bluuebunny/crossref_metadata_2025 huggingface dataset:
A PROBABILISTIC MODEL FOR INITIATION AND PROPAGATION OF SURFACE CRACKS IN WELDED JOINTS
Authors: Lecsek R. L. , Yee R. , Lambert S. B. , Burns D. J.
Published: August 2010

Abstract Monte Carlo simulations of fatigue in welded joints have been performed using an approach which combines a model for initiation with a multiple surface crack propagation model. The results have been compared with experiments on T‐plate welded joints in which initiation and propagation of surface cracks were monitored using potential drop techniques. Predictions of initiation life using a local strain approach were conservative. Despite this underprediction of initiation life, predictions of total fatigue life were very good as a consequence of accurate simulations of propagation life and the fact that initiation represented on average only 12 to 22% of total life, depending on stress level. The initiation model considered variability due to the local weld toe angle and radius, and material strain–life behaviour. The only variability considered in the propagation analysis was the position and timing of initiation events, which leads to variability in coalescence. The underprediction of the variability in propagation and total lives was attributed to the underprediction of initiation life and the fact that out‐of‐plane coalescence effects were ignored.
STRUCTURE AND FRACTURE BEHAVIOR OF t‐ZrO2/Al2O3 LAMELLAR COMPOSITES
Authors: Prakash O. , Sarkar P. , Nicholson P. S.
Published: August 2010

Abstract The structure and mechanical properties of electrophoretically deposited t‐ZrO2/Al2O3 lamellar composites are described. The fracture behavior of, and crack paths in, such composites with strong interfaces has been evaluated using indentation and 4‐point bend tests. The effect of residual and induced stresses is considered. It is shown that significant crack deflection can be obtained in t‐ZrO2/Al2O3 composites by incorporating a number of closely spaced, thin Al2O3 layers.
INFLUENCE OF TRANSFORMATION‐INDUCED CRACK CLOSURE ON SLOW FATIGUE CRACK GROWTH UNDER VARIABLE AMPLITUDE LOADING
Authors: Mayer H. R. , Stanzl‐Tschegg S. E. , Sawaki Y. , Hühner M. , Hornbogen E.
Published: August 2010

Abstract Fatigue crack growth under constant and random loading conditions was investigated for a metastable austenitic‐bainitic steel in comparison with a ferritic chromium steel at very low crack growth rates. Experimentally determined random crack growth was compared with linear Miner calculations on the basis of constant amplitude results. It was found that the measured crack growth rates in transforming material are a factor of 10 lower than the calculated values, whereas the difference is only a factor of 2 for the ferritic steel. The reason for the pronounced crack growth retardation in the metastable alloy is transformation of part of the austenitic phase into martensite in the stress field of the crack tip, accompanied by a volume increase and, consequently, residual compressive stresses. Rare high load cycles in the random sequence increase the closure level, which then leads to pronounced retardation of fatigue crack growth for the numerous successive low amplitude cycles.
APPLYING NUMERICAL METHODS FOR DETERMINING THE SERVICE LIFE OF GEARS
Authors: Pehan S. , Hellen T. K. , Flašker J.
Published: August 2010

Abstract The service life of gears with a crack in a tooth root can only be determined by numerical methods. An initial fatigue crack is assumed on the tensile side of a tooth root at the site of maximum reference stress and is assumed to commence growth perpendicularly to the surface. An initial assessment can be made using a two dimensional analysis, which is relatively quick and cheap to perform. However, if we wish to take into account the influence of the contact area of load transfer, which can be distributed along the tooth width in different ways, the gear has to be treated by a three dimensional finite element analysis. Crack profile advance is made in stages, each using the strain energy release rate criteria and giving ultimately the stress intensity factor as a function of average crack depth. With known fracture mechanics material characteristics of different gear layers, through which the crack propagates, the service life of a gear is then determined by numerical integration of Paris' equation. A one‐sided contact area causes the crack to propagate several times faster than the preferred load distribution across the middle of the tooth.
ELEVATED TEMPERATURE FRACTURE OF PARTICULATE‐REINFORCED ALUMINUM PART II: MICROMECHANICAL MODELLING
Authors: Somerday B. P. , Leng Y. , Gangloff R. P.
Published: August 2010

Abstract Micromechanical fracture‐toughness models are applied to experimental results for a metal‐matrix composite (2009/SiC/20p‐T6) to understand the temperature dependencies of toughness and fracture mechanisms, as well as to test quantitatively a continuum fracture‐mechanics approach. Models which couple the crack‐tip strain field, characteristic fracture‐process distance and measured intrinsic micro void‐fracture resistance predict the temperature dependencies of fracture‐initiation (KJICi) and crack‐growth (TR) toughnesses from 25°C to 316°C. The temperature dependencies of KJICi and TR result from the interplay between the fracture resistance and the crack‐tip strain field, each being temperature‐dependent. Strain‐based models are equally valid for void nucleation‐ or growth‐controlled fracture. A scenario for fracture is nucleation‐controlled damage within Sic‐particle clusters, corresponding to KJICi, followed by cluster‐damage growth to coalescence under increasing stress intensity. Void growth is stabilized increasingly at elevated temperatures.
THE EFFECT OF PRESSURE OVERLOAD ON FATIGUE CRACK GROWTH IN PRESSURE VESSELS
Authors: Shaniavski A. A.
Published: August 2010

