bluuebunny/crossref_metadata_2025 huggingface dataset:
Similarities of stress concentrations in contact at round punches and fatigue at notches: implications to fretting fatigue crack initiation
Authors: Giannakopoulos  , Suresh  , Chenut
Published: March 2003

A linear elastic model of the stress concentration due to contact between a rounded flat punch and a homogeneous substrate is presented, with the aim of investigating fretting fatigue crack initiation in contacting parts of vibrating structures including turbine engines. The asymptotic forms for the stress fields in the vicinity of a rounded punch‐on‐flat substrate are derived for both normal and tangential loading, using both analytical and finite element methods. Under the action of the normal load,P, the ensuing contact is of width 2bwhich includes an initial flat part of width 2a. The asymptotic stress fields for the sharply rounded flat punch contact have certain similarities with the asymptotic stress fields around the tip of a blunt crack. The analysis showed that the maximum tensile stress, which occurs at the contact boundary due to tangential loadQ, is proportional to a mode II stress intensity factor of a sharp punch divided by the square root of the additional contact length due to the roundness of the punch,Q/(√(b − a)√πb). The fretting fatigue crack initiation can then be investigated by relating the maximum tensile stress with the fatigue endurance stress. The result is analogous to that of Barsom and McNicol where the notched fatigue endurance stress was correlated with the stress intensity factor and the square root of the notch‐tip radius. The proposed methodology establishes a ‘notch analogue’ by making a connection between fretting fatigue at a rounded punch/flat contact and crack initiation at a notch tip and uses fracture mechanics concepts. Conditions of validity of the present model are established both to avoid yielding and to account for the finite thickness of the substrate. The predictions of the model are compared with fretting fatigue experiments on Ti–6Al–4V and shown to be in good agreement.
An improved method of calculating the peak stress distribution for a broad‐band random process
Authors: Lü  , Jiao
Published: March 2003

An improved method of fast fatigue life prediction under broad‐band random loading is proposed, which is based on the power spectral density of stress in the critical points of structures and the peak stress distribution of a stationary Gaussian random process. The improved method has higher precision than other existing approximate methods that are based on the peak stress distribution.
Review of fatigue monitoring of agile military aircraft
Authors: Molent L. , Aktepe B.
Published: March 2003

Fatigue monitoring of airframes has developed over the decades to the stage where it is now incumbent for all fighter type aircraft to be fitted with an airborne fatigue monitoring system. These systems typically collect operational data for the calculation of the safe‐life or the inspection interval of the airframe.

This paper presents a state‐of‐the‐art review of fatigue monitoring systems of agile military aircraft. It reviews and comprehensively examines the techniques used in individual aircraft fatigue monitoring programs, and examines current systems and practises. Based on experience from Australian fatigue monitoring programs, it highlights some of the potential pitfalls in the systems and techniques. It also investigates the issues of strain gauge utilization and calibration, collection of flight parameter data, data integrity, comparisons with fatigue test results and fatigue damage models. Some of the problems with current systems are highlighted and requirements for future fatigue monitoring systems are suggested.

This review has determined that there is little uniformity in the fatigue management practices of operators and that many aspects of the fatigue management process have been overlooked by some structural integrity managers. Also, very few of the papers reviewed specified the philosophy or aims of their monitoring systems.
Fatigue life predictions of spot welds using coarse FE meshes
Authors: Henrysson H.‐F.
Published: March 2003

A new engineering method for fatigue life prediction of spot welds is presented. The method starts with a coarse finite element representation of each spot weld using shell elements and one beam element. Forces and moments at the spot weld are calculated using the finite element method and used in an analytical calculation of the stresses around the spot weld. Mode I and II stress intensity factors are calculated from these stresses. Thereafter, an equivalent stress intensity factor is calculated and the fatigue life prediction is made using one unique ΔK–Ncurve for spot welds. Good agreement is found between a ΔK–Ncurve derived from the Paris law and several experimental results from the literature, although in order to achieve this, a shear correction factor is required. This factor is discussed in relation to results from the literature.
Mechanical and microstructural investigations into the crack arrest behaviour of a modern 2¼Cr‐1 Mo pressure vessel steel
Authors: Bouyne E. , Joly P. , Houssin B. , Wiesner C. S. , Pineau A.
Published: March 2003

Tests were performed on a 2¼ Cr–1 Mo steel to measure the fracture toughness at initiation, KIc and at arrest, KIa . The results were compared with those obtained on another pressure vessel steel (A508) of similar strength. Two techniques were used to measure KIa : (i) isothermal compact crack arrest (CCA) tests, and (ii) specially designed thermal shock experiments using an externally notched ring. These specimens were cooled to −196 °C and then heated by induction in the centre of the ring to produce very steep thermal gradients. This caused crack initiation from the notch. The crack propagates very rapidly (∼500 m s−1 ) and stopped when it reached the warmer region of the specimen. The specimens were analysed using an elastic–plastic finite element method to determine KIa values. These tests reveal a greater temperature shift (∼100 °C) between KIc and KIa in 2¼ Cr–1 Mo steel than in A508 steel. Detailed metallographical examinations of the micromechanisms of crack propagation and arrest in the 2¼ Cr–1 Mo steel showed that this involves the nucleation of a three‐dimensional network of cleavage microcracks which change their direction at bainitic packet boundaries. The remaining uncracked ligaments between the cleavage microcracks break by ductile rupture mechanism
Theoretical fatigue–effective notch stresses at spot welds
Authors: Radaj D. , Lehrke H.‐P. , Greuling S.
Published: March 2003

