Dynamic fracture experiments were performed in PMMA over a wide range of velocities and reveal that the fracture energy exhibits an abrupt 3-folds increase from its value at crack initiation at a well-defined critical velocity, below the one associated to the onset of micro-branching instability. This transition is associated with the appearance of conics patterns on fracture surfaces that, in many materials, are the signature of damage spreading through the nucleation and growth of microcracks. A simple model allows to relate both the energetic and fractographic measurements. These results suggest that dynamic fracture at low velocities in amorphous materials is controlled by the brittle/quasi-brittle transition studied here.PACS numbers: 46.50.+a, 62.20.M-, 61.43.-j Dynamic fracture drives catastrophic material failures. Over the last century, a coherent theoretical framework, the so-called Linear Elastic Fracture Mechanics (LEFM) has developed and provides a quantitative description of the motion of a single smooth crack in a linear elastic material [1]. LEFM assumes that all the mechanical energy released during fracturing is dissipated at the crack tip. Defining the fracture energy Γ as the energy needed to create two crack surfaces of a unit area, the instantaneous crack growth velocity v is then selected by the balance between the energy flux and the dissipation rate Γv. This yields [1]:where c R and E are the Rayleigh wave speed and the Young modulus of the material, respectively, and K(c) is the Stress Intensity Factor (SIF) for a quasi-static crack of length c. K depends only on the applied loading and specimen geometry, and characterizes entirely the stress field in the vicinity of the crack front. Equation (1) describes quantitatively the experimental results for dynamic brittle fracture at slow crack velocities [2]. However, large discrepancies are observed in brittle amorphous materials at high velocities [3][4][5][6]. In particular (i) the measured maximum crack speeds lie in the range 0.5 − 0.6c R , i.e. far smaller than the limiting speed c R predicted by Eq. (1) and (ii) fracture surfaces become rough at high velocities (see [3,4] for reviews). It has been argued [7] that experiments start to depart from theory above a critical v b ≃ 0.4c R associated to the onset of micro-branching instabilities [8]: for v > v b the crack motion becomes a multi-cracks state. This translates into (i) a dramatic increase of the fracture energy Γ at v b , due to the increasing number of micro-branches propagating simultaneously and (ii) a non-univocal relation between Γ and v [7]. The micro-branching instability hence yielded many recent theoretical efforts [9]. However, a number of puzzling observations remain at smaller velocities. In particular, even for velocities much lower than v b , (i) the measured dynamic fracture energy is generally much higher than that at crack initiation [7,[10][11][12] and (ii) fracture surfaces roughen over length scales much larger than the microstructure scale ("mist" patterns...

Dynamic crack propagation drives catastrophic solid failures. In many amorphous brittle materials, sufficiently fast crack growth involves small-scale, high-frequency microcracking damage localized near the crack tip. The ultrafast dynamics of microcrack nucleation, growth, and coalescence is inaccessible experimentally and fast crack propagation was therefore studied only as a macroscale average. Here, we overcome this limitation in polymethylmethacrylate, the archetype of brittle amorphous materials: We reconstruct the complete spatiotemporal microcracking dynamics, with micrometer/nanosecond resolution, through post mortem analysis of the fracture surfaces. We find that all individual microcracks propagate at the same low, load-independent velocity. Collectively, the main effect of microcracks is not to slow down fracture by increasing the energy required for crack propagation, as commonly believed, but on the contrary to boost the macroscale velocity through an acceleration factor selected on geometric grounds. Our results emphasize the key role of damage-related internal variables in the selection of macroscale fracture dynamics.dynamic fracture | crack speed | polymeric glass | fracture energy | fracture process

Background: Six percent of the Chilean population has a disability requiring assistance with daily-living-activities and 69% of these individuals (Cronbach alpha =0.84 and 0.87, respectively), respectively) and stability reliability respectively (Rev Méd Chile 2009; 137: 657-65).

We study experimentally the fracture dynamics during the peeling at a constant velocity of a roller adhesive tape mounted on a freely rotating pulley. Thanks to a high speed camera, we measure, in an intermediate range of peeling velocities, high frequency oscillations between phases of slow and rapid propagation of the peeling fracture. This so-called stick-slip regime is well known as the consequence of a decreasing fracture energy of the adhesive in a certain range of peeling velocity coupled to the elasticity of the peeled tape. Simultaneously with stick slip, we observe low frequency oscillations of the adhesive roller angular velocity which are the consequence of a pendular instability of the roller submitted to the peeling force. The stick-slip dynamics is shown to become intermittent due to these slow pendular oscillations which produce a quasistatic oscillation of the peeling angle while keeping constant the peeling fracture velocity (averaged over each stick-slip cycle). The observed correlation between the mean peeling angle and the stick-slip amplitude questions the validity of the usually admitted independence with the peeling angle of the fracture energy of adhesives.

Linear Elastic Fracture Mechanics (LEFM) provides a consistent framework to evaluate quantitatively the energy flux released to the tip of a growing crack. Still, the way in which the crack selects its velocity in response to this energy flux remains far from completely understood. To uncover the underlying mechanisms, we experimentally studied damage and dissipation processes that develop during the dynamic failure of polymethylmethacrylate (PMMA), classically considered as the archetype of brittle amorphous materials. We evidenced a well-defined critical velocity along which failure switches from nominally-brittle to quasibrittle, where crack propagation goes hand in hand with the nucleation and growth of microcracks. Via postmortem analysis of the fracture surfaces, we were able to reconstruct the complete spatiotemporal microcracking dynamics with micrometer/nanosecond resolution. We demonstrated that the true local propagation speed of individual crack fronts is limited to a fairly low value, which can be much smaller than the apparent speed at the continuum-level scale. By coalescing with the main front, microcracks boost the macroscale velocity through an acceleration factor of geometrical origin. We discuss the key role of damage-related internal variables in the selection of macroscale fracture dynamics.

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In this work we discuss the morphology and self-affine properties of the slowfracture surfaces of soda-lime glass obtained by a bending process under the effect of applied water vapor. The fractographic analysis showed the presence of secondary cracks in the mirror zone, whereas in the mist-hackle region step-like morphologies were observed and over them we found fine undulations. The self-affine analysis, performed by two methods, showed the existence of two different statistical distributions for the roughness exponent, ζ . At the beginning of the mirror zone ζ = 0.5, in the mist-hackle region we detected the same value for fine length scales, whereas at large length scales we observed ζ = 0.8. This scenario may be described by a qualitative model in which the deterministic mirror-mist-hackle pattern coexists with an irregular topography, the two observed regimes are thus characterized by two different roughness exponents, with the 0.5 value dominating at low-speed/fine-scales and the 0.8 value governing the high-speed/large-scales regimes.

O presente estudo teve como objetivos: a tradução e adaptação de um questionário de clima de escola para estudantes portugueses (Delaware School Climate Survey–Student - DSCS-S); compreender as relações entre clima de escola e envolvimento dos alunos na escola e o seu sucesso escolar anterior. Foi também aplicada a Escala Quadridimensional de Envolvimento dos Estudantes (EAE-E4D) a 442 estudantes de escolas básicas de Évora, Portugal. Os resultados demonstram a importância das relações professor/a – aluno/a na forma como o clima de escola se relaciona com percursos escolares bem-sucedidos. Foram encontradas diferenças significativas no envolvimento afetivo e comportamental entre alunos com e sem sucesso escolar. Verificou-se uma correlação positiva e moderada entre o clima de escola e o envolvimento na escola.