Showing papers on "Fatigue limit published in 2009"

Effect of strain ratio and strain rate on low cycle fatigue behavior of AZ31 wrought magnesium alloy

Abstract: Magnesium alloys are increasingly used in automotive and aerospace industries for weight reduction and fuel economy improvement. Low cycle fatigue (LCF) behavior of these alloys is an important consideration for the structural applications. The objective of the present investigation was to identify influences of strain ratio and strain rate on cyclic deformation characteristics and fatigue life of an AZ31 extruded alloy. As the strain ratio decreased, stronger cyclic hardening rate, more asymmetric hysteresis loop, smaller stress amplitude, lower mean stress, and higher initial plastic strain amplitude were observed due to increasing compressive stresses. This was considered to be associated with the twinning during cyclic deformation in the compressive phase, and detwinning in the tensile phase. The residual twins acting as barriers to dislocation slip and pile-up were considered to be the main cause for the occurrence of cyclic hardening. Fatigue life increased with decreasing strain ratio and increasing strain rate. Fatigue crack initiation occurred at the specimen surface due to the presence of larger grains near the surface, and fatigue crack propagation was characterized by a mixture of striations and dimple-like ductile fracture features.

The effects of deep rolling and shot peening on fretting fatigue resistance of Aluminum-7075-T6

Abstract: The effect of shot peening (SP) and deep rolling (DR) on fretting fatigue life of Aliminum7075-T6 is investigated in this work. The results show that fretting fatigue reduces the normal fatigue life by 67%. For low cycle fatigue, SP while being superior to DR, increases the fretting fatigue life by 300% for the specimens tested in this work. For high cycle fatigue, however, the effect of DR on fretting fatigue resistance is more profound than SP such that an increase of about 700% is observed for DR. Numerical simulation of SP and DR is also performed in this work. The results indicate that DR gives rise to a larger compressive residual stress than SP under the conditions considered in the simulations. This agrees quantitatively with the experimental results obtained from high cycle fatigue tests.

Fatigue behavior of high-Mn TWIP steels

Abstract: Fatigue behavior of three high-Mn TWIP steels with the grain sizes between 4.5 and 55 μm were investigated in reversed plane bending in the high-cycle regime. Crack initiation and propagation stages were examined by optical, scanning electron and atom force microscopy. It was found that the fatigue limit, which is the stress amplitude leading to fatigue life beyond 2 × 10 6 cycles, is quite high, about 400 MPa, for these steels. This value is between 0.42 and 0.48 of the tensile strengths, similarly as for austenitic stainless steels. Refining the grain size still increases the fatigue resistance. Neither martensite is formed nor mechanical twinning takes place in the course of cycling, but intense slip bands are created with extrusions and intrusions. Fatigue cracks tend to nucleate at an early stage of fatigue life, and preferentially on grain and twin boundaries, especially in the intersection sites of slip bands and boundaries, besides slip bands. However, the crack propagation takes place mainly transgranularly creating ductile striations and protrusions on fracture surfaces.

A Consideration of Allowable Equivalent Stresses for Fatigue Design of Welded Joints According to the Notch Stress Concept with the Reference Radii r ref = 1.00 and 0.05 mm

Abstract: In the literature, allowable stresses (FAT-values) for the fatigue design of welded joints are established according to the notch stress concept with the reference radii rref = 1.00 mm for thick connections (t ≥ 5 mm) and 0.05 mm for thin connections (t < 5 mm). However, it is not clear for which strength hypothesis they are valid. As local equivalent stresses may be calculated by the principal stress or von Mises hypotheses, it is necessary to distinguish between the applied hypotheses. The FAT-values according to the principal stress and von Mises hypotheses are compiled for steel, aluminium and magnesium for the reference radii rref = 1.00 and 0.05 mm. The allowable stresses are derived from normal as well as from shear stresses. However, the values derived from pure normal loading (axial or bending) and from pure torsion are not compatible when the principle stress or the von Mises hypotheses are applied. Therefore, in case of biaxial loading, the stated incompatibility between the values obtained from different loading modes should be overcome by the Gough-Pollard relationship.

