Investigation of the Fracture Mechanical Behavior of Amorphous Polymers Considering Crack Tip Heating

Abstract

Due to the steadily growing use of plastics, also for technically demanding applications, the selection of materials and their design are becoming increasingly important. A major part of the plastic design is the consideration of the temperature, whereas the heat development of the component under mechanical load has hardly been taken into account until today. Standard mechanical methods are often unable to describe molecular processes and the failure dynamics. Fracture mechanics methods in combination with imaging techniques offer the possibility to investigate the local failure much more precisely and represent a useful supplement to the standard testing methods. The unforeseen changes in the individual plastic properties due to the increased internal temperature changes during crack formation and the corresponding local softening can be considered in much greater detail. Thus, it has not yet been possible to clarify whether plastification at the crack tip inhibits or promotes crack growth. In order to be able to investigate this question, a test setup was implemented that determines basic fracture mechanics parameters and, in combination with a high-resolution thermographic camera provide temperature data with spatial and temporal resolution for each point on the so-called crack resistance curves. Three amorphous plastics were investigated in this study. These include a polystyrene and two polycarbonates with different chain lengths. To determine the mechanical properties, a tensile load is applied to pre-notched test specimens. In a first series of tests, the setup was used to determine the temperature change at the crack tip for test speeds between 1 mm/min and 250 mm/min. Due to the different polymer structure and the resulting different forces of attraction between the molecular chains of the polymers, a clear difference in the maximum temperatures at the crack surface between 45°C up to 90°C occurred. In addition, the material behavior had a major influence on the shape of the fracture process zone and showed a difference in the temperature data and strain rate recorded with the digital image system.