Papers by Keyword: Microplasticity

Abstract: Pariss law of fatigue crack propagation rate is well applied in the defect-tolerance fatigue approach. When carry out same approach in the very high cycle fatigue domain, the understanding of mechanism about fatigue crack propagation threshold which is obviously important, is helped. In the present work here, the fatigue crack propagation threshold of a surface crack for an Armco iron loaded in the VHCF regime was investigated by a new approach which combines the fracture surface analysis and the temperature recording on the surface during the test by an infra-red camera. The experiments were carried out on a sheet specimen under a 20 kHz ultrasonic frequency loading with IR images registration. Three stages of fatigue crack were identified with different mechanisms. It is found that the transition between initiation and crack propagation corresponds to the intrinsic fatigue threshold. It takes more than 99% of the gigacycle fatigue life to achieve this transition size.

Abstract: The strain-amplitude dependence of internal friction in Cu-0.41Ni-0.11P (mass%) alloys has been evaluated to reveal the relation between the amplitude-dependent internal friction and the stress relaxation performance. Annealing at 250°C after cold rolling causes a suppression of the strain-amplitude dependence with increasing annealing time in the range between 10 s and 10⁴ s. Analysis of the amplitude-dependent internal friction reveals the plastic strain of the order of 10^-9 as a function of effective stress on dislocation motion. It is found that the microflow stress at a constant level of plastic strain increases with increasing annealing time. This result is in line with the improvement in the stress relaxation performance but disagrees with a decrease in the tensile strength and yield stress after annealing. We believe that the increase in the microflow stress after annealing is caused by inhibition of dislocation motion due to Ni-P clusters, which were revealed by three-dimensional atom probe (3DAP) experiments.

Abstract: nternal friction behaviour of B2 FeAl alloys has been examined to reveal the correlation of the microplasticity and thermal vacancies. The internal friction peak for Fe₆₀Al₄₀ appears at around 550 K, and the peak height increases with increasing quenching temperature. The curves of internal friction against the strain amplitude shift to larger strain amplitude as the quenching temperature increases. Analysis of the amplitude-dependent internal friction provides the plastic strain of the order of 10^-9 as a function of effective stress on dislocation motion. It is found that the microflow stress at the plastic strain of 1×10^-9 increases linearly with the square root of the net peak height. Remarkably, the microflow stress decreases with rising temperature but turns to increase above 500 K when measured after holding for 1 h at test temperatures. The anomalous increase in the microflow stress is caused by the creation of thermal vacancies at intermediate temperatures.

Abstract: In order to fully understand the thermomechanical phenomena involved in high-cycle fatigue, a method for determining the dissipative thermal sources in a loaded metallic specimen from the spatio-temporal integration of the heat diffusion equation is proposed. Temperature fields obtained through a focal array infra-red camera are processed with this technique. After a refined analysis of the sensitivity of the method, preliminary tests have shown that it is possible to detect a burst of heat sources within the first couple of cycles for the specimen tested above a certain level of tensile stress. This behaviour is thought to be related to the microplasticity level.