Showing papers on "Fatigue limit published in 1999"

There is no infinite fatigue life in metallic materials

Abstract: Generally, the shape of the S-N curve beyond 10 7 cycles is unknown except in some statistical approaches, and this is also true for the fatigue limit. In the case of a statistical approach, the standard deviation applied to the average fatigue limit is certainly not the best way to reduce the risk of rupture in fatigue. Only the exploration of the life range between 10 6 and 10 10 cycles will create a safer basis for modelling. Today, some piezoelectric fatigue machines are very reliable, capable of producing 10 10 cycles in less than 1 week. We based our research on accelerated fatigue tests which were performed at 20 kHz in the gigacyclic fatigue regime in order to study several typical alloys from the aeronautical and space industries.

Factors influencing the mechanism of superlong fatigue failure in steels

Abstract: When the fatigue life N f of a specimen of 10 mm in thickness is longer than 10 8 cycles, the average fatigue crack growth rate is much less than the lattice spacing (∼0.1 A or 0.01 nm) that is 10 -11 to 10 -12 m/cycle. In the early stage of the fatigue process, the crack growth rate should be much less than the average growth rate, and accordingly we cannot assume that crack growth occurs cycle by cycle. In this paper, possible mechanisms for extremely high cycle fatigue are discussed. Of some possible mechanisms, a special focus was put on a newly found particular fatigue fracture morphology in the vicinity of the fracture origin (non-metallic inclusions) of a heat-treated alloy steel, SCM435, which was tested to N ≥ 10 8 . The particular morphology observed by SEM and AFM was presumed to be influenced by the hydrogen around inclusions. The predictions of the fatigue limit by the √area parameter model are ∼ 10% unconservative for a fatigue life of N f = ∼10 8 , though it successfully predicts the conventional fatigue limit defined for N = 10 7 . Thus, the fatigue failure for N ≥ 10 8 is presumed to be caused by a mechanism which induces breaking or releasing of the fatigue crack closure phenomenon in small cracks. In the vicinity of a non-metallic inclusion at the fracture origin, a dark area was always observed inside the fish-eye mark for those specimens with a long fatigue life. Specimens with a short fatigue life of N f = ∼10 5 do not have such a dark area in the fish-eye mark. SEM and AFM observations revealed that the dark area has a rough surface quite different from the usual fatigue fracture surface in a martensite lath structure. Considering the high sensitivity of high-strength steels to a hydrogen environment and the high hydrogen content around inclusions, it may be concluded that the extremely high cycle fatigue failure of high-strength steels from non-metallic inclusions is caused by environmental effects, e.g. hydrogen embrittlement coupled with fatigue.

Mechanical surface treatments on titanium, aluminum and magnesium alloys

Abstract: Generally, mechanical surface treatments induce high dislocation densities in near-surface regions. Due to the local plastic deformation, residual stresses are developed and the surface topography is changed. These changes can have contradictory influences on the fatigue strength. Results on the influence of mechanical surface treatments on magnesium are presented in relation to the relatively well-studied titanium and aluminum alloys.

Stepwise S-N curve and fish-eye failure in gigacycle fatigue

Abstract: Fatigue failure is normally initiated at the surface of a material. For some materials, failure can be initiated both at the surface and the interior. This twofold materials behaviour in fatigue is represented by a stepwise shape in the S-N curve. An internal failure mode is especially important for fatigue life in the gigacycle range, as this mode is predominant at low stress ranges. Materials with a hardened surface fail from the surface only at high stresses, and at low stresses from the inside, forming a fish-eye facet on the fracture surface. Exactly the same behaviour can be observed for materials without a hard surface, even at elevated temperatures. This paper displays some of the results obtained at NRIM and discusses possible interpretations.

