In order to increase the efficiency of inspection using ultrasound lock-in thermography (ULT) and obtain optimal results

they are investigated which include ultrasound modulation

sequences of the thermal images processing and optimal parameters selection. The principle of ULT is represented

and theory analysis is researched. Then inspection system is developed

and experiments are carried out on a metal plane with surface microcracks. In inspection

transient sequences are collected by infrared camera during amplitude modulated ultrasonic excitation by square signal and processed using method of lock-in processing to obtain phase image and amplitude image. For given A3 steel sample

results indicate that the modulation frequency which equal to 0.5Hz should be selected; the pressure force between the sample and the ultrasonic transducer is about 0.25kN

which should be small so that the transducer just can be pressed against the sample during injection ultrasound. Moreover

the distance between the injection position and crack has few effects on inspection and appropriate injection position should be depended on specific structure of sample. Using the inspection system

reliable detection of surface microcracks (micrometer level) has to be done for a few seconds.