Stress Amplitude Level Research Articles

疲労 高炭素クロム軸受鋼のＳ‐Ｎ曲線と内部き裂発生挙動

In order to investigate high cycle long life fatigue characteristics, cantilever-type rotary bending fatigue tests were conducted in an open environment at room temperature using hourglass shaped specimens of high carbonchromium bearing steel. The S-N curve obtained was clearly classified fracture modes into two groups of different crack origin. One was governed by a crystal slip on the specimen surface which occurred in the region of short fatigue life and high stress amplitude level. The other was governed by nonmetallic inclusion at subsurface which occurred in the region of long fatigue life and low stress amplitude. The inclusion governed fish-eye fracture mode was distributed in a wide range of stress amplitude not only below the fatigue limit defined as the threshold of fracture due to the slip governed mode but also above the fatigue limit. This remarkable shape of the S-N curve was different from a step-wise one reported in previous literature and was characterized as duplex S-N curves which composed of two different S-N curves corresponding to the respective fracture modes. An examination of the initial stress intensity factor range of fracture origin, ΔKini, showed that the ΔKini of the slip governed fracture mode was larger than the threshold stress intensity factor range of the surface crack growth, ΔKth. On the other hand, cracks originate and propagate from inclusion at subsurface with smaller ΔKini than the slip-originating crack and the ΔKth.

Fatigue of Cord—Rubber Composites: II. Strain-Based Failure Criteria

Abstract Fatigue failure mechanisms under low-frequency loading and their dependence on the strain properties were assessed for the rubber matrix composite of bias aircraft tire carcass reinforced by nylon cords as well as two model rubber composites reinforced by steel wire cables. Under cyclic tension at constant stress amplitude, these angle-plied, cord—rubber composite laminates exhibited localized damage in the form of cord—matrix debonding, matrix cracking, and delamination. The process of fatigue damage accumulation in the cord—rubber composite laminate was accompanied by a steady increase of cyclic strain (dynamic creep) and moderate temperature changes. The fatigue life was found to be linearly proportional to the inverse of the dynamic creep rate, i.e., the time required to increase cyclic strain by a unit amount. Regardless of the associated level of stress amplitude or fatigue life, the gross failure under low-frequency loading occurred when the total strain accumulation, i.e., cumulative creep strain, reached the static failure strain. The use of higher stress amplitude resulted in a decrease of fatigue life by simply shortening the time to reach the critical level of strain for gross failure. This observation indicates that the damage initiation and eventual structural failure of angle-plied, cord—rubber composite laminates are “ strain-controlled” processes. These critical strain properties appear to be controlled by the process of interfacial failure between the cord and matrix. Under static tension, the strain levels for cord—matrix debonding and gross failure of composite laminates showed no significant dependence on the level of carbon black loading of the matrix compound, despite the fact that carbon black loading strongly affected the modulus, strength and strain properties of the matrix. Also the number of debonding sites around the cut ends of cords increased at almost the same rate as the static strain increased regardless of the variation of matrix properties.

繰返し応力下における花こう岩の変形特性に及ぼす構造的異方性の影響に関する研究

The effects of structural anisotropy on the deformation properties of granite under cyclic loading have not been clarified. In this paper, four types of granite specimens are prepared with an angle θ of the structural anisotropic plane to the loading axis of 0°, 30°, 60° and 90° respectively; and cyclic loading tests are performed under uniaxial compression. The stress amplitude levels used in this experiment are 0-30%, 20-50% and 40-70% of the uniaxial compressive strength of this granite. As the results, for the stress amplitude level of 0-30%, the elastic modulus under the cyclic loading is approximately constant. However, it is recognized that the increase of axial strain during the cyclic loading is largest for the specimen with θ of 30°, and its value under ten thousands cyclic numbers corresponds to 1.27 times as much as the axial strain given by 20% stress for the uniaxial compressive strength, and that its uniaxial compressive strength after cyclic loading is lowest for the all stress amplitude conditions. Further more, it is clarified that its volumetric strain indicates only a negative value under 50% and 70% stress levels under cyclic loading.

