We developed a uniaxial tension hydraulic servo fatigue-testing machine optimized for obtaining gigacycle fatigue data. Main development goals of this machine were as follows: (1) to assure high accuracy of each cyclic loading, (2) to minimize a bending stress generated in a specimen without alignment before the test, (3) to bear high durability enough for a number of gigacycle fatigue tests, (4) to have good user-friendliness for changing testing conditions freely including stress ratio and frequency, (5) to get twice or three-fold higher response compared with a conventional fatigue testing machine. To accomplish these goals, we introduced a miniaturization of the hardware, an alignment free mechanism, fluid lubrication bearings without a sealing system, and a digital controller. As a result, this machine achieved the following performance: (a) high accuracy with loading errors less than ±0.1%, (b) good alignment with bending stress in the specimen less than 3%, (d) high response with 150Hz of ±9.8kN loading, and (e) friendly user interface for setting testing conditions.
The influence of the loading history and stress ratio on the behavior of fatigue crack propagation was investigated through the computer simulation based upon the emission and movement of dislocations from the crack-tip. The result of the simulation showed that there is a power relation between the stress intensity range in the preceding cycles and the threshold value where the crack stops propagating. It was also found that the threshold value obtained by the simulation is closely connected with the stress ratio through the Walker relation. The existence of those relations was explained from the viewpoint that the above two parameters control not only the maximum stress intensity but also the crack opening stress intensity level which is determined by the degree of the shielding effect due to the residual dislocations. This idea was supported by the simulation result that the crack propagation rate was clearly related to the effective stress intensity range which was defined as the difference between the maximum stress intensity and the above crack opening stress intensity.
Mechanism of interior inclusion induced fracture of bearing steel in long life region was experimentally examined under cantilever-type rotating bending, and discussed from microscopic observations by SEM, TEM and SIM, respectively. Fish-eye was usually formed on the fracture surface in the interior inclusion induced fracture, and an inclusion was always found at the center of the fish-eye. Furthermore a characteristic rough surface area was necessarily observed in the vicinity around the inclusion. Based on TEM observation of longitudinal section of fracture surface at the rough surface area, it was found that structural changes were caused around the inclusion. Fine grains were caused around the inclusion in the long sequence of the cyclic loading. The sizes of these grains were within 40nm. It was finally found that the fatigue crack in such a long life region took place through the boundary disadjoinings of the above fine grains. Thus the characteristic surface area around the inclusion was well interpreted as the fine granular area (FGA) produced by the above boundary separations.
To investigate the fatigue characteristics and the fracture feature of Ti-6Al-4V in very high cycle region, uniaxial tension fatigue tests were carried out. The S-N diagram showed a complex folding shape, which corresponded to the locations of the fracture origins; i. e., surface-originating fractures in high stress level and interior-originating fractures in low stress level. In this study, observed also were interference type fractures in which the main crack was formed by the coalescence of a surface-originating crack and interior-originating cracks in low stress level. Interference type fractures occurred in shorter life region than interior-originating fractures. Therefore, it is inappropriate and risky to evaluate a fatigue life in low stress level without consideration of interference type fractures. When characteristics of this type of material are investigated, it is important to make a model in consideration of distribution of crack initiation sites. A clear difference was observed on the fracture surfaces of structure sensitive stage 2a regions between the surface-originating fracture and the interior-originating fracture. The initial interior crack propagation is characterized by unclear slip marks and a generally round ductile feature with aggregation of a few micro-meter size granular substances. This tendency was not observed on the surface cracks. This seemed to result from the different surrounding environments of surface and interior cracks. Synergistic effects of a vacuum-like environment inside of material and compressive load may cause forming of granular substances.
