OS2121 高輝度放射光ラミノグラフィを用いた高強度鋼中の転動疲労き裂の観察

The flaking failure in rolling contact fatigue (RCF) results from crack initiation and propagation has been believed to originate from non-metallic inclusions located beneath the surface. With conventional microscopies, however, damage process in the internal region of materials could not be observed, then RCF crack initiation and propagation behaviours were observed by using synchrotron radiation computed laminography (SRCL) in the brightest synchrotron facility in Japan, and the effect of the inclusion orientation on the RCF property was examined. In our previous studies, crack initiation and propagation behaviours caused by extended MnS inclusions distributed in depth or transverse (width) direction was observed by the SRCL. In the present study, the fracture mechanism under RCF was discussed on specimens with MnS inclusions distributed in the rolling direction. As a result, vertical cracks were initiated on the surface, parallel to the ball-rolling direction in specimens. The crack propagation direction was then changed perpendicular to the rolling direction. Thereafter, similar with our previous studies, vertical cracks caused the horizontal cracks beneath the surface, when the vertical cracks reached to a critical length. The ratio of the vertical crack initiation life to the flaking life was higher than specimens with other inclusion orientation.

In rolling contact fatigue (RCF), cracks initiate and propagate from inclusions beneath the surface. In the present study, crack propagation behavior under RCF was observed by synchrotron radiation computed laminography (SRCL), which is suitable for thin plates, in SPring-8 (Super Photon Ring-8 GeV). Specially fabricated inclusion-rich steel (modified JIS SUJ2) plate specimen, which includes extended inclusions distributed in depth direction, was employed in the experiments. Specimen with surface crack from inclusion and after the occurrence of flaking was observed by SRCL, and flaking defect and crack under the surface was successfully detected. It was found that the tensile-type vertical crack, which was perpendicular to the rolling contact surface, was formed from a cylindrical inclusion. After the vertical crack propagated along inclusion, the shear-type horizontal crack, which was parallel to the rolling contact surface, was found to initiate and then flaking occurred as the result of the propagation of the shear-type crack. It is considered that the vertical crack is an important factor of flaking process and crack propagation behavior under RCF from inclusions is clarified by SRCL.

In rolling contact fatigue (RCF), cracks usually initiate from inclusions beneath the surface and propagate to the contact surface. In the present study, synchrotron radiation computed laminography (SRCL) imaging was performed to observe flaking defects during the RCF of a high-strength steel. Specially fabricated inclusion-rich steel plate specimens were employed in the experiments. For the in situ observation of crack propagation, a compact RCF testing machine was developed, and a 4D analysis scheme was applied to the data obtained by SRCL. RCF tests were carried out near the measurement hatch of the beam line used SRCL to enable the successive observation of crack initiation and growth behaviors. Specimens before and after the occurrence of flaking were observed by SRCL, and flaking defects and cracks under the surface were successfully detected. As a result, details of the crack initiation and flaking process in RCF could be discussed. Shear-type horizontal cracks were found to initiate after the initiation and propagation of tensile-type vertical cracks along inclusions, where the face of the vertical cracks was perpendicular to the rolling direction and rolling surface. Therefore, the formation of vertical cracks is considered to affect shear-type crack formation and flaking, where the shape and length of inclusions also affect the initiation and propagation of vertical cracks.

Effects of inclusion size and orientation on rolling contact fatigue crack initiation observed by laminography using ultra-bright synchrotron radiation

In rolling contact fatigue (RCF), cracks initiate and propagate from inclusions beneath the surface. In the present study, crack propagation behavior under RCF was observed by laminography using ultra-bright synchrotron radiation, which is suitable for thin plates, at SPring-8 (Super Photon Ring - 8 GeV). The material had intentionally contains a high concentration of sulfur to enable the observation of crack initiation from MnS inclusion. Fatigue tests were interrupted to conduct laminography by a new developed compact RCF test machine. It was found that vertical cracks, whose face was perpendicular to the rolling direction, were first appeared, then horizontal cracks, those faces were parallel to the sample surface were formed after the vertical crack propagated. And then flaking occurred as the result of the propagation of the horizontal crack. The initiation life of the vertical cracks and horizontal cracks depended on the length and width of the MnS inclusions. And it is considered that the vertical crack is an important factor of flaking life. These mechanisms of flaking process, which was directly observed by laminography using synchrotron radiation, were completely different from conventional RCF mechanism supposed from the surface observations by conventional microscopies.