Mechanisms for fracture and fatigue-crack propagation in a bulk metallic glass

Abstract: The fracture and fatigue properties of a newly developed bulk metallic glass alloy, Zr41.2Ti13.8Cu12.5 Ni10Be22.5 (at. pct), have been examined. Experimental measurements using conventional fatigue precracked compact-tension C(T) specimens (∼7-mm thick) indicated that the fully amorphous alloy has a plane-strain fracture toughness comparable to polycrystalline aluminum alloys. However, significant variability was observed and possible sources are identified. The fracture surfaces exhibited a vein morphology typical of metallic glasses, and, in some cases, evidence for local melting was observed. Attempts were made to rationalize the fracture toughness in terms of a previously developed micromechanical model based on the Taylor instability, as well as on the observation of extensive crack branching and deflection. Upon partial or complete crystallization, however, the alloy was severely embrittled, with toughnesses dropping to ∼1 MPa % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9vqpe0x% c9q8qqaqFn0dXdir-xcvk9pIe9q8qqaq-dir-f0-yqaqVe0xe9Fve9% Fve9qapdbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaOaaaeaaie% aacaWFTbaaleqaaaaa!3A30! $$\sqrt m $$ . Commensurate with this drop in toughness was a marginal increase in hardness and a reduction in ductility (as measured via depthsensing indentation experiments). Under cyclic loading, crack-propagation behavior in the amorphous structure was similar to that observed in polycrystalline steel and aluminum alloys. Moreover, the crack-advance mechanism was associated with alternating blunting and resharpening of the crack tip. This was evidenced by striations on fatigue fracture surfaces. Conversely, the (unnotched) stress/life (S/N) properties were markedly different. Crack initiation and subsequent growth occurred quite readily, due to the lack of microstructural barriers that would normally provide local crack-arrest points. This resulted in a low fatigue limit of ∼4 pct of ultimate tensile strength.

Fundamentals of Machine Elements

Abstract: Part I - Fundamentals Introduction What is Design? Design of Mechanical Systems Design as a Multidisciplinary Endeavor Design of Machine Elements Computers in Design Catalogs and Vendors Units Unit Checks Significant Figures Summary Load, Stress, and Strain Introduction Critical Section Load Classification and Sign Convention Support Reactions Static Equilibrium Free-Body Diagram Supported Beams Shear and Moment Diagrams Stress Stress Element Stress Tensor Plane Stress Mohr's Circle Three-Dimensional Stresses Octahedral Stresses Strain Strain Tensor Plane Strain Summary Introduction to Materials and Manufacturing Introduction Ductile and Brittle Materials Classification of Solid Materials Stress-Strain Diagrams Properties of Solid Materials Stress-Strain Relationships Two-Parameter Materials Charts Effects of Manufacturing Summary Stresses and Strains Introduction Properties of Beam Cross Sections Normal Stress and Strain Torsion Bending Stress and Strain Transverse Shear Stress and Strain Summary Deformation Introduction Moment-Curvature Relation Singularity Functions Method of Superposition Strain Energy Castigliano's Theorem Summary Failure Prediction for Static Loading Introduction Stress Concentration Fracture Mechanics Modes of Crack Growth Fracture Toughness Failure Prediction for Uniaxial Stress State Failure Prediction for Multiaxial Stress State Summary Fatigue and Impact Introduction Fatigue Cyclic Stresses Strain Life Theory of Fatigue Fatigue Strength Fatigue Regimes Stress Concentration Effects The Modified Endurance Limit Cumulative Damage Influence of Nonzero Mean Stress Influence of Multi-Axial Stress States Fracture Mechanics Approach to Fatigue Linear Impact Stresses and Deformations Summary Lubrication, Friction, and Wear Introduction Surface Parameters Conformal and Nonconformal Surfaces Hertzian Contact Bearing Materials Lubricant Rheology Regimes of Lubrication Friction Wear Summary Part II - Machine Elements Columns Introduction Equilibrium Regimes Concentrically Loaded Columns End Conditions Euler's Buckling Criterion Johnson's Buckling Criterion AISC Criteria Eccentrically Loaded Columns Summary Stresses and Deformations in Cylinders Introduction Tolerances and Fits Pressurization Effects Rotational Effects Press Fits Shrink Fits Summary Shafting and Associated Parts Introduction Design of Shafts for Static Loading Fatigue Design of Shafts Additional Shaft Design Considerations Critical Speed of Rotating Shafts Keys, Roll Pins, Splines and Set Screws Retaining Rings and Pins Flywheels Couplings Summary Hydrodynamic and Hydrostatic Bearings Introduction The Reynolds Equation Thrust Slider Bearings Journal Slider Bearings Squeeze Film Bearings Hydrostatic Bearings Summary Rolling-Element Bearings Introduction Historical Overview Bearing Types and Selection Geometry Kinematics Separators Static Load Distribution Elastohydrodynamic Lubrication Fatigue Life Variable Loading Summary General Gear Theory Spur Gears Introduction Types of Gears Gear Geometry Gear Ratio Contact Ratio and Gear Velocity Tooth Thickness and Backlash Gear Trains Gear Manufacture and Quality Gear Materials Loads Acting on a Gear Tooth Bending Stresses in Gear Teeth Contact Stresses in Gear Teeth Elastohydrodynamic Film Thickness Gear Design Synthesis Summary Helical, Bevel, and Worm Gears Introduction Helical Gears Bevel Gears Worm Gears Summary Fasteners, Connections, and Power Screws Introduction Thread Terminology, Classification, and Designation Power Screws Threaded Fasteners Riveted Fasteners Welded, Brazed, and Soldered Joints Adhesive Bonding Integrated Snap Fasteners Summary Springs Introduction Spring Materials Helical Compression Springs Helical Extension Springs Helical Torsion Springs Leaf Springs Gas Springs Belleville Springs Wave Springs Summary Brakes and Clutches Introduction Thermal Considerations Thrust Pad Clutches and Brakes Cone Clutches and Brakes Block or Short-Shoe Brakes Long-Shoe, Internal, Expanding Rim Brakes Long-Shoe, External, Contracting Rim Brakes Symmetrically Loaded Pivot-Shoe Brakes Band Brakes Slip Clutches Summary Flexible Machine Elements Introduction Flat Belts Synchronous Belts V-Belts Wire Ropes Rolling Chains Summary Appendices Index

