bluuebunny/crossref_metadata_2025 huggingface dataset:
Duality of Rate Coding and Temporal Coding in Multilayered Feedforward Networks
Authors: Masuda Naoki , Aihara Kazuyuki
Published: February 2003

A functional role for precise spike timing has been proposed as an alternative hypothesis to rate coding. We show in this article that both the synchronous firing code and the population rate code can be used dually in a common framework of a single neural network model. Furthermore, these two coding mechanisms are bridged continuously by several modulatable model parameters, including shared connectivity, feedback strength, membrane leak rate, and neuron heterogeneity. The rates of change of these parameters are closely related to the response time and the timescale of learning.
Determination of Firing Times for the Stochastic Fitzhugh-Nagumo Neuronal Model
Authors: Tuckwell Henry C. , Rodriguez Roger , Wan Frederic Y. M.
Published: February 2003

We present for the first time an analytical approach for determining the time of firing of multicomponent nonlinear stochastic neuronal models. We apply the theory of first exit times for Markov processes to the Fitzhugh-Nagumo system with a constant mean gaussian white noise input, representing stochastic excitation and inhibition. Partial differential equations are obtained for the moments of the time to first spike. The observation that the recovery variable barely changes in the prespike trajectory leads to an accurate one-dimensional approximation. For the moments of the time to reach threshold, this leads to ordinary differential equations that may be easily solved. Several analytical approaches are explored that involve perturbation expansions for large and small values of the noise parameter. For ranges of the parameters appropriate for these asymptotic methods, the perturbation solutions are used to establish the validity of the one-dimensional approximation for both small and large values of the noise parameter. Additional verification is obtained with the excellent agreement between the mean and variance of the firing time found by numerical solution of the differential equations for the one-dimensional approximation and those obtained by simulation of the solutions of the model stochastic differential equations. Such agreement extends to intermediate values of the noise parameter. For the mean time to threshold, we find maxima at small noise values that constitute a form of stochastic resonance. We also investigate the dependence of the mean firing time on the initial values of the voltage and recovery variables when the input current has zero mean.
Modeling Short-Term Synaptic Depression in Silicon
Authors: Boegerhausen Malte , Suter Pascal , Liu Shih-Chii
Published: February 2003

We describe a model of short-term synaptic depression that is derived from a circuit implementation. The dynamics of this circuit model is similar to the dynamics of some theoretical models of short-term depression except that the recovery dynamics of the variable describing the depression is nonlinear and it also depends on the presynaptic frequency. The equations describing the steady-state and transient responses of this synaptic model are compared to the experimental results obtained from a fabricated silicon network consisting of leaky integrate-and-fire neurons and different types of short-term dynamic synapses. We also show experimental data demonstrating the possible computational roles of depression. One possible role of a depressing synapse is that the input can quickly bring the neuron up to threshold when the membrane potential is close to the resting potential.
Indexed Families of Functionals and Gaussian Radial Basis Functions
Authors: Sandberg Irwin W.
Published: February 2003

We report on results concerning the capabilities of gaussian radial basis function networks in the setting of inner product spaces that need not be finite dimensional. Specifically, we show that important indexed families of functionals can be uniformly approximated, with the approximation uniform also with respect to the index. Applications are described concerning the classification of signals and the synthesis of reconfigurable classifiers.
Plato as "Architect of Science"
Authors: Zhmud Leonid
Published: February 2003

The figure of the cordial host of the Academy, who invited the most gifted mathematicians and cultivated pure research, whose keen intellect was able if not to solve the particular problem then at least to show the method for its solution: this figure is quite familiar to students of Greek science. But was the Academy as such a center of scientific research, and did Plato really set for mathematicians and astronomers the problems they should study and methods they should use? Our sources tell about Plato's friendship or at least acquaintance with many brilliant mathematicians of his day (Theodorus, Archytas, Theaetetus), but they were never his pupils, rather vice versa - he learned much from them and actively used this knowledge in developing his philosophy. There is no reliable evidence that Eudoxus, Menaechmus, Dinostratus, Theudius, and others, whom many scholars unite into the group of so-called "Academic mathematicians," ever were his pupils or close associates. Our analysis of the relevant passages (Eratosthenes' Platonicus, Sosigenes ap. Simplicius, Proclus' Catalogue of geometers, and Philodemus' History of the Academy, etc.) shows that the very tendency of portraying Plato as the architect of science goes back to the early Academy and is born out of interpretations of his dialogues.
Plotinus on the Structure of Self-Intellection
Authors: Crystal Ian
Published: February 2003

