AL-Legierung bei niedriger Beanspruchung


Fatigue life in the very high cycle fatigue (VHCF) regime is typically determined by the crack initiation phase. However, cracks initiated from pre-existing flaws may cause a significant shortening of the crack initiation phase with a notable amount of lifetime spent in the crack propagation phase. Long lifetimes can still be achieved in the case of very small cyclic loads, which keep the crack in the near-threshold regime but may lead to unexpected crack extension behavior. This phenomenon is related to the fact that the crack tip field interacting with the microstructure is short-ranged and microstructural features can consequently block or deviate this field completely. It is hence important to include the effect of cyclic loads in the near-threshold regime of crack extension in VHCF research.

Material und Methods

Aluminum alloy EN AW-6082 in peak-aged (6PA) and overaged (6OA) condition was studied, its microstructure being defined by a rolling texture. Flat dog-bone specimens were machined from sheet material both in rolling (L-S plane, -LS) and transverse direction (T-S plane, -TS). Following mechanical and electrolytic polishing, a part-through notch was cut in the specimen radius using a razor blade polishing technique. Compression pre-cracking was used to introduce an initial crack that is open when unloaded. Afterwards, experiments were performed at nominally constant ΔK-values close to the threshold initially determined by continuous load increase. The fractured specimens were analyzed in a SEM (ZEISS Ultra Plus) as well as in a µ-CT (ZEISS Xradia 520 Versa). Moreover, crystal plasticity simulations using the spectral formulation coupled to DAMASK were performed with the aim of getting more in-depth information on the crack extension mechanisms. To this end, digitized models of the experimentally characterized microstructure were generated with the help of EBSD and X-ray tomography. A three-dimensional imaging of the grain structure was realized by wetting the grain boundaries with liquid Ga.

Results and Conclusion

Two mechanisms were identified keeping the crack from continuous extension. First, the crack front was pinned by primary precipitates leading to ductile bridges on the fracture surfaces (Fig. 1). Overaging was observed to generally enhance the pinning potential of the precipitates. The second mechanism was crack extension in a shear-dominated mode (Fig. 2). This effect was rather pronounced in the case of cracks propagating parallel to the elongated grains and to the lines of primary precipitates. Crystal plasticity simulations based on microstructural models of the fractured samples (Fig. 3) show that this effect can be traced back to the multiaxial stress field induced by grain anisotropy and is thus different from the single slip line associated with stage-I microcracks (Fig. 4).

Fig. 1: Close-up of fracture surface showing local pinning of the crack front
Fig. 2: Crack tip field and SEM-SE image revealing shear-dominated crack extension
Fig. 4: Equivalent (von Mises) strain εvM obtained by simulation