Gigacycle fatigue of ferrous alloys

Abstract: The objective of this paper is to determine the very long fatigue life of ferrous alloys up to 1 x 10 10 cycles at an ultrasonic frequency of 20 kHz. A good agreement is found with the results from conventional tests at a frequency of 25 Hz by Renault between 10 5 and 10 7 cycles for a spheroidal graphite cast iron. The experimental results show that fatigue failure can occur over 10 7 cycles, and the fatigue endurance stress S max continues to decrease with increasing number of cycles to failure between 10 6 and 10 9 cycles. The evolution of the temperature of the specimen caused by the absorption of ultrasonic energy is studied. The temperature increases rapidly with increasing stress amplitudes. There is a maximum temperature between 10 6 and 10 7 cycles which may be related to the crack nucleation phase. Observations of fracture surfaces were also made by scanning electron microscopy (SEM). Subsurface cracking has been established as the initiation mechanism in ultra-high-cycle fatigue (> 10 7 cycles). A surface-subsurface transition in crack initiation location is described for the four low-alloy high-strength steels and a SG cast iron.

Estimating fatigue curves with the random fatigue-limit model

Abstract: In a fatigue-limit model, units tested below the fatigue limit (also known as the threshold stress) theoretically will never fail. This article uses a random fatigue-limit model to describe (a) the dependence of fatigue life on the stress level, (b) the variation in fatigue life, and (c) the unit-tounit variation in the fatigue limit.We fit the model to actual fatigue datasets by maximum likelihood methods and study the fits under different distributional assumptions. Small quantiles of the life distribution are often of interest to designers. Lower confidence bounds based on likelihood ratio methods are obtained for such quantiles. To assess the fits of the model, we construct diagnostic plots and perform goodness-of-fit tests and residual analyses.

