Finite element modeling of magnetoacoustic emission and of stress-induced magnetic effects at seam welds in steel pipes

Prior finite element modeling (FEM) has shown how creep damage reduces the voltage signal in a secondary coil wrapped around a C-core electromagnet po sitioned above a seam weld in a steel pipe. Prior hysteresis modeling has a lso demonstrated the reduction of magnetoacoustic emission (MAE) due to uni form creep damage in a steel specimen. In this article, it is shown how to evaluate the MAE due to a nonuniform distribution of creep damage at a seam weld using FEM. Also, FEM is used to evaluate the C-core secondary emf due to a nonuniform stress distribution, which is developed at a seam weld due to the internal pressure of hot steam passing through the pipe. In this la tter case, use is made of the stress distribution computed at the seam weld by Stevick. It is found that nonuniform creep damage also reduces the MAE signal, as is found experimentally. The stresses due to internal pressure i n the pipe are shown by FEM to increase the C-core secondary emf, which is opposite to the effect of creep damage. A comparison is made between stress ed and unstressed cases, with and without creep damage. (C) 2001 American I nstitute of Physics.