bluuebunny/crossref_metadata_2025 huggingface dataset:
FATIGUE LIMIT OF BLUNT‐NOTCHED COMPONENTS
Authors: Chapetti  , Kitano  , Tagawa  , Miyata
Published: March 2003

The position and effective resistance of microstructural barriers and their relation to the fatigue strength of blunt‐notched specimens are analysed and modelled for three low‐carbon steel microstructures. A relationship for the notch size effect on the basis of the experimental evidence that the fatigue limit (both plain and notched) represents the threshold stress for the propagation of the nucleated microstructurally short cracks, was derived. The derived relationship characterizes the fatigue notch sensitivity by means of the parameter ktd defined as the stress concentration introduced by the notch at a distance d from the notch root surface equal to the distance between microstructural barriers, and was experimentally verified for two notch geometries in three microstructures: ferrite, ferrite–bainite and bainite–martensite.
Stress singularities in composite wedge‐shaped materials
Authors: Rudge M.R.H. , Tiernan D.M.
Published: March 2003

Calculations of the stress singularity in the vicinity of the tip of two bonded wedges are described. Plasticity in the near‐tip region is accounted for within the framework of the Ramberg–Osgood relation. It is shown that the singularity can be larger for two bonded wedges than for a single wedge, and figures are presented to indicate how the stress singularity depends on the relevant parameters.
An apparatus for quantitative fretting testing
Authors: Wittkowsky  , Birch  , Dominguez  , Suresh
Published: March 2003

The design and construction of an apparatus for performing quantitative fretting fatigue experiments is described. The device allows accurate measurement and control of normal contact force, tangential contact force, relative displacement between contacting surfaces and bulk fretting loads, as well as measurement of average friction coefficients. Its design is simple, and includes interchangeable fretting contact pads, allowing the use of various pad geometries without major adjustment. The device incorporates many points of adjustment for alignment and compliance, making it a robust frame for a wide variety of fretting fatigue conditions involving different materials. The capabilities of this device are also verified by results of fretting fatigue experiments conducted on a 7075‐T6 aluminium alloy.
Modelling of crack surface interference under cyclic shear loads
Authors: Yu  , Abel
Published: March 2003

Crack surface interference under cyclic shear loads is studied by an analytical method. The proposed model simulates the effects arising from both the residual stresses and the asperity interactions. A closed‐form and a discrete approach are presented in obtaining the crack surface interference solutions. Backlashes of shear displacements, peeling or group sliding behaviours and induced cyclic mode I stress intensities are predicted under three configurations of residual stress distributions. The effects of a static mode I load, the facet angle and the frictional angle are also analysed. The predicted relationship between the effective shear mode stress intensity range and theR‐ratio is discussed together with the experimentally observed ‘contrasting’Reffects.
Measuring residual stress and the resulting stress intensity factor in compact tension specimens
Authors: Prime
Published: March 2003

The measurement of residual stress through the remaining ligament of a compact tension specimen was studied. In the crack compliance method, a slot or notch is successively extended through the part, and the resulting strain is measured at an appropriate location. By using a finite element simulation of a specimen preloaded beyond yield, three techniques for determining the original residual stress from the measured strains were compared for accuracy and sensitivity to measurement errors. A common beam‐bending approximation was substantially inaccurate. The series expansion method proved to be very versatile and accurate. The fracture mechanics approach could determine the stress intensity factor caused by the residual stresses with a very simple calculation. This approach offers the exciting possibility of determining the stress intensity factor prior to a fatigue or fracture test by measuring strains during the specimen preparation.
Stress intensity factors for edge‐cracked plates under arbitrary loading
Authors: Fett
Published: March 2003

Stress intensity factors for edge‐cracked plates are commonly available for long plates with height‐to‐width ratios H/W > 1. In the present note, stress intensity factor solutions are reported for short plates in tension and bending. In order to allow the computation of stress intensity factors under arbitrary loadings, two possibilities are considered. For the treatment of known stress distributions in the uncracked plate, the weight function is an appropriate tool. For arbitrarily prescribed tractions at the plate ends, a superposition method based on sectionally constant tractions is described and illustrated by examples.
Effects of frequency on fatigue crack growth at elevated temperature
Authors: Tong  , Byrne
Published: March 2003

The effects of frequency on fatigue crack growth behaviour have been studied in a prealloyed powder material, Udimet 720Li, at 650 °C. Fracture mode and fatigue crack growth behaviour were studied at frequencies ranging from 0.001 to 5 Hz using a balanced triangular waveform. Tests were carried out under constant ΔKcontrol, with load ratio and temperature being held constant. A mechanism map was constructed where predominantly time, mixed and cycle‐dependent crack growth behaviour were identified. The results were verified by SEM analyses. Cycle‐dependent crack growth data were obtained at room temperature, while fully time‐dependent crack growth data were generated under sustained loads at 650 °C.

