Low Cyclic Stress Research Articles

Low cycle fatigue behavior of a SiCp reinforced aluminum matrix composite at ambient and elevated temperature

Based on an investigation of low cycle fatigue life and cyclic stress response characteristics of SiC particulates reinforced pure aluminum and unreinforced matrix aluminum at 298 K and 441 K, the following observations were made. (1) Cyclic stress response of the unreinforced matrix aluminum, in the as-extruded condition, revealed initial cyclic hardening, cyclic stability and second hardening at ambient temperature. With a contrast, the unreinforced aluminum at elevated temperature showed progressively cyclic softening behavior without initial hardening. (2) The cyclic stress response characteristics of the composite were different from that of its unreinforced matrix at room temperature. In spite of the initial hardening, the composite showed progressive softening in most of the fatigue life. At elevated temperature the composite also displayed continuous cyclic softening behavior. The reason for the softening behavior probably was that the dislocation tangles in the composite specimen with a likely work-hardened status was not stable and could be changed under a cyclic loading. (3) The SiCp/Al composite and the unreinforced matrix followed the Coffin-Manson law. The low cycle fatigue resistance of the composite at room temperature was lower than that of the unreinforced matrix. A decrease in the fatigue endurance due to a rise in testmore » temperature was observed for the composite and the unreinforced matrix especially at low cyclic plastic strain ranges. The induction of fatigue life of the unreinforced aluminum was faster than that of the composite, so the fatigue resistance of the composite was stronger than that of the unreinforced aluminum under lower cyclic strain ranges at elevated temperature.« less

The inhibition effect of sodium molybdate on the initiation of corrosion fatigue of 1020 mild steel in aqueous solution

The influence of sodium molybdate as an inhibitor on the corrosion fatigue behavior of 1020 mild steel has been evaluated. The corrosion fatigue tests were carried out based on a cyclic loading normally referred to as a high stress-low cycle on V-notched specimens under a plane bend fatigue condition. Furthermore, the tests were performed in tap, distilled, and brine water without and with addition of the inhibitor at room temperature. A concentration of the inhibitor ranging between 0.05 M to 0.03 M was used in most of the solution. In general, the fatigue crack initiation time of the steel was found to increase as the inhibitor concentration increased in all solutions.

Cyclic Deformation and Fracture Behavior of Al Alloy 6061 under the Action of Positive Mean Stresses

Cyclic deformation and fatigue fracture behavior of Al alloy 6061 were studied over an extensive range of positive mean stress. In the regime of low cyclic stress amplitude, both secondary stage creep rate and fracture process are retarded by decreasing mean stress, but they are accelerated if the cyclic stress ratio is smaller than 0. 75. The superimposition of cyclic compressive stress is found to decrease the creep rate, but it has no effect on the relationship between fatigue life and mean stress. Characteristic patterns on the specimen fracture surfaces, representing the interaction between creep and fatigue and also the mechanism of the fatigue-creep deformation in different mean stress ranges, are discussed analytically. Finally, the concept of equivalent creep rate is proposed and a method to predict fatigue life with positive mean stresses is established and verified using the present experimental results.

FATIGUE CRACK GROWTH UNDER BIAXIAL LOADING

Fatigue crack growth under biaxial loading for long cracks subjected to low cyclic stress levels was investigated. The biaxial stress ratio λ ranging from ‐0.5 to + 1.0 was considered. The strain energy density factor range was used as the criterion for predicting the crack growth rates and crack path. The agreement between prediction and experimental results was reasonable for crack growth rates and marginal for crack paths. The investigation highlighted the inherent difficulties for crack path prediction and indicated the increased sensitivity to initial crack angle and biaxial stress ratio when the biaxial stress ratio approaches unity.

