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Recently in various fields, numerous researches are going on for the assessment of material damage on the basis of crack initiation and propagation. Various methods are available in NDT for this purpose, among which analysis using released acoustic emission (AE) waves due to crack propagation is very effective due to its dynamic monitoring features. Various approaches are proposed for long time to make it an ideal method for accurate monitoring of crack behaviors in materials. In fragmentation theory there are some proportionality among the relations of AE event, AE energy, area and volume of cracks etc., which are calculated from the released AE waves from any dynamic crack. It has been found that the necessity of calculating the fractal dimension is important in verifying these relationships. This parameter is emphasized for determining the geometry of the irregularity in crack surface and crack volume. In this paper a novel approach based on image processing is proposed to find out the fractal dimension for analyzing the crack propagation characteristics. Finally, the proportionality relationships of AE parameters with crack propagation behavior in ferrite cast iron under fatigue loading are demonstrated experimentally.

Mechanical properties, especially fatigue strength, of ferritic spheroidal graphite cast iron might depend not only on the graphite size but also on the ferrite grain size, little systematic research has been made on these factors. To clarify the influences of these structural factors as well as loading condition, fatigue tests have been carried out on ferritic spheroidal graphite cast iron with different sizes of graphite nodules and ferrite grains, under the axial loading with mean stress from -70MPa to 240MPa. The results obtained in this study are summarized as follows: (1) The fatigue limits are decreased with increasing graphite nodule and matrix grain sizes. (2) The fatigue limit decreases with increasing tensile mean stress. The fatigue limit at an arbitrary mean stress has been estimated by the modified Goodman diagram as well. (3) The parameter α is evaluated from the experimental results relating to the stress ratio R, for the mean stress acting on the tensile side and on the compressive side separately. For the case of R0, the value of α is 0.324. The result has been applied to the fatigue limit evaluation equation of Y.Murakami et al, the fatigue limit ratio falls within the range of about ±10%.

A method of improving the fatigue life and crack growth behavior of a center holed specimen was investigated. Local plastic deformation was applied around the hole by indentation to achieve the purpose. A series of fatigue tests was conducted on aluminum-alloy 2024-T3. Push-pull tests were performed under a stress ratio of R= -1 and a frequency of 10Hz. The observations of the crack initiation and growth were performed with a microscope, and hardness around the hole was measured by Vickers hardness testing machine. In the present study, the longest fatigue life was observed in the case of an indentation specimen with the highest load. The indentation was performed on both sides of the hole edges. The crack growth rate was decreased by indentation or expansion of the material around the hole. From the experimental results, it is found that the fatigue life and crack growth behavior of a holed or notched specimen can be improved by a simple technical method that is related to the local plastic working.

The present investigation attempts to evaluate the improvement of working by bolt on the fatigue limit of structural steel sheets with drilling a circular hole. The material used in this study is structural steel (JIS SM400A). And the specimens are identified to two types of the non-worked specimen and the worked specimen. The results obtained in this study can be summarized as follows: (1) The fatigue limit of worked specimens increases than that of non-worked specimen. Futhermore, there has a suitable value of the torque for improving the fatigue limits. (2) The reasons of enhancing the fatigue strength of the worked specimens are due to the elastic deformation, the plastic deformation and the changing of stress concentration part. Our investigations confirm the advantages of this working method which repaired the structural steel sheets by bolt.

This study is focused on the fatigue properties of automobile high-strength bolts, including the effect of mean stress level, pre-processing schedule and the residual stresses. And the mean stress levels are 0.3, 0.5 and 0.7 times to the tensile strength (σ B ) of the material respectively. The main results obtained are as follows: 1) the fatigue strength increases under the mean stress loading, but the differences between the loading levels are not so evident; 2) most of the cases in this study are broken from the bottom of the screw thread, and the crack initiated from the impurities.

Diamond-like carbon (DLC) is an amorphous hard carbon, which has very high hardness, high resistivity, and dielectric optical properties. Economically and technologically attractive properties have drawn almost unparalleled interest towards the coatings. Eutectoid steel is a kind of material that has been widely used in shafts and various kinds of industrial components. Three kinds of fatigue specimens with different DLC conditions were used in this study. Fatigue test had been performed to investigate the effects of DLC on fatigue properties of eutectoid steel. The fractography was analyzed by a scanning electron microscope (SEM), and surface hardness was also evaluated. The fatigue limits of the DLC coated specimens did notincrease after DLC process, though the compressive residual stress which produced by DLC process can prevent fatigue fracture. According to the results of fatigue test, the optimal DLC method for improving the fatigue properties of eutectoid steel is determined and the relationship between fatigue limits and coating bias are obtained.

