Torsional fatigue tests were conducted for spring steels whose Vickers hardness were 430, 480, 550, and 620. The effects of corrosive environment and surface artificial pits on the fatigue strength were studied. The fatigue strength of smooth specimens was maximum at 550HV for fatigue in air and 480HV for corrosion fatigue. Intergranular fracture surfaces were observed on the fatigue fracture surface corresponded to the decrease in the fatigue strength of the high-hardness materials. The fatigue fracture of smooth specimens was generated from shear crack for fatigue in air and from corrosion pit in corrosion fatigue. The stress intensity factor for crack initiation from corrosion pit was nearly the same for all materials. The fatigue strength reduction factor was constant under fatigue loading in air, while decreased with increasing fatigue life and hardness in corrosion fatigue. For the case of long of the material with 620HV in corrosive environment, a significant influence of the artificial pit on the fatigue strength was not seen. Many small cracks were observed at the sites except for artificial pit on the surface of the 620HV specimens fatigued at low stress levels.
Axial-reversed loading fatigue tests under three applied stress ratios of –1, 0 and 0.5 were carried out in air at room temperature using hour-glass shaped specimen of high carbon-chromium bearing steel, JIS SUJ2, to investigate fatigue fracture characteristic in very high cycle regime. A granular bright facet (GBF) was observed around subsurface inclusion in all specimens ruptured at long-life fatigue over 106cycles. Three types of fracture mode were observed from the fracture surface, such as surface-induced fracture, subsurface inclusion-induced fracture without GBF area and that with GBF area. Number of cycles to transition in the fracture mode depended on the applied stress ratio. Shape of the S–N curve obtained from the axial loading fatigue was a smooth and continuous in spite of the occurrence on three types of fracture mode, and was different from that of rotary bending fatigue. The characteristic of fatigue fracture behavior was discussed from point of view in fracture mechanics and also residual stress distribution on the specimen surface.
In order to simulate interior-originating cracks of Ti-6Al-4V in very high cycle fatigue, crack growth tests using CT specimens were conducted in a high vacuum. Morphogenetic factors on granular fracture surface of interior-originating fatigue were investigated based on the effect of vacuum on crack growth process. As a result, the following were obtained. (1) Vacuum decreased da/dN and increased ΔKth compared with air. This results from a stronger closure effect induced by plasticity in a vacuum. (2) Fine concavo-convexo patterns consisting of a few micrometer size granular features were observed on the fracture surface only in a vacuum. This granular region is same as the region observed in the interior-originating fatigue. (3) The magnitude of da/dN or ΔK had no relation with the formation of granular region. (4) To clarify the effect of crack closure on granular region, an additional test, which makes one of fracture surfaces contact with other, was conducted. This clarified that the area of granular region increased and the fine concavo-convexo patterns became more distinct. Therefore, granular fracture surface of interior-originating fracture in Ti-6Al-4V results from repeating contacts of crack surfaces in a high vacuum or similar environment.
In order to understand internal fracture of a shot peened material, rotating bending and uniaxial loading fatigue tests were carried out for specimens of a spring steel equivalent to SUP12, shot peened under various conditions and configurations of fish-eyes observed on fracture surfaces were discussed. A fatigue limit was not recognized in S–N curves and the internal fracture with the fish-eye was dominant under the conditions. The value of d/dcp, where d and dcp are depths of the origin of the fish-eye from the specimen surface and of the crossing point of residual stress distribution respectively, ranged from 0.8 to 2.0 for the rotating bending condition, whereas the values ranged from 1.0 to 11.0 for the uniaxial loading condition. This result indicates that internal fatigue cracks initiate at an area where the maximum stress σmax of cyclic stress by superposing the residual stress and the applied cyclic stress is high. The values of rs/rc and b/a were paid attention to describe the shape of the fish-eye. Where rs and rc are distances from the origin to the edges of the fish-eye in the directions of surface and center respectively, and a and b are widths of the fish-eye in the diametral and circumferential directions of the specimen respectively. The value of rs/rc depends on d/dcp independent of shot peening conditions and it changed from 0.3 to 1.5 with increasing for d/dcp from 0.8 to 2.0 under rotating bending conditions. The relationship between rs/rc and d/dcp under uniaxial loading conditions is the same as that under rotating bending conditions as long as small values of d/dcp. The value of rs/rc being less than unity means that crack propagation toward the surface is delayed compared with toward the center. This phenomenon was considered to relate to the compressive residual stress existing at the specimen surface layer. There is a good correlation between decreasing for the value of rs/rc less than unity and increasing for the value of b/a more than unity.