Long fiber reinforced polyamide and polycarbonate composites. II: Fatigue behavior and morphological property

AbstractFatigue behavior and morphology of long glass fiber reinforced semicrystalline polyamide (nylon 6,6) and amorphous polycarbonate (PC) composites were investigated. The fiber length distribution in the molded samples was calculated by image analyzer. The tension‐tension fatigue loading tests at various levels of stress amplitudes were studied. The two‐parameter Weibull distribution function were applied to obtain the statistical probability distribution of experimental data. A good correlation existed between the experimental data and the Weibull distribution curves. Straight line SN curves of long glass fiber reinforced semicrystalline polyamide and amorphous polycarbonate composites at various probabilities were established. The stiffness of the composite under tension‐tension fatigue loading was measured. The thermal stress history was also investigated by thermo‐imaging techniques during fatigue life testing. Further, failure morphology was examined by scanning electron microscopy (SEM). The results showed that the fracture behavior of the ductile damage in polyamide is different from the brittle damage in polycarbonate.

Fatigue crack propagation in a cast TiAl intermetallic compound at high temperature

High-temperature fatigue crack propagation tests were conducted on Ti-34 wt% Al intermetallic compound with lamellar structure, in order to elucidate the effects of lamellar boundaries and creep on the resistance to translamellar cracking. The results obtained are summarized as follows. (1) Slow crack growth during fatigue is found to traverse the lamellae, and its rate is independent of whether the cracking direction is perpendicular to the lamellae or along them. (2) The fatigue crack propagation rate, d l /d N , at 1073 K is correlated well with the effective stress intensity factor range, Δ K eff . (3) The fatigue crack propagation rate at room temperature is little slower than that at 1073 K. (4) Under creep-fatigue conditions, the crack propagation exhibits a time-dependence and its rate per cycle, d l /d N , is governed by the creep J -integral range, Δ J c . (5) In transverse crack propagation, the d l /d N -Δ J c relationship is independent of the crack direction to the lamellar surface. (6) Compared with some heat-resisting steels and alloys, the resistance of the present TiAl to fatigue crack propagation is remarkably low at high levels of stress amplitude while the resistance to creep- fatigue crack propagation is almost the same.

Fatigue behavior of quasi‐isotropic carbon fiber/PEEK laminates under tension‐tension loading

AbstractFatigue behavior of carbon fiber reinforced poly(etheretherketone)(PEEK) laminates was investigated. The static tensile measurement, tension‐tension fatigue loading tests, and residual tensile strength measurement of the [0/45/90/‐45]2s AS‐4/PEEK laminates were performed at various levels of stress amplitudes. The influences of stress amplitude on the fatigue life and the residual tensile strength were investigated. The experimental results for fatigue life and residual tensile strength under different stress amplitudes are analyzed by the median rank method. The S‐N curves at various survival probabilities are also presented by the pooled Weibull distribution function. Furthermore, a residual strength degradation model is used to predict the residual strength for the composites subjected to a number of fatigue cycles and to simulate the effects of the stress amplitude on the fatigue life. The agreement between experiment and theory is good.

The hygrothermal effect on the fatigue behavior of carbon fiber/ peek laminate composites (I)

The hygrothermal effects on the fatigue behavior of the Carbon/PEEK laminated composites before and after impact damage were examined in this study. The [0/45/90/-45]2s AS-4/PEEK laminated composites were immersed in 80°C hot water for 45, 90 and 200 days,and subjected to falling weight impact with an energy of 8.58 J and then immersed in 80°C hot water for 45 days. It was found that the tensile strength of AS-4/PEEK laminated composites decreased with the increase of exposure period. The injured AS-4/PEEK composites were subjected to a static load and a tensiontension fatigue load at various levels of stress amplitudes. The effect of stress amplitude on the fatigue life was studied. The experimental fatigue life under different stress amplitude tests were estimated by the median rank order statistic cumulative distribution function. Then,the fitting curves for estimated data were analyzed by the Weibull distribution function. The S-N curves for a series of cyclic loads at various survival probabilities were presented. The damage behaviors of composites after fatigue load test were also investigated by scanning electron microscope(SEM). Results indicated the fatigue lives of immersed specimens were shorter than those without hygrothermal effect, the impact damage affects the fatigue life of composite significantly.