It was found that mixed time/cycle‐dependent behaviour is of most significance for this material at the temperature and frequencies studied. Data for other nickel‐based superalloys from various sources in the literature were compiled and compared with those of U720Li alloy at a given stress intensity and temperature in the mixed regime. An analysis was developed to rationalize the observed effect of frequency on fatigue crack growth rate.
Smoothing and differentiating load–displacement data using a low‐pass filter for improved crack opening load estimates
Authors: Daniewicz
Published: March 2003

The presence of even small amounts of noise in experimental data significantly influences numerically determined derivatives. Dynamic programming is used to construct a low‐pass filter enabling the smoothing and differentiation of noisy data. The filter is used to smooth and differentiate load–displacement data for a propagating fatigue crack to obtain smoothed compliance as a function of applied load. The smoothed compliance is shown to allow for improved subsequent estimates of the crack opening load.
Application of ultrasound for fatigue testing of lightweight alloys
Authors: Mayer  , Lipowsky  , Papakyriacou  , Rösch  , Stich  , Stanzl‐Tschegg
Published: March 2003

The use of aluminium and magnesium alloys offers a great potential for weight reduction in automotive applications. Load‐bearing car components are subjected to 108 cycles and more during service, and the high‐cycle fatigue properties of construction materials are therefore of great interest.

The time‐saving ultrasound fatigue testing method has been used to study the fatigue properties of a high‐pressure, die‐cast magnesium alloy AZ91 hp and a post‐forged, cast‐aluminium alloy AlSi7Mg0.3 in ambient air and saltwater (5wt% sodium chloride) spray. In ambient air, fatigue cracks in AZ91 hp emanate from voids, and it is possible to correlate void areas with the numbers of cycles‐to‐failure. Post‐forging of AlSi7Mg0.3 reduces the numbers and size of voids. The remaining small voids (void areas smaller than 9000 μm2 ) do not significantly reduce lifetimes. Saltwater deteriorates the fatigue properties of both the lightweight alloys. With increasing numbers of cycles, the influence of the corrosive liquid on fatigue strength becomes more pronounced.
A numerical investigation on the effect of an overload on fatigue crack opening and closure behaviour
Authors: Ellyin F. , Wu J.
Published: March 2003

The effect of a superimposed single tensile overload or a compressive overload in a block of constant‐amplitude cycles on the crack opening and closing stresses is investigated using an elastic–plastic finite element analysis. The results obtained are in basic agreement with the experimental observations. Following an applied tensile overload cycle, the crack opening and closing stresses increase instantaneously, while the imposition of a compressive overload cycle results in a small decrease of the crack opening and closing stresses. A detailed discussion of the residual stress and strain patterns caused by the plastic deformation during the overload cycle and the corresponding crack profiles is included. The main governing mechanism resulting in the change of crack growth due to an overload is pointed out.
Fatigue behaviour of a box‐welded joint under biaxial cyclic loads
Authors: Takahashi I. , Ushijima M. , Takada A. , Akiyama S. , Maenaka H.
Published: March 2003

Multiaxial fatigue behaviour of box‐welded (wrap‐around) joints in a JIS SM400B steel (12‐mm‐thick plate) was examined using a biaxial fatigue test facility. For the specimen, two stiffeners were attached to a main plate by a CO2semi‐automatic welding procedure. Residual stress measurements and finite element (FE) analyses were also performed. Fatigue tests were performed under both uniaxial and biaxial (mainly out‐of‐phase) cyclic loads, and both results were compared and examined.

It was found that fatigue cracks in the biaxial fatigue test specimens were initiated at the boxing‐weld toes and propagated almost in the direction of the lateral loads. This is considered to be due to the dominant direction of tensile residual stresses from welding and the stress concentration in the vicinity of the boxing‐weld toe.

From the relation between the strain range near a weld toe, Δε5 , and the fatigue lives, it was found that crack initiation life,Nc , was almost equivalent in the biaxial and uniaxial fatigue tests, while the failure life,Nf , was slightly longer in the biaxial tests. However, when the fatigue lives are put in order using the stress range near a weld toe, Δσ5 , the crack initiation life,Nc , in the out‐of‐phase biaxial tests (phase difference ofπ) is ~30% lower than in the in‐phase biaxial and uniaxial tests, while the failure life,Nf , was almost equivalent in the biaxial and uniaxial tests. From these results, it is concluded that an increase in Δσ5(lowering of the minimum value ofσ5 ), induced by the out‐of‐phase lateral loads, leads to an increase in fatigue damage where the high tensile welding residual stresses exist in the vicinity of the boxing‐weld toe.

