Studying And Characterizing The Fracture Behavior Of Polyolefin Materials By The Essential Work Of Fracture Method

Polymer fracture behavior of is a important factor for using and choosing polymer materials. The methods which are used to characterize polymer fracture, such as linear elastic fracture mechanics and J-integral analysis, are very difficult to evaluate the ductile polymer and ductile thin polymer sheets or film. A new method, the essential work of fracture (EWF) method, can overcome the difficulties and has been used to determine the fracture toughness of ductile polymer. In this paper, the essential work of fracture method is employed to analyze the fracture behavior of polyolefin materials and their blends, and several factors which effect using and results of the essential work of fracture method are also studied. Some valuable information which will be useful for using and studying the essential work of fracture method are obtained, main work and conclusion are as follows:1. Effect of specimen dimension and moulded process on fracture behavior of polyolefin was investigated. It was found that the specific essential work of fracture decreased with increasing the thickness of specimens, but the specific nonessential work of fracture did not change. The temperature of injection moulding effected application of the method of essential work of fracture, and a set of load-displacement curves for a range of ligament lengths of the DENT specimens were more geometrically similar when the specimens were mouldedat higher processing temperature, and the results of EWF were more reliable. It was also found that the load-displacement curves of polypropylene with ductile fracture behavior were more geometrically similar than that of polypropylene with brittle fracture behavior, and the EWF results shown more reliable than the latter. The different processing methods also influenced the fracture behavior of polymer, that was the inject-compressed samples shown more obvious necking at fracture process than injected samples.2. Fracture behavior of four types of co-polypropylene resins for pipe, which had different long term behavior, were analyzed. It was found that the values of yield strength of the four materials were close to each other, and among the materials with similar impact strength, the we of the material with good long term behavior was higher than that of the materials with bad long term behavior. The co-polypropylene with high impact strength was also probably without good long term fracture behavior. So, it could be said that the EWF parameters were more useful to characterize long term fracture behavior of polymer than yield strength and impact strength. An increase in crystal grain size of co-polypropylene probably resulted in a decrease in the specific nonessential work of fracture, which induced less ability in preventing fracture for long time.3. Fracture behavior of PP/POE blends was analyzed. It was found that low content of POE could toughen PP by cavitation, cracking and shear yielding in matrix. The load-displacement curves were more geometrically similar when the POE content was more than 5%, and the results of EWF were more reliable. It was also found that we,y of PP was improved greatly when POE content was less than 5%, and improved lightly with POE content at 5-7%. Fracture process of PP and PP/POE blends DENT samples includes crack growth region and fracture region.