Size effects of near plane-strain fracture parameters of polypropylene thin films with different molecular weight, tacticity and crystalline structure using the essential-work-of-fracture approach

01.10.2007 - 31.12.2011
Proprietary project
The essential-work-of-fracture (EWF) concept is based on the assumption that, for plane-stress conditions, the total work of fracture W dissipated in the deformation zone can be divided into two parts, one corresponding to a component We characterizing the inner (core) or fracture process zone and another component Wp corresponding to an outer or plastic zone. The specific work of fracture w is obtained after dividing W by the ligament area B·l: w = EWF + ßwp·l (l - ligament length, ß - shape of plastic zone). For pure plane strain or the plane strain/plane stress transition range, w = f(l) has been rarely investigated. Based on the fact that the intrinsic fracture process takes place in the inner zone, the term EWF, the "essential work of fracture", has the meaning of a "crack-moving force" comparable to that of the physical crack initiation values if the J-integral concept is valid. EWF is experimentally determined by extrapolation of w as a function of l to zero ligament length. For this, several specimens having different ligament lengths but are identical in all other respects are monotonically loaded using a universal testing machine. bwp is the "non-essential work of fracture" as a measure of the resistance against stable crack propagation. For application of the EWF concept the following conditions must be fulfilled, which are combined with an inspection of measured load-displacement diagrams and the samples: (i) crack initiation after plastification of the ligament, (ii) well-defined state of stress (mostly plane stress, rarely near plane strain), (iii) self-similarity of the load-deformation diagrams. In the first stage of the project simple types of materials such as polypropylene (PP) homopolymers have to be used due to clearly separate the influence of molecular architecture, crystalline structure and molecular weight as well as the size of the specimen (thickness) on fracture mechanics behaviour. A very few investigations using essential-work-of-fracture parameters as a function of crystalline structure, the molecular weight and the specimen thickness are available for PP material. But such a combination of influencing factors as intend to vary in the present project has never been investigated before. (1) Crystalline structure: non-nucleated and beta- and alpha-nucleated isotactic PP homopolymer (standard molecular weight): The alpha-PP/beta-PP ratio will be varied by controlled thermal treatment (monoclinic alpha-phase, hexagonal beta-phase, mixed alpha/beta-phase). The smectic phase will be produced by quenching. Additionally, the pure smectic phase can be used as a basic material for annealing using different temperatures to obtain alpha-PP with less ordered crystallites. The size of spheroliths and the degree of crystallinity have to keep approximately constant. (2) Variation of the tacticity of standard-molecular-weight PP homopolymer (from isotactic PP to syndiotactic PP). (3) Variation of the molecular weight of PP homopolymer including PP type with standard molecular weight and PP types with molecular weights lower and higher than that of the standard PP. (4) Variation of the film thickness B (typically several ten micrometer) of sharply single-edge or double-edge-notched tension (SENT, DENT) specimens having width of W ~10 mm and gauge length of ~20 mm; the ligament length l (unnotched part of the width) has to varied between ~2 mm and ~8 mm.

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Keywords

GermanEnglish
Wahre BrucharbeitEssential work of fracture
Polypropylen-HomopolymerPolypropylene homopolymer
Kristalline StrukturCrystalline structure
TaktizitätTacticity
MolekulargewichtMolecular weight

External partner

  • Borealis Polyolefine GmbH

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