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

摘要

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 ℃ 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% < ε_1 < 70%). For more important strains (ε_1 > 70%), no colouration is observed and the degradation is stopped by limitation of the soda diffusion in the material [1,2]. 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 ε_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.