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Experimental techniques for measuring the crack length and crack closure behavior of single edge crack tension [SE(T)] specimens with fixed ends were developed with compliance method employing a high-resolution laser displacement meter together with data processing for noise elimination. Then, an automatic force-shedding fatigue crack propagation test system was developed. Using this system, we measured the crack length and crack closure behavior of thin films with a thickness of 5 or 10 µm and width of 10 mm, with which the accuracy of crack length measurement was better than 0.46 mm. The crack closure behavior could also be identified by the compliance method. The relationship between the crack propagation rate da/dN and the stress intensity factor range ΔK for nanocrystalline (NC) and ultrafine-grained (UFG) thin film was independent of the force ratio, and the relationship for the 10 µm-thick film was almost identical to that for the 5 µm thick film. No crack closure was observed in the UFG and NC thin films, whereas it occurred in the medium-size-grained (MG) thin film. The da/dN–ΔKeff relationships for thin films were independent of the force ratio and film thickness. In the Paris law region, da/dN for the MG film was higher than that for an MG plate at comparable ΔKeff, showing that crack closure could not account for the thickness effect. However, no difference in ΔKeff,th was observed for both the MG film and the MG plate. In the Paris law region (Region B), no effect of grain size on the da/dN–ΔK relationship was observed for the 10-µm-thick film; however, the crack propagation rate was higher for a film with smaller grain size and thickness from 100 to 180 µm. For NC and UFG films, the crack propagation rate of the 100-µm-thick film was higher than that of the 10-µm-thick film, but it was almost identical for MG films; however, the rate for a thin film was higher than that for a bulk plate. The threshold stress intensity factor range was always lower for a film with smaller grain size.

Since rolling contact fatigue (RCF) is known to be affected by non-metallic inclusions, the RCF crack initiation and propagation mechanisms in high-strength steels were observed using ultrabright synchrotron radiation laminography. The material of the sample was a high-carbon chromium bearing steel, which intentionally contained a high concentration of sulfur to enable the observation of crack initiation from MnS inclusions. To conduct an RCF test nearby the experimental hatch of a beam line of a synchrotron radiation facility, a special RCF testing machine was developed. Fatigue tests were interrupted to conduct laminography. The fatigue life for flaking depended on the length, width, and orientation of inclusions. For every length and orientation of inclusions, vertical cracks, whose faces were perpendicular to the rolling direction, first appeared, then horizontal cracks, whose faces were parallel to the sample surface, were formed after the vertical cracks reached a critical length. The initiation life of the vertical cracks and horizontal cracks depended on the length, width, and orientation of the MnS inclusions. The depth of horizontal crack initiation site, however, was almost independent of these parameters. The mechanism of the flaking process, which was directly observed by laminography using synchrotron radiation, was completely different from that previously supposed from observations by conventional microscopy.

To examine near-threshold fatigue crack propagation in commercially pure (CP) titanium with a bimodal harmonic structure, which is defined as a coarse grained structure surrounded by a network of fine grains, K-decreasing tests were conducted. The crack growth rates for harmonic-structured CP titanium were higher than those for homogeneous material with coarse grains at comparable stress intensity range, while the threshold stress intensity range was lower. This phenomenon was attributed to a reduction in the extent of crack closure and the effective threshold stress intensity range, resulting from the presence of fine grains in the harmonic structure.

TiB-reinforced Ti-3Al-2.5V, in which TiB whiskers are oriented parallel to the direction of heat extrusion, was fabricated. To investigate the effects of TiB whisker orientation on fatigue properties in Ti-3Al-2.5V, four-point bending fatigue tests were conducted for plate-type specimens having three different orientations of TiB whiskers. The fatigue limit and fatigue life of Ti-3Al-2.5V having TiB whiskers, which are oriented parallel to the loading direction, were high. This is because the resistances of fatigue crack initiation and propagation were changed with the TiB orientation in Ti-3Al-2.5V. Thus, fatigue properties in the Ti-3Al-2.5V having TiB whiskers depended on TiB orientation.