Fatigue tests under variable amplitude loading were conducted on a high carbon chromium steel, JIS SUJ2. The aim in the present study is to clarify the damage mechanisms under variable amplitude loading for a high strength steel showing the transition of failure mode in long life regime. Tests were performed using cantilever-type rotating bending fatigue testing machines in laboratory air. Load sequences employed were two-step block loading (high→low and low→high) and two-step multiple block loading (high↔low). Fatigue damages were estimated by the Palmgren-Miner rule. When failure mode was the same at two stress levels employed, i. e. subsurface crack initiation, fatigue damages were evaluated properly by the Palmgren-Miner rule. Also when the cycle ratio at low stress level became large and multiple block loading was applied, the estimation of fatigue damages was made successfully. According to SEM analysis, it was found that the morphology of fish-eyes formed under variable amplitude loading was the same as that obtained under constant amplitude loading.
In order to study the mechanism of the subsurface crack initiation and propagation of high strength steel in very high-cycle region, computational simulation with fracture surface topographic analysis (FRASTA) method was carried out for specimens of high-carbon-chromium bearing steel, JIS SUJ2, obtained from the rotary-bending fatigue test in air. A remarkable area formed around the nonmetallic inclusion inside the fish-eye zone on the fracture surface, which is a feature on the fracture surface in superlong fatigue and named as GBF (granular-bright-facet), was observed in detail by a scanning probe microscope and a three-dimensional SEM. The GBF area, in which a rich carbide distribution was detected by EPMA, revealed a very rough and granular morphology in comparison with the area inside the fish eye. It was clearly simulated by the FRASTA method that multiple microcracks initiate dispersively by the decohesion of spherical carbide from the matrix around a nonmetallic inclusion and coalesce each other into the GBF area during fatigue process. After formation of the GBF area, interior crack grew radially and the fish-eye pattern formed on the fracture surface.
The objective of the present paper is to clarify the effect of the core hardness and the case depth of the carburized steel on low-cycle-impact fatigue strength, which is required for automotive differential gear. Low-cycle-impact bending fatigue tests for carburized steels were carried out. As a result, it was found that crack initiation life increased with increasing the core hardness and the case depth. Furthermore, change of crack initiation life can be explained by the change of strain range, which was dominated by the core hardness and the case depth, generated in the fillet of test specimen.
The objective of the present paper is to clarify the effect of slip condition, ie. slip ratio, outer surface velocity, and lubrication oil temperature on the contact fatigue strength for pitting (pitting strength) of the carburized steel. Rolling contact fatigue (RCF) tests were conducted on 26mm diameter specimens made of carburized steel under oil lubrication. Then, slip ratio, outer surface velocity, and lubrication oil temperature as the slip condition were changed on the RCF tests. As a result, pitting strength decreased with the increase of slip ratio, the decrease of outer surface velocity, and the decrease of lubrication oil temperature. Furthermore, relationships between the above-mentioned three parameters, friction coefficient, and contact surface temperature were evaluated. Then slip ratio, outer surface velocity, and lubrication oil temperature influenced to friction coefficient and contact surface temperature. The friction coefficient slightly decreased when contact surface temperature increased. Lastly, it was found that the friction coefficient was main parameter which directly determines the pitting strength.
Rotating bending fatigue tests were carried out for maraging steels nitrided under three conditions in order to investigate the influence of nitriding on fatigue strength. The fatigue strength was increased due to nitriding. The increase in fatigue strength and the fracture mechanism were markedly influenced by nitriding condition. That is, fracturing of the steel nitrided at under-aging condition, 480°C-1h and 5h, occurred from the surface of the specimen in the short life region, and from the interior in the long life region, although all of the specimen aged at under and over-aging conditions and nitrided at over-aging condition, 570°C-1h, fractured from the surface. Consequently, S-N curves in the former steel showed a two-step shape. The fatigue strength decreased with the increase in nitriding time in the short life region where the surface fracture occurred, though there is no or little influence of nitriding time on the fatigue strength in the long life region where the internal fracture occurred. The decrease in the fatigue strength was related to the surface brittleness caused by longer nitriding.