Abstract— The pressure‐overload method for proof‐testing pressure‐vessels was investigated for hydrofilters made of an Al‐alloy. After a vessel had been overloaded using a hydrostatic fluid, the fatigue crack growth rate of the material was studied at three levels of hydrodynamic pressure. The period of fatigue crack growth Np and the uniaxial equivalent stress σe were calculated on the basis of fractographic analyses for various combinations of the hydrostatic overload and the hydrodynamic pressure. Increasing the overload level increased Np, and decreased σe. For a pressure overload of 3 times the normal pressure the initiation of a fatigue crack did not occur after 2 × 106 cycles of loading at an hydrodynamic pressure of 20.6 MPa. Striation spacing could not be correlated against the value of the hydrodynamic pressure after an overload of 2.5 times the normal pressure. A unified relationship between the durability Nd and the ratio Np/Nd (=Ndp) at different hydrodynamic fluid pressures was demonstrated.
CRACK INTERACTION, COALESCENCE AND MIXED MODE FRACTURE MECHANICS
Authors: Wang Y.‐Z. , Atkinson J. D. , Akid R. , Parkins R. N.
Published: August 2010

Abstract— Multiple cracks are often observed in engineering structures; and their interaction and coalescence may significantly affect the lifetime of component or structure. In this paper, the general features associated with crack interaction and coalescence are presented, and the conditions for coalescence in various forms are reviewed. A model has been developed based on the principles of mixed mode fracture mechanics, to provide a rational explanation for the phenomenon of crack interaction and to predict the coalescence conditions. The latter is found to agree reasonably with the experimental observations.
MODE II STABLE CRACK GROWTH
Authors: Mourad A.‐H. I. , Maiti S. K.
Published: August 2010

Abstract Mode II stable crack extension has been examined for an aircraft grade aluminium alloy D16AT. Both theoretical and experimental results are presented. The experimental observations include load displacement diagrams, plastic wake, crack front tunnelling and scanning electron micrographs of the fracture surfaces. The crack shows a tendency for in‐plane extension, and the fracture surface is very flat, smooth and free of any dimples. The crack front advances with neghgible tunnelling at all stages of extension. The span of mode II stable crack growth (SCG) is longer than in the case of mode I SCG reported earlier for the same material and there is also more extensive plastic deformation. In the presence of a slight mode I load, the crack grows out‐of‐plane and the fractured surface facets resemble that of a mode I or mixed‐mode dimpled fracture. The theoretical study is based on a finite element analysis using small deformation theory and incremental plasticity. Some of the experimental results have been theoretically predicted using the COA criterion as the governing criterion. The theoretical results include load‐displacement diagrams, crack edge displacement curves, plastic zones and the J resistance curves. There is good agreement between the load‐displacement diagrams. The initiation and maximum loads differ by less than 15%. The J resistance curve has a constant slope over the whole span of stable crack growth.
OBSERVATIONS OF TENSION/COMPRESSION ASYMMETRY IN THE CYCLIC DEFORMATION OF ALUMINUM ALLOY 7075
Authors: Meininger J. M. , Dickerson S. L. , Gibeling J. C.
Published: August 2010

Abstract— The cyclic deformation of AA7075 in the T6 and T651 tempers has been investigated at room temperature under plastic strain control at amplitudes from 0.1% to 1.0%. Although both tempers represented peak aged conditions, their cyclic deformation behavior differed. The T6 material exhibited a slight amount of cyclic hardening to saturation, with softening evident only at the highest amplitudes. At low amplitudes, the peak compression stresses were higher than the tension peaks throughout the test. The extent of this asymmetry in peak stresses diminished at higher amplitudes. In contrast, the peak compression stresses for the T651 temper were initially lower than the tension peaks. The tension peaks remained relatively stable throughout each test, but the compression peaks exhibited hardening such that they became greater in magnitude than the tension peaks. A nonlinear elastic strain contribution is shown to account for asymmetries in hysteresis loop shape, but not the asymmetric peak stresses. We hypothesize that the tension/compression asymmetry results from strain localization caused by the inhomogeneous microstructure created during quenching and aging. In addition, stretching prior to aging gives rise to the unusual behavior of the T651 material.
Influence of nanometre‐sized notch and water on the fracture behaviour of single crystal silicon microelements
Authors: Minoshima* K. , Terada T. , Komai K.
Published: August 2010

