A study of mixed-mode dynamic crack initiation in PMMA

The effect of mode-mix on crack initiation (toughness and trajectory) is well documented both theoretically and experimentally for quasi-static fracture. Dynamic fracture (single or mixed-mode) analysis is generally more complicated since inertial effects cannot be neglected. Recent (mode II) shear impact experiments in steel and polycarbonate have shown the existence of fracture mode transitions (brittle to ductile. including shear band formation) as a function of the impact velocity. Here we report experimental results for mixed-mode dynamic crack initiation in commercial PMMA as a model for brittle materials. The two-dimensional combination of a major and a minor mode (1-11 or 11-1 respectively) was systematically achieved using two distinct specimen geometries impacted at various velocities. The results show that, regardless of the dominant loading mode, the initial kink angle between the crack and the original notch line increase with the impact velocity. These results are described and discussed in terms of a simple criterion

Modelisation of dynamic crack propagation criteria

The aim of this study is the analysis of the dynamic crack propagation criteria in materials under impact loading. We want to define which mechanical quantities monitor the propagation, then use these parameters in numerical simulations and finally compare the results with dynamic propagation experiments. First, we propose a 1D semi-analytical model. In this model several mechanical quantities can be used to define the propagation criterion, like the energy release rate... The effect of the applied loading and the reflections of waves is also very important, leading sometimes to a stick-slip propagation. Next, we present a 2D modelisation. In this case, it is not possible to study analytically the effect each criterion and we need to use a numerical simulation. We propose a numerical method based on finite element. The propagation is simulated by releasing nodes with a the release criterion defined from the propagation criterion. The numerical scheme is chosen to respect the equivalence between the total dissipation and the fracture dissipation. In parallel, we have performed dynamic propagation experiments on PMMA. The samples are DCB specimens loading with a wedge in a Kolsky set-up. During each test, the location of the crack has been recorded 4 times with high speed cameras (1 microsecond shutter)

Musique et discothèques : pratiques des professionnels et pratiques des publics

Les nouvelles pratiques d’écoute et l’évolution des espaces « musique ». Table ronde menée par Michel Sineux, ancien président de la FFCB et rédacteur en chef d’Écouter Voir, avec : Yves Alix, Xavier Galaup, Christian Massault et Gilles Pierret, Médiathèque musicale de Paris

Bibliothèques jeunesse : quelles évolutions ?

Co-organisée avec lassociation Livres de jeunesse en fête (LJF) qui, depuis 2002, regroupe des bibliothécaires de plusieurs établissements du Loiret, cette journée détude bénéficiait du soutien de la Mairie dOrléans et de la DRAC Centre. Une cinquantaine de collègues ont pu y trouver quelques réponses pratiques à leurs interrogations toujours nombreuses

The Influence of Mode-Mixity on Dynamic Failure Mode Transitions in Polycarbonate

The transition of a shear to opening type of failure mechanism has been reported for side impact experiments of notched or cracked plates (metallic alloys and Polycarbonate). The present paper addresses additional aspects of the phenomenon in relation to mode-mixity for actual fatigue cracks in Polycarbonate specimens. Two distinct experimental setups are used and systematically compared throughout the work: dominant mode II and dominant mode I loading. The experimental results show that the same characteristic failure mechanisms operate irrespective of the specimen geometry (loading mode) and crack-tip nature for a given impact velocity (evolution of the stress intensity factors).La transition des modes de rupture d'ouverture en cisaillement a été observée lors d'essais d'impact latéral de plaques entaillées ou fissurées (alliages métalliques et Polycarbonate). Cet article traite d'autres aspects de la transition en relation avec la mixité du chargement pour des fissures de fatigue dans du Polycarbonate. Nous utilisons et comparons systématiquement deux types d'expériences distinctes: chargement en mode 11 dominant ou en mode I dominant. Les résultats montrent que des mécanismes de ruine identiques opèrent dans les deux cas, en fonction de la vitesse d'impact (evolution des facteurs d'intensité des contraintes) indépendemment de la géometrié de l'éprouvette ou de la stature de la fissure