In order to investigate the effects of alumina short fiber volume fraction on the fatigue property at room and elevated temperature, the rotating bending fatigue tests were carried out on alumina short fiber reinforced aluminum alloy composites (MMCs). A6061 aluminum alloy and the three kinds of MMCs with 10%, 18% and 25% volume fraction were prepared and the five temperature conditions with room temperature, 200, 350, 400 and 450°C were performed. Fracture surface observation by FE-SEM of all specimens and surface crack observation using replication technique at room temperature and 200°C were performed. As results, the followings were found: The fatigue strength at 107 cycles increased with increase in alumina short fiber volume fraction at all test temperature conditions. Fatigue crack origins of all MMCs were large size alumina short fibers, some kinds of cluster of short fibers and large size alumina particles independently of the alumina short fiber volume fraction. The short fatigue crack of all MMCs propagated with avoiding the short fiber. But the da/dN-ΔK relation of all MMCs agreed with the A6061 one's. The effect on fatigue strength of short fiber compounding was more remarkable in the higher test temperature region.
It has been recognized that the threshold stress intensity factor range ΔKth of short crack is dependent on many factors. Fatigue tests were performed on materials containing small defects under a wide range of mean stress for three grades of steels with different hardness. Strong dependencies of ΔKth on material hardness, mean stress and defect size were observed. Major tendencies such that softer material had lower ΔKth and smaller defect had lower ΔKth were obtained, which were quite different from those of long cracks. The causes of these effects were investigated based on the crack closure behavior of short crack. The crack closure measurement on a very shallow crack was done. Test results showed that softer material and shorter crack had lower crack closure stresses. It was shown that the threshold effective stress intensity factor range (ΔKeff)th was a unique value for defects deeper than 0.1mm. The crack closure was the major cause of the abovementioned effects of material hardness and mean stress for defects deeper than 0.1mm. On the contrary, (ΔKeff)th significantly decreased compared with that for long cracks when the defect was shallower than 0.1mm. This is another short crack effect which could not be explained only by the crack closure. It was shown that (ΔKeff)th was no more a unique value when the defect depth was shallower than 0.1mm.
Coiled tubings, which are used in drilling and washing up oil wells, receive various bending and straightening plastic deformations with operating internal pressure. Therefore the low cycle fatigue of coiled tubings is an important issue. In this report, as objective to clarify fatigue properties of coiled tubings and to develop a fatigue life prediction method of coiled tubings, we carried out full scale fatigue tests, fatigue tests with small size specimen, and FEM analysis simulated the full scale fatigue tests. As a result, it was clarified that fatigue lives of high strength coiled tubings (T95) were longer than those of low strength coiled tubings (L80) with high internal pressure, and predicted fatigue lives were satisfied general allowable difference, factor of two.
This paper deals with the problem of dynamic stability of cross-ply laminated cylindrical shells under impact hydrostatic pressure is described. First of all, the motion of cylindrical shells under impact hydrostatic pressure is defined as axially symmetric motion. Following this definition, certain perturbations are superimposed on this motion and their effect on the behavior of the shell is investigated. The symmetric state of motion of the shell is called stable if the perturbations remained bounded. The solutions for the prebuckling motion and the perturbed motion are obtained using the Galerkin's method. Stable regions are determined by utilizing Mathieu's equation. The inevitability of dynamically unstable behavior is proved analytically and the effects of various factors, such as hydrostatic pressure ratio, number of layers, stacking sequences and dynamic unstable mode, are clarified.
Dental implant placement is one of the modern treatments for the patients who lost their original tooth. A numerous treatment using titanium implants has been increased in the last decade. With the increase of the implant treatments, the implant failure also increases. Most of the implant failure cases result from the physical rejection or the infections, but a few cases of the mechanical fracture in the implant system are also reported. Most of the mechanical failures in the implant system are caused by the cyclic fatigue damage, but the analyses based on the mechanical viewpoints have not been reported. In the present work, the fractography analyses were applied for the two cases of the dental implant failure. The titanium implants in the maxillofacial bone were removed from the patients, as the implant fracture was confirmed in the radiographs. The striations were clearly observed on both fracture surfaces of the removed dental implants. The cyclic stress range subjected to the dental implant was also estimated from the tooth root angles and the mastication force. The factors that induced the fatigue fracture of the titanium implant in maxillofacial bone were discussed from the viewpoints of the mechanics and the oral surgery.
