Determination Of The Fracture Toughness Of Thermoformed Polypropylene Cups By The Essential Work Method

In this work the anisotropic fracture behavior of neat and rubber-modified polypropylene cups induced by a vacuum thermoforming process is studied. The effect of different draw-ratios is also examined. An elasto-plastic fracture mechanics approach, based on the method of the essential work of fracture, is applied to samples cut from the thermoformed cups both longitudinally and transversally to the drawing direction. For all the products examined a strong anisotropy in the fracture behavior is observed. The fracture resistance for cracks propagating along the drawing direction is lower than in the transverse direction, this fracture anisotropy being enhanced at higher draw-ratio. The presence of the rubber particles results in an increase in the fracture toughness for cracks running transversally to the drawing direction, but, at the same time, it seriously reduces the fracture resistance for cracks propagating along the drawing direction. In order to better understand the dependence of the fracture behavior upon the processing condi-tions and material structure, dynamic mechanical thermal analysis, transmission electron microscopy, and X-ray diffraction measurements are also carried out.