FATIGUE BEHAVIOUR OF THREE VARIANTS OF THE ROLLER BEARING STEEL SAE 52100

AbstractStress‐controlled, low‐cycle, push‐pull fatigue tests were performed on three variants of the bearing steel SAE 52100 with slightly different compositions and heat treatments. The experiments demonstrated differences in the cyclic plastic behaviour of differently hardened steels (bainitically‐hardened and martensitically‐hardened, respectively), whereas the two martensitic variants, which differ in composition, behaved very similarly. Bainitically‐hardened SAE 52100 steel exhibited initial hardening followed by cyclic softening above a stress amplitude level of 1200 MPa. In contrast, the martensitically‐hardened variants showed a pronounced cyclic hardening. The deformation behaviour of the martensitically‐hardened bearing steel in a monotonic tensile test and during the first cycles can be well understood on the basis of the transformation of retained austenite. This process leads to an onset of plastic deformation at lower stresses compared to the bainitically‐hardened bearing steel. As a result of the subsequent cyclic hardening of the martensitic variants, the CSS curves are almost identical for the differently hardened conditions under investigation. Additional tests under pulsating compression documented that a high negative mean stress enhances the cyclic plasticity.

Overview of fatigue behavior in copper single crystals—IV. Strain and load interaction effects for tests under variable amplitude

How different levels of cyclic strain or stress amplitudes, applied in sequences, affect the mechanisms of fatigue failure, is one of the most difficult and complicated problems in fatigue. In an attempt to understand some of the factors involved, multistep tests have been performed in both stress control and strain control on copper single crystals. The influence of low-high and high-low sequences have been studied on saturation stress, cyclic stress strain behavior, PSB activity and strain localization behavior in PSB's. The paper illustrates numerous effects of local strain variation on the damage processes and how these effects cause non-linear damage summation.

Relationship Between Fatigue Life Distribution, Notch Configuration, and S-N Curve of a 2024-T4 Aluminum Alloy

The objective of this study is to identify the most closely related variable to the distribution of fatigue life in unnotched and three kinds of notched 2024-T4 aluminum alloy specimens. Carefully designed fatigue tests under a constant temperature and humidity condition provided fatigue life distributions over a wide range of stress amplitude. This study used about 1000 specimens. On the basis of the test results, the dependence of the scatter in fatigue life on notch configuration, the period to crack initiation, the level of stress amplitude, the median fatigue life, and the slope of the median S-N curve is investigated, and the relationship between the distributional form of fatigue life and the shape of the median S-N curve is discussed. It is concluded that the slope and shape of the median S-N curve in the vicinity of the test stress level are closely related to the scatter and distributional form of fatigue life respectively. This is common to the unnotched and three kinds of notched specimens. A variability hypothesis to correlate the median S-N curve with fatigue life distributions is examined.

Experimental measurements of material damping in free fall with tunable excitation

An experimental method of measuring material damping of a specimen in free fall has been developed. An initially simply-supported test specimen was simultaneously excited and lofted into free fall with a spring-mass launcher. The launcher could be tuned to produce varying initial stress amplitude levels in the specimens. Damping information was obtained from the transient response of the specimens while in free fall in a vacuum. A dynamic model of the launch process was developed which indicates the range of frequency and initial specimen stress amplitude over which tests can be performed. To verify the facility, material damping was measured in 2024-T3 aluminum, and (+ or - 45)2s AS1/3501-6 graphite/epoxy. Free-free frequencies ranged from 20-357 Hz in aluminum, and from 18-171 Hz in the (+ or - 45)2s specimens. Damping values in aluminum were found to be close to theoretical values and independent of stress levels below 130 MPa (18.7 Ksi). Damping ratios in the (+ or - 45)2s graphite/epoxy specimens were found to be largely independent of stress levels below 43.8 MPa (6.35 Ksi) and only slightly dependent on frequency. A small dependence of the frequency of free vibration on specimen stress amplitude was also noted.

