Showing papers on "Fatigue limit published in 1995"

Effect of shot peening on residual stress and fatigue life of a spring steel

Abstract: This study describes shot peening effects such as shot hardness, shot size and shot projection pressure, on the residual stress distribution and fatigue life in reversed torsion of a 60SC7 spring steel. There appears to be a correlation between the fatigue strength and the area under the residual stress distribution curve. The biggest shot shows the best fatigue life improvement. However, for a shorter time of shot peening, small hard shot showed the best performance. Moreover, the superficial residual stresses and the amount of work hardening (characterised by the width of the X-ray diffraction line) do not remain stable during fatigue cycling. Indeed they decrease and their reduction rate is a function of the cyclic stress level and an inverse function of the depth of the plastically deformed surface layer.

A high‐cycle fatigue criterion applied in biaxial and triaxial out‐of‐phase stress conditions

Abstract: A high-cycle fatigue criterion suitable for multiaxial non-proportional stress loading is proposed in this paper. The criterion is based on some microscopic considerations related to the crystalline structure of metals. The purpose of the present paper is mainly the application of this criterion in two loading cases : (a) biaxial loads involving two normal stresses or one normal and one shear stress, and (b) triaxial load with two normal stresses and one shear stress. Stress states of these kinds are very common in piping assemblies. Application of the proposed criterion in the case of triaxial loading, where the three stress components are of the same frequency, but out-of-phase, leads to a simple analytical formula. This formula is the equation of a bounding surface that delimits in the space of the above three stresses the safety domain against fatigue crack initiation. A remarkable theoretical result concerns the phase difference of the shear stress, which does not appear in the derived formula. Consequently, according to our proposal the safety domain (i.e. the limiting fatigue endurance) under combined out-of-phase biaxial normal stress loading and torsion is independent of the phase difference of the torsion. Obviously this result holds also for the simpler case of axial load and torsion. On the contrary the phase difference between the two normal stresses has a strong detrimental effect on the fatigue endurance of a metal. As is shown these theoretical conclusions are in good agreement with fatigue limit test data found in the scientific literature.

Effect of microstructure on fatigue and tensile properties of the gamma TiAl alloy Ti-46.5Al-3.0Nb-2.1Cr-0.2W

Abstract: The relationships between the microstructure and tensile and axial load-controlled fatigue properties of the alloy Ti-46.5Al-3.0Nb-2.1Cr-0.2W (atomic per cent) have been studied. Two different microstructures, i.e. duplex (grain size, 20 μm) and fully lamellar (grain size, 300 μm), were produced, through two-step forging and subsequent heat treatments, giving similar yield strengths at room temperature. The fracture strains at room temperature were about 1.1% and 2.9% for the materials with the fully lamellar and the duplex microstructures respectively. At 600 °C, the duplex material shows a 15% higher fatigue strength than that of the fully lamellar material. At this temperature, the gamma alloy of both microstructures reaches high ratios of the fatigue strength at 10 7 cycles to the ultimate tensile strength (UTS), i.e. about 0.95. At 800 °C, the fully lamellar material exhibits higher fatigue strength values above 10 5 cycles, and both microstructures result in a two-stage behavior, in contrast to the test at 600 °C. The second stage features the characteristic conventional fatigue behavior, with a broad amplitude stress range, while the first stage is characterized by a narrow band of fatigue stress levels near the UTS. The fracture modes for the duplex material showed a general trend from transgranular to intergranular failure with increasing temperature. For the fully lamellar material, a change from predominantly translamellar failure to a mixture of inter lamellar and translamellar failure was observed, resulting in a microscopically and macroscopically rough fracture surface. The strain rate sensitivity of the fully lamellar material was negligible in the temperature range tested.