In this paper, I argue that Plotinus offers us a new and interesting account of self-intellection. It is an account which is informed to some extent by a dilemma that Sextus Empiricus raised about the intellect being to apprehend itself. The significance of Sextus' dilemma is that it sets out the framework within which such a cognitive activity is to be dealt with, namely the intellect must apprehend itself qua part or qua whole, both of which according to him are impossible. Plotinus, I think, successfully gets around this dilemma and is able to explain how the intellect can think itself qua whole. In the process of doing so, he offers an account of self-intellection in which the thinking subject or thinker becomes active in terms of generating its intellectual content, namely itself; a move which is a break from the traditional Platonic/Aristotelian account of the intellect. The paper itself is set up as follows. I start by mentioning the dilemma which Sextus raises about self-intellection. Then I attempt, through an analysis of the noetic intellect's structure, to show how Plotinus is able to offer an account of self-intellection in terms of whole apprehending whole. I conclude with Plotinus' analysis of the light analogy as a means of explaining how this intellectual process works.
Post-embryonic growth of longidorid nematodes
Authors: Boag Brian , Yeates Gregor
Published: February 2003

To investigate changes in body size of Longidoridae during growth, we used published dimensions of stages to calculate volumes of the juvenile and adult stages of 33 species. A consistent increase in body volume between the juvenile stages was found with proportionally more growth occurring between the smaller stages. In species where three, rather than four, juvenile stages are present, the ultimate size of adults was correspondingly smaller. In the Heteroderidae, greatest growth occurs in later stages and this indicates different adaptations to plant parasitism. Analysis of further groups of free-living and parasitic nematodes is required to increase understanding of body growth and life histories, both within and between families.
Suppressive effects of composts against the root-knot nematode Meloidogyne javanica on tomato
Authors: Oka Yuji , Yermiyahu Uri
Published: February 2003

Suppressive effects of two composts, from cattle manure and grape marc, on the root-knot nematode Meloidogyne javanica were tested in pot and in vitro experiments. No root galls were found on tomato roots grown in soils containing 10 or 25% (v/v) cattle manure compost, and very few on those grown in 50% grape marc compost. Significant reductions in galling index were also found on tomato plants grown in soils containing lower concentrations of this compost. Chemical analysis of the composts and leachates from the soils showed that the cattle manure compost had higher electrical conductivity (EC) and higher concentrations of nitrogen, especially N–NH4, than the grape marc compost. Water extract of the cattle manure compost showed high nematicidal activity to the nematode juveniles and less activity toward the eggs in vitro. Water extract of the grape marc compost showed weaker nematicidal activity to the juveniles and eggs. Washing composted soils with excess water before nematode inoculation and tomato planting led to better plant growth, but the nematode-suppressive effect was decreased. These results suggest that high nitrogen concentrations, especially N–NH4, and high EC values contribute to the nematode suppressiveness of the composts.
Development and pharyngeal gland activities of Heterodera schachtii infecting Arabidopsis thaliana roots
Authors: Tytgat Tom , Verreijdt Liesbeth , Claeys Myriam , Vanholme Bartel , De Meutter Jan , Coomans August , Gheysen Godelieve
Published: February 2003

As plant-nematode research continues to use the model host plant Arabidopsis thaliana more and more, we made a detailed life cycle study of Heterodera schachtii on this host, and investigated pharyngeal gland activities at the most crucial steps during the establishment of the parasitic interaction. In our experimental set up, in most cases induction of syncytia occurred during day 2, and the second moult during day 6. In pre-parasitic juveniles, both subventral glands contained numerous secretory granules, which gradually disappeared during intracellular migration of the juvenile through the roots. At the same time, both glands decreased in size and, after induction of the feeding cell, new kinds of secretory granules could be observed in the cytoplasm. During migration and induction of the feeding cell, the dorsal pharyngeal gland continuously increased in size. Secretory granules were already present in the dorsal gland of pre-parasitic juveniles, and a continuous synthesis of these granules was observed thereafter. Once the syncytium was induced, however, different types of secretory granules were formed.
Effects of the biological nematicide, DiTera®, on movement and sensory responses of second stage juveniles of Globodera rostochiensis, and stylet activity of G. rostochiensis and fourth stage juveniles of Ditylenchus dipsaci
Authors: Perry Roland , Warrior Prem , Twomey Ursula , Rolfe Richard
Published: February 2003

DiTera® (Valent BioSciences Corp.), a novel biological nematicide obtained from fermentation with a hyphomycete fungus, adversely affected activity and movement through sand columns of second stage juveniles (J2) of Globodera rostochiensis. Electrophysiological tests demonstrated that exposure to 1% DiTera® prevented sensory perception of potato root diffusate. Electropharyngeograms of stylet protractor muscle activity showed that prior exposure of fourth stage juveniles of Ditylenchus dipsaci and J2 of G. rostochiensis to 1% DiTera® significantly reduced stylet thrusting when nematodes were transferred to artificial tap water or 10 mM serotonin, respectively. The modes of action and the control potential of this biological nematicide are discussed.
Responses of Vietnamese Musa genotypes to Meloidogyne spp. under field conditions
Authors: Nguyet Duong Thi Minh , Nhi Ho Huu , Van Den Bergh Inge , Tuyet Nguyen Thi , De Waele Dirk
Published: February 2003

