Авіаційно-космічна техніка та технологія (Jul 2019)
ANALYSIS OF RESIDUAL STRESSES WHEN USING CONSTRUCTIVE AND TECHNOLOGICAL METHODS OF INCREASING FATIGUE LIFE. MESSAGE 1. FREE HOLE
Abstract
Increasing the fatigue life of elements of aircraft structures by methods based on local plastic deformation of the material in the hole zone directly depends on the magnitude and sign of the residual stresses. This paper presents the results of the study of residual stresses, which are formed when using constructive-technological methods to improve the durability of elements of aircraft structures with a free hole. The processes of cold expansion and barrier compression of the material in the area of the hole are considered. The studies were performed using the finite element method for two aluminum alloys D16AT and V95pchT2, which are widely used in the domestic aircraft industry. Material properties are given in the form of monotonous strain diagrams obtained by tensile testing of standard samples. The contact problem was solved in a physically nonlinear formulation, taking into account the friction between the tool and the structural element. The nonlinear behavior of the material is modeled using a multilinear model with kinematic hardening, taking into account the Bauschinger effect. Characteristics of the stress-strain state arising in the structure after the completion of the process of refining and barrier compression have been obtained. Equivalent stresses calculated according to the energy theory of strength were analyzed as residual stresses. In this case, the sign of stress is taken to be equal to the sign of the largest in magnitude principal stress. In the study of the process of cold expansion, a variable parameter was interference fit. The dependence of the residual stress in the hole on the magnitude of the interference fit is obtained. In the case of barrier compression, the variable parameters were the geometry of the tool being introduced (width, grip angle), the thickness of the structural element and depth of implementation. The dependences of the change in residual stresses on the parameters considered are constructed. It is shown that such parameters can be selected, at which the effect of compression will be most significant. But this will increase the effort required to complete this process.
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