Nearly 90% of failures of machines and mechanical parts are caused at the area of stress concentrated in the structural components. Hence, it is important to investigate the method of improving fatigue strength for notched parts. In this paper, the fatigue tests have been performed to investigate the effect of the plastic-working on fatigue strength of specimens with notch. The main results obtained in this study are as follows: (1) The fatigue limit of notched specimen chamfered by the plastic-working increases by 45% than that of specimen without chamfer. (2) The reason of enhancing the fatigue strength may be to attributed to the work-hardening and compressive residual stress which suppress the fatigue crack initiation and propagation in early stage.

In this study, the effect of plastic work on the fatigue strength of a structural steel was investigated in detail. Fatigue tests were performed using specimens made of a conventional structural steel, JIS S25C. One set of tests employed monotonically pre-strained specimens with pre-strain ranging from 2% until 8% in tension. In this case, the fatigue limit compared to that for non-pre-strained samples, decreases under small pre-strain and then gradually increases with pre-strain. The increase in the fatigue limit is only 7% relative to that for non-pre-strained specimens for a pre-strain of 8%. The other set of tests employed roller worked specimens with the roller deformation value varying from 0.25 to 1.50 mm in the diameter direction. In this case, the fatigue limit remarkably increases to about 230% of the original value with increased roller working with the effect gradually saturating and then deteriorated a little, so that there exists an optimum roller working value for improving the fatigue limit. This remarkable increase in fatigue limit is caused by the existence of compressive residual stress, elongated microstructure and work hardening.

This study is developed to investigate the effect of monotonic plastic deformation on the torsional fatigue properties of a structural steel. Five different kinds of tensile pre-strain, 2%, 5%, 8%, 12% and 22%, were applied to the specimens, respectively. And the maximum pre-strain value is near to the necking strain of the test material. The effects of tensile pre-strain on surface hardness, fatigue crack initiation and propagation behaviors, and the behavior of non-propagating cracks. The main results obtained are: The fatigue limits are 145, 160,175, 200 and 215MPa for specimens with tensile pre-strain of 2% 5%, 8%, 12% and 22%, which are improved to 104%, 114%, 125%, 143% and 153% of the fatigue limit for non-pre-strained specimens, respectively. The torsional fatigue limit increases with the tensile pre-strain increasing, until the pre-strain value being near to the necking strain ratio. However, the fatigue limit increase becomes more slowly for high tensile pre-strained specimens than the lower ones. The fatigue cracks of the tensile pre-strained specimens initiated earlier than that of the non-pre-strained specimens, and the propagation is also accelerated, but there is no effect on the fatigue crack initiation point and the branch point. Non-propagating crack length becomes shorter with increasing of tensile pre-strain until the value near the necking strain, and the quantity of non-propagating crack increases at the same time.

In this research, the authors have tried to decrease the weight of circular-digging large size bucket as far as possible with keeping the reliability of whole of the construction machine. In the case of main bucket, the authors have evaluated the strength reliability of this bucket and proposed more optimum design changing the thickness and kind of the material, though the expected weight reduction ratio becomes only 5% with increasing the rigidity of the main bucket. In the case of sub-bucket, it is desirable to change the thickness of the material and to attach the reinforcement in the both side. Therefore, the designer could reduce the weight by 11% with keeping the rigidity of the sub-bucket.

The effect of stress concentration factor on the fatigue properties of typical austenitic stainless steel SUS304 have been investigated using the circumferentially notched specimens. The notch of specimens has six kinds of radii, i.e. ρ = ∞ (i.e. plain specimen), 2.0, 1.0, 0.6, 0.3, and 0.1 mm with constant notch depth (t=0.2mm). Though the fatigue cracks in the specimens with a blunt notch initiate at one point, those in the specimens with a sharp notch initiate at several points. There exist the slip bands in the surface of the specimen under the stress amplitude of fatigue limit by 1×107 cycles, and do not exist the non-propagating micro-cracks in all kinds of the specimens. Furthermore, it has been found that notch sensitivity of austenitic stainless steels is higher than that of a typical plain carbon steels under the higher stress concentration factor region.