Eight Vietnamese Musa genotypes, belonging to the AA, AAA, ABB and BB genome groups, as well as FHIA-01 (AAAB), FHIA-02 (AAAA) and Yangambi Km 5 (AAA) were evaluated for their host-plant response to Meloidogyne spp. under field conditions. FHIA-01, Ngu Thoc (AA), Tay (ABB) and Com Lua (AAA) were found to be less susceptible to Meloidogyne spp. FHIA-01, Ben Tre (AAA) and Bom (AAA) were less sensitive to knot-forming activity of Meloidogyne spp. The population dynamics of the nematodes were also assessed. The number of juveniles recovered from the roots was strongly influenced by the weather. During the cool dry season, the numbers dropped drastically. The number of egg-laying females in the roots was much less influenced by environmental conditions. Their numbers did not increase during the cooler and dry season. No effects of the nematodes on plant growth were found. The number of nematodes in the roots was related to the physiological stage of the plants, with most nematodes present during flowering.
Use of a cold-active entomopathogenic nematode Steinernema kraussei to control overwintering larvae of the black vine weevil Otiorhynchus sulcatus (Coleoptera: Curculionidae) in outdoor strawberry plants
Authors: Edmondson Rodney , Richardson Paul , Willmott Deena , Hart Andrew , Long Steve
Published: February 2003

The susceptibility of overwintering black vine weevil larvae Otiorhynchus sulcatus to a cold-active entomopathogenic nematode, Steinernema kraussei (isolate L137) and a commercial proprietary biopesticide, Steinernema carpocapsae (Exhibit) was assessed on outdoor potted strawberry plants. Nematodes were applied at a range of doses in early winter at a field site located in Warwickshire, England. Three months later, infestation and mortality of vine weevils were assessed. There was a clear dose response observed for S. kraussei applications. Up to 81% of vine weevil control was recorded by this nematode at the highest dose of 60 000 nematodes per pot, whereas treatments with S. carpocapsae caused no significant mortality at the dose rates used. Dissection of vine weevil larvae showed infective juveniles had developed to adults within the host. Recovery of nematodes at the end of the experiment indicated that S. kraussei (L137) was able to survive winter field conditions including prolonged exposure to low temperatures (averaging 2.7°C during the experiment) in contrast to S. carpocapsae which showed poor survival. Of the two extraction methods used, the Baermann funnel technique was found to be more efficient than Galleria mellonella baiting, with up to 44% of the original highest dose of S. kraussei nematodes being recovered by Baermann funnel compared to 8% with G. mellonella at 18°C. These results suggest S. kraussei (L137) has potential as a commercial biocontrol agent against O. sulcatus at low temperatures.
Description of Caenorhabditis japonica n. sp. (Nematoda: Rhabditida) associated with the burrower bug Parastrachia japonensis (Heteroptera: Cydnidae) in Japan
Authors: Hironaka Mantaro , Kiontke Karin , Sudhaus Walter
Published: February 2003

Caenorhabditis japonica n. sp. is described from Parastrachia japonensis from Japan. The species is closely related to species of the Caenorhabditis elegans group and shares many characters with them. It differs from these species in having blunt spicule tips of complex shape and in lacking a terminal notch in the bursa velum. Caenorhabditis japonica n. sp. is further characterised by an anterior end with the lips fused in pairs, long and pointed stegostomal teeth, long fringes on the anterior bursa margin and the form of the genital papillae (GP4 reduced). The species is integrated into the phylogenetic tree of Caenorhabditis. Some resulting consequences for character evolution within Caenorhabditis are discussed. Caenorhabditis japonica n. sp. is associated with a burrower bug, thereby adding a new component to the diverse ecology of Caenorhabditis species.
The extent to which nematode communities are affected by soil factors-a pot experiment
Authors: Bengough A. Glyn , Neilson Roy , Griffiths Bryan , Trudgill David
Published: February 2003

Four similar, agricultural soils with distinct nematode communities were used to determine the extent to which soil and inoculum factors affected nematode community structure. The soils all had a sandy loam texture from the same geographical area and had been in pasture or arable rotation for the last 10 years. Treatments were established in pots containing a middle layer of frozen defaunated soil, sandwiched between an inoculum that was either fresh soil from the same site ('self') or a mixture of soils to give a more diverse inoculum ('mixed'). Principal component analysis indicated that a single soil type given different inocula developed different community structures (i.e., the community under 'self' differed from that under 'mixed') suggesting an inoculum effect. It was also true that different soil types under a single inoculum soil also developed different community structures (i.e., community under 'mixed' differed with soil type), suggesting a soil effect. It is likely that the nematode community structure is influenced by a combination of antecedent land use, soil factors, species introductions and inter-species competition, which should be considered in any interpretation of nematode communities as a biotic indicator.
The cellular structure of the female reproductive system within the Heteroderinae and Meloidogyninae (Nematoda)
Authors: Geraert Etienne , Van Driessche Rita , Karssen Gerrit , Bert Wim
Published: February 2003

