Thermographic Crack Detection by Liquid Gas in Materials with Low Emissivity

A new approach for detection of cracks in metals with low emissivity was investigated. Liquid gas is sprayed in a linear movement across the test object. The gas propagates into cracks open to the surface by capillary effects and by the pressure of spraying. This causes cooling and subsequent warming up. It was found that the time profiles of the temperature close to the crack have characteristic properties which are different from the time profiles of a pure surface disturbation of the emissivity. Cracks of different depth were investigated in aluminium and steel. An advantage of the technique compared to liquid penetrant testing is the residual-free evaporation of the gas that allow testing in a single step.