Study On Key Techniques In Nonlinear Ultrasonic Testing For Early Diagnosis Of Shafts Fatigue Damages

Shafts are widely used in engineering machinery and equipment. As the mechanical supporting and driving parts, they inevitably suffer from metal fatigue under long-term cyclic loading. Fatigue failure can cause life-threatening accidents, as well as enormous economic loss. 45 # steel is commonly used in shaft parts. So it can be the basic research object for the fatigue tests of shaft parts. Nonlinear ultrasonic nondestructive testing is able to evaluate the mechanical performance degradation of the material and assess the state of metal fatigue resulting from the interaction of an acoustic wave with fatigue-generated dislocation substructures that predictably evolve during the metal fatigue process. The interaction is quantified by the acoustic nonlinearity parameter ? extracted from acoustic harmonic generation measurements. It can diagnose the early fatigue of shaft parts and predict the remaining service life, thus discovering potential threats and preventing fatigue fracture accidents of shaft parts.The purpose of this paper is to diagnose and evaluate early fatigue of the material. The specimen at this stage is accompanied by the degradation of material properties, but without producing any macroscopic damage or flaws. Therefore, higher harmonic is applied to characterize 45 # steel specimens under different tensile loading cycles. Higher harmonic generation is the most classical phenomenon that the waveform of incident wave is distorted by the nonlinear elastic response of medium to the incident wave, and so that higher harmonic waves are generated in the transmitted wave. The following four key techniques were developed on nonlinear ultrasonic testing for the early fatigue diagnosis of the specimens.Famous finite element emulation software, ABAQUS, was used to simulate the deformation of the specimen under tensile pressure. Dynamic simulation was also carried out due to its good performance in simulating wave propagation, thus observing the attenuation and reflection rule of the ultrasonic stress wave.The nonlinear ultrasonic experiment system was established. The system stability was verified through repeated measurements, and was successfully applied in the field experiments.The normalized slope value of the fitting line of data points A2 vs.A12 was used as the relative ?, to characterize different specimens. It is equivalent to averaging the testing values after repeated measurements, thus improving measurement precision and reliability.The normalized slope value was respectively obtained as the nonlinear characteristic through direct extraction method and pulse inversion technology. While the normalized fundamental wave amplitude A1 was used as the linear characteristic. By comparing the experimental results of the three methods in characterization of specimens at different stage of fatigue, it turns out that only pulse inversion can discover the ? parameters typically increase monotonically with increasing cycles of the specimens. And it can potentially serve as a master curve for fatigue evaluation based on nonlinear ultrasonic measurements.