Chobin Makabe

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.

Effects of Small Surface Cracks on Ductility Loss in Low Cycle Fatigue of 70/30 Brass

The loss of fracture ductility after strain cycling in low cycle fatigue tests is not due to so-called weakening or losing of the strength quality caused by irreversible slips in crystals but rather is attributable to the existence of small surface cracks. The degree of loss of fracture ductility depends on the crack length l. If l is larger than the critical length lc, the fatigue crack causes macroscopic shear fracture in the tensile test after strain cycling. On the other hand, if l

Deceleration and Acceleration of Crack Propagation after an Overload under Negative Baseline Stress Ratio

In a previously reported study, it was found that the fatigue crack growth rate actually accelerated after a tensile overload under a negative value of baseline stress ratio R. This type of crack propagation behavior is related to the change in the compression residual stress to tensile residual stress distributed in the vicinity of the crack tip upon unloading to minimum load after an overload. In the present investigation, focused on the negative stress ratio, it was found that the deceleration and acceleration of crack propagation were associated with crack opening displacement at the overload point and the conditions of applied baseline cyclic stress. The fatigue crack growth rate decelerated after being overloaded at a positive R value. However, that rate accelerated rather than decelerated in some stress conditions at a negative R value. Therefore, the residual fatigue life after overloading became longer or shorter, depending on the overload level and baseline stress conditions.

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.

FATIGUE LIFE AND CRACK GROWTH BEHAVIOR IN ANNEALED AND NORMALIZED 0.83% CARBON STEEL

The variations of fatigue limit and fatigue life of a plain specimen of annealed and normalized 0.83% carbon steel were investigated. This material is used for cutting tools and the original microstructure includes a spherical microstructure. After heat treatment under some conditions, the microstructure changed to a lamellar microstructure. However, the fatigue lives of the plain specimens of this material showed almost the same tendency even after heat treatment under some conditions. In those cases, the initial crack length in the fatigue process is related to the size of the crystal structure and related to the distribution of ferrite. In the present study, the relationship between the distribution of hardness and the fatigue limit was investigated. Also, it was discussed that the fatigue limit of heat-treated 0.83% carbon steel could be evaluated by a relationship in which the parameters are the hardness and initial crack length. Finally, the tendencies of fatigue life of heat-treated 0.83% carbon steel were discussed based on the observations of crack growth behavior.

Improvement of Fatigue Crack Growth Behavior in the Case of the Cracked Specimen with Relatively Narrow Width

The improvement of acceleration behavior of crack growth was investigated with constant stress amplitude under negative stress ratio R=-1. Then a technical method to detect the fatigue crack growth was discussed. For example, when the stress amplitude exceeds a critical value, crack growth rate of overloaded specimen became higher than that of baseline which was obtained by crack growth test without applying overload. In some experimental cases, the acceleration of crack growth was observed and that could be happened on practical cases. Stop-holes were drilled at crack tips or in the vicinity of crack tips to remove the plastic zone and the effect of that on crack growth behavior were investigated. Also, steel pins were inserted into the stop-holes and its effect was discussed. Finite element method (FEM) was used to analyze the stress concentration at the edge of stop-holes. Positions of center of the stop-holes were varied at different distance from the crack tips to investigate the effect of stop-holes on fatigue crack growth. Also, stress intensity of base and stop-holed specimen was calculated. Then, the effect of stop-hole was discussed by both the experimental and analytical results. Specially, it was discussed whether the crack growth behavior was improved or not in the case of relatively smaller width specimen.