Temperature-dependent Damage-Tolerance Behaviors Of An Aerospace Material

With the continuous development of modern aeronautics science and technology and for the needs of the aircraft's comprehensive performance,the aircraft structure design ideas are constantly updated.Under satisfying the static strength,the damage tolerance design based on fracture mechanics has become one of the key technologies and widely used in aircraft structural design.Furthermore,considering the effect of service environment on aircraft structures,the analysis of the damage tolerance of aircraft structures under environmental influence has begun to receive more and more attention.The study of the static strength and fatigue fracture properties of materials under service environment is one of the important foundations of the damage tolerance design.In this paper,the static tension tests and fatigue fracture tests at room temperature and 175°C were performed for aerospace structural materials 7075-T651aluminum alloy.The macroscopic mechanical properties and microscopic mechanism of materials under different temperature environments were studied.According to the fatigue crack growth test results of nickel-based powder metallurgy superalloy FGH97at elevated temperatures,the main factors affecting crack propagation and the contribution of each factor were studied.The paper has obtained innovative research results with certain engineering application value in the following aspects.1.The consistency of the elastic modulus of the material measured by the extensometer and the resistance strain method was verified.The error of the two methods was less than 1%.The static tension test shows that the plasticity in the rolling direction?L direction?is slightly better than the T direction at room temperature,the fracture mode is mixed fracture and the shaft dimple is visible on the fracture.2.The effect of temperature and crystalline grain orientation of specimen on the crack propagation behavior of aluminum alloy was investigated by the compliance method.It is found that the effect of crystalline grain orientation on the rate of fatigue crack propagation in the Paris region is very small.The crack growth rate of the two crystalline grain orientations of the specimen at 175°C is lower than that at room temperature.The micro-fracture characteristics indicate that oxides appear on the fracture at 175°C and the fatigue strip is covered with an oxide layer;In addition,secondary cracks that extend in the depth direction at the grain boundaries also reduce the rate of crack growth.3.The trinomial model describing the fatigue crack growth of superalloys is improved,and a two-term model describing the high temperature fatigue crack propagation of materials by cycle-dependent term?f₁??K??and time-dependent term?f₂??K,t_h??is constructed.The model was used to conduct the high temperature fatigue crack growth data of FGH97 and the contribution of different factors in the total crack growth rate was discussed.4.The effect of crystalline grain orientation of specimen and temperature on the fracture toughness K_Q of the material was discussed.The fracture toughness test results were compared with the fatigue crack growth rate data.The conclusions of the two methods were found to be consistent,and the main factor causing the difference in fracture toughness of the material is the change in the plastic properties for variable temperature.