Finally, a simple life estimation for the biaxial fatigue tests was made using FE analyses and the results of the uniaxial fatigue tests, proving that the effects of the lateral loads should be taken into consideration.
A discrete‐integral strength criterion for complicated stress states
Authors: Kornev  , Kurguzov
Published: March 2003

The atomic structure behaviour at a crack tip in closely‐packed layers of atoms is studied. The deformation of a three‐atomic chain for the complicated stress state is analysed. This study includes the analyses of bifurcation and post‐bifurcation behaviour as well as the sensitivity of the limit strength to fixing conditions. The numerically obtained values of theoretical strengths are compared with the available experimental results and the theoretical estimates. The discrete‐integral strength criterion for complicated stress states is proposed. Contrary to the classical criteria, the proposed criterion allows a limiting transition of the crack length parameter and describes the strength of cracked and crack‐free solids.
An upper bound limit load for overmatched scarf‐joint specimens
Authors: Alexandrov  , Goldstein
Published: March 2003

A simple kinematically admissible velocity field involving single jump lines is proposed to find an upper bound limit load for overmatched scarf‐joint specimens. It is shown that the solution based on the chosen velocity field depends on two dimensionless parameters, while the number of essential dimensional parameters is four. This is a great advantage for engineering applications. The results, for a particular case, are compared with a similar solution for centre‐cracked tensile specimens and one which is obtainable from the new solution as a particular case.
Analytical modelling of fatigue crack propagation
Authors: Bolotin
Published: March 2003

An analytical approach to the prediction of fatigue crack growth is discussed from the viewpoint of analytical mechanics and mechanics of solids. Conditions of fatigue crack non‐propagation and stability are formulated in terms of the principle of virtual work for systems with unilateral constraints. Additional variables are introduced for the description of damage accumulated ahead of cracks and conditions at the damaged crack tips. Applications are given to high‐cycle and low‐cycle fatigue cracks in metallic structures. A brief survey is included of recent studies of fatigue and related phenomena based on the presented theory.
Crack tip mechanics effects on environment‐assisted cracking of beta‐titanium alloys in aqueous NaCl
Authors: Somerday  , Young  , Gangloff
Published: March 2003

The objective of this research is to understand the effects of crack tip mechanics on environment‐assisted cracking (EAC) of α‐precipitation hardened β‐Ti alloys. Precracked Ti–8V–6Cr–4Mo–4Zr–3Al and Ti–15Mo–3Nb–3Al are prone to severe EAC in aqueous NaCl when stressed under fixed or rising displacement. The latter is more damaging and establishes a lower bound threshold of the stress intensity (K ) well belowKIC . EAC is intergranular and occurs at fast growth rates (da/dt, up to 150 μm/s) for five orders of magnitude of loading rate, dK/dt. Cracking, due to hydrogen‐environment embrittlement, is rate limited by lattice diffusion of hydrogen. EAC at fast da/dtor high dK/dtrequires process zone embrittlement sites very near to the crack tip or enhanced H transport. Subcritical da/dtversusKdepends on loading format through crack tip strain rate (ε˙CT ) differences governed by dK/dt, da/dtand creep. A continuum model approximates the contributions of dK/dtand da/dtto ε˙CTand EAC, but it is inconsistent within situmeasurements of crack tip strain. Intergranular cracking is exacerbated by high ε˙CTthat destabilizes the crack tip passive film and enables hydrogen uptake from an electrochemical reaction. EAC is mitigated when ε˙CTis insufficient in these regards.
Response of notched carbon/PEEK and carbon/epoxy laminates subjected to tension fatigue loading
Authors: Aymerich F. , Found M. S.
Published: March 2003

In this study a comparison is made between the tensile static and fatigue behaviours of quasi‐isotropic carbon/PEEK and carbon/epoxy notched laminates, selected as separate representatives of both tough and brittle matrix composites. Damage progression was monitored by various non‐destructive (ultrasonic scanning and x‐radiography) and destructive (deply and microscopic examinations) techniques, and by continuously measuring the change in stiffness, in order to identify the effect of damage on mechanical properties.

The experimental observations indicated that fatigue damage in carbon/epoxy laminates consists of a combination of matrix cracks, longitudinal splitting and delaminations which attenuate the stress concentration and suppress fibre fracture at the notch; as a consequence, fatigue failure can be reached only after very high numbers of cycles while tensile residual strengths continuously increase over the range of lives investigated (103–106cycles). Due to the superior matrix toughness and the high fibre‐matrix adhesion, the nature of fatigue damage in carbon/PEEK laminates strongly depends on the stress level. At high stresses the absence of early splitting and delaminations promotes the propagation of fibre fracture therefore resulting in poor fatigue performances and significant strength reductions; while at low stress levels damage modes are matrix controlled and this again translates into very long fatigue lives. These results indicate a strong influence of the major damage mechanisms typical of the two material systems on the behaviour of the laminates, with the nature, more than the amount, of damage appearing as the controlling parameter of the material response up to failure.
The effect of contact load reduction on the fatigue life of pearlitic rail steel in lubricated rolling–sliding contact
Authors: Fletcher D. I. , Beynon J. H.
Published: March 2003

