Crack Initiation in Plates with Multiple Corrosion Pits This work focuses on the characterization of materials degradation caused by corrosion pitting in structural materials like aluminum alloys. We carried out theoretical modeling using FEM and BIEM to calculate the effect of multiple corrosion pits. Coupled with laboratory testing, we found that collectively, these defects are detrimental to the load carrying capablility of aluminum. The understanding we developed will enable countermeasures to extend the life of aging aircraft as well as preventing failure of structural components made out of these materials.

SEM Photomicrograph of an Corrosion Pit Color Charted Stress Level Calculated using FEM

Force-Moment Representation of a Mixed Mode Crack Front Comparison of Theoretical and Experimental Waveform in the same Mixed Mode Crack

Waveform-Based Acoustic Emission (AE) in Aluminum Alloys We calculated solution of the three-dimensional wave propagation in transversely anisotropic and isotropic plate. The displacement in the frequency domain is given as a single integral; its numerical evaluation is simple and rather straightforward. By using laminate theory with a general force system, we formulated the wave propagation problem in laminated composite plate. We also presented the relationship between the force systems and the Lamb wave modes on the surface in simple forms. The general theoretical model is validated through laboratory experiments with aluminum plates.

Comparison of Theoretical and Experimental Waveforms in a Mode I Crack

Fracture Wave Detector for Acquiring AE Signal Sample AE Signal

Acoustic Emission (AE) from Fatigue Fracture Fatigue fracture of high strength aluminum alloys generates numerous acoustic emission (AE) events that are detectable. With proper analysis, these AE events can be used to identify failure mechanisms in real-time. Conventional AE methods of using event counting, event energy, signal amplitude, duration, and rise time, however, are highly operator dependent and have severe limitations in the proper characterition of AE signals. For fatigue AE detection, methods using sensor output waveforms are needed to identify unknown events, evaluate their significance, and correlate identified events to the fracture modes. These methods allow qualitative interpretations based on event feature analysis and identification of the nature of the emission source.

Characterization of AE Signals from Corrosion Pits in Aluminum Alloys under Fatigue Loading In a series of studies, we used a waveform-based acoustic emission technique to study the characteristics of signals emanating from fatigue crack propagation and penetration behavior in 6061, 2024-T4, and 5052 aluminum alloy plates. We also studied the effects of prior thermal and mechanical treatment, rolling directions of the test specimen, especially microcrack initiated at corrosion pits to the resulting AE signals. Acoustic signals arising from crack propagation, plastic deformation at the crack tip, avalanche motion of dislocations, fracture of brittle particles, and the debonding of these particles from alloy matrix, were recorded by broadband transducers for analysis.

Corrosion Pits at Notch Tip

Modified 9/98