Super-Arrhenius dynamics for sub-critical crack growth in disordered brittle media

Taking into account stress fluctuations due to thermal noise, we study thermally activated irreversible crack growth in disordered media. The influence of material disorder on sub-critical growth of a single crack in two-dimensional brittle elastic material is described through the introduction of a rupture threshold distribution. We derive analytical predictions for crack growth velocity and material lifetime in agreement with direct numerical calculations. It is claimed that crack growth process is inhibited by disorder: velocity decreases and lifetime increases with disorder. More precisely, lifetime is shown to follow a super-Arrhenius law, with an effective temperature theta - theta_d, where theta is related to the thermodynamical temperature and theta_d to the disorder variance.Comment: Submitted to Europhysics Letter

Slow crack growth in polycarbonate films

We study experimentally the slow growth of a single crack in polycarbonate films submitted to uniaxial and constant imposed stress. The specificity of fracture in polycarbonate films is the appearance of flame shaped macroscopic process zones at the tips of the crack. Supported by an experimental study of the mechanical properties of polycarbonate films, an analysis of the stress dependence of the mean ratio between the process zone and crack lengths, during the crack growth, show a quantitative agreement with the Dugdale-Barenblatt model of the plastic process zone. We find that the fracture growth curves obey strong scaling properties that lead to a well defined growth master curve

Thermal activation of rupture and slow crack growth in a model of homogenous brittle materials

Slow crack growth in a model of homogenous brittle elastic material is described as a thermal activation process where stress fluctuations allow to overcome a breaking threshold through a series of irreversible steps. We study the case of a single crack in a flat sheet for which analytical predictions can be made, and compare them with results from the equivalent problem of a 2D spring network. Good statistical agreement is obtained for the crack growth profile and final rupture time. The specific scaling of the energy barrier with stress intensity factor appears as a consequence of irreversibility. In addition, the model brings out a characteristic growth length whose physical meaning could be tested experimentally.Comment: To be published in : Europhysics Letter

A dynamical law for slow crack growth in polycarbonate films

We study experimentally the slow growth of a single crack in polycarbonate films submitted to uniaxial and constant imposed stress. For this visco-plastic material, we uncover a dynamical law that describes the dependence of the instantaneous crack velocity with experimental parameters. The law involves a Dugdale-Barenblatt static description of crack tip plastic zones associated to an Eyring's law and an empirical dependence with the crack length that may come from a residual elastic field

Subcritical crack growth in fibrous materials

We present experiments on the slow growth of a single crack in a fax paper sheet submitted to a constant force $F$. We find that statistically averaged crack growth curves can be described by only two parameters : the mean rupture time $\tau$ and a characteristic growth length $\zeta$. We propose a model based on a thermally activated rupture process that takes into account the microstructure of cellulose fibers. The model is able to reproduce the shape of the growth curve, the dependence of $\zeta$ on $F$ as well as the effect of temperature on the rupture time $\tau$. We find that the length scale at which rupture occurs in this model is consistently close to the diameter of cellulose microfibrils

Force-induced breakdown of flexible polymerized membrane

We consider the fracture of a free-standing two-dimensional (2D) elastic-brittle network to be used as protective coating subject to constant tensile stress applied on its rim. Using a Molecular Dynamics simulation with Langevin thermostat, we investigate the scission and recombination of bonds, and the formation of cracks in the 2D graphene-like hexagonal sheet for different pulling force $f$ and temperature $T$. We find that bond rupture occurs almost always at the sheet periphery and the First Mean Breakage Time of bonds decays with membrane size as $ \propto N^{-\beta}$ where $\beta \approx 0.50\pm 0.03$ and $N$ denotes the number of atoms in the membrane. The probability distribution of bond scission times $t$ is given by a Poisson function $W(t) \propto t^{1/3} \exp (-t / )$. The mean failure time that takes to rip-off the sheet declines with growing size $N$ as a power law $\propto N^{-\phi(f)}$. We also find $\propto \exp(\Delta U_0/k_BT)$ where the nucleation barrier for crack formation $\Delta U_0 \propto f^{-2}$, in agreement with Griffith's theory. $$displays an Arrhenian dependence of$$ on temperature $T$. Our results indicate a rapid increase in crack spreading velocity with growing external tension $f$.Comment: 12 pages, 10 figures, LaTeX, misprints correcte

Nonmonotonic fracture behavior of polymer nanocomposites

Polymer composite materials are widely used for their exceptional mechanical properties, notably their ability to resist large deformations. Here, we examine the failure stress and strain of rubbers reinforced by varying amounts of nano-sized silica particles. We find that small amounts of silica increase the fracture stress and strain, but too much filler makes the material become brittle and consequently fracture happens at small deformations. We thus find that as a function of the amount of filler there is an optimum in the breaking resistance at intermediate filler concentrations. We use a modified Griffith theory to establish a direct relation between the material properties and the fracture behavior that agrees with the experiment

ЦИТОГЕНЕТИЧЕСКИЙ СТАТУС БОЛЬНЫХ РАКОМ ЖЕЛУДКА

The aim was to identify cytogenetic disorders, proliferative activity and apoptosis in cells exfoliated buccal and nasal epithelium in patients with gastric cancer. The study involved 10 patients with an established diagnosis. The control group included 30 healthy people. It has been revealed that all this parameters were increased in buccal and nasal epithelium in patients with gastric cancer than in healthy people. The detected changes are systemic in nature and reflect the overall condition of the body. Moreover, we show reduction of karyological parameters after radical treatment, which indicates their prognostic significance.Целью было выявление цитогенетических нарушений, нарушений пролиферативной активности клеток и апоптоза в эксфолиативных клетках буккального и назального эпителия у больных с впервые диагностированным раком желудка. Обследовано 10 пациентов с установленными диагнозами. В группу контроля вошли 30 условно здоровых людей. Показано увеличение доли клеток с цитогенетическими нарушениями в буккальном и назальном эпителии у больных раком желудка по сравнению со здоровыми. Выявленные изменения носят системный характер и отражают общее состояние организма. Кроме того, показано снижение уровня кариологических показателей после радикального лечения, что свидетельствует об их прогностическом значении.