The influence of a notch and a water environment on the quasi‐static and fatigue fracture behaviour was investigated in single crystal silicon microelements. The tests were conducted in smooth and notched microcantilever beam samples. Smooth specimens were prepared by micromachining (photo‐etching) of (110) silicon wafers. For some specimens, a nanometre‐sized notch was machined 100 μm away from the sample root by using a focused ion beam system. A machining condition was optimized, and the V‐shaped notch was successfully introduced. The radius of curvature of the notch, measured by an atomic force microscope (AFM), decreased with an increase in notch depth, and ranged from about 20 to 100 nm. Single‐crystal Si microelements deformed elastically until final failure, which was of a brittle nature. The maximum fracture strength of a smooth microcantilever specimen reached about 7.7 GPa, which was higher than that obtained in millimetre‐sized single crystal Si samples. However, the fracture strength decreased with an increase in notch depth, even though the notch depth was of the order of a nanometre. This means that a nanometre deep notch, which is often regarded as surface roughness in ordinary‐sized mechanical components, caused a decrease in the fracture strength of Si microelements. The fracture initiated at the notch, and then the {111} crack propagated in the direction normal to the sample surface. Fatigue tests were also conducted in laboratory air and in pure water at a stress cycle frequency of 0.1 Hz and a stress ratio of 0.1. In laboratory air, no fatigue damage was observed even though the surface was nanoscopically examined by an AFM. However, when the fatigue tests were conducted in pure water, the fatigue lives in water were decreased. Crack formation on the {111} plane was promoted by a synergistic effect of the dynamic loading and the water environment. Atomic force microscopy was capable of imaging the nanoscopic cracks, which caused failure in water.
A numerical study of fatigue crack closure induced by plasticity
Authors: ANTUNES F. V. , BORREGO L. F. P. , COSTA J. D. , FERREIRA J. M.
Published: August 2010

Crack closure delays the intrinsic mechanisms responsible for crack growth, therefore, it must be considered in fatigue crack growth modelling. The objective of this work is to develop a numerical procedure to predict crack closure induced by plasticity. First the crack closure was experimentally measured on M(T) 6082‐T6 aluminium alloy specimens of 3 mm thickness. A pin microgauge was used with the compliance technique. Then different parameters of the numerical procedure were analysed, namely the finite element mesh and the crack propagation scheme. The size of crack‐tip elements has an important influence and it is recommended to be of the same order of cyclic plastic zone. Crack‐opening levels only 10% lower than experimental results were obtained considering kinematic hardening and two load cycles in each increment.
Probabilistic fatigue crack growth analysis of spot‐welded joints
Authors: NI K. , MAHADEVAN S.
Published: August 2010

This paper presents a probabilistic fatigue crack growth life prediction methodology for spot‐welded joints under variable amplitude loading history. The loading is multi‐axial and is obtained from transient response analysis of a vehicle model using finite‐element analysis. A three‐dimensional (3D) finite element model of a simplified joint with four spot welds is developed, and the static stress analysis of this joint is performed. Then the fatigue crack inside the base material sheet is modelled as a surface crack. Probabilistic crack growth model is combined with the stress analysis result to develop a probabilistic fatigue crack growth life prediction methodology for spot welds. This new method is implemented with MSC/NASTRAN and MSC/FATIGUE and is useful for the reliability assessment of spot‐welded joints against fatigue crack growth.
Fatigue limit evaluation of notches, small cracks and defects: an engineering approach
Authors: LIVIERI P. , TOVO R.
Published: August 2010

In this paper, the average stress method for the fatigue limit evaluation of stress raising geometrical features is revised and extended. In particular, an analytical close‐form approach was used and the linear elastic stress equations were modified by taking into account the effect of nominal stress on the local stress distribution. Hence, the average tangential stress was correctly evaluated over a distance of 2a0, where a0 was El Haddad's short crack constant, for long and small notches as well as for crack‐like notches. When this model is applied to a wide range of geometrical features subjected to mode I fatigue loading, the classical shape of the curves of the Kitagawa–Takahashi diagram was obtained for changes in crack‐like notch size. Similarly, notch sensitivity was estimated by reducing the notch tip radius. The accuracy of the proposed method in predicting fatigue limits was then checked by using experimental data taken from the literature and generated on testing specimens weakened by rounded and sharp notches as well as by small artificial defects.
Can pure mode III fatigue loading contribute to crack propagation in metallic materials?
Authors: POKLUDA J. , PIPPAN R.
Published: August 2010

The possibility of pure mode III crack growth is analysed on the background of theoretical and experimental results obtained in the last 20 years. Unlike for modes I and II, there is no plausible micromechanistic model explaining a pure mode III crack growth in ductile metals. In order to realize ‘plain’ mode III fracture surface, we propose the propagation of a series of pure mode II cracks along the crack front. Fractographical observations on crack initiation and propagation in a low alloy steel under cyclic torsion support such a model. The authors have not seen any clear indication of a pure mode III crack growth micromechanism in metals until now.
Parametric study of oblique edge cracks under cyclic contact loading
Authors: BEGHINI M. , BERTINI L. , FONTANARI V.
Published: August 2010

The problem of a two‐dimensional elastic body, carrying an inclined edge crack and loaded by a cylinder rolling on the surface, is solved by the weight function method. The load induced by the cylinder on the cracked body was represented by the Hertzian pressure distribution, and the nominal stress distribution in the uncracked body was numerically evaluated by the superposition principle. The crack opening displacement components were obtained by an analytical Green's function. The partial crack closure was considered and the influence of the mutual forces between the crack faces included in the analysis, by which the effective stress intensity factors KI and KII could be evaluated. By considering different friction conditions between the crack surfaces and several crack inclinations, the evolutions of the effective KI and KII during typical loading cycles were analysed.
Prediction of crack opening stress levels for 1045 quenched and tempered steel under service loading spectra
Authors: KHALIL M. , TOPPER T. H.
Published: August 2010

