Research On Multiple Sites Damage Of The Win Skin Structure

Widespread fatigue damage usually refers to the existence of tiny cracks either on multiple components or couples of similar details,which will lead to connecting together under the action of fatigue loading,and the residual strength of structure will be too weak to bear the load leading to the structure failure ultimately.Widespread fatigue damage exists in the skin of the aircraft structure widely,it reduces the carrying capacity of the skin of the aircraft structure and affect the safety and life of aircraft seriously,especially in aging aircraft.Widespread fatigue damage includes multiple element damage and multiple site damage,this paper mainly studies the widespread fatigue damage.Based on the skin structure and 2024 aluminum alloy material which was commonly used in the wing skin,according to the general development of multiple site damage,these problems was studied,including the probability of occurring Multiple Sites Damage,rule of redistribution of internal forces and residual strength of 2024 aluminum alloy structure.The study has important reference value to the further understanding of the formation and development process of multiple site damage,but also provides a powerful reference for safety design and life assessment of aircraft structure.It also has the significant practical significance to Chinese aviation and national defense construction.For the study of occurrence probability of Multiple Sites Damage,firstly,give the criterion of multiple site damage,under the assumption that each details have the same fatigue characteristic and the crack occurred randomly and independently,according to the theory of fracture mechanics equivalent initial crack distribution and the three parameter Weibull distribution established the mathematical model to predict the probability of occurrence of multiple site damage in multi detail structure,specifically established the function relationship between load cycles and probability of occurrence.Finally,carried out fatigue test with 2024 aluminum alloy sheet single detail and three detail to verify therationality of the model and draw the conclusion: the model can better predict the occurrence probability of multiple sites damage of multi detail structure,but the prediction result is conservative.As for the internal force redistribution of multiple site damage,firstly,simplify the complex multiple site damage problem,according to a certain civil aircraft wing skin structure obtained two basic simplified model of multiple site damage,and then use Abaqus and Franc3 d to establish the finite element crack model to research structure comprehensive correction factor of structure intensity factors in the two basic multiple site damage structure,after obtain the conclusion that there exists function relationship between the combined correction factor and local load redistribution factor and the main crack length,through this function relationship study the change rule between combined correction factor and local load redistribution factor and the main crack length,and thus study the change rules of internal force redistribution indirectly.With a certain load,internal load of containing Multiple Sites Damage structure is distributed continuously,and the distribution is related to the structure and distribution of cracks,etc.For the study of residual strength in multiple site damage,firstly,according to the internal force redistribution conclusions and test standards,designed two test pieces with similar details and different crack distribution which contains multiple site damage,the test piece material using 2024 aluminum alloy.Through the residual strength test of the two types of multiple site damage test piece,discusses the applicability of three kinds of commonly used residual strength calculation method in the residual strength calculation of multiple sites damage structure,the methods are net section yield calculation method,the calculation method based on Swift connecting standards,and the calculation method under the condition of linear elastic fracture.Finally draws the conclusion: different crack distribution with the same details can influence the residual strength and net section yield calculation method is superior to other methods.