Effect of hardness on multiaxial fatigue behaviour and some simple approximations for steels

Abstract: Constant-amplitude in-phase and 90° out-of-phase axial-torsional fatigue tests were conducted on tubular specimens made from a medium-carbon steel with three hardness levels obtained from normalizing, quenching and tempering and induction hardening to find the effect of hardness on multiaxial fatigue behaviour. In addition, the same loadings were applied on the normalized solid specimens to investigate the effect of specimen geometry on multiaxial fatigue life. Similar fatigue life variation as a function of hardness was found for in-phase and out-of-phase loadings, with higher ductility beneficial in low-cycle fatigue (LCF) and higher strength beneficial in high-cycle fatigue (HCF). Multiaxial fatigue data were satisfactorily correlated for all hardness levels with the Fatemi-Socie parameter. Furthermore, in order to predict multiaxial fatigue life of steels in the absence of any fatigue data, the Roessle-Fatemi hardness method was used. Multiaxial fatigue lives were predicted fairly accurately using the Fatemi-Socie multiaxial model based on only the hardness level of the material. The applicability of the prediction method based on hardness was also examined for Inconel 718 and a stainless steel under a wide range of loading conditions. The great majority of the observed fatigue lives were found to be in good agreement with predicted lives.

Solution to the problem of the poor cyclic fatigue resistance of bulk metallic glasses

Abstract: The recent development of metallic glass-matrix composites represents a particular milestone in engineering materials for structural applications owing to their remarkable combination of strength and toughness. However, metallic glasses are highly susceptible to cyclic fatigue damage, and previous attempts to solve this problem have been largely disappointing. Here, we propose and demonstrate a microstructural design strategy to overcome this limitation by matching the microstructural length scales (of the second phase) to mechanical crack-length scales. Specifically, semisolid processing is used to optimize the volume fraction, morphology, and size of second-phase dendrites to confine any initial deformation (shear banding) to the glassy regions separating dendrite arms having length scales of ≈2 μm, i.e., to less than the critical crack size for failure. Confinement of the damage to such interdendritic regions results in enhancement of fatigue lifetimes and increases the fatigue limit by an order of magnitude, making these “designed” composites as resistant to fatigue damage as high-strength steels and aluminum alloys. These design strategies can be universally applied to any other metallic glass systems.

Effect of aging treatment on fatigue behaviour in extruded AZ61 and AZ80 magnesium alloys

Abstract: The effect of aging treatment on fatigue behaviour in extruded Mg alloys AZ61 and AZ80 was studied. Rotary bending fatigue tests were performed using as-received and aged specimens. Two different aging conditions, T6 and T5, were evaluated. Mg 17 Al 12 precipitated near grain boundaries and within grains by aging treatments. In both materials, the T6-treated specimens showed significantly lower fatigue strength than the as-received specimens, which was more remarkable in AZ80, while the T5-treated specimens exhibited slightly higher fatigue strength in AZ80. It was indicated that the effect of aging treatment on fatigue strength was mainly attributed to the change in fatigue crack initiation mechanism.

Enhanced fatigue strength of commercially pure Ti processed by severe plastic deformation

Abstract: The high-cycle fatigue behavior of smooth and notched samples of ultrafine-grained titanium prepared by severe plastic deformation is compared with the corresponding properties of conventional titanium. It is shown that the combination of high strength and enhanced ductility of ultrafine-grained titanium lead to an increase of the fatigue endurance limit. Using a combination of equal-channel angular pressing and subsequent thermal and mechanical treatment, it was possible to increase the fatigue endurance limit of commercial-purity titanium by a factor of 1.5. Furthermore, it is shown that post-deformation annealing can additionally enhance the ductility of the ultrafine-grained Ti and lower fatigue-notch sensitivity particularly in comparison with Ti-6Al-4V.