Small defects and Inhomogeneities in fatigue strength: experiments, models and statistical implications

Abstract: The method explained in this paper for quantitative evaluation of fatigue limit for materials containing defects is based on the experimental evidences that inhomogeneities and micro-notches can be treated like cracks. First, the basic concept of the √area parameter model is explained introducing the various data obtained by the first author's group for over last 15 years. Evidences are shown that small cracks, defects and nonmetallic inclusions having the same value of the square root of projection area, √area, have the identical influence on the fatigue limit regardless of different stress concentration factors. Various applications of these concepts to various defect types and microstructural inhomogeneities are shown. Since the estimation of fatigue strength is related to the estimation of the size of maximum defects occurring in a piece, the methods for searching the defects and the quality control of materials with respect to inclusion or defect rating as well as their statistical implications are discussed.

Technical note High-cycle fatigue crack initiation and propagation behaviour of high-strength sprin steel wires

Abstract: An attempt has been made to characterize high-cycle fatigue behaviour of high-strength spring steel wire by means of an ultrasonic fatigue test and analytical techniques. Two kinds of induction-tempered ultra-high-strength spring steel wire of 6.5 mm in diameter with a tensile strength of 1800 MPa were used in this investigation. The fatigue strength of the steel wires between 106 and 109 cycles was determined at a load ratio R = −1. The experimental results show that fatigue rupture can occur beyond 107 cycles. For Cr–V spring wire, the stress–life (S–N ) curve becomes horizontal at a maximum stress of 800 MPa after 106 cycles, but the S–N curve of the Cr–Si steel continues to drop at a high number of cycles (>106 cycles) and does not exhibit a fatigue limit, which is more correctly described by a fatigue strength at a given number of cycles. By using scanning electron microscopy (SEM), the crack initiation and propagation behaviour have been examined. Experimental and analytical techniques were developed to better understand and predict high-cycle fatigue life in terms of crack initiation and propagation. The results show that the portion of fatigue life attributed to crack initiation is more than 90% in the high-cycle regime for the steels studied in this investigation.

Static and Fatigue Performance of RC Beams Strengthened with CFRP Laminates

Abstract: The paper presents the results of a study involving the static and fatigue performance of reinforced concrete beams strengthened with externally bonded carbon-fiber-reinforced plastic (CFRP) sheets The main parameters in the static test study were the concrete compressive strength, the number of CFRP laminates, and the placement of CFRP reinforcement The static test program shows that the application of CFRP to reinforced concrete beams results in increased strength and enhanced performance Accelerated fatigue testing is performed on several specimens receiving various amounts of the CFRP lamination system, including one member that is fatigued for over half the expected fatigue life, then rehabilitated with the CFRP, and fatigued again until failure Comparisons are made for the standard section and equivalent sections with two and three layers of CFRP involving the improvements in fatigue behavior, stiffness, and capacity The results from the fatigue study indicate that fatigue life of reinforced co

Study of fatigue resistance of chemical and radiation crosslinked medical grade ultrahigh molecular weight polyethylene

Abstract: The aim of this work is to understand the role of chemical and radiation induced crosslinking on the fatigue crack propagation resistance of medical grade ultrahigh molecular weight polyethylene (UHMWPE). In recent years, the need to improve the tribological performance of UHMWPE used in total joint replacements has resulted in the widespread utilization of crosslinking as a method to improve wear resistance. Although crosslinking has been shown to drastically improve the wear resistance of the polymer, the potential trade-off in fatigue properties has yet to be addressed. Fatigue crack propagation resistance is a concern in tibial inserts where large cyclic stresses are sufficient to drive the growth of subsurface cracks that potentially contribute to delamination wear mechanisms. For clinical relevance, the combined effects of sterilization and aging are examined in two commercially available crosslinked resins. Nonsterile and unaged resins serve as a control. To evaluate the effect of crosslinking, a comparison is made to uncrosslinked resins. Scanning electron microscopy is used to provide an understanding of fatigue fracture mechanisms in the crosslinked polymers. The results of this study show that the current level of crosslinking used in orthopedic resins for enhanced wear resistance is not beneficial for fatigue crack propagation resistance.

