N251-015 TITLE: Predictive Lifing Tool for Coupled Corrosion, Pitting, and Fatigue Degradation

OUSD (R&E) CRITICAL TECHNOLOGY AREA(S): Advanced Materials;Sustainment

OBJECTIVE: Develop and demonstrate predictive modeling and lifing capability for coupled corrosion, pitting, fracture, and fatigue mechanisms of naval aero-engine propulsion materials.

DESCRIPTION: Critical propulsion components are carefully lifed to uphold system safety and reliability in support of the warfighter’s mission. Lifing analysis relies on an understanding of the fracture and fatigue mechanics of component materials to establish inspection, maintenance, and removal intervals throughout the life of the weapons system. Often, component lifing is based on empirically derived material properties, S-N curves, and Weibull analyses of inspection and failure events. However, the roles of the austere naval operational environment and environmental degradation of material properties are difficult to evaluate, and it becomes even more complicated to predict the resulting impact on fatigue and service life.

In naval aviation, the warfighter must operate within maritime atmospheric environments, and critical propulsion components are exposed to harsh cycles of high humidity, temperature, and salt exposure, which contribute to corrosion degradation and pitting. In recent years, there has been significant research into coupled corrosion, pitting, fracture, and fatigue mechanisms. This research has uncovered numerous contributing mechanisms, such as galvanic coupling, high temperature oxidation, saltwater corrosion, cyclic environmental corrosion, hydrogen embrittlement, coupled erosion/corrosion or impact/corrosion, among others. This interconnection of mechanisms has made the development of predictive corrosion modeling very difficult and very complex.

However, there is an ever-growing need for predictive lifing capabilities. The material, maintenance, and manpower costs of repairing or replacing components due to corrosion has become a significant hindrance to the affordability, availability, and safety of the warfighter. Currently, the U.S. Navy spends between $3 to 4 billion per year to combat corrosion. Part of this cost is driven by the undetermined effect corrosion degradation has on component life; out of an abundance of caution, components are repaired or replaced prematurely when there may be additional, safe operational life still available. Notably, this need exists not only across naval aviation but also across surface and undersea naval material applications, so predictive corrosion fatigue lifing capabilities may be disseminated across the Department of the Navy.

Thus, the U.S. Navy seeks the development of a predictive modeling and lifing tool for coupled corrosion, fracture, and fatigue of naval aero-engine propulsion materials to support more accurate, reliable, and safe lifing of warfighter components. This lifing tool should address as many of the specific capabilities listed here as feasible. The proposed solution may be a self-contained tool encapsulating all the capabilities listed. It may be an add-on tool compatible with a commercial-off-the-shelf predictive model, such that the combination addresses the target capabilities. It may also be a series of independent tools that each address the different capability needs. The intent of this Description is to outline the desired capabilities of the final lifing tool but to allow the proposer flexibility for how to achieve and package these capabilities.

The tool should address the following capabilities:

