As a material is subjected to mechanical, chemical and/or thermal stress, changes are induced at the atomic and molecular level that can change the macroscale properties of that material. We have found that the optical second harmonic generation (SHG) process is affected by important material changes, and are working to determine how much information can be gained from the SHG signal. Because SHG can be done through fiber optic leads, and is relatively cheap and simple to do, it has the potential to be a powerful and versatile nondestructive testing (NDT) technology. SHG is the simplest form of nonlinear optics and occurs when an input laser beam is frequency-doubled at the surface of interest, e.g. 532 nm visible light becomes 266 nm UV light. The magnitude of this conversion process depends on the chemical and physical makeup of the sample. Our preliminary, proof of concept work has focused on the investigation of two types of changes with SHG: mechanical deformation in aerospace grade aluminum, and the chemical changes in naval grade aluminum related to sensitization.

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