Research & DevelopmentDevelopment and validation of an experimental test protocol to examine the initiation of a fatigue

crack from corrosion damage below the damage tolerant flaw size

Refining the understanding of how corrosion inhibitors can affect corrosion fatigue damage

Expanding the database for K-solution validity in finite width plates to aid in the development of

better life predictions

Designing and implementing new large structure test systems for the application of an

atmospheric environmental spectrum in conjunction with load spectrum.

AA7075-T651 Single Edge

Notch (SEN) Sample.

Dimensions in mm.

Bulk Solution Test Cell

(500 mL)

Reduced Volume

Test Cell (0.1 mL)

Atmospheric (Thin Film)

Exposure [3]

Investigation and development of new environmentally friendly, biologically based corrosion

fatigue inhibition methods

Corrosion damage can be detrimental to the structural integrity of aircraft through theinitiation of fatigue cracks. A test methodology is lacking for quantifying how the corrosiondamage to fatigue crack transition process is altered by environment, corrosion inhibitors orload spectrum. The test protocol being developed covers everything from the pitting &welding procedures for the dcPD fatigue sample to the post-test processing required so thereis a robust, repeatable & validated test methodology available.

A preferential pit (< 150µm) is placed at the corner of the sample center hole. This pit is rough and designed to mimic real world

corrosion damage on a micro/nano scale level.

Direct current potential drop wires are then single point spot welded on

either side of the pit (< 1mm spacing).

The sample is tested under fatigue loading with a marker band spectrum; post test the marker bands are mapped.

Using the dcPD data, crack growth predictions can be

made. Eventually the predictions will be integrated

with AFGROW . This test protocol is currently being transitioned into corrosive

NaCl environments.

Accurate quantification of crack tip stress intensity values isparamount in the analysis of damage tolerant structures. Thisprogram uses the three dimensional virtual crack closure technique(3D VCCT) and a well-structured, completely hexahedral, elementmesh. Stress intensity values are generated for single quarter-elliptical corner cracks emanating from centrally located holes. Arange of crack depth to crack length ratios (a/c = 0.01 to 10), crackdepth to sheet thickness ratios (a/T = 0.01 to 0.99), hole radius tosheet thickness ratios (R/T = 0.1 to 10), and sheet width to holediameter ratios (W/D = 1.1 to 20) are being completed under remotetension, bending, and pin loading. These new K- solutions will beadded to the AFGROW damage tolerant analysis framework.

The majority of work pertaining to the effect of corrosion inhibitors on fatiguecrack propagation has been completed using highly soluble salts (Na2MoO4/CrO4)added to a bulk NaCl solution. While these studies have shown that corrosioninhibitors can slow FCGR; the studies did not take into account the fact that tooffer protection on aircraft, low solubility inhibitors must leach from polymericcoatings. Testing is underway to determine if corrosion inhibitors in the amountsleached from polymeric coatings can slow corrosion fatigue damage.

Three aircraft corrosion geometries (bulk area, lap joint, atmospheric) were identified based on areas of common corrosion damage. The surface area of coating/volume of solution ratio was calculated and

leaching studies used to determine the amount of inhibitor expected in

solution for each configuration.

Based on the leaching results corrosion fatigue testing is being completed using low solubility

inhibitors (CaMoO4/SrCrO4). Currently the low level inhibitors

are not proving as effective at inhibiting FCG at a ∆K=6 MPa√m.

The continuing direction for the program is to examine the effects of aging (UV light, ozone, heat) on coating leaching

under atmospheric thin film conditions, as well as to vary the stress ranges being examined (∆K= 4, 6 & 14 MPa√m) to define the boundaries of corrosion fatigue protection. An analysis of

the oxide layer morphology in the fatigue crack using ESBD would also be performed to understand how the layer changes

with the protection provided.

It has been found that the gram-negative, biofilm forming bacteria Ralstoniapickettii can substantially slow fatigue crack growth rates in aqueousenvironments. This effect is particularly exciting as the military and otherindustries are looking for more environmentally friendly corrosion preventionmethods. Research is underway to understand the conditions which cause thebiofilm to form and the mechanism behind the corrosion fatigue protection. Avariety of theories are being pursued including (1) desalination of the testsolution, (2) production of a new oxide layer within the crack, (3) uptake of aprotective metal into the bacteria cell wall, (4) the polymeric biofilm coverage.While R. pickettii reduces the salinity of the test solution by about 50% in 48hours, it does not appear to be the sole cause of the fatigue protection. Currentresults suggest the bacteria may effect the natural oxide layer to provideprotection. Ideally this bacteria, which has proved hardy in a wide range ofenvironments, could be applied for corrosion protection in a coating form toaircraft or other applications.

When added to a bulk NaCl solution R. pickettii can slow fatigue crack growth rates to near those of the

best known corrosion inhibitor (chromate) and a chromate replacement (molybdate).

AA2024: pH 7 AA2024: 37C AA2024: 0.6M NaCl

The structure of growth can change based on the material, temperature pH and growth environment. A

live/dead stain was used to quantify growth.

New test methodologies and procedures are needed to move corrosion fatigue research toward more realistic test designs. Atmospheric corrosion, a thin hydrated salt film environment controlled by relative humidity, and the ability to apply an environmental spectrum in conjunction with a load spectrum on an aircraft representative component are research areas of interest. The apparatus/instrumentation needed to conduct this cutting edge research are not available commercially and must be designed in house.

Atmospheric corrosion requires the application of a

highly uniform salt film. A printing method has proved

highly effective for controlling the amount of salt applied and producing the desired

droplet morphology.

A test chamber (H=25”;D=11”) has been

designed for the application of an environmental

spectrum (ozone, UV light relative humidity). Design of

the control system for the chamber underway.

DTA life prediction comparisons using the VCCT method versus older stress intensity solutions.The comparison of the VCCT K-solutions to the current AFGROW solutions show a wide variationunder certain conditions. A well structured completely hexahedral mesh is used for the FE model.

Corrosion fatigue test configurations were designed to mimic each aircraft geometry