Determination of stiffness and higher gradient coefficients by means of the embedded atom method: An approach for binary alloys

For a quantitative theoretical description of phase separation and coarsening reliable data of stiffness constants and the so called Higher Gradient Coefficients (HGCs) are required. For that reason pair potentials of the Lennard-Jones type were used in [1] to provide a theoretical tool for their quantitative determination. Following up on this work these quantities are now calculated by means of the Embedded-Atom Method (EAM), a recently developed approach to describe interatomic potentials in metals. This is done, first, to achieve a better agreement between predicted and experimentally observed stiffness data as well as to avoid artifacts, such as the Cauchy paradox, and, second, to increase the trustworthiness of the HGCs for which experimental data are rarely available. After an introduction to the fundamentals of EAM it is outlined how it can be used for calculating stiffness constants and HGCs. In particular, Johnson's modification of EAM for nearest neighbor interactions [3] is applied to present explicit numerical results for a case study alloy, Ag-Cu, which has a ``simple" face-centered-cubic crystal structure and where it is comparatively easy to obtain all the required analysis data from the literature and to experimentally compare the predictions of mechanical data

A higher gradient theory of mixtures for multi-component materials with numerical examples for binary alloys

A theory of mixture for multi-component materials is presented based on a novel, straightforward method for the exploitation of the Second Law of thermodynamics. In particular the constitutive equations for entropy, heat and diffusion flux as well as the stress tensor are formulated as a consequence of the non-negative entropy production. Furthermore we derive the established Gibbs equation as well as the Gibbs Duhem relation which also follow from the formalism. Moreover, it is illustrated, how local mechanical strains due to eigenstrains or external loadings, modify the free energy and, consequently, change the chemical potentials of the components. All consecutive steps are illustrated, first, for simple mixtures and, second, for a system containing two different phases. So-called higher gradients of the concentrations are considered, which take the nonuniform composition into account. It will also become apparent that more/other variables of modified/different physical pr oblems beyond the illustrated ones can be easily treated within the presented framework. This work ends with the specification to binary alloys and with the presentation of various numerical simulations

Modified Shape of Dynamic Master Curves due to Adiabatic Effects

AbstractWithin a joint project of IWM/Freiburg and MPA/Stuttgart the fracture toughness of a 22 NiMoCr 3 7 steel (A 508 Cl.2) was characterized at IWM with SE(B)10/10- und SE(B)40/20-specimens at -20 °C and high crack loading rates in the range of 103 to 106 MPa√m s-1, see Böhme et al. (2012 and 2013). The single temperature Master Curve evaluation according to ASTM E1921 and Wallin (2011) resulted in part in 5%-lower-bound fracture toughness versus temperature curves below the deterministic ASME lower bound KIR-reference-curve. At a first glance, this seems to violate the ASME KIR-concept, however, possibly this just indicates, that the conventional MC-evaluation has to be modified for elevated loading rates. Adiabatic heating in the vicinity of the crack tip could be one reason for that, as already argued in Schindler (2013 and 2015).Therefore, additional SE(B)-tests at temperatures of -20 °C, 0 °C and +20 °C were performed at IWM within the current follow-up joint IWM-MPA project. The new IWM-results show in agreement with previous investigations by Viehrig et al. (2010) and Schindler et al. (2013 and 2015) that the Master Curves at elevated loading rates are steeper than at quasistatic loading, probably due to local adiabatic heating in the vicinity of the crack tip. Therefore, the temperature field around the crack tip has been measured with a high speed infrared camera and has been compared to results of a numerical simulation. Up to crack initiation, a local adiabatic rise in temperature of the order of magnitude of about 60 K was measured and calculated in the vicinity of the crack tip at a crack loading rate of about 106 MPa√m s-1. In order to take into account this adiabatic effect, the dynamic master curves were evaluated by applying an adjusted MC shape parameter. This finally leads to more plausible results for the dynamic Master Curves. Thus, the choice of a rate dependent shape parameter p should be considered for future modifications of the elevated loading rate appendix of ASTM E1921

Reprodução em uma taxocenose de lagartixas (Squamata: Phyllodactylidae) na região de Marañon (Peru) e comentários sobre a maior lagartixa do Novo Mundo

Reproduction is described for four phyllodactylid geckos, Phyllopezus maranjonensis, Phyllodactylus delsolari, P. thompsoni and P. reissii from the upper Marañon Valley, Peru. Observations were made between March and May 2010 and voucher specimens, already housed in the Zoologisches Forschungsmuseum Alexander Koenig (ZFMK), were X-rayed to check for gravid females. Gravid female P. maranjonensis, P. delsolari and P. reissii contain two eggs, whereas P. thompsoni produces a single egg that is deposited in leaf litter or crevices of stone walls. It is likely that females of all of these species produce multiple clutches in one year; the reproductive period seems to be extended in at least two species. Furthermore, measurements of numerous specimens of all four species are presented herein, revealing that Phyllopezus maranjonensis is the largest species of New World geckos.Descrevemos aqui a reprodução de quatro lagartixas da familia Phyllodactylidae, Phyllopezus maranjonensis, Phyllodactylus delsolari, P. thompsoni e P. reissii, do Vale do Alto Marañon, Peru. As observações foram feitas entre março e maio de 2010, e os espécimes-testemunhos, depositados no Zoologisches Forschungsmuseum Alexander Koenig (ZFMK), foram radiografados para verificar a presença de fêmeas ovígeras. As fêmeas ovígeras de P. maranjonensis, P. delsolari e P. reissii portavam dois ovos, enquanto P. thompsoni produz um único ovo, que é depositado na serapilheira ou em frestas de paredões rochosos. As fêmeas de todas as espécies são mais provavelmente capazes de produzir desovas múltiplas ao longo do ano; o período reprodutivo parece prolongado em pelo menos duas espécies. Adicionalmente, medidas de diversos espécimes das quatro espécies são apresentadas aqui, revelando que Phyllopezus maranjonensis é a maior espécie de lagartixa do Novo Mundo