In order to study the effect of plastic working on fatigue strength of notched specimen, pulsating fatigue tests had been performed on notched deformed stainless steel specimens including on notched non-deformed specimens in order to evaluate the influence of mean stress on fatigue strength. The test results showed that the fatigue limits of plastic worked specimens are higher than that of non-worked one. This difference value would be caused by residual stress, work hardening and fiber texture due to plastic working. When degree of plastic deformation equal zero ("t=0), the fatigue limit ratio (σw /σB) of SUS430 is the higher than that of SUS304. On the other hand, the fatigue limit of worked specimen for SUS304 increases as the plastic deformation value increases to 0.5 mm and then it does not significantly increase from 0.5 mm to 1 mm. It is necessary to investigate an optimal deformation value.

Structural steel is widely used in engineering even in nowadays and many new materials have been developed or are under developing. As one of the most common type, torsional fatigue properties are normally calculated from the rotating bending fatigue properties. In this study, the torsional fatigue properties of a structural carbon steel (S45C) with different tensile pre-strain ratio is investigated, especially on the effects of Mode II crack before it branches under different stress amplitudes. Based on the experimental results and microstructure observation, the main results obtained are: (1)Torsional fatigue strength is increased after tensile pre-strain deformation, and the fatigue limits increase with increasing of tensile pre-strain ratio, the highest improvement can be up to 125% of that of the plain specimen. (2) Torsioanl fatigue cracks initiate from the torsional slip lines in ferrite grains, and there is no obvious effect by the tensile pre-strain. With increasing of tensile pre-strain ratio, the fatigue crack initiation life ratio becomes later and the growth rate becomes faster. (3) The mode II crack length along the axial direction becomes longer with increasing of tensile pre-strain ratio, and the crack branch direction does not affected by the tensile pre-strain. Moreover, the length along axial direction is not affected by the stress amplitude change for specimens with the same tensile pre-strain ratio. (4) The length of non-propagating crack becomes shorter with increasing of tensile pre-strain ratio.

In this study, the authors have investigated and compared the effects of roller working and ion nitriding on fatigue properties of eutectoid steel. Five kinds of roller worked and two kinds of ion nitrided specimens were used in this test. The fatigue test was performed using a rotating bending fatigue testing machine to evaluate the fatigue strength of roller worked and ion nitrided eutectoid steel. The fatigue test result shows that roller working is more effective on improving the fatigue strength of the material than ion nitriding. On the other hand, the ion nitriding can much greatly increase the surface hardness than the roller working. In the case of roller working, the fatigue properties are improved by the three main factors which are compressive residual stress, work hardening and fiberized micro-structure.

Three kinds of materials, pure aluminum (Al1050), carbon steel (S25C) and eutectoid steel (NHH) with V-notch are used in this study to compare the effect of roller working on the fatigue properties among the test materials. As the result, all of the fatigue strengths are improved after roller working and the optimum deformation values exist for every test materials. Fiberized microstructure at the notch bottom, work hardening and compressive residual stress caused by roller working are considered as the main reasons for the fatigue strength improvement. The highest fatigue strength improvement of Al1050, S25C and NHH was 30%, 118% and 155%, respectively. The optimum fatigue improvement of NHH is the highest and it is much higher than that of Al1050, and the optimum deformation value of Al1050 is much lower than that of the other materials. Moreover, the roller working effect is much remarkable for the materials with better hardenability.

This paper is aimed to investigate the effect of roller working on the torsional fatigue properties of a typical low carbon steel referring by the resutls of FEM analysis. Three types of specimens had been prepared with plastic deformation value of 0, 0.5 and 1.0mm, respectively. The main results obtained in this study are as follows: (1) The torsional fatigue strength of roller-worked specimen with 1.0mm plastic deformation is increased more than twice of that of non-rollerworked specimens. (2) Surface hardness of roller worked specimen is much higher than that of the non-roller worked one, and compressed and elongated structure is formed at the notch bottom of the specimen. (3) Crack length along the axial direction of roller worked specimens is longer than that of the non-roller worked specimen, and plural cracks initiated and propagated in the roller worked specimens. (4) The residual stress distribution at the specimen’s notch bottom was analysed using FEM analysis and it is verified the same tendency as the experimental results.

