This paper presents the fatigue behavior of a high-nitrogen austenitic stainless steel, type 304N2. Rotating bending fatigue tests have been performed in laboratory air and in 3%NaCl solution, and the results obtained were discussed through the comparison with those of type 304. Type 304N2 showed higher static strength and fatigue strength in both environments than those of type 304. Since the crack growth rates in both materials were almost similar, the resistance to crack initiation was the primary cause of the observed higher fatigue strength of type 304N2. In 3%NaCl solution, fatigue strength of both materials increased slightly compared with those in laboratory air due to the cooling effect of the solution. Under stress-incremental tests, failure stress did not increase significantly in type 304N2 compared with type 304, indicating a less pronounced coaxing effect in type 304N2. The difference in the coaxing effect between both materials seemed to be due to strain-induced martensite transformation that occurred significantly in type 304. No strain-induced martensite transformation took place in type 304N2 because of higher stability of austenitic phase, thus the coaxing effect in type 304N2 was attributed to both work hardening and strain aging.
The purpose of the present study is to evaluate aging and fatigue behaviour at elevated temperatures in ferritic stainless steels with different Cr contents. The materials used were ferritic stainless steels, Type 430 (16.3Cr), 444 (18.72Cr) and 447 (30.66Cr). In all the alloys, hardness was increased by aging around 500℃ due to the 475℃ embrittlement and the most remarkable increase was observed in Type 447 with the highest Cr contents. Fully reversed axial fatigue tests have been performed at ambient and elevated temperatures (480∼520℃). At both ambient and elevated temperatures, Type 430 and 444 had nearly the same fatigue strength and Type 447 showed higher fatigue strength than those alloys. When fatigue strength was characterized in terms of fatigue ratio (σ/σB), the difference of fatigue strength among three alloys became considerably small, but Type 447 still exhibited lower fatigue strength. Brittle fracture was seen at elevated temperature in Type 447, which could be responsible for the observed lower relative fatigue strengths. It was indicated by TEM analysis that the 475℃ embrittlement occurred due to the spinodal decomposition of Cr phase that the decomposition was suppressed by stress cycling.
It is shown that 2 wt% clay-reinforced nylon and 4 wt% clay-reinforced nylon have similar tensile strengths, which are increased by about 30% compared to those of nylon 6. With an increase in loading rate, the ultimate tensile strength increases for both nylon 6 and nanocomposites, and the ductility decreases for nylon 6 while it does not change obviously or even increase a little for nanocomposites. The fatigue strength of 2 wt% clay-reinforced nylon is also increased by about 30% compared to that of nylon 6, but the fatigue strength of 4 wt% clay-reinforced nylon is slightly increased or similar to that of nylon 6. The cyclic deformation has been examined using stress-strain hysteresis loops. The small cyclic deformation in 4 wt% clay-reinforced nylon is attributed to the no increase in fatigue strength. The strain incompatibility near the phase boundary leads to the relaxation of three dimensional stress field in 2 wt% nano-composite, while leads to the extraction of clay platelets in 4 wt% nano-composite. Furthermore, time-dependent behavior and microscopic damage mechanisms are discussed.
Fatigue crack propagation behavior is essential to investigate the mechanism of fatigue fracture. The replication technique has been used to observe the behavior on the surface of various materials; however, the technique does not provide the information on the shape of cracks inside materials. In the present study, the authors have attempted micro computed tomography (μCT) with synchrotron radiation of SPring-8 to observe the cracks inside the specimens. Cracks were introduced through rotating-bending fatigue loading on aircraft aluminum alloy A7050-T7451 specimens. The crack images were intermittently visualized by μCT during the propagation. The interference and coalescence of the two fatigue cracks were evaluated, which were closely located in a plane or parallel planes perpendicular to the axis of each specimen. Fatigue crack propagation rates were calculated using the pairs of crack images with different fatigue cycles. Stress intensity factors were estimated by the Paris's law with the calculated crack propagation rate, which revealed that the high points in stress intensity factor corresponded with those by other researchers for the axial loading of plate specimens.
To discuss an effect of stress ratio on high cycle fatigue properties of the extruded magnesium alloy, axial loading fatigue tests have been performed under three conditions of stress ratio, R, of 0, -1 and -1.5 in laboratory air at ambient temperature using hourglass shaped specimens of extruded AZ61 magnesium alloy. Clear fatigue limit existed on the S-N curve obtained from the testing condition of R = 0, while under the tests of R = -1 and -1.5, specimens showed a step-wise S-N curve on which two knees appear. From the detailed observation with a SEM, fracture surface was divided characteristic five regions depending on the applied stress amplitude level. Facet-like smooth area was observed at crack origin only under low stress amplitude level and long fatigue life regime of R = -1 and -1.5, and whole stress amplitude level of R = 0. Fatigue crack initiation mechanism changed from the twin-induced failure mode at high stress amplitude level to the slip-induced one at low stress amplitude level. This transition was occurred by the relation between the minimum stress during a cycle and the offset 0.2% compressive yield stress at which deformation twin occurred. The deformation twin is formed during fatigue process when the minimum stress exceeds the yield stress in compression that is smaller than that in tension. It was pointed out that the high cycle fatigue properties were strongly affected by the formation of twin due to anisotropy of the materials processed with extrusion and crystallographic nature.