Gonads from living young females, representing 23 different species, were extracted to study the cellular structure of the female genital structure within the Meloidogyninae and Heteroderinae. All genera studied can be characterised by their cellular spermatheca morphology. Within Meloidogyne a spherical spermatheca is found with lobe-like protruding cells, most species having 16 to 18 spermatheca cells with interlaced cell boundaries while M. microtyla and M. ichinohei have more spermatheca cells with different cell boundaries. Heterodera and Globodera reveal a comparable gonad structure. The spermatheca cells of Heterodera are columnar and arranged in a restricted number of rows, whereas in Globodera the spermatheca cells are squarish to rounded, depending on the species. The gonad morphology of Afenestrata koreana is clearly different from what would be expected based on the related genera Globodera and Heterodera. The apparently simplest genital system was found in Meloidodera floridensis where the uterus has a limited number of cells. In the other genera studied a large and variable cell number was found.
Susceptibility of cabbage root fly Delia radicum, in potted cauliflower (Brassica oleracea var. botrytis) to isolates of entomopathogenic nematodes (Steinernema and Heterorhabditis spp.) indigenous to the UK
Authors: Chandler David , Hart Andrew , Richardson Paul , Long Steve , Willmott Deena
Published: February 2003

The susceptibility of larvae of the cabbage root fly (Delia radicum) to ten isolates of entomopathogenic nematodes was examined in a glasshouse experiment using cauliflower (Brassica oleracea var. botrytis) as the host plant. Steinernema affine Horticulture Research International (HRI) code 179 provided the highest level of control, killing approximately 46% of the cabbage root fly within 28 days. Two Heterorhabditis isolates (HRI code K122 and UK211) and Steinernema E1 and F1 (HRI code 194 and GWE63) did not give any significant control. In a second experiment, S. affine was applied against cabbage root fly larvae at 8000 to 64 000 nematodes per pot, and its performance was compared to identical dose applications of Nemasys®, a commercial formulation of S. feltiae. Averaged over the four dose rates, S. affine controlled significantly more cabbage root fly (36%) than Nemasys® (10.4%).
Bursaphelenchus rainulfi sp. n. (Nematoda: Parasitaphelenchidae), first record of the genus Bursaphelenchus Fuchs, 1937 from Malaysia
Authors: Braasch Helen , Burgermeister Wolfgang
Published: February 2003

Wood samples were taken from pine tree plantations in different regions of Malaysia and investigated for the occurrence of Bursaphelenchus species. Among 13 samples collected from damaged or dead pines in West Malaysia (near Kuala Lumpur) and Sabah (Kinabalu National Park and Sepilok), only one sample of a dead Pinus caribaea tree with bark beetle attack from a plantation near Kuala Lumpur revealed the presence of a new species of Bursaphelenchus. Bursaphelenchus rainulfi sp. n. is characterised by a relatively small stylet lacking distinct basal knobs but with slight basal swellings, lateral field with two lines, female with a small vulval flap, postuterine branch occupying about 33-50% of vulva-anus distance, female tail slim, conoid, with a finely rounded, ventrally bent terminus, male spicules relatively small with high condylus, distinct rostrum but no cucullus, and a small terminal 'bursa' on the male tail. It is similar to B. hellenicus, B. hylobianum and B.abietinus in the shape of the spicules and the female tail as well as in the presence of only two lines in the lateral field. It is distinguished by a number of characters including spicule size, shape of the 'bursa', female tail shape and excretory pore position. Bursaphelenchus rainulfi sp. n. is further distinguished from these morphologically similar species by molecular studies using the ITS-RFLP technique.
New approach to determination of maximum temperatures in welding using travelling heat sources
Authors: Tušek J. , Tomc J. , Radkovič M.
Published: February 2003

The present paper considers methods of calculation of maximum temperatures in welding using a constant welding speed and a point heat energy source. A comparison of theoretically obtained times for the occurrence of maximum temperatures with those obtained experimentally shows slight deviations. The latter are dependent only on the accuracy of temperature measurements and the workpiece size (i.e. plate size in the present work). In the present case, the limitations refer only to the workpiece size. The advantage of this new approach is in the determination of a plane curve which is an envelope of all the points in the workpiece that are simultaneously at the maximum temperature in arc welding.
Effect of nitrogen addition to shielding gas on residual stress of stainless steel weldments
Authors: Tseng K. H. , Chou C. P.
Published: February 2003

The objective of the present study was to investigate the effect of nitrogen additions to the shielding gas on the ferrite content and residual stress in austenitic stainless steels. Autogenous gas tungsten arc (GTA) welding was applied on austenitic stainless steels 304 and 310 to produce a bead on plate weld. The delta ferrite content of the weld metals was measured using a Ferritscope. The residual stress in the weldments was determined using the hole drilling strain gauge method. The present results indicated that the retained delta ferrite content in type 304 stainless steel weld metals decreased rapidly as nitrogen addition to the argon shielding gas was increased. The welding residual stress increased with increasing quantity of added nitrogen in the shielding gas. It was also found that the tensile residual stress zone in austenitic stainless steel weldments was extended as the quantity of added nitrogen gas in the argon shielding gas was increased.
Use of static stress for modification of welding residual stresses
Authors: Munsi A. S. M. Y. , Waddell A. J. , Walker C. A.
Published: February 2003

