The purpose of this study is to evaluate the effect of mean stress and casting defect on fatigue strength of Ni based castalloys, 246 and 100, using ultrasonic fatigue tests, which were conducted under stress ratios, R = -1, 0 and 0.3, at a cyclic frequency of 20 kHz. The results revealed that fatigue strength decreased by subjecting mean stress. SEM observation for the 246 and 100 alloys near the fracture origin exhibited that cracks initiated from a casting defect near the specimen surface for the majority of the specimens, where a flat fracture surface was formed in the vicinity of the defects. Also conducted were ultrasonic fatigue tests for shot-peened specimens under R = 0 in order to change the location of the fracture origin. The results indicated that the influence of the casting defect near the specimen surface became negligible in the very high cycle fatigue regime. Stress intensity factor range for the casting defect, ΔKCD, which was calculated by the axial projected area of the defect and the stress range, correlated with the fatigue life, Nf. The influence of mean stress and the casting defect was evaluated by the relationship between Nf and the equivalent stress intensity factor range, ΔKeq, calculated by taking account of mean stress. The result suggested that the effect of casting defect could be expressed uniformly by the evaluation of ΔKeq-Nf relationship.
The purpose of this study is to evaluate fatigue crack growth (FCG) characteristics and its threshold of Ni based castalloys, 246 and 100. Results of FCG tests by 4-points bending show that the FCG path is highly deflected when the FCG rate is low, while it is macroscopically straight when FCG rate is high. The former and the latter behaviors are denoted mixed mode and mode I crack growths, respectively. The effective stress intensity factor range, ΔKeff, in mode I obtained by displacement field around a crack tip measured by digital image correlation (DIC) is dominant on FCG rate even under the mixed mode crack growth. To determine the threshold value and to evaluate FCG rate under extremely slow region, maximum stress intensity factor, Kmax, -constant test using ultrasonic fatigue testing machine is performed. As the result, fracture surface roughness and FCG rates are correlated and the role of crystal orientation on the fracture surface formation is clarified. The threshold condition for FCG is constant independently of the crystal orientation near crack tips and threshold stress intensity factor range, ΔKth, is about 1 MPa√m.
Quantitative mechanical analyses by nano indentation were performed for two types of ferritic heat-resisting steel that contained 12mass% chromium and 2mass% tungsten (12Cr-2W). One of the 12Cr-2W with a superior creep-fatigue property showed transgranular fractures, whereas the other steel with an inferior creep-fatigue property did intergranular fractures. In the inferior steel, coarse subgrains in the coarse blocks neighboring the prior austenite grain boundaries were formed during the creep-fatigue testing. Nano-scale hardness of the coarse subgrains or blocks neighboring the grain boundaries were markedly lower than those of the fine blocks far from the grain boundaries after the creep-fatigue test. Moreover, a pop-in behavior (in the relationship between force and penetration depth) occurred at only the coarse blocks neighboring the prior austenite grain boundaries. The pop-in behavior indicates that the dislocation density in the coarse blocks should be extremely lower by the recovery or rearrangement of dislocation by the creep-fatigue process at high-temperature. Therefore local deformations were assisted in the coarse blocks neighboring to grain boundaries and introduce the intergranular fractures.
In order to examine the effects of cleanliness and induction hardening on the very high cycle fatigue properties of low alloy forged steel (40CrMo8) used for crankshafts of Diesel engines for marine use, rotating bending fatigue tests were performed for specimens with different cleanliness (super clean steel or conventional steel) and thickness of hardened layer (0.2 mm or 0.4 mm). The super clean steel showed higher fatigue limit than the conventional steel because the fatigue fracture at non-metallic inclusion was suppressed; however, the induction hardened super clean steel failed in the subsurface fracture mode from the singular microstructure. The induction hardened steel showed lower fatigue limit than the untreated steel. This result was attributed to the fact that the tensile residual stress was generated beneath the surface hardened layer at the fracture origin. Therefore, fatigue limit of the induction hardened steel was determined by the fatigue strength of the matrix at the fracture origin.
