Computer modelling of crack growth in rubber-toughened polymers.

We present results from a computer modelling study of fracture propagation in a rubber-toughened polymer subject to external uniaxial tension. Using a simple lattice model comprising two bond types (one for rubber and one for polymer), simulations have been performed on both 2- and 3-dimensional systems. Each bond is characterised by two parameters : an elastic constant and a mechanical strength. All bonds behave in a purely elastic manner up to fracture; this limits the model's applicability to impact failure of brittle glassy polymers. We employ a stochastic breaking process based on Eyring's reaction rate theory, and allow the system to equilibrate fully after each bond breakage. It is found that the Conjugate Gradient method is the most efficient technique for calculating these equilibria. The model allows straightforward measurement of the total damage at failure, which is a crude approximation of the material toughness, and the stress distribution during the damage evolution.We show that, for a wide range of bond parameters, as the rubber loading is increased the damage goes through a sequence of behaviour : embrittlement; toughening; saturation; and weakening. This sequence is consistent with experimental data for rubber-toughened Poly (methyl methacrylate) (RT-PMMA). Our results show that the embrittlement and saturation stages are sensitive to the ratio of the two elastic constants but that they disappear as this ratio approaches unity. The toughening achieved is sensitive to both the concentration and the mechanical strength of the rubber bonds, the most damaged systems being those with large rubber mechanical strength at approximately 50% concentration. We also find that the damage is dependent on the spatial distribution of the rubber bonds, the greatest toughness being achieved using a homogeneous rubber distribution.We also find that the evolution of the system's failure goes through four regimes. In the first of these, the damage is evenly distributed throughout the system but showsa propensity for occuring adjacent to rubber bonds. Subsequently, short cracks develop between some of these weakly damaged sites. As more of these develop, a number of long rubber-bridged cracks appear. In the third regime, the stabilising effect of the bridging rubber bonds forces the damage to extend throughout the system, leading to branching and interconnecting of rubber-bridged cracks. Finally, the stress experienced by the crack-bridging rubber bonds becomes unsustainable and catastrophic failure occurs. Zones of material consistent with each of these four regimes can be observed in experimentally fractured RT-PMMA. The main toughening mechanisms in operation in our model are, therefore, crazing (which occurs mainly in the second regime) and rubber-bridging (which occurs in the third). The former is enhanced by using a lower rubber elastic constant, whereas the latter is prolonged by a large rubber mechanical strength.We also apply the model to simulations of 2D and 3D systems containing rubber macro-particles, i.e. large volumes of connected rubber bonds, and we present visual results of the damage and the stress field evolution in such systems. In these, the damage appears to develop in accordance with the four regimes described above. The stress maps of multi-particle 2D systems show the gradual stress transfer which takes place as cracks develop, from initially high-stressed inter-particle polymer regions to the crack tips and, then, to the rubber particles along the crack lines. These visual results also highlight three possible modes of particle-crack interaction : particle debonding, particle splitting and crack reflection

Molecular dynamics calculation of elastic constants in Gay-Berne nematic liquid crystals

<F4.417e+05> In this paper we present a molecular dynamics calculation of the Frank elastic constants of a nematic liquid crystal. We study two well-known variants of the Gay-Berne potential, and determine the elastic constants by measuring orientational fluctuations as a function of wavevector, using reasonably large system sizes in the range 1000--8000 molecules. For some of the simulations, a set of Lagrangian constraints was applied in order to keep the director fixed along one of the box axes, facilitating the measurement of fluctuations in components of the reciprocal-space order tensor <F5.754e+05><F5.273e+05><F5.754e+05><F5.273e+05> Q(k)<F4.417e+05> in the director frame. <F5.754e+05>1 Introduction<F4.417e+05> In a nematic liquid crystal, the distribution of molecular positions is translationally invariant but the orientational distribution shows preferential alignment along the director. Deviations from uniform alignment occur principally through the existence of defects (..