Previous studies have suggested that the application of stress during weld cooling could change the microstructure and residual stress state of a weld. Flat bar specimens were welded in a prestressed condition of tensile or compressive stress. The longitudinal residual stresses were found to decrease with application of tensile pre-induced stresses and to increase with application of compressive pre-induced stresses. Away from the weld toe the transverse residual stresses were found to decrease and near the weld toe the residual stresses were found to increase with application of tensile pre-induced stresses. With application of compressive pre-induced stresses the transverse residual stresses were found to increase. The present study suggests that the prestressing method should not be used as a residual stress relieving mechanism.
Determination of laser weld properties for finite element analysis of laser welded tailored blanks
Authors: Dry D. , Waddell W. , Owen D. R. J.
Published: February 2003

The present paper details two techniques that were employed to determine laser weld material properties, and outlines how the weld properties were used in the finite element analysis simulation of simple small scale axisymmetric tests. From these simulations, an understanding of some fundamental laser welded tailored blank formability issues was gained, which would otherwise have required substantial practical tests to be conducted. The materials studied in the present paper were high strength steels, including a relatively newly developed ultrahigh strength steel, for application in structural body in white components.
Ultrasonic spot weld testing with automatic classification
Authors: Roberts D. , Mason J. , Lewis C.
Published: February 2003

Spot welds can be tested non-destructively using ultrasound. By interpreting the signals received when testing, the quality of the weld may be determined. The method has not been widely accepted in the past, possibly owing to difficulties in manually quantifying the information contained in the signals. An automatic system to interpret the signals and classify welds has been developed in an attempt to aid human operation and as a step towards automatic, non-destructive spot weld testing. Details of a trial used to assess the performance of the system are given. By comparing the results with manual testing using ultrasound, the potential of automating signal interpretation is assessed. The results also indicate that the success of the ultrasonic technique, manual or automatic, is limited.
Plasma augmented laser welding and its applications
Authors: Page C. J. , Devermann T. , Biffin J. , Blundell N.
Published: February 2003

The present paper is concerned with hybrid laser-arc welding processes. Previous work is reviewed, and attention is then focused on plasma augmented laser welding. Features, application parameters, and mechanisms of the process are discussed, followed by a summary of the research and development work undertaken by the present authors, concentrating on application areas where the process can offer distinct advantages, such as tailor welded blanks, lap welding of coated steels, welding using additional filler material, and high speed welding of thin sheet metals.
Scaling analysis of momentum and heat transport in gas tungsten arc weld pools
Authors: Chakraborty S. , Sarkar S. , Dutta P.
Published: February 2003

A systematic procedure is outlined for scaling analysis of momentum and heat transfer in gas tungsten arc weld pools. With suitable selections of non-dimensionalised parameters, the governing equations coupled with appropriate boundary conditions are first scaled, and the relative significance of various terms appearing in them is analysed accordingly. The analysis is then used to predict the orders of magnitude of some important quantities, such as the velocity scale at the top surface, velocity boundary layer thickness, maximum temperature increase in the pool, and time required for initiation of melting. Some of the quantities predicted from the scaling analysis can also be used for optimised selection of appropriate grid size and time steps for full numerical simulation of the process. The scaling predictions are finally assessed by comparison with numerical results quoted in the literature, and a good qualitative agreement is observed.
Finite element analysis of resistance spot welding in aluminium
Authors: Theddeus M. P.
Published: February 2003

In the present work, an integrated simulation model has been developed for analysis of the resistance spot welding process in aluminium alloys via the finite element method. A coupled electrothermal analysis for an axisymmetric sheet-electrode geometry has been carried out that can predict the expected nugget diameter, penetration, and electrode face heating at any instant throughout the welding time. Several calculations have been carried out for different welding currents, welding times, and electrode forces and for different surface conditions of the aluminium sheets. Non-linear, temperature dependent, thermophysical material properties have been considered. An observation of interest is that in most instances the maximum nugget diameter is reached well within 0·02-0·04 s and further flow of welding current simply increases the electrode face heating. Also, the initial surface condition influences the growth of the fusion zone to a great extent. Various other conclusions have also been drawn.
Prediction of steady state dilution in multipass hardfacing overlays deposited by self shielded flux cored arc welding
Authors: Francis J. A. , Bednarz B. , Bee J. V.
Published: February 2003

A method of predicting the steady state dilution of multipass hardfacing overlays deposited via self shielded flux cored arc welding is described. A new variable is defined and used to describe the steady state dilution of multipass overlays. A mathematical model for dilution is presented in terms of this variable. Eight multipass overlays were deposited, using different combinations of welding parameters in each instance. The steady state dilutions for these overlays were measured and compared with those predicted by the model for dilution. It is found that, with an empirical correction, it is possible to predict the dilution of multipass hardfacing overlays over a wide range of welding conditions. This predictive capability establishes a relationship between welding process variables and the steady state composition, and hence the microstructure, of high chromium white iron weld overlays. Microstructure–wear relationships can then be applied to complete the link between welding parameters and the wear performance of the overlay.
Electroslag cladding of low alloy steel with stainless steel
Authors: Prasad V. V. Satya , Sambasiva Rao A. , Prakash U. , Baligidad R. G.
Published: February 2003