Fatigue Behaviour of Welded High-Strength Steels after High Frequency Mechanical Post-Weld Treatments

Abstract: Investigations have been carried out regarding the fatigue strength of welded details improved by high frequency treatment methods. These methods increase the fatigue strength by cold forming of the surface, so that the weld toe is smoothened, the surface hardness is increased and compressive residual stresses are present up to a depth of 1 to 1.5 mm. In this paper, the surface residual stresses at the weld toe are investigated before and after different loading conditions and for different steel grades. It is shown that only high tensile fatigue loading can lead to a slight residual stress relaxation for low-strength steels. The fatigue crack behaviour is analysed in more detail. The crack propagation rates with and without surface treatment are investigated, using defined lines of rest. The study shows that crack propagation in the edge layers is reduced. Several cracks may start in the UIT-treated zone but will not propagate further, until one final crack, often close to the edge of the specimens, will lead to failure. The results of fatigue tests for butt welds and longitudinal stiffeners improved by high frequency hammer peening are presented. The fatigue strength is seen to be doubled. For high-strength steels, the improvement at different load levels is identical, but for lower-strength steels, high stress ranges lead to reduced improvement. This fact results in flatter SN-curves and can be explained by the lower maximum of residual stresses and residual stress relaxation.

The effect of bainite morphology on the mechanical properties of a high bainite dual phase (HBDP) steel

Abstract: 4340 steel bars were austenitized at 850 °C for 1 h followed by heating at 700 °C for 90 min and quenching into a salt bath at the temperature range of 300–450 °C for 1 h to obtain dual structures with 34 vol.% fraction ferrite and various bainite morphologies. SEM studies showed that by increasing the austempering temperature, bainite morphology varies from lower to upper bainite. Tensile, impact and hardness tests revealed that increasing the austempering temperature from 300 to 400 °C leads to a reduction in yield and ultimate tensile strength, hardness, uniform and total elongation and impact energy. But in dual phase steel produced by austempering at 450 °C, yield and tensile strength and hardness increased and severe reduction in total elongation and impact energy obtained. Fractography of tensile specimens showed brittle behavior for this austempering temperature. Fatigue test results showed that fatigue limit decreases with increasing austempering temperature from 300 to 400 °C. Finally, fractography studies showed cleavage fracture at the surface of fatigue specimens austempered at 400 °C, which confirms the tendency to brittle behavior.

Microstructure and fatigue properties of the ultrafine-grained AM60 magnesium alloy processed by equal-channel angular pressing

Abstract: This paper reports on the microstructures and fatigue properties of the ultrafine-grained AM60 magnesium alloy processed by equal-channel angular pressing (ECAP) at various temperatures. After ECAP processing, samples exhibited an increase in fatigue endurance limit, which correlates well with a decrease in grain size. For the lowest ECAP temperature, the mean grain size is as small as 1 μm which leads to an increase in the fatigue endurance limit by 70% in comparison to coarse-grained alloy. The temperature of ECAP not only governs the grain size but also the volume fraction of precipitates, thus affecting the probability of twinning and grain growth after fatigue treatment.

Practical Application of Viscoelastic Continuum Damage Theory to Asphalt Binder Fatigue Characterization

Abstract: This paper describes how the ability of the binder phase of an asphalt mixture to resist fatigue damage can have a profound effect on the service life of an asphalt pavement. An accurate and efficient test method for evaluating the fatigue characteristics of asphalt binder has thus far been elusive due to excessive time requirements or equipment limitations. This paper presents a new attempt to use existing testing procedures (the Dynamic Shear Rheometer) to estimate fatigue resistance in a relatively short period of time. The test involves subjecting binder specimens to a monotonic constant strain-rate shear testing at intermediate temperatures. To evaluate fatigue performance a parameter is derived from the test results by integrating the area under the stress-strain curve to the maximum stress value. Initial validation is carried out by testing binders used in the FHWA Accelerated Loading Facility (ALF). The results collected at the same temperature used in the ALF are shown to accurately rank the fatigue resistance of pavements tested in full scale. The data collected in the new test can also be used for a fundamental analysis procedure based on the Viscoelastic Continuum Damage (VECD) theory to estimate fatigue of binders under varying levels of traffic and pavement conditions. The procedure is modeled after the extensive work already published on mixtures. There are, however, challenges in applying the theory to modified binders that are believed related to the non-linear behavior of some modified binders. These challenges can prove to be critical in accurately applying the fundamental approach to modified binders.