Key Engineering Materials Vols. 345-346

Abstract: The creep-fatigue crack-growth behavior of HAYNES® 188, a cobalt-based superalloy, was studied at the temperatures of 649, 816, and 927 oC under isothermal conditions. Various hold times at the maximum load were introduced to study the effects of hold time and temperature on the crack-growth behavior. The experiments were conducted under constant stress-intensity-factorrange control modes. Crack lengths were measured by a direct current potential method. The introduction of hold times led to an increase in the cyclic crack-growth rate. As the temperature increases, the time-dependent crack-growth behavior was dominant.

Abstract: Effect of nitriding on fatigue crack initiation and growth rate has been studied on Ni-Cr-Mo steel. Specimens were nitrided for 15hr at 680°C. The fatigue limit of nitrided specimens were superior to those of annealed(680°C, 15 hr) specimens. Based on detailed observations of slip band and micro crack initiation, it is concluded that the excellent fatigue limit of nitrided specimens is attributed to improved slip initiation resistance by nitriding. The characteristic of fatigue crack growth rate of nitrided specimens was investigated by comparing with those of annealed specimens. It was found that by nitriding the crack growth rate was markedly decreased and the threshold stress intensity factor range was improved. It is concluded that the excellent fatigue limit of nitrided specimens is also attributed to improved fatigue crack growth rate and threshold stress intensity factor range by nitriding.

Abstract: Single phase bainite structure which is obtained by the conventional austempering treatment reduces the ductility of ductile cast iron. Because of the reduction of ductility it is possible to worsen the fatigue properties. Therefore, semi austempered ductile iron which is treated from
+ϒ is prepared to investigate the static strength and fatigue properties in comparison with fully austempered ductile iron (is treated from ϒ). In spite of semi austempered ductile iron shows the 86% increase of ductility. Also, semi austempered ductile iron shows the higher fatigue limit and lower fatigue crack growth rate as compared with fully austempered ductile iron. By the fractographical analysis, it is revealed that the ferrite obtained by semi austempering process brings about the plastic deformation(ductile striation) of crack tip and gives the prior path of crack propagation. The relatively low crack growth rate in semi austempered specimen is caused by above fractographical reasons

Abstract: The effect of axial loading resulting from torque on the fatigue life of bolts has been investigated. It was found that the fatigue limit of bolts increased by an amount proportional to the increase in torque. There was a linear relationship between torque and fatigue limit. Using the Shigley method, the prediction of fatigue limit from calculated stress concentration factors was also examined. A good correlation was found between the experimental and predicted fatigue limit. The Shigley method was appropriate for the prediction of fatigue life of bolts.

Abstract: A clear understanding of fatigue properties for the pipeline steel and its weld is important to provide information for pipeline design during pipeline construction and predict pipeline fatigue life during pipeline operation. The materials used in this study are API 5L X65 pipeline steel generally used for natural gas transmission. This pipeline was welded by gas tungsten arc welding (GTAW) and shielded metal arc welding (SMAW) with V-groove configuration. The fatigue crack growth behaviors of pipeline steel and its girth weld according to crack growth directions and stress ratios were investigated over a wide range of stress intensities in laboratory air.

Abstract: The fatigue behaviour of newly developed Mg2Si-dispersed magnesium (Mg) alloys produced by solid-state synthesis was studied. Rotary bending fatigue tests have been performed using smooth specimens of materials produced with fine and coarse AZ31 alloy powders. Both Mg2Si-dispersed Mg alloys exhibited lower fatigue strength than a conventional extruded AZ31 alloy and the powder size dependence of fatigue strength was clearly recognized, where the material produced with fine alloy powder showed considerably higher fatigue strength than the counterpart. Fatigue cracks invariably initiated at large Mg2Si particles immediately after cyclic loading was applied and subsequent small crack growth was faster than the extruded AZ31 alloy. It was concluded that the lower fatigue strength of Mg2Si-dispersed Mg alloys was attributed to premature crack initiation at Mg2Si particles and faster small crack growth, and the observed powder size dependence of fatigue strength was due to difference in the size of the particle from which the crack initiated.

Abstract: In order to investigate the influence of hydrogen on the fatigue strength of Type 304 meta-stable austenitic stainless steel, specimens were cathodically charged with hydrogen. Hydrogen-charging led to a marked decrease in fatigue crack growth life. Crack growth paths and slip bands morphology were changed by the hydrogen-charging. To elucidate the mechanism of the degradation by hydrogen, the surfaces of both the uncharged and charged specimens were examined by the hydrogen microprint technique (HMT). In the uncharged specimen, no hydrogen emission from specimen surface was observed. On the other hand, in the hydrogen-charged specimen, a hydrogen emission was observed, especially in the vicinity of fatigue cracks. Hydrogen was mainly emitted from slip bands. These results suggest that the degradation of fatigue crack growth resistance in hydrogen-charged specimens was caused by the diffusion of hydrogen to slip bands, which accelerated the dislocation mobility and thereby facilitated the fatigue crack growth.

Abstract: The present paper summarizes the fully reversed strain-controlled creep-fatigue tests conducted on thin-walled tubular specimens of SUS304 austenitic stainless steel at 973K in air under push-pull, cyclic torsion, in-phase straining and 90deg out-of-phase straining of push-pull and cyclic torsion. It is shown that, as the results of analysis of the experimental data by the strain-range partitioning method
and the critical plane model parameter, a new inelastic-strain based parameter was proposed for life estimation of SUS304 subject to nonproportionally combined push-pull and cyclic torsion by the strain-range partitioning method.