Replicated Data and Domain Decomposition Molecular Dynamics Techniques for the Simulation of Anisotropic Potentials

The implementation of parallel molecular dynamics techniques is discussed in the context of the simulation of single-site anisotropic potentials. We describe the use of both replicated data and domain decomposition approaches to molecular dynamics and present results for systems of upto 65536 Gay-Berne molecules on a range of parallel computers (Transtech i860/XP Paramid, Intel iPSC/860 Hypercube, Cray T3D). We find excellent parallel speed-ups are possible for both techniques, with the domain decomposition method found to be the most efficient for the largest systems studied. I. INTRODUCTION In recent years molecular dynamics (MD) has become one of the most widely used techniques for studying condensed systems. It can be applied to a wide range of problems including the simulation of liquids [1], biological systems [2], and the investigation of complex fluids such as liquid crystals [3], colloids [4] and micellar systems [5]. In most of these cases molecular dynamics involves the s..

Le principat d’Auguste

À la suite de la bataille d’Actium, Auguste est parvenu à fonder un régime politique durable, communément appelé principat, dont la nature continue à faire débat. L’apparition au début des années 20 av. J.-C. du thème de la Res publica restituta, à la fois slogan et programme politique qui faisaient d’Auguste le restaurateur de l’État romain, n’est pas le moindre des paradoxes. Si l’historiographie reste sensible aux ruptures qui résultaient de la mainmise sur l’État d’un seul homme et de sa famille, il y a place également pour une étude des continuités que le nouveau régime n’a cessé de mettre en avant à ses débuts. Les organisateurs du colloque de Nantes, Fr. Hurlet et B. Mineo, ont rassemblé une équipe internationale d’une vingtaine de chercheurs pour susciter un échange de points de vue sur le sujet entre littéraires, historiens, historiens de l’art et archéologues. Les auteurs augustéens – historien comme Tite-Live, poètes comme Virgile, Horace ou Ovide – ont été l’objet d’une attention particulière à travers l’étude de leur perception du nouveau régime et de la nature de leurs relations avec le prince ; l’enquête a été étendue aux auteurs postérieurs à Auguste qui ont traité de la naissance du principat (Tacite et Dion Cassius). Les épigraphistes et les numismates ont analysé la manière dont le nouveau régime cherchait à se présenter sur les inscriptions et les monnaies afin de mieux saisir toutes les subtilités du discours officiel. La mise en forme des pouvoirs impériaux et l’attitude de l’aristocratie au moment de la mise en place du principat ont fait également l’objet d’études spécifiques. Ont été prises en compte les images liées au nouveau pouvoir – monuments, demeure d’Auguste sur le Palatin, statues, reliefs, cérémonies religieuses – comme support matériel de l’idéologie impériale. Un tel faisceau de points de vue permet de mieux se représenter dans toute leur complexité les fondements du principat augustéen à sa naissance

SEIS: Insight’s Seismic Experiment for Internal Structure of Mars

By the end of 2018, 42 years after the landing of the two Viking seismometers on Mars, InSight will deploy onto Mars’ surface the SEIS (Seismic Experiment for Internal Structure) instrument; a six-axes seismometer equipped with both a long-period three-axes Very Broad Band (VBB) instrument and a three-axes short-period (SP) instrument. These six sensors will cover a broad range of the seismic bandwidth, from 0.01 Hz to 50 Hz, with possible extension to longer periods. Data will be transmitted in the form of three continuous VBB components at 2 sample per second (sps), an estimation of the short period energy content from the SP at 1 sps and a continuous compound VBB/SP vertical axis at 10 sps. The continuous streams will be augmented by requested event data with sample rates from 20 to 100 sps. SEIS will improve upon the existing resolution of Viking’s Mars seismic monitoring by a factor of ∼2500 at 1 Hz and ∼200000 at 0.1 Hz. An additional major improvement is that, contrary to Viking, the seismometers will be deployed via a robotic arm directly onto Mars’ surface and will be protected against temperature and wind by highly efficient thermal and wind shielding. Based on existing knowledge of Mars, it is reasonable to infer a moment magnitude detection threshold of ∼3 at 40∘ epicentral distance and a potential to detect several tens of quakes and about five impacts per year. In this paper, we first describe the science goals of the experiment and the rationale used to define its requirements. We then provide a detailed description of the hardware, from the sensors to the deployment system and associated performance, including transfer functions of the seismic sensors and temperature sensors. We conclude by describing the experiment ground segment, including data processing services, outreach and education networks and provide a description of the format to be used for future data distribution