B. Zoofan

Effect of pitting corrosion on fatigue crack initiation and fatigue life

Abstract Fatigue crack initiation and growth from artificial pits of different depths has been studied. To analyze the experimental results a simple three-dimensional fracture-mechanical model has been developed. The model shows very good agreement with experiments including for small cracks, in describing the initiation and growth of a fatigue crack emanating from a pit and in predicting the dependence of reduction of fatigue life on pit size. Based on experimental data an empirical relation between the depth of the corrosion pit and the fatigue life has been established. Also, a microradiographic method for pitting corrosion depth determination has been described.

Pit Growth Study in Al Alloys by the Foil Penetration Technique

This work was supported by Major H. DeLong at the Air Force Office of Scientific Research, under contract F49620-96-0042.

In Situ X-Ray Radiographic Study of Intergranular Corrosion in Aluminum Alloys

Abstract In situ x-ray radiography was used to characterize intergranular and exfoliation corrosion in high-strength Al alloys. The samples were either exposed to sodium chloride (NaCl) solution at a controlled potential or to high humidity after an electrochemical pretreatment in NaCl solution. Intergranular corrosion (IGC) growth rates in solution for samples encased in epoxy and exposed to solution were distributed over a range with an upper limit equal to the rates determined by the foil penetration technique. Unencased AA2024-T3 (UNS A92024) samples that were held at a controlled potential in solution exhibited IGC and then exfoliation corrosion. AA2024 and AA7178 (UNS A97178) samples encased in epoxy developed sharp intergranular fissures during exposure to 96% RH following an electrochemical pretreatment. Unencased samples given the same treatment exhibited exfoliation in the high-humidity environment.

Phase-contrast x-ray imaging for nondestructive evaluation of materials

Phase-contrast x-ray imaging has been studied intensively in the past, mainly with applications to low density biological materials. In this work experimental and theoretical aspects of phase-contrast microfocus x-ray imaging of structural materials (metals and polymers) are considered. The diffraction field in an object from a point x-ray source is simulated and the effects of geometrical and material parameters on the x-ray phase-contrast image are obtained. It is shown that higher-order terms should be retained in the asymptotic expansion of the Green function in the image plane to accurately image discontinuities in structural materials. Experimental phase-contrast images of small defects are obtained with a 5μm microfocal x-ray source and compared with those from computer simulation as a function of geometrical magnification and photon energy. The phase-contrast x-ray imaging provides enhanced image contrast and improved edge definition and is important for further development of nondestructive evalu...

Transition from Intergranular Corrosion to Intergranular Stress Corrosion Cracking in AA2024-T3

This work was supported by the United States Air Force Office of Scientific Research through grant no. F49620-02-1-0148 and FA9559-04-1-0368.

In Situ X-Ray Radiographic Study of Stress Corrosion Cracking in AA2024-T3

Abstract Despite the many studies that have addressed stress corrosion cracking (SCC) in high-strength aluminum alloys, the separate contributions of stress and electrochemistry to the SCC process ...

MICRORADIOGRAPHIC CHARACTERIZATION OF PITTING CORROSION DAMAGE AND FATIGUE LIFE

Due to the increasing number of aging aircraft, the inspection and assessment of corrosion and fatigue damage on airframe structures is of major concern [1]. Among the different types of corrosion damage, pitting corrosion is an important factor in triggering widespread fatigue crack initiation and reduced fatigue life. Therefore, the ability to predict surface crack origination from pitting corrosion is necessary for timely maintenance of aging aircraft.

Localization of Dwell Fatigue Cracks in Ti‐6242 Alloy Samples

An in‐situ ultrasonic guided wave technique is employed for real‐time monitoring of crack initiation and evolution during dwell, cyclic fatigue and creep tests of Ti‐6242 alloy samples. Ultrasonic signals are acquired continuously during the test at different levels of fatigue load using a high‐speed data acquisition system. The initiation time and growth history of primary and multiple secondary cracks are assessed. Localization of the secondary cracks is performed by both the in‐situ ultrasonic method and an ultrasonic immersion scanning method which we call “vertical C‐scan” (VC scan). The VC scan is developed for imaging small cracks aligned normal to the fatigue sample axis. The fusion of ultrasonic and microradiographic images exhibits good agreement in crack location. Joint use of the three techniques provides location, shape, and size of the secondary cracks.