Analysis of localization in brittle materials through optical techniques

This paper is devoted to the experimental analysis of strain localization in rock like specimens subjected to uniaxial compression. Two different techniques are involved in this research: stereophotogrammetry and laser speckle photography. The strain fields obtained by these techniques exhibit a homogeneous response followed by a strain localization before the peak of the load verses displacement curve. These observations allow one to choose the appropriate mechanical constitutive equations to describe the behavior of such materials.     

Optimal feedback control policies for stochastic,distributed-parameter systems

This paper considers optimal feedback control policies for a class of discrete stochastic distributed-parameter systems. The class under consideration has the property that the random variable in the dynamic systems depends only on the time and possesses the Markovian property with stationary transition probabilities. A necessary condition for optimality of a feedback control policy, which has form similar to the Hamiltonian form in the deterministic case, is derived via a dynamic programming approach.     

Discontinuous crack growth in poly (vinyl fluoride) by mechanochemical ageing in sodium hydroxide

The mechanochemical degradation of poly (vinyl fluoride) (PVDF) was studied in sodium hydroxide solution (soda) in the presence of various gradients of imposed mechanical deformation. Compact tension (CT) and low-necked tensile specimens were used to study ageing in soda (pH 14) at 90 °C during up to 35 days. This made it possible to impose various strain fields during ageing. An image correlation method was used to estimate the displacement fields at the surface of the specimen during the mechanical tests. PVDF degradation in soda usually results in a reddish brown colouring and is essentially localised in the “fairly deformed zones” (30% < ?₁ < 70%). For more important strains (?₁ > 70%), no colouration is observed and the degradation is stopped by limitation of the soda diffusion in the material [1] and [2]. In the presence of an imposed strain field, there is a level of mechanochemical degradation “Stress Corrosion Cracking” (SCC) in soda for which the formation of microcracks is observed within a layer of exposed surface of degraded polymer. Cracking always starts at the border of the zone coloured by chemical degradation. The mechanical deformation in this bordering zone is approximately ?₁ ≈ 30%. The cracking continues via a discontinuous propagation of propagations and crack arrests usually called “stick-slip”. The fracture surfaces are marked by a succession of distinct coloured striations representing a temporary crack arrests characteristic of discontinuous crack growth. Cracking can stop with the elimination of one of the two combined causes of SCC, mechanical or chemical.