The influence of loading frequency on the fatigue crack propagation behaviour of Al-Zn-Mg alloy at low cyclic stress intensity level in 3.5 wt.% NaCl solution: Chun, Y.-G., Pyun, S.-I and Lee, S.-M. J. Mater. Sci. Lett.10 24 (15 Dec 1991) 1439–1442

The influence of loading frequency on the fatigue crack propagation behaviour of Al-Zn-Mg alloy at low cyclic stress intensity level in 3.5 wt.% NaCl solution: Chun, Y.-G., Pyun, S.-I and Lee, S.-M. J. Mater. Sci. Lett.10 24 (15 Dec 1991) 1439–1442

Effect of cyclic stress on enthalpy relaxation in polycarbonate

AbstractPossible effects of cyclic stress on physical aging in polycarbonate were investigated using differential scanning calorimetry (DSC) measurements. When the enthalpy overshoot by DSC of specimens of different previous thermophysical aging histories is measured as a function of the cyclic stress amplitudes, two characteristic regimes are observed. By correlating with optical microscopic observations, these regimes are identified as the incubation and crazing stages (denoted regimes I and II, respectively). The enthalpy relaxation behavior in Regime I is similar to thermophysical aging, indicating that the glassy structure as a whole is initially shifted to one where molecular mobility is retarded by relatively low amplitude cyclic stress. A strong interaction is also seen between the enthalpy overshoot and previous physical aging. That is, the more the material is previously aged, and the shorter the incubation period, the longer the crazing region is. As a result, brittle failure occurs over a wider load range compared with less aged specimens.

Finite element prediction of the fatigue limit of steel

The fatigue properties of a material are traditionally described by a stress-life (S-N) curve, i . e . , the relationship between the applied cyclic stress and the number of cycles required to cause failure. At low cyclic stresses, the fatigue life extends to 107 to 108 cycles and effectively defines a fatigue endurance limit or fatigue strength. Recently, we described a method tq of predicting this highcycle regime for precipitation-hardened aluminum alloys. The basic concept is that the rate-controlling process is the development of persistent slip bands (PSBs). We represented a PSB by a finite element model and calculated the strain field of the PSB and its environs under the action of a remote stress. We proposed that the elongation of a PSB is controlled by the plastic strain (ep) n e a r its tip and derived a fatigue damage (D) criteflon to describe the weakening of the matrix:

The influence of loading frequency on the fatigue crack propagation behaviour of Al-Zn-Mg alloy at low cyclic stress intensity level in 3.5 wt % NaCl solution

The influence of loading frequency on the fatigue crack propagation behaviour of Al-Zn-Mg alloy at low cyclic stress intensity level in 3.5 wt % NaCl solution

Isothermal and Nonisothermal Fatigue Behavior of a Metal Matrix Composite

The isothermal and nonisothermal fatigue resistance of a metal matrix com posite (MMC) consisting of Ti-15V-3Cr-3Al-3Sn (Ti-15-3) matrix reinforced by 33 v/o continuous SiC fibers was investigated. The fibers were nominally oriented parallel to the specimen axis. Isothermal fatigue tests were performed in air at 300 and 550°C. In non isothermal fatigue tests, each load excursion at 300°C or 550°C was preceded by a tem perature excursion from 300 to 550°C at zero load. The MMC had good isothermal fatigue resistance at 300 and 550°C at low cyclic stresses, with fatigue cracks initiating from fiber-matrix interfaces and foil laminations. At high cyclic stresses, stress relaxation in the matrix reduced isothermal composite fatigue resistance at 550°C. Nonisothermal fatigue loading substantially degraded composite fatigue resistance. This degradation was produced by a thermomechanical fatigue damage mechanism associated with the fiber-matrix interfaces.

ポリカーボネートの回転曲げ疲労試験片におけるき裂の発生と進展

Fatigue fracture behavior of the smooth specimens of polycarbonate was investigated in the range of low cyclic stress. In such fatigue tests, specimens fail during the process of crack initiation and propagation like metal. An experimental program was set up to examine the process of initiation and growth of a surface crack. This was accomplished by careful observations by using the transmitted light photographs of a surface crack on the smooth specimen of polycarbonate during the rotating bending fatigue test at the speed of 2Hz at room temperature. It was shown that the initiation of the surface crack appeared from a very small inclusion near the surface of specimen at 10-30% of the fatigue life. The experiment showed that the surface crack propagation was hindered by the shear bands formed at the both ends of a crack. The final length of surface crack was discussed in terms of a combination of the stress amplitude and the stress intensity factor at the end of crack. From these results, it became evident that the characteristic behavior of the crack initiation and growth in the smooth specimen of polycarbonate was affected significantly by the mechanical properties and yield behavior of the material.

