Showing papers on "Fatigue limit published in 2008"

Mechanical biocompatibilities of titanium alloys for biomedical applications.

Abstract: Young's modulus as well as tensile strength, ductility, fatigue life, fretting fatigue life, wear properties, functionalities, etc., should be adjusted to levels that are suitable for structural biomaterials used in implants that replace hard tissue. These factors may be collectively referred to as mechanical biocompatibilities. In this paper, the following are described with regard to biomedical applications of titanium alloys: the Young's modulus, wear properties, notch fatigue strength, fatigue behaviour on relation to ageing treatment, improvement of fatigue strength, fatigue crack propagation resistance and ductility by the deformation-induced martensitic transformation of the unstable beta phase, and multifunctional deformation behaviours of titanium alloys.

Fatigue Properties of Highly Oriented Polypropylene Tapes and All-Polypropylene Composites:

Abstract: This paper describes the fatigue behaviour of newly developed all-polypropylene (all-PP) tapes and composites, with reference to the composite processing conditions, testing temperature and making a comparison with commercial alternatives. All-PP tapes are highly oriented and their failure behaviour follows that of other highly oriented polymers. All-PP woven composites fail ultimately due to PP tape failure. However, this failure mode is accompanied by delamination of fabrics in the woven structure. Consolidation pressure plays a decisive role in controlling the interlaminar properties and hence the delamination resistance and furthermore the fatigue limit of the composite. Comparison of all-PP woven composites with commercial alternatives based on glass and natural fibres reveals the excellent relative performance of all-PP composites under fatigue loads. Fatigue properties of all-PP composites are however sensitive to the testing temperature, and elevated temperatures can lead to a rapid reduction of the fatigue resistance of these all-polymer systems.

Crack initiation mechanism of extruded AZ31 magnesium alloy in the very high cycle fatigue regime

Abstract: Ultrasonic fatigue testing as well as conventional fatigue testing has been conducted on commercial extruded AZ31 magnesium alloy. The S-N curve for this alloy appears to have a continuous decreasing trend in the very high cycle regime. Fatigue strength at 109 cycles is 88.7 +/- 4.1 MPa. The ratio of endurance limit at one billion cycles to the tensile strength (sigma(-1)/sigma(b)) is 0.301. Fatigue failure mainly originated from the specimen surface or near surface, where deformation twins were found after cyclic loading. Fatigue cracks were observed along twin bands. Based on cyclic deformation irreversibility caused by twinning, fatigue damage mechanisms are proposed in terms of surface/near surface crack initiation. (C) 2008 Elsevier B.V. All rights reserved.

Microstructure and fracture characteristics of spot-welded DP600 steel

Abstract: Microstructural characterization, microhardness tests, tensile shear tests, and fatigue tests of spot-welded hot dipped galvanized (HDG) DP600 steel were conducted. The effects of weld expulsion on the microstructural characteristics, mechanical properties, and fracture modes were investigated using optical microscopy, image analysis and scanning electron microscopy (SEM) together with energy dispersive X-ray spectroscopy (EDS). The hardness in the weld nugget was observed to be over two times higher than that in the base metal due to the formation of lath martensite. Fatigue tests showed a slightly lower fatigue limit for the joints with the expulsion. The crack initiation was observed to occur basically at the boundary of the weld nugget and at the interface between two sheets. The causes for the interfacial fracture (IF) were discussed in relation to the microstructure and weld parameters.

Effect of casting imperfections on the fatigue life of 319-F and A356-T6 Al–Si casting alloys

Abstract: Casting imperfections, such as porosity, in cast aluminum components greatly influence their fatigue properties The effect of porosity on the fatigue life of 319-F and A356-T6 aluminum alloys was studied, where the porosity characteristics on the fracture surfaces of fatigue-tested samples were examined using SEM and image analysis The results show that porosity has the greatest detrimental effect on fatigue life: 92% of all tested samples fractured as a result of porosity which acted as the main crack initiation site In the absence of casting imperfections, other microstructural aspects such as slip bands may be held responsible (4%) Porosity was investigated in terms of the pore size at the sample fracture surface It was found that fatigue life decreases as the size of the surface pore increases A comparison was made between the fatigue behavior of low-pressure-permanent mold-cast 319 alloy and lost foam-cast A356-T6 alloy The results show that the 319 alloy provides greater fatigue strength compared to the 356 alloy, which may be explained by taking into consideration the nature of the surface porosity (single pore versus multiple shrinkage pores) that initiated the fatigue crack in the two alloys The microstructural characteristics are of secondary importance in this regard