The Reciprocity Gap Functional for Identifying Defects and Cracks

International audienceThe recovery of defects and cracks in solids using overdetermined boundary data, both the Dirichlet and the Neumann types, is considered in this paper. A review of the method for solving these inverse problems is given, focusing particularly on linearized inverse problems. It is shown how the reciprocity gap functional can solve nonlinear inverse problems involving identification of cracks and distributed defects in bounded solids. Exact solutions for planar cracks in 3D solids are given for static elasticity, heat diffusion and transient acoustics

Water retention in unsaturated soils subjected to wetting and drying cycles

The suction is an essential parameter to describe and understand the behavior of unsaturated soils. The ability of unsaturated soils to retain water is quantified by determining the water retention curves (WRC), which express the hydraulic behavior of porous materials such as soil. These curves are determined by subjecting samples to several drying and wetting cycles. The curve during drying path is located above the wetting curve, developing a hysteresis phenomenon [1], and value of content water at a given suction value depends on the path used to reach this point. The aim of this paper is to present a study on the hydraulic behavior of soil, water retention capacity due to drying and wetting cycles, pointing out the hydro-mechanical behavior of unsaturated soils. In the first part, the effect of physical and mechanical properties of soil [32] (initial void ratio, particle size, cohesion, density...) on the water retention is presented. In the second part, a complete numerical model was developed, based on the empirical model of Van Genuchten [18], to model the two boundary curves, and the experimental scanning data were bestfitted using the same theory of Mualem model [13]. This complete model requires 4 parameters. This model has been validated with experimental data on different type of soils: sand [10], [34], U.S. Silica F-95 sand [30]

Identity and parentage of two alpine grape cultivars from Switzerland (Vitis vinifera L. Lafnetscha and Himbertscha)

Four closely related white grape cultivars from the Western Alps (Switzerland) - Humagne Blanc, Completer, Lafnetscha and Himbertscha - and three putative relatives or synonyms - Gouais Blanc, Plantscher and Bordeaux Blanc - were analyzed with up to 50 microsatellite markers. Humagne Blanc and Completer are ancient cultivars from the Haut-Valais and Graubünden regions, respectively. Lafnetscha and Himbertscha are lesser-known cultivars scarcely cultivated in Haut-Valais. Lafnetscha is frequently considered as synonym of Completer. Himbertscha might be related to Gouais Blanc, one of the parents of Chardonnay, Gamay, etc. Plantscher, a putative synonym of Lafnetscha, is scarcely cultivated in Haut-Valais (Switzerland) and Bordeaux Blanc (or Gros Bourgogne) is a cultivar of unknown origin (despite its names) cultivated in Switzerland. Our results allowed us to determine the true-to-type Lafnetscha and confirmed that Lafnetscha is not a synonym of Completer. Plantscher is not a synonym of Lafnetscha but a synonym of Bordeaux Blanc (or Gros Bourgogne) and is a likely parent or progeny of the Hungarian Furmint. Himbertscha is not related to Gouais Blanc and shares at least one allele at each locus with Humagne Blanc, providing strong evidence of a parent/progeny relationship. Given that Humagne Blanc is an older cultivar, we propose that it is the parent of Himbertscha. Alleles at 49 out of 50 microsatellite loci are consistent with Lafnetscha being the progeny of Completer and Humagne Blanc. The exception is a 10-base pair discrepancy at one locus (VVMD 36), most likely due to the occurrence of a null allele, since this parentage is supported at other markers by very high likelihood ratios. With Lafnetscha = Completer x Humagne Blanc, we present the second grape cultivar parentage showing a multiple repeat discrepancy at one locus. This study emphasizes that one multiple repeat unit discrepancy is not sufficient to reject a parentage, and that the greater is the number of loci, the greater are the chances to encounter null alleles or clonal mutations