Notch stress formulae are derived for the application of a notch stress approach to the fatigue assessment of spot welds. A keyhole notch is assumed to describe the edge of the weld spot between the overlapping plates. The stress fields at the keyhole notch under ‘singular’ and ‘non‐singular’ in‐plane loading modes inclusive of the stress concentration factors Kt are derived from the relevant Airy stress functions. The formulae are applied to typical loading cases of spot welds and compared with finite element solutions. Fatigue‐effective notch stresses inclusive of fatigue notch factors Kf are calculated by applying the microstructural support hypothesis of Neuber. The notch stresses at the keyhole are also derived for out‐of‐plane shear loading based on the relevant harmonic stress functions. The multiaxial notch stresses at the weld spot edge are thus completely described.
Fatigue crack growth and crack closure in an AlMgSi alloy
Authors: Borrego L. P. , Ferreira J. M. , Costa J. M.
Published: March 2003

This study reports an experimental investigation of fatigue crack propagation in AlMgSi1‐T6 aluminium alloy using both constant and variable load amplitudes. Crack closure was monitored in all tests by the compliance technique using a pin microgauge. For the constant amplitude tests four different stress ratios were analysed. The crack closure parameterUwas calculated and related with ΔKand the stress ratio,R. The threshold of the stress intensity factor range, ΔKth , was also obtained. Fatigue crack propagation tests with single tensile peak overloads have been performed at constant load amplitude conditions. The observed transient post overload behaviour is discussed in terms of the overload ratio, ΔKbaseline level andR. The crack closure parameterUtrends are compared with the crack growth transients. Experimental support is given for the hypothesis that crack closure is the main factor determining the transient crack growth behaviour following overloads on AlMgSi1‐T6 alloy for plane stress conditions.
Interim fatigue design recommendations for fillet welded joints under complex loading
Authors: Maddox S. J. , Razmjoo G. R.
Published: March 2003

Current fatigue design methods for assessing welded steel structures under complex combined or multiaxial loading are known to be potentially unsafe. This has led to a number of research projects over the past 10 years. Some progress has been made in developing better methods, but they are not yet suitable for general design. This paper presents an interim solution based on a review and analysis of relevant published data; all referring to fatigue failure from a fillet weld toe. These indicate that Eurocode 3/IIWS–Ncurve FAT80/3 (negative inverse slope of 3) is suitable for combined normal and shear stresses acting in phase, and possibly for out‐of‐phase (i.e. non‐proportional loading) bending and shear if the shear stress is not due to torsion. However, a shallower curve FAT80/5 is necessary for out‐of‐phase torsion and bending or tension. Both curves are used in conjunction with the nominal maximum principal stress range occurring during the loading cycle.
A mechanistic approach to critical‐distance methods in notch fatigue
Authors: Taylor D.
Published: March 2003

One approach to the prediction of notch fatigue limits is the Neuber method in which stresses are averaged over a critical distance ahead of the notch. In recent years this theory has been updated by the discovery of an analytical method for finding the critical distance for a given material, which shows that the appropriate distance is 2ao ,aobeing El Haddad’s short‐crack constant. The present author has advocated this approach, which he has called the Line Method (LM) and has tested it extensively against experimental data. However the method still remains essentially empirical; the aim of the present paper was to try to link this approach to the known mechanisms of crack growth at a notch. It is proposed that the LM is successful because it expresses the conditions necessary for growth of a small crack located at the notch root. The argument is developed by starting from the resistance curves used to predict non‐propagating cracks and by linking the LM with the expression for stress intensity,K, derived by the crack‐line loading method. The results provide some mechanistic justification for the use of the LM for sharp, crack‐like notches; its success for other types of notch (i.e. blunt notches and short notches) requires a different explanation.
Low‐costJ‐Rcurve estimation based on CVN upper shelf energy
Authors: Wallin K.
Published: March 2003

J‐R curve testing is costly and difficult. The results may also sometimes be unreliable. For less demanding structures, J‐R curve testing is therefore not practical. The only way to introduce tearing instability analysis for such cases is to estimate the J‐R curves indirectly from some simpler test. The Charpy‐V notch test provides information about the energy needed to fracture a small specimen in half. On the upper shelf this energy relates to ductile fracture resistance and it is possible to correlate it to the J‐R curve. Here, 112 multispecimen J‐R curves from a wide variety of materials were analysed and a simple power‐law‐based description of the J‐R curves was correlated to the CVNUS energy. This new correlation corresponds essentially to a 5% lower bound and conforms well with the earlier correlations, regardless of the definition of the ductile fracture toughness parameter.
An overview of the damage approach of durability modelling at elevated temperature
Authors: Chaboche J.‐L. , Gallerneau F.
Published: March 2003

