Semi-ductile fracture

For this semi-ductile fracture mode, a method using two material parameters has been proposed to characterize the impact resistance of the polymer [11], The method takes account of an average value of the fracture energy during the stable propagation of the crack, and also the fracture energy at instability. In this case, the total energy absorbed by the sample to break, U, has been shown 

It is worth noting that this semi-ductile behavior has been found in other polymers Newmann and Williams [4] observed stable crack propagation before brittle fracture in ABS over the temperature range from —40 to 0°C Bernal and Frontini also observed this type of behavior in a rubber-modified thermoplastic at room temperature [12]. 

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This conclusion was only partly confirmed by scanning electron microscopy micrographs of RuC>4 stained surfaces taken at the crack tip of deformed specimens at 1ms-1, where the non-nucleated and /3-nucleated materials showed, respectively, a semi-brittle and semi-ductile fracture behavior. While some limited rubber cavitation was visible for both resins, crazes—and consequently matrix shearing—could not develop to a large extent whether in the PP or in the /1-PP matrix (although these structures were somewhat more pronounced in the latter case). Therefore, a question remains open was the rubber cavitation sufficient to boost the development of dissipative mechanisms in these resins ... 

Figure 27.2 Fracture surface of semi-ductile fracture (a) PA 11 at room temperature (b) HIPS at -40 C (c) variables considered in semiductile model...

Fig. 5 Evolution of the fracture energy, Gtot, with the temperature, T, for non-nudeated and /S-nucleated resins with different flowabilities a MFR 0.3 dgmin-1 and b MFR 2 dgmin-1. The ductile-brittle transition temperature was chosen in a somewhat arbitrary manner as the temperature corresponding to half of the maximum of Gt01 in the considered MFR range. It reflects the transition from a semi-ductile to a fully ductile behavior, without breaking of the tested specimen. The test speed was about 1.5 ms-1, the specimens were injection molded...

Karger-Kocsis recorded the different fracture behaviors of non-nucleated and -modified PP (MFR 0.8 dg min 1) tested in a three-point bending configuration at 1 ms-1 at 23 °C, a-PP was semi-ductile and /3-PP ductile with a plastic hinge at - 40 °C a-PP was brittle, /i-PP ductile [72], The descriptors from the linear elastic fracture mechanics (LEFM), Kq, the stress intensity factor, and Gc, the energy release rate, used to quantify the toughness correlated well with the fracture picture. This conclusion is also valid for... 

With increasing temperature, the fracture mode changes from brittle to semi-ductile at about -50 °C and then becomes ductile at temperatures higher than... 

The variation of fracture energy as a function of temperature for HIPS at a loading rate of 2 m/s is shown in Figure 27.7. Depending on the type of fracture involved, the fracture energy corresponding to brittle, semi-ductile or ductile behavior was determined according to Equation (2), (9) or (12), respectively. 

It can be seen that the transition from brittle to ductile behavior occurs with a peak in the value of fracture energy. As the temperature increases, a semi-ductile behavior is observed before the peak and the onset of ductile behavior of fracture. In the semi-ductile behavior, a certain amount of stable crack... 

Fig. 6. Impact properties of PET and blends. Percentage of ductile fractures as a function of temperature. Comparison between semi-crystalline (a) and amorphous PET (b). Results are obtained using a falling mass apparatus (initial energy of 147 J, initial velocity of 3.6 ms striker diameter of 25 mm and plaque diameter of 40 mm).

Selecting the ductile - semi-ductile transition as ductile-brittle transition was attempting but not realistic it lead admittedly to substancial reduction of the global fracture energy, but it is known to be highly geometry. Chosing between the semi-ductile - semi-brittle transition or... 

Figure 10.21 (a) A fractured semi-ductile steel tensile test specimen, showing necking and cup and cone fracture, (b)... 

This study has explored evaluation of fracture toughness for thin-waU injection moulded plastics from semi-ductile material to ductile material by means of EWF test. Stress criteria of EWF tests were investigated. It was understood that the current stress criteria may not be sufficient for some materials. In conclusion, the results presented in this paper, indicates the possibility of further inprovement in stress criteria therefore, in reproducibiUty of EWF test. 

Keff is always higher than Kimax- In the ductile range, Kjmax/Keff 0.70 0.03 for both grades whatever the test conditions. In case of unstable crack propagation, Kimax Ketr The values of Ken(iPP/EPR-l)/KeH(iPP/EPR-2) for given test conditions are close to those of Kimax(iPP/EPR-l)/Kimax(iPP/EPR-2) when both grades exhibit the same macroscopic behaviour. In other words, Kimax is a semi-quantitative toughness parameter, whereas Kefr provides a quantitative description of the fracture resistance. 

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