Microstructure and mechanical properties of pearlitic gray cast iron

Abstract: Mechanical properties of structural materials strongly depend on their microstructure. The aim of this work, which essentially has an experimental character, is to correlate microstructure and mechanical properties of pearlitic gray cast iron. Because of the industrial interest behind to this work, the material under study is extracted from identical sand-cast parts produced by three different foundries. Laboratory tests show that mechanical properties, as tensile and fatigue strength, of gray cast iron of the same grade vary from foundry to foundry. A noticeable statistical data scatter is found also, depending on the location of the castings from which the specimens are machined. The cause of this behaviour is ascribed to the inhomogeneity that characterizes the gray cast iron microstructure. Metallographic sections are observed to quantitatively measure the relevant microstructural parameters, as graphite lamellas morphology, eutectic cell size and inclusions content. Results are correlated to the measured mechanical properties: reduced graphite content increases the tensile and the fatigue strength, both fine eutectic cells structure and high eutectic phosphide percentage improve the fatigue properties.

Strain-Controlled Low-Cycle Fatigue Properties of a Newly Developed Extruded Magnesium Alloy

Abstract: To reduce fuel consumption and greenhouse gas emissions, magnesium alloys are being considered for automotive and aerospace applications due to their low density, high specific strength and stiffness, and other attractive traits. Structural applications of magnesium components require low-cycle fatigue (LCF) behavior, since cyclic loading or thermal stresses are often encountered. The aim of this article was to study the cyclic deformation characteristics and evaluate LCF behavior of a recently developed AM30 extruded magnesium alloy. This alloy exhibited a strong cyclic hardening characteristic, with a cyclic strain-hardening exponent of 0.33 compared to the monotonic strain-hardening exponent of 0.15. With increasing total strain amplitude, both plastic strain amplitude and mean stress increased and fatigue life decreased. A significant difference between the tensile and compressive yield stresses occurred, leading to asymmetric hysteresis loops at high strain amplitudes due to twinning in compression and subsequent detwinning in tension. A noticeable change in the modulus was observed due to the pseudoelastic behavior of this alloy. The Coffin–Manson law and Basquin equation could be used to describe the fatigue life. At low strain ratios the alloy showed strong cyclic hardening, which became less significant as the strain ratio increased. The lower the strain ratio, the lower the stress amplitude and mean stress but the higher the plastic strain amplitude, corresponding to a longer fatigue life. Fatigue life also increased with increasing strain rate. Fatigue crack initiation occurred from the specimen surface and crack propagation was mainly characterized by striation-like features. Multiple initiation sites at the specimen surface were observed at higher strain amplitudes.

Improving performance of GFRP/aluminum single lap joints using bolted/co-cured hybrid method

Abstract: The present study proposes a bolted/co-cured hybrid joining method, and experimentally investigates the joint strength The bolted/co-cured hybrid joints combine co-cured adhesive joints and bolted joints without damaging reinforcing fibers The method allows for low scatter strength in static and fatigue loading for easily manufactured co-cured joints Testing of the static tensile lap-shear and fatigue strengths is performed using aluminum alloy A5052-F and knit fabric glass epoxy composites The results show that the hybrid joints have 184 times higher maximum shear strength and a quarter of the standard deviation compared with conventional co-cured joints Furthermore, less stress concentration and undamaged glass fibers in the hybrid joints contribute to a much higher fatigue strength than that of the bolted joint

Effect of microstructure on the fatigue of hot-rolled and cold-drawn NiTi shape memory alloys

Abstract: We present results from a systematic study linking material microstructure to monotonic and fatigue properties of NiTi shape memory alloys We consider Ni-rich materials that are either (1) hot rolled or (2) hot rolled and cold drawn In addition to the two material processing routes, heat treatments are used to systematically alter material microstructure giving rise to a broad range of thermal, monotonic and cyclic properties The strength and hardness of the austenite and martensite phases initially increase with mild heat treatment (300 °C), and subsequently decrease with increased aging temperature above 300 °C This trend is consistent with transmission electron microscopy observed precipitation hardening in the hot-rolled material and precipitation hardening plus recovery and recrystallization in the cold-drawn materials The low-cycle pseudoelastic fatigue properties of the NiTi materials generally improve with increasing material strength, although comparison across the two product forms demonstrates that higher measured flow strength does not assure superior resistance to pseudoelastic cyclic degradation Fatigue crack growth rates in the hot-rolled material are relatively independent of heat treatment and demonstrate similar fatigue crack growth rates to other NiTi product forms; however, the cold-drawn material demonstrates fatigue threshold values some 5 times smaller than the hot-rolled material The difference in the fatigue performance of hot-rolled and cold-drawn NiTi bars is attributed to significant residual stresses in the cold-drawn material, which amplify fatigue susceptibility despite superior measured monotonic properties