The mechanism of internal casting defect-originating fracture of aluminum alloy die-casting in the ultra-high cycle regime was studied from the observation of fracture surface and cross section perpendicular to fracture surface. The main results were as follows : (1) Fine irregular fracture surface around the crack initiation point was clearly observed after 108 cycles, and the boundary between porosity and fine irregular fracture surface was not clear beyond 109 cycles, and a partial chip of spherical porosity was observed at crack initiation point. (2) From SIM observation of cross section perpendicular to fracture surface, fine structure of nanocrystalline was observed at fine irregular fracture surface and fine structure of nanocrystalline was not limited to fine irregular fracture surface. Fine structure of nanocrystalline was also observed at the vicinity of fine irregular fracture surface. (3) The boundary between fine structure of nanocrystalline and matrix structure was clear. This feature was similar to the feature of fine structure of nanocrystalline obtained from the severe plastic deformation process. (4) From the observation of fracture surface and cross section perpendicular to fracture surface, it seems to be that the contact between the fracture surface could occur at the crack initiation point and around the crack initiation point of the internal casting defect-originating fracture, and the repetition of the contact between the fracture surface seems to influence the formation of fine structure of nanocrystalline.
In order to investigate the mechanism of fatigue crack initiation in Alloy 718 at elevated temperature, rotating bending fatigue tests were carried out at room temperature and 500℃ for the alloy with different grain sizes of 18 and 88μm. Fatigue strength in the long life region decreased markedly with increase in grain size at both temperatures. Non-propagating cracks of one or two grain sizes were observed in both fine and coarse grained specimens under stress levels near a horizontal line in each S-N curve, though the S-N curve of fine grained specimens showed a duplex one at 500℃. All of fractures occurred from surface in the life range up to 108cycles except for those originated from the subsurface of fine grained specimens at 500℃ beyond∼107cycles. Cracks nucleated from not only slip bands but also annealing twin boundaries, irrespective of grain size and fracture type. The frequency for twin boundary cracking increased with increases in grain size and temperature and with decrease in stress.
In order to investigate the effect of aging condition on initiation and propagation properties of a fatigue crack, rotating bending fatigue tests were carried out using partially notched specimens of Ni-base supper alloy, Alloy 718, at 500℃. Aging conditions selected were the under-aged and the over-aged conditions with nearly the same hardness. Fatigue strength of the over-aged alloy was higher than that of the under-aged one in wide life region till 108 cycles. Moreover, fatigue strength at 500℃ was larger than that at room temperature in both alloys. Most of fatigue life was occupied by the growth life of a small crack in both alloys. The main reason for higher fatigue strength of the over-aged alloy was an increase in crack growth resistance. Many slip bands were observed on the specimen surface of the over-aged alloy in comparison with results of the under-aged one. This related to the difference in crack growth resistances in both alloys.
Slip band-like microstructural change formed under the cyclic shear stress coupled with hydrogen has a crucial importance to clarify the basic mechanism in the presence of hydrogen of Mode II fatigue failure. The authors' previous research works have indicated that the slip band-like microstructural changes are formed by the hydrogen enhanced slip deformation, and this microstructural change occurrence due to the hydrogen is detrimental when compared with a surface inclusion for the fatigue fracture life. The slip band-like microstructural change was observed by the electron backscattering diffraction (EBSD), scanning ion microscopy (SIM) and transmission electron microscopy (TEM). Numerous Mode II fatigue cracks and slip band-like microstructural changes were observed only in the hydrogen-precharged specimens. The EBSD analysis revealed that the blocks in the martensite structure in the vicinity of the slip band-like microstructural change were arranged along the slip band-like microstructural change. The SIM and TEM analyses revealed that the slip band-like microstructural change consists of band structures and that structure differed from base tempered martensite. Therefore, it is presumed that cyclic shear stresses act on the base tempered martensite and hydrogen enhanced slip deformation in the hydrogen-precharged specimens, as a result base tempered martensite structure is deformed, and eventually produces slip band-like microstructural change along the two shear stress directions.
Spalling is one of the typical rolling contact fatigue failure of railway wheels. In this study, in order to clarify the mechanism and the condition of fatigue crack initiation in wheel tread with white layer, elastic-plastic FEM analyses were conducted. The results are summarized as follows: Dang Van model, which is one of the multiaxial fatigue strength evaluation methods, was applied to evaluate the damage level of white layer. Accordingly, it is clear that fatigue crack has a tendency to originate at the leading edge of white layer particularly under slip condition. Furthermore, it is possible to identify the detailed position of fatigue crack initiation site by using accumulation damage value which was calculated by Dang Van model. According to plastic strain distribution of base metal around the white layer, the area which indicates high plastic strain value at the leading edge of white layer was larger than at the trailing edge. It is concluded that these plastic strain area of base metal at the leading edge of white layer contributes the fatigue damage around the white layer.