The opening stresses of a crack emanating from an edge notch in a 1045 quenched and tempered steel specimen were measured under two different Society of Automotive Engineers (SAE) standard service load histories having different average mean stress levels. The two spectra are the Grapple Skidder history (GSH), which has a positive average mean stress, and the Log Skidder history (LSH), which has a zero average mean stress. To capture the behaviour of the crack opening stress in the material, the crack opening stress levels were measured at 900X using an optical video microscope, at frequent intervals for each set of histories scaled to two different maximum stress ranges.A crack growth analysis based on a fracture mechanics approach was used to model the fatigue behaviour of the steel specimens for the given load spectra and stress ranges. Crack growth analysis was based on an effective strain‐based intensity factor, a crack growth rate curve obtained during closure‐free loading cycles and a local notch strain calculation based on Neuber's rule.The crack opening stress (Sop) was modelled and the model was implemented in a fatigue notch model, and the fatigue lives of the specimens under the two different spectra scaled to several maximum stress levels were estimated. The average measured crack opening stresses were between 6 and 12% of the average calculated crack opening stresses. In the interest of simplifying the use of Sop in design, the average Sop was correlated with the frequency of occurrence of the cycle reducing the Sop to the average crack opening stress level. The use of an Sop level corresponding to the cycle causing a reduction in Sop to a level reached once per 10 cycles gave a conservative estimate of average crack opening stress for all the histories.
Analysis of fatigue crack behaviour based on dynamic response simulations and experiments for tensile‐shear spot‐welded joints
Authors: SHANG D.‐G. , BARKEY M. E.
Published: August 2010

Fatigue crack initiation and propagation behaviours were studied based on the dynamic response simulation by the three‐dimensional finite‐element analysis (FEA) and dynamic response experiments for tensile‐shear spot‐welded joints. The entire fatigue propagation behaviour from the surface elliptical cracks at the initiation stage to the through thickness cracks at the final stage was taken into consideration during the three‐dimensional FEA dynamic response simulations. The results of the simulations and experiments found that the fatigue cracks of spot‐welded joint from initial detectable crack sizes to crack propagation behaviour could be described by three stages. Approximately one‐half of the total fatigue life was taken in stage I, which includes micro‐crack nucleation and the small crack growth process; 20% of the total fatigue life in stage II, in which the existing surface crack propagates through the thickness of sheet and 30% of the total fatigue life in stage III, during which the through thickness crack propagates along the direction of plate width to the final failure. According to the relationship between the crack length and depth and the dynamic response frequency during the simulated fatigue damage process, the definition of fatigue crack initiation and propagation stages was proposed. The analysis will provide some information for the fatigue life prediction of the spot‐welded structures.
A critical plane fatigue model with coupled meso‐plasticity and damage
Authors: HUYEN N. , FLACELIERE L. , MOREL F.
Published: August 2010

The work proposed in this paper is a possible way of modelling some local observations at the surface of mild steel specimens submitted to uniaxial and multiaxial loads. It is clearly seen that local plasticity, controlled by local microstructural heterogeneities, plays a fundamental role in microcrack nucleation and damage orientation is closely related to the applied loading mode. The framework of irreversible thermodynamics with internal variables for time‐independent, isothermal and small deformations has been used to build a critical plane damage model by assuming the existence of a link between mesoplasticity and mesodamage. Non‐associated plasticity and damage rules allow the evolution of some plastic slip before any damage nucleation, as seen during the observations. A key feature of this proposal is the capacity to reflect nonlinear damage accumulation under variable amplitude loading.
Fracture analysis using parametric meshless Galerkin method
Authors: HOSSEINI‐TOUDESHKY H. , MUSIVAND‐ARZANFUDI M.
Published: August 2010

The recently introduced parametric meshless Galerkin method (PMGM) uses meshless shape functions which are mapped from a parametric space. The PMGM improves the efficiency of the meshless methods via the capability of using the meshless shape functions of an existing elements library and no necessity to construct all meshless shape functions in the running time. In this paper, the application of the PMGM to calculate the crack‐tip stress intensity factors (SIFs) is studied. Some modifications are performed on the original PMGM to eliminate the restrictions arisen from the compulsory use of the whole of the parametric data. In doing so, some of the mapped nodes are inactivated due to their confliction with the problem geometry or extra node patterns and some of the Gaussian cells are also replaced with new ones. Numerical examples show a computational time saving of more than 20% as well as accurate obtained SIFs.
Control volumes and strain energy density under small and large scale yielding due to tension and torsion loading
Authors: LAZZARIN P. , BERTO F.
Published: August 2010