High-cycle fatigue of hybrid carbon nanotube/glass fiber/polymer composites

Abstract: Glass fiber polymer composites have high strength, low cost, but suffer from poor performance in fatigue. Mechanisms for high-cycle (>104 cycles) fatigue failure in glass fiber composites consist primarily of matrix-dominated damage accumulation and growth that coalesce and propagate into the fibers resulting in ultimate fatigue failure. This investigation shows that the addition of small volume fractions of multi-walled carbon nanotubes (CNTs) in the matrix results in a significant increase in the high-cycle fatigue life. Cyclic hysteresis measured over each cycle in real time during testing is used as a sensitive indicator of fatigue damage. We show that hysteresis growth with cycling is suppressed when CNTs are present with resulting longer cyclic life. Incorporating CNTs into the matrix tends to inhibit the formation of large cracks since a large density of nucleation sites are provided by the CNTs. In addition, the increase in energy absorption from the fracture of nanotubes bridging across nanoscale cracks and nanotube pull-out from the matrix is thought to contribute to the higher fatigue life of glass composites containing CNTs. High-resolution scanning electron microscopy suggests possible mechanisms for energy absorption including nanotube pull-out and fracture. The distributed nanotubes in the matrix appear to inhibit damage propagation resulting in overall improved fatigue strength and durability.

Universality behind Basquin's Law of Fatigue.

Abstract: Basquin's law of fatigue states that the lifetime of the system has a power-law dependence on the external load amplitude, tf approximately sigma 0- alpha, where the exponent alpha has a strong material dependence. We show that in spite of the broad scatter of the exponent alpha, the fatigue fracture of heterogeneous materials exhibits universal features. We propose a generic scaling form for the macroscopic deformation and show that at the fatigue limit the system undergoes a continuous phase transition. On the microlevel, the fatigue fracture proceeds in bursts characterized by universal power-law distributions. We demonstrate that the system dependent details are contained in Basquin's exponent for time to failure, and once this is taken into account, remaining features of failure are universal.

Uniaxial ratchetting and low-cycle fatigue failure of the steel with cyclic stabilizing or softening feature

Abstract: The ratchetting behaviour and low-cycle fatigue failure, as well as the interaction between them were investigated by uniaxial cyclic stressing tests for 42CrMo steel with annealing or tempering heat-treatment. The ratchetting strain and fatigue life of the material were measured in the uniaxial cyclic stressing with different loading levels. The effects of mean nominal stress, nominal stress amplitude and stress ratio on the ratchetting strain and final failure life were discussed. Simultaneously, the variations of responded strain amplitude with the number of cycles were illustrated to discuss the interaction between ratchetting and low-cycle fatigue failure behaviour. The experimental results show that the ratchetting and fatigue failure behaviours of the annealed 42CrMo steel are different from those of the tempered steel, since different cyclic softening/hardening features are caused by different heat treatments, i.e., the annealed 42CrMo steel presents cyclic stabilizing feature, but the tempered steel is cyclic softening. Two kinds of failure modes (i.e., ratchetting failure with obvious necking due to large ratchetting strain and fatigue failure due to low-cycle fatigue with nearly constant responded strain amplitude) take place, depending on mean nominal stress, nominal stress amplitude and stress ratio of uniaxial cyclic stressing, as well as the heat-treatment.

About the role of residual stresses and surface work hardening on fatigue ΔKth of a nitrided and shot peened low-alloy steel

Abstract: Nitriding and shot peening are surface treatments widely used to improve the fatigue strength of mechanical and structural components. Both treatments enhance the mechanical properties of the surface layer of material: nitriding mainly by means of chemical transformations and formation of a very hard case, shot peening mainly due to compressive residual stress field in the sub-surface layer of material. The combined application of nitriding and shot peening has not been adequately investigated in literature and the known data do not allow to define a general criterion to assess if and when shot peening application after nitriding can be useful. In this paper the effect of nitriding plus shot peening on fatigue strength of a low-alloy steel is investigated by means of experimental tests carried out on specimens containing a micro-hole, acting as a pre-crack. To analyse the role played by shot peening induced residual stresses, a series of specimens was heat-treated after shot peening, being the aim the partial removal of residual stresses without strongly modifying the mechanical properties of the surface layer of material. After a critical discussion of the results in terms of residual stress, micro-hardness trend and fatigue strength, an original fracture mechanics based approach is proposed to predict the threshold value of the stress intensity factor of nitrided and shot peened steels. The results are in good agreement with the experimental evidence.