Inspection of both inner and outer cracks in aluminum tubes using double frequency circumferential current field testing method

Aluminum and alloy tubes are widely used in industrial fields because of the advantages of good corrosion resistance, high thermal conductivity and light weight. Due to the stress corrosion cracking (SCC) and fatigue corrosion cracking (FCC), both inner and outer cracks generate in the aluminum tube. It is still a challenge to inspect all inner and outer surface cracks in the thick-wall aluminum tube in real time by one pass scan using the nondestructive testing (NDT) method. A double frequency circumferential current field testing (CCFT) method is presented for the inspection of both inner and outer cracks in the aluminum tube by one pass scan. A simulation model is proposed to extract characteristic signals of inner and outer cracks at two excitation frequencies. The bobbin-type probe with sensor array is developed and excited by the synthetic double frequency excitation signal. Both inner and outer cracks are tested by the double frequency CCFT system. Results show that both inner and outer cracks can be identified, distinguished and evaluated by the double frequency CCFT method by one pass scan.