In the recent literature some researchers proposed the use of the mean value of the Strain Energy Density (SED) over a well‐defined control volume for static and fatigue strength assessment of components weakened by sharp V‐shaped notches. In those papers the SED was expressed in terms of Notch Stress Intensity Factors (NSIFs), whose accurate evaluation needs a very fine mesh when based on local stress determined along the notch bisector. This contribution shows that when the material behaviour is ideally linear elastic or obeys a power hardening law, the mean value of the SED over the control volume can also be precisely determined from a coarse mesh. This result is of interest in the practical application of the SED approach to real components. Eventually, NSIFs can be evaluated a posteriori, just on the basis of the local SED. While discussing some results from elastic‐plastic analyses carried out on a V‐notched plate under tension loading and on a V‐notched round bar under torsion, the different roles played by local and large scale yielding are highlighted. The result is used here to provide a justification for the different slopes, 3.0 and 5.0, reported by Eurocode 3 and other Standards in force for welded details subjected to tensile or shear stresses, respectively.
Analysis of key factors for the interpretation of small punch test results*
Authors: LACALLE R. , ÁLVAREZ J. A. , GUTIÉRREZ‐SOLANA F.
Published: August 2010

In recent years, great advances have been made in non‐destructive or quasi‐non‐destructive test techniques for the determination of fracture toughness. One of the most interesting of these techniques is the one known as Small Punch, in which a punch acts on a small plane specimen, deforming it until fracture. From the continuous records of force and displacement of the punch taken during the test, it is possible to quantify the energy absorbed and then to determine the classic parameters of fracture mechanics such as ductile‐to‐brittle transition temperature or even the fracture toughness of the material. In this paper, fundamental aspects of the interpretation of the results of this kind of test have been studied. The effect of specimen thickness has been analysed. Moreover, a study has been carried out with the aim of clarifying the relation between the orientations in which it is possible to machine Small Punch specimens and the orientations of traditional specimens used in the determination of fracture toughness.
Tensile and fatigue behaviour of wrought magnesium alloys AZ31 and AZ61
Authors: CHAMOS A. N. , PANTELAKIS Sp. G. , HAIDEMENOPOULOS G. N. , KAMOUTSI E.
Published: August 2010

The mechanical behaviour of two hot rolled magnesium alloys, namely the AZ31 and AZ61, has been evaluated experimentally under both monotonic and cyclic loading. Both longitudinal (L) and long transverse (LT) directions were evaluated. The tensile behaviour of the L and LT directions is similar and differs only in the offset 0.2% yield strength for both materials. This difference is attributed to the angular spread of basal poles toward the rolling direction and is more pronounced for the case of AZ31. A distinct hardening response is obvious in both directions. Twinning formation was observed; it is more pronounced in the longitudinal direction while the fracture mode is intergranular and equiaxed facets are present in the fracture surfaces of the specimens. The S–N curves exhibit a smooth transition from the low to high cycle fatigue regime. AZ61 exhibits an overall better fatigue behaviour compared to AZ31. A transgranular crack initiation mode is observed in all tested specimens while the propagation of the cracks is characterized as intergranular.
δ25 Crack opening displacement parameter in cohesive zone models: experiments and simulations in asphalt concrete
Authors: SONG S. H. , WAGONER M. P. , PAULINO G. H. , BUTTLAR W. G.
Published: August 2010

Recent work with fracture characterization of asphalt concrete has shown that a cohesive zone model (CZM) provides insight into the fracture process of the materials. However, a current approach to estimate fracture energy, i.e., in terms of area of force versus crack mouth opening displacement (CMOD), for asphalt concrete overpredicts its magnitude. Therefore, the δ25 parameter, which was inspired by the δ5 concept of Schwalbe and co‐workers, is proposed as an operational definition of a crack tip opening displacement (CTOD). The δ25 measurement is incorporated into an experimental study of validation of its usefulness with asphalt concrete, and is utilized to estimate fracture energy. The work presented herein validates the δ25 parameter for asphalt concrete, describes the experimental techniques for utilizing the δ25 parameter, and presents three‐dimensional (3D) CZM simulations with a specially tailored cohesive relation. The integration of the δ25 parameter and new cohesive model has provided further insight into the fracture process of asphalt concrete with relatively good agreement between experimental results and numerical simulations.
Influence of core hardness on bending strength of tooth in cylindrical gears under quasistatic loading conditions
Authors: BOZCA M.
Published: August 2010

Dynamically loaded mechanical components can withstand overloading if they are designed by considering sufficient safety of margin for the static strength. In this study, the influence of core hardness on bending static strength of the tooth in cylindrical hardened gears is investigated experimentally. For this purpose, shot peened and unpeened gears having different core hardness and comparable surface hardness are tested on a pulsator test rig, which provides a quasi‐static loading condition by means of a servomotor. All test gears are made of 20MnCr5 steel and have the same geometry (module m = 5 mm). Average static load for fracture is measured. The relationship between the applied load and bending stress is known. Hardness profiles from the surface to the core of the test gears were measured. Strength is assumed to be a function of depth depending hardness to justify fracture initiation on the surface under bending static load conditions. Tooth root failures originating from the surface were observed experimentally. It is found that the shot peening treatment and core hardness have no considerable influence on the bending strength of the tooth under static loading conditions.
Dynamic stress intensity factors due to scattering of harmonic waves by a crack in an infinite medium
Authors: ESLAMI S. H. , AMINI F.
Published: August 2010

