Influence of Welding and Composition on the Short-Term Stable Crack Propagation Through Polyolefin Single- and Bilayered Structures

Abstract

The overall stable crack initiation and propagation behaviour of fracture mechanics specimens cut from plastic pipes that were composed of different polyolefin materials were investigated using concepts of elastic–plastic fracture mechanics including the crack propagation kinetics. The effect of specimen shape, orientation, welding and lading rate on the crack resistance (R) behaviour of these materials has been thereby assessed. It was found in principle that specimen shape, orientation and welding have an influence indeed but only an unexpected small one on crack initiation behaviour and, particularly, on crack propagation behaviour. The crack initiation toughness is not sensitive to the orientation in most cases. In contrast, the crack propagation toughness is significantly affected by the orientation where the values for crack propagation in extrusion direction are larger than ones for crack propagation crosswise to that. This confirms that the morphology affects the stable crack propagation behaviour more than the stable crack initiation behaviour. In agreement with results of the microindentation test, fracture mechanics investigations also show that a lower welding pressure and a larger welding temperature, respectively, have no or a positive effect on the mechanical and fracture mechanics properties, whereas a larger pressure and a lower temperature, respectively, result in deterioration of the performance of the welded joint. Furthermore, the R-curve behaviour was investigated using specimens cut from bilayer pipe segments. It has been shown that an additional layer has a clear impact on the R-curve behaviour compared to the crack propagation in single-layer pipes, which can be explained thereby that the plastic constraint was affected by this additional layer. For clarification of the toughness-in- or -decreasing effect of an additional layer (with differing mechanical characteristics) on the layer where the crack was growing, R-curve ratios were introduced, that showed that the asymmetric mechanical properties of different layers were directly reflected in an asymmetric impact on the stable crack initiation and propagation behaviour.