Propagation energy

Material Initiation energy, f Propagation energy, f Total fracture energy, ... 

In particular, standard specimens contain a sharp notch so that it is propagation energy rather than initiation energy which is the dominant factor. In general the standard tests are useful for quality control and specification purposes but not... 

To explain this inconsistency in the case of fracture with ductile behavior, another approach taking into account the crack initiation and crack propagation energies in the material is needed and has been proposed [18,19]. This approach assumes that the fracture energy of the polymer with ductile behavior varies linearly with crack extension and is given by... 

Figure 27.19 shows the variation of U/A as a function of A, according to the proposed model of crack initiation and propagation energies. From the intercept of this plot, a fracture energy value of 4.7 kJ/m2 can be obtained for ABS. This value is much lower than that determined above, using the method developed for brittle fracture or the constant fracture energy consideration. 

The effect of crystallinity on the PP fracture behaviour was observed from tests on the neat polymer, by using different crystallisation temperatures and annealing treatment spherulite sizes range from 20 pm to 80 pm, and crystallinity X. from 64% to 75% [20 -21]. As the crystallinity is increased, the elastic modulus is enhanced and the toughness (both critical energy Jq 2 and propagation energy) is considerably reduced a ductile to brittle transition is observed at Xg > 70% This is coherent with results from Ouedemi [22]. 

Fig. 1 Evaluation of the elastic-plastic crack propagation energy rate, J, from incremental crack growth measurements.

The Method Used to Define the Transitions. In determining these three transitions, the most important parameters are the relative size of the whitened area on the fracture surface (normalized by the entire surface of the ligament), and the decrease in propagation energy. These parameters define the transitions marked by arrows in Figure 5. The parameters are listed in Table II. Figure 5 shows an example of how the transitions were defined using the data for 2 L15. The results obtained for all materials, as well as the crack velocities measured, are summarized in Table III. 

Figure 4. Notched tensile impact propagation energy versus piston speed for PA-6-EPR blends with different rubber contents. Key A, 0 vol% , 6.3 vol% +, 12.5 vol% , 18.4 vol% and O, 24.3 vol% (13).

As discussed in Part 12.4.3 Fracture Mechanics Testing, it is important to determine the material parameters (the initiation and the propagation energies) using the specimen geometry that reduces the effects of geometry to an acceptable level. 

---