On-line interferometric study on the mechanical fracture behaviour by crazing observed in stretched polypropylene fibres

Abstract

Two-beam polarizing light interference microscope devised by Pluta is used for in situ investigation of fracture mechanism of as-spun isotactic polypropylene (iPP) fibres by crazing during cold drawing process. The study includes characterization of crazing of polypropylene fibres (involving craze initiation, craze propagation and craze breakdown) as a function of crazing strain, crazing temperature and stretching speed. The investigation of craze damage showed that it is increased rapidly with stretching speed and then increased slowly to level off. Also it was found that, stretching iPP fibre at relatively low speed (lower than 0.015 cm/sec) at constant temperature 19 °C would reduce the effect of fracture on the stretched filaments. The time to crazing, the time to failure and the areal craze density of iPP fibres during cold drawing process are estimated. Finally, observing the craze formation at different temperatures showed that there was a critical value of stretching temperature for the formation of crazes in iPP fibre, and it was found to be 40 °C. The obtained results are correlated to the corresponding variations in some optical and structural properties of iPP fibres due to stretching.