Twin‐disc contact simulation tests were carried out to investigate the influence of contact pressure variation on rail steel fatigue life. Both a colloidal suspension of molybdenum disulphide in an oil carrier fluid (similar to a commercial flange lubrication product) and water were used as lubricants. It was found that the reduction from 1500 to 900 MPa of the maximum Hertzian contact pressure (at which a molybdenum–disulphide‐lubricated and previously worn rail sample was tested) extended the fatigue life of the rail steel by over five times. For water lubrication a similar reduction in contact pressure produced only a marginal increase in fatigue life. The results were found to be in qualitative agreement with the predictions of the newly developed Three Mechanism (TM) model of rolling contact fatigue, which is introduced here. This model combines the mechanisms of ratcheting and the fracture mechanics‐based mechanisms of both shear stress‐ and tensile stress‐driven, fluid‐assisted, crack growth.
Long‐life torsion fatigue with normal mean stresses
Authors: Marquis  , Socie
Published: March 2003

Relatively simple fatigue tests have been performed on two common engineering materials, cast ductile iron and low‐carbon steel, using two stress states, cyclic torsion and cyclic torsion with static axial and hoop stresses. Tests were designed to discriminate between normal stress and hydrostatic stress as the most suitable mean stress correction term for high cycle fatigue analysis. Microscopy shows that cracks in low‐carbon steel nucleate and grow on maximum shear planes, while for cast iron pre‐existing flaws grow on maximum normal stress planes. The data illustrate that tensile normal stress acting on a shear plane significantly reduced fatigue life and is an appropriate input for fatigue analysis of ductile materials. Static normal stresses did not significantly affect the fatigue life for the cast iron because the net mean stress on the maximum normal stress plane was zero. Mean torsion significantly reduced the fatigue strength of the cast iron. A critical plane long‐life parameter for nodular iron which accounts for both stress state and mean stress is proposed, and is found to accurately correlate experimental data.
Finite element analyses of deformation around holes near a crack tip and their implications to theJ‐resistance curve
Authors: Gu I.
Published: March 2003

The initiation phase of ductile crack growth is investigated in a detailed finite element analysis. The process is modelled by the growth and coalescence of a series of holes in front of a blunted crack tip, representing the growth of holes separated from large inclusions. The reductions of each ligament between neighbouring holes due to hole growth are calculated as well as crack‐tip opening displacements (CTOD). All the ligaments except the first and last show similar behaviour of reduction, with the reduction curves separated in an equal CTOD increment. The CTOD increment as a function of effective crack extension due to hole growth defines crack‐tip opening angle, a measure of crack‐growth resistance for a given condition. As the CTOD increment changes substantially due to crack extension in small‐scale yielding, crack initiation is defined at the point of deviation from the linear increment of CTOD againstJfor a stationary crack. The effective crack tip is defined at each hole with the ligament reduction of crack initiation. Therefore, theJ‐resistance curve may be derived from the hole growth model, and hence the effects of specimen geometry, crack length and specimen size on the R‐curve are discussed. The calculations for a hole spacing 10 times of the initial hole diameter are in agreement with the experimental R‐curve of 2024‐T351 aluminium alloy.
Restraint of fatigue crack growth by wedge effects of fine particles
Authors: Takahashi I. , Takahashi C. , Kotani N.
Published: March 2003

This paper presents some experimental results which demonstrate restraint of fatigue crack growth in an Al–Mg alloy by wedge effects of fine particles.

Fatigue test specimens were machined from a JIS A5083P‐O Al–Mg alloy plate of 5 mm thickness and an EDM starter notch was introduced to each specimen. Three kinds of fine particles were prepared as the materials to be wedged into the fatigue cracks, i.e. magnetic particles and two kinds of alumina particles having different mean particle sizes of 47.3 μm and 15.2 μm. Particles of each kind were suspended in an oil to form a paste, which was applied on the specimen surface covering the notch zone prior to the fatigue tests. In order to make some fracture mechanics approaches, in situ observations of fatigue cracks were performed for the two cases using a CCD microscope, with a magnification of ×1000. The crack length and the crack opening displacement (COD) at the notch root, δ, were measured.

First it was ensured by control tests that the wedge effect of the oil itself was negligible. Then it was found that the large size alumina particles were not effective in restraining crack growth because the paste was difficult to make due to the large particle size and the particles could not enter the cracks properly.

However, both of the magnetic particles and the small size alumina particles effectively restrained crack growth, especially the latter which produced 143–350% increase in the lifetime to failure.

From the in situ observations, in the case of the small size alumina particles, a pronounced retardation of crack growth was observed immediately after the crack length exceeded 0.4 mm, and this is considered to be due to the range of COD value, δmax − δmin , being strongly affected by the wedge effects of the particles. The crack retardation effect continues almost through the entire lifetime if the alumina paste is re‐applied at specified intervals, while the effect is apparently lost after the crack length exceeds ∼2 mm when such re‐painting is not continued.