An analytical method for calculating dynamic stress intensity factors in the mixed mode (combination of opening and sliding modes) using complex functions theory is presented. The crack is in infinite medium and subjected to the plane harmonic waves. The basis of the method is grounded on solving the two‐dimensional wave equations in the frequency domain and complex plane using mapping technique. In this domain, solution of the resulting partial differential equations is found in the series of the Hankel functions with unknown coefficients. Applying the boundary conditions of the crack, these coefficients are calculated. After solving the wave equations, the stress and displacement fields, also the J‐integrals are obtained. Finally using the J‐integrals, dynamic stress intensity factors are calculated. Numerical results including the values of dynamic stress intensity factors for a crack in an infinite medium subjected to the dilatation and shear harmonic waves are presented.
Alcohol consumption enhances antiretroviral painful peripheral neuropathy by mitochondrial mechanisms
Authors: Ferrari Luiz F. , Levine Jon D.
Published: August 2010

A major dose‐limiting side effect of human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS) chemotherapies, such as the nucleoside reverse transcriptase inhibitors (NRTIs), is a small‐fiber painful peripheral neuropathy, mediated by its mitochondrial toxicity. Co‐morbid conditions may also contribute to this dose‐limiting effect of HIV/AIDS treatment. Alcohol abuse, which alone also produces painful neuropathy, is one of the most important co‐morbid risk factors for peripheral neuropathy in patients with HIV/AIDS. Despite the prevalence of this problem and its serious impact on the quality of life and continued therapy in HIV/AIDS patients, the mechanisms by which alcohol abuse exacerbates highly active antiretroviral therapy (HAART)‐induced neuropathic pain has not been demonstrated. In this study, performed in rats, we investigated the cellular mechanism by which consumed alcohol impacts antiretroviral‐induced neuropathic pain. NRTI 2′,3′‐dideoxycytidine (ddC; 50 mg/kg) neuropathy was mitochondrial‐dependent and PKCε‐independent, and alcohol‐induced painful neuropathy was PKCε‐dependent and mitochondrial‐independent. At low doses, ddC (5 mg/kg) and alcohol (6.5% ethanol diet for 1 week), which alone do not affect nociception, together produce profound mechanical hyperalgesia. This hyperalgesia is mitochondrial‐dependent but PKCε‐independent. These experiments, which provide the first model for studying the impact of co‐morbidity in painful neuropathy, support the clinical impression that alcohol consumption enhances HIV/AIDS therapy neuropathy, and provide evidence for a role of mitochondrial mechanisms underlying this interaction.
Cortical cholinergic abnormalities contribute to the amnesic state induced by pyrithiamine‐induced thiamine deficiency in the rat
Authors: Anzalone Steven , Vetreno Ryan P. , Ramos Raddy L. , Savage Lisa M.
Published: August 2010

Although the key neuropathology associated with diencephalic amnesia is lesions to the thalamus and/or mammillary bodies, functional deactivation of the hippocampus and associated cortical regions also appear to contribute to the memory dysfunction. For example, there is loss of forebrain cholinergic neurons and alterations in stimulated acetylcholine (ACh) levels in the hippocampus and cortex in animal models of diencephalic amnesia associated with thiamine deficiency. In the present study, the pyrithiamine‐induced thiamine deficiency rat model was used to assess the functional relationships between thalamic pathology, behavioral impairment, ACh efflux and cholinergic innervation of the hippocampus and cortex. In pyrithiamine‐induced thiamine deficiency‐treated rats, ACh efflux during behavioral testing was blunted to differing degrees in the hippocampus, medial frontal cortex and retrosplenial cortex. In addition, significant reductions in cholinergic fiber densities were observed in each of these regions. However, only hippocampal cholinergic fiber density correlated significantly with ACh efflux in the same region, suggesting that the reduction in cortical ACh efflux in cases of diencephalic amnesia cannot be fully explained by a loss of cholinergic fiber innervation. This notion supports the emerging theory that the functional consequences of the distal effects of lesions go beyond simple deafferentation. Specifically, some frontal cortical regions exhibit hypersensitivity to deafferentation that is only detected during behavioral and/or physiological demand.
Effects of antiepileptic drugs on associative LTP‐like plasticity in human motor cortex
Authors: Heidegger Tonio , Krakow Karsten , Ziemann Ulf
Published: August 2010