1. Develop a Material Model with Customizable Loading Conditions. Any predictive lifing tool should be capable of reflecting realistic or structurally relevant loading conditions. For example, airworthiness considerations may require conventional tension and tension fatigue testing, steady and cyclic applied loading, flexural loading, and uniform and non-uniform loading conditions. Solutions are sought that are capable of modeling different mechanical loading scenarios and can be validated with common tensile, flexural, or other experimental method. Upon award, the Navy technical point of contact (TPOC) can help identify the most crucial mechanical loading configurations to model.
2. Many different alloys, coatings, greases, and other materials are used in the U.S. Navy fleet. The material model should be developed and demonstrated with the material properties of (a) a representative aluminum alloy, and (b) a representative stainless steel. Upon award, the Navy TPOC can assist the team to identify the most appropriate material systems based on the required data inputs per the strategy described above and available property data. Common aluminum alloys used in the fleet are F357, AA 2024, and AA 7076, and common stainless steels are 17-4PH, A286, and M152.
3. Simplify Material Corrosion Degradation. Corrosion mechanisms are highly complex and interdependent. Thus, truly predictive modeling of those mechanisms is very challenging, likely involving coupled chemical and environmentally-driven reaction kinetics and likely beyond the scope of a single project. Solutions are sought, which propose a reasonable and achievable simplification of naval aero-engine corrosion mechanisms within the material model. For example, it may be reasonable to apply some environment-dependent scaling coefficient or stress riser to the stress intensity factor driving crack growth. It may also be possible to model material degradation due to corrosion by reducing the material to an effective, load-bearing geometry or by imposing a pre-existing defect, like a pit or crack, to the material model. Furthermore, aero-engine corrosion is highly cyclic, so models that incorporate some cyclic progression of corrosion degradation will be prioritized. Continuing from the previous examples, this cyclic degradation may take the form of periodic reduction of the load-bearing geometry or of pre-existing surface defects of the material model. These are just examples of simplifications and are not meant to constrain proposed corrosion degradation modeling. Upon project award, the Navy TPOC can provide experimental data on observed corrosion rates, pitting, and pre-existing defect features as well as some operational data, which may describe the frequency and magnitude of environmental cycles.
4. Correlate Impacts to Fatigue Life. Once a strategy to model the corrosion degradation is identified, that strategy should be implemented within the material model with customizable loading configurations to form the predictive lifing tool. This lifing tool should be capable of quantifying a predicted fatigue strength over a range of loading conditions in the form of an S-N curve. S-N curves are common graphical depictions of the load fatigue strength relationships of a material or component and provide an easy, visual means of comparing the strength properties of different materials. The key output of this predictive lifing tool should be the S-N curves of one representative aluminum alloy and one representative stainless-steel alloy across a realistic range of loads and cyclic content. Upon project award, the Navy TPOC can provide guidance about engine loading and cyclic content.
5. Predict the Fatigue Life Impacts of Pre-existing, Surface and Near-Surface Cracks and Pits. The predictive lifing tool should be capable of representing the effect of pre-existing, surface and near-surface cracks and pits. Many degradation mechanisms begin with the generation of surface and near-surface cracks and pits, and the predictive capability to model the fracture and fatigue performance of a material will inform allowable limits on pre-existing pits, cracks, and porosity within an as-manufactured material. The tool should be capable of modeling the impact of crack or pit size, depth, and density within the material. The resulting fatigue performance should be represented in S-N curves.
6. Validate the Fundamental Fatigue Life Predictions. The predictions of the corrosion fatigue lifing tool should be validated for both the representative aluminum alloy and the representative stainless-steel alloy in at least two disparate loading configurations (e.g., two different tensile loads, one tensile load and one torsion load, etc.). Tool predictions should be made for each material in an ideal, uncorroded state; a minor-to-moderately corroded state; and a moderate-to-heavily corroded state. The nature of the modeled corrosion state may depend on the proposed strategy to model corrosion degradation. Upon award, the Navy TPOC may help to identify the appropriate model prediction and validation conditions, but these conditions should be producible in a laboratory environment. The proposer should identify the test facility capable of performing the validation experiments, mimicking the test conditions in the tool prediction to enable a direct comparison of results.
7. Package the Predictive Lifing Tool into a Testbed. The predictive corrosion fatigue lifing tool should be packaged into a testbed deliverable, integrating the required capabilities previously described. This testbed may take the form or forms most appropriate to deliver these capabilities to NAWCAD engineering personnel. For example, the testbed may consist of a single, standalone software package or model, or it may consist of a series of add-on packages for existing commercial software. If awarded funding, teams should work with the Navy TPOC early in the tool development to identify the form of the testbed deliverable to comply with any Navy software or computational tool restrictions. In addition to providing key benefits to the Navy, the lifing tool is anticipated to extend corrosion fatigue lifing capabilities to other ground, surface, and aerial commercial and military vehicles operating in highly corrosive environments, so the proposer should outline a commercialization pathway for the lifing tool.
8. Provide Workforce Training for the Tool. To facilitate the delivery of the testbed lifing tool, the proposer should plan for and conduct either an on-site or virtual training for NAWCAD engineering personnel. The objective of the training should be to provide NAWCAD personnel an understanding of the tool’s function with respect to the required capabilities listed above, of how to use the tool, and of the application of the tool to other material and environmental conditions (including identifying necessary user inputs for such applications).

PHASE I: Demonstrate the feasibility and probability of success via the initial development of a tool framework.

1. Technical Challenges Assessment. Perform a review of the technical challenges facing a proposed solution and assess these challenges. Consider the availability of environmental and material data, the proposed strategy for corrosion modeling simplification, the difficulty and time required for the tool development, among other obstacles. This assessment should inform the Solution Feasibility Assessment, which capabilities will be addressed, and to what extent they will be addressed.
2. Strategic Work Plan. Based on the Technical Challenges Assessment, develop a strategic work plan for the development of a corrosion fatigue lifing tool to address the listed capabilities and to clearly identify the scope of work and tasking associated with development of each capability. This work plan should carefully consider and identify the assumptions, advantages, and limitations of the proposed strategy to model corrosion degradation described in the section "Simplify Material Corrosion Degradation." However, this work plan should also address the other capabilities, validation, testbed delivery, and training. The work plan should lay out a schedule of tasking and activities for Phase I and subsequent phases of work if awarded. Outline specific tasks, objectives, milestones, and go/no-go decision points to track the progress and feasibility of the proposed solution in context with the Technical Challenges Assessment and Risk Assessment.
3. Risk Assessment. Evaluate the Strategic Work Plan and Technical Challenges and identify potential sources of risk. Develop a risk mitigation plan that outlines specific strategies and/or go/no-go decision points in the outlined work plan.
4. Preliminary Feasibility Assessment. Begin execution of the Strategic Work Plan to demonstrate the initial feasibility and likelihood of success of the proposed solution. Phase I progress will be evaluated based on accomplishments made against the Strategic Work Plan and towards Capabilities 1, 2, 3, and 4.

