Papers by Author: Da Bo Wu

Abstract: In order to investigate material behavior at the stage of early fatigue damage and critical fatigue fracture, a new nondestructive testing called metal magnetic memory (MMM) technology, has been introduced in this paper. The fatigue experiments are carried out on the PQ1-6 rotary bending fatigue machine. The MMM fatigue characteristics are presented and analyzed. It has been found that Hp(x) is increasing up to the peak value while the fatigue crack is initiating and growing. After that peak, Hp(x) decreases gradually with the fatigue crack deepening and stress releasing. The gradual increasing Hp(x) is the sign of the early fatigue damage and the Hp(x) peak predicts the critical fracture state. The validity of MMM results has been certified by microscopic metallographic examination. Furthermore, the MMM fatigue behavior is deduced theoretically based on the dislocation mechanics and fracture mechanics.

Abstract: It is important that monitoring the fatigue crack propagation in engineering. This paper studied crack growth rate with the metal magnetic memory technique. Three-point bending fatigue tests of center-cracked specimens were carried out and the normal component of magnetic field signals Hp(y) on the surface of specimens were measure by TSC-1M-4. The correlation of crack growth rate da/dN and metal magnetic memory normal signal Hp(y) was studied. The specimens, with different heat treatment, quenching and thermal refining, were tested in fatigue-testing Machine to study the influence of heat treatment on the metal magnetic memory signal Hp(y). The curve of da/dN ~ Hp(y) was built.

Abstract: The evaluation of stresses in engineering structures to provide early indications of stress status is a fast growing area in non-destructive testing. Metal magnetic memory technique is the magnetic stress measurement technique for ferromagnetic materials using passive magnetic fields to monitor changes in the magnetic properties of materials. This paper studied the physical formation process of metal magnetic memory effect. This paper studied the physical mechanism between stress and magnetic field in metal magnetic memory effect, through the influence of stress-induced magnetic anisotropy on magnetization process of ferromagnetic crystal.

Abstract: Metal magnetic memory (MMM) method, based on the magneto-elasticity and mechanical effect theory, is a new nondestructive testing technology. This paper aims at the material property and failure analysis for steam turbine blade with MMM method. The failure blade and its fracture face are tested with MMM technology at workshop. It has been found that the MMM signal variation gradient of the crack transition zone in the fracture face is minimal, that of the crack initiation zone is in the middle, and that of the tear fracture zone is maximal. MMM testing results reflect material behavior degradation of the failure blade. In order to proof the MMM results, the dynamic stress of the steam turbine blade under the operation loading is calculated by WESTHOMES software and analyzed theoretically. It has proved the validity of the MMM method applied on material property evaluation and failure analysis for steam turbine blade.