Molekulni mehanizmi dinamike lanca i ilavost polimera / Molecular chain dynamics and polymer fracture toughness

Nedavni primjeri pristupa modeliranju i ra unalnim prora unima plasti nosti i loma u polimernim sustavima dokazuju da se njihove karakteristike razlikuju od takvih procesa u kovinama. Fizi ke mehanizme plasti nosti i loma prate procesi lokalizacije. Plasti nost i smi no te enje dovode do zapo injanja i rasta smi nih vrpci koje su posljedica unutarnjeg omek avanja nakon te enja, za kojim slijedi usmjereno o vr ivanje. Do loma uobi ajeno dolazi zbog mikronapuklina, s lokaliziranim deformacijama u njima koje se pro iruju zbog mjesnoga plasti nog te enja vrlo malih veli ina. Porast mikronapuklina u polimerima mo e se, za staklaste polimere u vrstom stanju, opisati modelom gibanja polimera gmizanjem. Tek su nedavno razvijeni konstitutivni modeli plasti nosti i modeli kohezivne plohe za napukline koje sadr avaju bitne karakteristike tih procesa. To se posebice potvr uje usporedbom s vremenskom ljestvicom na kojoj su prou avani plasti nost i lom. Tako je jo uvijek ve ina pitanja o polimernom lomu neodgovorena, pa tomu mogu ra unalna i modelna mehanika znatno pridonijeti. / The examples of recent modelling and computational studies given in this paper serve to demonstrate that plasticity and fracture in polymeric systems have particular features that distinguish them from those in metals. Both in plasticity and fracture, the physical mechanisms involve localization. Plasticity, or shear yielding, leads to the initiation and propagation of shear bands, which are a consequence of intrinsic softening after yield followed by orientational hardening. Fracture is commonly caused by localization of deformation in the craze, which propagates because of the local plastic flow at a very small size scale. A constitutive model for plasticity and a cohesive surface model for crazing that capture these essential features have been developed only recently. This is especially true when compared to the time frame over which plasticity and fracture have been studied. Thus, there are still many open issues in polymer fracture and to resolve them a lot can be done using computational and modelling mechanics.