Effects of injection molding-induced morphology on the work of fracture parameters in rubber-toughened polypropylenes

The effects of the injection-molding induced skin-core morphology on the fr acture behavior of rubber-toughened polypropylene (RTPP) systems were studi ed by employing the essential work of fracture (EWF) method. RTPP with 31 w t% ethylene/propylene rubber (EPR) showed no skin-core structure after mold ing and the EWF approach worked well in this case. In contrast, RTPP with 1 0 wt% EPR exhibited a pronounced skin-core morphology: EPR depletion and en richment was observed in the skin and core region, respectively. This morph ology caused necking instead of crack growth in deeply double edge-notched (DDENT) specimens under tensile loading along the mold filling direction (M FD). The necking process not only was accompanied by a large scatter but al so yielded highly unrealistic specific essential work of fracture (w(e)) va lues. This skin-core structure was also the reason for an anistropic EWF re sponse of this system observed by loading the specimens both in longitudina l (L) and transverse (T) directions to the MFD. The failure sequence and it s characteristics were studied by light microscopy (LM) and infared thermog raphy (IT). It was concluded that the EWF approach cannot be applied for RT PP with a prominent skin-core structure. Since yielding preceded the limite d crack growth prior to necking in the loading direction for the DDEN-T spe cimen of RTPP with 10 wt% EPR the yielding-related specific essential work (w(e,y)) was used for toughness comparison. In case of RTPP with 31 wt% EPR , where yielding was less pronounced prior to the crack growth, the work of fracture until the maximum load was assigned to the yielding-related work of fracture (w(f,y)) used for computing w(e,y). The latter value seems to b e closely matched to the plane-strain essential work of fracture value.