A fatigue life of die-casting dies depends on heat checks, which are fine shallow cracks on a surface due to cyclic thermal stress. According to the recent study, the laser-processing is effective to repair cracks and extend the life of dies. Therefore, the purpose of this study is to evaluate the fatigue strength of laser-processed hot work tool steel to validate the reliability of repaired dies. In this study, the rotating bending fatigue test for the laser-processed hot work tool steel specimens has been carried out. Because test results have scatter, S-N curves are decided based on the evaluation of fatigue strength distribution. As a result, the fatigue strength of the laser-processed specimens decrease remarkably in compared with that of the non-processed ones. The notch sensitivity might have risen because the structure became brittle due to rapid cooling. However, it can be recovered by heat-treatment at 773K. This could be due to a high yield strength resulted in a secondary-hardening effect. The fractgraphic results also demonstrate the same tendency. The fracture surface of heat-treated specimen is similar to that of base metal, while that of laser-processed one shows different appearance. The laser-processing method can be carried out at relatively low cost and effective for the repair of heat check. Therefore, we can conclude that this method is available for the extension of the life span of dies.
In a previous report, we proposed strengthening and toughening mechanisms in nanocomposites based on thermally induced residual stresses around nano-particles within matrix grains. These residual stresses generate dislocations at elevated temperatures and the dislocations release tensile residual stresses along grain boundaries in the matrix. Conversely, the dislocations in alumina are immovable at room temperature, act as nano-crack nuclei when a main crack tip approaches and expand the size of the frontal process zone. In this paper, alumina matrix/silicon carbide dispersed nanocomposites were fabricated to clarify the strengthening mechanism in nanocomposites by means of hot-press processing. To compare the effects of the annealing process on the mechanical properties of sintered specimens, heat treatment was carried out at 1300°C (1573K) in Ar gas atmosphere after sintering at 1600°C (1873K) for 1h. Bulk densities of the specimens were more than 98% of relative density, but some specimens had relative densities lower than 95%. The mean values of fracture strength and fracture toughness were 862MPa for the 5vol% SiC and 3.90MPa·m1/2 for the 3vol%SiC, respectively. Even though the fracture strength of the nanocomposites was 2 to 3 times higher than that of the monolithic alumina, the fracture toughness was almost same as the monolithic alumina.
Phenolic resin has originally advantages of heat proof, fire retardant and less smoke during burning and these advantages are suitable properties in the construction field. In this paper, we present a development of pultrusion technique of phenolic foam composite materials composed of phenolic resin foam as matrix and roving glass fibers as a reinforcement in order to apply this composite to materials in the field of construction. A pultrusion technique can produce composites having the same cross section and optional length, and it is very useful because the volume and arrangement of fibers in the cross section of composite can be almost kept constant. In the case of molding phenolic foam composites, it is important to control the foaming time, and the foaming time can be controlled by a certain environmental temperature under cooling and heating. Furthermore it is also important to feed constant resin, and we develop the original feeder machine. Next, this paper also shows mechanical strengths of the phenolic foam composite comparing with those of the candidate materials in the construction. As a result, the phenolic foam composite shows a possibility of application in the field of construction like natural woods. Additionally, measurement results of the thermal conductivity and flammability of phenolic foam composite are also shown.
For measuring the residual stress on ceramics, X-ray method is frequently used, but Vickers indentation method (ID method) is also as it can be applied easily. Recently, this method was adopted as a standard test method by the Society of Materials Science, Japan (JSMS-SD-4-01), but it is not recommended for materials that induced transformation occurs during processing. ID method can be applied on Al2O3 and Si3N ceramics, and the authors reported the precise test conditions for these materials. In this study, ID method was applied for estimating the residual stress on ZrO2 ceramics, which shows phase transforma-tion by stress. SiC was tested for reference. The residual stress value obtained by ID method on ZrO2 was close to the value by X-ray method, when the indentation load was 294N. For estimating the residual stress on ZrO2 by ID method, the influ-ence of the phase transformation by the indentation shall be added onto the original residual stress, when the indentation small. The influence became smaller when the indentation load was large. It was found that ID method is effective for esti-mating the residual stress on ZrO2 ceramics, if the applied indentation load is between 294N and 490N.