| Vibrothermography is a nondestructive evaluation (NDE) technique that is used to detect surface and sub-surface defects such as cracks, disbonds, and delaminations through observations of vibration-induced frictional heat generation at defects. Frictional heating is observed using an infrared (IR) camera and is used to determine the presence and location of defects. There is a large industrial interest in vibrothermography due to its ability to rapidly detect defects over a large area. Another motivation for using this technology is its ability to find defects, such as tightly-closed cracks, that can be missed using other common NDE techniques. A major hindrance to the widespread application of vibrothermography has been an inability to quantify the reliability and capability of the inspection due to insufficient knowledge of the underlying physics of vibrothermography.The purpose of this work is to further understand the physics controlling defect detection in vibrothermography. The influence of vibration was studied through the use of synthetic defects and noncontact measures of vibration. Numerous samples of aluminum, brass, titanium, and carbon fiber-reinforced polymer composites were used to study the physics of heat generation to isolate the different sources of heat generation in metals and composites. The effects of crack closure on heat generation were studied and a method was developed to accurately measure crack closure stresses using vibrothermography. Finally, the effect of friction and heat generation on rubbing crack faces was observed using techniques such as profilometry, optical microscopy, and scanning electron microscopy. This work describes some of the fundamental parameters affecting heat generation and methods to improve defect detection reliability. This research provides a foundation for creating statistical models to improve the defect detection process using vibrothermography. |