Čovjek i lacertidni gušteri na sredozemnim otocima: perspektive zaštite

The aim of the present study is to synthesise the available data on the occurrence of lacertid lizards on Mediterranean islands better to understand how far human influence is involved in the present distributional patterns of insular lacertid lizard fauna. At present, the Mediterranean islands are inhabited by several lacertid genera, including endemic species of the genera Algyroides (viz. fitzingeri), »Lacerta« Archaeolacerta (viz. bedriagae) and Podarcis (atratus, filfolensis, gaigae, lilfordi, milensis, pityusensis, raffonei, tiliguerta, waglerianus). Recently reported data shed new light on the relationships between these endemics and some continental species that have recently colonised the islands. The arrival of the latter might be directly related to the progressive human colonisation of these islands. The arising conservation problems are also considered.Cilj ovog rada bio je sakupiti sve dostupne podatke o pojavljivanju lacertidnih guštera na otocima Sredozemlja da bi se bolje razumjelo koliko je čovjek utjecao na sadašnje uzorke rasprostranjenosti otočne faune lacertidnih guštera. Danas sredozemne otoke naseljuje nekoliko rodova lacertida, uključujući endemične vrste rodova Algyroides (viz. fitzingeri), »Lacerta« Archaeolacerta (viz. bedriagae) i Podarcis (atratus, filfolensis, gaigae, lilfordi, milensis, pityusensis, raffonei, tiliguerta, waglerianus). Nedavno objavljeni podaci bacili su novo svjetlo na odnose tih endemičnih i nekih kontinentalnih vrsta koje su nedavno naselile otoke. Dolazak potonjih mogao bi biti u direktnoj vezi s uznapredovalim naseljavanjem čovjeka na te otoke. Razmatraju se nastali problemi zaštite

Dynamics of the Size and Orientation Distribution of Microcracks and Evolution of Macroscopic Damage Parameters

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.We are dealing with damage of brittle materials caused by growth of microcracks. In our model the cracks are penny-shaped. They can only enlarge but not heal. For a single crack a Rice–Griffith growth law is assumed: There is crack growth only if tension is applied normally to the crack surface, exceeding a critical value. Our aim is to investigate the effect of crack growth on macroscopic constitutive quantities. A possible approach taking into account such an internal structure within continuum mechanics is the mesoscopic theory. A distribution of crack lengths and crack orientations within the continuum element is introduced. Macroscopic quantities are calculated as averages with the distribution function. A macroscopic measure of the progressing damage, i.e., a damage parameter, is the average crack length. For this scalar damage parameter we derive an evolution equation. Due to the unilateral growth law for the single crack, it turns out that the form of this differential equation depends explicitly on the initial crack length distribution. In order to treat biaxial loading, it is necessary to introduce a tensorial damage parameter. We define a second-order tensor damage parameter in terms of the crack length and orientation distribution function

Notes on the systematic value of the tongue morphology in different genera and species groups of lacertid lizards (Reptilia: Lacertidae)

The tongue shape and its epidermal structure was examined in a stock of palearctic and paleotropical lizards from Europe, Africa, the Middle East and south eastern Asia. The results of this preliminary report clearly point out the systematic importance of this morphological feature in providing valuable generic and specific characters. The fundamental pattern described for the genera Podarcis, Lacerta sensu stricto and Gallotia, e. g., is very distinctive despite several features in common, likely due to ancient phyletic relationships. A number of similar morphological trends stress the reciprocal affinities between Lacerta (Archaeolacerta) and the widely spread genus Podarcis which is still undergoing speciation. Another case is the minor but significant affinities linking the genera Lacerta sensu stricto and Gallotia, the latter also undergoing more complicated speciation due to its insular isolation. The interspecific differences in the taxa belonging to the so-called “species groups” such as Podarcis, are scanty or insignificant. Instead there are striking differences in the generic patterns of the tongue morphology in Afro-Asian lacertid lizards. However, a similar pattern was found in the African genera Adolfus and Holaspis or Centromastix (= Gastropholis according to Arnold, 1989) and Ichnotropis.Material digitalizado en SEDICI gracias a la colaboración del Dr. Jorge Williams (FCNM-UNLP).Facultad de Ciencias Naturales y Muse