Showing papers on "Fatigue limit published in 2002"

Metal fatigue : effects of small defects and nonmetallic inclusions

Abstract: Chapter headings. Mechanism of fatigue in the absence of defects and inclusions. Stress concentration. Notch effect and size effect. Effect of size and geometry of small defects on the fatigue limit. Effect of hardness HV on fatigue limits of materials containing defects, and fatigue limit prediction equations. Effects of nonmetallic inclusions on fatigue strength. Bearing steels. Spring steels. Tool steels : effect of carbides. Effects of shape and size of artificially introduced alumina particles on 1.5Ni-Cr- Mo (En24) steel. Nodular cast iron. Influence of Si-phase on fatigue properties of aluminium alloys. Ti alloys. Torsional fatigue. The mechanism of fatigue failure of steels in the ultralong life regime of N >107 cycles. Effect of surface roughness on fatigue strength.

A bi-parametric Wöhler curve for high cycle multiaxial fatigue assessment

Abstract: This paper presents a method for estimating high-cycle fatigue strength under multiaxial loading conditions. The physical interpretation of the fatigue damage is based on the theory of cyclic deformation in single crystals. Such a theory is also used to single out those stress components which can be considered significant for crack nucleation and growth in the so-called Stage I regime. Fatigue life estimates are carried out by means of a modified Wohler curve which can be applied to both smooth and blunt notched components, subjected to either in-phase or out-of-phase loads. The modified Wohler curve plots the fatigue strength in terms of the maximum macroscopic shear stress amplitudes, the reference plane - where such amplitudes have to be evaluated - being thought of as coincident with the fatigue microcrack initiation plane. The position of the fatigue strength curve also depends on the stress component normal to such a plane and the phase angle as well. About 450 experimental data taken from the literature are used to check the accuracy of the method under multiaxial fatigue conditions.

A study of fibre and interface parameters affecting the fatigue behaviour of natural fibre composites

Abstract: The tension–tension fatigue behaviour of different natural fibre reinforced plastics was investigated. The composites used were made of flax and jute yarns and wovens as reinforcements for epoxy resins, polyester resins and polypropylene. Fibre type, textile architecture, interphase properties, fibre properties and content were found to affect the fatigue behaviour strongly as illustrated with damping versus applied maximum load curves. It was found that natural fibre reinforced plastics with higher fibre strength and modulus, stronger fibre–matrix adhesion or higher fibre fractions possess higher critical loads for damage initiation and higher failure loads. In addition, damage propagation rates were reduced. Furthermore, unidirectional composites were less sensitive to fatigue induced damage than woven reinforced ones.

A state-of-the-art review on fatigue life prediction methods for metal structures

Abstract: Metals are the most widely used materials in engineering structures, and one of the most common failure modes of metal structures is fatigue failure. Although metal fatigue has been studied for more than 160 years, many problems still remain unsolved. In this article, a state-of-the-art review of metal fatigue is carried out, with particular emphasis on the latest developments in fatigue life prediction methods. All factors which affect the fatigue life of metal structures are grouped into four categories: material, structure, loading, and environment. The effects of these factors on fatigue behavior are also addressed. Finally, potential problems to be resolved in the near future are pointed out.

On ‘multi-stage’ fatigue life diagrams and the relevant life-controlling mechanisms in ultrahigh-cycle fatigue

Abstract: In recent years, several fatigue studies on different metallic materials have indicated that fatigue failures can occur even at amplitudes below the conventional high-cycle fatigue (HCF) fatigue limit in the gigacycle or ultrahigh-cycle fatigue (UHCF) range (number of cycles to failure in excess of ca. 10 7 -10 8 ). In the latter case, fatigue failures were observed to originate from internal defects (non-metallic inclusions). The S-N curves displayed a multi-stage shape, sometimes with a second lower fatigue limit in the UHCF range. The present paper specifies the conditions under which internal and/or surface fatigue failure can occur at low load levels below the conventional HCF fatigue limit. The relevant fatigue thresholds are discussed for two classes of materials, namely pure single-phase metallic materials (type I) without internal defects and materials such as steels or cast materials (type II) with internal defects (inclusions or pores). In the case of type II materials, the probability of surface versus internal fatigue is discussed in terms of the volume density, size and location of the inclusions and the relevant cracking mechanisms. It is argued that, in both type I and type II materials, multi-stage S-N curves or Manson-Coffin plots can be expected under certain conditions.

Fatigue resistance of two implant/abutment joint designs.

