Nondestructive and Noncontact Evaluation of Corrosion and Fatigue by LaserSpeckle Sensor (LSS) and Laser Moire

摘要

Five NDE techniques have been exploited for the detection of damage caused bycorrosion and fatigue. They are (1) ESPI (Electronic Speckle Pattern Interferometry), (2) Digital Speckle Correlation, (3) TADS (Time Average Digital Specklegraphy), (4) Modified Projection Grating Technique and (5) Fractal Damage Sensor. Except the first one whose basic principle is known, the rest are all new techniques. While different techniques have different advantages and disadvantages, the most versatile is probably TADS and the Modified Projection Grating technique, whereby large areas (such as 5 M x 5 M or larger) can be evaluated simultaneously. We also investigated the corrosion and fatigue processes and their synergism. We find that corrosion in general reduces the fatigue life of a metallic alloy. However, if a modest amount of initial fatigue is followed by a modest amount of corrosion, the total fatigue life is actually enhanced. This is attributed to the blunting of micro crack tips by corrosion.