Lifetime prediction techniques for components working at elevated temperature are revisited. Two damage approaches in which time effects at high temperature are introduced in different ways are discussed in greater detail. First, a creep–fatigue damage model considers the interaction of the two processes during the whole life before macrocrack initiation; and second, a creep–fatigue–oxidation model separates the fatigue life into two periods: during initiation the environment‐assisted processes interact with fatigue, although bulk creep damage only interacts during the micropropagation period. The second model is illustrated by its application to a coated single‐crystal superalloy used in aerojet turbine blades. Its capabilities are illustrated in a number of isothermal and thermomechanical fatigue tests. Anisotropy effects are also briefly discussed and a special test, introducing cyclic thermal gradients through the wall thickness of a tubular component, demonstrates the predictive capabilities for actual engine conditions.
Fatigue behaviour of riveted Glare lap joints
Authors: Lazzeri L.
Published: March 2003

Fibre metal laminates, such as Glare, are a family of materials with very interesting properties for fatigue‐critical applications. This article describes the results of a research programme carried out to evaluate the fatigue and damage tolerance characteristics of riveted Glare lap joints, representative of fuselage longitudinal joints. The comparison with the behaviour of metallic joints shows that different contributions are made to the total fatigue life, with the crack propagation life being by far the longer one for the Glare material whereas the crack nucleation life covers almost the entire fatigue life for metallic joints. Design rules should take this peculiar behaviour into consideration, to achieve the maximum benefit in the use of this class of materials in fatigue‐critical applications.
Mechanisms for corrosion fatigue crack propagation
Authors: SHIPILOV S. A.
Published: March 2003

ABSTRACTThe corrosion fatigue crack growth (FCG) behaviour, the effect of applied potential on corrosion FCG rates, and the fracture surfaces were studied for high‐strength low‐alloy steels, titanium alloys, and magnesium alloys. During investigation of the effect of applied potential on corrosion FCG rates, polarization was switched on for a time period in which it was possible to register the change in the crack growth rate corresponding to the open‐circuit potential and to measure the crack growth rate under polarization. Due to the higher resolution of the crack extension measurement technique, the time rarely exceeded 300 s. This approach made possible the observation of a non‐single mode effect of cathodic polarization on corrosion FCG rates. Cathodic polarization accelerated crack growth when the maximum stress intensity (Kmax) exceeded a certain well‐defined critical value characteristic for a given material‐solution combination. WhenKmaxwas lower than the critical value, the same cathodic polarization, with all other conditions (specimen, solution, pH, loading frequency, stress ratio, temperature, etc.) being equal, retarded or had no influence on crack growth. The results and fractographic observations suggested that the acceleration in crack growth under cathodic polarization was due to hydrogen‐induced cracking (HIC). Therefore, critical values ofKmax, as well as the stress intensity range (ΔK) were regarded as corresponding to the onset of corrosion FCG according to the HIC mechanism and designated asKHICand ΔKHIC. HIC was the main mechanism of corrosion FCG atKmax>KHIC(ΔK> ΔKHIC). For most of the material‐solution combinations investigated, stress‐assisted dissolution played a dominant role in the corrosion fatigue crack propagation atKmax<KHIC(ΔK< ΔKHIC).
Initiation and early growth of fatigue cracks in an aerospace aluminium alloy
Authors: Barter S. A. , Sharp P. K. , Holden G. , Clark G.
Published: March 2003

Abstract Material imperfections usually play a substantial role in the early stages of fatigue cracking. This article presents some observations concerning fatigue crack initiating flaws and early crack growth in 7050‐T7451 aluminium alloy specimens and in full‐scale fatigue test articles with a production surface finish. Equivalent initial flaw size (EIFS) approaches used to evaluate the fatigue implications of metallurgical, manufacturing and service‐induced features were refined by using quantitative fractography to acquire detailed information on the early crack growth behaviour of individual cracks; the crack growth observations were employed in a simple crack growth model developed for use in analysing service crack growth. The use of observed crack growth behaviour reduces the variability which is inherent in EIFS approaches which rely on modelling the whole of fatigue life, and which can dominate EIFS methods. The observations of realistic initial flaws also highlighted some of the significant factors in the fatigue life‐determining early fatigue growth phase, such as surface treatment processes. Although inclusions are often regarded as the single most common type of initiating flaw, processes which include etching can lead to etch pitting of grain boundaries with significant fatigue life implications.
Mode I crack in a soft ferromagnetic material
Authors: LIANG W. , FANG D. , SHEN Y.
Published: March 2003