Abstract: Statement of Problem. Because of reported mechanical failures, alternative implant system components with suggested optimized strength have been manufactured. However, the endurance of these products has not been well investigated. Purpose. This study was designed to assess the effect of joint design on the fatigue strength and failure mode of 2 single-tooth implant systems: Branemark and ITI, in which a hex mediated-butt joint and 8-degree internal conical implant/abutment interface are used, respectively. Material and Methods. Seven 10-mm implants from each implant system were embedded to a depth of 7 mm in cylindrical acrylic resin blocks. CeraOne and Solid abutments with cement-retained castings were assembled to the Branemark and ITI implants, respectively. The assembled units were mounted in a lever-type-testing machine that was equipped with an automatic counting device and shutoff sensors, enabling the recording of the number of cycles till failure. A cyclic load of 100 N was applied perpendicular to the long axis of the assemblies at a rate of 75 cycles/min. To investigate specimen resistance to fatigue during 6 years of simulated function, a target of 1,800,000 cycles was defined. Specimen preparation and testing was performed by the same operator. The association of the joint design with the occurrence of failure was verified by Fisher's exact probability test ( P Results. For the Branemark group, the gold alloy abutment screw in all specimens fractured between 1,178,023 and 1,733,526 cycles with a standard deviation of 224,477 cycles. For the ITI group, all specimens had no failure until 1,800,000 cycles. Statistical analysis showed a highly significant difference between the 2 groups ( P =.000582). Conclusion. Within the limitations of this in vitro study, the effect of joint design on the fatigue strength and failure mode of the ITI single tooth implant system was significantly better ( P >.001) than the Branemark single-tooth implant system tested. (J Prosthet Dent 2002;88:604-10.)

High-cycle rotating bending fatigue property in very long-life regime of high-strength steels

Abstract: In order to investigate the fatigue properties of high-strength steels in the very long-life regime up to over 10 9 cycles, cantilever-type rotating bending fatigue tests were carried out for two kinds of high-strength steels, SUJ2 and SNCM439, which were machined by grinding and finished by electropolishing after grinding. And also, the residual stress on the specimen surface of the ground specimen was examined by X-ray diffractometer in order to investigate effects of the residual stress on the fatigue properties. From the investigations, the S-N curves clearly have a tendency to decrease again in the longer-life range over 10 7 cycles for both types of specimen and for both steels. From observations of fracture surfaces, it was found that fatigue crack origins could be grouped into two types: (i) 'surface crack origin type' in the shorter-life regime and (ii) 'internal crack origin type' in the longer-life regime.

Fatigue failure in polysilicon not due to simple stress corrosion cracking.

Abstract: In the absence of a corrosive environment, brittle materials such as silicon should be immune to cyclic fatigue However, fatigue effects are well known in micrometer-sized polycrystalline silicon (polysilicon) samples tested in air To investigate the origins of this phenomenon in polysilicon, we developed a fixed-grip fracture mechanics microspecimen but could find no evidence of static stress corrosion cracking The environmental sensitivity of the fatigue resistance was also investigated under cyclic loading For low-cycle fatigue, the behavior is independent of the ambient conditions, whether air or vacuum, but is strongly influenced by the ratio of compressive to tensile stresses experienced during each cycle The fatigue damage most likely originates from contact stresses at processing-related surface asperities; subcritical crack growth then ensues during further cyclic loading The lower far-field stresses involved in high-cycle fatigue induce reduced levels of fatigue damage Under these conditions, a corrosive ambient such as laboratory air exacerbates the fatigue process Without cyclic loading, polysilicon does not undergo stress corrosion cracking

Fatigue design of structures under thermomechanical loadings

Abstract: This paper presents a global approach to the design of structures that experience thermomechanical fatigue loading, which has been applied successfully in the case of cast‐iron exhaust manifolds. After a presentation of the design context in the automotive industry, the important hypotheses and choices of this approach, based on a thermal 3D computation, an elastoviscoplastic constitutive law and the dissipated energy per cycle as a damage indicator associated with a failure criterion, are first pointed out. Two particular aspects are described in more detail: the viscoplastic constitutive models, which permit a finite element analysis of complex structures and the fatigue criterion based on the dissipated energy per cycle. The FEM results associated with this damage indicator permit the construction of a design curve independent of temperature; an agreement is observed between the predicted durability and the results of isothermal as well as non isothermal tests on specimens and thermomechanical fatigue tests on real components on an engine bench. These results show that thermomechanical fatigue design of complex structures can be performed in an industrial context.

Fatigue of as-extruded AZ61A magnesium alloy

Abstract: Samples prepared from as-extruded AZ61A bars (18 mm in diameter) were used in a rotating bending test. The relation between stress amplitude and cycles to failure has been constructed, as well as the cycles to failure at two specific stress amplitudes. The probability of failure at these two specific stress levels was also analyzed. This study finally provided the predicted fatigue strength at 10 7 cycles with different probabilities (10 to 90%). A fractography study indicated that fatigue cracking initiated from subsurface or surface inclusions. These inclusions near the surface served as stress raisers and induced clusters of slip bands during the rotating bending test. After initiation, the cracks grew under the dominant shear stress and resulted in a cleavage fracture over a large area. Microscopic cracks occurred, resulting from the induced deformation twins that developed from the blunting process. Consequently, the propagation of cracks followed the existence of microscopic cracks and resulted in a transgranular fracture.

