Investigation of Low Cycle Fatigue Damage of Al 7075-T6 by Experimental and Numerical Methods.
Year: Sept. 2008- Feb. 2011.
One of the most important and widely used alloys of aluminum in aerospace structures is 7075-T6. On the other hand, many aviation parts are under low-cycle fatigue loading. Therefore, the analysis of 7075-T6 alloy under low-cycle fatigue loading is very important. In this thesis, prediction of failure of 7075-T6 aluminum alloy in the low cycle fatigue loading with damage mechanics approach is applied by numerical and experimental methods. A fully coupled constitutive elastic-plastic-damage model is developed and implemented inside the finite element commercial FORTRAN code, for ABAQUS VUMAT subroutines. The main advantage of developing a user-subroutine inside a commercial FE code is the access to a wide range of element types, material models and other facilities such as efficient equation solvers.First, damage parameters and coefficients for 7075-T6 aluminum, under three-dimensional stress conditions consistent with actual state are obtained from experimental results. Therefore, an experimental procedure to identify the damage parameters under triaxial stress conditions is proposed. To identify and extract the parameters, three tests are developed and regulated. The experiments included tensile tests, for simple and notch tensile specimens, measuring damage test and low-cycle fatigue test. To measuring damage, the voltage drop and measuring Young's modulus methods for specimen under tensile test, are used. Modulus measured by strain gauges and extensometer. Various experiment and measuring methods have been compared, and parameters are obtained from the average results. Extensometer data are very similar to the strain gauge data while electrical data have an offset to the strain gauge data. Then, by introducing the damage, basic definitions of damage mechanics are discussed. Lemaitre’s damage model and the constitutive equations based on small deformation are introduced. The next step is introducing two algorithms for solving the equations. In one of the algorithms, the stress updates neglecting the kinematic hardening by solving one non-linear scalar equation. In the complete algorithm, the combination of the isotropic and kinematic hardenings is considered, all variable update by solving two scalar nonlinear equations. The discussed algorithm can be able to trace the different loadings and low cycle fatigue. The non-linear obtained equations are solved by Newton-Raphson iterative procedure. The compound algorithm is able to model the material’s behavior in accordance to the experimental results, while the simplified algorithm does not predict these behaviors under the non-uniform and fatigue loading.
Keywords:Damage mechanics, Al 7075-T6, Low cycle fatigue, Kinematic hardening.