After the fatigue tests, some macro‐ and microfractographic analyses were performed using a CCD microscope, a SEM and an EPMA (electron probe microanalyser), in order to examine the mechanism of fatigue crack restraint by the wedge effects of the fine particles. From those analyses, it was reasoned that the fine particles that entered a fatigue crack are subjected to cyclic pressures between the crack faces and then form a kind of wedge which causes significant levels of crack closure that restrain crack growth.
Finite element analysis of corner point displacements and stress intensity factors for narrow notches in square sheets and plates
Authors: Pook L. P.
Published: March 2003

It is possible to model a crack as a narrow parallel‐sided notch with a semicircular tip. Surface displacements of narrow notches in the vicinity of a corner point where the notch tip intersects a free surface were investigated, using finite element analysis, for each of the three modes of crack tip surface displacement. Some reasonably accurate relevant stress intensity factors were determined by analysis of notch tip stresses. The extent of corner regions in which stress intensity measures, rather than stress intensity factors, dominate crack tip stresses was determined by analysis of the notch surface displacements and of relevant stress intensity factors. Under nominal Mode III loading corner point effects are local, but under nominal Mode I and nominal Mode II loadings they are a combination of local and global effects. Volterra distorsioni are useful in the description of crack, and narrow notch, surface displacements under load.
Fatigue of casting flaws at a notch root under an SAE service load history
Authors: Dabayeh A. A. , Topper T. H.
Published: March 2003

Smooth and notched specimens of a 319 cast aluminium alloy were fatigue tested under a Society of Automotive Engineers service load history in the as‐cast and hipped conditions. The hipping process, which includes subjecting the cast material to a high pressure at high temperature and then slowly cooling down to eliminate internal flaws, decreased the flaw size and improved the fatigue life of cast Al 319 smooth specimens. A 0.6‐mm‐diameter hole was drilled at the notch root of notched specimens to simulate a natural flaw at the notch root. Specimens with two different notch sizes were tested. Circular edge notches reduced the fatigue strength and a 0.6‐mm‐diameter drilled hole at the notch root resulted in a further reduction.

The fatigue lives of smooth specimens, notched specimens and notched specimens with a flaw at the notch root subjected to the service load history were predicted using the strain‐life approach, an effective strain‐life approach and a strain‐based intensity factor crack growth model. In crack growth modelling of the fatigue life of smooth cast aluminium specimens the flaw was modelled as a circular edge notch having the same diameter as the flaw. However, in the case of a flaw at a notch root the flaw was modelled as a three‐dimensional cavity subjected to the notch stress field and the crack length was predicted in the longitudinal and transverse directions of the specimen cross‐section. The strain‐life approach was unconservative for all specimen geometries studied. The effective strain‐life approach gave good predictions for smooth and blunt notched specimens but gave very conservative predictions for the specimens with flaws in the notch roots. The crack growth calculations gave accurate predictions for all the specimen geometries.
A rolling contact fatigue crack driven by squeeze fluid film
Authors: BOGDAÑSKI S.
Published: March 2003

ABSTRACT A new model of surface breaking rolling contact fatigue (RCF) crack driven by a coupled action of a squeeze oil film built up in the crack interior and a pressure exerted at the external contact interface was developed. The model can be applied to the ‘nominally dry’ contact couples with an occasional presence of liquid in the crack interior (wheel/rail contact) as well as to the elastohydrodynamic lubrication (EHL) conditions. In the first case, the contact load is a result of solid/solid interaction and can be determined by solving the FE contact problem, but the liquid contained in the crack interior forms a thin film between the crack faces changing their interaction into the type of liquid/solid. This liquid is being periodically squeezed under contact load and acts as a ‘squeeze film’ known from the lubrication theory. In the second case, the liquid (lubricating oil) is permanently present in the contact area and consequently in the vicinity of the crack mouth. This creates conditions for filling the crack with oil. Similarly as in the first case, the ‘squeeze oil film’ is built between the crack faces. The contact load in this case results from a liquid/solid interaction and can be approximated by the pressure and traction distributions obtained from the numerical solution of the elastohydrodynamic contact problem. In both cases the model can be used to determine the Linear Elastic Fracture Mechanics (LEFM) crack tip stress intensity histories during cyclic loading and consequently to predict the crack growth rate and direction.

An example of applying the model to the EHL case is given to explain the mechanisms and phenomena leading to the crack front loading. The cycle of rolling a roller over the crack was numerically simulated to obtain the mixed mode (I and II) SIF histories. In the analysis, the EHD pressure and traction were determined through the full solution of the EHD line contact problem accounting for the presence of a crack, whilst the pressure in the crack was found with the use of the wedge shaped squeeze oil film (SOF) model. Possible effects of the mode I and mode II stress intensity cycles on crack growth rate and direction are discussed. The solution indicates high pressure in the neighbourhood of the crack tip, exerted on the crack faces by the squeeze oil film. This leads to the ranges of the mode I and mode II SIF variations, which are larger than for the ‘dry’ and ‘fluid entrapment’ models, and can be an explanation for the crack growth rate observed in practice
Fatigue crack initiation and growth in AlMg4.5Mn butt weldments
Authors: Brandt U. , Lawrence F. V. , Sonsino C. M.
Published: March 2003