Fatigue strength and crack propagation behavior of fine particle peened A7075 aluminum alloy was investigated. Residual stress due to fine particle peening treatment was about -350 MPa at the surface and gradually declined inside and disappears at about 80 μm from the surface. The fatigue strength at 107 cycles of the fine particle peened specimen was 1.3 times larger than that of the unpeened specimen. The crack propagation behavior was also investigated under three kinds of stress ratios. The displacement distribution around the crack tip was measured in detail by the digital image correlation method to determine the effective stress intensity factor range and crack tip opening stress. Fine particle peeing contributes to suppressing the crack opening especially when the crack length is small. It was also clarified that the retardation effect of crack propagation by the fine particle peening treatment is remarkable when the stress ratio is small. If the crack starts to open, the crack opening ratio is almost the same as the unpeened condition and this means that the effective stress intensity factor range can be calculated from the crack opening displacement. When the crack propagation rates are arranged by the effective stress intensity factor range, all data collapse into a single narrow scatter band. Therefore, the effective stress intensity factor range dominates the surface crack propagation rate, even when the material is subjected to fine particle peening treatment.
A new surface modification technique, scanning cyclic press (SCP), was developed. SCP scans a metal surface with a vibrating indenter under precise loading control based on servo fatigue testing machine and can apply a variable cyclically compressive load. This study applied SCP to magnesium alloy AZ31 to investigate the effect on fatigue properties. After applying SCP, the surfaces of specimens were observed by using a laser scanning microscope and uniaxial push-pull fatigue tests were conducted. As a result, surface roughness of SCP-treated specimens slightly increased; however, the fatigue life became longer than that of untreated specimens. To clarify the reason for the improvement effect, fracture surfaces of the specimens were investigated in detail on the basis of SEM observation. SEM observation showed differences between the fracture surfaces of the untreated and SCP-treated specimens. The origin of fracture was at the surface in the untreated specimen. In the SCP-treated specimen, however, the fracture origins were sub-surface and a band-like layer just beneath the surface was observed on fracture surface, whereas the layer did not exist on that of untreated specimen. The surface hardness of SCP-treated specimen increased to almost twice as much after applying SCP. The result suggests that SCP modified surface layer of specimen and this layer suppressed crack initiation from surface.
Titanium alloys such as Ti-6Al-4V are used worldwide in the field of aerospace, and the applied results are abundant, and the data are accumulated as structural members. On the other hand, commercial pure (CP) titanium which is cheaper than the alloys is widely used in the field of general industry in Japan because it is superior to seawater corrosion resistance and biocompatibility. However, CP titanium has anisotropy to consist of only hcp crystal structure, therefore, the strength data are insufficient because of the short history as structural materials, and an unidentified part of the mechanical materials properties is remaining. In this study, the effect of mean stress and stress concentration on fatigue strength of CP titanium was evaluated for the application range expansion of CP titanium which has a superior characteristic as structural members in the field of marine. As the result, the following conclusions were provided. Endurance limit of longitudinal (L) direction was 80-84% of endurance limit of transverse (T) direction on smooth specimens, on the other hand, no significant difference was found in endurance limit of both directions on notched specimens. It is necessary to apply at least Sa-0.5Su line to design the safe side in CP titanium. Around Kt=2, the notch sensitivity factor of CP titanium in T-direction is larger than that of carbon steel and it in L-direction is smaller than that of carbon steel. On the other hand, in Kt=3.77, it in L-direction is at the same level as that of S15C or S35C and it in T-direction is at the same level as that of S45C. In the design of the welding structure composed by CP Grade2 titanium which is the nearest to mechanical strength of ship’s classifications mild steel, it seems that the design fatigue strength of the steel welding structure is applicable.
Rolled pure titanium films were fatigued. The film was adhered to a through elliptical hole in a base plate and was fatigued in accordance with the displacement constraint along the hole-circumference in the base plate subjected to a cyclic stress. In this testing method, a stress intensity factor range was decreased with a crack propagation toward the hole-edge because of the difference in thickness between the film and the base plate. Consequently, the fatigue crack propagation rate was decreased and was arrested near the hole-edge under the constant stress amplitude to the base plate. As a result, fatigue cracks propagated faster in the specimen loaded to the rolling direction than in that loaded to the transverse direction and the threshold stress intensity factor ranges, ΔKth, are lower in the specimen loaded to the rolling direction than in that loaded to the transverse direction. Crack opening displacements were measured belong the fatigue crack and the curvature of crack tip was estimated. The fluctuation in the crack propagation rate was related to the curvature and relations between the crack propagation rate and the estimated crack tip opening displacement based on the Dugdale model were almost the same between the specimens loaded toward the rolling direction and the transverse direction.