ABSTRACT In the existing magnetoelastic theories, stress is proportional to the square of magnetic intensity and the linear model developed is usually used to analyse magnetoelastic problems. For a crack problem, the perturbation of the magnetic field caused by deformation is not much less than the applied field. In this paper, complex potentials for a mode I crack with a nonlinear relation for magnetic intensity are developed. The boundary conditions on crack faces are represented in terms of the continuity of the magnetic field. A solution of the crack problem is obtained by solving the Riemann‐Hilbert problem. Making use of the solution, the effects of the boundary conditions on the crack faces on the magnetoelastic coupling are discussed.
The characteristics of fatigue under isothermal and thermo‐mechanical load in Cr–Ni–Mo cast hot work die steel
Authors: FANG J. R. , JIANG Q. C. , GUAN Q. F. , WANG S. Q.
Published: March 2003

ABSTRACTHigh temperature isothermal fatigue (IF) and in‐phase thermo‐mechanical fatigue (TMF) tests in load control were carried out in cast hot work die steel. At the same load amplitude, the fatigue lives obtained in the in‐phase TMF tests are lower than those obtained in the isothermal tests. Observations of fracture surface and the response of stress–strain reveal that cyclic creep in the tensile direction occurs and the intergranular cracks dominate in TMF tests, whereas cyclic creep in the compressive direction occurs and the path of the crack growth is mainly transgranular in IF tests. A model of life prediction, based on the Chaboche law, was discussed. Damage coefficients that are functions of the maximum temperature and the variation of temperature are introduced in the model so as to evaluate TMF lives in load control. With this method, the lifetime prediction gives results corresponding well to experimental data.
Characteristics of microcrack initiation in CuNiAl shape memory alloy
Authors: LU Y. H. , QIAO L. J. , CHU W. Y.
Published: March 2003

The mechanism of microcrack initiation in CuNiAl shape memory alloy was investigated through in situ tensile tests under optical microscopy, scanning electron microscopy and transmission electron microscopy. The results indicated that various martensite phases appeared first around a notch tip upon loading, and then microcracks initiated along the martensite/parent interface. The small martensite regions could transform back to the parent phase not only upon unloading but also upon loading or during crack propagation. There was no dislocation emission and motion before martensite formation. The slip bands formed after the crack had propagated a definite distance, and the microcracks nucleated and the cracks propagated either along the slip bands or the martensite/parent interface.
A non‐local theory applied to high cycle multiaxial fatigue
Authors: Morel F. , Palin‐Luc T.
Published: March 2003

ABSTRACT The stress gradient effect on the fatigue limit is an important factor which has to be taken into account for an efficient transfer of fatigue data from laboratory tests to design of industrial components. A short review of some multiaxial high cycle fatigue criteria considering this effect is presented. On the basis of the two local mesoscopic approaches of Papadopoulos, two new non‐local high cycle multiaxial fatigue criteria are developed. These proposals are based on the concept of volume influencing fatigue crack initiation. Their predictions are compared with experimental multiaxial fatigue data on four materials (a mild steel, two high strength steels and a spheroidal graphite cast iron). The accuracy of the two local Papadopoulos criteria and of the non‐local proposals are compared and discussed, together with the physical interpretation of the threshold defining the volume influencing fatigue crack initiation.
Variable amplitude fatigue of spot welds
Authors: HENRYSSON H.‐F.
Published: March 2003

A new method for fatigue life prediction of spot welds subjected to variable amplitude loads is proposed. The method is based on the concept of crack closure and is experimentally verified with three different specimens and four different load signals with variable amplitude. Experimental fatigue lives were found to be within a factor of three from the predicted lives. To start with, the stress intensity factor history at the spot weld is calculated with a finite element analysis. Then, crack closure is taken into account: the crack opening stress intensity factor, which is assumed to be constant, is determined from the maximum and minimum in the history. All stress intensities lower than the crack opening level are filtered from the calculated history. The filtered history is then analysed with rain flow count. Finally, fatigue life is predicted with the Palmgren–Miner cumulative damage rule together with an effective (closure‐free) curve for spot welds. In addition, single overload tests were carried out to investigate the assumption of a constant crack opening stress.
Material aspects and design methods for high‐pressure‐components of hard steels subject to cyclical loads
Authors: BACHER‐HÖCHST M. , LINKEWITZ T. , MÜHLEDER F.
Published: March 2003

This paper deals with high cycle fatigue design methods for internal pressure loaded components of diesel injection systems. Intensive studies of material fatigue behaviour and the derivation of fatigue calculation methods in combination with adequate finite element calculations led to successful prediction methods. One interesting approach is the description of notch effects with volume integrals, which is outlined in more detail.
Linking aphid honeydew, throughfall, and forest floor solution chemistry of Norway spruce
Authors: Stadler Bernhard , Michalzik Beate
Published: March 2003