The fatigue life of full‐penetration and partial‐penetration 5 and 25 mm thickness AlMg4.5Mn (AA5083) aluminium alloy butt weldments was investigated under (R = 0 andR = − 1) constant amplitude loading. The fatigue lives of the tested specimens were predicted using an analytical model which estimated both the crack initiation and crack growth portions of the total fatigue life. The fatigue life of partial‐penetration weldments was found to be substantially less than that of full‐penetration weldments because of the greater stress concentrations of the incomplete joint penetration and the consequent absence of a substantial crack‐initiation life period. Tensile mean stresses (R = 0 versusR = − 1‐test conditions) markedly reduced the fatigue life of the weldments studied and greatly diminished the duration of the fatigue crack growth period. The extra material provided by the weld reinforcement noticeably increased the fatigue life of the partial‐penetration weldments. Weld angular distortion‐induced bending stresses greatly affected the smaller thickness (5 mm) full‐penetration weldments offsetting the fatigue strength bonus anticipated for small‐size weldments. Except for the predictions for R = − 1 full‐penetration weldments at long life, which the analytical model underestimated, the agreement between experiment and analytical prediction was within a factor of 2, that is, as good as can be generally expected.
A multiparameter approach to the prediction of fatigue crack growth in metallic materials
Authors: Soboyejo A. B. O. , Shademan S. , Foster M. , Katsube N. , Soboyejo W. O.
Published: March 2003

A multiparameter approach is proposed for the characterization of fatigue crack growth in metallic materials. The model assesses the combined effects of identifiable multiple variables that can contribute to fatigue crack growth. Mathematical expressions are presented for the determination of fatigue crack growth rates, da/dN, as functions of multiple variables, including stress intensity factor range, ΔK, stress ratio,R, crack closure stress intensity factor,Kcl , the maximum stress intensity factorKmax , nominal specimen thickness,t, frequency,Ω, and temperature,T. A generalized empirical methodology is proposed for the estimation of fatigue crack growth rates as a function of these variables. The validity of the methodology is then verified by making appropriate comparisons between predicted and measured fatigue crack growth data obtained from experiments on Ti–6Al–4V. The effects of stress ratio and specimen thickness on fatigue crack growth rates are then rationalized by crack closure considerations. The multiparameter model is also shown to provide a good fit to experimental data obtained for HY‐80 steel, Inconel 718 polycrystal and Inconel 718 single crystal. Finally, the implications of the results are discussed for the prediction of fatigue crack growth and fatigue life.
Low‐cycle fatigue/high‐cycle fatigue interactions in notched Ti‐6Al‐4V*
Authors: Lanning D. , Haritos G. K. , Nicholas T. , Maxwell D. C.
Published: March 2003

Combined low‐cycle fatigue/high‐cycle fatigue (LCF/HCF) loadings were investigated for smooth and circumferentially V‐notched cylindrical Ti–6Al–4V fatigue specimens. Smooth specimens were first cycled under LCF loading conditions for a fraction of the previously established fatigue life. The HCF 107 cycle fatigue limit stress after LCF cycling was established using a step loading technique. Specimens with two notch sizes, both having elastic stress concentration factors of Kt = 2.7, were cycled under LCF loading conditions at a nominal stress ratio of R = 0.1. The subsequent 106 cycle HCF fatigue limit stress at both R = 0.1 and 0.8 was determined. The combined loading LCF/HCF fatigue limit stresses for all specimens were compared to the baseline HCF fatigue limit stresses. After LCF cycling and prior to HCF cycling, the notched specimens were heat tinted, and final fracture surfaces examined for cracks formed during the initial LCF loading. Fatigue test results indicate that the LCF loading, applied for 75% of total LCF life for the smooth specimens and 25% for the notched specimens, resulted in only small reductions in the subsequent HCF fatigue limit stress. Under certain loading conditions, plasticity‐induced stress redistribution at the notch root during LCF cycling appears responsible for an observed increase in HCF fatigue limit stress, in terms of net section stress.
Weibull stress model for cleavage fracture under high‐rate loading
Authors: Gao X. , Dodds Jr R. H. , Tregoning R. L. , Joyce J. A.
Published: March 2003

This paper examines the effects of loading rate on the Weibull stress model for prediction of cleavage fracture in a low‐strength, A515‐70 pressure vessel steel. Interest focuses on low‐to‐moderate loading rates (K˙I < 2500 MPa √m s−1 ). Shallow cracked SE(B) specimens were tested at four different loading rates for comparison with previous quasi‐static tests on shallow notch SE(B)s and standard C(T)s. To utilize these dynamic experimental data, we assume that the Weibull modulus (m) previously calibrated using quasi‐static data remains invariant over the loading rates of interest. The effects of dynamic loading on the Weibull stress model enter through the rate‐sensitive material flow properties, the scale parameter (σu ) and the threshold Weibull stress (σw‐min ). Rate‐sensitive flow properties are modelled using a viscoplastic constitutive model with uniaxial, tension stress–plastic strain curves specified at varying plastic strain rates. The analyses examine dependencies ofσw‐minandσuonK˙I . Present results indicate thatσw‐minandσuare weak functions of loading rateK˙Ifor this pressure vessel steel. However, the predicted cumulative probability for cleavage exhibits a strong sensitivity toσuand, consequently, the dependency ofσuonK˙Iis sufficient to preclude use of the staticσuvalue for high loading rates.
Propagation of short fatigue cracks from notches in a Ni base superalloy: experiments and modelling
Authors: Sansoz F. , Brethes B. , Pineau A.
Published: March 2003

