Fatigue Growth Analysis of Pre Induced Surface Defects Using Piezoelectric Wafer Based Impedance Method and Digital Image Correlation System

Defects in real mechanical, industrial and aerospace structures frequently have complex shapes. Most real structures are typically prone to multiple fatigue cracks and their propagation can be monitored by observing changes in the structural stiffness resulting from strength reduction as a function of the number of loading cycles. In addition, strain variations on the structural surface can be captured using digital equipments. The present work monitors two specimens (AISI 4340 steel) with electrode sparked hemispherical defects on their surfaces. Multiple-cracks emanating from these defects, under fatigue cyclic loading were monitored using piezoelectric wafer based electromechanical impedance (EMI) technique, and digital image correlation (DIC) system. EMI technique uses signature comparison of healthy and damaged state of the structure to depict the occurrence of crack and its growth. Images of DIC system captures initial sightings of surface hair-line cracks from the corners of machined defects and their propagation till merging. Thus, a signature analysis based technique such as EMI and image processing technology such as DIC were found to complement each other to expedite the prediction of early crack and their appearance on the surface.