Cladding of a low alloy steel slab with stainless steel was carried out using a modified electroslag remelting technique. It is shown that the thickness of the cladding that can be achieved via electroslag remelting is dependent on the fill ratio used. The effect of power input on the joint profile obtained is reported. A combination of low fill ratio and relatively low power input is essential to minimise penetration of the base slab by the liquid metal. A satisfactory joint profile and defect free joint can be obtained via the optimisation of these process parameters. The clad product was successfully forged and rolled, which indicates satisfactory strength of the clad joint.
Phase change problem and effects of geometrical constants on weld pool geometry in sheet metal laser welding
Authors: Tanriver U. , Longobardi J. , Latham W. P. , Kar A.
Published: February 2003

A three-dimensional quasi-state heat conduction model is presented for the solid/liquid interface in laser welding of thin sheet metals. The vapour plasma plume effect is incorporated into the model by considering the electromagnetic field equation to determine the laser power reaching the substrate surface after passing through the plume region. Optical properties, such as the dielectric constant and electrical conductivity of plume, and the thermal conductivity of the vapour of stainless steel 316 in the plume region are estimated. Elliptic-paraboloid weld pool geometry is considered and analytic expressions for temperature distributions are obtained in the solidand liquidregions based on theweld pool geometry. Experimental and theoretical results for the weld depths and widths areillustratedfor process parameters such as the laser power, welding speed, and weld pool shape factors in x and y directions.
Weld growth mechanisms during resistance spot welding of two and three thickness lap joints
Authors: Harlin N. , Jones T. B. , Parker J. D.
Published: February 2003

Understanding the mechanisms of weld growth during resistance spot welding is a prerequisite for the development of optimum welding conditions which ensure high levels of joint quality in autobody manufacture. This study compared the heat development/weld growth of double and triple sheet lap joints as a means of understanding the role of welding conditions, sheet thickness configuration, and physical material properties on joint microstructure and performance. High temperature heat patterns have been determined using metallographic and thermographic techniques for double and triple sheet lap joints in uncoated low carbon steel. In the case of double sheet joints, initial heat generation and weld nugget formation was observed to occur where the resistance to the flow of current was the greatest, i.e. at the sheet/sheet interface. This was also the case for heat generation in triple sheet joints, but in this more complex case weld nugget formation was dependent on sheet thickness and joint configuration. Furthermore, the incubation period to weld nugget formation was dependent on total joint thickness with a longer weld time required for joints of greater thickness. For non-uniform triple sheet joints, heating was observed to be concentrated around the thinnest sheet as a result of the way heat was dissipated by the steel substrate.
Laser welding of complex aluminium structures
Authors: Sekhar N. C. , Bjørneklett B.
Published: February 2003

Automotive manufacturing is making significant strides towards producing lighter but stronger vehicles. Developments in high strength steels and lightweight materials, especially aluminium, have contributed to these advances. The body shell contributes nearly 25% of a car's weight. Current predictions estimate the contribution of aluminium in a car to be of the order of 10% of the total vehicle weight, whereas the use of aluminium alloys in car body manufacture could lead to potential weight savings of up to 43%. The use of aluminium is expected to grow in body in white and tailored welded blank applications within the automotive sector. Complex joint configurations are encountered in spaceframe assembly. One such joint structure, which involves the welding of a rectangular section to a flat plate, has been considered in the work reported. Traditionally, a MIG welding solution would be adopted to manufacture such a joint. Work was performed to establish the feasibility of using laser welding to manufacture the component and to compare the laser and MIG processes. A fibre optic delivered Nd-YAG laser beam, manipulated with an articulated arm robot, was successfully used to weld the tube to the aluminium plate. Spatter free welds with no externally visible porosity were achieved and no cracking was observed in any weld cross-section examined. Mechanical tests indicated that the static strength of the laser welds was lower than that achieved by MIG welding, which is attributed to the lower load bearing area in the laser welded joints.
Fracture toughness test for flux cored wire welded joint
Authors: Zhang G. D. , Yu S. F. , Li Z. Y.
Published: February 2003

