Stephen W. Smith

Terahertz NDE application for corrosion detection and evaluation under shuttle tiles

Pulsed Terahertz NDE is being examined as a method to inspect for possible corrosion under Space Shuttle Tiles. Other methods such as ultrasonics, infrared, eddy current and microwave technologies have demonstrable shortcomings for tile NDE. This work applies Terahertz NDE, in the frequency range between 50 GHz and 1 THz, for the inspection of manufactured corrosion samples. The samples consist of induced corrosion spots that range in diameter (2.54 to 15.2 mm) and depth (0.036 to 0.787 mm) in an aluminum substrate material covered with tiles. Results of these measurements are presented for known corrosion flaws both covered and uncovered and for blind tests with unknown corrosion flaws covered with attached tiles. The Terahertz NDE system is shown to detect all artificially manufactured corrosion regions under a Shuttle tile with a depth greater than 0.13 mm.

Microstructure-Scale In-Situ Image Correlation-Based Study of Grain Deformation and Crack Tip Displacements in Al–Cu Alloys

Mechanistic understanding based on experimental measurements of grain and crack tip deformation at relevant length scales is critical for improvement of physics-based modeling of microstructurally-sensitive fatigue crack propagation and for the computationally-assisted design of more durable materials. In this study, single-crystal, bi-crystal, and large-grain multi-crystal specimens of Al–Cu alloys are fabricated, characterized using electron backscattered diffraction (EBSD), and mechanically tested. In addition to conventional optic al–imaging based 3D image correlation (IC), scanning electron microscope (SEM) based high-resolution 2D IC is used to measure displacements within grain interiors, near crack tips, and grain boundaries. The use of micro-scale IC and EBSD-based experiments is discussed as they relate to the development of crystal-plasticity-based finite element models (CP–FEMs). This article specifically highlights experimental methods that have been developed to obtain the relevant data needed for CP–FEM calibration and validation.

Residual Strain Effects on Bridging Stress of Cracked and Delaminated Fiber Metal Laminates

An analytical model is developed that accounts for the effects of both residual thermal strain and residual post-cure stretching strain in the prediction of the bridging stress and the crack opening displacement (COD) of cracked and delaminated fiber metal laminates. Two GLARE® tensile specimen configurations with center cracks in the aluminum layers, one with elliptically shaped delaminations and the other with rhomboidally shaped delaminations, are analyzed with the governing solutions of the analytical model. The shear deformations at the delamination front of either the resin rich region at the metal-fiber layer interface or the fiber layer itself are modeled. Analytical results show that the presence of residual thermal strain reduces the bridging stress while residual post-cure stretching strain increases the bridging stress. A high post-cure stretching level, inducing a greater bridging stress for lowering the COD and also affecting crack closure at crack tips, can result in an increased fatigue life. The use of either of the two shear models has negligible effect on the calculated bridging stress and COD. The analytical results are correlated with available test data of CODs for validating the analytical model.

Application of Terahertz Radiation to the Detection of Corrosion under the Shuttle's Thermal Protection System

There is currently no method for detecting corrosion under Shuttle tiles except for the expensive process of tile removal and replacement; hence NASA is investigating new NDE methods for detecting hidden corrosion. Time domain terahertz radiation has been applied to corrosion detection under tiles in samples ranging from small lab samples to a Shuttle with positive results. Terahertz imaging methods have been able to detect corrosion at thicknesses of 5 mils or greater under 1” thick Shuttle tiles and 7‐12 mils or greater under 2” thick Shuttle tiles.