Novel NDT Test Method For Stress Concentrations And Fatigue Cracks

摘要

Residual stresses and stress concentration in structures impinge on the mechanical properties, erosion-resistance, dimensional precision and cause fatigue failure. Therefore, detecting and evaluating these stresses nondestructively are necessary for analyzing the state of equipments in use. A variety of nondestructive testing (NDT) techniques, such as X-ray diffraction method, Barkhausen Noise technology, Ultrasonic method, and etc, are used to evaluate stress. However, all the methods have the weakness of being time-consuming. More recently, a new magnetoelastic method, which was a non-contact, fast and non-exciting method, has been used for stress concentration testing of ferromagnetic materials. Further more, this method could test fatigue cracks for normal flaws. Usually the magnetoelastic stress testing method is conducted under a magnetizing field. The new method uses the net magnetic moment induced by stress and does not needs magnetic excitation. The domain structures and magnetic properties of polycrystalline ferromagnetic materials under stresses, especially uneven stresses, without a magnetizing field, have not been noted. The first time spontaneous magnetization phenomena were noticed in production was in 1970's by Mr. Zhong Wei-Chang. He found that demagnetized ferromagnetic materials were intensely magnetized by cutting or operation. The same phenomenon was observed in Russia at the beginning of 1980's. The two phenomena were caused by uneven stresses, however, the phenomenon has not been applied to stress concentration testing.