Variation features of MMM signals of X80 steel in tensile fatigue process under geomagnetic excitation

Abstract

Fatigue failure and stress damage are the main damage forms of marine risers.To study the variation rule of surface magnetic field signals of X80 steel during tensile fatigue process under geomagnetic excitation, the fatigue tensile test of X80 steel specimens under typical working conditions was carried out with an MTS810 250 fatigue tensile testing machine. During the experiment, the variation of magnetic field on the surface of the tensile specimen was monitored by a magnetic memory sensor, the fatigue failure of the specimen was observed by a handheld microscope, and the morphology of the fracture was analyzed with the help of scanning electron microscope(SEM).The results show that the magnetic memory is sensitive to the tensile fatigue damage. Thus, the characteristic parameters of magnetic memory detection for tensile fatigue damage of X80 steel were proposed, which took the extreme value of magnetic memory signal with background removed HE(X) and the line slope of the extreme value of tangential gradient signal SK(X) as the eigenvalues. As a result,zero crossing phenomena occur to HE(X) and SK(X) in the stage of crack initiation. The acceleration of HE(X) and SK(X) speeds up with the increase of the fatigue cycle, which is consistent with the crack growth law during the fatigue process. Generally, HE(X) and SK(X)can reflect the fatigue failure process of the specimen, and they can be used to monitor the fatigue damage of marine risers. This research results could provide a reference for the field application of the magnetic memory detection method in the fatigue damage detection of marine risers.

Keywords

• marine risers
• x80 steel
• tensile fatigue
• metal magnetic memory(mmm)
• characteristic parameters