Detection of surface breaking defects, such as rolling contact fatigue (RCF) cracks, is an on-going topic of research within the context of rail inspection. At present, detection and classification of RCF cracks using an ACFM sensor is based on low speed walking stick systems where negligible sensor lift-off changes result in a reasonably stable background signal and hence the defects can be automatically detected using a threshold method. However, in the case of high speed inspection systems, the inevitable lift-off variations (e.g. owing to the dynamics of the train bogie etc.) lead to a varying background ACFM signal which renders the threshold method ineffective. A novel method for the detection of isolated RCF cracks, namely combined threshold and signature match (CTSM), has previously been applied to ACFM scans over RCF cracks. The method proved to be effective for automatic detection of isolated RCF cracks, however, it was observed to perform poorly in response to areas of multiple RCF cracks, which often appear in clusters. This paper investigates the application of an enhanced CTSM algorithm to the detection of clustered RCF cracks. The algorithm has been applied to low speed scans over sections of rails removed from service containing real RCF cracks and to ACFM scans obtained at high speed (up to 48 km/h) over a spinning rail rig containing clusters of artificial cracks. Results suggest that the extended CTSM algorithm is effective in automatically detecting multiple RCF cracks with a high detection rate (> 90%). Further, in the case of widely spaced (> 5 mm) multiple cracks, the algorithm can also provide extra characterisation information about the number and position of cracks within a cluster.

Effect of defect shape on rolling contact fatigue crack initiation and propagation in high strength steel

The objectives of the present paper are to clarify the rolling contact fatigue (RCF) crack path in high strength steel with artificial defects using synchrotron radiation micro-computed tomography (SR micro CT) imaging and to discuss the mechanism of RCF crack propagation by finite element (FE) analysis. Two crack types, a vertical crack and a horizontal crack, were observed around the artificial defect by SR micro CT. The stress intensity factor (SIF) of horizontal cracks, calculated by FE analysis, increased by the existence of vertical cracks. The interaction between these two crack types dominates RCF crack propagation originating from artificial defects.

The overall stable crack initiation and propagation behaviour of fracture mechanics specimens cut from plastic pipes that were composed of different polyolefin materials were investigated using concepts of elastic–plastic fracture mechanics including the crack propagation kinetics. The effect of specimen shape, orientation, welding and lading rate on the crack resistance (R) behaviour of these materials has been thereby assessed. It was found in principle that specimen shape, orientation and welding have an influence indeed but only an unexpected small one on crack initiation behaviour and, particularly, on crack propagation behaviour. The crack initiation toughness is not sensitive to the orientation in most cases. In contrast, the crack propagation toughness is significantly affected by the orientation where the values for crack propagation in extrusion direction are larger than ones for crack propagation crosswise to that. This confirms that the morphology affects the stable crack propagation behaviour more than the stable crack initiation behaviour. In agreement with results of the microindentation test, fracture mechanics investigations also show that a lower welding pressure and a larger welding temperature, respectively, have no or a positive effect on the mechanical and fracture mechanics properties, whereas a larger pressure and a lower temperature, respectively, result in deterioration of the performance of the welded joint. Furthermore, the R-curve behaviour was investigated using specimens cut from bilayer pipe segments. It has been shown that an additional layer has a clear impact on the R-curve behaviour compared to the crack propagation in single-layer pipes, which can be explained thereby that the plastic constraint was affected by this additional layer. For clarification of the toughness-in- or -decreasing effect of an additional layer (with differing mechanical characteristics) on the layer where the crack was growing, R-curve ratios were introduced, that showed that the asymmetric mechanical properties of different layers were directly reflected in an asymmetric impact on the stable crack initiation and propagation behaviour.

Synchrotron radiation computed laminography (SRCL) was carried out at SPring-8 to visualize a fatigue crack inside a friction stir welded plate and to investigate the propagation behavior. The surface fatigue crack shape and its size reproduced by SRCL agree well with the observation by an optical microscope. A fatigue crack propagation curve obtained by SRCL corresponded with that obtained by replication technique. There are two inflection points on a curve describing the relation between the crack propagation rate and the surface crack length. The cause of the inflection was discussed.

Characterization of intersecting and bifurcating rolling contact fatigue (RCF) cracks in railway rails using ACFM sensor

Degradation of a sintered Cu nanoparticle layer studied by synchrotron radiation computed laminography

In order to visualize a fatigue crack inside a friction stir welded plate, synchrotron radiation computed laminography (SRCL) was carried out at SPring-8. As the results of SRCL, the surface fatigue crack shape and its size were well in agreement with the observation by an optical microscope. Furthermore, it was clear that the crack inside the specimen was also observable m a non-destructive manner by reconstructing slice images at various depths. The fatigue crack propagation behavior inside the plate specimen could be investigated by SRCL without destruction of the specimen.

4D observations of rolling contact fatigue processes by laminography using ultra-bright synchrotron radiation

Experimental and theoretical investigation of the phenomenon of filling the RCF crack with liquid