Comparison of Inelastic Response of Type 316 Stainless Steel and HT-9 Materials on the Fusion Reactor First Wall

An analysis based on available materials property data has been performed to compare the inelastic response of first-wall structural materials. The first wall is assumed to be operated under the conditions of the pulse surface heat load, coolant pressure, and bombardment from energetic particles. An axisymmetric inelastic stress analysis calculates the long-term redistribution of the stress in a thin-walled plate element of a cylindrical module that is subjected to membrane load. The plate is free to expand but is constrained from bending. The redistribution is caused by inelastic deformation from irradiation creep and swelling. The present effort has concentrated on the performance of two candidate structural materials, namely, Type 316 stainless steel and HT-9 ferritic steel. The results obtained indicate a lower cyclic stress and a lower mean stress for the HT-9 ferritic steel than for stainless steel under the conditions of interest. Therefore HT-9 ferritic steel is quite attractive for future application of the fusion reactor first wall.

Fatigue Fracture of Ti-5Al-2.5Sn ELI Alloy at Liquid Helium Temperature

S-N curves of Ti-5Al-2.5Sn ELI alloy were determined at liquid helium temperature (4K) for both the base and welded materials.The base material had a longer fatigue life at 4K than at 77K. Welding deteriorated the fatigue properties at 4K. Internal crack initiation was seen at lower cyclic stress in both the base and the welded materials. For the base material, internal crack initiation occurred at 4K, and there were no obvious defects near the initiation sites. On the other hand, fatigue cracks generally initiated at porosity in the weldment. Internal crack initiation is considered one of the causes of reduced fatigue life.High cyclic frequency brought about a large temperature increase in the specimen, making the testing temperature uncertain in fatigue tests at cryogenic temperature.

The Influence of Several Combined High/Low Cycle Loading Parameters on the Crack Growth of a Turbine Disk Alloy

The interactive effects of high- and low-cycle loading on crack growth were evaluated for a high-temperature engine disk alloy (Inconel 718) over a wide range of low-cycle hold times, low-cycle stress intensity factor range (ΔK), high-cycle ΔK, and high-cycle frequency up to 1825 Hz. A series of fatigue crack growth experiments were carried out with a constant low-cycle ΔK and an increasing high-cycle ΔK. The high-frequency ΔK and crack length were measured with sufficient precision to capture details of the transition from the low-cycle dominated regime to the high-cycle dominated regime. In these experiments, the low- and high-cycle dominated growth regimes remained distinct over the entire range of low cycle ΔK, low cycle hold time and frequency investigated. An interactive effect apparent in the low-cycle dominated regime, was a retardation of the low-cycle growth rate when high-cycle loading was applied. The degree of retardation varied with low-cycle ΔK and frequency.

液体ヘリウム温度におけるTi-5Al-2.5Sn ELI合金の疲れ破壊

S-N curves of Ti-5Al-2.5Sn ELI alloy were determined at liquid helium temperature (4K) for both the base and welded materials.The base material had a longer fatigue life at 4K than at 77K. Welding deteriorated the fatigue properties at 4K. Internal crack initiation was seen at lower cyclic stress in both the base and the welded materials. For the base material, internal crack initiation occurred at 4K, and there were no obvious defects near the initiation sites. On the other hand, fatigue cracks generally initiated at porosity in the weldment. Internal crack initiation is considered one of the causes of reduced fatigue life.High cyclic frequency brought about a large temperature increase in the specimen, making the testing temperature uncertain in fatigue tests at cryogenic temperature.

The effect of low-frequency cyclic stresses on the initiation of stress corrosion cracks in X60 line pipe steel in carbonate solutions

The initiation and early stages of growth (< 100 μm) of stress corrosion cracks in X65 line pipe steel have been studied with superimposed low frequency cyclic stresses. The effects of frequency, cyclic amplitude and the level of background tensile stress have been examined.