A notched specimen containing a semicircular slot (0.1 mm deep) was designed to simulate the growth of three‐dimensional short cracks under a stress concentration. Fatigue tests were performed on N18 superalloy at 650 °C with trapezoidal loading cycles. A high‐resolution optical measurement technique proved to be capable of detecting half‐surface crack increments as small as 10 μm, and the potential drop method was found to be inappropriate for very small crack lengths. The stress intensity factor, ΔK, was calculated using a weight functions method. Non‐uniform stress fields were determined by FEM modelling using elasto‐viscoplastic constitutive equations. The plasticity‐induced crack closure effect was calculated within the specimen using viscoplastic FEM modelling. The prediction of crack aspect ratio was used to investigate differences of closure along the crack front. The role of notch plasticity on these differences is discussed. Using these calculations, it is shown that the apparent differences between the growth behaviour of short and long cracks can be largely accounted for.
Strength, stability and size effects in the brittle behaviour of bonded joints under torsion: theory and experimental assessment
Authors: Pugno N. , Carpinteri A.
Published: March 2003

This paper describes a simple Griffith fracture energy criterion to predict the brittle failure load for tubular or non‐tubular bonded joints subjected to torsion. The theoretical solution generalizes to non‐tubular bonded joints an analogous result already presented in the literature and experimentally validated for particular tubular bonded joints. The stability of brittle crack propagation and the size effects on mechanical collapse behaviour, as well as the ductile–brittle transition, are analysed. Experimental measurements of failure loads under torsion for non‐tubular bonded joints agree satisfactorily with the theoretical predictions.
Effects of mean stress and crack closure on fatigue life of spot welds
Authors: HENRYSSON H.‐F.
Published: March 2003

The effects of mean stress and crack closure on fatigue life of spot welds were investigated. A review showed that several of the previously proposed mean stress corrections give similar results. Fatigue tests on shear and peel loaded specimens were carried out, and the results agreed with the corrections reviewed. The present study shows that crack closure explains the mean stress effects observed. The crack opening force for spot welds was obtained, both experimentally from F–N curves with different load ratios and analytically from the available mean stress corrections. This was verified with detailed finite element simulations. Finally, the experiments and simulations indicate that the use of linear damage accumulation in fatigue life prediction of spot welds can be non‐conservative.
Secondary elastic crack tip stresses which may influence very slow fatigue crack growth
Authors: PARIS P. C.
Published: March 2003

ABSTRACT Secondary elastic crack tip stress intensity factors are defined in a convenient way consistent with the primary stress intensity factor definition. Methods of evaluation of these secondary stress intensity factors are developed. They are evaluated for example configurations with cracks. It is shown that the first secondary term may be sufficient to evaluate the effects of secondary stresses beyond the usual stress intensity factor and nominal normal stresses parallel to the crack direction.
A fracture mechanics interpretation of the DNV brittle fracture criteria for ships and mobile offshore units
Authors: WALLIN K. , NEVASMAA P. , LAUKKANEN A.
Published: March 2003

ABSTRACT Design procedures against brittle fracture are either based on empirical experience based methods or theoretical fracture mechanics based methods. Frequently, the fracture mechanics based assessment procedures are suspected to be tools by which to get acceptance for poor quality. In reality, these suspicions are somewhat unfounded, since the manufacturing standards, implicitly, also assume some specific defect size. The standards contain specific inspection criteria, which in themselves lead to some specific possible accepted defect size. The problem has been that the manufacturing standards have not been evaluated based on an advanced fracture mechanics analysis. Here the Det Norske Veritas (DNV) rules for ships and mobile offshore units are analysed, applying the state‐of‐the‐art fracture mechanics assessment method SINTAP. Based on the assessment, the consistency of the DNV rules, with respect to strength level and inspection criteria is revealed. The assessment, also creates the possibility to extend the rules to thinner and higher strength steels.
Do Bertalanffy's growth curves result from optimal resource allocation?
Authors: Czarnołe‘ski Marcin , Kozłowski Jan
Published: March 2003

Bertalanffy's equation is commonly used to model indeterminate growth. Bertalanffy claimed that this growth pattern results from growth potential decreasing with age. An alternative approach provided by life history theory predicts that indeterminate growth is optimal for organisms in a seasonal environment and results not from decreasing growth potential but from allocating increasingly less energy with age into growth, and more into reproduction. Bertalanffy's curves are the result of evolutionary optimization and should not be used in optimization models as an assumption, but they can be used as a tool to describe the indeterminate growth pattern phenomenologically.
Viability in a pink environment: why “white noise” models can be dangerous
Authors: Morales
Published: March 2003