In a seminatural manipulation experiment with artificial irrigation we followed throughfall and forest floor solution chemistry collected underneath aphid infested and uninfested Norway spruce. Solutions underneath infested trees showed significantly higher concentrations of dissolved organic carbon (DOC) but lower concentrations of dissolved organic nitrogen (DON), NO3‐N, and NH4‐N in throughfall solutions and of NH4‐N in forest floor solutions. Average concentrations were 40.5% (DON), 27.5% (NO3‐N), and 46.2% lower (NH4‐N) underneath infested trees in throughfall solutions, and 19.5% (DON), 9.4% (NO3‐N), and 42.0% (NH4‐N) lower in forest floor solutions. Differences in throughfall were more pronounced than in forest floor leachates. It is likely that honeydew is fuelling the metabolism of micro‐organisms and thus critically affects above and below ground nutrient cycles. We emphasize the importance of linking the biology of herbivores and micro‐organisms with geochemical processes.
The cost of an immune response: vaccination reduces parental effort
Authors: Råberg L. , Nilsson J.‐Å. , Ilmonen P. , Stjernman M. , Hasselquist D.
Published: March 2003

A fundamental assumption of theories of the ecology and evolution of inducible defences is that protective responses to attacks by parasites or predators should not only have benefits, but also costs. The vertebrate immune system is by far the best studied example of an inducible defence, yet little is known about the costs of an immune response, especially in natural populations. To test if an immune response per se is costly, we induced an antibody response in female blue tits, Parus caeruleus, by immunising them with human diphtheria–tetanus vaccine, and compared their nestling‐feeding rate with that of saline‐injected controls. We found that vaccinated females reduced their nestling feeding rate, thus demonstrating a cost of the immune response in the currency of parental effort.
Increased ecosystem variability and reduced predictability following fertilisation: Evidence from palaeolimnology
Authors: Cottingham K.L. , Rusak J.A. , Leavitt P.R.
Published: March 2003

We tested the hypothesis that fertilisation increases community and ecosystem variability while reducing predictability using annual fossil records from fertilised Lake 227, Experimental Lakes Area, Ontario, Canada. Comparison of fossil pigments from unperturbed and eutrophied periods using a median‐log Levene’s test demonstrated that variability increased significantly during enrichment for total algae (chlorophyll a, sum of carotenoids), cyanobacteria (aphanizophyll, lutein‐zeaxanthin), chlorophytes (pheophytin b, lutein‐zeaxanthin), and cryptophytes (alloxanthin), but not for other algal taxa (chrysophytes, dinoflagellates) or herbivory (pheophorbides). Dynamic linear models (DLMs) of individual time series showed that forecast accuracy declined during enrichment for taxa which showed increased variability, while forecast uncertainty increased for all fossil pigments. DLMs of simulated data identified a strong inverse relationship between variability and predictability, suggesting that predictability will decline whenever variability increases. These findings imply that anthropogenic eutrophication of ecosystems may destabilise lakes, obscure impacts of global change, and reduce the sensitivity of whole‐ecosystem experiments.
Environmental effects on constitutive and inducible resin defences of Pinus taeda
Authors: Lombardero M.J. , Ayres M.P. , Lorio Jr P.L. , Ruel J.J.
Published: March 2003

The ecological literature abounds with studies of environmental effects on plant antiherbivore defences. While various models have been proposed (e.g. plant stress, optimal allocation, growth‐differentiation balance), each has met with mixed support. One possible explanation for the mixed results is that constitutive and induced defences are differentially affected by environmental conditions. In this study, constitutive oleoresin flow from Pinus tadea was least during periods of rapid tree growth and most when drought conditions limited growth; this is as expected if constitutive secondary metabolism is a function of the carbohydrate pool size after growth has been maximised. Induced increases in resin flow, however, were greatest in the fastest growing trees during the season of greatest growth. Apparently, resin production becomes an allocation priority after wounding but not before. Understanding environmental effects on plant antiherbivore defences requires physiological and evolutionary models that account for the differences between constitutive and induced secondary metabolism.
The carbon–nutrient balance hypothesis: its rise and fall
Authors: Hamilton J.G. , Zangerl A.R. , DeLucia E.H. , Berenbaum M.R.
Published: March 2003

The idea that the concentration of secondary metabolites in plant tissues is controlled by the availability of carbon and nitrogen in the environment has been termed the carbon–nutrient balance hypothesis (CNB). This hypothesis has been invoked both for prediction and for post hoc explanation of the results of hundreds of studies. Although it successfully predicts outcomes in some cases, it fails to such an extent that it cannot any longer be considered useful as a predictive tool. As information from studies has accumulated, many attempts have been made to save CNB, but these have been largely unsuccessful and have managed only to limit its utility. The failure of CNB is rooted in assumptions that are now known to be incorrect and it is time to abandon CNB because continued use of the hypothesis is now hindering understanding of plant–consumer interactions. In its place we propose development of theory with a firm evolutionary basis that is mechanistically sophisticated in terms of plant and herbivore physiology and genetics.
Complementary representation and zones of ecological transition
Authors: Gaston K.J. , Rodrigues A.S.L. , Van Rensburg B.J. , Koleff P. , Chown S.L.
Published: March 2003