Improvement of the fatigue strength of AISI 4140 steel by an ion nitriding process

Abstract: The influence of plasma nitriding on the fatigue behaviour of AISI 4140 low-alloy steel was investigated under varying process conditions of temperature (500–600 °C), time (1–12 h), heat treatment before ion nitriding (quenched and tempered, normalized) and gas mixture (50% H 2 50% N 2 ). A rotating bending fatigue machine was used to determine the fatigue strength. It was found that the plasma nitriding improves the fatigue strength and increases the fatigue limit depending on the surface hardness of the case depth. The microstructure of surface and diffusion layers was examined by optical microscopy. The fracture surface of specimens and the origin of fatigue cracks were observed by scanning electron microscopy.

Fatigue behaviour of polymer composite sandwich beams

Abstract: The static and flexural fatigue characteristics of foam cored polymer composite sandwich beams are investigated. The skins of the beams are made from hybrid glass-aramid fibres set in epoxy resin and the core materials are linear and cross-linked polymer foams. The applied load in a ten-point configuration approximates a uniformly distributed load throughout the span of the beam which is simply supported at the ends. The testing frequencies are 0.33-0.91 Hz. Failure modes relate to both core shear and skin failure.

Energy Approach to the Fatigue of 60/40 Solder: Part I—Influence of Temperature and Cycle Frequency

Abstract: This study correlates previously published fatigue life data with the hysteresis energy, and compares this to the previous correlation’s with the applied plastic strain. It was found that, while the hysteresis energy could be used to describe the fatigue life, corrections must be made to account for the temperature and strain rate dependence of the flow stress. Because of these factors, it is believed that the plastic strain, and not the hysteresis energy, is the true governing factor in determining the fatigue life. Part I (described here) covers only the influence of temperature and cycle frequency for symmetric cycling. Part II, to be published, will deal with hold time and asymmetric cycling.

Grain size effects on the mechanical properties of some squeeze cast light alloys

Abstract: Mechanical property-grain size relationships have been examined for squeeze cast Al-4.5% Cu alloy, for an aluminium alloy with a composition corresponding to wrought 7010, and for a magnesium alloy AZ91. The general trend of the results obtained showed that the tensile properties and the fatigue strength improved as grain size decreased and the reverse was found to be the case for the fatigue crack propagation resistance and fracture energy of these castings. However, the results also showed that no simple common relationship existed between grain size and the tensile properties of the different alloys. The results are discussed in respect of their microstructures.

Mechanisms of fretting fatigue

Abstract: Fretting fatigue is one of the most important phenomena for inducing a dramatic reduction of fatigue strength and consequently, of unexpected failure accidents. There are numerous works on fretting fatigue. The objective of the present review is to clarify fretting fatigue mechanisms based on the vast amount of research work. A fretting fatigue crack is initiated at the highest point of tangential force on a contact surface in the very early stage of fatigue life. The tangential force is the dominant factor in enhancing crack initiation and propagation and hence, in reducing fatigue strength. Further studies on short crack and mixed-mode crack growth behavior in fretting fatigue are needed.

Preliminary Evaluation of Hybrid Titanium Composite Laminates

Abstract: In this study, the mechanical response of hybrid titanium composite laminates (HTCL) was evaluated at room and elevated temperatures. Also, the use of an elastic-plastic laminate analysis program for predicting the tensile response from constituent properties was verified. The improvement in mechanical properties achieved by the laminates was assessed by comparing the results of static strength and constant amplitude fatigue tests to those for monolithic titanium sheet. Two HTCL were fabricated with different fiber volume fractions, resin layer thicknesses, and resins. One panel was thicker and was more poorly bonded in comparison to other. Consequently, the former had a lower tensile strength, while fewer cracks grew in this panel and at a slower rate. Both panels showed an improvement in fatigue life of almost two orders of magnitude. The model predictions were also in good agreement with the experimental results for both HTCL panels.

