The new technology utilizes laser ultrasonic imaging to identify fatigue cracks initiated from pitting corrosion sites on various types of material surfaces, and can be used to reduce the risk of catastrophic failure in aircraft structural components.

The researchers conducted a detailed, microscopic characterization of stress-corrosion cracking processes for electro-chemically pitted aluminum specimens in a high-cycle fatigue environment.

Measurements were acquired using scanning laser ultrasonic detection of electromagnetic waves propagating along the material surface.

This technique provided an advanced crack imaging capability that proved to be extremely useful for micro-characterization of crack growth processes, and also provided a wealth of information on micro-features of cracks initiated from pitting corrosion sites.

This information is not available using any other advanced method.

The researchers also conducted microscopic evaluations of crack extent and position, growth rate and depth using near-field ultrasonic scattering signatures and an examination of various pit depths, material surfaces and volume loss to ensure their conclusions were accurate.

The effects of corrosion, fatigue and cracking are becoming a major concern as aerospace systems are asked to perform well beyond their intended design lives.

As this concern continues to grow, so does the need for efficient, affordable, more accurate NDE detection systems.

Several studies have concluded that corrosion in structurally significant components can eventually lead to catastrophic failures, when gone unchecked.

Corrosion fatigue and stress-corrosion cracking in particular have been identified as two of the primary mechanisms contributing to reduced service life in aircraft.

Although these detailed mechanisms are not yet well understood, it is generally accepted that localized corrosion pitting can act as a crack nucleation site,

dramatically accelerating the onset of fatigue cracks and crack growth rates.

The objective of this research effort, conducted by the directorate's Nondestructive Evaluation Branch, was to use near field scanning inter-ferometry (NFSI) to image surface breaking cracks generated by corrosion fatigue and stress-corrosion cracking processes.

Basic scanning inter-ferometry had been used previously to image several different types of cracks, such as a sharp thru-the-thickness crack, a large irregular surface-breaking crack, and small microscopic fatigue cracks; however, no information was provided on the dynamic growth characteristics of these cracks.

NFSI imaging was possible at that time but the measurements were made on static, crack-reference samples, which necessarily narrowed the scope of the research and the findings.

Conversely, the thrust of this effort was to advance the basic crack-imaging concept to an active fatigue environment and study the initiation and propagation characteristics of the cracks.

The researchers employed an advanced laser-ultrasonic-based NDE system to "image" corrosion fatigue cracks at various stages of fatigue life.

They introduced surface acoustic waves into the fatigue specimens using a traditional transducer/wedge system and then obtained images of the displacement field using a scanning laser inter-ferometry system.

Next, the researchers identified corrosion fatigue cracks and imaged them according to their near-field ultrasonic scattering features.

They observed displacement field increases in the immediate vicinity of the surface-breaking crack sites.

This permitted them to readily distinguish the cracks from background displacement levels, and also provided a means for characterizing the detailed morphology of a crack and its local surroundings.

The research team determined basic crack initiation and crack growth rates during the evaluation of several different electrochemically-generated corrosion pits.

Their findings have provided important insight into the complicated interactions between realistic corrosion pitting sites and how they respond to applied stresses, material fatigue levels, and ultimately, crack initiation and growth processes.

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