N13A-T007 TITLE: Multi-scale Peridynamics Theory for Corrosion Fatigue Damage Prediction

TECHNOLOGY AREAS: Air Platform, Ground/Sea Vehicles, Space Platforms

ACQUISITION PROGRAM: PMA-261

OBJECTIVE: Develop innovative techniques using peridynamics theory to predict corrosion fatigue across length and time scales in Naval aircraft.

DESCRIPTION: Corrosion damage remains a major challenge in aging Navy aircraft fleet with implications for both the safety and economic operation of components and structures. Of particular interest is corrosion fatigue, which can severely limit material lifetime and performance. Corrosion fatigue is the degradation of a material due to interaction of corrosion and mechanical stress due to cyclic loading. The crack propagation is driven by stress and electrochemistry, as the applied stress may open up cracks that allow easier diffusion of corrosion products away from the crack tip, allowing crack tip to corrode faster. Additionally, hydrogen tends to be attracted to regions of high tensile stress in metal structures, such as the region around a crack tip. This leads to embrittlement of the metal, making fracture easier. As a significant factor in cost, airworthiness of the aircraft and fleet availability, a pro-active approach in design of new aircraft as well as sustainment of legacy fleet to address the corrosion damage is of great importance.

The difficulties in accurate prediction of corrosion damage point to the fact that corrosion fatigue is inherently a multi-scale process in both length and time. Although qualitative effects from three basic sources of corrosion, i.e., design, environment, and maintenance, are well understood, prediction of corrosion fatigue damage in service has been a quite a challenge despite several decades of research in corrosion. This is because of the inherent time and loading frequency dependence of crack initiation and propagation and the dependence of damage developed within a wide range of interacting mechanical, material, and environmental variables.

Computational modeling of corrosion fatigue must bridge damage phenomenon occurring across length and time scales, capturing the interactions between cyclic loading and electrochemistry. To capture microscale corrosion electrochemistry, various modeling and simulation techniques exist such as continuum mass transport, kinetic Monte-Carlo, density functional theory, and molecular dynamics with classical interatomic potentials. Similarly, to capture the macroscale response, many continuum and field methods of analysis and approximation theories also exist. However, inherent difficulties are encountered in multi-model coupling approaches such as intensive computational resources and time requirements in addition to defining and deriving suitable parameters to bridge scales from one level to the next.

An alternative theory, known as the peridynamic theory, is a nonlocal extension of classical continuum mechanics that is based on integral equations, in contrast with classical theory of continuum mechanics, which is based on partial differential equations, and has the capability to handle multi-scale modeling for both length and time, and address discontinuities and non-linearity. The peridynamic theory has the potential to serve as a basic model across all scales avoiding the difficulties inherent in multi-model coupling in addition to the ability to efficiently link with many microscale models including molecular dynamics.

The proposed computational models must underpin the true physical processes rather than empirical correlations and deal with mechanisms operating at different length scales. Model should also minimize computational resources and time requirement.

PHASE I: Propose a suitable analytical technique/method for corrosion fatigue damage prediction using peridynamics. Demonstrate the feasibility of applying this method for example case studies. Outline approach for further development of the proposed technique in Phase II.

PHASE II: Develop the proposed approach to predict key chemical reactions and threshold stress levels controlling corrosion damage kinetics and factors affecting them. Implement the proposed model in a continuum code for damage prediction in simulated service conditions. Demonstrate the prototype model with available experimental data.

PHASE III: Transition the peridynamic dynamic model developed for use with commercially available computational tools to assess the effects of corrosion fatique damage for Navy aircraft platforms.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Corrosive enviroment concerns are faced by many industries besides such as automotive, heavy industry, and chemical plants. The developed technology could be integrated with existing software to address design and in-service maintenance issues faced by these industries.

REFERENCES:
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