Fatigue Behavior of Steel Fiber Reinforced Concrete in Uniaxial and Biaxial Compression

Abstract: The fatigue strength and behavior of plain and fiber concrete are investigated. Seventy-two steel reinforced concrete specimens, with 1 volume percent of 25-mm (1-in)-long fibers, were tested under compression fatigue loading. The S-N curves were obtained under four stress ratios of 0 (uniaxial), 0.2, 0.5, and 1.0, resulting in a series of fatigue stress envelopes for fiber concrete. Deformations in all three principal directions were measured. The S-N curves, strength envelopes, failure modes, and cyclic deformations of fiber concrete are compared to those of plain concrete. It is found that addition of fibers does not increase the endurance limit but is beneficial above the endurance limit in the low-cycle region. Furthermore, adding fibers to concrete increases its ductility, and changes failure modes from splitting-type to faulting-type. All these phenomena can be explained by the development of microcracks in the cement-sand matrix and at the bond surfaces between the matrix and aggregates.

Relation between pore structure and fatigue behavior in sintered iron-copper-carbon

Abstract: A statistically designed study of the high cycle fatigue of an iron-copper-carbon alloy (Fe-2w/oCu-0.8w/oC or FC-0208) was performed to isolate the controlling pore microstructure factors with respect to endurance strength. The study included four variations in powder characteristics and three variations in density, with a sintering cycle of 30min at 1120 o C in an atmosphere of 11v/o hydrogen and 89v/o nitrogen. Twenty samples were fatigue tested for each of the 12 powder density test combinations to assess the fatigue strength distribution. Quantitative metallography provided insight to the controlling microstructure effects on fatigue life. Regression analysis between the various mechanical properties and microstructure parameters shows that the fatigue endurance strength is linked to the porosity, average pore curvarture, and separation distance between pores. The powder characteristics optimal for fatigue resistance depend in the density level. At the lower density (6.65g/cm 3 ), endurance is enhanced by increasing the proportion of small particles, thereby generating smaller, smoother pores in sintering. But an increased proportion of large particles is desirable for fatigue resistance at the higher densities (7.15g/cm 3 )

In vitro fatigue testing of a dental bonding system on enamel

Abstract: The aim of this study was to investigate the in vitro fatigue behavior of a dental bonding system [ScotchBond Multi Purpose (SBMP)] by using a cyclic shear loading test. Cylinders of a light-cured hybrid composite resin (Z100) were formed on, and bonded with SBMP to, the flattened, acid-etched enamel surface of human teeth. Thirty-two samples (eight per group) were used to determine the 1-h, 1-day, 7-day, and 30-day shear bond strengths. For the shear fatigue test, the stress was cycled, at a rate of 1Hz, between 0 and a preset value in the 12–28 MPa range (from -3 SD to + 1 SD around the 24-MPa mean bond strength of the 1-h specimens). The applied stress and the number of cycles to failure were recorded for each of the 75 samples tested (15 samples per set-stress). Three stress versus number of cycles curves (low, median, and high S-N curves) were obtained and an approximate endurance limit of 10 MPa was identified for the SBMP bonding system. The data obtained explain, in part, the discrepancy between in vitro and in vivo results and underline the difficulty of predicting the in vivo behavior based on results of in vitro bond strength tests alone. Shear fatigue test data, however, could provide a better insight into the long-term in vivo behavior of a dental bonding system. © 1995 John Wiley & Sons, Inc.

The development of an accurate model for the fatigue assessment of doubly curved cracks in tubular joints

Abstract: Fatigue tests on tubular joints have shown that as a crack propagates through the chord wall, it curves under the weld toe. This produces, at the brace-chord intersection, a doubly curved semi-elliptical crack emanating from the weld toe. A doubly curved crack in a tubular joint is a very complex geometry which has proved to be difficult to model. In consequence, previous work on the evaluation of stress intensity factors in tubular joints adopted a simplified approach, ignoring the crack curvature under the weld toe. However, in the absence of benchmark solutions, the effects of any modelling approximation on accuracy are impossible to quantify. To address this problem and as part of the research on fatigue assessment methodologies, a technique which is able to accurately model doubly curved cracks in tubularT-joints has been developed at University of Wales, Swansea. This paper describes a detailed account of the generation of the finite element model and the procedure for evaluating the stress intensity factor solutions. The validation results are also presented to demonstrate the reliability of the model developed.

