3.9.2 Fracture behaviour and toughening mechanisms
The deformation zones of the crack tip of SENT was studied using a transmission optical microscopy. Fig.19 shows the bright field images of the three samples, where the cavitates appear dark in the micrographs. The dark area on the TOM image is due to the stress whitening phenomenon in the crack tip of SENT. The observed stress whitening zone in the samples is either due to debonding of the EPDM particles from the PP matrix or to internal rubber cavitation which led to dilatation shear bands. The EPDM particles in the matrix act as stress concentration points, resulting in a distribution of strain constraints and improve the fracture toughness 46, 47. According to the Fig. 19, it was observed that the stress whitening zone of the PP/EPDM nanocomposite fabricated by FSP were significantly larger than the other two samples. It is clear in Fig. 19 (c) that a large number of dilatation shear bands in crack tip of FSP sample have been developed whereas the thickness of these bands is very smaller than those observed in PP/EPDM blends.The reason for the FSP sample larger stress whitening is attributed to the smaller size of EPDM particles and their uniform distribution in the PP matrix. A better dispersion of EPDM and nanoclay particles brings a more uniform stress distribution in the PP matrix which in turn causes a larger volume of the material to participate in energy dissipation, shock absorption process and improve fracture toughness. A larger deformation zone with smaller size of EPDM particles in the PP matrix has also been reported by Khodabandelou et al 47.
The Fig. 20 shows the SEM micrographs of the EWF test fractured surfaces for PP/EPDM blend and PP/EPDM nanocomposites fabricated by TSE and FSP. From Fig. 20 it is clear that fracture surface is comprised of micro and nanovoids and stretched fibrils which reflect a ductile crack growth mechanism. According to Fig. 20 (a) it was observed that in the PP/EPDM sample large voids were formed because of the debonding of EPDM particles from the matrix and also by internal cavitation and tearing of rubber. In this sample, the fibrils with larger thickness resulted from the larger distance between rubbers particles. Furthermore, it can be seen from Fig. 20 (b) that the stretched fibrils thickness and size of voids decreases with the addition of nanoclay to PP/EPDM blend. One may hypothesize that the nanoclay acts like a weak nucleating agent owing to its silicate nature and hence it decreases the size of PP spherulites of PP and the size of EPDM particles. Similar results were observed by Khosrokhavar et al and Pahlavanpour et al 7, 48. Some voids in fracture surface of TSE sample may be related to debonding of nanoclay aggregations from surrounding matrix because nanoclay particles cause stress concentration in the matrix and voids have been formed around of these particles. In this sample, it can be seen some stretched fibrils with smaller length that it is related to the presence of clay aggregations and larger voids around it in fibrils which leads to earlier tearing these fibrils. Fig. 20 (c) clearly shows that the size of voids and fibrils thickness is much smaller compared to other samples. In this sample length of stretched fibrils is higher so that after the fibrils tearing its covers the voids.This is in accordance with the results of EPDM particle size and interparticle distance obtained for three samples.
Fig. 21 illustrates the subsurface SEM micrographs obtained from the core of ligaments for three samples at two distances from crack plane (marked as FPZ and OPZ), according to Fig 4 (c). Figs 21 (a) and 22(a) illustrates the differences between the FPZ and OPZ for PP/EPDM composite sample. In the EWF test, initial stage is plastic deformation at the tip of crack of DENT specimen as demonstrated by stress whitening evidence on the sample. The second step is a full ligament yielding initiated by stress whitening that grows in the plane perpendicular to the loading direction from tip of crack to middle of ligament, which are formed the OPZ. Also in this step, voids growth in the direction of loading took place and it led to the formation of fibrils. After full ligament yielding, onset of crack propagation is observed, which is the stage where the crack will begin to propagate and continuously grow along the ligament with tearing of fibrils. Finally fracture occurs in FPZ with unstable crack propagation in the matrix. Fig 21 (a) – (c) shows deforming mechanism in FPZ which voids growth and fibrils formed. It is clear that with addition of clay to PP/EPDM blend the cavitation, fibrils thickness and void in the FPZ of the TSE sample decreases and stretched fibrils with longer length can be observed. In PP/EPDM nanocomposite fabricated by FSP the fibrils length is very long and this thickness is very smaller which these results conform with the larger stretched fibrils in SEM micrographs of fractured surfaces for this sample compared to other samples. Fig 22 (a) – (c) shows the subsurface SEM micrographs from three samples for OPZ which it is clear from the relative comparison of the subsurface zone in PP/EPDM and FSP sample that the small size of rubber particle leads to smaller initial voids in the FSP samples.In PP/EPDM sample it can be observed that the size of voids is larger and number of this voids is low which led to smaller stress whitening and fast crack growth in this sample compared to FSP sample. Similar results were reported by other researchers such as Zebarjad and Khodabandelou et al 47, 49.
Therefore, the failure mechanisms in PP/EPDM blend and PP/EPDM nanocomposite that occurred during EWF test in the followed thesesteps: (1) micro voids formation around of EPDM becauseof the lack of affinity between PP and EPDM droplatesand/or internal void formed in the rubber particle or in the nanocompsoites, as well as micro and nano voids because of debonding of clay particle from surrounding matrix, (2) void growth and formation of fibrils, (3) crazing formation (or craze-like), (4) diffused crazing and crack initiation by tearing of fibrils and (5) unstable crack propagation in the surrounding matrix andfracture.