CORROSION FATIGUE RESEARCH

Research Related to Corrosion/Fatigue -- V. Chandrasekaran and D.W. Hoeppner

 

Modeling corrosion growth on aircraft structure state of the art review

Correlation of pit depth to fatigue life

Correlation of pit depth to short fatigue crack growth rates


Modeling corrosion growth on aircraft structure state of the art review

A state of the art review was prepared on corrosion and corrosion fatigue predictive modeling for NCI Information systems, Fairborn, OH. The major sections of the report discussed the following three major areas:

  • Corrosion in Aircraft Structural Aluminum Alloys,
  • Pitting Corrosion,
  • Microstructure and environment effects on "short" crack behavior of materials.

The review of the literature clearly showed that much progress has been made on modeling the effects of corrosion on material behavior and structural integrity. It is clear that to date the models have centered around characterizing the corrosion and modeling the effects of the corrosion as one or more of the following:

  • section change that affects the area/volume that modifies the stress.
  • nucleation of localized debris that may modify the stress (part of pillowing) that modifies the stress or stress intensity.
  • nucleation of intergranular corrosion that is involved in pillowing that modifies the stress or stress intensity.
  • nucleation of localized corrosion (pitting, fretting, etc.) that modifies the local stress and may ultimately nucleate cracks.
  • production of products of corrosion that produce localized embrittlement effects that may alter the material behavior and produce accelerated crack propagation.

Correlation of pit depth to fatigue life

An experimental study was conducted to characterize corrosion damage quantitatively in relation to fatigue life of 2024-T3 aluminum alloy specimens. The primary focus of this study was to correlate corrosion "damage" using pit depth as a controlling factor with the fatigue life of prior corroded 2024-T3 aluminum alloy specimens. It is believed that pit size in terms of pit depth has an important role in crack nucleation. Under fatigue loading conditions, "small" cracks nucleate from pits reducing the residual strength and fatigue life of the materials.

Confocal microscopy was used to quantify corrosion damage in terms of pit depth and it was correlated to the fatigue life of prior corroded aluminum alloy specimens. This report includes results from experiments to date.

In general, the greater the range of pit depth, the lower the fatigue life of the specimens. In addition, an extensive fractographic study was conducted and the analysis of the fracture surfaces revealed that cracks propagated deeper from the pitted area when compared to the uncorroded area of the specimens. Also, the fractography results indicated that fatigue cracks that nucleated from the corroded area resulted in the final fracture of the specimens. This observation was supported by the appearance of fatigue striations and secondary cracks on the fracture surface corresponding to the prior corroded area of the specimen. Moreover, the SEM analysis of fracture surfaces clearly showed that fatigue cracks that formed from an uncorroded area of the specimens had a negligible effect on the fatigue life of the prior corroded 2024-T3 aluminum alloy specimens. In other words, cracks that nucleated from an uncorroded area of the specimens remained as surface cracks and did not propagate further into the specimen thickness. This observation was supported by the fractographic analysis as the characteristic features of fatigue fracture were not seen on the fracture surfaces corresponding to the uncorroded area of the specimens.


Correlation of pit depth to short fatigue crack growth rates

An experimental study was conducted to characterize corrosion damage quantitatively in relation to "short" fatigue crack growth rates of 2024-T3 aluminum alloy specimens including a specimen made from a dismantled JSTAR fuselage panel. The primary objective of this study was to correlate corrosion "damage" using pit depth as a controlling factor with the "short" fatigue crack growth rates of plain (uncorroded) as well as prior-corroded 2024-T3 aluminum alloy specimens. It is believed that pit size in terms of pit depth has an important role in crack nucleation. Under fatigue loading conditions, "small" cracks nucleate from pits reducing the residual strength and fatigue life of the materials. Confocal microscopy was used to quantify corrosion damage in terms of pit depth and it was correlated to fatigue crack growth rates particularly in the "short" regime of prior corroded aluminum alloy specimens. This report includes results from experiments to date.

In all fatigue experiments, the cycles to ‘first detectable crack length’ was recorded. A video recording system was used to observe the specimen surface, to record real time video as well as to measure cracks during fatigue experiments. The initial detectable crack length recorded using this system was in the range 0.15 to 0.21 mm. From the plots of ‘a’ vs. N for all of the specimens, it can be observed that the shorter the range of pit depth, the longer the fatigue cycles to form the first detectable crack length. In addition, it was observed that the number of fatigue cycles to the first detectable crack for the uncorroded specimen was much higher when compared to the prior-corroded as well as the JSTAR specimen. Therefore, the number of fatigue cycles to a detectable crack size at a given stress level is accelerated by prior-corrosion in the 2024-T3 aluminum alloy specimens tested in this study. This alone indicates that prior-corrosion indeed has an effect in the fatigue crack formation process.

Moreover, in general, the crack growth rates for the prior-corroded specimens and the JSTAR specimen were greater than the uncorroded specimen. At low D K values (for example @ 0.1 MPaÖ m and 0.25 MPaÖ m), the crack growth rates of the prior-corroded specimens in the "short" region were considerably faster than the uncorroded specimen. In addition, as expected, the greater the range of pit depth, the lower the fatigue life of the specimens. Also, the AFGROW program was used to perform a preliminary analysis of the experimentally generated data using the Harter-T method with the "user defined" through crack model. In general, the AFGROW program over-predicted the fatigue life of prior-corroded 2024-T3 aluminum alloy specimens. However, the fatigue life estimation by the AFGROW for the uncorroded and the JSTAR specimens was conservative.

Finally, fractography analysis revealed some interesting results. The origins of the cracks were as expected for the most part. On the uncorroded specimens the origins were at the corner of the rivet hole where the stress concentration was highest, due to no other damage on the surface. The prior corroded specimens cracked at the rivet hole also at the points of large corrosion pits. The most important result was the crack origin of the JSTAR specimen, which was inside the rivet hole. This shows the extent of the damage inside the rivet holes that may go unseen but that may result in failure.

From the experimentally generated results, it is thereby concluded that prior-corrosion indeed affects the fatigue crack growth rates of 2024-T3 aluminum alloy specimens in the "short" crack region as well as it alters the cracking mechanism(s) as observed from fractographic analysis.


 © 2011 FASIDE International Inc., Salt Lake City, Utah, USA. All rights reserved.