Antiepileptic drugs (AEDs) are used extensively in clinical practice but relatively little is known on their specific effects at the systems level of human cortex. Here we tested, using a double‐blind randomized placebo‐controlled crossover design in healthy subjects, the effects of a single therapeutic oral dose of seven AEDs with different modes of action (tiagabine, diazepam, gabapentin, lamotrigine, topiramate, levetiracetam and piracetam) on long‐term potentiation (LTP)‐like motor cortical plasticity induced by paired associative transcranial magnetic stimulation (PAS). PAS‐induced LTP‐like plasticity was assessed from the increase in motor evoked potential amplitude in a hand muscle contralateral to the stimulated motor cortex. Levetiracetam significantly reduced LTP‐like plasticity when compared to the placebo condition. Tiagabine, diazepam, lamotrigine and piracetam resulted in nonsignificant trends towards reduction of LTP‐like plasticity while gabapentin and topiramate had no effect. The particularly depressant effect of levetiracetam is probably explained by its unique mode of action through binding at the vesicle membrane protein SV2A. Enhancement of gamma‐amino butyric acid‐dependent cortical inhibition by tiagabine, diazepam and possibly levetiracetam, and blockage of voltage‐gated sodium channels by lamotrigine, may also depress PAS‐induced LTP‐like plasticity but these mechanisms appear to be less relevant. Findings may inform about AED‐related adverse effects on important LTP‐dependent central nervous systems processes such as learning or memory formation. The particular depressant effect of levetiracetam on LTP‐like plasticity may also relate to the unique properties of this drug to inhibit epileptogenesis, a potentially LTP‐associated process.
β‐Secretase‐1 elevation in aged monkey and Alzheimer’s disease human cerebral cortex occurs around the vasculature in partnership with multisystem axon terminal pathogenesis and β‐amyloid accumulation
Authors: Cai Yan , Xiong Kun , Zhang Xue‐Mei , Cai Huaibin , Luo Xue‐Gang , Feng Jia‐Chun , Clough Richard W. , Struble Robert G. , Patrylo Peter R. , Chu Yaping , Kordower Jeffrey H. , Yan Xiao‐Xin
Published: August 2010

Alzheimer’s disease (AD) is the most common dementia‐causing disorder in the elderly; it may be related to multiple risk factors, and is characterized pathologically by cerebral hypometabolism, paravascular β‐amyloid peptide (Aβ) plaques, neuritic dystrophy, and intra‐neuronal aggregation of phosphorylated tau. To explore potential pathogenic links among some of these lesions, we examined β‐secretase‐1 (BACE1) alterations relative to Aβ deposition, neuritic pathology and vascular organization in aged monkey and AD human cerebral cortex. Western blot analyses detected increased levels of BACE1 protein and β‐site‐cleavage amyloid precursor protein C‐terminal fragments in plaque‐bearing human and monkey cortex relative to controls. In immunohistochemistry, locally elevated BACE1 immunoreactivity (IR) occurred in AD but not in control human cortex, with a trend for increased overall density among cases with greater plaque pathology. In double‐labeling preparations, BACE1 IR colocalized with immunolabeling for Aβ but not for phosphorylated tau. In perfusion‐fixed monkey cortex, locally increased BACE1 IR co‐existed with intra‐axonal and extracellular Aβ IR among virtually all neuritic plaques, ranging from primitive to typical cored forms. This BACE1 labeling localized to swollen/sprouting axon terminals that might co‐express one or another neuronal phenotype markers (GABAergic, glutamatergic, cholinergic, or catecholaminergic). Importantly, these BACE1‐labeled dystrophic axons resided near to or in direct contact with blood vessels. These findings suggest that plaque formation in AD or normal aged primates relates to a multisystem axonal pathogenesis that occurs in partnership with a potential vascular or metabolic deficit. The data provide a mechanistic explanation for why senile plaques are present preferentially near the cerebral vasculature.
Stimulus‐locked responses on human arm muscles reveal a rapid neural pathway linking visual input to arm motor output
Authors: Pruszynski J. Andrew , King Geoffrey L. , Boisse Lysa , Scott Stephen H. , Flanagan J. Randall , Munoz Douglas P.
Published: August 2010

Previous studies have demonstrated that humans are sometimes capable of initiating arm movements towards visual stimuli at extremely short latencies, implying the presence of a short‐latency neural pathway linking visual input to limb motor output. However, little is known about the neural mechanisms that underlie such hastened arm responses. One clue may come from recent demonstrations that the appearance of a visual target can elicit a rapid response in neck muscles that is time‐locked to target appearance and functionally relevant for orienting gaze (head and eye) towards the target. Because oculomotor structures thought to contribute to ‘visual responses’ on neck muscles also target some arm muscles via a tecto‐reticulo‐spinal pathway, we hypothesized that a similar visual response would be present in arm muscles. Our results were consistent with this hypothesis as we observed the presence of rapid arm muscle activity (< 100 ms latency) that was time‐locked to target appearance and not movement onset. We further found that the visual response in arm muscles: (i) was present only when an immediate reach towards the target was required; (ii) had a magnitude that was predictive of reaction time; (iii) was tuned to target location in a manner appropriate for moving the arm towards the target; and (iv) was more prevalent in shoulder muscles than elbow muscles. These results provide evidence for a rapid neural pathway linking visual input to arm motor output and suggest the presence of a common neural mechanism for hastening eye, head and arm movements.
Performance of the green spruce aphid, Elatobium abietinum (Walker) on previously defoliated Sitka spruce
Authors: Williams D. T. , Straw N. A. , Day K. R.
Published: August 2010

Abstract  1 The green spruce aphid, Elatobium abietinum, is an important defoliator of Sitka spruce in the U.K. However, it is usual for years in which high E. abietinum populations occur to be followed by a year with low aphid densities. The possibility that the performance of E. abietinum is reduced on previously infested Sitka spruce, and that this is the cause of year‐to‐year fluctuations in population density, was investigated by comparing population development and the growth rate of individual aphids on experimentally defoliated trees.