The authors have studied and clarified that ion nitriding was able to improve the fatigue properties of tool steel. Five kinds of ion nitriding methods (ion nitriding condition is different) were used in this study. The fatigue test had been performed using a rotating bending fatigue testing machine to investigate the effects of ion nitriding on fatigue properties of tool steel. The fractography was analyzed using a scanning electron microscope (SEM), and hardness distribution was also investigated using a microhardness tester. As a result, the fatigue strength and hardness of the ion nitrided specimen increased after ion nitriding processing. It is considered that the compressive residual stress which produced by ion nitriding processing in the layer reduced fatigue fracture, and the altered surface composition improved surface hardness. According to the results of the fatigue test, the optimal ion nitriding method on improving the fatigue limit of tool steel was determined. The hardness of the specimens remarkably increased after ion nitriding processing.

The object in this study is to investigate the pulsating fatigue properties of ferritic and martenstic stainless steels using replica method. The main results obtained in this test are as follows; two kinds of ferrritic stainless steels show the same fatigue strength, even if there is difference in micro-structures due to charge of heat treatment condition. In addition, the fatigue limit of martenstic stainless steel shows higher than that of ferritic one by 230%. It is considered that the static mechanical properties relate to the mean value of micro-structures and the fatigue strength relates to the weakest portion of structures. In addition, the ratio between fatigue limit and hardness number becomes considerably smaller than that of conventional structural steels.

In order to study the effect of plastic deformation on fatigue behaviors of plastically deformed specimen, bending fatigue tests had been performed on notched deformed stainless steel specimens. Also pulsating fatigue tests were done on notched non-deformed specimens to evaluate the influence of mean stress on fatigue behavior of notched non-deformed specimens. The result showed that according to increase of deformation value, the fatigue limits of these specimens also significantly increase. Fatigue limit of rolled specimen does not linearly increase with increase in plastic deformation value. Based on fatigue limit diagram, the effect of compressive residual stress on fatigue limit improvement of stainless steel is higher than that of work-hardening. In case of non-deformed specimen, when the compressive mean stress increases, the fatigue limit and the number of cycles to failure increase. In case of tensile mean stress, this kind of mean stress decreases the fatigue limit.

Nearly 90% of failures of machines and mechanical parts are caused at the area of stress concentrated in the structural components [1]. Hence, it is important to investigate the method of improving fatigue strength for notched parts, and method of evaluation the fatigue strength of notched parts for mechanical engineering. In this paper, the tests were carried out for improving the fatigue strength of specimens, with the hole notches that have been chamfered by die-pressing. The fatigue strength of notched part is enhanced to around 100% by die-pressing, comparing with notched specimen without die-pressing. Based on the above results, the new method for evaluation the fatigue strength with both the effects of work hardening and residual stress caused by the cold working. The estimated results by the method revealed good agreement with the tested results.

The present study focused to the effect of ion nitriding on fatigue properties of eutectoid steel. Two different temperatures (550°C and 500°C) and two different types of specimens (smooth and notched ones) were used in this study to investigate the mechanism of modification. Residual stress and EPMA analysis were used to derive the quantity relationship among case depth, residual stress and fatigue strength. The results confirmed the efficiency of ion nitriding in improving the fatigue properties of eutectoid steel. Surface hardening, refined microstructure and compressive stress were the three main factors being responsible for the modification of fatigue properties.

This study was focused on the effects of pre-strain on the torsional fatigue properties of three kinds of medium carbon steels, including the fatigue strength, surface hardness, microstructure, and the crack initiation and propagation behaviors. The effect of pre-strain on the non-propagating cracks was also discussed. The main results obtained in this test are as follows: 1) the fatigue limits increase with the increasing of tensile pre-strain ratio for all kinds of the test materials; 2) under certain stress amplitude, with the increasing of tensile pre-strain ratio, the fatigue crack initiates a little earlier and propagates faster; 3) the length of non-propagating crack decreases with the increasing of tensile pre-strain ratio.

This study is focused to the effects of plastic working on the fatigue strength of Ti-6Al-4V alloy with notch. In general, the fatigue strength of plastic worked specimen is higher than that of non-worked one. However, the potential of hardening ability of Ti-6Al-4V alloy is very limited. Accordingly, the effect of work hardening on fatigue strength about this material is very small. In addition, the surface of the worked part becomes rougher with increasing plastic deformed value and the fatigue cracks initiate at this part. Consequently, the fatigue limit of the plastic worked specimen is lower than that of the non-plastic-worked one.