Improvement of Fatigue Strength of Aluminum Alloy by Cavitation Shotless Peening

Abstract: Cavitation impact, which normally produces severe damage in hydraulic machinery, can be used to modify surfaces in the same way as shot peening. Cavitation impact enables the surface of a material to be peened without the use of shot, thus it is called cavitation shotless peening. As there are no solid body collisions occurring in this peening process. the roughness of the peened surface should be less than that produced by shot peening. This characteristic makes it suitable for peening soft metals. In order to demonstrate the improvement of the fatigue strength of aluminum alloy be this process, specimens were subjected to the process, and then tested in a rotating bending fatigue test. Cavitation impacts were produced and controlled by using a submerged high speed water jet with cavitation, i.e., a cavitating jet. It was revealed that the fatigue strength of an aluminum alloy specimen treated by this peening process was 50% stronger than that of a specimen without peening.

Recommended Hot-Spot Analysis Procedure For Structural Details of Ships And FPSOs Based On Round-Robin FE Analyses

Abstract: As part of the Joint Industry Project FPSO Fatigue and Fracture Capacity, a Special Task Group of 9 participants performed investigations regarding finite element (FE) modelling and analysis of typical structural details in FPSOs (Floating Production, Storage and Offloading Units) as well as in ships. The purpose of this special effort was to develop recommendations on appropriate hot-spot stress methods and S-N data for fatigue strength design. In total, 5 details with different characteristics, from the perspectives of both geometry and fatigue loading, were selected, for which stress measurements and fatigue tests are available. Participants developed various FE models, using different types and sizes of elements, modelling and stress evaluation techniques as well as FE programs. Comparisons between the analysis results and measured stresses near the weld toes allow conclusions to be drawn. Three different stress extrapolation techniques for predicting hot-spot stresses at the weld toes were investigated. The resulting hot-spot stresses, together with the estimated fatigue lives, are compared against the existing design S-N curves published by the International Institute of Welding (IIW). It was concluded that the hot-spot stresses predicted using the 3 stress extrapolation techniques, where in most cases the element sizes and stress evaluation points are determined by the plate thickness, can be used with the current design S-N curves. One of the recommended methods requires no stress extrapolation, which is considered an attractive and practical alternative to the existing practices developed by classification societies.

Mechanical property improvement of quenchable steel by grinding

Abstract: This paper studies a method of surface heat treatment by making use of grinding heat and stress to create favorable microstructures and promote high wear and fatigue resistance It was found that the thickness of the treated surface layer could be up to 600 μm The beneficial microstructure of the layer was created by an enhanced martensite transformation, intensive dislocations and a more desirable carbon distribution It is highly possible that the method can be used to incorporate grinding and surface hardening into a single grinding operation to develop a cost-effective production method

Toward a Micromechanics-Based Procedure to Characterize Fatigue Performance of Asphalt Concrete

Abstract: A lattice-based micromechanics approach is proposed to characterize the cracking performance of asphalt concrete. A random truss lattice model was introduced and investigated for simulating the following: (a) linear elastic and viscoelastic deformation of homogeneous materials in axial compression and shear loading experiments, (b) linear elastic deformation and the stress field in heterogeneous materials in an axial compression loading experiment, and (c) damage evolution in elastic solids under an indirect tensile test. The simulation results match well with the theoretical solutions and show excellent promise in predicting cracking patterns in the indirect tensile test. A brief discussion about ongoing work is also presented.

Simple Performance Test for Fatigue Cracking and Validation with WesTrack Mixtures

Abstract: Viscoelastic characterization of asphalt concrete in indirect tensile (IDT) testing and development of a simple performance test for fatigue cracking are described A 50-mm gauge length was adopted to measure the horizontal and vertical deformations with surface-mounted linear variable differential transducers on an IDT specimen with a 100- or 150-mm diameter and 38-mm thickness The effect of a concentrated load under loading strips on vertical displacement within the 50-mm gauge length was evaluated by the digital image correlation method, a noncontact, full-field displacement and strain measurement technique The theory of viscoelasticity was used to develop analytical solutions for creep compliance and center strain from displacements measured on the specimen surface These solutions were verified by three-dimensional finite element viscoelastic analysis IDT creep and strength tests were performed on fine and coarse mixtures from WesTrack with various asphalt contents and air void contents, and vario