Minimum complementary sets of sites that represent each species at least once have been argued to provide a nominal core reserve network and the starting point for regional conservation programs. However, this approach may be inadequate if there is a tendency to represent several species at marginal areas within their ranges, which may occur if high efficiency results from preferential selection of sites in areas of ecological transition. Here we use data on the distributions of birds in South Africa and Lesotho to explore this idea. We found that for five measures that are expected to reflect the location of areas of ecological transition, complementary sets tend to select higher values of these measures than expected by chance. We recommend that methods for the identification of priority areas for conservation that incorporate viability concerns be preferred to minimum representation sets, even if this results in more costly reserve networks.
Poor male function favours the coexistence of sexual and asexual relatives
Authors: Britton  , Mogie
Published: March 2003

Classical models of the evolution of sex typically assume that an asexual lineage, once derived, is reproductively separate from the sexual lineage from which it was derived. However, many asexuals, including hermaphrodite plants, produce male gametes capable of fertilising the eggs of co‐existing sexuals, giving rise to sexual and asexual progeny. This male function of asexuals may be poor, and it has been proposed that this could favour sexuality and adversely affect the successful establishment of asexual lineages. We show that things are more complicated than this; the effect is frequency‐dependent and poor male function may sometimes favour asexuality. In a spatially distributed population of flowering plants, it can prevent the successful invasion of either reproductive mode by the other via long‐range dispersal. Consequently invasions must be driven by short‐range dispersal, and are therefore extremely slow. Thus poor male function favours long‐term co‐existence of sexuals and asexuals. When coupled with the superior ability of asexuals to colonise virgin territory after an Ice Age, it may explain current ecological distribution patterns.
Subsidized Island Biogeography Hypothesis: another new twist on an old theory
Authors: Anderson W.B. , Wait D.A.
Published: March 2003

We present a new hypothesis for predicting and describing patterns of species diversity on small islands and habitat fragments. We have modified the traditional island biogeography equilibrium theory to incorporate the influence of spatial subsidies from the surrounding matrix, which vary among islands and habitat fragments, on species diversities. The modification indicates three possible directions for the effects of spatial subsidies on diversity, which depend on where the focal community falls on the hypothesized unimodal curve of the productivity–diversity relationship. The idea is novel because no recent syntheses of productivity–diversity–area relationships examine the role of allochthonous resources on recipient communities’ diversity patterns.
Positive effect of predators on prey growth rate through induced modifications of prey behaviour
Authors: Peacor Scott D.
Published: March 2003

Many prey modify behaviour in response to predation risk and this modification frequently leads to a foraging rate reduction. Although this reduction can have a clear direct negative effect on prey growth rate, theory predicts that a net positive effect can occur when the combined reduction in foraging by the entire population leads to a large increase in resource level. Here, I present experimental results that corroborate this counterintuitive prediction: the predation threat of ‘nonlethal’ caged larval dragonflies (Anax longipes) caused a net increase in small bullfrog (Rana catesbeiana) growth. A behavioural response (i.e. a reduction in activity level and microhabitat usage) was likely to have negatively affected growth, but was offset by a positive effect on growth from a large increase in resource levels (measured using a bioassay). Further, the positive Anax effect was dependent on nutrient level, illustrating the role of the resource response magnitude. Results of this study are discussed in the context of studies in which Anax had the opposite (i.e. negative) effect on tadpole growth. Predator‐induced modifications in prey behaviour can have large negative or positive effects on prey growth, the sign and magnitude of which are dependent on relative species density and resource dynamics.
The ‘abundant centre’ distribution: to what extent is it a biogeographical rule?
Authors: Sagarin Raphael D. , Gaines Steven D.
Published: March 2003

Several ecological and evolutionary hypotheses are based on the assumption that species reach their highest abundance in the centre of their range and decline in abundance toward the range edges. We reviewed empirical tests of this assumption, which we call the ‘abundant centre’ hypothesis. We found that of 145 separate tests conducted as part of 22 direct empirical studies, only 56 (39%) support the abundant centre hypothesis. More problematic than the percentage of studies that support the hypothesis is the finding that most studies inadequately sampled the species’ ranges. Only two of the studies analysed data that were collected throughout the species’ range. The remaining studies relied on data from a small number of points in their analysis, meaning that the range edges were severely under‐sampled. Patterns of abundance across the entire range must be known to draw testable hypotheses about the consequences of species’ geographical abundance distributions. Indirect tests of the abundant centre hypothesis, in which ecological or evolutionary expectations of abundant centre distributions were examined, did not support or reject the abundant centre hypothesis overall. We conclude that more exploration of species’ abundance distributions is necessary and we suggest methods to use in future studies.
Connecting geographical distributions with population processes
Authors: Maurer Brian A. , Taper Mark L.
Published: March 2003

