Low cycle lifetime assessment of Al2024 alloys
Creators: name: Khan, Shehzad Saleem
Publisher: name: Technische Universität Dortmund
Year: 2011
Subjects: subject: Aluminium alloys
subject: Continuum damage mechanics (CDM)
subject: Low cycle fatigue
subject: Quasi-brittle damage
subject: 620
subject: 670

description: Summary and Conclusions Many research groups around the globe take a direct route from failure undermonotonic loading to high cycle fatigue. Surprisingly, low cycle fatigue being a border phenomenon between the aforementioned failure phenomena, is often not considered. In the present thesis, the gap between failure under monotonic loading and HCF was closed by thorough LCF analyses. As a practically relevant material, the high-strength aluminium alloy Al2024 has been chosen. The SRµCT investigations have made it evident that micro-mechanics based porous plasticity damage models are not suitable for this particular alloy due to a high number of dense particles. The particles are arranged in continuous layers throughout the micro-structure. Studying the microstructure and fracture mechanisms, it can be safely stated that bigger inclusions in Al2024 sheets and plates are responsible for damage initiation in the LCF regime. Al2024 thick plate, due to its underlying manufacturing process develops precipitates in the middle of the plate, creating layers within. A transition of fracture mode, from surface to internal fracture, was observed with increasing plastic range in cyclic experiments. In an ordinary low cycle regime (40-200 cycles), a fatal crack is generated by the propagation and frequent coalescence of small surface cracks. Eventually, a mesocrack initiates from the surface of the specimen due to the presence of bigger intermetallics. For a 100 mm thick plate the S-direction has been found to be very brittle when compared to L and T-directions (rolling directions) respectively. Although there is a ductility observed, the resulting failure occurs without macroscopic softening. An identical response was also observed in 4 mm thin sheets. For allowing symmetric strain amplitudes (R = -1) in such sheets, a oating antibuckling guide has been proposed which successfully prevents buckling at high compressive loading without affecting adversely the mechanical behaviour of the specimen. In summary, the material behaviour in S-direction can be characterised as neither completely brittle nor completely ductile. More precisely, plastic strain accumulation as damage driving process as well as small fatigue crack growth as brittle damage mechanism can be seen for LCF. This led to a conclusion that for modelling low cycle fatigue for these materials, a material model which incorporates both these effects (coupled ductile and brittle) was required. For that purpose, a phenomenological CDM approach was considered in the present thesis. In our earlier analyses in the past, only ductile damage was taken into account. However, since the microstructural arrangement of Al2024, as observed in our experiments favour also brittle failure modes, a fully coupled ductile-brittle model has been proposed. While ductile damage has been modelled in a similar fashion as advocated formerly by other authors (see, e.g. Lemaitre &amp; Desmorat (2005)), a novel approach was elaborated for brittle damage. In sharp contrast to the ductile damage model, material degradation can already evolve below the yield limit. Since early work on a large number of smooth and notched specimen demonstrated that wide variations in commercial aluminium alloys caused little or no detectable differences in fatigue strength, the novel coupled damage model is also applicable to a wider range of aluminium alloys. The material parameters necessary to use the proposed model can be easily obtained from a hysteresis loop test in order to determine the cyclic stress-strain curve. The predicted LCF lifetimes for A12024 alloy are in good agreement the respect experiments verifying the underlying assumptions.