Available time and dubious zone size of welding crack by metal magnetic memory method

Test plates with prepared welding cracks were made by designed experiment, the different areas of the test plates were detected by Metal Magnetic Memory (abbreviation MMM)method in different times. The MMM detection signal characteristics was analyzed by Fourier transformation. And then the detected signal and its characteristics analyzed by Fourier transformation were studied at different zones of the crack in different times.It is concluded that the dubious welding crack zone is 70 mm from theMMM signal character position, and the available time when the welding crack is detected by MMM after the weld joint is cooled below 100℃.