Some aspects of the fracture mechanics of thin flexible films

A theory is presented for the determination of a number of physical constants which affect the fracture behaviour of thin plastic and viscoelastic materials. The theory is based on a relatively simple consideration of the energy balance in a tensile-test-strip with an edge-crack. The total energy needed up to fracture was measured, for each material, for 90 samples approximately, varying both the total length of the test-trips, and the length of the initial cracks. In the first instance, the width of the test-strips was kept constant at a value of 15 mm. Then, with the help of a special version of a multiple linear regression, values of physical parameters were obtained. Three materials were investigated, viz. a polyethylene, a polyester/polyethylene laminate and a polyamide/polyethylene laminate. Initial crack lengths between 0.1 and 1 mm were investigated, in two different ways, viz. with a static tensile test and with an impact tensile test. For each material, considerable differences between static and dynamic behaviour has been measured; a qualitative explanation, however, of these differences can not yet be given. Secondly, the number of data per material was reduced, in order to get an impression of the reliability of the applied method of approximation. It appeared that even a set of four test-strips for one material produced significant results. Next, an effort has been made to extrapolate the values obtained for test-strips with a width of 10 and 5 mm, respectively. In general, this extrapolation appeared to be inadmissable, implying that the underlying theory should only be considered as a rough approximation of a very complex reality. From the experiments and calculations it is concluded that in general the method adopted is suitable to quantify the fracture behaviour of thin films in industrial practice. © 1973.