2 Separate experiments were performed to determine whether aphid performance was reduced either in the autumn immediately after defoliation in the spring, or was reduced in the spring of the next year. Different rates of initial defoliation on trees used to test aphid performance were created by artificially infesting the trees with aphids in the spring before the experiments, and varying the time of infestation.

3 Population development and the mean relative growth rate (MRGR) of individual aphids on previously defoliated and undefoliated Sitka spruce did not differ significantly in the spring of the next year. No differences were observed in the nutrient content of the 1‐year‐old needles of previously defoliated or undefoliated trees at this time.

4 In the autumn and winter immediately after spring defoliation, aphid MRGR was significantly higher on trees that had been heavily defoliated earlier in the season compared with trees that had been lightly defoliated. However, the difference in MRGR decreased over the winter period. Nitrogen, phosphorous and potassium concentrations were 9.4–12.2% higher, at the beginning of the autumn, in the current year needles of heavily defoliated trees than in the current year needles of lightly defoliated trees.

5 The experiments indicate that high populations of E. abietinum in the spring do not induce any defensive mechanisms in Sitka spruce that adversely affect subsequent generations of the aphid. By contrast, the results suggest that high spring densities of the aphid improve the nutritional quality of the current year's foliage for autumn generations.
Effects of altitude on aphid‐mediated processes in the canopy of Norway spruce
Authors: Mühlenberg Eva , Stadler Bernhard
Published: August 2010

Abstract  1 The abundance of aphids and their honeydew are important in shaping the ecology of food web interactions and nutrient cycling in forests of Norway spruce. Here, the effects of the different environmental conditions at two study sites located at different altitudes (500 m, 765 m a.s.l.), in the Fichtelgebirge, north‐eastern Bavaria, Germany, on the abundance of Cinara pilicornis and their influence on epiphytic microorganisms on shoots of Picea abies were compared. Subsequent changes in throughfall fluxes were measured over a period of 12 weeks beneath infested and reference trees. In a laboratory experiment, the effects of ultraviolet (UV) radiation on microbial mortality and C and N concentrations in leachates were determined.

2 The warmer and drier conditions at the low altitude site favoured an early onset to aphid multiplication in spring compared with the high altitude site, where aphid numbers peaked 3 weeks later.

3 The presence of honeydew was associated with a significant increase in the total number of cultured epiphytic filamentous fungi, yeasts and bacteria in 12 of the 18 sample units, indicating better culturability or growth, whereas altitude had no significant effect on cultured cell numbers. By contrast to the reference trees, the high dissolved organic carbon (DOC) and hexose‐C fluxes beneath infested trees at the peak in aphid abundance, in June and July, resulted in a concomitant decrease in the fluxes of total inorganic nitrogen beneath infested trees (low altitude: −19.7%; high altitude: −52.3%). Fluxes of organic nitrogen were significantly higher beneath infested trees at the time of infestation. Similarly, potassium fluxes in throughfall increased 1.6–2.0‐fold in response to aphid infestation.

4 The exposure of infested and uninfested shoots of Norway spruce to UV‐A and UV‐B radiation only weakly affected epiphytic microbial mortality and did not affect the concentrations of the different nitrogen compound in leachates. However, bacteria, tended to be more active in the leachates collected from infested shoots, which resulted in the higher concentrations of aminosugar‐N. The aphids had a more pronounced effect on the concentrations of DOC in leachates, with average DOC concentrations being 4.2‐fold higher than in leachates from uninfested shoots.

5 It is suggested that, even at very low densities, aphids exert a strong influence via honeydew on the performance of microorganisms, and nutrient and energy flow, in spruce forests.
Life table for Leptoglossus occidentalis Heidemann (Heteroptera: Coreidae) and prediction of damage in lodgepole pine seed orchards
Authors: Bates Sarah L. , Borden John H.
Published: August 2010

Abstract  1 Fecundity, egg hatch and nymphal survivorship of Leptoglossus occidentalis Heidemann were determined and used to develop a damage prediction formula for seed bugs in lodgepole pine seed orchards.

2 A single overwintered L. occidentalis female may lay up to 80 eggs during early and mid‐season cone development.

3 Up to 83.4% of eggs that were attached to branches with tape hatched successfully, compared with 53.5% of eggs attached to needles with wire. Egg parasitism was only observed in egg masses attached with wire.

4 Egg masses attached with wire suffered approximately 30% mortality from parasitism, primarily by Gryon pennsylvanicum (Ashmead).

5 Approximately 15.1 and 2.5% of first instars survived to adulthood in 2000 and 2001, respectively. Survival of caged nymphs that were protected from predators was 14.5 and 17.8%, respectively.

6 A composite survival table indicated that 9.3% of eggs laid by L. occidentalis are expected to survive to adulthood.

7 Based on previously determined damage estimates for each life stage of feeding during three periods of cone development, a hypothetical density of one seed bug per tree early in the season will result in an expected seed loss of approximately 310 seeds.