The Phase I effort will include prototype plans to be developed under Phase II.

PHASE II: Fully execute the Strategic Work Plan developed in Phase I and demonstrate a predictive corrosion fatigue lifing tool.

1. Testbed Development. The Strategic Work Plan should be fully executed to deliver a predictive corrosion lifing tool satisfying the listed capabilities.
2. Testbed Validation. Validation of the testbed should be completed as described in Capability 5.
3. Updated Risk Assessment. Upon completion of the testbed, the Risk Assessment from Phase I should be revisited and updated to reflect resolved or new risks, and to identify appropriate mitigation to sustain use of the lifing tool. Future risks or opportunities for future development should also be identified, which may bring added capability.

PHASE III DUAL USE APPLICATIONS: Complete delivery of the testbed corrosion fatigue lifing tool.

1. Testbed Delivery and Training. Delivery and training of the testbed with NAWCAD engineering personnel should be completed as described by Capabilities 6 and 7. A commercialization pathway beyond the Navy should also be identified.
2. Technical Challenges Re-Assessment. Perform a re-assessment of the technical challenges identified in Phase I. These challenges would have informed the proposed strategy for corrosion modeling simplification, such as for example the simplification to the effective load-bearing geometry. This re-assessment should consider how the lifing tool may require future development to meet the specific needs of commercial and military end users and how corrosion model simplifications may or may not be appropriate for different users. Identify a pathway to re-address those challenges and to feasibly avoid model simplifications. Continuing with the previous example, if the model was simplified to a load-bearing geometry, describe the future tasking required to introduce the reaction kinetics that may instead inform a more representative depiction of the material degradation. Identify what information or knowledge gaps need to be resolved to avoid such simplifications, what testing or analysis is needed to address those gaps, and assess how feasible it would be to incorporate updates to the lifing tool.

The solution is expected to be highly applicable to both military and commercial aviation propulsion systems and materials. Military operations, by their nature, are more strenuous than commercial, and involve operations in the harshest environments, which contribute to accelerated corrosion. However, commercial aviation is also experiencing significant degradation due to corrosion, particularly commercial operators who fly in and out of coastal and subtropical regions. As such, this corrosion fatigue lifing tool may benefit commercial operators to predict how that degradation will affect component lifecycles and to use the tool to provide informed engineering judgement for component maintenance and sustainment activities. While the tool targets naval aviation applications, the basic function of the tool may also be extended to ground and surface vehicles operating in highly corrosive environments. Air-breathing propulsion systems, like gas turbines, are used for power generation in surface vehicles and use equivalent designs and materials to aviation systems. Lightweight and structural materials (i.e., stainless steels and aluminums), are also common across ground, surface, and aerial vehicles, meaning that material-customizable configurations of the tool may be readily transferred among applications.

REFERENCES:

1. Staroselsky, A.; Keat, W. D. and Voleti, S. "Constitutive model for corrosion fatigue crack growth in 3D parts." Engineering Fracture Mechanics, 279, 109013, 2023. https://doi.org/10.1016/j.engfracmech.2022.109013

2. De Meo, D. and Oterkus, E. "Finite element implementation of peridynamic pitting corrosion damage model." Ocean Engineering, 135, 2017, pp. 76-83. https://doi.org/10.1016/j.oceaneng.2017.03.002

3. Rusk, D. T.; Hoppe, W.; Braisted, W. and Powar, N. "Modeling and prediction of corrosion-fatigue failures in AF1410 steel test specimens." Technical Report No. NAWCADPAX/TR-2008/60, 12 January 2009. https://apps.dtic.mil/sti/citations/ADA492308

KEYWORDS: Naval Aviation Propulsion; Corrosion; Fatigue; Cyclic Environment; Corrosion Fatigue Modeling; Lifing Tool

TPOC 1: Clifton Bumgardner
(301) 342-0836
Email: [email protected]

TPOC 2: Christine Myers
(301) 757-050
Email: [email protected]

TPOC 3: Steven Kopitzke
(301) 342-128
Email: [email protected]

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