Search

Based on biaxial tests with cruciform specimens, the scenario of fatigue cracks in the upper fuselage section between the wings and the tail unit of a commercial single aisle aircraft has been studied. Two different types of specimen were designed, namely for the scenario of a circumferential crack across a broken stringer and for a longitudinal crack parallel to the stringers. With crack length up to 2a ≈ 610 mm, maximum mode I stress intensity factors of 160 MPa√m were obtained for the commercial aluminum alloys AA2024-T351 and AA5028-H116. All experiments were supported by digital image correlation to obtain the actual deformation fields of the specimens. Based on these data stress intensity factors and J integrals were computed with a line integral procedure. Furthermore, plastic zone and crack closure effects were studied. [ABSTRACT FROM AUTHOR]

The plastic zone at a crack tip of the commercially available aluminum alloy AA2024-T3 was energetically analyzed by digital image correlation and coupled FEM simulations. In mode I, mode II and mixed mode experiments the crack tip was cyclically loaded by the same maximum equivalent stress intensity factor. In every load maximum and minimum a digital image of the crack tip area was taken to compute the displacement field using digital image correlation (DIC). The computed displacement field was mapped on the nodes of a 2d FE-model. Using a bilinear isotropic hardening model the stresses and the plastic strain energy as a measurement for the damage accumulation in the plastic zone was computed. Beyond that a postprocessor for the DIC based strain fields has been programmed to calculate the actual stress intensity factors during cyclic loading. [ABSTRACT FROM AUTHOR]

The different types of plastic zones found at a crack tip under cyclic loading of a commercially available aluminium alloy are studied based on 3-dimensional finite element simulations and mechanical testing. During the experiments, the local strain field at the crack tip is computed based on digital image correlation analysis of the specimen’s surface under different load levels. The simulations with an elastic-plastic material model (bilinear isotropic hardening) predict the experimental surface strain field at the crack tip pretty well. The combination of experimental results and finite element analysis allows for the accurate distinction into monotonic and cyclic plastic zone, and moreover, into backward cyclic plastic zone and forward cyclic plastic zone. Furthermore, it is possible to calculate the energy accumulation ahead of the crack tip which represents a very useful quantitative measure for the analysis of local fatigue damage accumulation inside the plastic zone ahead of a crack tip. [ABSTRACT FROM AUTHOR]

The formation of dislocations patterns within the plastic zone ahead of a crack tip introduced by fatigue loading in AA2024-T3 sheets is studied by electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD). The aim lies in correlating local plastic effects at grain level with digital image correlation (DIC) results obtained at coarser resolution. While dislocation patterns are observed already prior to fatigue loading, the presence of dislocation sources and dislocation entanglements increases towards highly deformed regions near the crack path. Also, deformation veins are revealed by EBSD in grains located along the crack path. Based on ECCI and EBSD the dislocation density was determined and shows a gradient towards the crack tip. The results help linking the process of fatigue deformation at grain-level to the deformation field probed at the meso-scale level. [ABSTRACT FROM AUTHOR]