Test techniques using unloading compliance single specimens to determine crack opening displacement (COD) resistance curves for flux cored arc welded joints were investigated in detail. Computer based methods for data acquisition and processing were incorporated in the test procedure. The causes of the shorter calculated crack length in comparison with the length measured according to unloading compliance single specimen techniques were studied, and the counter measures used to correct the crack length were analysed. The COD resistance curves for YJ657 basic flux cored wire (FCW) weld metal and joint line regions were then obtained using these techniques. The correlation coefficient using the least squares method to fit the curves was greater than 0.99. The results showed that the present test system and computer based data acquisition and processing techniques offered advantages such as flexibility of operation, stability of performance, and high accuracy when they were used to measure the COD resistance curves for FCW welded joints. The test results also indicated that the characteristic value of fracture toughness (extrapolated COD at zero crack length) for the YJ657 basic FCW weld metal and joint lines at 253 K was greater than 0.1 mm, indicating high toughness. This was because the weld metal microstructure consisted of a large proportion of acicular ferrite plates which were in different orientations and therefore able to deflect propagating cleavage cracks.
Modelling turbulent thermofluid flow in stationary gas tungsten arc weld pools
Authors: Hong K. , Weckman D. C. , Strong A. B. , Zheng W.
Published: February 2003

The effects of turbulence in stationary gas tungsten arc (GTA) welds in AISI 304 stainless steel were examined using a finite element thermofluids model. The model includes buoyancy, Lorentz and Marangoni driven fluid flow, a large deformation model of the free surface, and a k–∊ turbulence model. To facilitate implementation of the wall function boundary conditions for the k–∊ turbulence model, a dynamic numerical grid remapping technique was used to clearly separate elements in the liquid from those in the solid. The influences of sulphur content of AISI 304 stainless steels on the turbulent viscosity, fluid flow, and weld pool dimensions were simulated. Good correlation between experimentally observed and predicted weld pool shapes and dimensions was obtained. Also, the effect of sulphur concentration on AISI 304 weld pool dimensions was correctly predicted. The simulations indicate that the flow in such stationary GTA weld pools in AISI 304 stainless steel is not laminar and that quantitatively accurate predictions of weld pool fluid flow and weld shapes and dimensions will only be possible if the effects of turbulence in GTA weld pools are modelled correctly.
Power beam welding of thick section nickel base superalloys
Authors: Sekhar N. C. , Reed R. C.
Published: February 2003

A study is presented of the power beam welding of two precipitation strengthened nickel base superalloys, Waspaloy and Udimet 720Li, which are widely used in the gas turbine industry. Processing was carried out on 4·3 mm thickness rectilinear sections using (i) an electron beam welding apparatus operating at 150 kV and 12 mA, and (ii) a Nd-YAG laser capable of delivering 3·5 kW at the workpiece. The laser beam welds yielded ‘hourglass shaped’ fusion profiles, due to the blooming of the laser beam; conversely, the profiles of the electron beam welds were very much more uniform. Radiography indicated considerable porosity in the laser welds, which was attributed to the entrapment of gases due to keyhole collapse. In Waspaloy, the fusion zone was about 40% softer than the baseplate following welding; in contrast, the Udimet 720Li fusion zone was about 15% harder. This was attributed to the different equilibrium fractions of the γ’ phase, and the lack of any detectable in weld γ’ precipitation in Waspaloy compared with very fine precipitation in Udimet 720Li. Under any given welding conditions, Udimet 720Li exhibited a higher degree of distortion, due to the greater shrinkage stresses resulting from its superior elevated temperature properties.
Investigation of effect of boronising on welding zone
Authors: Meric C. , Sahin S. , Uzkut M.
Published: February 2003

In the present study, AISI 1040 and AISI 8620 steels are joined by arc welding and gas metal arc welding, and then subjected to a solid boronising process at 850, 900, 950, and 1000°C using EKabor HM (tradename) for 4 h. The microhardness and thickness of the boride layers produced on the surfaces of the air cooled specimens are measured and the microstructures are photographed using optical microscopy. The effect of boronising in and around the welding zone is investigated.
Analysis of processes involving heat deposition using constrained optimisation
Authors: Lambrakos S. G. , Milewski J. O.
Published: February 2003

Aspects are presented of a general approach based on constrained optimisation for the analysis of processes involving heat deposition, such as welding processes. The methodology of the present approach entails generating functions and subdomain elliptic solvers useful for the practical application of constrained optimisation for the calculation of thermal histories. The emphasis in the present work is on the general properties associated with the methodology of this approach as these properties relate to its application. In addition, prototype weld analyses are presented that serve to demonstrate many of the details associated with the practical application of this methodology.
Effect of chromium addition on brazing of copper to alumina: Some microstructural aspects
Authors: Zhang J. X. , Chandel R. S. , Seow H. P.
Published: February 2003

Experiments were carried out to study the effect of chromium additions to copper on the wettability of the latter on alumina substrates. The results showed that chromium can enhance the wettability of copper on alumina ceramic. To investigate the phase evolution at the interface after introducing chromium additions, X-ray diffraction was used to study the interface between the metal and alumina. It was found that two different types of copper aluminium oxide, namely, CuAlO2 and CuAl2O4, were formed near the interface between pure copper and alumina ceramic. For the copper with chromium additions, only chromium oxide was found at the interface. The reaction products were analysed and possible reaction mechanisms were proposed.
Strength of laser welded thick plate lap joint
Authors: Kitagawa A. , Takeda S. , Fukuda N. , Matsunawa A.
Published: February 2003