Prevention of slack in fastening bolt has been an important researching subject since fatigue failure can be generally caused by loosening nut. According to the above reason, the super lock bolt with double screw mechanism, which is non-loosening, has been developed for preventing such kind of bolt failure. The objects of the present study are as follows: (1) To clarify the fatigue strength characteristics. (2) To investigate the initiation and propagation behaviour of fatigue crack regarding to the thread root of first pitch. In addition, the mechanism of “fatigue strength of super lock bolt” is also discussed in this paper and it is confirmed that this bolt shows excellent fatigue strength and non-loosening properties.

As though titanium and its alloy have specific tensile strength, excellent corrosion resistance, ect., they have not been widely used until now mainly due to their extremely high processing cost. Therefore, it is considered that the above barrier will become smaller if it is possible to improve the mechanical properties of Ti-alloys by applying surface treatments on properties, especially fatigue strength of about 6kinds of surface treatment Ti-6Al-4V alloys. This is, surface treatments by 2kinds of plasma, 3kinds of DLC (diamond-like carbon) coating and the electroness plating method, have been studied for Ti-6Al-4V alloy . This study presents an analysis of fatigue behavior from the viewpoint of micro-hardness and adhesion work through a new technique in Ti-6Al-4V alloy. In addition, the specimen's surface has been successively observed by the replica method during tests and it seems clear that the fatigue strength is improved by the DLC treatment in comparison with that of the conventional Ti-6Al-4V alloy by about 20% (fatigue limit improvement by 70MPa). Though all the hardness value of these specimens increased, the fatigue limit improved only in the case of two kinds of specimens by DLC coating.

This paper presents the results of a research on the effect of cold rolling on fatigue properties of notched eutectoid steel. The results indicate that the fatigue strength of the material is improved significantly following cold rolling. Non-propagating cracks are found on the surfaces of all four cold-rolled specimens used in the test and the depths of the cracks increase with the increasing of the degree of plastic deformation. Compressive residual stresses, work hardening and fiberized microstructure are the three main factors responsible for the modification of fatigue properties. Compressive residual stresses can effectively improve the resistance to fatigue crack propagation into the interior region of the specimens. The fundamental mechanisms for fatigue strength improvement are also discussed.

The fatigue properties of austenitic stainless steels become inevitably important when using in structural materials. The authors have performed fatigue tests to investigate the effect of nitrogen content on fatigue properties of typical austenitic stainless steels (SUS304) and two kinds of nitrogen-contained SUS304 (SUS304N and YUS170). The main results obtained in this study are as follows; (1) The knee point in S-N curves exists for SUS304 and SUS304N, but does not exist for YUS170. (2) The fatigue limit of SUS304N is higher than that of SUS304. On the other hand, the fatigue strength by 107 cycles of YUS170 is lower than those of SUS304 and SUS304N. (3) For SUS304, the transformation ratio increases with increase in stress amplitude. On the other hand, the transformation ratio of SUS304N is very small and that of YUS170 remains essentially zero.

About 90% of steel structure™s failure was induced from the notched parts due to the stress concentration. Therefore it was very important to research how to improve the fatigue property of notched parts when the stainless steel was used as structural material. Rotating bending fatigue test was preformed to investigate the effect of different notch-machining process (Conventional lathe-machining, Combination of lathe-machining and roller-working) on fatigue strength of the typical austenitic stainless steel SUS304. Fatigue limit of roller-worked specimen was improved to 285% compared to that of lathe-machined specimen. It was found that, work hardening effect induced in roller-working process played an important role to improve the fatigue strength. In addition, the residual compressive stress and the fibriform structure in the specimen surface-layer induced by roller-working were also effective to improve the fatigue strength. Non-propagating fatigue crack was observed in the notch surface of the roller-worked SUS304 for the first time.

The fatigue strength have been investigated about the plastic deformed specimens by roller working in Ti-6Al-4V alloy with a notch by 4 kinds of plastic deformation ratio. The plastic deformation values (Δ t) are employed 0.05, 0.1, 0.25 and 0.4 mm on notched specimens of Ti-6Al-4V alloy by roller-working. The hardness test, fatigue test and SEM observation was carried out on non-roller worked specimens and roller-worked specimens at room temperature. Based on the results of these tests, Though the hardness number of roller worked materials are higher than those of non-deformed Ti-6Al-4V alloy, the fatigue limit of roller worked materials becomes deteriorated in spite of roller working. Fatigue fracture had occurred in the non- deformed area where tensile residual stress exists. From the results of the SEM observation, stepped defect could be notch effect for reducing the fatigue strength of Ti-6Al-4V alloy.

