Papers by Author: Nu Yan

Abstract: Rotating bending fatigue tests were carried out to investigate the effects of temperature on the fatigue strength and the fracture mechanism of an 18 % Ni maraging steel at room and elevated temperatures of 473K and 673K. Fatigue strength was higher at elevated temperatures than at room temperature, though static strength was decreased by softening at elevated temperature. There was no effect of temperature on crack morphology and fracture mechanism. On the other hand, during fatigue process at elevated temperature, the specimen was age-hardened and the specimen surface was oxide. That is, the increase in fatigue strength at elevated temperature was mainly caused by the increase in hardness due to age-hardening and suppression of a crack initiation due to surface oxidation.

Abstract: The design of small-sized single pass boilers is usually based on the construction code for small-sized boilers, in Japan. Design methods in this construction code and other country codes have not been clearly defined, although most of small-sized boiler’s headers are constructed from two concentric circular shells and cover plates. Therefore, Finite Element Analysis (FEA) with a PC and measurement by strain gauges are mainly used in strength design of these boilers. A T type welded joint of SS400 was adopted for the header of small-sized single pass boilers was studied. The stress concentration was evaluated by the FEA, and S-N curves were re-drawn using maximum elastic stress, Ktσmax.

Abstract: In order to improve the heat transfer efficiency, the fins are commonly used in the industrial boiler in Japan. In actual application, the thermal fatigue due to the cyclic change of temperature usually occurs in the fins. The thermal fatigue tests were carried out by using the thermal fatigue apparatus designed. The designed testing apparatus can apply thermal load to the fin by giving heating and cooling alternatively. The fatigue cracks can be observed in the vicinity of the toe in the thermal fatigue test. The heat transfer coefficient and the thermal stress were calculated by using Finite Element Analysis (FEA) method. The fatigue experiments of the fins were also conducted using electro-hydraulic servo fatigue test machine in the laboratory. The results of thermal fatigue experiments were discussed by comparing with those of the mechanical fatigue experiments.

Abstract: The effect of plasma radical nitriding treatment on fatigue properties of SCM435 steel in super long life region was investigated. Fatigue tests were carried out using a dual-spindle rotating bending fatigue-testing machine at room temperature in air for the specimens nitrided at 773 K and 823 K for 3 hrs. The fatigue strength of nitrided specimen was greater than that of un-nitrided specimen and the crack initiation mode changed from the surface cracking of un-nitrided specimen to the subsurface cracking of nitrided specimen. Hardening layer and compressive residual stress were formed by nitriding, which resulted in the improvement of the fatigue strength. The stress intensity factor was calculated using facet area in Fish-eye fracture mode. As a result, the stress intensity factor indicated almost constant value, ~ 3-4 MPa·m1/2, regardless of the number of cycles to failure.

Abstract: In order to investigate the influence of loading frequency on the fatigue behaviors of the high strength steel, ultrasonic fatigue tests were carried out for a high-carbon-chromium steel and the results were compared with those of fatigue tests using conventional rotary bending fatigue test machine with a frequency of 52.5Hz. The different of fatigue strength at ultrasonic frequency level and conventional frequency level is very small and the S-N curve obtained from 20 kHz or 52.5 Hz shows the step-wise shape. The fatigue crack occurred from inclusions on the subsurface site in the long life regime and the typical surface fracture occurred in the short life one though the loading frequency level is different. It is indicated that ultrasonic fatigue method is an effective method to investigate the fatigue properties in super-long life region.

Abstract: Fatigue properties of Inconel 718 at 500°C were investigated under rotating bending. Fatigue strength was higher at 500°C than at room temperature. Fracture occurred by the initiation and propagation of a surface crack in short life region. The early propagation of a surface crack smaller than 20-30 μm was suppressed by oxidation at 500°C. This is a main reason for the higher fatigue strength at 500°C. However, the crack growth after the retardation of a small crack growth at 500°C was accelerated reflecting the lower static strength. On the other hand, in long life region, though a surface crack initiated at the early stage of fatigue life similar to the case in short life region, the crack did not propagate over 20-30 μm, and an internal fracture occurred. Consequently S-N curve at 500°C showed a duplex property and the fatigue limit was not recognized. The subsurface damage was initiated at the early stage of fatigue life.