Design-of-Experiment Approach to Hydrogen Re-Embrittlement Evaluation

WP-2152

Objective

Solvent substitution for maintenance and overhaul operations of military systems has been a primary environmental concern for many years. Cadmium replacement in these systems has been targeted for decades. Both of these areas have a common obstacle for implementation of any potential alternate--hydrogen embrittlement of high strength steel. High strength materials are sensitive to the phenomena, and the source of the hydrogen can be within the fabrication process, maintenance practice, or the natural corrosion cycle. Historically, the various aerospace defense contractors have each tested in their own manner, which has led to the national standard incorporating many approved test geometries and somewhat ambiguous procedures. This standard test is a pass/fail, go/no-go type test, and the ambiguous procedures lead to conflicting test results and perceived risk and roadblocks when it comes to implementation of proposed alternative chemicals and coatings.

The objective of this project is to identify the most viable and reliable test geometry and utilize it to evaluate the most prospective environmentally friendly maintenance chemicals and cadmium alternative coatings that have had their use limited by the perceived risk of hydrogen embrittlement. This project will evaluate the chemicals and coatings over a range of material strength, load level, and hydrogen emitting environment, which will demonstrate performance over parameter ranges and not as pass/fail while developing lifetime predictive models for each case. This small detail will greatly increase the applications for which the replacements will be considered.

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Technical Approach

A design of experiment (DOE) approach will be used over a range of material strength for aerospace grade 4340 steel, load level, and hydrogen environment. Five geometries from ASTM F 519 will be tested and load levels will be monitored to determine a precise time to fracture at specific percentages of notch fracture strength (NFS), material strength, and hydrogen emitting environment (sodium chloride solution). Comparisons among the geometries will be made and a single geometry with which to proceed will be chosen. A life prediction model will then be created for aerospace grade 4340 steel, over a range of material strength and applied load (simulated service load), while exposed to the five most prospective maintenance chemicals over a range of concentration. A second life prediction model will be created for coated aerospace grade 4340 steel, over a range of material strength and applied load in combination with prospective cadmium replacements while in a hydrogen producing environment with a defined severity range. Tri-service, ASTM F07.04 committee, and aerospace industrial partners will be canvassed for the best possible alternatives and impact.

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Benefits

In comparison to the existing standard, greater information will be gleaned from the life predictive models beyond pass/fail. Incorporating the failure time, load, and stress levels into the design of experiment failure models will present predictive equations over broad ranges. Instead of a yes/no answer, one will be able to reliably predict material strengths and load levels where the prospective alternatives may be safely employed. This will remove existing implementation hurdles for the prospective maintenance chemicals and cadmium replacement coatings, leading to an increase in their service utilization in many applications. (Anticipated Project Completion - 2014)

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Interim Results

As an initial step toward refining ASTM F 519 Annex 5, it was determined that a "ruggedness test" in accordance with ASTM E 691 Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method should be completed. The Boeing Company took the lead in designing and executing the task. The factors were (1) surface treatment (LHE cadmium plated versus bare); (2) notch condition (bare notch root versus Cd plated notch root); (3) ratio of fluid volume to surface area of specimen exposed as ml/cm2; (4) fluid concentration in volume percent; (5) fluid temperature in degrees Fahrenheit; (6) fluid exposure time (hrs); and (7) test load (% NFS). Analysis of the resulting data determined that test load was the single most influential factor followed by fluid temperature and then volume per surface area. The results of this experiment were used to refine Annex 5.

Testing is now nearing completion for the aerospace grade 4340. Once completed, life prediction models will be formed and compared with the base 4340 material. These comparisons will help to refine the test matrix and geometry utilized for the FY12 and FY13 program. During those years, the project will investigate the most commonly used maintenance chemicals and coatings in the aerospace industry as defined by the project team to have maximum impact on alleviating implementation barriers.

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Points of Contact

Principal Investigator

Mr. Scott Grendahl

US Army Research Laboratory

Phone: 410-306-0819

Fax: 410-306-0806

Program Manager

Weapons Systems and Platforms

SERDP and ESTCP

Document Types

  • Fact Sheet - Brief project summary with links to related documents and points of contact.
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