Defect detection with thermal imaging and phase shifting methods in lock-in thermography

This paper presents the results of experimental investigation of thermal phenomena in a square shape STS 304 specimen with artificial defects of varying depth and diameter at the back side. The material is aimed to be tested by means of lock-in thermography. The temperature field of the front surface of material tested is observed and analyzed at several excitation frequencies. Phase contrast method was used for better identification and analysis of the existing defects of the specimen. The phase contrast between the defective and sound regions illustrates the qualitative and quantitative investigation of defects. The two, three, four and five step phase shifting methods are investigated to obtain the information on defects. Phase contrast with four-step phase shifting at an optimum frequency of 0.01 Hz provides excellent results.