Microstructure-dependent fatigue damage process zone and notch sensitivity index

Abstract

The development of simulation methods for calculating notch root parameters for purposes of estimating the fatigue life of notched components is a critical aspect of designing against fatigue failures. At present, however, treatment of the notch root stress and plastic strain field gradients, coupled with intrinsic length scales of grains or other material attributes, has yet to be developed. Ultimately, this approach will be necessary to form a predictive basis for notch size effects in forming and propagating microstructurally small cracks in real structural materials and components. In this study, computational micromechanics is used to clarify and distinguish process zone for crack formation and microstructurally small crack growth, relative to scale of notch root radius and spatial extent of stress concentration at the notch. A new nonlocal criterion for the fatigue damage process zone based on the distribution of a shear-based fatigue indicator parameter is proposed and used along with a statistical method to obtain a new microstructure-sensitive fatigue notch factor and associated notch sensitivity index, thereby extending notch sensitivity to explicitly incorporate microstructure sensitivity and attendant size effects via probabilistic arguments. The notch sensitivity values obtained for a range of notch root radii using the new statistical approach presented in this

Micromechanical modeling for the probabilistic failure prediction of stents in high-cycle fatigue

“…An other approach, proposed by Owolabi et al [40], provides a definition of the fatigue damage process zone based on a ''fatigue indicator parameter" denoted DC. This method consists in characterizing the susceptibility to fatigue crack initiation in each grain thanks to an equivalent mechanical quantity DC and to define the process zone with the grains whose DC values are greater or equal to a threshold value.…”

A new approach to estimating the fatigue notch factor of Ti-6Al-4V components

“…The probabilistic framework used to obtain the fatigue notch factor and the associated probability of fatigue at the scale of grain follows the framework presented in [14][15] with a few modifications based on the physical mechanisms of fatigue crack formation and growth for heterogeneous aero-engine materials. This is important as fatigue is manifested by extremal microstructure attributes that promote slip intensification and hence does not conform to homogenization.…”

“…This is important as fatigue is manifested by extremal microstructure attributes that promote slip intensification and hence does not conform to homogenization. Following the framework presented in [14][15], the probability of survival of a notched component having a fatigue damage process zone of volume V d , divided into small volume elements, dV with probability of survival of a sufficiently small volume element is given as: (5) where ξ,  th and σ 0 represent the Weibull shape, location and scale parameters respectively. The shape parameter, ξ , is a measure of the scatter in the stress, distribution among grains within the damage process zone V d at the notch root;  th is the threshold stress below which no microdamage will occur at a given number of cycles in the high cycle fatigue (HCF) regime, and V 0 is a reference volume.…”

“…The probabilistic framework presented in [14][15] focuses on the formation of a fatigue crack on the scale of a single grain, and will therefore be applicable to the high cycle fatigue 1 0 0…”

“…It is therefore very difficult to link the k f obtained using these methods to the realistic microstructure of the material such as grain size, grain orientation, multi-phases, and the notch root geometry. Owolabi et al [14][15] recently developed probabilistic models based on the weakest link theory and extreme-value statistics where elements of crystal plasticity were combined with new probabilistic methods for notch sensitivity based on computed slip at the notch root within a well-defined fatigue damage process zone for homogenous oxygen free high conductivity copper. The purpose of this paper is to extend this model to heterogeneous dual-phase aeroengine materials such as the titanium alloy.…”