In the present study, fatigue tests have been performed to study the effect of pre-strain on fatigue properties of NHH (New Head-Hardened) rail. The objectives of this study were: (1) to observe the microscopic behavior of specimens during pre-strain process, (2) to research the influence of pre-strain on fatigue strength of NHH rail and (3) to investigate initiation and propagation behavior of the fatigue crack. The results showed that plastic pre-strain decreased the fatigue strength of NHH rail; fatigue limits had no obvious variation among the different pre-strain ratios. Fatigue cracks initiated in the microscopic cracking or slip lines that were originated in the pre-strain process and propagated from these sites in the later fatigue test.

Carbon fiber/metal matrix composites were fabricated by powder metallurgy process, spark plasma sintering followed by rotary swaging, to achieve high heat conduction and low thermal expansion. Pitch-based carbon shortfiber was mixed with the powder of aluminum or copper. A volume fraction of the fiber was 40%. Orientation angles of the fiber to the compacting direction were measured in the longitudinal sections of the composites. Hermans' orientation parameter was adopted to evaluate the fiber orientation. The swaging process caused the parameter to change markedly from -0.9 to 0.9 in both of Al and Cu matrix composites. These values mean that the fiber is dispersed perpendicularly to the compacting direction in the sintered objects and dispersed in parallel to that one in the objects with swaging process. Thermal conductivity of the composites increases with increasing Hermans' orientation parameter. The thermal conductivity of Al and Cu composites was 210 and 420 W⋅m-1⋅K-1 at room temperature, respectively. Coefficients of thermal expansion of both Al and Cu matrix composites decreased approximately 25% as compared with those of monolithic Al and Cu metals.

論文(Article)

Eutectoid steel is one of the most important industrial materials. In addition, these materials are usually pre-strained before use during production or machining process. In this study, fatigue test has been performed to research the effect of pre-strain on fatigue properties of eutectoid steel with different plastic pre-strain ratio at ep=0, 2%, 4% and 6%, respectively. At the same time, microscopic behavior of the crack initiation and propagation are also investigated by the successively taken replica method during the pre-strain and fatigue test. The results of the research indicate pre-strain directly affect the fatigue strength and crack initiation and propagation behavior. All fatigue limits of specimens with different pre-strain ratio are lower than that of specimens without pre-strain (ep=0) and the fatigue limits of pre-strained specimens have no obvious change from ep=2% to 6%. Fatigue cracks initiate from slip lines or micro-cracks that are generated in the process of plastic pre-strain. However, for non-pre-strained one, fatigue cracks initiate from boundary or inter-lamellar of the pearlite. The mechanism for this difference is discussed in this paper.

Research results of the application of hard-rolling treatment to aluminum surface modification are presented. The main objective of the research presented here was to study the change in fatigue properties, particularly fatigue crack behavior of notched aluminum specimens after hard rolling. The results indicated an obvious improvement in fatigue properties of the aluminum specimens after hard rolling, but the extent of enhancement of fatigue strength was nearly the same or even decreased slightly with the increase of the hard-rolling value. Fatigue crack initiation and propagation behavior displayed different characteristics for specimens with different hard-rolling values. The microstructural change and the appearance of compressive residual stress at the notch were employed to explain the effect of hard rolling on the fatigue properties of aluminum. In addition, a saturation condition was observed in the compressive residual stress introduced by hard rolling. This phenomenon was explained in terms of the intrinsic properties of the material studied in this work.

Fatigue properties of tungsten fiber reinforced Ti-6Al-4V (W/Ti-6Al-4V) alloy has been investigated. W/Ti-6Al-4V composites were fabricated by 2 kinds of Hot Isostatic Pressing (HIP) conditions and elongated by 2 kinds of the following process by rotary swaging (RS), die forging (DF). The fatigue properties of these MMCs (metal matrix composites) were compaired with conventional Ti-6Al-4V alloy and HIP treated Ti-6Al-4V one. According to the results in the present study, the average hardness and fatigue strength of HIP-DF-0 showed higher values in comparison with those of conventional Ti-6Al-4V by about 20-30% and 15% respectively. However, the fatigue limit of MMCs was unexpectedly deteriorated in comparison to the conventional ones. The deterioration of the fatigue limit was considered to be due to the defects in the microstructure of the composites such as clustering of fibers, fiber orientation, micro-cracks and porosity.

