László Oláh

The role of ultimate elongation in the determination of valid ligament range of essential work of fracture tests

In this study, the feasibility of a previously proposed ultimate elongation-based criterion of essential work of fracture (EWF) method was reconsidered for several materials. Suitable ductility level (DL) ranges for the EWF testing of polymers were discussed, too. It was shown that there is a relationship between the DL terms and the experimentally estimated crack opening displacement values. Based on Coterell’s estimate of the size of the fracture process zone a lower ligament limit of EWF concept was also presented. This lower bound should be considered for thin samples where the original thickness-dependent criteria seemed to be not severe enough.

Investigation of Time-Dependent Behavior of Starch-Based, Injection Molded Biodegradable Polymer

Nowadays as a result of the increasing environmental friendly consciousness and the decreasing oil reserves, natural origin based materials are developed. These materials are not only based on renewable, natural resources, but capable of biodegradation in compost. These biodegradable polymers (biopolymers) can be inserted into the nature’s recycling process. One of the most important biopolymers is starch. Starch can be processed like a thermoplastic material by adding plasticizers, creating the so called thermoplastic starch. However the price of starch is rather low, it has certain drawbacks retarding its wide industrial usage. It is sensitive to moisture, disrupts in pure water, has low mechanical properties, and in time it ages. This ageing or time-dependent behavior was analyzed with injection molded tensile specimens. The shrinkage and mechanical properties of the specimens were determined as a function of ageing time. The fracture surface was analyzed by scanning electron microscope. The results of the measurements were compared with previous examinations.