The geographical distribution of a species is determined by a large number of complex processes operating over spatial scales spanning 10 orders of magnitude. Patterns in population processes have been described at numerous scales. We show that two patterns, measured at different scales, jointly allow us to infer heretofore unknown patterns in the distribution of demographic patterns across the geographical range of a species. The resulting model describes three fundamentally different modes of geographical variation in vital rates of populations. One mode is characterized by a positive nonlinear relationship between the maximum rate of population growth and the intensity of intraspecific competition across a geographical range. That is, populations that grow rapidly are also those where individuals experience the greatest per capita negative effect of the presence of other individuals. The second mode of behaviour is described by a negative nonlinear relationship between maximum growth rate and density dependence. Under this scenario, populations with low capacity to grow rapidly have highest intensities of intraspecific competitive effects. A third mode of behaviour is characterized by a weak positive relationship between growth rate and intraspecific competition, with very little geographical variation in maximum growth rate. A survey of studies relating temporal means and variances in population abundance for a variety of species indicate that the second mode of geographical variation in population dynamics across species ranges is the most common, though a few species appear to be characterized by the third mode.
Effects of regional pool size on local diversity in small‐scale annual plant communities
Authors: Valone Thomas J. , Hoffman Catherine D.
Published: March 2003

The shape of the relationship between local and regional richness has been used to infer the relative strength of local vs. regional processes in structuring communities. While most empirical work has demonstrated linear relationships, recent reviews suggest that prior work has contained biases by selecting communities from different biological provinces and by defining large‐scale local communities. We evaluate the utility of inferring interaction strength from local–regional relationships by examining 11 years of data from small‐scale, annual plant communities at one site with strong local interactions. Both winter and summer annual plant assemblages exhibited linear rather than saturating relationships over a wide range of regional pool sizes. Our empirical work supports recent theory suggesting that local–regional relationships cannot be used to infer the strength of interactions in communities.
Spatio‐temporal variation in Markov chain models of subtidal community succession
Authors: Hill M. Forrest , Witman Jon D. , Caswell Hal
Published: March 2003

In this paper we ask whether succession in a rocky subtidal community varies in space and time, and if so how much affect that variation has on predictions of community dynamics and structure. We describe succession by Markov chain models based on observed frequencies of species replacements. We use loglinear analysis to detect and quantify spatio‐temporal variation in the transition matrices describing succession. The analysis shows that space and time, but not their interaction, have highly significant effects on transition probabilities. To explore the ecological importance of the spatio‐temporal variability detected in this analysis, we compare the equilibria and the transient dynamics among three Markov chain models: a time‐averaged model that includes the effects of space on succession, a spatially averaged model that include the effects of time, and a constant matrix that averages over the effects of space and time. All three models predicted similar equilibrium composition and similar rates of convergence to equilibrium, as measured by the damping ratio or the subdominant Lyapunov exponent. The predicted equilibria from all three models were very similar to the observed community structure. Thus, although spatial and temporal variation is statistically significant, at least in this system this variation does not prevent homogeneous models from predicting community structure.
Jasmonate‐deficient plants have reduced direct and indirect defences against herbivores
Authors: Thaler Jennifer S. , Farag Mohamed A. , Paré Paul W. , Dicke Marcel
Published: March 2003

Plants employ a variety of defence mechanisms, some of which act directly by having a negative effect on herbivores and others that act indirectly by attracting natural enemies of herbivores. In this study we asked if a common jasmonate‐signalling pathway links the regulation of direct and indirect defences in plants. We examined the performance of herbivores (direct defence) and the attraction of natural enemies of herbivores (indirect defence) to wild‐type tomato plants and mutant plants that are deficient in the production of the signalling hormone jasmonic acid. Wild‐type plants supported lower survivorship of caterpillars compared with jasmonic acid‐deficient plants. Damaged wild‐type plants were more attractive to predaceous mites compared with undamaged wild‐type plants, whereas damaged jasmonate‐deficient plants were not more attractive to predators. Damaged wild‐type plants induced a greater production of volatile compounds (primarily the sesquiterpene β‐caryophyllene and the monoterpenes α‐pinene, β‐pinene, 2‐carene and β‐phellandrene) compared with damaged jasmonate‐deficient plants. Treating jasmonate‐deficient plants with exogenous jasmonic acid restored both the direct and indirect defence capabilities, demonstrating that jasmonic acid is an essential regulatory component for the expression of direct and indirect plant defence.
Molecular evidence for shifts in polysaccharide composition associated with adaptation of soybean Bradyrhizobium strains to the Brazilian Cerrado soils
Authors: Da Costa Coutinho Heitor Luiz , Kay Hazel E. , Manfio Gilson Paulo , Neves Maria Cristina Prata , Ribeiro José Roberto A. , Rumjanek Norma G. , Beringer John E.
Published: March 2003