The effect of crack length on the retardation of fatigue crack propagation caused by single overload was investigated using carbon steels S25C and S45C. The retardation and crack arrest occurred even in short cracks ranging from 50 to several hundred microns. Retardation was caused by the increase of crack closure stress caused by overload. The occurrence of retardation, however, showed a complex dependency on crack length, yield strength of material and stress ratio of baseline stress R. At R = -1, retardation did not occur for very small crack as small as 50μm deep in the case of S25C whose yield strength is relatively low. This was because the increase of crack closure stress was prevented by the compressive stress component of baseline load. The plastically deformed layer of crack flank was collapsed by the compressive stress component of baseline load, which prevented the development of crack closure. Retardation occurred even in 50μm deep crack at R = -1 in the case of S45C which has higher yield strength. On the contrary at R = 0, retardation occurred even in small crack as small as 50μm. Prevention of the development of crack closure did not occur in the case of R = 0.
Effects of shot peening on the bending fatigue limit of spring steel (SUP9A) containing an artificial semi-circular slit were investigated. Shot peening (SP) and stress shot peening (SSP) were conducted on the specimens containing an artificial semi-circular slit with a depth of a = 0.1, 0.2 and 0.3mm. Then, bending fatigue tests were conducted on the specimens. The fatigue limit was improved by SP and SSP. In the case of SP specimens and SSP specimens, the specimen having a semi-circular slit under a = 0.2mm fractured outside the slit, and they had considerably high fatigue limits almost equal to those of the shot-peened smooth specimens. Therefore, semi-circular slit whose depth of under a = 0.2mm could be made non-damaging by SP or SSP. It was found that the fatigue limit of specimens having a semi-circular slit which received SP or SSP determined by the threshold condition for non-propagation of fatigue cracks emanated outside the slit. The criterion that the semi-circular slit is non-damaging or not is decided by relationship between the stress intensity factor range of semi-circular crack and threshold stress intensity factor range.
Fatigue crack growth characteristics were investigated for sintered and thermal-sprayed tungsten carbide with cobalt (WC-Co) cermets. Acoustic emission (AE) techniques were applied to study the crack growth mechanism. The materials used were two types of sintered WC-Co materials, commercially available as G5 and KD20 with the WC grain sizes of 1.0∼8.0μm and 0.5∼1.5μm, respectively, and 7mm thick sprayed layer with the WC grain size of 0.5∼1.5μm. The Co content (mass %) of these materials were 12∼13%. Compact type (CT) specimens were employed for fracture mechanics experiments in accordance with ASTM E647-95a. AE source wave analysis enabled us to estimate the crack volume dynamically developed near the crack tip. The fatigue crack growth rate, da/dN, depended on the WC grain size as well as the stress ratio for the sintered materials, and it was accelerated in sprayed material because of the existence of micro-pores. When these da/dN were compared with structural steels and aluminum alloy as a function of effective stress intensity factor, ΔKeff, normalized by Young's modulus, E, they were much higher than those of the structural materials. This behavior was due to the brittle inter-granular (IG) fracture of WC particle, whose contributions were estimated by AE analysis and fracture surface observations. AE source wave analysis revealed that fracture volume of the IG fracture corresponded to the WC grain size and thus the IG fracture assumed to occur at the individual WC particles. The results of maximum stress intensity factor, Kmax, constant, ΔK decreasing tests suggested that the frequency of the IG fracture of the WC particle was controlled by Kmax. In the threshold region, the contribution of the IG fracture decreased and the threshold value of ΔKeff tended to be constant for sintered materials, while it was lower for sprayed material due to the micro-pores.
Fatigue tests were conducted for circumferentially notched bars of austenitic stainless steel, JIS SUS316L, under cyclic torsion with and without static tension. For the case of cyclic torsion without static tension, the fatigue life of notched bars was found to be longer than that of smooth bars and to increase with increasing stress concentration under the same amplitude of the nominal torsional stress. This notch-strengthening effect is anomalous for the conventional fatigue design criterion. On the other hand, the fatigue life decreased with increasing stress concentration, when the static tension was superposed on cyclic torsion. To investigate the anomalous behavior, the initiation and propagation of fatigue cracks at the notch root were monitored by the direct-current electrical potential method and the compliance method. For the case of cyclic torsion without static tension, the crack propagation life increased with increasing stress concentration, while the crack initiation life decreased. The anomalous behavior of the notch effect was ascribed to the larger retardation of fatigue crack propagation by crack surface contact for sharper notches. The superposition of static tension reduced the retardation due to the smaller amount of crack surface contact, which gave rise well-known notch-weakening of the fatigue strength.