminates,the main damage forms such as matrix cracking,delamination and fiber breaking are quantitatively described by finite element modeling,and the influence of different damage forms on the natural frequency of the laminates is discussed.The damage forms are superimposed to some extent according to the fatigue stages,and then the natural frequency in the fatigue process is discussed.(3)The fatigue dynamic response test of carbon fiber laminates was carried out to verify the rationality of the theory and finite element analysis,and the fatigue natural frequency degradation law of composite laminates was summarized;the natural frequency fatigue degradation model was established based on the test datas,and the rationality of the model is verified by predicting the fatigue life of laminates.