Application of J-integral approach for welded polymer structures

Some fire-service vehicles are equipped with tanks for quenching liquids made from extruded polypropylene copolymer (PP(RC)) sheets. This bulk polymeric material reveals pronounced ductility. Several hundred meters of extrusion welding joints are used in these structures, which may contain a large number of defects. Hence, this welded structure was considered as a model component for elastic-plastic fracture mechanics investigations. The first part of this study focuses on fracture tests performed on both 15 and 20 mm thick bulk extruded sheets of a polypropylene copolymer (PP(RC)) and on 4 configurations of their welded joints. The fully ductile fracture range was determined by rate dependent tests on single CT specimens and fracture toughness values (J_Fmax and CTOD_Fmax) were derived at the peak loads. These values were determined for stable crack extension based on the J-delta a and/or CTOD-delta a R-curves using single and multiple specimens. As expected, both methods revealed distinct differences between the bulk materials and the welded joints. In the second part of the study, the crack tip deformation behaviour of model subcomponents containing various process induced defect models were simulated. J-integral values and CTOD-delta a curves were calculated based on rate dependent elastic-plastic models. Due to the high data scatter and uncertainty of both the simulated crack tip loading in terms of J-integral and the experimentally measured fracture toughness in terms of J_c, a direct comparison of the nominal values for these parameters can be misleading and leads to over- or underestimation of the severity of the ductile fracture. Their applicability to proper dimensioning necessitates both for experimental and numerical fracture values on the same physical basis and with the same mathematical background. In spite of single nominal fracture toughness values, the entire R-curve functions and the complex relationships of their distribution function might be efficiently used for fracture assessment of components.