Hideo Kaneshiro

Dislocation structures in the strain localized region in fatigued 70/30 brass and the interaction with grain boundary

Long-term fatigue tests of polycrystalline 70/30 brass were carried out under low strain amplitudes in vacuum, and dislocation structures of the strain localized regions developed were examined by means of transmission electron microscopy In the planar dislocation structures, the strain localized regions (SLRs) bounded by a pair of parallel active glide layers were frequently observed in favorably oriented grains. Where the SLRs impinge on grain boundaries (GBs), extrusion-type deformations were sometimes formed notwithstanding the restraints of the neighboring grains. On the basis of observations, the mechanism of crack initiation at the GB is discussed.

Dislocation structures in the strain localized region in fatigued 85/15 brass

The correlation between the formation of extrusions and the dislocation structures in polycrystalline 85/15 brass subjected to a long term stress cycling with a low strain amplitude in vacuum was examined by means of transmission electron microscopy. The overall dislocation structures consisted of two types of structures,i.e., copper-type and 70/30 brass-type. Within them the strain localized regions bounded by two closely located active slip layers were frequently observed. In these boundary layers appeared a fringe pattern which was suggestive of intensive slip. Extrusions were formed in close association with the two closely located layers, and the initiation and growth of fatigue cracks occurred along one of them. On the basis of these observations, the mechanism of extrusion formation and of fatigue crack initiation are discussed.

Detection of 1 mm Fatigue Crack Initiation Using Strain Waveform

A method of continuous monitoring for fatigue crack initiation is examined which uses the strain waveform of running machine. A notched section of the equipement is taken as the inspection point because may fracture accidents originate in fatigue cracks initiated from such a notch. Fatigue crack initiation can easily be detected by observation of the waveform of a strain function composed of the strains in the vicinity of the notch, because it changes shape on crack initiation. This change of the waveform is brought about by the change of the compliance of the material due to the crack closure behavior

A Detection Method of Fatigue Crack Initiation by Analyzing Strain Waveform

A detection method of fatigue crack initiation at some weak section of machine equipment during operation is examined using a partial-notched specimen. The waveform of the strain function composed of strains in the vicinity of the partial notch was observed successionally. It is found that such strain waveform changes its figure at the time when the crack length is 1 mm, due to the crack closure behavior. Therefore, in this experiment, the crack initiation could be detected when the crack length is about 1 mm. Since the engineering crack initiation size is frequently taken to be 1 mm, the present method of detecting crack initiation can be applied to real machine equipment. Also, this waveform was analyzed with FFT (Fast Fourier Transformation). Power spectrum density of the waveform varies immediately after the crack length reaches 1 mm. Furthermore, the figure of the coherence of two waveforms composed of the same strains clearly varies at the same crack length of 1 mm. Thus, it is thought that the engineering size crack initiation can be detected by the analysis of the strain waveform.

A Consideration of Evaluation of Fatigue Crack Propagation Rate from Effective Stress Intensity Factor Range

The fatigue crack propagation law was investigated for the two cases of crack propagation in a region of welding residual stress and after overloading. In the former case, the crack propagation rate da/dN was found to depend on the stress ratio R defined by σmin/σmax where σmin, σmax are the minimum and maximum cyclic stresses. Due to the tensile residual stress, the crack propagation rate increased as the stress ratio decreased in the range −2 ≤ R ≤ 0. In the latter case, the crack propagation rate da/dN after overloading was higher than that for the constant stress amplitude test at first and then became lower. The crack propagation rate was thus affected by both the residual stress and the overloading. However, the crack propagation behavior correlated well with the crack closure behavior. Therefore, in the cases considered here, the crack propagation law can be reasonably evaluated from the effective stress intensity factor range ΔKeff.

Effects of Microcracks and Artificial Surface Cracks on Fracture Ductility of a Torsional Prestrained Specimen

The diminution of fracture ductility of smooth specimens and small cracked specimens on surfaces subjected to torsional prestrains was investigated. For smooth specimens, the transition of tensile fracture ductility after a critical torsional prestrain is independent of the existence of microcracks and attributed to the embrittlement of the layers of the specimen surfaces due to the formation of anisotropic helical structures. For cracked specimens, the degree of fracture ductility depends on the crack area and the magnitude of torsional prestrain. Where torsional prestrain is lower than a critical value, the surface crack starts in the direction close to the maximum shear stress, and ductility is somewhat lost from virgin state. If the torsional prestrain exceeds a critical value, the specimen is broken in a brittle manner, and the crack extension shows a helical pattern. This behavior is due to the embrittlement of the specimen surface layer.

A Method of Detecting Fatigue Crack Initiation and an Application of an Overload Using Local Strains

A method of detecting a fatigue crack which initiates from a notch and an application of an overload during a stress cycle were investigated. A crack initiated from a notch can be detected by analyzing the waveform of strain function which was calculated from the local strains in the vicinity of the notch. The local compliance of the material changed after initiation of the crack. This is the reason why the crack initiation can be detected by the strain waveform. This technique can also be applied to detection of an overload application during stable crack growth. The waveform shape of strain function changed after overloading. Especially, the present method is useful for cases of crack propagation under negative stress ratio conditions.Copyright

Fatigue Properties and Dislocation Structures of Nitrided Rolled Steel.

Fatigue properties of the nitrided rolled steel are investigated on the basis of the observations of the substructures produced by nitriding and fatigue. It is found that the fatigue limit of the nitrided rolled specimens is much higher than that of the nitrided annealed ones and the substructures before the nitriding are effective for the improvement of the fatigue strength. The substructure of the rolled steel consists of cells with dislocation clusters within them. By means of nitriding treatment, cell structures and twins appear in the inner part of the compound layer, while uniformly distributed dislocation clusters in the region just below the compound layer and the inner part of the diffusion layer. After fatigue test, most of these substructures are preserved except for the region just below the compound layer where the cell structures due to active dislocation motions were formed.

Fatigue Properties and Dislocation Structures of Nitrided SS 400.

Fatigue properties of the nitrided SS 400 are investigated on the basis of the observation of dislocation structures, surface topography and fracture surface by means of optical and electron microscopy. Both slip bands and dislocation structures during stress cycling are not formed in the compound layer. The dislocation morphology of the diffusion layer is composed of the untangled and uniformly distributed dislocations and the clusters of the dislocation dipoles at lower stress amplitude, but well defined cell structures at higher stress amplitude. It is also found that the fatigue cracks of the nitrided specimen are initiated without the formation of slip bands and the separation between the compound layer and the diffusion layer occurs during cyclic loading. On the basis of these observations, a mechanism of fatigue fracture for the nitrided SS 400 is proposed.

Fatigue Properties and Dislocation Structures of Polycrystalline Copper Film.

Fatigue properties of polycrystalline copper films subjected to pulsating stresses are investigated on the basis of observations of surface topography, fracture surface and dislocation structure. Fatigue limit corresponding to 107 cycles of a rolled copper film is 2. 3 times higher than that of an annealed one, but the fatigue limit ratio of the rolled specimen is lower than that of annealed one. From the observations of the surface crack and fracture surface, it is found that fatigue crack initiation and early-stage crack growth mechanisms of annealed bulk copper are not necessarily applicable to those of film. No striations are formed in both films. In annealed films, cell structures are formed under high-cycle fatigue whereas no band/ladder structures are formed. Dislocation structures near the fatigue crack in annealed films are similar to those of bulk specimens except for the absence of intensely elongated cells observed in the shear flow regions in the bulk specimen. No appreciable change during fatigue was found in the dislocation structures of rolled films.