Fatigue Strength of Adhesive/Rivet Combined Lap Joints

Abstract: Adbesive/rivet combined bonding has attracted special interest recently as a joining technique of highstrength steel because of its high joint efficiency. In the present study, the strength characteristics of adhesive/rivet combined lap joints were investigated. To darify bonding conditions capable of improving the fatigue strength of combined joints, fatigue tests were conducted on the rivet, adhesive and adhesive/rivet combined joints with different lap widths, adhesive and rivet strengths. Furthermore, to compare fatigue crack initiation and propagation behavior of the adhesive joint with that of the combined joint, the strain changes were measured by strain gauges bonded onto the adherend plate near the lap end. The results indicate that the fatigue strength of adhesive joints can be improved through combination with rivets of nearly equal or slightly higher fatigue strength than the adhesive joint. Furthermore, we also confirmed that fatigue cracks propagate more gradually in combined joints...

Fatigue behavior of a plain-woven glass fabric laminate under tension/torsion biaxial loading

Abstract: Fatigue strength and stress-strain response of a plain-woven glass fabric laminate subjected to pulsating tension/pulsating torsion biaxial loading were investigated. Biaxial loads were proportionally applied. S-N curves at different biaxial stress ratios were obtained. For the S-N diagram, the normalized fatigue strength defined by the ratio between the cyclic biaxial stress and the static strength, was used. All data roughly locate on a slightly wide band on the normalized S-N diagram, although the tendency that the slope of the S-N curve becomes low with an increase of the shear stress component is distinguishable. Modulus decays both in tension and shear with respect to loading cycles under biaxial loading are almost the same as those under uniaxial tension and pure torsion loadings, respectively. The direction of matrix cracks is dependent on the principal stress direction under biaxial loading although it does not coincide with the direction of the principal plane.

Fatigue-Life Prediction Methodology Using a Crack-Closure Model

Abstract: This paper reviews the capabilities of a plasticity-induced crack-closure model and life-prediction code, FASTRAN, to predict fatigue lives of metallic materials using small-crack theory. Crack-tip constraint factors, to account for three-dimensional state-of-stress effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range ({Delta}K{sub eff}) under constant-amplitude loading. Some modifications to the {Delta}K{sub eff}-rate relations were needed in the near-threshold regime to fit small-crack growth rate behavior and endurance limits. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched specimens made of several aluminum alloys and a titanium alloy under constant-amplitude and spectrum loading. Fatigue lives were calculated using the crack-growth relations and microstructural features like those that initiated cracks for the aluminum alloys. An equivalent-initial-flaw-size concept was used to bound the fatigue lives for the titanium alloy. Results from the tests and analyses agreed well.

The effect of grain size on fatigue crack growth in an aluminium magnesium alloy

Abstract: Fully reversed uniaxial fatigue tests were performed on aluminium magnesium alloy Al 5754 with four different grain sizes in order that the effect of grain size on fatigue crack growth could be examined. Surface cracks were monitored by a plastic replication technique. Fatigue strength was shown to improve with a decrease in grain size. The endurance stress is a function of the inverse square root of the grain size and is described empiricdty by a Hall-Petch type relation. The effect of grain size on fatigue crack growth is most significant when the crack length is of the order of the microstructure. Fluctuations in the growth rate of microstructurally short cracks are most marked in a fine grained microstructure and may be related to the need to transfer slip to adjacent grains. Crack path deviation is greatest in the coarsest grained microstructure and SEM fractography reveals a more pronounced crack surface roughness in the coarser grained alloy than in the finer grained alloy.