In order to investigate the hardness dependence on the fatigue strengths for surface fracture and internal fracture, rotating bending fatigue tests were carried out for maraging steels using specimens with different hardness obtained by heat-treating at different under-aging conditions. Fatigue limit for surface fracture was evaluated in electro-polished specimens and the one for internal fracture was in shot-peened ones, where fatigue limits were defined as fatigue strengths at 107 cycles for surface fracture and 108 cycles for internal fracture, respectively. Fatigue limit for internal fracture increased with increase in hardness similar to the one for surface fracture. However, the proportional relationship between the fatigue limit and the hardness existed till higher hardness in the fatigue limit for internal fracture than in the one for surface fracture. This difference in the hardness dependence on fatigue limit was discussed from the viewpoint of the influence of humidity on the fatigue strength.
The effect of laser peening to retard fatigue crack propagation has been demonstrated through rotating bending fatigue tests conducted on pre-cracked AC4CH casting aluminum alloy. The apparent maximum stress intensity factor at the surface of laser peened specimens was nearly equal to four times of the threshold stress intensity factor of long surface fatigue cracks just starting the propagation on AC4CH. The increase of the threshold stress intensity factor was due to the effect of compressive residual stress introduced by laser peening. The three dimensional (3D) image of fatigue cracks was reconstructed by micro computed tomography (μCT) using synchrotron radiation of SPring-8. The reconstructed images were used to visualize the crack propagation behavior inside the specimens, which could not be attained by usual replication technique. The 3D image of a laser peened specimen showed a legible plastic deformation on the surface of the specimen.
The purpose of present study is to propose a new technical method for retarding fatigue crack reinitiation and growth from the crack tip introduced-stop-drilled hole by providing additional holes. To demonstrate the validity of the proposal, analyses using software for two-dimensional elastic problems based on body-force method were done. Furthermore, a series of experiments was carried out for comparison. The calculated results showed that the additional holes give positive or negative effects on lowering stress concentration at the stop-drilled hole, which varies depending on combination of arrangement and size of both holes. Calking pin into the additional holes which yields internal pressure on the periphery, was found also advantageous for reducing stress concentration at the drilled hole. Finally, the calculated results showed relatively good agreement with the experimental results.
All tests were performed for high-strength aluminum alloy plate with a notch on both sides. Firstly, the proposed simplified method for estimating the stress and strain at the notch root was investigated. The strain at the notch root was measured by a strain gage. The estimated strains and strain ranges were in good agreement with the experimental results. Next, fatigue tests were carried out under constant amplitude loading and two-step loading. In two-step loading, two stress levels were used as the primary stress amplitude and three cycle ratios were combined. The cycle ratio was defined as the ratio of cycles at the primary stress amplitude to the corresponding fatigue life. Fatigue test results were discussed using the equivalent stress amplitude, which was estimated by the stress amplitude and the mean stress at the notch root. Both the fatigue life and the fatigue limit under the secondary stress amplitude tended to lower, as the cycle ratio under the primary stress amplitude increased. However, the fatigue life under the secondary stress amplitude did not depend upon the magnitude of the primary stress amplitude, if the cycle ratio was same. Hence, the fatigue life under the secondary stress amplitude can be estimated by the equivalent stress amplitude without the effect of the primary stress amplitude.
Spalling is one of the typical rolling contact fatigue (RCF) failure of railway wheels, initiating at white layers, as the result of unexpected wheel slides. As for spalling, evaluation methods of fatigue strength have not been tried and tested. The purpose of this study is to propose a prediction method of RCF strength for crack initiation at white layers. In this study, 1) RCF tests in order to evaluate spalling behavior of wheel steels, 2) axial fatigue tests to obtain fatigue strength of white layer itself, and 3) elastic-plastic FEM analyses simulating the RCF tests to calculate stress distribution in/around white layers were conducted. As the results, the following facts were clarified. 1) In RCF tests, cracks were initiated at the leading edge of white layers particularly under slip condition. 2) Fatigue strength of the white layer can be evaluated accurately using the endurance limit diagram estimated by a straight line passing the true fracture strength and fully reversed fatigue strength. 3) According to FEM analysis results, the maximum stress at the leading edge of white layer is larger than at the trailing edge, and the maximum stress of the white layer increases by taking into consideration of small surface profile change during RCF test. Moreover, Dang Van model, which is one of the multiaxial fatigue strength evaluation methods, was applied to predict the white layer's fatigue strength in RCF tests. It can be concluded that the present prediction method is reasonably acceptable for crack initiation assessment of spalling and that in case of the existence of relatively apparent surface profile change, stress evaluation has to be done by taking it into consideration.
