Crack Paths 2012

Advances in Fatigue CrackGrowthModeling

Huseyin Sehitoglu1, Piyas Chowdhury1, Garrett Pataky1, Rick Rateick2,

HansJ. Maier3

1 Department of Mechanical Science and Engineering – University of Illinois – Urbana

Illinois 61801 (USA)

2 Advanced Technology, Honeywell Aerospace – South Bend, Indiana 46628(USA)

3 Lehrstuhl für Werkstoffkunde, Universität Paderborn Pohlweg 47-49 D-33098 Paderborn

(Germany)

huseyin@illinois.edu

Experiments

The concept of irreversibility

is significant in understanding the crack growth behavior in

metallic materials. W e provide an overview of the literature highlighting the important

contributions. W e consider several cases to illustrate the importance of irreversibility of

plastic strains at crack tips on fatigue crack growth behavior. The work is divided to two

parts: experiments and modeling.

In our experimental work, we utilize extensive digital image correlation of cracks growing

in single crystals and polycrystals, and we provide direct measurement of crack tip plastic

W econsider cracks that are growing normal to principal stress in certain

strain irreversibility.

single crystal orientations and also cases where the crack growth is crystallographic on {111}

planes, hence the crack is subjected to a considerable shear stress component. Digital image

correlation was used to measure crack growth rates and displacements during fatigue loading

cycles. Stress intensity factors, KI and KII, and the T-stress for the isotropic and anisotropic

cases were determined. Plastic zones associated with the fatigue crack growth were

determined for all specimens using an anisotropic yield criterion. Slip irreversibility in front

of the crack tip was measured and showed an increasing trend with increasing crack length;

this information is critical for describing the crack growth behavior.

Modeling

In our modeling work, molecular dynamics and numerical simulations based on discrete

dislocation formulations are performed to model fatigue crack growth. Irreversibility of crack

tip emitted screw dislocations as imposed by obstacles (twins) upon cyclic loading is studied.

A method is developed to quantify the irreversibility of slip as a function of microstructural

dimensions - the twin lamella width and the crack-tip to twin spacing. The energy barrier for

cyclic slip-twin interaction is calculated. Discrete dislocation da/dN formulations are derived

for the cases ranging from single to multiple screw dislocations emission from the crack-tip

over cycles. Atomistically calculated energy barriers are incorporated in these numerical

simulations to understand the role of thickness of the nano-twins as well as the crack-tip to

twin spacing on the F C Gmetrics. Results demonstrate that both for the cases of decreasing

nano-twin thickness or lowering of crack-tip to twin spacing, F C G properties improve to a

substantial extent complying with recent experimental findings in the literature.

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