The use of neural networks for the prediction of fatigue lives of composite materials

Abstract: Constant-stress fatigue data for five carbon-fibre-reinforced plastics and one glass-reinforced plastic laminate have been used to evaluate possible artificial neural network architectures for the prediction of fatigue lives and to develop network training methods. It has been found that artificial neural networks can be trained to model constant-stress fatigue behaviour at least as well as other current life-prediction methods and can provide accurate (and conservative) representations of the stress/R-ratio/median-life surfaces for carbon-fibre composites from quite small experimental data-bases. Although their predictive ability for minimum life is less satisfactory than that for median life, and is non-conservative, the procedures developed in this work could nevertheless be used in design with little further modification. Some success has been achieved in modelling fatigue under block-loading conditions, but this problem is more difficult and requires much more effort before ANNs could be used with confidence for variable-stress conditions.

The fatigue limit and its elimination

Abstract: The classical fatigue limit of ferrous metals is a consequence of testing materials at a constant range of cyclic stress and determining the cyclic stress range below which fatigue failures do not occur. This classical fatigue limit of a material is equated to the condition for which fatigue cracks can not propagate beyond microstructural barriers. This paper discusses the causes, leading to the elimination of this fatigue limit, including the introduction of transitory cyclic-dependent mechanisms and time-dependent processes that will permit a previously non-propagating crack to grow across the different threshold states expressed in terms of linear-elastic fracture mechanics (LEFM), elastic-plastic fracture mechanics (EPFM) and microstructural fracture mechanics (MFM). These transitory mechanisms and processes include different loading and environmental conditions, which in a long-life engineering plant (e.g. 30 years lifetime) can lead to apparently premature failures. Of greater concern is the creation of a new crack-initiation zone, i.e. a transfer from a surface-generated crack to an internal-generated crack that eventually dominates the fatigue failure event. The impact of these conditions on the elimination of the classical fatigue limit necessitates changes in Design Codes of Practice, and such changes are discussed in relation to the extremely long-lifetime regime (10 7

On the life‐controlling microstructural fatigue mechanisms in ductile metals and alloys in the gigacycle regime

Abstract: The high-cycle fatigue (HCF) behaviour of ductile metals and alloys, and the life-controlling microstructural fatigue mechanisms known from HCF are reviewed critically with respect to their possible role in the gigacycle or ultra-high-cycle fatigue (UHCF) regime. Arguments are presented to support the hypothesis that, at the very low amplitudes of the UHCF regime, fatigue crack initiation, resulting from cyclic strain localization, and slow early Stage I fatigue crack propagation are the life-controlling mechanisms and that these processes can essentially be described in terms of the microstructurally irreversible portion of the cumulative cyclic plastic strain. Emphasis is placed on the important role of the so-called slip irreversibility which decreases as the amplitude becomes lower and lower. Finally, the Manson-Coffin law is reformulated for very low amplitudes in terms of microstructurally relevant parameters, and a fatigue life diagram is developed, based on these preceding microstructural considerations. Important features of this diagram are: (i) the plastic strain fatigue limit in the HCF regime which is related to the threshold for cyclic strain localization in persistent slip bands; and (ii) the transition from this plastic strain fatigue limit to a threshold of negligible slip irreversibility at still lower amplitudes in the UHCF regime.

Fatigue Strength Prediction under Multiaxial Stress

Abstract: A multiaxial fatigue failure criterion for composite materials is presented in this paper along with an assessment of the capability it offers for design under multiaxial constant or variable amplitude stresses. The applicability of this criterion, based on the well known quadratic failure tensor polynomial criterion for static loading, is validated through comparisons with uniaxial and multiaxial fatigue experimental data. Static and fatigue tests were carried out during this study on glass/polyester specimens cut off-axis from a multidirectional laminate at different angles. The agreement between experimental values and theoretical predictions is good. The proposed criterion is also compared to existing fatigue criteria and an overall assessment of their performance is given. Some theoretical design considerations for the case of irregular stress spectra, introducing the concept of multiaxial Miner coefficient, are finally presented.