Analysis of long time series suggests that environmental fluctuations may be accurately represented by 1/f  noise (pink noise), where temporal correlation is found at several scales, and the range of fluctuations increases over time. Previous studies on the effects of coloured noise on population dynamics used first or second order autoregressive noise. I examined the importance of coloured noise for extinction risk using true 1/f  noise. I also considered the problem of estimating extinction risk with a limited sample of environmental variation. Pink noise environments increased extinction risk in random walk models where environmental variation affected the growth rate. However, pink noise environments decreased extinction risk in the Ricker model where environmental variation modified the carrying capacity. Underestimation of environmental variance almost always yielded underestimation of extinction risk. For either population viability analysis or management, we should carefully consider the long‐term behaviour of the environment as well as how we include environmental noise in population models.
The distribution of deer biomass in North America supports the hypothesis of exploitation ecosystems
Authors: Crête
Published: March 2003

The hypothesis of exploitation ecosystems (EEH) predicts that, along a productivity gradient in terrestrial environments, predators will regulate herbivores at a relatively constant density whenever primary productivity exceeds 700 g m−2 y−1; under this threshold, or if predators are absent, forage production determines herbivore density. I tested EEH using the pattern of deer biomass distribution over North America, the dominant family of large herbivores. Deer biomass increased from the High Arctic to the north of the boreal forest and remained in the same range southward within the gray wolf range; for the same latitude, deer biomass increased by a factor of 5 in the absence of wolves. South of the wolf range, there existed a clear relationship between actual evapotranspiration, a proxy of primary productivity, and deer biomass. Highest deer densities occurred in the south‐east of the continent where only white‐tailed deer are present. The observed pattern lends support to EEH and suggests that the removal of large predators in southern North America may have imposed an unprecedented pressure on plants eaten by deer.
Flee or fight uncertainty: plant strategies in relation to anticipated damage
Authors: Järemo  , Ripa  , Nilsson
Published: March 2003

In order to cope with damage, plants have evolved a number of strategies. We incorporate two of those strategies, compensatory regrowth and escaping damage in time, into a mathematical model in an attempt to outline under what circumstances one or the other of these phenotypic traits will evolve. Escaping damage in time is accomplished by flowering and setting seeds at a point of time when the risk of damage is low, whereas a compensatory capacity is made possible by activating a proportion of meristems that are left dormant. Our analysis suggests that damage that is predictable in time will favour phenotypes that flower late in the season and that have a good compensatory capacity. As damage becomes less predictable in time, a strategy that implies flowering as early as possible in the season and with no compensatory capacity at all, becomes advantageous.
Color polymorphism and sex ratio distortion in a cichlid fish as an incipient stage in sympatric speciation by sexual selection
Authors: Seehausen  , Van Alphen  , Lande
Published: March 2003

We investigated a Lake Victoria cichlid with a complex colour polymorphism that apparently represents one original species and two incipient species, all of which are sympatric. In laboratory breeding experiments we observed sex ratio distortion in certain matings between original and incipient species. Mate choice experiments show that males of the incipient species exhibit mating preferences against the original species, and males and females of the original species exhibit strong mating preferences against the incipient species. Mating preferences might evolve by sex ratio selection to avoid matings with distorted progeny sex ratios. Phenotype frequencies in nature suggest that mating preferences translate into mating frequencies, thus restricting gene flow and exerting disruptive sexual selection between the original and incipient species. The incipient species do not differ in morphology or ecology from the original species, implying that colour polymorphism, associated with sex ratio distortion, can be an incipient stage in sympatric speciation, and that disruption of gene flow can precede ecological differentiation.
Biogeographic effects of red fire ant invasion
Authors: Gotelli N.J. , Arnett A.E.
Published: March 2003

The red imported fire ant, Solenopsis invicta, was accidentally introduced to North America over 60 years ago and has spread throughout the southeastern United States. We document the biogeographic consequences of this invasion. We censused ground‐foraging ant communities on a 2000 km transect from Florida through New York that passed through invaded and intact biotas. Native ant species density peaks at mid‐latitudes in the eastern United States, and the location of this peak corresponds to the range limit of S. invicta. In uninvaded sites, ant species co‐occur less often than expected by chance. In the presence of S. invicta, community structure converges to a random pattern. Our results suggest that the effects of S. invicta on native ant communities are pervasive: not only does the presence of S. invicta reduce species density at local scales, it alters the co‐occurrence patterns of surviving species at a biogeographic scale.
Using sex ratios to estimate what limits reproduction in parasitoids
Authors: West S.A. , Rivero A.
Published: March 2003

The reproductive success of a foraging parasitoid may be limited by the number of eggs that she produces and/or the number of hosts that she can locate. Despite the significance for population dynamics and numerous areas of behavioural ecology, the relative importance of these factors remains an issue of contention. Attempts to resolve this controversy have been hindered because estimating the importance of factors limiting reproduction in the field can be extremely laborious and time consuming. We show how sex ratio data can be used as a relatively easy method to indirectly estimate the relative importance of the factors limiting reproduction. Sex ratio data from 48 samples of eight species suggest that: (a) the extent of host or egg limitation in a species varies between site collected and time of year; and (b) on average, species are at an intermediate position on the egg/host limitation continuum, with a bias towards host limitation.