The purpose of the present study is to evaluate the static strength of thick plate laser welded lap joints. The strength of thin sheet laser welded lap joints of thickness less than 1·6 mm has already been reported, but the strength of thick plate laser welded lap joints having a thickness greater than 10 mm is not known. It is the opinion of the present authors that application of such a thick laser welded lap joint to sandwich panel fabrication presents revolutionary opportunities for heavy industry. Mechanical tests have been conducted on 10 mm–6 mm thickness laser welded lap joints to determine their static strength. The measured nominal shear stress is found to be almost the same as the tensile strength of the weld metal. This result is significantly different to that for thin sheet laser welded lap joints. In the present report, the fracture criteria for thick plate laser welded lap joints are clarified.
Bonding zone formation in magnetic pulse welds
Authors: Stern A. , Aizenshtein M.
Published: February 2003

The microstructural features of the bonding zone in magnetic pulse welds of similar and dissimilar metal pairs have been investigated. The nature of the reactions and phase formation that occur in the magnetic pulse joints displaying a discontinuous pocket type or a continuous transition layer along the bond interface is explained in terms of local melting followed by rapid solidification. The most significant feature of the transition zone created during the magnetic pulse welding process is the hardness increase of the interface layer. For similar base metals the main cause of the hardness increase is the fine grained microstructure created during the rapid local solidification. In the dissimilar base metal welds the increase in hardness is a result of the formation of stable or metastable intermetallic phases and of the fine grained microstructure of the bonding zone.
Analysis of friction stir welds using an inverse problem approach
Authors: Fonda R. W. , Lambrakos S. G.
Published: February 2003

A preliminary analysis of friction stir welds is presented using an inverse problem approach which entails representing the rotating tool as a three-dimensional surface distribution of heat sources. In this approach, the heat generated from these heat sources is proportional to the local velocity of the tool surface, but the resulting temperature field can be adjusted within this constraint so that the shapes of the calculated three-dimensional temperature isotherms are consistent with experimental weld cross-sections. In the present work, experimental information about the local mechanical properties and transformation microstructures observed in the heat affected zone is used to scale the calculated temperature field in a consistent manner. In addition, procedures are discussed which can extend this approach to include more complex aspects of friction stir welding and determine an absolute scale for the calculated temperature field.
Finite element modelling of resistance spot welding of aluminium with spherical tip electrodes
Authors: De A.
Published: February 2003

A FEM based simulation model of the resistance spot welding process has been developed. The current simulation accounts for the non-uniform current density distribution in the sheet-electrode geometry and the elastoplastic deformation of the sheet due to electrode force, especially with spherical tip electrodes, since these effects are driven by and contribute to the main heat transfer analysis, which is governed by the internal heat generation within the sheet-electrode geometry as well as along the contact surfaces. The latent heat of transformation during melting or solidification, the variation of sheet/sheet contact resistance with temperature, and temperature dependent thermophysical material properties have been incorporated. The model also calculates the local time-temperature history of the sheet-electrode geometry that can be coupled with appropriate metallurgical reactions to determine metallurgical changes and subsequent mechanical properties in both the fusion zone and the HAZ.
Dissimilar 2·25Cr/9Cr and 2Cr/0·5CrMoV steel welds: Part 1: Characterisation of weld zone and numerical simulation
Authors: Kozeschnik E. , Pölt P. , Brett S. , Buchmayr B.
Published: February 2003

When dissimilar welds are heat treated or exposed to in service conditions, long range diffusion of rapidly diffusing elements can be induced by strong gradients in chemical potential across the fusion zone. This diffusion is usually accompanied by precipitation or dissolution of carbides and nitrides. In the present work, six 2·25Cr/9Cr and 2Cr/0·5CrMoV welds in different heat treatment conditions have been characterised by means of optical and electron microscopy. Carbon profiles have been obtained with wavelength dispersive X-ray analysis. A general thermokinetic model for simultaneous precipitation and diffusion, which is implemented in the software MatCalc, has been used to predict the time dependent phase distributions and the carbon profiles based on multicomponent thermodynamic and kinetic data. The calculations and the experimental data show good agreement.
Dissimilar 2·25Cr/9Cr and 2Cr/0·5CrMoV steel welds: Part 2: Identification of precipitates
Authors: Kozeschnik E. , Warbichler P. , Letofsky-Papst I. , Brett S. , Buchmayr B.
Published: February 2003

The long range diffusion of rapidly diffusing elements in dissimilar welds, which occurs during heat treatment and in service exposure, initiates heavy microstructural changes on both sides of the weld. Various carbides and nitrides either dissolve or precipitate as a function of the distance from the fusion line. The type and amount of these particles can be predicted by a numerical method for solving problems of simultaneous precipitation and diffusion. Predicted precipitate distributions for a 2·25Cr/9Cr and a 2Cr/0·5CrMoV weld are verified using experimental data obtained with analytical and energy filtering transmission electron microscopy. The phase diagrams for the high chromium side and the low chromium side of the welds are evaluated and compared with the experimentally determined precipitate distribution.