Observations of internal small fatigue crack growth in Ti-6Al-4V were conducted using synchrotron radiation μCT imaging at SPring-8 to obtain the internal crack growth rate. The just-initiated internal cracks and their growth processes were successively observed. The internal small cracks propagated very slowly with the growth rate of less than 10-10 m/cycle. The crack growth rate had a lot variability in the smaller crack length regime. In order to discuss the reason for the small growth rate of internal crack in terms of the effect of environment inside the crack, the internal crack growth rate was compared with the surface crack growth rate in air and ultra-high vacuum. As a result, the internal crack growth rate was lower than the surface crack growth rate in air, and matched with that in ultra-high vacuum in the longer crack length regime. However, the growth rate of internal crack was smaller than that of surface crack in ultra-high vacuum in the relatively smaller crack length regime. These results indicate the possibility that the differences in vacuum pressure have a dominant effect on the small crack growth length, and that the gas contained in the alloy plays an important role in the internal small crack growth.
Pure titanium films with rolling texture were fatigued. In order to discuss correlation between fatigue crack propagation and active slip systems, crystal orientation around the fatigue crack was measured. The crystallographic information was analyzed using the Electron Back-scatter Diffraction (EBSD) system and a misorientation vector was estimated as the axis of misorientation angle. As a result, fatigue cracks propagated faster in the specimen loaded to the rolling direction than in that loaded to the transverse direction. Using the misorienation vector, the active slip system with crack propagation was estimated. It is found that the fatigue crack propagated by two prismatic slip systems in the specimen loaded to the rolling direction. On the other hand, the fatigue crack seems to propagate by one basal slip system in the specimen loaded to the transverse direction. The basal slip hardly operates because the slip is limited in one plane and the fatigue crack propagation is decelerated in the specimen loaded to the transverse direction.
Generally, JIS type gutter is used in road construction. However, in the case of backfill, there's a possibility that compaction is insufficient when it is using. Therefore a rectangular section gutter was developed by the authors to improve constrains of construction caused by JIS type gutter. However, the range of strength of its abutment varies in relation to the load position as the shape of abutment is corbel and the shape changes depending upon the size of each product. In this study, first, the strength and deformation behavior up to peak load of a test specimen was measured during experiments. Furthermore, 2-dimensional RBSM (Rigid Bodies Spring Model) analysis was carried out to simulate experiments and very similar results are obtained. After that, the strength of abutment (Size 250) of an actual size product was estimated by using 2-dimensional RBSM analysis.
It was reported that the lack of the structural stability of semiconductor silicon micro-pattern induced the lateral undulation buckling. Our previous report revealed that the intrinsic stress of the oxide film on the surface of amorphous silicon produced the compressive stress which induces the buckling failure. However, actual amorphous silicon contains hydrogen atoms. Therefore, in this study, we realize the surface oxide film fabrication on hydrogenated amorphous silicon and clarify the relationship between hydrogen concentration and intrinsic stress due to surface oxidation. As a result, regardless the hydrogen concentration, surface oxide layer contains no hydrogen atoms. In addition, it is found that the intrinsic stress is generated in the sub-oxide layer where oxidation process is not completed. As the hydrogen concentration increases, the integral value of the compressive stress decreases linearly. The stress decreases about 30 % when the hydrogen concentration reaches 25 at%. Decrease in the stress would be caused by the sparse silicon structure due to hydrogen atoms and resulting release of the strain due to surface oxidation.
The structural components used in power generating plants sometimes fail due to the corrosion fatigue. The corrosion fatigue cracks often initiate from the pits. In order to prevent the corrosion fatigue failure, 12Cr stainless steel is employed for the components used in the corrosive environments. Even if employing 12Cr stainless steel, the components failed by the fatigue and the corrosion pits were observed at the origins of fatigue cracks. In this study, the effect of surface finishing on the nucleation of corrosion pits was investigated using 12Cr stainless steel. The corrosive environment is pure water with chlorine ion concentration. The initiation and growth of the corrosion pits are remarkably affected by the chlorine ion concentration. So-called “One-third rule” was obtained and the aspect ratio, that means the ratio of pit depth to length on the surface, was estimated as about 1/4. The surface finishing affected the nucleation and growth of corrosion pits. Especially they are reduced when the surface was finished to the mirror-like surface by the emery paper polishing. The fatigue tests were conducted in air at the ambient temperature and in de-oxygenated water at 463K. The fatigue strength decreased with the increasing the corrosion pits. The reduction of fatigue strength can be explained by the short crack theory. On the other hand, all the fatigue cracks initiated from the corrosion pits in the high temperature water. The surface observation of test specimens suggested that the corrosion pits were mainly nucleated from the manganese sulfide. This indicates that the corrosion fatigue strength could be improved by controlling the chemical compositions and the impurity atoms.