Some aspects to strength analysis of aircraft structures with respects to fatigue and fracture mechanics

by Prof. Stevan Maksimovic (Militarz Scientific Technical Institute, Department for Aeroronautics Serbian Armz)

Wednesday 30 Mar 2011, 18:00 → 19:00 Europe/Belgrade

301 F (Mathematical Institute)

Abstract:

The fatigue life of structural components is generally divided into crack initiation and crack growth. Where plastic strain predominates, at short lives, crack growth accounts for the major part of cyclic to failure. At long lives, elastic strain is dominant, as are the cycles occupied by crack initiation. Even in the case of nominal elastic loading, some zones have stress concentrations leading to plastic fatigue. In summary, fatigue analysis may be thought of as a process of initiating and then growing a crack, which finally causes the structure to break into two or more pieces. The mathematical models used to simulate the initiation and propagation processes are quite different. The initiation phase is usually modelled using strain-life and cyclic stress-strain curves while the propagation phase uses crack growth rate versus stress intensity curves.
Attention in this consideration is focused on developing computation methods of damaged structural components with respect fatigue and fracture mechanics. Considered computation methods are based on combining singular finite elements to determine stress intensity factors for cracked structural components with corresponding crack growth lows that include effect of load spectra on number of cycles or blocks up to failure. Crack growth analyses are considered using two aproaches: conventional and strain energy density (SED) approach.
For the lifetime evaluation of structural elements until the occurrence of initial damage in the low-cycle fatigue domain the relations for which the magnitudes of low-cycle material behaviour properties have to be obtained experimentally are being used. For the crack propagation analysis and evaluation of residual lifetime of structures two approaches can be used. First approach is based on conventional crack propagation laws such as Paris` crack propagation law, for which it is necessary to experimentally obtain dynamic properties of the material. The second approach for crack propagation analysis is based on the use of Strain Energy Density Method. This approach uses the low-cycle properties of the material, which are also being used for the lifetime evaluation until the occurrence of initial damage. Therefore experimentally obtained dynamic properties of the material such as Paris` constants are not required when this approch is concerned.
To demonstrate efficient computation procedure in fatigue life estimation here numerical examples are included. Attention in this work is focused on design of aircraft wing-fuselage joints. Computation procedure to strength analize with respects fatigue and fracture mechanic is applied to cracked aircraft structural components. Computation results are compared with correspond experiments.