A STUDY OF TRANSVERSE AND LONGITUDINAL SIZE EFFECTS IN HIGH-STRENGTH POLYETHYLENE FIBERS

The results of an experimental study of the effects of fibre diameter and testing length on the strength of polyethylene fibres prepared by the gel-spinning-hot-drawing process are reported. These size effects have been studied in fibres covering a range of mechanical properties in order to establish how size effects are related to fibre structure. It was found that both types of size effect studied are present in po lyethylene fibres, and that the magnitude of both changes with increas ing modulus of elasticity of the fibre; length effects become weaker a nd disappear completely, whereas diameter effects become more pronounc ed as the fibre modulus increases. This behaviour shows that transvers e and longitudinal size effects cannot be described simultaneously wit hin the framework of statistical strength theory. A model is proposed, based on the well known Griffith approach, which shows that a strengt h-diameter dependence may arise owing to geometrical effects rather th an from the presence of flaws and imperfections.