Pyrolysis mass spectrometry (PyMS) and DNA fingerprinting (RAPD and RSα hybridization) were used to characterize soybean inoculant strains and root nodule isolates of bradyrhizobia from the Brazilian Cerrado soils. Most isolates were shown to be derived from the inoculant strains on the basis of genotype comparisons by DNA fingerprinting. Phenotypic analysis (using PyMS) of the strains and separately of the polysaccharides derived from them showed that the nodule isolates differed from the parental strains, suggesting adaptation to the Cerrado soil environment. The extent of the differences between the derivatives and inoculant strains was similar for comparisons made on the basis of whole‐cell preparations or from the isolated polysaccharides, indicating that the adaptation was caused by changes in the composition of the polysaccharides produced.
Anaerobic degradation of naphthalene by a pure culture of a novel type of marine sulphate‐reducing bacterium
Authors: Galushko  , Minz  , Schink  , Widdel
Published: March 2003

Incubation of marine sediment in anoxic, sulphate‐rich medium in the presence of naphthalene resulted in the enrichment of sulphate‐reducing bacteria. Pure cultures with short, oval cells (1.3 by 1.3–1.9 μm) were isolated that grew with naphthalene as the only organic carbon source and electron donor for sulphate reduction to sulphide. One strain, NaphS2, was characterized. It affiliated with completely oxidizing sulphate‐reducing bacteria of the δ‐subclass of the Proteobacteria, as revealed by 16S rRNA sequence analysis. 2‐Naphthoate, benzoate, pyruvate and acetate were used in addition to naphthalene. Quantification of substrate consumption, sulphide formation and formed cell mass revealed that naphthalene was completely oxidized with sulphate as the electron acceptor.
Effect of temperature on sulphate reduction, growth rate and growth yield in five psychrophilic sulphate‐reducing bacteria from Arctic sediments
Authors: Knoblauch Christian , Jørgensen Bo Barker
Published: March 2003

Five psychrophilic sulphate‐reducing bacteria (strains ASv26, LSv21, PSv29, LSv54 and LSv514) isolated from Arctic sediments were examined for their adaptation to permanently low temperatures. All strains grew at −1.8°C, the freezing point of sea water, but their optimum temperature for growth (Topt) were 7°C (PSv29), 10°C (ASv26, LSv54) and 18°C (LSv21, LSv514). Although Topt was considerably above the in situ temperatures of their habitats (−1.7°C and 2.6°C), relative growth rates were still high at 0°C, accounting for 25–41% of those at Topt. Short‐term incubations of exponentially growing cultures showed that the highest sulphate reduction rates occurred 2–9°C above Topt. In contrast to growth and sulphate reduction rates, growth yields of strains ASv26, LSv54 and PSv29 were almost constant between −1.8°C and Topt. For strains LSv21 and LSv514, however, growth yields were highest at the lowest temperatures, around 0°C. The results indicate that psychrophilic sulphate‐reducing bacteria are specially adapted to permanently low temperatures by high relative growth rates and high growth yields at in situ conditions.
Preferential occurrence of diazotrophic endophytes, Azoarcus spp., in wild rice species and land races of Oryza sativa in comparison with modern races
Authors: Engelhard Margret , Hurek Thomas , Reinhold‐Hurek Barbara
Published: March 2003

Several diazotrophic species of Azoarcus spp. occur as endophytes in the pioneer plant Kallar grass. The purpose of this study was to screen Asian wild rice and cultivated Oryza sativa varieties for natural association with these endophytes. Populations of culturable diazotrophs in surface‐sterilized roots were characterized by 16S rDNA sequence analysis, and Azoarcus species were identified by genomic fingerprints. A. indigens and Azoarcus sp. group C were detected only rarely, whereas Azoarcus sp. group D occurred frequently in samples of flooded plants: in 75% of wild rice, 80% of land races of O. sativa from Nepal and 33% of modern cultivars from Nepal and Italy. The putatively endophytic populations of diazotrophs differed with the rice genotype. The diversity of cultured diazotrophs was significantly lower in wild rice species than in modern cultivars. In Oryza officinalis (from Nepal) and O. minuta (from the Philippines), Azoarcus sp. group D were the predominant diazotrophic putative endophytes in roots. In contrast, their number was significantly lower in modern cultivars of O. sativa, whereas numbers and diversity of other diazotrophs, such as Azospirillum spp., Klebsiella sp., Sphingomonas paucimobilis, Burkholderia sp. and Azorhizobium caulinodans, were increased. In land races of O. sativa, the diazotrophic diversity was equally high; however, Azoarcus sp. was found in high apparent numbers. Similar differences in populations were also observed in a culture‐independent approach comparing a wild rice (O. officinalis) and a modern‐type O. sativa plant: in clone libraries of root‐associated nitrogenase (nifH ) gene fragments, the diazotrophic diversity was lower in the wild rice species. New lineages of nifH genes were detected, e.g. one deeply branching cluster within the anf (iron) nitrogenases. Our studies demonstrate that the natural host range of Azoarcus spp. extends to rice, wild rice species and old varieties being preferred over modern cultivars.