Relationship between friction forces and the endurance limit of light alloys

1. The stress determined by the friction forces preventing dislocation movement is the parameter controlling the processes of plastic deformation in cyclic loading the given alloys and, consequently, the cumulation of fatigue damage and failure. 2. the friction force of the aged aluminum-based alloys decreases during cyclic loading. 3. A correlation was found between the magnitude of the friction force and the safe cycle stress amplitude. The level of the safe stress amplitude in the aluminum alloy does not depend on the mean cycle stress. The level of the free stress amplitude in the titanium alloy in the tests with high values of πm is controlled by the friction forces. In the tests with the stress ratio R=0, the endurance limit is determined by the macroscopic limit of elasticity.

ガス軟窒化処理したＣｒ‐Ｍｏ鋼平滑試験片の両振平面曲げにおけるき裂の発生と伝ぱ

The long life fatigue failure of a case hardened unnotched steel specimen starts just under the hard surface layer and “Fish eye” is found on the fractured surfaces. In the previous experiment, it was found that the rotating bending fatigue strength of carbonitrided unnotched specimens was fairly higher than the value estimated based on the strength for fatigue crack initiation and propagation at the critical portion in the specimen cross section.To examine the reason for the difference between the experimental and estimated fatigue strengths, fully reversed bending fatigue tests were made on two kinds of Cr-Mo steel specimens of rectangular test section with surface layers hardened by low temperature gas carbonitriding. Specimen A was provided with hard cases only on the upper and lower surfaces of the test section, and specimen B was carbonitrided on the whole surface.By using specimens A, crack initiation and propagation on the side surfaces was watched during the fatigue tests. Cracks were initiated just below the hard layer (0.4∼0.6mm distant from upper or lower surface) and propagated to fracture.At a considerable wide range of stress amplitude (about 10kgf/mm2) below the fatigue fracture limit, cracks were initiated but soon arrested due to compressive residual stress of the hard layer. The minimum stress amplitude where non-propagating cracks were observed, coincided mostly with the estimated fatigue limit based on the strength of the critical portion.In the fatigue tests of specimen B, “Fish eye” was found on the fracture surfaces of all specimens failed beyond 3×105 stress cycles. The mean depth of the fish eye center from upper or lower surface of test section was 0.47mm. Comparison of the S-N relation of specimen B with that of specimen A, suggested the slow propagation of internal crack in specimen B, but the difference in fatigue limit between B and A was comparatively little.Three specimens of kind B which endured to 107 cycles at the stress amplitude levels less than 4kgf/mm2 below the fatigue limit, were fractured under rather high stress levels to show the internal failure. A non-propagated internal crack was found on one of these forced fracturd surfaces.The higher fatigue strength of case hardened unnotched specimen than the estimated value is supposed to depend mainly on the following factors.(1) Arrest of crack initiated in the critical portion due to compressive residual stress in the hard layer.(2) Slower propagation of the internal crack than the external one.

アルミ合金の腐食疲労強度に及ぼす引張予ひずみの影響に関する研究

In order to investigate the effect of tensile prestrain on the corrosion fatigue strength, high cycle fatigue tests were performed on aluminium alloy (2017) sheet specimens prestrained by pulling under completely reversed plane bending stress in laboratory air and in salt water (3.0% NaCl). The conclusions obtained are summarized as follows;(1) Corrosion fatigue strength for the prestrained specimens decreased in comparison with that for the as-received specimens, and it decreased monotonously with increasing prestrain regardless of the applied stress amplitude level.(2) Cyclic work hardening and subsequent softening behaviours were observed repeatedly twice for the as-received and 3% prestrained specimens during the corrosion fatigue process. In the specimens prestrained above 3%, the saturation of work hardening took place at about 20% of the fatigue life and thereafter the work softening occured continuously until fracture under subsequent stress cycling.(3) The number of cycles to crack initiation in corrosion fatigue decreased with increasing tensile prestrain. On the other hand, the number of cycles in the crack propagation period was independent of the tensile prestrain.(4) It is suggested that the experimental results obtained in this test can be explained from a consideration that the barriers to the motion of dislocations are easily relaxed by the corrosive dissolution of the specimen surface prestrained during the stress cycling and the plastic deformation of grains easily occurs near the specimen surface.