Stress relief groove has been used to improve the fretting fatigue strength of fitted part between mechanical components. However, the effect of groove has not been fully understood and there are not enough investigations to determine the optimal groove shape. In this study, the evaluation of fretting fatigue strength of specimens which have various shapes of stress relief groove was conducted by fretting fatigue tests and FEM analyses in order to find an index for the selection of groove shape. The groove shape was changed systematically with parameters of groove radius R and tangential angle θ. In the fretting fatigue test, fatigue limit of grooved specimen had a peak with increase of θ. The maximum improvement of fatigue limit was achieved at the transition point of the failure mode from the fretting fatigue at contact part to the fatigue at groove root. It was experimentally shown that fatigue limit had a good correlation with the parameter θd, where d is the groove depth. FEM analyses were done to understand the effect of stress relief groove. It was found that the fretting fatigue limit of grooved specimen could be evaluated using the peak axial stress near the contact edge. The estimation of fretting fatigue limit using a relationship between Kt/Kt0 and θd gave a good correlation with the experimental results and it would be a useful method to select the optimal groove shape.
This paper deals with the compressive strength of Terfenol-D type giant magnetostrictive materials fabricated with the powder metallurgy. Static and fatigue compressive tests for the specimens with different porosity were performed. Static compressive tests indicated that the compressive strength of this material can be estimated with the porosity and strength of dense material with same components. Compressive fatigue tests with the same specimens showed that the compressive fatigue strength of this material is 90% to the static compressive strength. Observations of cracks in the specimens were carried out before and after compressive loading. The effects of the compressive stress to the numbers and length of the cracks were examined. Observations showed significant increase of the number of the cracks with length of 5 to 20μm after static and fatigue compressive loading. Fracture processes for static and cyclic loading were proposed.
In the research of fatigue of metals, fatigue properties of ultra low carbon steels are important as a fundamental reference data for structural carbon steels. Thus, fatigue test data for low carbon steel of S10C and pure iron in rotating bending were extracted from the “Database on Fatigue Strength of Metallic Materials” published by JSMS. Applying the JSMS standard (JSMS-SD-6-04) “Standard Evaluation Method of Fatigue Reliability for Metallic Materials –Standard Regression Method of S-N Curve–”, S-N curves were analyzed on the basis of the “semi-logarithmic bilinear model with fatigue limit” for the respective series of fatigue data. It was found that S-N property of the individual material was well normalized by tensile strength of σB. Based on this aspect, a number of fatigue data extracted were pooled altogether and P-S-N property was also analyzed by the above standard (JSMS-SD-6-04). Analytical results thus obtained are usefully applied as the fundamental data for the reliability design of the mechanical structures in the various areas.
In order to accelerate the development of structural materials and improve reliability under an actual environment over extended and practical lifetimes, a multi fatigue testing system, the test efficiency of which is twelve times greater than that of the conventional fatigue testing system, has been developed. The fundamental components of this system are (1) a unified hydraulic power unit, (2) six individual testing units, each of which has an actuator and a frame for accurate test control, (3) original software that enables stable control and has a graphical user interface with a labor-saving function, such as an automatic start function after elevating and holding the test temperature. The cost of this system is very low, because of the elimination of the exclusive controller and unification of the hydraulic power unit. Using this system, fatigue tests of six specimens can be simultaneously carried out with respective testing conditions, without interference among testing units, while other tests are still in progress. In addition, the multi fatigue testing system enables stable and accurate fatigue tests over a long period of time. Furthermore, the newly developed system can reduce the operating time remarkably, by employing convenient software. As a result, the evaluation period for the fatigue properties of materials could be remarkably shortened in comparison with the conventional fatigue testing system. The reliability of the proposed fatigue testing system was verified based on the stability of the waveform and the load amplitude, the relative deviation among testing units, and the reproducibility of the S-N diagrams obtained by the conventional system. As a result, the proposed multi fatigue testing system was confirmed to provide high performance.
Recently, the concern with acrylic resin as a new repair material has been growing. The acrylic resin is characterized by high quality, low viscosity, highly elongation percentage and so on. Several studies have been made on the crack repair by using acrylic resin, but little is known about the effect of acrylic resin as a crack repair material for concrete. The purpose of this study is to examine the applicability of acrylic resin for using as a crack repair material for the concrete structures. The fundamental mechanical properties of acrylic resin by using super lightweight powder were examined. And the repair effects of acrylic resin as crack repair material were examined by the flexural loading test of the concrete member repaired by crack injection technique. As a result, acrylic resin could adjust the viscosity by using super lightweight powder, and the elongation percentage of acrylic resin was superior to epoxy resin. The crack repair effects of acrylic resin were equivalent to the epoxy resin under drying condition of the crack surface of concrete member.