NDT.net - September 1999, Vol. 4 No. 9

In-Service NDE of Aerospace Structures -- Emerging Technologies and Challenges at the End of the 2nd Millennium

Yoseph Bar-Cohen
Jet Propulsion Laboratory, Caltech, (82-105)
4800 Oak Grove Drive Pasadena, CA 91109-8099
Corresponding Author Contact:
Email: yosi@jpl.nasa.gov , Web: http://ndeaa.jpl.nasa.gov




Emerging technologies that are affecting multiple NDE methods

Fig 8: An aging aircraft with well-documented flaws at AANC (Sandia National Labs).

Emerging NDE Techniques

NDE issues and Challenges

Table1: Corrosion types, which can inflict damage to aircraft strictures, and their characteristics
Corrosion type Source Appearance By-Product Notes
Crevice Afflicts mechanical joints, e.g., coupled pipes or threaded connections. Triggered by local environment composition differences (O2 concentration). Localized damage in the form of scale and pitting. Same as scale and pitting.
  • Caused by differential aeration.
  • Difference in oxygen concentration produces potential difference and leads to flow of electrical currents across aerated (cathode) and derated (anode) portions of the metal.
  • Causes localized corrosion failure.
Filiform High humidity around fasteners, skin joints or breaks in coating cause an electrolytic process. Meandering, fine, thread-like trenches spreading from the source. Similar to scale
Galvanic Corrosion Corrosive condition that results from contact of different metals. Uniform damage, scale, surface fogging or tarnishing. Emission of mostly molecular hydrogen gas in a diffused form.
  • Slow growth rate. Expressed as penetration/year or weight loss per unit-area/day, e.g. the rate for aluminum in open atmospheric conditions of Los Angeles, CA is 0.02-mil/yr.
  • For Ti and Al alloys the rate is slow and therefore it does not pose serious structural problems
  • The metal with the most negative potential suffers the most damage.
Inter-granular Presence of strong potential differences in grain or phaseboundaries. Appears at the grain or phases boundaries as uniform damage. Produces scale type indications at smaller magnitude than stress corrosion.
  • The severity and rate of growth depends on the material microstructure crystalinity and segregation.
  • This type of corrosion can merely result increased susceptibility to attack in the form of pitting or stress cracking.
  • Exfoliation occurs in layered grains (e.g. rolled sheets) in the form of laterally 24 extended damage.
  • Aluminum is susceptible to such attack mainly in an environment of chlorine ions and dissolved oxygen.
Microbial Bacterial, fungus or yeast in contaminated kerosene-type jet engine fuel. Appears in integral fuel tanks. Combination of by-product of pitting and scale. Can be eliminated by a proper maintenance of the fuel in the tank.
Pitting Impurity or chemical discontinuity in the paint or protective coating. Localized pits or holes with cylindrical shape and hemispherical bottom. Rapid dissolution of the base metal.
  • Expressed in terms of pitting depth (i.e. pitting factor).
  • Pitting can be critical to the structural integrity.
  • Can be detected by AC impedance or electrochemical impedance spectra analysis
  • Usually pitting is accompanied by an order of magnitude change in the local resistance and capacitance.
Stress Corrosion Cracking Mechanical tensile stresses combined with chemical susceptibility. Localized micro- macro-cracks at shielded or concealed areas. Produces initially scale type indications at a large magnitude that progresses to cracking.
  • Causes critical failure of structures
  • Failure rate is determined by the stress levels
  • Corrosion fatigue occurs under cyclic stresses.
  • Stress and rubbing action remove protection and lead to fretting corrosion as a result of contact of the metal surface with particles introduced by the abrasion process.
Thermo-galvanic Corrosion Caused by thermal gradients parallel to the metal surface. Localized attack correlated with temperature distribution. Produces scale indications. Hot portion of the metal serves as cathode whereas the cold portion as anode.




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