炭素鋼予ひずみ材の腐食疲労寿命とき裂進展

The effects of tensile prestrain on corrosion fatigue life and crack growth rate in 0.16%C carbon steel were studied under completely reversed plane bending stress in salt water (3.0%NaCl). The conclusions obtained are summarized as follows.(1) The corrosion fatigue strength of the prestrained specimen in the region of high cyclic stress amplitude increased in comparison with that of the annealed specimen and it increased monotonously with increasing prestrain and yield strength of the specimen. On the other hand, the effect of tensile prestrain on corrosion fatigue life disappeared in the region of low cyclic stress amplitude.(2) It was observed by an optical microscope that 80% of cracks were initiated from corrosion pits on the specimen surface under low cyclic stress amplitude. They were independent of the degree of tensile prestrain. The number of cycles to crack initiation, Ni, was not affected by tensile prestrain under the corrosion fatigue of low stress amplitude. In the region of high cyclic stress amplitude, it was found that Ni and the number of cycles during crack propagation increased with increasing tensile prestrain.(3) Micro-cracks on the specimen surface subjected to corrosion fatigue of low cyclic stress amplitude were initiated in the early stage of fatigue life and after that the surface crack density did not change with stress cycling. The growth rate of each surface crack on the specimen depended on the stress intensity factor in the early stage of fatigue life. But in the late stage of life, the growth rate of surface crack per cycle was constant. This constant value varied for each crack and followed normal distribution because of interaction and connection of distributed cracks.

The fracture of constituent particles during fatigue

Localized plastic strains within individual surface grains in aluminum alloy 2219-T851 were measured, using a reference gauge technique, at intervals during fatigue. Consistent with contemporary hypotheses regarding the fracture of constituent particles, we found that critical values of localized plasticity led to particle fracture; however, we also found that cyclic hardening of the surface reduced the rate of accumulation of plasticity from that predicted by previous models. Consequently, fewer numbers of particles were found to fracture in aluminum alloy 2219-T851 during fatigue at low stress amplitudes than theories predict. Various methods to incorporate the cyclic hardening of the surface into a particle fracture model are discussed. One such model is used successfully to predict the reduced number of constituent particles in aluminum alloy 2219-T851 that fracture in specimens which are first conditioned by pre-fatigue at a low cyclic stress amplitude and compared with unconditioned specimens when both are cycled at higher cyclic stress amplitudes.

The influence of overlay coatings on high cycle fatigue of an advanced tantalum carbide strengthened eutectic composite

High cycle fatigue of coated and uncoated NiTaC 3-116A2, a directionally solidified tantalum carbide strengthened eutectic, has been examined over a range of temperatures. In the uncoated condition, the fatigue strength of the alloy increased with temperature from room temperature to 600 °C, but was slightly lower at 900 °C than at 600 °C, which is consistent with the variation in 0.2 pct yield strength with temperature. At room temperature and 600 °C, the influence of several plasma sprayed coatings in reducing the fatigue life was found to be sensitive to the coating composition and alternating stress level. A Ni-20Cr coating resisted fatigue damage on the basis of its ductility and therefore performed better at high cyclic stress levels than at low cyclic stress levels. By contrast, Ni-20Cr-10Al-2Hf-0.1C. and Ni-20Co-20Cr-10Al-2Hf-0.1C coatings were stronger and so exhibited high fatigue strengths at long lives but, because of their limited ductility, cracked readily when tested at high stress levels. At 900 °C the coatings had little effect on fatigue life. On the basis of the present results and the metallographic observations of early crack growth through the coatings, it is concluded that the selection of a mechanically compatible coating requires a knowledge of the fatigue cycle. To ensure a minimum effect on fatigue lives for the anticipated stress cycle, it is necessary to match the coating and substrate mechanical properties as closely as possible.

A dislocation mechanism of grain boundary crack nucleation and growth under low cyclic stresses

A dislocation mechanism of grain boundary crack nucleation and growth under low cyclic stresses

A SYSTEMATIC STUDY OF THE FRACTOGRAPHY OF FATIGUE IN HIGH STRENGTH ALUMINIUM ALLOYS

Abstract A systematic study has been made of the fractography of high stress low cycle fatigue of two high strength aluminium alloys, 2024 T351 and 7075–T6, stressed in pulsating tension at low frequency. Problematic features, one such being multiple fine ridges, were often observed adjacent to the fracture origin. These could not be explained in terms of Stage I or Stage II. As the crack front advanced the ridges began to display fatigue striations. The density of fatigue striations decreased with the progress of the crack, as expected, and well-defined relationships were found between the total number of striations along the length of the crack and the total number of applied cycles. Separate relations were found for smooth and notched samples and for the different alloys. These results can be of significant practical value, although their generality remains to be determined.