Crystal plasticity modeling of microstructure influence on fatigue crack initiation in extruded Al6082-T6 with surface irregularities

Abstract

The influence of surface roughness and microstructure heterogeneities in the vicinity of macroscopic stress concentrations are investigated by crystal plasticity simulations. It is shown that in the extruded Al6082-T6 alloy under consideration, twin grain boundaries and other highly misoriented grain interfaces, which constitute significant barriers to plastic slip, provide internal domains in the material where initiation of fatigue damage is more likely to occur than at the stress concentrations which are due to the surface roughness. In addition, the microstructure influence on the appearance of stress gradients is found to be significant. Further, the present study indicates that frequently used fatigue initiation parameters (FIP) such as the locally accumulated plastic strain or stored energy may be insufficient to identify fatigue crack initiation. Such parameters should be complemented or replaced by other FIP which take additional characteristics of the microstructure into account. This is exemplified in the present study by employing two additional FIPs, based on a modified Fatemi-Socie critical-plane parameter and on the Dang Van criterion, respectively.

A gradient-based multiaxial criterion for fatigue crack initiation prediction in components with surface roughness

“…There are many alternative ways of deciding on this length: depth of notch (Morel et al, 2009), extent of notch field, distance from notch to where the stress field gradient changes sign (Qylafku et al, 1999), or some multiple of grain size (Amargier et al, 2010). In our case, the critical distance corresponds quite well to the governing initiation notch depth (Hallberg et al, 2018), extent of the notch field (Hallberg et al, 2018), grain size (Ås et al, 2005).…”

“…The surface irregularities are of the same magnitude as the grain size of this aluminium alloy. Hallberg et al (2018) analyzed with CPFEM two of the specimens analyzed by FEM herein (X22 and X40). The computational domain was 400µmx200µm, and 10 grains with different orientations obtained from ESBD measurements were analyzed.…”

“…One can argue that using finite elements of micrometer size is violating the continuum assumption since the elements are of same size as the microstructural elements and less than representative volume elements (RVE). In recent years, accounting for microstructure in fatigue crack inititation analyses has received increased focus, e.g., by CPFEM methods (McDowell, 2007, Bennet and McDowell, 2003, Manonukul and Dunne, 2004, Li et al, 2015, Hallberg et al, 2018. This can provide more details on fatigue damage processes and is very important for increased understanding of the crack intitation phenomenon.…”

PRISMS-Fatigue computational framework for fatigue analysis in polycrystalline metals and alloys

“…FIPs were introduced to serve as computable surrogate measures of the fatigue crack formation driving force at the scale of individual grains/phases or subgrains 1 . FIPs can capture the effects of intrinsic and extrinsic microstructure properties (e.g., grain size 20 , phase volume fraction 20 , surface roughness 21 , porosity 22 , presence of inclusions 23 , etc.) and loading conditions (e.g., applied strain state and magnitude 17,24 ) on the probability of failure.…”

Micromechanical Modeling of Fatigue Crack Nucleation around Non-Metallic Inclusions in Martensitic High-Strength Steels

“…In the past, different types of materials have been the focus of micromechanical investigations on fatigue crack initiation in the absence of NMI. Selected works are for example: On polycrystalline ferritic and ferritic-pearlitic steels [24][25][26], on martensitic steels [27][28][29], on aluminum alloys [30], on nickel-base alloys [18,23,31,32] and finally on ideal brittle materials [33,34]. The listed works are exemplary and there is no claim to completeness.…”