In this paper a plane bending fatigue tests have been performed to investigate the effect of die pressing on fatigue strength of the plate specimens with a central circular hole. The main results are as follows : (1) The fatigue limit of notched specimen by die pressing is increased by 60% as compared to that of the same notched specimen without die pressing and is equal to that of plain specimen. (2) The reason of increase in the fatigue limit is attributed to the work hardening and compressive residual stress, which are generated by die pressing to the edge of the hole, which increase the resistance against crack initiation and suppress its initial growth. (3) In the case of die pressing, the lifetime summations of the period of the fatigue crack initiation and short crack propagation within 1mm is about 80% of the total number of cycle to failure.

Fatigue test has been performed to investigate the effect of strain aging on fatigue properties of pre-strained carbon steel (S45C) using Ono-type rotary bending fatigue testing machine. Main results summarized in this study are as follows : (1) It is considered that both of the pre-strain and annealing condition influence the fatigue limit of pre-strained specimen. (2) Both the fatigue limit and hardness are increased by annealing. (3) Under the same annealing condition, the fatigue limit of pre-strained specimen increases with increase of pre-strain. (4) The optimal heat treatment temperature for obtaining the maximum value of fatigue limit decreases with increase of pre-strain.

In this study, the fatigue test has been carried out to investigate the effect of plastic deformation value by roller-working on the fatigue strength of notched specimen. Herein, the authors have tested four types of specimens whose deformation values have been changed by roller-working and final shape and dimensions are the same. The reason why fatigue strengh changes with different plastic deformation value, has been examined by analyzing the residual stress, hardness distribution and crack tip opening displacement etc. The main results obtained in this test are as follows : (1) The resistance of the fatigue crack initiation increases with the increase of plastic deformation value. (2) When deformation value becomes too large by roller working, the residual stress may be released by cycle stress and the critical stress of non-propagating crack tip opening becomes decrease. (3) The fatigue limit of notched specimen by roller-working tends to deteriorate gradually when the plastic deformation value becomes too large, the optimum deformation value would exist for improvement of fatigue limit of notched specimen by roller-working.

The objective of this study is to establish an optimized manufacturing process of the metal-matrix composite (MMC) made from tungsten fiber reinforced Ti-6Al-4V alloys using a hot isostatic pressing (HIP). The rod type MMC was made by mixing W fibers of W fibers of 20 ㎛ in diameter and 1㎜ long and Ti-6Al-4V powders of of 70 ㎛ in the mean diameter in a V-mill blender, pressing the mixed powders using in argon atmosphere at 950℃ for 1 hr. under applied pressure of 1000㎫, followed by swaging at RT with 70% cold work. The MMC with the tungsten fibers varying from 0 to 12 vol.% were subjected to tensile tests, fatigue tests and hardness measurement ar RT. Comparing to the mechanical properties of conventional Ti-6Al-4V alloy, the rod-type MMC made by HIP had increased hardness with increasing volume percent of W fibers, the peak tensile strength at the 9vol.% W fibers but lower fatigue strengths for all the volume percent of W fibers added. The effect of the W fiber concentration on the mechanical properties of the MMCs was discussed based on the orientation of the W fibers in the matrix, porosity and the secondary cracks which were evaluated by microstructural examination of the MMCs after mechanical tests.

Axial cyclic tension-compression fatigue test has been performed about the typical austenitic stainless steel (SUS 304) using a servo-controlled electro-hydraulic fatigue tester. The main object of this study is to investigate the effect of strain aging and overstress on fatigue property of SUS 304, which is applied just after fatigue micro-crack initiation. The main results obtained in this study are summarized as follows : (1) The effect of strain aging on fatigue property which has not been observed in high cycle rotating bending fatigue test, is obviously recognized in low cycle axial compression-tension test. (2) The fatigue life (σa=300MPa) is enhanced 211.1 times by the combination of overstress and aging treatment. The reason concerning in this outstanding effect is that, strain aging does play a much more significant role for improving the fatigue strength besides the strain hardening and austenite-martensite transformation caused by overstress. (3) Non-propagating fatigue micro-crack is observed in the specimen, which has been suffered under the stress amplitude of fatigue limit by 1×107 cycles, with combining overstress (525MPa, 50 sec) with aging treatment (350°C×2hrs).