Nondestructive inspection of turbine blades with lock-in thermography

The combination of lock-in technique and commercial IR-thermography allows for a fast, remote, and nondestructive inspection of components. Even at low modulation frequencies (e.g. 0.03 Hz) the measurement time is only about 3 minutes. Thermal waves are known to be very sensitive to the boundary situation between coating and substrate [1]. Therefore they are suited to investigate layered materials and to detect areas of thickness variations, inclusions, or disbonds. A phase image can be considered as an image of thermal thickness which describes local variations of material properties and/or thickness variations, For turbines the quality control of functional coatings such as thermal barrier coatings and wall thickness of turbine blades are very important. The results obtained on some relevant problems related with turbines are presented here, these include, detection of subsurface structures and flaws, correlation of phase angle measurements with mechanical properties of coatings (e.g. density, hardness, and porosity), monitoring of wall thickness variations of turbine blades including a wax model used for casting.